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University of Wollongong Thesis Collections
2013
Thermally sensitive conducting hydrogels
Nuchalee Thipmonta
University of Wollongong
Recommended Citation
Thipmonta, Nuchalee, Thermally sensitive conducting hydrogels, Doctor of Philosophy thesis, Department of Chemistry, University
of Wollongong, 2013. http://ro.uow.edu.au/theses/3961
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Thermally sensitive conducting hydrogels
A thesis submitted in fulfilment of the
requirements for the award of the degree
DOCTOR OF PHILOSOPHY
From
UNIVERSITY OF WOLLONGONG
By
Nuchalee Thipmonta
B.Ed. (Science Physics), M.Sc. (Physics)
March, 2013
I
“This thesis is dedicated to my beloved dad, mom and
husband for their powerful and endless true love,
huge encouragement and kindness done
without expecting any return.”
II
CERTIFICATION
I, Nuchalee Thipmonta, declare this thesis, submitted in fulfilment of the
requirements for the award of Doctor of Philosophy, in the department of
Chemistry, University of Wollongong, is wholly my own work unless otherwise
referenced or acknowledged. The document has not been submitted for
qualifications at any other academic institution.
Nuchalee Thipmonta
March, 2013
III
Publications and conference presentations
Manuscript in preparation
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, “Electrochemically
Produced Responsive Hydrogel Coatings”
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, "Shrinkable Electronic
Conductor: Carbon nanotube/Hydrogel Composites”
Oral presentation
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, “Electrochemically
Produced Responsive Hydrogel Coatings”, 23rd Chemistry & IPRI Conference
2009.
Poster presentation
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, "Gel encapsulation of
carbon nanotube electrodes", the International Conference on Nanoscience and
Nanotechnology ICONN 2010, 22-26 February 2010, Sydney Convention &
Exhibition Centre, Darling Harbour, NSW, Australia.
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, "Gel encapsulation of
carbon nanotube electrodes", ACES End-user Technology Showcase, ARC Centre
of Excellence for Electromaterials Science (ACES) 14th October 2010 at Ocean
Room iC Central, Innovation Campus, Squires Way, Fairy Meadow. University of
Wollongong, NSW, Australia.
IV
N. Thipmonta, P.G Whitten, S.E. Moulton, G.G. Wallace, "Shrinkable Electronic
Conductor: Carbon nanotube/Hydrogel Composites”, 2011 Electromaterials
Symposium, ARC Centre of Excellence for Electromaterials Science (ACES) 9-11
February 2011 at Ocean Room iC Central, Innovation Campus, Squires Way,
Fairy Meadow. University of Wollongong, NSW, Australia.
V
ABSTRACT
This thesis aims to synthesize thermally sensitive electrically conducting
hydrogels. Poly(N-isopropylacrylamide) (PNIPAM) is the most common
thermally responsive hydrogel and exhibits a lower critical solution
temperature (LCST) which can be reversibly switched from a hydrophilic,
swollen state to a hydrophobic, collapsed state. For constructing thermally
sensitive electrically conducting hydrogels, this project is limited to the use of
single-walled carbon nanotubes (SWNTs) as these materials are especially good
candidates from a variety of carbon nanotubes because of they have relatively
low bending stiffness. PNIPAM hydrogel composites containing SWNTs are
expected to be promising materials because they can combine two or more
dominant properties from both materials to produce novel structures with new
properties.
The investigation of hydrogel composite materials began with the encapsulation
a SWNT Buckypaper with the thermally sensitive PNIPAM hydrogel. SWNT
Buckypaper was fabricated using a vacuum-filtration method. The fabrication of
PNIPAM hydrogel coating on SWNT Buckypaper was achieved using
electrochemically induced free radical polymerization the based on the
electrochemical reduction of persulfate anion. In addition, the encapsulation of
gold coated Mylar was also investigated as a background electrode for
comparison with Buckypaper. The PNIPAM hydrogel was formed within and/or
adjacent to the electrode surfaces. It could be readily observed by eye and in
VI
more detail using scanning electron microscopy (SEM). In addition, the
formation of PNIPAM on gold coated Mylar, RVC and Buckypaper electrodes
was confirmed by cyclic voltammetry which was performed before and after
polymerisation, the FTIR spectra of PNIPAM exhibit all of the bands expected
for PNIPAM such as amide II, C=O, -CH2-CH3 and CONH2 and cyclic
voltammograms performed after polymerisation in the presence of potassium
ferricyanide comparing with bare electrodes. A PNIPAM hydrogel coated on
gold coated Mylar underwent a transition when varying the temperature of the
electrolyte in an aqueous solution containing sulfate with the LCST transition
occurring between 10C and 20C. The effect of salt on the LCST of PNIPAM
coated on gold Mylar was studied by UV-vis determining the change in
absorbance and shows a decrease in LCST of the gel as a function of increasing
salt (
) concentration in the hydrogel. Importantly, electrochemically
induced free radical polymerisation can be used to deposit smart coatings on
electrode which regulate permeability of reactants based on temperature.
The second investigation was carried out with the use of SWNTs for the purpose
of electronic conducting fillers within a thermally sensitive hydrogel.
PNIPAM/SWNT hydrogel composites as shrinkable electronic conductors
studied here were successfully synthesised with various SWNT loadings: 0,
0.05, 0.1, 0.5 and 1.0 wt% through free radical polymerisation. The SWNTs were
homogeneously dispersed in the aqueous solution of PNIPAM using SDBS as a
surfactant under sonication prior to polymerisation as evidenced by optical
microscopy. The ability of hydrogel composites to swell in water decreased with
further increasing SWNT loading. Thus, the swelling ratio of the composite gels
VII
depends mainly on the crosslinking density. For the temperature-response
studied, the onset of the LCST was measured to be 30C. All hydrogels except
0.5 and 1.0 wt%, reached the equilibrium thickness at 40C, with 0.5 and 1.0
wt% reaching equilibrium at 45C. The extreme volume change of hydrogels
with high SWNT loadings (0.5 and 1.0 wt%) are related to low equilibrium
swelling ratios and equilibrium water contents due to SWNTs decreasing the
extent of chemical crosslinking. In addition, the level of compression strength
and breaking strain and compressive modulus increased as function increasing
SWNT concentrations into the hydrogel matrix. The changes of the mechanical
properties of the gels are observed corresponding to the large change in
swelling ratio. However, the hydrogel containing low concentration of SWNT
demonstrated no considerable differences in the mechanical properties, while
0.5 and 1.0 wt% SWNT can be significantly improved the mechanical properties.
The electronic properties of SWNT/NIPAM thermally sensitive composites were
quantified by EIS plots below and above the LCST. The composite hydrogels has
been thermally cycled between two temperatures through the heating-cooling
cycles. The resistance of hydrogels decreased as a function of increasing SWNT
concentration. The extreme resistance changes of hydrogels with high SWNT
loadings (0.5 and 1.0 wt%) are related to volume changes and low equilibrium
swelling ratios and equilibrium water contents.
The final investigation was on the preparation hydrogel composites using
Laponite XLG ([Mg5.34Li0.66Si8O20(OH)4]Na0.66) clay as physical crosslinker
without the need of a chemical crosslinker by free radical polymerisation at
room temperature. The FTIR spectra of PNIPAM exhibit all of the bands
VIII
expected for PNIPAM such as amide II, C=O, -CH2-CH3 and CONH2 and for
synthetic Laponite clay. The swelling ratio decreased with increasing clay
loading. Clay concentration is the most important parameter of PNIPAM/clay
gel for going from solid-like to liquid-like behaviour with these chemical
compositions in this study as PNIPAM/clay hydrogels were observed that the
extrudable testing of the gels strongly depends on clay content. Moreover, the
elastic modulus significantly improved as a function of increasing clay contents
as resulted from rheological oscillatory frequency sweeps, creep-recovery and
shear yield experiment applying shear strain at constant shear rate tests. In
addition, the elastic modulus of the hydrogel composites containing SWNTs
significantly improved with increasing SWNT concentrations. The LCST
transition-dynamic
mechanical
behaviour
of
the
PNIPAM/clay
and
SWNT/PNIPAM/clay composite hydrogels show that elastic moduli rapidly
increased around 35C to 40C in both composite gels. These approaches can be
used to form thermally sensitive injectable hydrogels which are highly desirable
in clinical applications.
IX
ACKNOWLEDGEMENT
I would like to thank my supervisors Prof. Gordon Wallace, Assoc. Prof. Simon
Moulton and Dr. Philip Whitten for their excellent supervision and great help.
Thank you very much for giving me the guidance and motivation that was
needed to lead me in the correct direction. Thanks for taking care of and being
patience with my thesis.
Given a great impression, thanks to Assoc. Prof. Chee On Too for reading and
commenting on my thesis and also giving me great advice on everything. Many
thanks to Dr. Damia Mawad, Dr. Klaudia Wagner and Dr. Lucie Viry for being my
good friends, sisters and advisors, always listening to me and giving me great
advice on everything. Thanks to Dr. Sina Naficy for reading and discussing some
parts of my thesis.
Huge thanks to all IPRI members and visitors who are still here or have left
including Tony, Pawel, Benny, Syed, Sanjeev, Johana, Patricia, Wid, Adrian,
Suriya, Satein, Ali, Dorna, Sina, Shayan, Sara, Mohammed, Jared, Cameron, Willo,
Troy, Mark, Sepidar, Peter, Bo, Wen, Weimin, Dennis, Paul, Natda, Kanlaya, Benz
and Yuth for their help and support in both work and personal life.
Financial support from Rajamangala University of Technology Srivijaya and
University of Wollongong for an International Postgraduate Tuition Award
(IPTA) are greatfully acknowledged.
Finally, I would like to thank my beloved dad (Mr. Satit Khunjan) who passed
away in December 2011 for all good things of his kindness done to me. He
taught, encouraged and supported me in every aspect of my life. His love is the
greatest love for me and unforgettable. He always stays beside me and will be in
my heart forever. I believe that he knows and that he is proud of me for fulfilling
his wish. I also would like to thank Khunjan and Thipmonta families for
supporting and encouraging me for four and a half year of my life in Australia.
X
Especially, I would like to thank my mom who is always supporting me and
praying for me to obtain this degree successfully. I respect her patience during
my absence from her all these years.
XI
TABLE OF CONTENTS
ABSTRACT...........................................................................................................................................VI
ACKNOWLEDGEMENT....................................................................................................................X
TABLE OF CONTENTS..................................................................................................................XII
LIST OF FIGURES............................................................................................................................XV
LIST OF TABLES...........................................................................................................................XXV
LIST OF ABBREVIATIONS.......................................................................................................XXVI
1 INTRODUCTION.......................................................................................................................... 3
1.1
General introduction ................................................................................................. 3
1.2
Hydrogel materials .................................................................................................... 4
1.2.1 Definition, classification and properties ....................................................... 4
1.2.2 The lower critical solution temperature (LCST) and the upper
critical solution temperature (UCST) ........................................................................... 8
1.2.3 Poly(N-isopropylacrylamide) ......................................................................... 10
1.2.4 Injectable hydrogels ........................................................................................... 13
1.3
Carbon nanotubes .................................................................................................... 15
1.3.1 Carbon nanotubes (CNTs) ................................................................................ 15
1.3.2 Single-walled nanotubes (SWNTs) ............................................................... 18
1.3.3 Dispersion of single-walled nanotubes ....................................................... 20
1.3.4 Electronic conduction in SWNT-polymer composites .......................... 22
1.4
Hydrogel composites............................................................................................... 24
1.4.1 Physically crosslinked clay – hydrogel composites ................................ 24
1.4.2 Carbon nanotubes-polymer hydrogel composites ................................. 29
1.5
Aims of the project ................................................................................................... 31
1.6
Chapter Summaries ................................................................................................. 31
1.7
References ................................................................................................................... 35
2 Gel Encapsulation of Carbon Nanotube Electrodes .................................................... 56
2.1
Introduction ................................................................................................................ 56
2.2
Experimental .............................................................................................................. 60
2.2.1 Chemical and Materials ..................................................................................... 60
2.2.2 Fabrication of Buckypaper ............................................................................... 61
2.2.3 Formation of hydrogel encapsulation on conducting electrodes ..... 63
XII
2.2.4 Morphology ............................................................................................................ 70
2.2.5 FTIR analysis ......................................................................................................... 72
2.2.6 Effect of chemical crosslinking ....................................................................... 75
2.2.7 Cyclic Voltammetry with Potassium Ferricyanide ................................. 75
2.2.8 UV-Vis-NIR spectrophotomer ......................................................................... 76
2.3
Results and discussions ......................................................................................... 79
2.3.1 Preparation of Buckypaper .............................................................................. 79
2.3.2 Hydrogel encapsulation of conducting electrodes.................................. 80
2.3.3 Cyclic Voltammetry before and after polymerisation on different
substrates. ............................................................................................................................. 86
2.3.4 FTIR analysis ......................................................................................................... 91
2.3.5 Morphology ............................................................................................................ 92
2.3.6 Effect of chemical crosslinking ....................................................................... 97
2.3.7 Cyclic Voltammetry with Potassium Ferricyanide ................................. 98
2.3.8 Effect of salt on the LCST of PNIPAM coated on gold coated Mylar
106
2.4
Conclusions .............................................................................................................. 111
2.5
References ................................................................................................................ 113
3 Shrinkable Electronic Conductor: Carbon nanotube /Hydrogel Composites 120
3.1
Introduction ............................................................................................................. 120
3.2
Experimental ........................................................................................................... 122
3.2.1 Chemical and Materials .................................................................................. 122
3.2.2 Preparation PNIPAM/SWNT hydrogel composites ............................. 122
3.2.3 Swelling properties .......................................................................................... 125
3.2.4 Phase transition behaviour ........................................................................... 126
3.2.5 Mechanical analysis ......................................................................................... 126
3.2.6 Electrochemical Impedance Spectroscopy (EIS) .................................. 127
3.3
Results and Discussions ...................................................................................... 132
3.3.1 Preparation of NIPAM/SWNT hydrogels................................................. 132
3.3.2 Swelling and Mechanical analysis .............................................................. 135
3.3.3 Phase transition behaviour ........................................................................... 140
3.3.4 EIS analysis.......................................................................................................... 142
3.4
Conclusions .............................................................................................................. 152
XIII
3.5
References ................................................................................................................ 155
4 Thermally sensitive injectable conducting hydrogels ............................................ 160
4.1
Introduction ............................................................................................................. 160
4.2
Experimental ........................................................................................................... 164
4.2.1 Chemical and Materials .................................................................................. 164
4.2.2 Preparation of PNIPAM/Clay hydrogel composites ............................ 164
4.2.3 Preparation of SWNT/PNIPAM/ Clay hydrogel composites ............ 166
4.2.4 Fourier Transform Infrared Spectroscopy ............................................. 166
4.2.5 Swelling properties .......................................................................................... 167
4.2.6 Extrudable testing ............................................................................................ 168
4.2.7 Rheological analysis......................................................................................... 170
4.3
Results and discussions ...................................................................................... 179
4.3.1 Appearance and network structure of the hydrogel composites... 179
4.3.2 Swelling properties .......................................................................................... 183
4.3.3 Extrudable testing of hydrogels .................................................................. 184
4.3.4 Rheological properties of PNIPAM/Clay hydrogels ............................ 188
4.4
Conclusions .............................................................................................................. 214
4.5
References ................................................................................................................ 217
5 SUMMARY, Conclusions and Recommendations for future work ..................... 223
5.1
Summary and conclusions ................................................................................. 223
5.2
Recommendations for future work ................................................................ 228
5.3
References ................................................................................................................ 229
XIV
LIST OF FIGURES
Figure 1.1 Insoluble network of polymer chains that swell in aqueous solutions.
Lines indicate polymer chains. Circles indicate crosslink sites. ........................ 5
Figure 1.2 (A) A sliding double ring crosslinking agent was produced by
chemically crosslinking two cyclodextrin molecules, each threaded on a
different PEG chain (end-capped with a bulky group, such as adamantan),
(B) double network hydrogel are a subset of interpenetrating networks
(IPNs) formed by two hydrophilic networks, one highly crosslinked, the
other loosely crosslinked and (C) clay nanocomposite hydrogels are
organic-inorganic hybrids where polymer is grafted on the clay surface by
one or two ends [14]. .......................................................................................................... 7
Figure 1.3 Chemical structure of poly(N-isopropylacrylamide). .............................. 10
Figure 1.4 At temperatures above the LCST, PNIPAM undergoes a reversible
phase transition from a swollen hydrophilic state to a collapsed
hydrophobic state [7]. ...................................................................................................... 11
Figure 1.5 Publication in database (The Chem Abstracts online search (CAS)
(done 16/07/12) was based on the search of poly(N-isopropylacrylamide)
with removal of duplicates). .......................................................................................... 13
Figure 1.6 Dr. Sumio Iijima was awarded the 2007 Balzan Prize for Nanoscience
for his discovery of carbon nanotubes, and in particular the discovery of
single walled carbon nanotubes and the study of their properties, by the
International Balzan Prize Foundation [142]. ........................................................ 16
Figure 1.7 Publication in database (The Chem Abstracts online search (CAS)
(done 16/07/12) was based on the search of carbon nanotubes with
removal of duplicates). .................................................................................................... 17
Figure 1.8 Some SWNTs with different chiralities. The difference in structure is
easily shown at the open end of the tubes where (a) armchair structure, (b)
zigzag structure and (c) chiral structure [160]. ..................................................... 19
Figure 1.9 Molecular structure of (A) triton X-100, (B) SDS and (C) SDBS ........... 22
Figure 1.10 Schematic representation of the preparation of clay aqueous
suspension and the formation of the house-of-cards structure: (A) clay, (B)
XV
dispersion (exfoliation) of clay platelets in water and (C) house-of-cards
structure [182]. ................................................................................................................... 25
Figure 1.11 (A) Schematic representation of a 100 nm cube of PNIPAM/clay gel
consisting of uniformly dispersed inorganic clay and two primary types of
flexible polymer chain,  and g, grafted onto two neighbouring clay sheets
and one clay sheet, respectively. (B) Elongated structure model for
PNIPAM/clay gel and (C) conventional PNIPAM/BIS gel network structure
model [188]. ......................................................................................................................... 27
Figure 1.12 A heat shrinkable electronic conductor undergoes a reversible
phase transition from a swollen hydrophilic to a collapsed hydrophobic
state. ........................................................................................................................................ 33
Figure 2.1 Setup of an electrochemical quartz crystal microbalance which tracks
the electrochemically induced polymerization of NIPAM on the front
electrode of a quartz crystal resonator [227]. ........................................................ 59
Figure 2.2 Sonicator ultrasonic (A) probe and (B) bath Systems and the Leica
DMF-52 optical microscope (C). ................................................................................... 62
Figure 2.3 (A) Schematic diagram (adapted from [239]) and (B) the experiment
setup showing fabrication of a vacuum-filtration method for making the
Buckypapers......................................................................................................................... 63
Figure 2.4 Setup of a three‐electrode electrochemical cell configuration for
electrochemically induced free radical polymerization of NIPAM on
Buckypaper; WE = working electrode (Buckypaper), RE = reference
electrode, AE = auxiliary electrode. ............................................................................ 64
Figure 2.5 Linear triangular potential excitation waveform of cyclic
voltammetry. ........................................................................................................................ 68
Figure 2.6 Typical cyclic voltammogram where ipc and
ipa
show the peak
cathodic and anodic current respectively for a reversible cyclic
voltammetry. ........................................................................................................................ 69
Figure 2.7 A typical SEM instrument, showing the electron column, sample
chamber, EDS detector, electronics console, and visual display monitors. . 71
Figure 2.8 (A) Infrared radiation is passed through the sample and (B) FTIR
instrumentation process: the sequence of steps involved in obtaining an IP
spectrum using an FTIR [251]. ..................................................................................... 73
XVI
Figure 2.9 FTIR spectrometer (IRPrestige-21, Shimadzu, Japan) ............................ 74
Figure 2.10 UV-Vis-NIR spectrophotometer (Varian Australia Pty Ltd, Cary500) ......................................................................................................................................... 77
Figure 2.11 A schematic of the monochromatic radiation passes through a
homogeneous solution in a cell. ................................................................................... 78
Figure 2.12 Typical morphologies of as-synthesised Buckypaper fabricated by
filtering a stable suspension of SWNTs. (A) Photograph of a piece of
Buckypaper, (B) low-magnification SEM image, (C) high magnification SEM
image of Buckypaper surface, and, (D) cross section image of neat
Buckypaper........................................................................................................................... 80
Figure 2.13 Electrochemical production of free radicals and formation of
hydrogel coating at the substrate surface. ............................................................... 81
Figure 2.14 Cyclic voltammograms of gold coated Mylar electrode taken at a
scan rate of 50mV/s, 5 cycles in the reaction mixture (CV, -1V to 0.2V) at
different temperatures (A) in an ice bath (approx. 4C), (B) at room
temperature, (C) at 40C. The dashed arrows indicate the direction of the
potential scan....................................................................................................................... 83
Figure 2.15
UV-vis absorption recorded at room temperature of PNIPAM
coated on gold coated Mylar polymerised at different temperatures. .......... 85
Figure 2.16 Photographs show the PNIPAM gel coating on (A) gold coated Mylar
(width= 1 cm), (B) RVC (60 ppi, width= 1 cm) and (C) Buckypaper (width=
0.5 cm), where arrows show the parts of PNIPAM coated on the electrodes.
Images from an optical microscope show the PNIAPM gel coating on (E)
RVC, 60 ppi where the black areas are RVC surfaces. .......................................... 87
Figure 2.17 Electrochemically induced free radical polymerisation was initiated
by applying a voltage of -0.8 V versus Ag|AgCl reference electrode for 60
min in the mixture of 0.3 M NIPAM, 3 mM MBA and 10 mM KPS adding into
0.4 M K2SO4 as an electrolyte solution for a PNIPAM coated on different
substrates in an ice bath where (A) gold coated Mylar, (B) RVC, 60 ppi and
(C) Buckypaper. Cyclic voltammograms (-1 to 0.2 V, 50 mV/s (gold coated
Mylar and RVC), 5 mV/s (Buckypaper)) were performed in the same
mixture before (bold line) and after (dash line) polymerisations for 5 cycles
(3rd cycles are shown). Arrows show the direction of potential scan. .......... 90
XVII
Figure 2.18 FTIR spectrum of a PNIPAM film which coated on gold coated
Mylar. The spectrum was taken in % transmittance mode. ............................. 92
Figure 2.19 SEM images of surface of gold coated Mylar coated with PNIPAM at
various magnifications; (A) x 500, (B) x 2,000 and (C) x 10,000. ................... 93
Figure 2.20 Cross-sectional SEM image of dried gold coated Mylar coated with
PNIPAM where 3 parts are resin (i), PNIPAM hydrogel (ii) and gold (iii). .. 94
Figure 2.21 SEM images of (A) surface of neat Buckypaper, (B) cross section of
neat Buckypaper, (C) surface of Buckypaper coated with PNIPAM and (D)
cross section of Buckypaper coated with PNIPAM where (i) is polymer
coating part and (ii) is Buckypaper part. .................................................................. 95
Figure 2.22 SEM images of dried sample cross section of (A) thickness of
Buckypaper coated with PNIPAM, (B) Buckypaper coated with PNIPAM
part and (C) high magnification image of the interface polymer coating Buckypaper coated with PNIPAM................................................................................ 96
Figure 2.23 The electrochemically induced free radical polymerisation was
initiated by applying a voltage of -0.8 V versus Ag|AgCl reference electrode
for 60 min in the mixture of 0.3 M NIPAM, 10 mM KPS and 0.4 M K 2SO4
without adding crosslinker for a PNIPAM coated on gold coated Mylar
substrate. Cyclic voltammograms (-1 to 0.2 V, 50 mV/s) were performed in
the same mixture before (bold line) and after (dash line) polymerisations
for 5 cycles (3rd cycles were picked). Arrows show the direction of
potential scan....................................................................................................................... 98
Figure 2.24 (A) Cyclic voltammograms of PNIPAM coated on gold coated Mylar
in K3[Fe(CN)6] (5 mM in 0.4 M aqueous potassium sulfate) with increasing
temperature from 10C to 50C and then decreasing temperature back to
10C at scan rate 50 mV/s at a scan rate of 50 mV/s. The arrows show the
direction of potential scan. (B) Ipc and Ipa are cathodic and anodic peak
current
respectively
for
cyclic
voltammograms
with
increasing
temperature from 10C to 20C where the lines are included to guide the
eye only. .............................................................................................................................. 102
Figure 2.25 Cyclic voltammograms of PNIPAM coated on (A) RVC and (B)
Buckypaper electrodes in K3[Fe(CN)6] (5 mM in 0.4 M aqueous potassium
sulfate) with increasing temperature from 10C to 50C and then
XVIII
decreasing temperature back to 10C at scan rate 50 mV/s. The arrows
show the direction of potential scan. ...................................................................... 105
Figure 2.26 Cyclic voltammograms of Buckypaper electrodes in K3[Fe(CN)6] (5
mM in 0.4 M aqueous potassium sulfate) with various scan rates from 5 to
50 mV/s. The arrows show the direction of potential scan. .......................... 106
Figure 2.27 (A) UV-vis of PNIPAM coated on gold coated Mylar in water with
increasing from 25C to 40C and then decreasing temperature with
(, backslash) at scan rate of 200 nm/min and (B) the sharp changes in
absorbance at wavelength 360 nm of samples measuring in 0 M, 0.1 M and
0.4 M K2SO4 (filled and opened makers represent heating and cooling
temperature, respectively). Bare gold coated Mylar was used as the
background in the respective electrolytes. The lines are included to guide
the eye only. ...................................................................................................................... 108
Figure 2.28 Comparison the LCST transition of PNIPAM gel as a function of the
concentration using (NH4)2SO4 (ref [229]) and K2SO4 (this study).
................................................................................................................................................ 110
Figure 3.1 Schematic representation: (A) top view of a square hole rubber
mould (1 cm x 1 cm x 0.15 cm) placed between parallel stainless mesh
electrodes and glass slides, (B) side view of the same where the indentation
was made for taking the rubber off after the gel formation and (C)
PNIPAM/SWNT hydrogel formed between parallel edge cut stainless
meshes. ................................................................................................................................ 124
Figure 3.2 Diagram of the compression test. ................................................................. 127
Figure 3.3 Sinusoidal current response in a linear system. ................................... 128
Figure 3.4 Bode plots i.e., |Zreal| versus log  and phase angle versus log  plots
[278-280]. .......................................................................................................................... 130
Figure 3.5 Chart of electrolyte temperature versus time of the impedance
measurements. ................................................................................................................. 132
Figure 3.6 Optical images of SDBS aqueous dispersions of the different SWNTs
which containing NIPAM (A) 0.05 wt%, (B) 0.1 wt%, (C) 0.5 wt% and (D)
1.0 wt% SWNT which were taken immediately after sonication (scale bar:
100 m)............................................................................................................................... 133
XIX
Figure 3.7 Schematic illustration of various surfactant assembly structures on a
SWNT, including; (A) SWNT encapsulated within a cylindrical surfactant
micelle, side and cross-section views, (B) SWNT covered with either
hemispherical micelles and (C) randomly adsorbed surfactant molecules,
where the blue is the alkyl carbon chain and the red is the charge head
group.[283]. ....................................................................................................................... 134
Figure 3.8. A black homogenous gels were formed separately (A) in syringe and
(B) between stainless mesh electrodes. ................................................................. 135
Figure 3.9 Compression strength dependence of breaking strains of the
hydrogel various SWNTs loadings. .......................................................................... 140
Figure 3.10 The normalized volume of the gel as a function of temperature.
Normalized volume of the gel was calculated from the ratio of volume of
the gel at each temperature to the volume of the gel at 20C. ....................... 141
Figure 3.11 Impedance spectra were obtained between frequencies of 0.1 Hz
and 100 kHz with AC amplitude of ±10 mV of parallel stainless mesh
electrodes (A) with no gel and (B) with PNIPAM hydrogel containing 0.1
wt% SWNT. All experiments, the samples were measured in various NaNO3
concentrations at room temperature and held for 30 min prior to
commencing measurement. ........................................................................................ 145
Figure 3.12 (A) Impedance spectra were obtained between frequencies of 0.1
Hz and 100 kHz with AC amplitude of ±10 mV of PNIPAM hydrogel
contained 1.0 wt% SWNT formed between stainless steel mesh electrodes.
The samples were measured in 1 mM NaNO3 and (B) impedance spectra at
100 kHz as a function of heating-cooling cycles between 20C ( ) and 40C (
) for 3 cycles. ..................................................................................................................... 147
Figure 3.13 Impedance spectra were obtained between frequencies of 0.1 Hz
and 100 kHz with AC amplitude of ±10 mV of different distances of blank
parallel stainless steel mesh electrodes of 1 to 4 mm which were measured
in 1 mM NaNO3 at (A) 20C and (B) 40C. (C) Relationship between
solution resistance (Rs) at 100 kHz and thickness using Equation
where
,
= resistivity (.mm), L = thickness between parallel electrodes
(mm) and A = cross sectional area (mm2). ............................................................ 150
XX
Figure 3.14 Comparisons of the change of Rs (at 100 kHz) over thickness (t) for
SWNT/PNIPAM composite hydrogels with various SWNT concentrations
which were performed in 1 mM NaNO3 at below 20C ( ) and above 40C (
) the LCST. .......................................................................................................................... 152
Figure 4.1 Laponite (a synthetic hectorite clay) [Mg5.34Li0.66Si8O20(OH)4]Na0.66
with particle size (single platelet) 20-30 nm   1 nm thick and cation
charge capacity = 1.04 mequiv/g. ............................................................................. 161
Figure 4.2 Hydrogel formed in a rubber gasket mould (thickness = 2.5 mm)
sandwiched between 2 glass slides.......................................................................... 166
Figure 4.3 (A) Schematic illustration of extrudable testing using a custom-built
syringe printer. (B) Different diameters of the tips for extrudable testing of
the hydrogels. (C-E) The hydrogels were extruded from 2.5, 1.5 and 0.5 mm
of diameter of the tips, respectively. ....................................................................... 169
Figure 4.4 Creep - recovery experiment where a constant stress () is applied
to the hydrogels (creep part) for a period of time. Afterwards the applied
stress is removed (recovery part). ........................................................................... 172
Figure 4.5 Generalised viscoelastic model (Generalised Voigt chain) ................ 177
Figure 4.6 Images of (A) PNIPAM/clay formed as sheet and (B)
SWNT/PNIPAM/clay extruded from syringe (C) SWNT/PNIPAM/clay gel in
a glass beaker containing water. ............................................................................... 180
Figure 4.7
FTIR spectra of (A) clay and (C) PNIPAM/Clay hydrogel were
analysed by using a KBr tablet containing clay and PNIPAM/Clay gel
powders. FTIR spectra of (B) chemically crosslinked PNIPAM/MBA
hydrogel film which was coated on gold coated Mylar by induced free
radical polymerisation are included for comparison (as shown in section
2.3.4). Spectra have been offset for clarity where the intensity has been
normalised. ........................................................................................................................ 181
Figure 4.8 Tensile test of 3.3 wt% clay of PNIPAM/Clay hydrogel sheet
(thickness =4.88 mm, width = 2.48 mm, length = 9.8 mm). (A) Image of the
gel composite before breaking point and (B) stress-strain curve which
shows breaking strain of 1254%, tensile strength at break of 6.4 kPa and
tensile modulus of 8.2 kPa. .......................................................................................... 182
XXI
Figure 4.9 Equilibrium-swelling ratio (ESR) as a function of clay concentration
of PNIPAM/Clay composite hydrogels. ................................................................... 184
Figure 4.10 Elastic (G’, filled symbols) and viscous (G”, open symbols) modulus
as a function of angular frequency of as-prepared PNIPAM/Clay hydrogel
composites various clay contents. Oscillatory frequency sweep test with a
controlled strain of  = 0.01 rad was performed at 25 °C and over the range
of angular frequency from 1 to 100 rad/s in triplicate. ................................... 189
Figure 4.11 Elastic (G’), viscous (G”) modulus and phase shift angle (𝜹) at 10
rad/s as a function of clay concentration of as-prepared PNIPAM/Clay
composite hydrogels. Oscillatory frequency sweep test with a controlled
strain of  = 0.01 rad was performed over the range of angular frequency
from 1 to 100 rad/s in triplicate. .............................................................................. 190
Figure 4.12 Elastic (G’), viscous (G”) modulus and phase shift angle ( ) at 10
rad/s as a function of clay concentration of as-prepared SWNT(0.1
wt%)/PNIPAM/Clay hydrogel composites. Oscillatory frequency sweep
test with a controlled strain of  = 0.01 rad was performed over the range
of angular frequency from 1 to 100 rad/s in triplicate. ................................... 191
Figure 4.13 Elastic (G’), viscous (G”) modulus and phase shift angle (𝜹) at 10
rad/s as a function of initiator APS of as-prepared SWNT(0.1
wt%)/PNIPAM/Clay hydrogel composites. Oscillatory frequency sweep
test with a controlled strain of  = 0.01 rad was performed over the range
of angular frequency from 1 to 100 rad/s in triplicate. The lines are
included to guide the eye only. .................................................................................. 192
Figure 4.14 Creep – recovery curves of PNIPAM/Clay hydrogels with various
contents of clay (wt%). Creep and recovery strains are shown as a function
of creep (0-300 s) and recovery times (300-600 s) under constant stress of
20 Pa. .................................................................................................................................... 193
Figure 4.15 (A) Creep–recovery curves of SWNT/PNIPAM/Clay hydrogels with
0.1 wt% SWNT. Creep and recovery strains are shown as a function of
creep (0-300 s) and recovery times (300-600 s) under various constant
stresses and (B) creep strain at 300 s as a function of applied various
constant stresses. ............................................................................................................ 195
XXII
Figure 4.16 Relationship between elastic modulus (E0), viscoelastic modulus
(E1), retardation time (), viscosity () and applied various constant
stresses of SWNT/PNIPAM/Clay hydrogels with 0.1 wt% clay resulting
from (A-D) creep and (E-H) recovery curve fitting by using the generalised
Voigt chain (a four-element model). ........................................................................ 197
Figure 4.17 (A) Creep–recovery curves of SWNT/PNIPAM/Clay hydrogels with
0.05 wt% SWNT. Creep and recovery strains are shown as a function of
creep (0-300 s) and recovery times (300-600 s) under various constant
stresses and (B) creep strain at 300 s as a function of applied various
constant stresses. ............................................................................................................ 198
Figure 4.18 (A) Creep–recovery curves of PNIPAM/Clay hydrogels. Creep and
recovery strains are shown as a function of creep (0-300 s) and recovery
times (300-600 s) under various constant stresses and (B) creep strain at
300 s as a function of applied various constant stresses. ................................ 199
Figure
4.19
Creep–recovery
curves
of
SWNT/PNIPAM/Clay
hydrogel
composites contained 0, 0.05 and 0.1 wt% SWNTs. Creep and recovery
strains are shown as a function of creep (0-300 s) and recovery times (300600 s) under a constant stress 20 Pa. ..................................................................... 200
Figure 4.20 Relationship between elastic modulus (E0), viscoelastic modulus
(E1), retardation time (), viscosity () and SWNT/PNIPAM/Clay hydrogels
contained various SWNT concentrations (0, 0.05 and 0.1 wt%) under a
constant stress 20 Pa resulting from (A-D) creep and (E-H) recovery curves
fitting by using the generalised Voigt chain (a four-element model). ........ 201
Figure 4.21 (A) Creep and (B) recovery curve fitting of 0.4 wt% clay hydrogel by
using a two-element, a three-element and a four-element models and their
equations as illustrated in Table 4.3. ....................................................................... 205
Figure 4.22 Relationship between elastic modulus (E0) and viscoelastic
modulus (E1), retardation time () and viscosity ()) and clay contents of
PNIPAM/clay hydrogels resulting from (A-D) creep and (E-H) recovery
curve fitting by using the generalised Voigt chain (a four-element model).
................................................................................................................................................ 206
Figure 4.23 Diagram of change where a constant shear rate is applied. ............ 207
XXIII
Figure 4.24 Shear yield experiments by applying shear strain at constant shear
rate of 0.8, 1.6 and 3.3 wt% clay contained in PNIPAM/Clay hydrogel
composites. ........................................................................................................................ 208
Figure 4.25 Comparison of modulus measured from different approaches of
PNIPAM/clay hydrogels as a function of clay concentration. Shear modulus
from
s
tensile
test
converted
by
Equation:
,
a
Poisson’s ratio of 0.5 is assumed (constant volume deformation). .......... 210
Figure 4.26 Phase transition of (A) PNIPAM/clay 1:3.3 wt% and (B)
SWNT/PNIPAM/clay 0.1:0.8:1 wt% hydrogels measuring by temperature
ramp step performed between 20 to 50C over the range of angular
frequency from 1 to 31.62 rad/s with a controlled stress of  = 5 Pa and 0.6
Pa, respectively. ............................................................................................................... 213
XXIV
LIST OF TABLES
Table 1.1 Thermally sensitive polymers which exhibiting the phase transition
above the LCST in aqueous solution with different side group structures. .. 9
Table 1.2 Typical properties of CNTs [163]. .................................................................... 20
Table 1.3 Characterization of a novel clay-polymer nanocomposite hydrogel
with a unique organic (PNIPAM)/inorganic (synthetic hectorite laponite
clay) network structure reported by Haraguchi et. al. [188]. Note: the
solution compositions for syntheses of NC-X (OR-Y) gels are as follows:
H2O/NIPAM/Clay(BIS)/TEMED/KPS = 30 g : 3 g : g : 0.198X g (0.021Y g) : 24
µL : 0.03 g............................................................................................................................... 27
Table 3.1 Equilibrium swelling behaviour and mechanical properties of PNIPAM
containing different SWNT concentrations. ......................................................... 136
Table 4.1 Creep-recovery response of linear spring, linear dashpot, Maxwell
model and Voigt model. ................................................................................................ 173
Table 4.2 Preparation and extrudable testing of composite hydrogels. ............ 185
Table 4.3 Three Voigt models and their equations ..................................................... 202
Table 4.4 Shear strength, breaking shear strain and shear modulus of PNIPAM
hydrogel composites containing different clay concentrations applying
shear strain at constant shear rate measurements. .......................................... 209
XXV
LIST OF ABBREVIATIONS
UOW
University of Wollongong
IPRI
Intelligent Polymer Research Institute
NIPAM
N-isopropylacrylamide
PNIPAM
Poly(N-isopropylacrylamide)
LCST
Lower Critical Solution Temperature
UCST
Upper Critical Solution Temperature
SWNTs
Single-Walled Carbon Nanotubes
CNTs
Carbon Nanotubes
HOPG
Highly oriented pyrolytic graphite
SDBS
Sodium Dodecyl Benzene Sulfate
BIS, MBA
N,N’-Methylene(bisacrylamide)
APS
Ammonium Persulfate
KPS
Potassium Persulfate
SEM
Scanning Electron Microscopy
QCM
Quartz Crystal Microgravimetry
TEM
Transmission Electron Microscopy
RVC
Reticulated vitreous carbon
MW
Molecular Weight
A
Ampere
AC
Alternating current
Ag/AgCl
Silver/silver chloride
Au
gold
DSC
Differential scanning calorimetry
XXVI
°C
Celsius degree
CV
Cyclic voltammetry or cyclic voltammogram
or cyclic voltametric
LSV
Linear sweep voltammetry
DC
Direct current
f
frequency (Hz or s-1)
[Fe(CN)6]3-
Ferricyanide
g
gram
g/cm3
gram per cubic centimetre
GPa
Gigapascal
KCl
potassium chloride
h
hour
Hz
Hertz
I
Current
Ipa
anodic peak current
Ipc
cathodic peak current
M
Molar
µm
micrometre
mg
milligram
mg/ml
milligram per millilitre
ml
millilitre
mM
millimolar
MPa
Megapascal
mV
millivolt
N2
Nitrogen
XXVII
NaCl
sodium chloride
nm
nanometre
Pa
Pascal
PBS
phosphate buffer saline
Pt
Platinum
Raman
Raman spectroscopy
RBM
radial breathing mode
S/cm
Siemen per centimetre
t
Celsius temperature (°C)
T
absolute temperature (K)
|G*|
|*|
dynamic viscosity
G’, E
storage modulus or elastic modulus
G”
loss modulus or viscous modulus

phase shift angle
rad/s
radian per second
,
strain

stress

viscosity
strain rate

relaxation time
XXVIII
CHAPTER 1
INTRODUCTION
Chapter 1
Introduction
1
CHAPTER 1
INTRODUCTION
Table of Contents
1 INTRODUCTION.......................................................................................................................... 3
1.1
General introduction ................................................................................................. 3
1.2
Hydrogel materials .................................................................................................... 4
1.2.1 Definition, classification and properties ....................................................... 4
1.2.2 The lower critical solution temperature (LCST) and the upper
critical solution temperature (UCST) ........................................................................... 8
1.2.3 Poly(N-isopropylacrylamide) ......................................................................... 10
1.2.4 Injectable hydrogels ........................................................................................... 13
1.3
Carbon nanotubes .................................................................................................... 15
1.3.1 Carbon nanotubes (CNTs) ................................................................................ 15
1.3.2 Single-walled nanotubes (SWNTs) ............................................................... 18
1.3.3 Dispersion of single-walled nanotubes ....................................................... 20
1.3.4 Electronic conduction in SWNT-polymer composites .......................... 22
1.4
Hydrogel composites............................................................................................... 24
1.4.1 Physically crosslinked clay – hydrogel composites ................................ 24
1.4.2 Carbon nanotubes-polymer hydrogel composites ................................. 29
1.5
Aims of the project ................................................................................................... 31
1.6
Chapter Summaries ................................................................................................. 31
1.7
References ................................................................................................................... 35
2
CHAPTER 1
INTRODUCTION
1 INTRODUCTION
1.1 General introduction
This chapter is intended as a general introduction to temperature-sensitive
polymer/carbon nanotube hydrogel composites focusing on the synthesis and
behaviour of the composites. The hydrogel composite referred to here are
composed mainly of poly (N-isopropylacrylamide) (PNIPAM) and single-walled
carbon nanotubes (SWNTs) using both chemical and physical crosslinkers.
PNIPAM is a temperature-responsive hydrogel which undergoes a physical
change in the presence of external thermal stimuli, putting this material into the
category of smart materials. However, PNIPAM hydrogel is an electrically
insulating polymer. To synthesize thermally sensitive electrically conducting
hydrogels, this research employed the use of SWNTs. SWNTs are an especially
good candidate from a variety of carbon nanotube types because of they have
relatively low bending stiffness. This research is focused on the use of SWNTs as
they offer a lower bending stiffness than MWNTs and DWNTs [1, 2] which is
essential for high compliance composites.
SWNTs incorporated within PNIPAM hydrogel matrices functions as effective
reinforcement to enhance the mechanical performance of hydrogel composites
[3, 4]. Hydrogels containing SWNTs are expected to be promising materials
because they can combine two or more desirable properties from both
materials to produce novel structures with new properties.
3
CHAPTER 1
INTRODUCTION
1.2 Hydrogel materials
1.2.1 Definition, classification and properties
Hydrogel (also called aqua gel) refers to three-dimensional networks of
hydrophilic polymer chains containing a large volume fraction of water.
Hydrogel materials are defined as crosslinked polymeric networks which can
absorb large amounts of water or aqueous solutions without dissolving (Figure
1.1). The water content can be 10-20 times their weight in the dry state [5-8].
As a classification of materials, hydrogels are solids which contain fluid [9].
Hydrogels are often biocompatible materials and applicable as drug and cell
carriers and as tissue engineering matrices [10]. Some are excellent candidates
for controlled release devices which can be injected in vivo as a liquid that gels
at body temperature [11-13]. Common disadvantages of using hydrogels
include their physical weakness, toughness after swelling, difficulty in handling,
and difficulty of sterilization [8, 14, 15].
4
CHAPTER 1
INTRODUCTION
Figure 1.1 Insoluble network of polymer chains that swell in aqueous solutions.
Lines indicate polymer chains. Circles indicate crosslink sites.
Hydrogel materials are broadly classified into two categories: physical and
chemical according to the mechanism underlying the network formation [16].
A chemical hydrogel (also called permanent gel) consists of a covalently
crosslinked network which is one molecule regardless of its size with the
average molecular weight between crosslinks defining its modulus [17-19].
Chemical crosslinks can be introduced by many methods including radical
polymerization [20-26], click chemistry [27-33] or Michael addition [34-37].
Chemical hydrogels maintain their original shape during and after swelling.
In contrast to chemical gels, physical gels are often reversible gels obtained by
physical interactions of polymers with each other or inorganic substances. The
polymeric networks are held together by polymer chain entanglements and/or
secondary forces present between the network polymers [6, 8, 38, 39]. The
secondary forces can be due to a variety of interactions such as stereocomplex
5
CHAPTER 1
INTRODUCTION
formation [40-43], hydrogen bonding [44-47], hydrophobic interactions [4851] and ionic interactions [52, 53].
Both chemical and physical crosslinking methods have their advantages and
drawbacks. In a chemical hydrogel, the network structure is characteristically
more stable than physical gels under a variety of conditions [8, 17]. In contrast,
a physical hydrogel network can be reversibly broken by external stimuli [54,
55] or high shear forces [56-58].
According to the starting materials, chemical and physical hydrogels can also be
divided into natural polymer hydrogels, synthetic polymer hydrogels and
natural/synthetic hybrid hydrogels. Hydrogel-forming natural polymers include
proteins such as collagen and gelatin, and polysaccharides such as alginate and
agarose. Synthetic gels are produced with excellent properties, e.g. hydrogel
implants [59-63], soft contact lenses [63-66], tissue engineering [67-70], drug
delivery [3, 71-74], cell culture [74-76] and sensing applications [77-79].
Recently developed methods in controlled radical polymerisation have
provided the potential to control the chain length, sequence and threedimensional structure; such as atom transfer radical polymerization (ATRP)
[70, 80, 81], reversible addition-fragmentation transfer (RAFT) polymerization
[82-84], nitroxide-mediated polymerization [85, 86] and combination methods
[87-89].
6
CHAPTER 1
INTRODUCTION
Many applications of hydrogels are limited by their weak mechanical
performance. Tough hydrogels which can resist the applied force without
failure are not common. Recently, tough synthetic polymer hydrogels have been
produced by four main approaches, namely introduction of sliding crosslinking
agents [90-92], double network hydrogels [93-95] and nanocomposite (clay
filled) hydrogels [96-98] and multi-valent crosslinks [99, 100].
Figure 1.2 (A) A sliding double ring crosslinking agent was produced by
chemically crosslinking two cyclodextrin molecules, each threaded on a
different PEG chain (end-capped with a bulky group, such as adamantan), (B)
double network hydrogel are a subset of interpenetrating networks (IPNs)
formed by two hydrophilic networks, one highly crosslinked, the other loosely
crosslinked and (C) clay nanocomposite hydrogels are organic-inorganic
hybrids where polymer is grafted on the clay surface by one or two ends [14].
Hydrogels can also be classified as responsive or non-responsive gels.
Responsive hydrogels undergo a physical change in the presence of external
stimulus [101] such as electrical current [102-104], magnetic field [105-107],
7
CHAPTER 1
INTRODUCTION
pH [18, 95, 108, 109], a host of biomolecules [110], temperature [54, 68, 70, 71,
109], ionic strength [108] and light [3].
1.2.2 The lower critical solution temperature (LCST) and the upper
critical solution temperature (UCST)
The behaviour of miscible polymer solutions often exhibits one or two solubility
boundaries, the lower critical solution temperature (LCST) and the upper
critical solution temperature (UCST). The words lower and upper indicate that
the critical temperatures are lower and upper bounds to the temperature
interval of partial miscibility [111]. At temperature below LCST or above UCST,
the system is completely miscible in all proportions [112]. In this thesis, the
LCST of hydrogel is used as a handle to develop heat shrinkable electronic
conductor which can be reversibly switched from swollen state to collapsed
state. These polymers can be mainly categorized into four classes according to
the side group structures (Table 1.1): amide [19, 113-116], ether [117-120],
oxazoline [121-123] and amino acrylate type [124, 125].
8
CHAPTER 1
INTRODUCTION
Table 1.1 Thermally sensitive polymers which exhibiting the phase transition
above the LCST in aqueous solution with different side group structures.
Side group
Polymers
LCST in
structures
References
aqueous
solution (C)
Amide
32
[115, 116]
62-82
[126]
26-35
[19]
Polyphosphazenes containing lactic
acid ester and methoxyethoxyethoxy
side groups
33-52
[117]
poly(N-acryloyl alanine Me ester)
(PAAME)
18
[118]
poly(N-acryloylglycine methyl ester)
(PNAGME)
62
[120]
62-65
[121]
39
[122]
35-80
[123]
14-50
[124]
50
[125]
Poly(N-isopropylacrylamide)
poly(ethylacrylamide)
poly(N,N-diethylacrylamide)
Ester
oxazoline
poly(2-ethyl-2-oxazoline) (PEtOx)
Poly(2-isopropyl-2-oxazoline)
(PiPrOx)
Poly[oligo(2-ethyl-2-oxazoline)
methacrylate]s
Amino
acrylate
type
poly(N,N-dimethylaminoethyl
methacrylate) (PDMAEMA)
poly(dimethylaminoethyl
methacrylate)
9
CHAPTER 1
INTRODUCTION
1.2.3 Poly(N-isopropylacrylamide)
Poly(N-isopropylacrylamide) is variously abbreviated to PNIPA, PNIPAAm,
PNIPAA , PNIPAm or PNIPAM; in this thesis PNIPAM is used. The molecular
structure of PNIPAM is represented in Figure 1.3. PNIPAM is the most published
thermally responsive synthetic hydrogel and exhibits a lower critical solution
temperature (LCST) of  32C in water [115, 116]. Thus, linear (un-crosslinked)
PNIPAM is miscible with water in all proportions below  32C but immiscible
at temperatures greater than  32C.
Figure 1.3 Chemical structure of poly(N-isopropylacrylamide).
PNIPAM exhibits a LCST because of the presence of both hydrophilic amide
groups and hydrophobic isopropyl groups in its repeat unit [116, 127]. Below
the LCST, the hydrogels absorb a large amount of water to lower its enthalpy
10
CHAPTER 1
INTRODUCTION
and exist in a transparent swollen structure. When heated from below the LCST
to above the LCST the hydrogels switch from a hydrophilic, swollen state to a
hydrophobic, collapsed state. This behaviour is due to the unique interactions of
PNIPAM with water molecules that is temperature dependent. At a temperature
above the LCST, the hydrogen bonds between the polymer chains and the water
molecules are broken, the polymer increases its configurational entropy as
water is expelled from the polymer network [128] as illustrated in Figure 1.4.
Swollen/Hydrophilic
Collapsed/Hydrophobic
Figure 1.4 At temperatures above the LCST, PNIPAM undergoes a reversible
phase transition from a swollen hydrophilic state to a collapsed hydrophobic
state [7].
As PNIPAM expels its liquid contents at a temperature near that of the human
body, PNIAPM has been an especially good candidate for applications in
11
CHAPTER 1
INTRODUCTION
biotechnology and in medicine; i.e. its LCST is close to, but importantly below
physiological temperature [129-132].
PNIPAM was evaluated as drug release carriers consisting of semi-IPN of
PNIPAM and poly(tetramethylene glycol) crosslinked with MBA, analysing the
release of indomethacin as a model drug, showing also an on-off pulsatile
release behaviour [130]. Poly(NIPAM-co-acrylic acid) hydrogels have been
applied as extracellular matrix for pancreatic islets in biohydrid artificial
pancreas [132]. NIPAM hydrogels were formed inside glass pores to make a
stable thermally controlled on-off device for drug delivery [129]. The drugs
were delivered across the composite membrane when the pores are opened
which changing from an expanded state to a collapsed state, by increasing
temperature above the LCST [129, 131].
PNIPAM is a temperature-responsive polymer that was first synthesised in the
1950s [116]. Over the past few decades, PNIPAM has been appearing with
increasing frequency in academic literature. As Figure 1.5 illustrates, the
number of PNIPAM references in the published literature went through a rather
explosive growth from 1990-2012.
12
CHAPTER 1
INTRODUCTION
Figure 1.5 Publication in database (The Chem Abstracts online search (CAS)
(done 16/07/12) was based on the search of poly(N-isopropylacrylamide) with
removal of duplicates).
1.2.4 Injectable hydrogels
1.2.4.1 Definition of injectable hydrogels
Injectable hydrogels are materials which can be extruded through a syringe and
then rapidly form a gel after extrusion.
1.2.4.2 Application of injection hydrogels
These hydrogels are highly desirable in clinical applications. Injectable
hydrogels can be introduced into the body via a syringe or tube and can avoid
the need for open surgery [133-135]. Applications include injectable constructs
13
CHAPTER 1
INTRODUCTION
in vivo tissue formation to reduce healing time, reduce scarring, decrease risk of
infection and ease of delivery compared with surgically implanted materials
[12, 13].
Thermosensitive injectable hydrogels are especially attractive and widely
studied due to their spontaneous gelation with the employment of body
temperature [11-13, 136]. Various thermoresponsive injectable hydrogels
based on synthetic polymers such as poly(N-isopropylacrylamide) (PNIPAM)
have been extensively studied. Most of these injectable hydrogels could be
injected as a liquid and then form a solid gel. Over the past decade, various
thermosensitive and injectable PNIPAM hydrogels have been developed and
employed in a variety of settings [135, 137-140]. A temperature-sensitive poly
(NIPAM-co-CSA) hydrogels was synthesised by the copolymerisation of acrylic
acid-derivatized Chitosan (CSA) and NIPAM in aqueous solution [138]. An
aminated hyaluronic acid-g-poly(N-isopropylacrylamide) (AHA-g-PNIPAM)
injectable hydrogel was synthesized by coupling carboxylic end-capped
PNIPAM (PNIPAM-COOH) to AHA through amide bond linkages [140]. Chen and
Cheng reported that a chitosan-graft-poly(N-isopropylacrylamide) injectable
hydrogel has been synthesised by grafting PNIPAM-COOH with a single carboxy
end group onto chitosan through amide bond linkages for cultivation of
Chondrocytes and Meniscus cells [139]. PNIPAAm-grafted hyaluronic acid
14
CHAPTER 1
INTRODUCTION
copolymer as temperature-sensitive injectable gels for use in controlled drug
delivery applications was studied by Ha et al. [137].
1.3 Carbon nanotubes
1.3.1 Carbon nanotubes (CNTs)
Carbon nanotubes (CNTs) were discovered during the study of fullerene (C60)
synthesis that revealed a crystalline form of carbon. The discovery is largely
attributed to the Japanese researcher Sumio Iijima of NEC in 1991 (Figure 1.6),
although the production and observation of carbon nanotubes have been done
under a variety of conditions prior to 1991 [141].
15
CHAPTER 1
INTRODUCTION
Figure 1.6 Dr. Sumio Iijima was awarded the 2007 Balzan Prize for Nanoscience
for his discovery of carbon nanotubes, and in particular the discovery of single
walled carbon nanotubes and the study of their properties, by the International
Balzan Prize Foundation [142].
Since their discovery in 1991, CNTs have been of great interest due to their
wide scope of possible applications such as composite reinforcement materials
[143-145], hydrogen containers [146, 147], field emission sources [148, 149],
super-capacitors [150-152], molecular sensors [153, 154] and scanning probe
tips [155-157]. As Figure 1.7 demonstrates, the number of CNT references in
the published literature has increased substantially from 1992-2011.
16
CHAPTER 1
INTRODUCTION
Figure 1.7 Publication in database (The Chem Abstracts online search (CAS)
(done 16/07/12) was based on the search of carbon nanotubes with removal of
duplicates).
CNTs are members of the fullerene structural family. A carbon nanotube is a few
nanometers in diameter and has a range of lengths. In general, the length is
significantly greater than the width creating a large aspect ratio. Carbon
nanotube is a long nano‐scaled hallow cylinder constructed from a graphene
sheet which is a monolayer of graphite. CNT materials can be constructed by
rolling up the graphitic sheets into a tube with at least one end typically capped
with a hemisphere of buckyball type structure.
CNTs are the strongest known material, with tensile strength and elastic
modulus up to 100 GPa and 1 TPa, respectively [158]. CNTs exhibit
17
CHAPTER 1
INTRODUCTION
extraordinary mechanical properties, very high aspect ratio and low density
making them ideal reinforcing materials for nanocomposites [53, 143-145,
159]. There are several types of CNTs; single‐walled (SWNT), double‐walled
(DWNT) and multi‐walled (MWNT), CNTs which consist of one cylinder, two
and many nested cylinders, respectively. CNTs come in a variety of diameters
and lengths, depending on the growth process and treatment, which can be
selectively produced.
Special properties of carbon nanotubes are determined to a large extent by
their nearly one dimensional structure. First, the chemical reactivity of a CNT is
directly related to the pi-orbital mismatch caused by an increased curvature of
its surface. Second, electrical conductivity of CNTs depends on their chiral
vector which can be either semi-conducting or metallic. The differences of
molecular structure that result in a different band structure affect differences in
conducting properties of CNTs. SWNT’s are relatively flexible because of their
small diameter and are suitable for applications in composite materials that
need anisotropic properties [160].
1.3.2 Single-walled nanotubes (SWNTs)
SWNTs can be imagined as long wrapped graphene sheets. SWNTs generally
have large lengths (up to several microns) and small diameter (a few
nanometres), resulting in length to diameter ratios of about 1000 and so they
18
CHAPTER 1
INTRODUCTION
can be considered as nearly one-dimensional structures [161, 162]. Figure 1.8
shows some SWNTs with different chiralities.
Figure 1.8 Some SWNTs with different chiralities. The difference in structure is
easily shown at the open end of the tubes where (a) armchair structure, (b)
zigzag structure and (c) chiral structure [160].
In this thesis, SWNTs are considered to be ideal fiber reinforcements which can
enhance the fracture toughness, strength, and electrical conductivity in
polymeric hydrogel type nanocomposites. The unique atomic structure, high
aspect ratio, high surface are, extraordinary mechanical properties and high
thermal conductivity (as shown in Table 1.2) make SWNT a potentially very
attractive material to be used in polymer/carbon nanotube composite
applications.
19
CHAPTER 1
INTRODUCTION
Table 1.2 Typical properties of CNTs [163].
Property
SWNT
MWNT
DWNT
Tensile strength (GPa)
50-500
10-60
23-63
Elastic modulus (TPa)
∼1
0.3-1
-
Elongation at break (%)
5.8
-
28
Density (g/cm3)
1.3–1.5
1.8–2.0
1.5
Electrical conductivity (S/m)
∼106
∼103
∼106
Thermal stability
Typical diameter
>700 C (in air)
1 nm
∼20 nm
∼5 nm
1.3.3 Dispersion of single-walled nanotubes
Since SWNTs were discovered, they have been widely investigated because of
their many unique properties. Applications of SWNTs have been limited by their
poor dispersibility in solvents, polymers, ceramics and metallic matrices [164].
To overcome this limitation, two different approaches have been widely used,
namely surface functionalization and surfactant stabilisation [3, 165-167].
Some procedures require harsh solvents such as liquid ammonia with extended
reaction times. Liang et al. 2004 used lithium and alkyl halides in liquid
ammonia that yielded sidewall functionalized SWNTs that are soluble in
common organic solvents [168]. Chattopadhyay et. al. 2005 prepared carbon
nanotube salts by treating SWNTs with lithium in liquid ammonia and they
react readily with aryl iodides to give SWNTs functionalized by aryl groups
20
CHAPTER 1
INTRODUCTION
[169]. Rasheed et. al. 2006 presented a novel approach to improve the
dispersion of oxidized SWNTs in a copolymer matrix by tuning hydrogenbonding interactions. However, Tian and co-workers have functionalized
SWNTs with cheap, green, convenient and efficient alcohols, 2008 [170].
However, Buffa et. al. reported that functionalization destroys the inherent
electrical conductivity of the tubes [171]. Thus, functionalization of SWNTs was
avoided in this thesis due to the desire to use SWNTs to improve electronic
properties of the composite gels.
Prichard and Vogt obtained stable dispersed SWNTs using non-ionic surfactants
based on poly(ethylene oxide) in either water or ethanol using sonication [172].
For SWNTs, non-ionic surfactants (Triton X-100 and Tween 80) are less
effective in debundling than ionic surfactants such as sodium dodecyl benzyl
sulfonate (SDBS), sodium dodecyl sulfate (SDS) and sodium cholate (cholate)
[173]. Blanch et al. reported that the concentration of SDBS was found to be a
more significant parameter on the resulting dispersion than the ratio of
surfactant to SWNTs by mass [174]. In addition, the dispersions of SWNTs with
SDBS were found to be very stable, even after many months of storage, through
slight changes in the UV-Vis-NIR absorption spectra. Islam and co-workers
[175] reported that a commercially obtained surfactant-SDBS can enhance the
disaggregation of bundles and stability of SWNTs in water because of its
benzene ring moiety, charged groups and alkyl chains. They also speculated that
SDBS and Triton X-100 disperse the tubes better than SDS because they have
21
CHAPTER 1
INTRODUCTION
benzene rings which facilitate physical bonding to the CNTs [175]. However,
Triton X-100 has a weaker interaction with a CNT surface compared to SDBS
because their head group is polar and larger than that of SDBS. Triton X-100,
SDS and SDBS molecular structures are shown in Figure 1.9.
10
(A)
(B)
(C)
Figure 1.9 Molecular structure of (A) triton X-100, (B) SDS and (C) SDBS
1.3.4 Electronic conduction in SWNT-polymer composites
One aim of this thesis is to achieve electronic conduction in PNIPAM hydrogels
by using SWNT as a conductive filler. The critical content of SWNTs that
characterises a drastic increase in electronic conductivity is referred to as the
electrical percolation threshold.
Conceptually, percolation refers to the
22
CHAPTER 1
INTRODUCTION
minimum concentration where a continuous conductive path is formed
between two opposing surfaces by direct contact of SWNTs.
The investigation of electrical percolation threshold of the electrically
conducting composites is significant for different electronic applications. Using
SWNTs as an electrically conducting filler to make conducting polymer hydrogel
composite shows dramatic improvements in electrical conductivity with low
percolation threshold around 0.05-0.1 wt% of SWNT loadings as reported by
Ramasubramaniam et al. [176]. However, Blanchet et al. [177] reported that
good dispersion of SWNTs in doped polyaniline enables a low percolation
threshold at 0.25 wt% SWNT loading, while the poor dispersion of SWNTs in
ethylcellulose leads to a high percolation threshold at 3.0 wt% SWNT loading.
Hernandez et al. prepared a conducting polymer nanocomposites based on
poly(ethylene terephthalate) (PET) and SWNT as additive which show a low
percolation threshold for electric conductivity 0.024 wt% of SWNT [178]. Zhang
et al. reported that the conductive property of SWNT- polyethylene (HDPE)
composite was improved by increasing SWNTs. Furthermore, the electrical
percolation threshold of SWNT-HDPE composite was  4 wt% of SWNT loading
while conductivity of the composite containing 5 wt% SWNTs increases to 10-3
S/cm from that of 2.6 wt% SWNTs of 10-10 S/cm. Regev et al. prepared SWNT–
SDS–PS composites using latex technology, which exhibits an excellent
conductive property with a percolation threshold of 0.28 wt% [179].
23
CHAPTER 1
INTRODUCTION
1.4 Hydrogel composites
1.4.1 Physically crosslinked clay – hydrogel composites
1.4.1.1 Synthetic hectorite clay (Laponite XLG)
A synthetic hectorite clay [180], Laponite XLG corresponds to the following
chemical formula [Mg5.34Li0.66Si8O20(OH)4]Na0.66. This clay is made of small
platelets or dish-shaped particles with average diameter of 30 nm, average
thickness of 1 nm and cation charge capacity of 1.04 mequiv/g [180, 181]. In
aqueous dispersions, the particles are ionized. The faces of the platelets have a
large number of negative charges while a few positive charges exist along the
edges exist [182]. These surface charges are compensated by Na+ counterions
which are distributed next to the negative surface charges and also in a diffuse
layer surrounding each particle [183]. The main interactions between particles
are electrostatic between the anisotropic charge distributions.
The inorganic clay was swollen when dispersed in water and gradually cleaved
to form discrete exfoliated-disklike particles (Figure 1.10 A and B).
The
resulting aqueous clay suspension, composed of exfoliated clay particles and
formed a house-of-cards structure through ionic interactions as illustrated in
Figure 1.10 C [184, 185].
24
CHAPTER 1
INTRODUCTION
(A)
(B)
(C)
Figure 1.10 Schematic representation of the preparation of clay aqueous
suspension and the formation of the house-of-cards structure: (A) clay, (B)
dispersion (exfoliation) of clay platelets in water and (C) house-of-cards
structure [182].
1.4.1.2 Clay-polymer hydrogel composites
Polymeric hydrogels have attracted much scientific interest and have been
proposed for many applications [14, 63, 70, 71, 87]. Hydrogels composed of
chemically crosslinked structures have several significant limitations; such as
structural inhomogeneity at high concentration of crosslinker [186] and
mechanical properties [186, 187]. Most of the potential applications of these
chemically crosslinked gels have not yet been realised [188]. In order to
25
CHAPTER 1
INTRODUCTION
improve the physical and mechanical properties of polymer hydrogels, clay
nanoparticles are currently of great interest as they both perform as physical
crosslinkers and strengthen the polymer network [189, 190]. A nanocomposite
hydrogel composed of organic (polymer)/inorganic (clay) network is
schematically shown in Figure 1.11. Clay-polymer hydrogel nanocomposites
have attracted much attention from researchers because of the great
interactions between exfoliated clay with the polymer due to their large
surfacel area for contact [188, 191, 192]. Furthermore, the chemical reactivity
of clay platelets is high at the clay[193].
(A)
(B)
(C)
26
CHAPTER 1
INTRODUCTION
Figure 1.11 (A) Schematic representation of a 100 nm cube of PNIPAM/clay gel
consisting of uniformly dispersed inorganic clay and two primary types of
flexible polymer chain,  and g, grafted onto two neighbouring clay sheets and
one clay sheet, respectively. (B) Elongated structure model for PNIPAM/clay gel
and (C) conventional PNIPAM/BIS gel network structure model [188].
Haraguchi et. al. first reported on novel clay-polymer nanocomposite hydrogels
with a unique organic (PNIPAM)/inorganic (synthetic hectorite laponite clay)
network structure [188]. The nanocomposite hydrogels were successfully
prepared by in-situ free radical polymerisation without a chemical crosslinker
and exhibit extraordinary optical, mechanical and swelling/de-swelling
properties as shown in Table 1.3.
Table 1.3 Characterization of a novel clay-polymer nanocomposite hydrogel
with a unique organic (PNIPAM)/inorganic (synthetic hectorite laponite clay)
network structure reported by Haraguchi et. al. [188]. Note: the solution
compositions
for
syntheses
of
NC-X
(OR-Y)
gels
are
as
follows:
H2O/NIPAM/Clay(BIS)/TEMED/KPS = 30 g : 3 g : g : 0.198X g (0.021Y g) : 24 µL :
0.03 g.
Hydrogel
Transparency at Temp.
Swelling
Tensile
relative to that at 1C
ratio
strength
Modulus
Elongation
Tg of
dried gel
(5C)
(20C)
(50C)
(Ww/Wdry)
[kPa]
[kPa]
[%]
[C][a]
NC1
97
98
78
110
41
1.5
1424
141
NC3
95
94
79
71
69
4.0
1112
141
NC5
94
96
80
48
109
9.9
857
142
OR1
99
94
1
29
-
-
-
145
OR5
99
4
0
14
-
-
-
152
[a] Tg of linear polymer (PNIPAM) was observed at 142C
27
CHAPTER 1
INTRODUCTION
Haraguchi et al. also reported the effects of clay content on various physical
properties, such as high swellability, rapid swelling/de-swelling rates and welldefined changes in transparency in response to thermal transitions of
nanocomposite hydrogels [182, 191, 192, 194]. In comparison, the deswelling
rate of two different types of PNIPAM/clay nanocomposite hydrogels and
conventional chemically cross-linked PNIPAM/BIS hydrogels was affected
markedly by the crosslinker content in both gels though in opposite directions
[191]. On increasing crosslinker contents, the deswelling rate of PNIPAM/clay
and PNIPAM/BIS gels decreases and increases, respectively. Haraguchi and coworkers also reported that the transparency (homogeneity) of PNIPAM
decreased as a function of increasing crosslinker (BIS) content [191] as
inhomogeneity generally occurs in polymeric hydrogels prepared with chemical
crosslinkers. However, PNIPAM/clay gels exhibit high transparency almost
regardless of clay concentration as a total transmittance was higher that 90%
although the gel contained a large amount of clay showing the structural
homogeneity. In 2006, Haraguchi and co-workers successfully demonstrated
the improvement in mechanical properties of PNIPAM/clay gels by increasing
clay concentration up to 25 mol%. They found that gels with high clay content
exhibit remarkable increases in strength and modulus [194].
In addition, the structural homogeneity of PNIPAM/clay gels could not be
achieved using other methods such as mixing clay and polymer solutions or by
in-situ polymerization in the presence of other inorganic nanoparticles, silica
28
CHAPTER 1
INTRODUCTION
and titania nanoparticles both of which are a few tens of nanometers in
diameter, instead of clay [182]. It indicates that the formation of
organic/inorganic network structures in PNIPAM/clay gels is highly specific
and only realized by in-situ free-radical polymerization in the presence of clay.
1.4.2 Carbon nanotubes-polymer hydrogel composites
The presence of the nanotubes can improve the strength and stiffness of
hydrogels as well as add multi-functionality (such as electrical conductivity) to
hydrogel based composite systems. Hydrogels containing CNTs are expected to
be promising materials because they can combine two or more useful
properties from both materials to produce novel structures with new
properties. SWNTs exhibit electrical conductivity which can show metallic or
semiconducting behaviour. Using SWNTs as an electrically conducting filler to
make conducting polymer hydrogel composite shows dramatic improvements
in electrical conductivity with low percolation threshold around 0.05-0.1 wt%
of SWNT loadings [176, 195]. SWNTs were successfully incorporated into a
poly(ethylene glycol) (PEG) hydrogel by covalent bonds to form a hybrid
material for bio-inspired applications [196]. He et al. reported a biocompatible
electric interface engineered from PEGDA hydrogel and SWNTs as a hydrogelbased hybrid for improving conductive properties as well as function as
mechanical fillers [197].
29
CHAPTER 1
INTRODUCTION
Homogeneously dispersed surfactant coated SWNTs at low concentration in
NIPAM gel can undergo a volume-compression transition; as reported by Islam
and co-workers [198]. The SWNTs were dispersed in water using SDBS
surfactant; resulting nanotube dispersions were 90 ± 5% single tubes with
average length L 516 ± 286 nm at a concentration of 0.1 mg/ml and stable in
suspension
for
concentrations
up
to
10
mg/ml.
In
addition,
a
thermoresponsive PNIPAM copolymer containing 1mol-% pyrene side groups
(p-PNIPAM) was produced and is capable of stabilizing both SWNTs and
MWNTs which could have potential applications in nano-electronics and
sensors [199]. Fujigaya and co-workers first reported the NIR-laser-driven
phase transition of SWNT-embedded PNIPAM gels [4]. The SWNT embedded
PNIPAM gel shows a reversible (more than 1200 cycles) phase transition (gel
volume change) via ON/OFF switching of NIR laser light irradiation due to the
SWNTs acting as a photon antenna that serves as an effective molecular heater
around the NIR region. Besides, MWNT/PNIPAM hydrogels synthesized by in
situ atom transfer radical polymerization (ATRP), showed temperature
switching assembly and disassembly behaviours in water, because of the
hydrophilic and hydrophobic transformation of the bonded PNIPAM chains at
30-35°C [200].
30
CHAPTER 1
INTRODUCTION
1.5 Aims of the project
This project aims to synthesize thermally sensitive electrically conducting
hydrogels. PNIPAM polymer is the most outstanding example of a thermally
responsive hydrogel which has a LCST that can be reversibly switched from a
hydrophilic, swollen state to a hydrophobic, collapsed state. For constructing
thermally sensitive electrically conducting hydrogels, this project employs
SWNTs because of their flexibility [115, 116, 201, 202]. Combining SWNTs with
PNIPAM hydrogel matrices is an effective method of enhancing the functionality
of these hydrogels.
The strategies to achieve the aim of this project are
described-below.
1.6 Chapter Summaries
In Chapter 2, an encapsulation a SWNT Buckypaper with the thermally sensitive
PNIPAM hydrogel was produced and investigated. Buckypapers (freestanding
sheet from SWNTs) were fabricated using a vacuum-filtration method. The
SWNTs were dispersed using surfactant to improve their dispersibility in
aqueous media prior to vacuum filtration through PVDF membrane to form
freestanding
Buckypapers.
PNIPAM
hydrogel
encapsulation
of
SWNT
Buckypaper electrode was synthesised using electrochemically induced free
radical polymerisation technique which is a particularly simple and flexible
method. The coatings are formed directly at the electrode surface by using the
decomposition of an electro-active initiator at an electrode to commence free31
CHAPTER 1
INTRODUCTION
radical polymerisation. Cyclic voltammetry was performed to characterise the
electrochemical properties and the electrochemically active surface area of the
electrodes both before and after the polymerisation process. FTIR was used to
verify that the hydrogel coating was PNIPAM. Permeability and temperature
transition of PNIPAM coatings on various substrates were investigated here by
a study of the oxidation and reduction rates of potassium ferricyanide by cyclic
voltammetry. Temperature transition of PNIPAM coatings on gold Mylar was
also measured using UV-vis measurements at temperatures below and above
the LCST.
In chapter 3, a heat shrinkable electronic conductors as composite hydrogels
were prepared by free radical polymerisation as illustrated in Figure 1.12. The
reaction mixture consisted of SWNTs, dispersed in an aqueous solution of SDBS
as a surfactant under sonication, mixed with monomer, crosslinker, initiator
and accelerator. The hydrogel composites were synthesised with various SWNT
concentrations. The swelling behaviour of the hydrogel composites was
investigated using the gravimetric method. The phase transition was verified
using the change of the hydrogel sample volumes immersed in water at
different temperatures. The mechanical properties of the swollen hydrogel
samples were determined by compression testing room temperature. Changes
of the mechanical properties of the gels were observed relative to the large
change in swelling ratio. The electronic properties of SWNT/NIPAM thermally
sensitive hydrogel composites were quantified by using electrochemical
32
CHAPTER 1
INTRODUCTION
impedance spectroscopy below and above the LCST. The phase transition
behaviour was also investigated through the change of the impedance.
Figure 1.12 A heat shrinkable electronic conductor undergoes a reversible
phase transition from a swollen hydrophilic to a collapsed hydrophobic state.
In chapter 4, thermally sensitive injectable clay-based hydrogels (PNIPAM/Clay
and SWNT/PNIPAM/Clay) by a free radical polymerisation technique are
reported [182, 191, 192, 203-205]. The injectable hydrogel composite was
prepared using initial aqueous solutions consisting of monomer (NIPAM),
physical crosslinker (inorganic clay, Laponite XLG), initiator (KPS and APS) and
accelerator (TEMED). The composition of monomer, crosslinker, initiator and
accelerator were varied during the hydrogel preparation to study which
parameter is important for the transition occurring from solid-like to liquid-like
33
CHAPTER 1
INTRODUCTION
behaviour. The swelling, rheological and mechanical properties of the resulting
hydrogels were characterised. The creep and recovery behaviour of injectable
hydrogels was investigated by applying a constant shear stress. The creeprecovery results were fitted using generalised Voigt chain to study the
viscoelastic properties. The structure of injectable thermally sensitive
hydrogels was confirmed by FTIR spectroscopy. The phase transition behaviour
of the PNIPAM/clay and SWNT PNIPAM/clay composite hydrogels was studied
by considering the change of elastic modulus of the gels as a function of
temperature using a rheometer.
34
CHAPTER 1
INTRODUCTION
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Schematic illustration of LCST behaviour of PNIPAM gel coated gold Mylar
forming by electrochemically induced free radical polymerisation and
the change of permeability of the gel at below and above the LCST
investigating electrochemical response signal of K3[Fe(CN)6].
53
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CARBON NANOTUBE ELECTRODES
Chapter 2
Gel Encapsulation of
Carbon Nanotube Electrodes
54
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CARBON NANOTUBE ELECTRODES
Table of Contents
2 Gel Encapsulation of Carbon Nanotube Electrodes .................................................... 56
2.1
Introduction........................................................................................................... 56
2.2
Experimental ......................................................................................................... 60
2.2.1
Chemical and Materials.................................................................................. 60
2.2.2
Fabrication of Buckypaper........................................................................... 61
2.2.3
Formation of hydrogel encapsulation on conducting electrodes 63
2.2.4
Morphology.......................................................................................................... 70
2.2.5
FTIR analysis ......................................................................................................72
2.2.6
Effect of chemical crosslinking.................................................................... 75
2.2.7
Cyclic Voltammetry with Potassium Ferricyanide .............................75
2.2.8
UV-Vis-NIR spectrophotomer...................................................................... 76
2.3
Results and discussions .................................................................................... 79
2.3.1
Preparation of Buckypaper.......................................................................... 79
2.3.2
Hydrogel encapsulation of conducting electrodes............................ 80
2.3.3
Cyclic Voltammetry before and after polymerisation on different
substrates................................................................................................................................. 86
2.3.4
FTIR analysis ......................................................................................................91
2.3.5
Morphology ......................................................................................................92
2.3.6
Effect of chemical crosslinking.................................................................... 97
2.3.7
Cyclic Voltammetry with Potassium Ferricyanide .............................98
2.3.8
Effect of salt on the LCST of PNIPAM coated on gold coated Mylar
....................................................................... .......................................................................106
2.4
Conclusions ......................................................................................................... 111
2.5
References ........................................................................................................... 113
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2 GEL ENCAPSULATION OF CARBON NANOTUBE ELECTRODES
2.1 Introduction
Carbon nanotube electrodes are being developed for a range of applications
including energy storage, biological electrodes and sensors [1]. Many of these
applications require a gel type coating on the nanotubes to control the interface
between electrode and the analyte. Gel coatings can be used to control the
biological response of electrodes planted in vivo within the body. The coatings
can also be a convenient route to forming a solid electronically insulating
electrolyte between electrodes for charge storage applications [2]. Carbon
nanotubes electrodes are challenging to coat with polymer. Polymerisation of
appropriate monomers for hydrogels in the presence of nanotubes has been
reported [3], but is often restricted by the nanotubes scavenging free radicals
thereby limiting achievable molecular weight.
In recent years, the preparation of surface-attached hydrogel layers on
conducting substrates has been extensively used in a wide variety of biomedical
applications [4-6], sensing [7,8], microfluidics [9], drug release [10,11] and
tissue engineering [10-12]. Examples of the various approaches of attaching
polymers to a solid surface are photo-crosslinking of preformed polymer chains
[8], in-situ atom transfer radical polymerisation (ATRP) [13], electron beam
irradiation [14], plasma polymerisation [15], anionic polymerisation [16,17],
and, electrochemically induced precipitation of polyelectrolytes [18].
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Electrochemically induced free-radical polymerisation technique has been
previously investigated in acrylic monomers utilizing different methods to
initiate polymerisation [19-22]. Electrochemically induced free radical
polymerization was employed for the fabrication of hydrogel coating based on
the electrochemical reduction of persulfate anion [20, 21, 23-27]. Lee and Bell
decomposed potassium persulfate on an aluminium alloy to initiate the
polymerisation of acrylamide [20]. Instead of an aluminium alloy electrode, a
silver electrode was utilized in the work of Yildiz et al. using ammonium
persulfate as an initiator [21]. Shi and co-workers fabricated amperometric
glucose biosensors using potassium sulphate on platinum electrode to initiate
the
polymerization
of
acrylic
acid
[23].
Electrochemically
initiated
polymerisation has produced coatings of polyacrylic acid (PAA) [25], poly (Nisopropylacrylamide) (PNIPAM) [24, 26-28] and copolymer consisting of PAA
and PNIPAM on a gold electrode surface [25].
The work in this chapter focuses on PNIPAM, which is thermally responsive
when immersed in water. Cross-linked PNIPAM, the most frequently reported
thermally responsive hydrogel, exhibits a fully reversible lower critical solution
temperature (LCST) at ~32C in aqueous solutions [29, 30]. PNIPAM in the form
of temperature responsive hydrogels has attached much scientific interest
because of the possibility of combining a thermal response to stimuli within an
in vivo environment. The polymer is a very good candidate for applications in
biotechnology and in medicine because its LCST is close to physiological
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temperature [31-33]. PNIPAM exhibits a LCST due to the presence of both
hydrophilic amide groups and hydrophobic isopropyl groups in its repeat unit
[34]. PNIPAM is swollen at temperatures below the LCST, but shrinks to a
collapsed hydrophobic state above its LCST. Reuber et al. observed
electrochemically induced polymerisation to produce thermoresponsive
PNIPAM hydrogel coatings on gold surfaces based on the scheme shown in
Figure 2.1 [24]. The polymerisation reaction was performed under
potentiostatic control in a standard three-electrode configuration. A custombuilt Teflon cell was used and consisted of a platinum plate as the counter
electrode, a saturated calomel reference electrode and gold coated a 5 MHz ATcut quartz crystal resonator as the working electrode.
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Figure 2.1 Setup of an electrochemical quartz crystal microbalance which tracks
the electrochemically induced polymerization of NIPAM on the front electrode
of a quartz crystal resonator [24].
This chapter presents research undertaken to encapsulate a carbon nanotube
electrode (Buckypaper) with the thermally sensitive PNIPAM hydrogel. PNIPAM
hydrogel was synthesised using electrochemically induced free radical
polymerization technique [23, 24, 28, 35]. In addition, the encapsulation of gold
coated Mylar was also investigated as a background electrode for comparison
with Buckypaper. Gold coated Mylar was used as flat working electrode in the
electrochemical system due to its electrical conductivity, corrosion resistance
and electrochemical stability in the analyte and potential ranges used
throughout the chapter. A hydrogel was formed within and/or adjacent to the
carbon nanotubes of the Buckypaper electrode and could be readily observed
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by eye and in more detail using scanning electron microscopy (SEM). Cyclic
voltammetry was performed to characterise the electrochemical properties and
the electrochemically active surface area of the electrodes both before and after
the polymerisation process. FTIR was used to verify that the hydrogel coating
was PNIPAM. Permeability and transition temperature of PNIPAM coatings on
gold coated Mylar and RVC were investigated here by a study of the oxidation
and reduction rates of potassium ferricyanide by cyclic voltammetry. K2SO4 is
used here as supporting electrolyte during the polymerisation process. It is
known the salts from the Hofmeister series can influence the LCST of PNIPAM
hydrogel [36-38]. Therefore, the LCST of PNIPAM coatings on gold coated Mylar
was also determined by measuring the transmission of light over a rage of
temperature as a function of anion (
) concentration [36, 37].
2.2 Experimental
2.2.1 Chemical and Materials
HiPCO Single-walled carbon nanotubes (SWNTs, batch no. P0900) were
purchased from CNI Nanotechnology, Rice University (Houston, USA) and used
without further treatment. Triton-X100 (Aldrich) was used as a surfactant and
Ethanol (Univar, 95 % purity) was used for the removal of Triton X-100 from
SWNT films. PVDF membrane (DURAPORE®MEMBRANE FILTERS, Millipore)
with a pore size of 0.22 µm was used for vacuum assisted filtration of the CNT
dispersion solution. N-isopropyl acrylamide (NIPAM, 97%, MW:113.16, Sigma60
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Aldrich, Batch#: MKBB3307), N,N'-methylene-bis-acrylamide (MBA, ≥ 98.0%,
MW:154.2, C7H10N2O2, Sigma), Potassium persulfate (KPS, K2S2O8, FW=270.32,
Sigma-Aldrich) and, N,N,N’,N’ -Tetramethyl-Ethylenediamine (TEMED, ≥ 99.0%,
MW:116.21, C6H16N2, Sigma) were used as monomer, crosslinker, initiator and
accelerator, respectively. Potassium sulfate (K2SO4, Ajax) and potassium
ferricyanide (K3[Fe(CN)6], Ajax) were used as electrolyte solution and All
reagents were used without further purification. All solutions were prepared
with deionised Milli-Q water (Resistance > 18.2 MΩ).
2.2.2 Fabrication of Buckypaper
120 mg SWNTs were suspended in 240 ml aqueous solution containing 1 ml
Triton X100 by ultrasonic horn sonication (Figure 2.2 A), operated at 30%
amplitude, with a pulse waveform of 1 s on followed by 1 s off (Sonics and
Materials 500 Watt Vibra cell). The total sonication “on” time was 2 h. The
suspension was then sonicated for 1 h in low energy ultrasonic bath sonication
(Branson B5500R-DTH 175 Watt) (Figure 2.2 B).
(A)
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(C)
(B)
Figure 2.2 Sonicator ultrasonic (A) probe and (B) bath Systems and the Leica
DMF-52 optical microscope (C).
A small droplet of the resulting dispersion was placed between a glass slide and
coverslip and imaged using a Leica DMF-52 optical microscope (Figure 2.2 C) in
transmission mode. Following dispersion, the solutions were vacuum filtered
over a 0.22m pore size hydrophobic PVDF membrane (5.5 cm x 8 cm) forming
porous sheets [39]. To remove the surfactant, the obtained freestanding
Buckypapers were washed with approximately 500 ml of Milli-Q water until the
bubbling of the surfactant disappeared. Then, the Buckypaper was rinsed with
50 ml ethanol to remove excess surfactant [40] and allowed to dry at room
temperature. The freestanding Buckypaper is obtained by peeling the CNT
assembly off the filtration membrane. The setup of a vacuum-filtration method
for making the Buckypaper is shown in Figure 2.3.
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(A)
(B)
Figure 2.3 (A) Schematic diagram (adapted from [39]) and (B) the experiment
setup showing fabrication of a vacuum-filtration method for making the
Buckypapers.
2.2.3 Formation of hydrogel encapsulation on conducting electrodes
Encapsulation of conducting electrodes with PNIPAM was performed using
electrochemically induced free radical polymerisation [24, 25, 27, 28]. Cyclic
voltammetry (CV) (as described in the following paragraph) was used to form
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the hydrogel encapsulation on gold coated Mylar, reticulated vitreous carbon
(RVC) and Buckypaper electrodes. Electrochemical experiments were
conducted with a potentiostat (eDAQ Pty. Ltd. Australia, EA 160 electrochemical
system) employing a three-electrode cell where the auxiliary (counter) and
reference electrodes are platinum mesh (2 x 2 cm2) and Ag|AgCl (3 M NaCl),
respectively (Figure 2.4). Prior to encapsulating gold coated Mylar, its surface
was cleaned with acetone and placed in an UVO cleaner for 10 min (Jelight
Co.Inc. USA, MODEL 42-220). This UV/Ozone cleaning process effectively
removes organic contaminants from the gold surface.
Figure 2.4 Setup of a three‐electrode electrochemical cell configuration for
electrochemically
induced
free
radical
polymerization
of
NIPAM
on
Buckypaper; WE = working electrode (Buckypaper), RE = reference electrode,
AE = auxiliary electrode.
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For electrochemical induced polymerization, an aqueous solution of 0.4 M
K2SO4 electrolyte solution was prepared and sonicated for at least 30 min. 0.3 M
monomer NIPAM, 3 mM crosslinker MBA and 10 mM initiator KPS were added
to the electrolyte solution following sonication. The working, reference and
auxiliary electrodes were immersed in a small vial containing the prepared
monomer solution. The solution was bubbled with nitrogen for 10 min prior to
polymerisation. Electrochemically induced free radical polymerisation was
initiated by applying a voltage of -0.8 V versus Ag|AgCl (3 M NaCl) reference
electrode for 60 min using the eDAQ system. The polymerisation was
performed at various temperatures, namely room temperature (22C), in an
ice bath (4C) and at 40C. After polymerisation, the PNIPAM coated electrodes
were removed from the cell and rinsed with Milli-Q water. Before and after the
polymerisation, a cyclic voltammogram (-1 to 0.2 V) was recorded for 5 cycles
to characterize the electrochemical properties and the electrochemically active
surface area of the electrodes. The scan rate used was 50 mV/s with a starting
potential of 0 V, scanning first to positive potential.
Cyclic voltammetry (CV) is one of the most commonly used eletrochemical
techniques where current is studied as a response to potential. This technique
is based on a linear potential waveform. The potential can be considered as a
linear triangular potential excitation signal (Figure 2.5). This signal sweeps the
potential of the electrode between two potential values sometimes called the
switching potential [41, 42]. Figure 2.6 shows the obtained voltammogram
65
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displays current (vertical axis) versus potential (horizontal axis) which can be
considered as time axis [41-46]. Cyclic voltammetry is widely used
electroanalytical technique for the study of redox process to understand
reaction intermediate and stability of reaction products [24, 45-47]. The
magnitude of the anodic peak current (Ipa), cathodic peak current (Ipc), anodic
peak potential (Epa) and cathodic peak potential (Epc) are the important
parameters of a cyclic voltammogram as labelled in Figure 2.6. Ipa and Ipc
measure involving extrapolation of a baseline current. The peak current for a
reversible system is described using Randles-Sevcik equation [48, 49] for the
forward seep of the first cycle as shown in Equation 2.1.
(2.1)
where ip is peak current in ampere, n is electron stoichiometry (number of
electrons transferred in the redox reaction), A is area of the electrode (cm2), D is
diffusion coefficient (cm2/second), Co is the concentration (mol/cm3), and v is
scan rate (V/second).
Reversible couples will display a ratio of the peak currents passed at reduction
(ipc) and oxidation (ipa) that is near unity (1 = ipa/ ipc). According to Equation 2.1,
ip is proportional to bulk concentration, square root of scan rate and number of
electrons transferred in the redox reaction. However, the ratio of peak currents
can be significantly affected by chemical reactions coupled to the electrode
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process. The current is recorded both in the forward and reverse scan direction
in any number of cycles during potential is applied at the working electrode as
shown in Figure 2.6. In the forward scan, the anodic current increases until the
maximum point (Epa) where the reduced species become oxidized species. In
the reverse scan, the cathodic peak current increases until it reaches the
maximum point (Epc) where most of the oxidized species become reduced
species. For electrochemically reversible system, a redox couple in which both
species rapidly exchange electrons with the working electrode. The number of
electrons transferred in the electrode reaction (n) for a reversible couple can be
determined from the separation between peak potentials as shown in Equation
2.2.
(2.2)
Thus, for a one-electron process, such as the reduction of [Fe(CN)6]3– to
[Fe(CN)6]4–, exhibited
of 0.059 V. However, quasi-reversible systems
display a larger peak separation in Ep. For a very slow charge-transfer or
irreversible system, the anodic and cathodic peaks are completely separated
and no reverse wave is observed.
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GEL ENCAPSULATION OF
CARBON NANOTUBE ELECTRODES
t1
t0
t2
Time
t3
t4
Figure 2.5 Linear triangular potential excitation waveform of cyclic
voltammetry.
Current (A)
Epa
ipa
Scan direction
Scan direction
ipc
Epc
Potential (V)
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Figure 2.6 Typical cyclic voltammogram where ipc and
ipa
show the peak
cathodic and anodic current respectively for a reversible cyclic voltammetry.
A capacitive current is an additional consideration in cyclic voltammetry which
is produced by the growth of a double electric layer on the interface between
the electrode and the solution [44]. Thus, the total current flowing through the
electrode is finally due to the sum of the charging current (capacitive current)
and the faradic current. The size of the capacitive current contribution depends
on the area of the electrode and the scan rate as shown in Equation 2.3.
ic = Cv
(2.3)
where ic is capacitance current (A), C is capacitance of the system (F) and v is
the potential scan rate (V/s)
In many cases, capacitive current is much smaller than the Faradaic component,
and can be effectively ignored. However, SWNT Buckypaper in this research is
used as an electrode in electrochemical experiments. The Buckypaper has very
high capacitance contributed by their high electro-active surface areas and high
porosity which formed double layer capacitive films on surfaces as reported by
Pacios et. al [50]. The high contribution from the capacitive charging current
may mask electrochemical redox signals.
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2.2.4 Morphology
An optical microscope (Leica, Germany) was used for visual observations to
study the surface morphology of PNIPAM coatings on gold coated Mylar and
RVC. A scanning electron microscopy (SEM, JOEL 7500F) (Figure 2.7) was used
to study both surface morphology and microstructure of neat and gel
encapsulation of Buckypapers. SEM is a microscope that uses a focused beam of
electrons instead of light to generate signals at the surface of a variety specimen
to form an image. The SEM uses electromagnets rather than lenses to control
the degree of magnification. Moreover, the SEM also has allowed to obtain high
resolution and high magnification images of sample surfaces. Therefore, the
SEM has been widely used to study the information about the sample including
external morphology (texture), chemical composition, and crystalline structure
and orientation of materials making up the sample. The samples require
controlled drying to allow the SEM chamber to reach high vacuum and to
prevent deformation of the sample at the SEM vacuum. General characteristics
for sample preparation, the sample must be conductive materials to prevent a
static electric field during scan and also vacuum compatible. For insulating
samples, the sample was coated with small amount of conductive material to
improve the conductivity and carry away the charging electrons. The coatings
need to keep the material thin and keep the coating layer particles small in
order to accurately image the samples. In this thesis, the Buckypapers imaged
by SEM were dried without metal sputter coating. For insulating samples, the
70
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hydrogel coated on electrodes was sputter coated with small amount of
conductive material to improve the conductivity.
Figure 2.7 A typical SEM instrument, showing the electron column, sample
chamber, EDS detector, electronics console, and visual display monitors.
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2.2.5 FTIR analysis
FTIR stands for Fourier Transform Infrared, the preferred method of infrared
spectroscopy. FTIR spectroscopy has been widely used in laboratory and
industry as a non-destructive technique for determination of chemical
compounds in liquids, gases, powders and films. In FTIR spectroscopy, IR
radiation is passed through (transmitted) a sample and some of it is absorbed
by the sample (Figure 2.8 A). Therefore, the resulting spectrum creates a
fingerprint of a sample, representing the molecular absorption and
transmission. A fingerprint of a sample with absorption peaks corresponds to
the frequencies of vibrations between the bands of the combination atoms
making up the unique material [51]. Therefore, no two unique compounds
produce the exact same infrared structure. This makes infrared spectroscopy an
excellent tool for quantitative analysis which can identify the molecular
structures of every different kind of material.
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(A)
(B)
Figure 2.8 (A) Infrared radiation is passed through the sample and (B) FTIR
instrumentation process: the sequence of steps involved in obtaining an IP
spectrum using an FTIR [51].
Figure 2.8 B illustrates infrared energy is emitted which passed through the
interferometer which controls the amount of energy presented to the sample. It
73
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is transmitted through or reflected off of the surface of the sample which is
uniquely characteristic of the sample. The detector measures the interferogram
signal. The Fourier transformation is performed by the computer to present
final infrared spectrum for quantitative analysis. Prior to the measurement of
the samples, a background spectrum must be measured in order to remove all
instrumental characteristics to ensure that the resulting spectrum is all spectral
features of the sample only. The instrument used in this experiment is the FTIR
spectrometer (IRPrestige-21, Shimadzu, Japan) (Figure 2.9).
Figure 2.9 FTIR spectrometer (IRPrestige-21, Shimadzu, Japan)
FTIR spectrometer has been used to investigate the chemical structure of
PNIPAM film coated on gold Mylar. Transmission infrared spectra of PNIPAM
film coated on gold coated Mylar was measured over the wavelength range of
74
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400 to 4000 cm-1. Prior to FTIR analysis, the polymer film was soaked in Milli-Q
water for 2 h, with the water changed once. Then, the water was changed and
the film was soaked in the water for another hour. The polymer film was then
dried by placing in an oven overnight at 60°C under vacuum. The samples
studied by FTIR consisted of gold coated Mylar coated with dried PNIPAM
network. A bare gold coated Mylar film was employed as background. The
spectrum was taken in transmittance mode.
2.2.6 Effect of chemical crosslinking
In general, a chemical crosslinker is introduced to create covalent inks between
polymer chains during synthesis forming a network. To investigate the effect of
chemical crosslinking, electrochemically induced free radical polymerisation
was performed with and without the addition of a crosslinker.
2.2.7 Cyclic Voltammetry with Potassium Ferricyanide
Potassium Ferricyanide (K3[Fe(CN)6]) undergoes oxidation and reduction in
aqueous solutions (E -436 mV at pH 7) [52, 53]. In order to investigate the
permeability of the gel layer encapsulating the electrodes, the PNIPAM
encapsulated electrodes were immersed into a solution of potassium
ferricyanide (K3[Fe(CN)6], 5 mM in 0.4 M aqueous potassium sulfate) where
cyclic voltammograms were acquired as a function of time [24]. Cyclic
voltammograms (-0.4 to 0.6 V) versus Ag|AgCl (3 M NaCl) were performed from
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10°C to 50°C increasing the temperature by 10°C after every CV at a scan rate of
50 mV/s. To study the reversibility of the phase transition from hydrophilic to
hydrophobic with change in temperature, cyclic voltammograms were also
recorded whist decreasing the temperature from 50°C to 10°C at 10°C intervals.
2.2.8 UV-Vis-NIR spectrophotomer
The absorbance of PNIPAM coated on gold coated Mylar immersed in Milli-Q
water was measured using a Cary-500 Scan UV-Vis-NIR spectrophotometer
(Varian Australia Pty Ltd, Cary-500) with a Cary temperature controller (Figure
2.10). The absorbance of gold coated Mylar coated with PNIPAM was performed
from 25°C to 40°C increasing the temperature by 5°C and measuring
absorbance of wavelengths between 300-900 nm. The samples were
investigated in different concentrations of K2SO4 as electrolyte (0 M, 0.1 M and
0.4 M) to quantify the effect of salt to the LCST of PNIPAM, and the reversibility
of the temperature transition changes.
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Figure 2.10 UV-Vis-NIR spectrophotometer (Varian Australia Pty Ltd, Cary500)
UV-vis or UV/Vis spectroscopy refers to absorption or reflectance spectroscopy
which using light near UV, visible and near-infrared (NIR) regions. UV-vis is a
common and versatile technique to determine molecular structures which work
in quantitative determination. The Beer-Lambert’s law states that the
concentration of a substance in solution and the path length of the radiation
through the sample are directly proportional to the absorbance of the solution.
However, the law is only true for monochromatic light. That is light of a single
wavelength or narrow band of wavelengths. It provided that the physical or
chemical state of the substance does not change with concentration. The
intensity of the emitted monochromatic radiation through a homogeneous
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solution in a cell depends upon the thickness (l) and the concentration (c) of the
solution as shown in Figure 2.11.
l
I0
c
I
Figure 2.11 A schematic of the monochromatic radiation passes through a
homogeneous solution in a cell.
I0 is the intensity of the incident radiation and I is the intensity of the
transmitted radiation. The ratio I/I0 is called transmittance which is expressed
as a percentage and referred to as % transmittance. Thus, mathematically,
absorbance is related to percentage transmittance T as illustrated in Equation
2.4.
(
)
(2.4)
where A is absorbance, L is the length of the radiation path through the sample,
c is the concentration of absorbing molecules in that path and k is the extinction
coefficient - a constant dependent only on the nature of the molecule and the
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wavelength of the radiation. The absorption spectrum of a compound can be
shown as a plot of the absorbance against wavelength.
2.3 Results and discussions
2.3.1 Preparation of Buckypaper
Carbon nanotubes have a high tendency toward self-aggregation due to strong
Van der Waals forces [54]. Black paper-like sheets (so called Buckypaper) of
SWNTs
were
successfully
fabricated
by
multi-step
microfiltration of a suspension of nanotubes (Figure 2.12).
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Figure 2.12 Typical morphologies of as-synthesised Buckypaper fabricated by
filtering a stable suspension of SWNTs. (A) Photograph of a piece of
Buckypaper, (B) low-magnification SEM image, (C) high magnification SEM
image of Buckypaper surface, and, (D) cross section image of neat Buckypaper.
Figure 2.12 A shows an optical photograph of a freestanding SWNT Buckypaper.
The Buckypapers are self‐supported CNT networks with a thickness of 40-50
μm. Generally, agglomerates of SWNTs present as bundles and ropes as well as
individual SWNTs are distributed in the Buckypaper which are held together via
non‐covalent interactions [55, 56]. As expected, SEM images of Buckypaper
exhibit the physical entanglement of SWNT bundles, ropes and individuals.
Figure 2.12 B, C and D show that SWNT Buckypaper contains many very smallinterconnected pores [39, 57]. Buckypaper was cut into rectangular strips (0.5
cm x 2.5 cm) for encapsulation by PNIPAM.
2.3.2 Hydrogel encapsulation of conducting electrodes
Electrochemically induced free radical polymerization was employed for the
fabrication of hydrogel coatings on conducting electrodes. It is an easy and
versatile polymerisation route utilizing a one-step construction. Based on the
electrochemical reduction of persulfate anion, a negative voltage (-0.8 V rel. to
Ag|AgCl) was applied to the electrode to be encapsulated which was immersed
into an aqueous solution of monomer, supporting electrolyte, crosslinker and a
redox-active initiator. Samec et al. investigated the redox behaviour of
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persulfate ions on gold electrodes in an aqueous medium [58-60]. Persulfate
anions (
located close to the cathode received an electron to form
. The radical ions then initiated free radical polymerization as
shown in Figure 2.13. Although high quality coatings have been reported, the
primary mechanism by which the PNIPAM hydrogel coatings adhere to the
electrodes have not been isolated. After preparation, the polymer films were
extensively rinsed and stored in Milli-Q water at room temperature. The
adhesion of the polymer coatings is good. The films did not peel off when stored
in water, even after several weeks [24, 27].
Figure 2.13 Electrochemical production of free radicals and formation of
hydrogel coating at the substrate surface.
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The preparation of surface-attached hydrogel layers on gold coated Mylar using
electrochemically induced free-radical polymerisation at different temperatures
shows that the large peak at -0.75 V from the decomposition of potassium
persulfate as shown in Figure 2.14. This peak intensity decreased dramatically
during the first few electrochemical cycles. The decrease in the reduction peak
at -0.75 V indicates that the diffusion coefficient of initiator was reduced due to
the formation of an attached PNIPAM hydrogel coating [25].
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Figure 2.14 Cyclic voltammograms of gold coated Mylar electrode taken at a
scan rate of 50mV/s, 5 cycles in the reaction mixture (CV, -1V to 0.2V) at
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different temperatures (A) in an ice bath (approx. 4C), (B) at room
temperature, (C) at 40C. The dashed arrows indicate the direction of the
potential scan.
With the voltammetric cycling before polymerisation (A) at 4C (below the LCST
for PNIPAM), the initial cathodic current at -1.0 V decreased from 0.75 to 0.45
mA/cm2 of 1st to 5th cycles (48 s each cycle), indicating that the polymeric
matrix was generated on the electrode surface. The current at -1.0 V after
polymerisation decreased to 0.20 mA/cm2. The decrease of the current during
polymerisation indicates that hydrogel was produced on the gold coated Mylar
electrode surface. At room temperature (Figure 2.14 B), the initial cathodic
current at -1.0 V decreased from 0.86 to 0.56 mA/cm2 after 5 cycles while the
current at -1.0 V after polymerisation decreased to 0.12 mA/cm2. Besides,
polymerisation at 40C (above the LCST for PNIPAM) (C), the initial cathodic
current at -1.0 V decreased from 1.25 to 0.19 mA/cm2 after 5 cycles and
continuously decreased to 0.08 mA/cm2 after the polymerisation. The
hydrogels are transparent when polymerised in an ice bath and at room
temperature while the polymer appears cloudy when polymerised at
temperature above the LCST. This is due to phase separation and the resulting
light scattering as the hydrophobic interaction between the polymer chains
become dominant [61].
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The rapid current decrease upon successive potential scans at temperature
above the LCST probably due to the polymerisation rate is faster than at
temperature in an ice bath and room temperature or polymer at above the LCST
has lower diffusion which could decrease current.
Figure 2.15
UV-vis absorption recorded at room temperature of PNIPAM
coated on gold coated Mylar polymerised at different temperatures.
UV-vis spectroscopy revealed markedly different absorption spectra of samples
produced at different polymerisation temperatures (Figure 2.15). UV-visible
absorbance spectra of film polymerised at 40C was larger than that in an ice
bath (4C), indicating the film polymerised at 40C is less transparent than
polymerised in an ice bath. Therefore, all polymerisation processes reported
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here were carried out in an ice bath in order to prepare a uniform PNIPAM
network structure.
2.3.3 Cyclic Voltammetry before and after polymerisation on different
substrates.
PINAM was electrochemically grown on a range of substrates using the
conditions outlined above, namely application of a voltage of -0.8 V versus
Ag|AgCl reference electrode for 60 min in a solution of 0.3 M NIPAM, 3 mM MBA
and 10 mM KPS added into 0.4 M K2SO4, an electrolyte solution. The polymeric
matrix was produced on conductive electrodes with one-step construction.
Under these polymerisation conditions a uniform hydrogel formed on gold
coated Mylar, RVC and Buckypaper (fabrication procedure outlined in section
2.2.2) electrodes which was readily observed by eye and by optical microscope
as shown in Figure 2.16 A-D. In Figure 2.16 A-D arrows show the regions of
electrodes coated by PNIPAM. The adhesion of polymer films on electrode
surfaces was sufficient to keep the coating attached during manual handling and
immersion in different electrolytes [24].
Figure 2.16 E and F are images from an optical microscope showing the PNIAPM
gel coating film on RVC foam with a pore size of 60 pores per inch (ppi). It can
be clearly seen that the polymer gel has grown as a film bridging the porous
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matrix of both electrodes. The PNIPAM gel bridges pores much larger than 100
µm in diameter. Most of the films have fully covered the pores while some of the
PNIPAM films contain cracks due to drying of samples (indicated by arrows in
Figure 2.16 E).
Figure 2.16 Photographs show the PNIPAM gel coating on (A) gold coated Mylar
(width= 1 cm), (B) RVC (60 ppi, width= 1 cm) and (C) Buckypaper (width= 0.5
cm), where arrows show the parts of PNIPAM coated on the electrodes. Images
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from an optical microscope show the PNIAPM gel coating on (E) RVC, 60 ppi
where the black areas are RVC surfaces.
Electrochemical behaviour of the PINPAM samples was examined by cyclic
voltammetry before and after polymerisation on different substrates. Cyclic
voltammetry was performed in the monomer solution over the potential range
-1 to 0.2 V (Figure 2.17). A smaller charging current after polymerisation
confirms the formation of PNIPAM due to the effective reduction the
electrochemically active surface area of the electrodes or a reduction in the rate
of diffusion of the initiator (KPS). The reduction currents recorded at -0.8V
before and after polymerisation of the hydrogel coated on gold coated Mylar
electrode, have greatly decreased, indicating that the polymer layers
significantly impeded the diffusion of the initiator to the electrode surface.
Moreover, a large reduction peak is clearly visible on gold coated Mylar (Figure
2.17 A) at approximately -0.8 V resulted from the decomposition of the KPS as
initiator which is similar to Reuber et al. work [24]. As mentioned in section
2.2.3, when the potential is scanned in the negative direction, the cathodic peak
current increases until it reaches the maximum point (Epc) where some of the
KPS is reduced at this point. As described by the Randles-Sevcik equation 2.1,
the peak current is proportional to the area of the electrode (A) or square root
of diffusion coefficient (D) if all other parameters (n is number of electrons
transferred in the redox reaction, v is scan rate) are kept constant. Thus, the
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decrease of accessibility of the active surface area of the electrode or a
reduction in the rate of diffusion of the initiator (KPS) will result in a decrease
of peak current as shown in Figure 2.17. However, the current peaks of RVC and
Buckypaper electrodes are slightly less in comparison. It is noted that
attachment of polymer on RVC and Buckypaper electrodes is slightly affected by
the impediment of the diffusion of the initiator (KPS) to the surface.
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(C)
Figure 2.17 Electrochemically induced free radical polymerisation was initiated
by applying a voltage of -0.8 V versus Ag|AgCl reference electrode for 60 min in
the mixture of 0.3 M NIPAM, 3 mM MBA and 10 mM KPS adding into 0.4 M
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K2SO4 as an electrolyte solution for a PNIPAM coated on different substrates in
an ice bath where (A) gold coated Mylar, (B) RVC, 60 ppi and (C) Buckypaper.
Cyclic voltammograms (-1 to 0.2 V, 50 mV/s (gold coated Mylar and RVC), 5
mV/s (Buckypaper)) were performed in the same mixture before (bold line)
and after (dash line) polymerisations for 5 cycles (3rd cycles are shown). Arrows
show the direction of potential scan.
2.3.4 FTIR analysis
PNIPAM films studied by FTIR consisted of gold coated Mylar coated with dried
PNIPAM networks. The FTIR spectra of PNIPAM (Figure 2.18) exhibit all of the
bands expected for PNIPAM [24, 62] including: 3300 cm-1, N-H stretching; 2973
and 2934 cm-1 C-H stretching; 1650 cm-1, amide-I band assigned to C=O
stretching; 1549 cm-1, amide-II band belonged to N-H in-plane bending
vibration. FTIR confirms that the hydrogel formed on gold coated Mylar is
PNIPAM.
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Figure 2.18 FTIR spectrum of a PNIPAM film which coated on gold coated
Mylar. The spectrum was taken in % transmittance mode.
2.3.5 Morphology
The surface morphologies of PNIPAM coated onto gold Mylar and Buckypaper
electrodes were examined by scanning electron microscopy (SEM) at various
magnifications as shown in Figure 2.19 and Figure 2.21, respectively. The dried
polymer coating is found to be relatively rough and porous when examined
under high magnification. It is not known whether this roughness resembles the
surface of the synthesised hydrogel, or, it is created by the drying process.
Furthermore, SEM was also performed on cross-sections of PNIPAM coated
electrodes. It is found that the thickness of dried PNIPAM coated on gold coated
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Mylar is 1.65 ± 0.19 µm as shown in Figure 2.20. Hence, a hydrated PNIPAM
hydrogel layer may be more than 15 µm thick (assuming 90% water).
Figure 2.19 SEM images of surface of gold coated Mylar coated with PNIPAM at
various magnifications; (A) x 500, (B) x 2,000 and (C) x 10,000.
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ii
iii
i
i
Figure 2.20 Cross-sectional SEM image of dried gold coated Mylar coated with
PNIPAM where 3 parts are resin (i), PNIPAM hydrogel (ii) and gold (iii).
The surface and cross-section of neat Buckypaper and Buckypaper coated with
PNIPAM at the same magnification show that the uncoated Buckypaper has a
relatively rough surface at both top-down and cross section images. Buckypaper
coated with PNIPAM clearly shows that PNIPAM has formed on the exterior of
the Buckypaper and shows a relatively smooth morphology (Figure 2.21 C). The
cross section image of Buckypaper coated with PNIPAM shows that the pores
are still visible (Figure 2.21 D).
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Figure 2.21 SEM images of (A) surface of neat Buckypaper, (B) cross section of
neat Buckypaper, (C) surface of Buckypaper coated with PNIPAM and (D) cross
section of Buckypaper coated with PNIPAM where (i) is polymer coating part
and (ii) is Buckypaper part.
In the SEM image of a cross-section (Figure 2.22), the thickness of dried
Buckypaper coated with PNIPAM film was roughly 100 µm while the thickness
of pure Buckypaper was in the range of 40 – 50 µm. The PNIPAM film thickness
was between 20 and 30 µm. For the cross section images of the coating polymer
surface on Buckypaper are found to be clearly rough and porous when
examined under high magnification.
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Figure 2.22 SEM images of dried sample cross section of (A) thickness of
Buckypaper coated with PNIPAM, (B) Buckypaper coated with PNIPAM part
and (C) high magnification image of the interface polymer coating - Buckypaper
coated with PNIPAM.
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2.3.6 Effect of chemical crosslinking
Generally, a chemical crosslinker is incorporated to create links between
polymer chains necessary to form a polymer network. In some cases, the selfcrosslinking of polymer chains can also lead to the formation of polymer gel
spheres instead of linear chains without the use of added crosslinker by chain
transfer reactions [63]. Figure 2.23 shows that the formation of PNIPAM using
electrochemically induced free radical polymerisation in the absence of a
crosslinker also produces PNIPAM films. It can be seen that the decomposition
peaks of the initiator at -0.75 V after polymerisation has decreased, indicating
that a coating significantly impedes the diffusion of the initiator to the electrode
surface. However, the PNIPAM coating film strength is very low as they could be
easily removed by Milli-Q water rinsing after the polymerisation. The films may
show a strong physical interaction between gold and PNIPAM as reported by
Reuber et al. [24].
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Figure 2.23 The electrochemically induced free radical polymerisation was
initiated by applying a voltage of -0.8 V versus Ag|AgCl reference electrode for
60 min in the mixture of 0.3 M NIPAM, 10 mM KPS and 0.4 M K2SO4 without
adding crosslinker for a PNIPAM coated on gold coated Mylar substrate. Cyclic
voltammograms (-1 to 0.2 V, 50 mV/s) were performed in the same mixture
before (bold line) and after (dash line) polymerisations for 5 cycles (3rd cycles
were picked). Arrows show the direction of potential scan.
2.3.7 Cyclic Voltammetry with Potassium Ferricyanide
In order to investigate the permeability of the hydrogel layer, cyclic
voltammetry (-0.4 to 0.6V, 50mV/s) was performed in a solution of potassium
ferricyanide K3[Fe(CN)6] (5 mM in 0.4 M aqueous potassium sulfate). The
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transport characteristics of the gel layer were determined by evaluating the rate
of Fe being oxidised or reduced at the electrode surface above and below the
LCST. The electrochemical response signal of K3[Fe(CN)6] exhibits redox peaks
relating to the redox switching of the Fe(CN)63-/ Fe(CN)64- couple as observed at
bare gold coated Mylar electrode and gold coated Mylar coated with PNIPAM
(Figure 2.24) [24]. The observations revealed that the bare gold coated Mylar
electrode offered a high availability to the Fe(CN)64-/Fe(CN)63- anions, while the
permeability was decreased at gold coated Mylar coated with PNIPAM. This
approach can be used to evaluate the change of permeability of the hydrogel at
different temperatures.
A PNIPAM hydrogel coated on gold coated Mylar underwent a transition when
varying the temperature of the electrolyte in an aqueous solution containing
sulfate with the LCST transition occurring between 10C and 20C. The change
of peak current in CV at the temperature recorded is lower than the reported
LCST at ~32C in aqueous solutions [29, 30]. This may be due to the addition of
salts (
0.4 M K2SO4) which is an important parameter because
electrochemical preparation requires a supporting electrolyte in order to
conduct the current. As presented by Bünsow and Johannsmann [26], the LCST
transition of PNIPAM gel is observed between 20-25C in solution of 0.4 M
ammonium sulfate. The initial onset values of the LCST for PNIPAM gel as a
function of
concentration occurred lower than 32C here, which is
consistent with published literature [26, 37, 38]. Figure 2.24 shows the
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magnitude of the peak currents decreases when the electrode coated with
PNIPAM film comparing to bare electrode because the film inhibited the
diffusion of ions to the electrode surface. However, the magnitude of the peak
currents also decreases when heated above the LCST due to the diffusion of ions
to the electrode surface being impeded by the collapsed hydrogel film [24]. This
process is reversible. This verifies that the electrochemically induced PNIPAM
hydrogel also shows temperature-induced reversible swelling or collapse at
below or above the temperature LCST, respectively. Figure 2.24 B shows the
magnitude of cathodic and anodic peak currents measured involving
extrapolation of a baseline current as described in section 2.2.3. Both peak
currents continued to decrease while temperature is further increased. It
indicates that the hydrogel changed from an expanded state to a collapsed state
that obstructs the diffusion of ions to the Au electrode surface.
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(A)
(B)
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Figure 2.24 (A) Cyclic voltammograms of PNIPAM coated on gold coated Mylar
in K3[Fe(CN)6] (5 mM in 0.4 M aqueous potassium sulfate) with increasing
temperature from 10C to 50C and then decreasing temperature back to 10C
at scan rate 50 mV/s at a scan rate of 50 mV/s. The arrows show the direction
of potential scan. (B) Ipc and Ipa are cathodic and anodic peak current
respectively for cyclic voltammograms with increasing temperature from 10C
to 20C where the lines are included to guide the eye only.
Figure 2.25 shows that the electrochemical response signal of K3[Fe(CN)6]
exhibits redox peaks relating to the redox switching of the Fe(CN)63-/ Fe(CN)64couple as observed for bare RVC electrode. Whilst cyclic voltammetry of the
bare Buckypaper in K3[Fe(CN)6] showed a slight reduction peak around +0.1 V,
the corresponding oxidation peaks are difficult to observe (Figure 2.25 B). This
may be due to the large capacitance feature of the Buckypaper attributed to its
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high surface area and high porosity resulting in large double layer capacitive
films on surfaces as reported by Pacios et. al [50]. The high contribution from
the capacitive charging current may mask electrochemical redox signals from
K3[Fe(CN)6]. In addition, the Fe(CN)63-/ Fe(CN)64- couple cannot completely
diffuse into the pores of the porous electrodes during each cycle at a fast scan
rate as reported by Yang et al. [47]. As presented in Figure 2.26, the
electrochemical response signal of K3[Fe(CN)6] clearly exhibits redox peaks at
low scan rates. It can be also implied that the oxidation peak at scan rate 50
mV/s might be greater than +0.6 V versus Ag/AgCl. It is known that the
capacitive current is proportional to scan rate while the Faradaic current is
proportional to square root of scan rate. Thus, when scan rate decreases the
capacitive current decrease much more than Faradaic current. It is seen that the
Faradic current can be observed when the background capacitive current
decreased at lower scan rates. In addition, peak separation (Equation 2.2)
decreases with the decrease of scan rate. Thus, the number of electron
transferred in the electrode reaction (n) which inversely proportional to
at
scan rate 50 mV/s, is lower than at other low scan rates. The resultants are slow
electron transfer and the slow diffusion through the pores of Buckypaper.
Therefore, Fe(CN)63-/ Fe(CN)64- couple can properly diffuse into the pores of
Buckypaper electrode at slow scan rates. However, cyclic voltammetry was
performed only at scan rate 50 mV/s on Buckypaper coated with PNIPAM in
this study.
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Cyclic voltammetry of RVC and Buckypaper coated with the hydrogel show the
peak currents are significantly different to the bare electrodes (Figure 2.25).
The magnitude of the peak currents decreased on all electrodes with coated
PNIPAM gels. This indicates that diffusion of ions to the electrode surface being
impeded by the hydrogel coating. With increasing temperature, the magnitudes
of the peak currents slightly decreased or were difficult to observe on RVC and
Buckypaper probably due to the high surface area and high porosity of both
electrodes. However, the small reduction peak was not observed in the
Buckypaper coated with PNIPAM gel. It is seen that redox peaks are difficult to
observe on Buckypaper coated with PNIPAM. This may be due to the gel
inhibiting the diffusion of Fe(CN)63-/ Fe(CN)64- couple.
(A)
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(B)
Figure 2.25 Cyclic voltammograms of PNIPAM coated on (A) RVC and (B)
Buckypaper electrodes in K3[Fe(CN)6] (5 mM in 0.4 M aqueous potassium
sulfate) with increasing temperature from 10C to 50C and then decreasing
temperature back to 10C at scan rate 50 mV/s. The arrows show the direction
of potential scan.
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Figure 2.26 Cyclic voltammograms of Buckypaper electrodes in K3[Fe(CN)6] (5
mM in 0.4 M aqueous potassium sulfate) with various scan rates from 5 to 50
mV/s. The arrows show the direction of potential scan.
2.3.8 Effect of salt on the LCST of PNIPAM coated on gold coated Mylar
Based on the preparation of PNIPAM gel coated electrodes using the
electrochemically induced free radical polymerisation, a supporting electrolyte
(containing salt) is an important parameter in an aqueous solution in order to
conduct the current. As CV resulted in section 2.3.7, PNIPAM hydrogel coated
gold Mylar underwent a transition when varying the temperature of the
electrolyte in containing sulfate with the LCST transition occurring between
10C and 20C which is lower than the ( ~32C) in neat water [29, 30]. The
LCST of PNIAPM gel coated on gold Mylar was studied by UV-vis to determine
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the change in absorbance as a function of temperature. In addition, samples
were immersed into electrolyte of different salt concentrations. The effect of
salt on the LCST of PNIPAM coated on gold Mylar is determined by the change in
light absorbance to compare with the results from cyclic voltammograms that
were taken in the presence of potassium ferricyanide in section 2.3.7. In
addition, comparison of the shift of the LCST from this study and from ref [26]
which determined the LCST from impedance analysis is presented.
Figure 2.27 A displayed UV-vis absorbance change of PNIPAM hydrogel coated
on gold coated Mylar in Milli-Q water with increasing temperature from 25C to
40C and then decreasing temperature over the same temperature range. It can
be seen that the absorbance of the gel changed sharply at all wavelengths. As
the temperature was increased from 30C and 35C the absorbance increased
which is associated with the hydrogel changing from transparent to opaque
indicating the temperature sensitivity of the hydrogel. The changes in
absorbance result from the scattering of light that occurs when the hydrogel
collapses forming a heterogeneous structure. Meanwhile, samples are
transparent and turn from clear to opaque between 30C to 35C, and then
transparent again after cooling down from 35C to 30C, indicating that the
LCST is set between 30C and 35C.
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(A)
(B)
K2SO4
concentration
Figure 2.27 (A) UV-vis of PNIPAM coated on gold coated Mylar in water with
increasing from 25C to 40C and then decreasing temperature with
(\, backslash) at scan rate of 200 nm/min and (B) the sharp changes in
absorbance at wavelength 360 nm of samples measuring in 0 M, 0.1 M and 0.4
108
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M K2SO4 (filled and opened makers represent heating and cooling temperature,
respectively). Bare gold coated Mylar was used as the background in the
respective electrolytes. The lines are included to guide the eye only.
The different concentration of electrolytes affected the LCST of PNIPAM as
reported by Bünsow and Johannsmann [26]. Whereas, LCST was inferred from
the temperature dependence of the frequency shift (f) and bandwidth shift,
() were determined by impedance analysis. The gel layer coated on quartz
crystal using an electrochemical quartz crystal microbalance (EQCM)
underwent a transition in salt solution with the transition occurring between
30-35C, 25-30C and 20-25C in aqueous solution of 0 M, 0.25 M and 0.4 M
ammonium sulfate (NH4)2SO4, respectively [26]. Figure 2.27 B shows the
measurement of the absorbance of the gold coated Mylar coated with PNIPAM
hydrogel in different concentrations of K2SO4 which occurring the sharp
changes in absorbance at wavelength 360 nm (indicated by the dashed line in
Figure 2.27 A). The LCST of PNIPAM decreased in the presence of sulfate. The
LCSTs of PNIPAM were set between 30-35C, 25-30C and 15-20C in 0 M, 0.1 M
and 0.4 M K2SO4, respectively. Therefore, the LCST of the gel decreased as a
function of increasing salt (
) concentration. It is seen that the sharp change
in absorbance can be attributed to the effect of salt on the LCST of PNIPAM
coated on gold coated Mylar. In comparison, the LCST transition of PNIPAM gel
as a function of
concentration (Figure 2.28), the shift of the LCST in 0.4 M
K2SO4 in this study is lower than ref [26]. This may be affected from the
109
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different characterisation technique employed. However, this change of the
LCST is similar to the results from cyclic voltammograms taken after
polymerization in the presence of potassium ferricyanide (5mM in 0.4 M
aqueous potassium sulfate) in section 2.3.7. The LCSTs of PNIPAM were set
between 15-20C and 10-20C which was determining the change in
absorbance and CV, respectively. This effect is pronounced for anions (
)
and is a general phenomenon in aqueous solution [37].
Figure 2.28 Comparison the LCST transition of PNIPAM gel as a function of the
concentration using (NH4)2SO4 (ref [229]) and K2SO4 (this study).
110
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2.4 Conclusions
Electrochemically induced free radical polymerisation using an eDAQ chart
system is an easy and versatile route for preparation of surface-attached
hydrogel films in one-step construction. All polymerisation processes reported
here were carried out in an ice bath in order to prepare the uniform PNIPAM
network structure. The application of cyclic voltammetry before and after
polymerisation confirms the formation of PNIPAM on gold coated Mylar, RVC
and Buckypaper electrode. The redox current recorded before and after
polymerisation of the hydrogel coated on all electrodes, have greatly decreased
due to the polymer layers significantly impeding the diffusion of the initiator to
the electrode surface.
The FTIR spectra of PNIPAM exhibit all of the bands expected for PNIPAM such
as amide II, C=O, -CH2-CH3 and CONH2. In addition, the PNIPAM hydrogel was
formed within and/or adjacent to the electrode surfaces could be readily
observed by eye and in more detail using scanning electron microscopy (SEM).
Moreover, the dried coating polymer surfaces are found to be relatively rough
and porous when examined under high magnification.
In order to investigate the permeability of the gel layer, a cyclic voltammogram
(-0.4 to 0.6V, 50mV/s) was applied in potassium ferricyanide. The results show
a change in hydrogel permeability for gold coated Mylar and RVC coated with
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PNIPAM. This change was not observed in the Buckypaper coated with PNIPAM
gel. However, the electrochemical response signal of K3[Fe(CN)6] clearly
exhibits redox peaks as observed at Buckypaper at low scan rates showing
Fe(CN)63-/ Fe(CN)64- couple can properly diffuse into the pores of Buckypaper
electrode. A PNIPAM hydrogel coated on gold coated Mylar underwent a
transition when varying the temperature of the electrolyte in an aqueous
solution containing sulfate with the LCST transition occurring between 10C
and 20C which is lower than the reported the LCST at ~32C in aqueous
solutions. The change in CV at the temperature recorded is lower than the
reported LCST, this may be due to the addition of salts (
) from the
supporting electrolyte (K2SO4) to the hydrogel. However, the sharp change in
absorbance of the gold coated Mylar coated with PNIPAM hydrogel in different
concentrations of K2SO4 can be attributed to the effect of salt on the LCST. The
LCSTs of PNIPAM were set between 30-35C, 25-30C and 15-20C in 0 M, 0.1 M
and 0.4 M K2SO4, respectively. Therefore, the LCST of the gel decreased as a
function of increasing salt (
) concentration. The change of the LCST from
this approach is related to the results from cyclic voltammograms taken after
polymerization in the presence of potassium ferricyanide. The most
importantly, electrochemically induced free radical polymerisation can be used
to deposit smart coatings on electrode which regulate permeability of reactants
based on temperature.
112
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Carbon nanotube/Hydrogel Composites
Chapter 3
Shrinkable Electronic Conductor:
Carbon nanotube/Hydrogel Composites
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Table of Contents
3 Shrinkable Electronic Conductor: Carbon nanotube /Hydrogel Composites 120
3.1
Introduction ............................................................................................................. 120
3.2
Experimental ........................................................................................................... 122
3.2.1 Chemical and Materials .................................................................................. 122
3.2.2 Preparation PNIPAM/SWNT hydrogel composites ............................. 122
3.2.3 Swelling properties .......................................................................................... 125
3.2.4 Phase transition behaviour ........................................................................... 126
3.2.5 Mechanical analysis ......................................................................................... 126
3.2.6 Electrochemical Impedance Spectroscopy (EIS) .................................. 127
3.3
Results and Discussions ...................................................................................... 132
3.3.1 Preparation of NIPAM/SWNT hydrogels................................................. 132
3.3.2 Swelling and Mechanical analysis .............................................................. 135
3.3.3 Phase transition behaviour ........................................................................... 140
3.3.4 EIS analysis.......................................................................................................... 142
3.3.4.1 Impedance in NaNO3 .............................................................................. 142
3.3.4.2 The effect of distance of electrodes and temperature .............. 147
3.3.4.3 The effect of SWNT concentration .................................................... 150
3.4
Conclusions .............................................................................................................. 152
3.5
References ................................................................................................................ 155
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3 SHRINKABLE ELECTRONIC CONDUCTOR: CARBON NANOTUBE
/HYDROGEL COMPOSITES
3.1 Introduction
A heat shrinkable electronic conductor using thermally sensitive hydrogels has
been fabricated. Embedding SWNTs into temperature-sensitive hydrogel
matrices is an effective method of enhancing the functions of PNIPAM
hydrogels. To combine the advantages of both SWNTs and PNIPAM polymer,
SWNTs must be dispersed in solution just prior to use. Since produced SWNTs
are aggregated because of - stacking, van der Waals and hydrophobic
interactions with their sidewalls in aqueous surroundings their dispersion is
limited [1-3]. Suspension of SWNTs in aqueous media using various surfactants
has been investigated using both chemical [4-8] and physical [9-12]
functionalization techniques.
Embedding nanoparticles into PNIPAM hydrogel matrices is an effective
method of enhancing the functions of these materials. Zhao et al. [13] reported
that the combination of PNIPAM hydrogel with Au nanoparticles as their
electronic properties show great promise for application in the field of sensing,
photoelectrochemical, and nano-electronic devices [14, 15]. The PNIPAM/Au
composite hydrogel shows excellent thermoswitchable properties with the
electronic conductivity changes due to the temperature reversible both above
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and below the LCST. Li et al. employed a PNIPAM hydrogel as a solid matrix for
the physical immobilization of Cadmium telluride (CdTe) [16]. CdTe
nanocrystals, often referred to as quantum dots QDs, exhibit strong
photoluminescence (PL) which could be synthesized in aqueous solution with
simplicity and high reproducibility. PNIPAM/CdTe hydrogel composites show a
heating/cooling cycle which is repeatable and reproducible as determined by
measuring maximum emission wavelengths changes upon varying temperature
[16]. The reversibility of response from PNIPAM/CdTe gels to temperature
makes it possible to develop a QD-based thermosensitive devices or sensor.
The SWNTs/NIPAM hydrogel composite described here were prepared in two
steps. First, SWNTs were dispersed with low molecular weight anionic
surfactant (SDBS) under sonication, which can exfoliate the SWNTs bundles into
individual tubes [17]. Second, the suspension was mixed with monomer,
crosslinker, initiator and accelerator and a PNIPAM hydrogel is synthesised.
The SWNTs dispersion is stable in the presence of NIPAM monomer solution
allowing the hydrogel to form through free radical polymerisation, allowing the
SWNTs to be attached to the polymer chains by covalent bonds [18].
Characterisation of the SWNT/NIPAM hydrogel composites was performed
below and above the LCST, with the electronic properties of thermally sensitive
composites (SWNTs/NIPAM) quantified by Electrochemical Impedance
Spectroscopy (EIS) under temperature stimuli.
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3.2 Experimental
3.2.1 Chemical and Materials
SWNTs were obtained from CNI (Batch#: P0900), USA and used without any
further treatment. Sodium dodecylbenzene sulfonate (SDBS, MW:348.48,
Sigma-Aldrich), N-isopropyl acrylamide (NIPAM, 97%, MW:113.16, SigmaAldrich, Batch#: MKBB3307), N,N'-methylene-bis-acrylamide (MBA, ≥ 98.0%,
MW:154.2, C7H10N2O2, Sigma), Ammonium persulfate (APS, (NH4)2S2O8, FW=
228.19, Ajax) and, N,N,N’,N’ -Tetramethyl-Ethylenediamine (TEMED, ≥ 99.0%,
MW:116.21, C6H16N2, Sigma) were used as surfactant, monomer, crosslinker,
initiator and accelerator, respectively. All reagents were used without further
purification. All solutions were prepared with deionised Milli-Q water (18.2
MΩ).
3.2.2 Preparation PNIPAM/SWNT hydrogel composites
In a typical experiment, 0.1 g (1 wt%) SDBS and different SWNT concentrations
[0.005 g (0.05 wt%), 0.01 g (0.1 wt%), 0.05 g (0.5 wt%) and 0.1 g (1.0 wt%)]
were added into 5.0 ml Milli-Q water. The mixture was sonicated for 30 min
using a Sonics VibraCell ultrasonicator with an amplitude of 30 % (500 Watts,
20 kHz) in an ice-water bath, with a 1.0 s ON and 1.0 s OFF pulsed waveform. To
determine the homogeneity of the dispersions, a small droplet of the resulting
dispersion was placed between two glass slides and evaluated using a Leica
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DMF-52 optical microscope in transmission mode. To the dispersion, 0.8 g (8.0
wt%) NIPAM dissolved in 5.0 ml Milli-Q water was added and then bath
sonicated. The quality of the dispersions was investigated immediately after
finishing sonication. Subsequently, 0.0135 g (0.135 wt%) MBA and 0.02 g (0.2
wt%) of APS were added to the 10 ml mixture and vigorously stirred for
another 30 min. The mixture was then placed into an ice bath and degassed by
bubbling nitrogen gas for 10 min to remove O2 before adding (20 µl) TEMED.
The mixture was then immediately transferred into moulds with consisted of
either syringes ( = 8.7 mm, 3.0 ml) or a rubber mould between parallel
stainless steel mesh electrodes (1 cm x 1 cm x 0.15 cm) (Figure 3.1). In-situ free
radical polymerisation was carried out at room temperature (~ 22 C)
overnight. After polymerisation, the samples were immersed in a large excess of
Milli-Q water in order to wash out unreacted monomer and initiator. The
samples were then kept in Milli-Q water prior to measurements for at least 1
week.
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Figure 3.1 Schematic representation: (A) top view of a square hole rubber
mould (1 cm x 1 cm x 0.15 cm) placed between parallel stainless mesh
electrodes and glass slides, (B) side view of the same where the indentation was
made for taking the rubber off after the gel formation and (C) PNIPAM/SWNT
hydrogel formed between parallel edge cut stainless meshes.
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3.2.3 Swelling properties
For the swelling measurements, hydrogel samples that were formed in syringes
( =8.7 mm) were cut into specimens of approximately 10 mm length and
immersed in Milli-Q water at room temperature for at least 1 week in order to
reach the equilibrium degree of swelling. The gravimetric method was
employed to study the hydrogel-swelling behaviour. The completely swollen
gels were removed from the swelling medium and weighed accurately after
removing the excess water on the hydrogel surface with tissue paper. The
hydrogel samples were then dried in an oven at 60C for 24 h prior to
measuring their mass again. The equilibrium-swelling ratio (ESR) and
equilibrium water content (EWC%) were calculated as follows:
ESR 
EWC % 
Me  Md
Md
(3.1)
Me  Md
 100
Me
(3.2)
where Me and Md are the weights of water present in the swollen hydrogels at
equilibrium swelling and dried hydrogels, respectively. The ESR value was
calculated as gram of water per gram of sample. All measurements were made
in triplicate.
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3.2.4 Phase transition behaviour
The gels formed between stainless mesh electrodes were used to study the
phase transition behaviour. The samples were placed in water-bath
temperature controller (controlled up to ±0.1C) at 20oC which increasing the
temperature by increment of 5oC and held at each temperature for 30 min prior
to measuring the thickness change. This procedure was repeated up to a
temperature of 60oC. The thickness changes were measured using a Vernier
caliper (Manostat Corp., Switzerland, the smallest division is 0.1 mm) to study
the phase transition of hydrogels. All measurements were performed in
triplicate.
3.2.5 Mechanical analysis
The mechanical properties of the swollen hydrogel samples were performed on
a Shimadzu EZ mechanical tester, in compression mode at a room temperature
(Figure 3.2). This test is used primarily to determine the relationship between
the average normal stress and average normal strain of the composite
hydrogels. For compression testing, gels were formed in syringes without
electrodes. The samples were compressed at a speed of 2 mm/min. The 500 N
load cell was selected. All measurements were made in triplicate. The plates
were covered with Naflon tape (TOMBO 9001) to reduce the effect of friction
between the gel and the plates during compression test. Sizes of samples are
between 10-12 mm in diameter and between 5-8 mm in height.
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Figure 3.2 Diagram of the compression test.
3.2.6 Electrochemical Impedance Spectroscopy (EIS)
Electrochemical Impedance Spectroscopy (EIS) is a well-established laboratory
technique where the cell or electrode impedance is plotted versus frequency.
Electrochemical impedance is measured by applying an AC potential to an
electrochemical cell and measuring the current through the cell. The current
response to a sinusoidal potential input will be a sinusoid at the same frequency
but shifted in phase in a linear system (Figure 3.3).
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Figure 3.3 Sinusoidal current response in a linear system.
The excitation signal expressed as a function of time has a form:
(3.3)
where
is the potential which changes sinusoidally with time t,
amplitude of the signal and
While, the response signal
is the radial frequency (
is shifted in phase () and has
is the

).
as a different
amplitude in a linear system.

Ohm's Law is used to calculate the impedance of the system as:
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

(3.5)

The impedance is therefore expressed in terms of a magnitude (modulus) │Z│,
and a phase shift, .
It is possible to express response current and potential as complex functions
which are described as,

(3.6)
(3.7)

Using Euler’s relationship (


 ), the impedance becomes,


| |


(3.8)
where
are real part and imaginary part of impedance, respectively.
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Figure 3.4 Bode plots i.e., |Zreal| versus log  and phase angle versus log  plots
[278-280].
The Bode plot is plotted with a log-frequency axis for showing the system's
frequency response. Since frequency appears as one of the axes on the Bode
plot, it is easy to understand from the plot how the impedance depends on the
frequency. Furthermore, the plot uses the logarithm of frequency to allow a
very wide frequency range to be plotted on one graph. The Bode plot also shows
the magnitude | | (| |
√
, the length of the vector) on a log axis
which can easily plot a wide impedance ranges on the same set of axes. This can
be an advantage when the impedance depends strongly on the frequency. In
addition, Rs and Rp can be read from the high and low frequency horizontal
plateau, respectively as shown in Figure 3.4. Furthermore, Bode plot allows a
more effective extrapolation of data from higher frequencies. Therefore, the
Bode plot is used to examine the magnitude as a function of frequency of
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SWNT/PNIPAM hydrogel composites at both below and above the LCST in this
work.
Electrochemical impedance spectroscopy (EIS) measurements were carried out
by using a Gamry Impedance system in a two electrode cell at open-circuit
potential (OCP). Impedance spectra were obtained between frequencies of 0.1
Hz and 100 kHz with AC amplitude of ±10 mV. The samples formed between
stainless steel mesh electrodes were used in EIS measurements. Due to the
limitation of the EIS instrument at high frequencies in Milli-Q water to measure
high resistance, small amounts of electrolyte was added to Milli-Q water. The
hydrogel composites formed between parallel stainless steel mesh electrodes
were placed in NaNO3 solution at various concentrations at room temperature
and held for 30 min prior to commencing each EIS measurement to study
effects of ionic strength and SWNT concentration. The lowest concentration of
the electrolyte which facilitated the generation of EIS spectra across the whole
frequency range was chosen for all EIS measurements. This is determined by
the impedance spectra at high frequency should be fitted into straight line. The
measurement of the impedance change of all samples through a heating-cooling
cycle was obtained by measuring the impedance spectra upon varying
temperature using water-bath temperature controller. For both the heating and
cooling processes, the samples were held in electrolyte for 30 min at 20C
(below the LCST) and 40C (above the LCST) prior to commencing
measurements as shown in Figure 3.5. The measurements were taken 30 min
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after the electrolyte had reached the target temperature. In order to understand
the relationship between impedance spectra and distance of blank parallel
stainless steel mesh electrodes (1 to 4 mm), the experiments were performed in
the chosen concentration of NaNO3.
Figure 3.5 Chart of electrolyte temperature versus time of the impedance
measurements.
3.3 Results and Discussions
3.3.1 Preparation of NIPAM/SWNT hydrogels
SWNTs were dispersed with SDBS under sonication, which can exfoliate the
SWNTs bundles into individual tubes. At low SWNT concentration, the SWNTs
dispersion remained stable in the presence of NIPAM monomer solution (Figure
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3.6). The quality of the dispersions is first confirmed using optical microscopy
immediately after sonication.
Figure 3.6 Optical images of SDBS aqueous dispersions of the different SWNTs
which containing NIPAM (A) 0.05 wt%, (B) 0.1 wt%, (C) 0.5 wt% and (D) 1.0
wt% SWNT which were taken immediately after sonication (scale bar: 100 m).
However, it was found that at high SWNTs loadings and a constant
concentration of SDBS produced dispersion that contained small agglomerates
which increased with increasing SWNT concentration. The dispersion
mechanism of SWNTs in aqueous solutions with surfactants is thought to be
primarily through hydrophilic and hydrophobic interactions [17]. The
hydrophobic tail of a surfactant molecule adsorbs on the surface of SWNT
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bundles while its hydrophilic head associates with water [22]. The electrostatic
repulsion between the hydrophilic head groups and SDBS-anionic surfactant is
able to produce stabilizing dispersions as reported by Vaisman et. al. [23].
Three most probable configurations of the adsorption mechanism on nanotube
walls are shown in Figure 3.7.
(A)
(B)
(C)
Figure 3.7 Schematic illustration of various surfactant assembly structures on a
SWNT, including; (A) SWNT encapsulated within a cylindrical surfactant
micelle, side and cross-section views, (B) SWNT covered with either
hemispherical micelles and (C) randomly adsorbed surfactant molecules, where
the blue is the alkyl carbon chain and the red is the charge head group [24].
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Hydrogel composites were formed in syringes (Figure 3.8 A) or between two
pieces of stainless steel mesh to obtain good electrical connection (Figure 3.8 B)
(B)
(A)
Figure 3.8. A black homogenous gels were formed separately (A) in syringe and
(B) between stainless mesh electrodes.
3.3.2 Swelling and Mechanical analysis
The hydrogel swelling behaviour was gravimetrically investigated. As can be
seen in Table 3.1, the Equilibrium Swelling Ratio (ESR) of the hydrogel
containing 0.05 wt% SWNT is higher with an almost two-fold increased when
compared to the neat gel (PNIPAM only). This can be attributed to a decrease in
efficiency of the initiators in the presence of SWNT which act as radical
scavenger [25-28]. Hence, crosslink density is less. However, after this initial
increase, ESR decreased as a function of increasing SWNT concentration,
indicating that the water absorption of the gels decrease with further increasing
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of SWNT loading. It can be interpreted that the swelling of the composite gels
depends mainly on the crosslinking density. The ability of hydrogels to swell in
water decreased with increasing SWNT content due to SWNTs decreasing the
extent of chemical crosslinking. However, this effect is overcompensated for by
the ability for SWNTs to act as physically crosslinkers.
Table 3.1 Equilibrium swelling behaviour and mechanical properties of PNIPAM
containing different SWNT concentrations.
SWNT
Equilibrium
Equilibrium
concen
Swelling
Water
tration
Ratio (ESR)
Content
(wt%)
Compressive
Strength
(kPa)
Breaking
Strain (%)
Compressive
Modulus
(0-10%
strain) (kPa)
Crosslink
density
(x103 chains
per m3)
(EWC%)
0
32.9±1.4
97.0±0.1
30.0  8.7
70.2  9.3
10.0  0.1
10.9  0.5
0.05
64.1±1.3
98.5±0.1
160.0  25.0
84.4  6.3
0.9  0.1
3.3  0.6
0.1
49.9±0.4
97.8±0.1
170.0  5.7
84.9  2.4
3.3  0.6
7.9  1.1
0.5
25.0±1.3
96.0±0.3
400.0  7.4
85.1  10.4
11.3  1.5
19.9  2.7
1.0
18.3±1.1
94.8±0.3
500.0  87.3
81.2  6.9
50.0  10.0
89.1  29.4
The crosslink density may also be calculated from the uniaxial stress-strain
measurements by the use of Uniaxial Affine model equation, which has been
used to describe the elastic behaviour in hydrogels. Elasticity theory leads to
the following equation for the load-compression behaviour of hydrogels using
the results at 1.1 extension ratio:
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*
+
3.9
where σn = nominal stress (load / initial cross-sectional area)
λ = compression ratio (final length / initial length) at 1.1 extension ratio
N = number of chains per unit volume
k = Boltzmann’s constant = 1.38 × 10-23 J.K-1
T = absolute temperature (295 K, room temperature 22C)
= the polymer volume fraction between the swollen state and
as-prepared state (without soaking in water).
where compression ratio = compression strain +1
The mechanical properties of the crosslinked hydrogels are highly dependent
on the polymer structure, especially the crosslinking density and the degree of
swelling [288]. As the swelling ratio of PNIPAM/SWNT hydrogels varied with
different SWNT concentrations, it is expected that the mechanical properties of
the composite hydrogels will also depend on SWNT concentration.
To investigate the effect of SWNT formation on the mechanical behaviour of
hydrogels, compression tests were performed on the various gel systems. The
results of the compression mechanical tests of the equilibrium swollen PNIPAM
hydrogels with the various SWNT concentrations at room temperature are
presented in Table 3.1. It is found that the level of compression strength,
breaking strain, compressive modulus and crosslink density changes
dramatically after adding a small amount of SWNT into the hydrogel matrix. The
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changes of the mechanical properties of the gels are observed corresponding to
the large change in swelling ratio. However, the strength and breaking strain of
the 0.05 wt% SWNT increased more than 5 times and 14% compared
respectively to the neat polymer network. This indicates that the increase of
strain is obviously due to the lower network density or the lower swelling
volume of the composite gel. The compressive modulus of this 0.05 wt% SWNT
composite gel is approximately 10 times less than that the gel without SWNT.
This is likely to be due to lower crosslinking density of the gel, which in turn
that could account for the greater equilibrium water content and swelling ratio.
Compressive modulus increased as a function of increasing SWNT
concentrations with further increasing of SWNT loading. The compression
breaking strain did not show any considerable difference between these
PNIPAM/SWNT hydrogel (0.05 – 1.0 wt%) networks (~ 81 – 85 %). The
compressive strengths of 0.5 and 1.0 wt% SWNT hydrogels exhibit
enhancements of more than 2.5 times compared to 0.05 and 0.1 wt% SWNT
hydrogels. It is seen that the increase of compressive modulus of the composite
gels also depends mainly on the increase of SWNT concentration, indicating that
a significant improvement in compressive mechanical properties due to SWNTs
efficiently reinforced hydrogel composites as reported by Xiao and co-workers
[30].
Crosslink density of swollen hydrogels at 1.1 extension ratio changed
dramatically with adding a small of SWNT into the hydrogel matrix. This change
is observed similar to the large change in compressive modulus. However,
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crosslink density increased with further increasing SWNT loadings. It is
observed that the increase of crosslink density of hydrogels depends mainly on
the increase of SWNT concentrations.
However, the comparison of compression strength dependence of breaking
strains of the hydrogel various SWNTs loadings is shown in Figure 3.9. It is
found that the compression strengths of 0, 0.05 and 0.1 wt% SWNT at 70.2 %
breaking strain are not much different, while of 0.5 and 1.0 wt% SWNT
significantly increased approximately two-fold and forth-fold respectively
compared to those low concentrations. The hydrogel containing low
concentration of SWNT demonstrated no considerable differences in the
mechanical properties, while 0.5 and 1 wt% SWNT can be significantly
improved the mechanical properties. It indicates 0.5 and 1 wt% SWNT
embedded in PNIPAM matrix effectively reinforced the hydrogel composites
[30].
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Figure 3.9 Compression strength dependence of breaking strains of the
hydrogel various SWNTs loadings.
3.3.3 Phase transition behaviour
The initial thicknesses of prepared samples are 1.61 mm (1.5 mm mould
thickness plus 2  0.055 mm mesh thickness). All sample thicknesses were
measured included the mesh thicknesses in triplicate using vernier caliper. The
phase transition was expressed using the change of the hydrogel sample
volumes with every 5oC increase in temperature using a temperature-controlled
water bath. The volumes of all as-prepared samples are increased after
immersing in Milli-Q water.
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Figure 3.10 The normalized volume of the gel as a function of temperature.
Normalized volume of the gel was calculated from the ratio of volume of the gel
at each temperature to the volume of the gel at 20C.
For all types of composite gels the thermo-induced swelling and shrinking are
completely reversible and a slight shift in the LCST takes place in cooling and
heating processes. It is seen that all the gels undergo a volume-phase transition
between the swollen and collapsed states in water. For the phase transition
behaviour of the PNIAPM gels containing various SNWT concentrations were
studied by recording the normalized volume of the gel as a function of
temperature as shown in Figure 3.10. The results show that onset of the LCST
was measured to be 30C. All hydrogels except 0.5 and 1.0 wt%, reached the
equilibrium volume at 40C with 0.5 and 1.0 wt% reaching equilibrium at 45C
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Pure PNIPAM and composite PNIPAM/SWNT hydrogels demonstrate a sharp
transition between 30-40C, which is in good agreement with the literature
[31]. Above the LCST, the volume of the hydrogels has changed as a function of
increasing SWNTs loadings. It can be interpreted that the extreme volume
change of hydrogels with high SWNT loadings (0.5 and 1.0 wt%) are related to
low equilibrium swelling ratios and equilibrium water contents (Table 3.1) due
to SWNTs decreasing the extent of chemical crosslinking. Moreover, at high
SWNT concentrations (0.5 and 1.0 wt%), the change in volume from swollen to
collapsed states is greater. The hydrophobic properties of SWNTs could account
for the volume change of hydrogels at these high concentrations.
3.3.4 EIS analysis
3.3.4.1 Impedance in NaNO3
The electronic properties of SWNT/NIPAM hydrogel composites were
characterised by EIS measurements under temperature stimuli. Small amount
of NaNO3 was added to Milli-Q water for overcoming the limitation of the EIS
instrument at high frequencies in high water resistance. Due to the occurrence
of high frequency inductive behaviour of hydrogel samples in pure water at
room temperature but this behaviour was not observed at higher temperature.
This behaviour can be expectedly caused by high water resistance. The
measured impedance relies on a measured AC current and voltage the system
does not control the AC current and voltage exactly at high frequency on cells
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with high solution resistance [32]. To avoid this occurrence, solution resistance
was lowered. Thus, the investigation of EIS spectra of the gel in various NaNO3
electrolyte concentrations was presented. It shows the impedance spectra at
very high frequency are fitted into straight lines with increasing NaNO3
concentrations as shown in Figure 3.11. This indicates that the concentration of
electrolyte affects the occurrence of an error of the impedance spectra from the
limitation of the instrument at high frequencies. The impedance spectra of the
parallel electrodes (see Figure 3.11) with and without hydrogel obtained
between frequencies of 0.1 Hz and 100 kHz in various NaNO3 concentrations
decreased with increasing NaNO3 concentrations. However, the change of
impedance spectra of PNIPAM hydrogel contained 0.1 wt% SWNT in 1 M NaNO3
at 100 kHz related to solution resistance (Rs) and the second flattening of the
curve between 200 and 1000 Hz related to polarisation resistance (Rp) were
clearly seen in Figure 3.11 B. Rs and Rp increased as a function of decreasing
electrolyte concentration, while Rp was larger than Rs at high concentrations.
Whereas, Rs increased and became greater than Rp due to the decrease of
electrolyte concentration. It can be seen that the impedance of 0.001 M NaNO3
did not drop at very high frequencies. It indicates that the high frequency
inductive behaviour is not evident in this electrolyte concentration. Thus,
0.001M NaNO3 was selected to characterise the impedance of composite
hydrogel instead of pure water to avoid the error of impedance at high
frequency on cells with high solution resistance.
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(A)
Impedance (kohm)
10
1
0.00001M
0.0001 M
0.1
0.001 M
0.01 M
0.1 M
1M
0.01
0.1
1
10
100
1000
10000
100000
10000
100000
Frequency (Hz)
100
(B)
Impedance (kohm)
10
1
0.00001M
0.0001 M
0.1
(Rp)
0.001 M
(Rs)
0.01 M
0.1 M
1M
0.01
0.1
1
10
100
Frequency (Hz)
144
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Figure 3.11 Impedance spectra were obtained between frequencies of 0.1 Hz
and 100 kHz with AC amplitude of ±10 mV of parallel stainless mesh electrodes
(A) with no gel and (B) with PNIPAM hydrogel containing 0.1 wt% SWNT. All
experiments, the samples were measured in various NaNO3 concentrations at
room temperature and held for 30 min prior to commencing measurement.
Figure 3.12 shows a bode plot for frequencies of 0.1 Hz and 100 kHz with AC
amplitude of ±10 mV of PNIPAM hydrogel contained 1.0 wt% SWNTs formed
between stainless steel mesh electrodes in 1 mM NaNO3 at 20C and 40C. The
impedance of the gel decreased upon heating above the LCST, indicated that the
hydrogel switching from a hydrophilic swollen state to a hydrophobic collapsed
state as mentioned in section 3.1 can influence the impedance. Furthermore, the
decrement of distance of electrodes and the increment of temperature of
electrolyte might affect the impedance of this gel. The composite hydrogel
became a rigid solid and the distance between the parallel electrodes become
closer due to hydrogen bonds breaking and water being expelled from the gel
film at the final temperature above the transition temperature as reported by
Zhang et. al. [33]. Figure 3.12 B shows the impedance values recorded at 100
kHz as a function of heating-cooling cycles between 20C and 40C has been
thermally cycled. The impedance decreased while the gel shrank when it was
warmed through its LCST, and then reversibly increased while the gel expanded
and swelled when it was cooled below the LCST.
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(B)
Figure 3.12 (A) Impedance spectra were obtained between frequencies of 0.1
Hz and 100 kHz with AC amplitude of ±10 mV of PNIPAM hydrogel contained
1.0 wt% SWNT formed between stainless steel mesh electrodes. The samples
were measured in 1 mM NaNO3 and (B) impedance spectra at 100 kHz as a
function of heating-cooling cycles between 20C ( ) and 40C ( ) for 3 cycles.
3.3.4.2 The effect of distance of electrodes and temperature
Due to the formation of the shrinkable thermally sensitive gels between the
electrodes as mentioned in section 3.2.2, the distance between electrodes and
temperature of electrolyte are also important factors that might affect the
impedance of these gels. Additional experiments are carried out to evaluate the
effects of various distances of electrodes and temperatures of electrolyte on the
impedance. In these experiments, different distances of blank parallel stainless
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mesh electrodes were investigated in 1 mM NaNO3 at 20C and 40C keeping
the surface area fixed. The impedance decreased as a function of decreasing the
distance of the electrodes as shown in Figure 3.13. Furthermore, the EIS spectra
show a significant reduction in impedance with increasing temperature [34,
35]. Figure 3.13 C shows the relationship between solution resistance (Rs) at
100 kHz and thickness using Equation
. It is found that the slope ( ) of the
relationship at 20C (0.4483 /mm) is greater than at 40C (0.2649 /mm).
Thus, resistivity ( ) at 20C is higher than at 40C. It can be interpreted that the
impedance change of the shrinkable hydrogel composites was affected from the
volume change of the gel but also the change of distance of electrodes and
temperature of electrolyte as followed Equation
. These
results are consistent with the theory that the resistivity of a solution
containing electrolyte decrease as a function of temperature and concentration
of the electrolyte [36].
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(A)
(B)
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(C)
Figure 3.13 Impedance spectra were obtained between frequencies of 0.1 Hz
and 100 kHz with AC amplitude of ±10 mV of different distances of blank
parallel stainless steel mesh electrodes of 1 to 4 mm which were measured in 1
mM NaNO3 at (A) 20C and (B) 40C. (C) Relationship between solution
resistance (Rs) at 100 kHz and thickness using Equation
, where
=
resistivity (.mm), L = thickness between parallel electrodes (mm) and A =
cross sectional area (mm2).
3.3.4.3 The effect of SWNT concentration
When embedding SWNT into the gel for constructing thermally sensitive
electrically conducting connectors it is necessary to investigate the effect of
varying the SWNT concentration has on impedance. Under varying conditions of
temperature, the various SWNT loadings were mainly investigated on the
impedance of the thermally sensitive hydrogels below and above the LCST.
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Figure 3.14 shows that the Rs/t (at 100 kHz) ratios of hydrogels are not
different at low SWNT concentrations (0, 0.05 and 0.1 wt%). While, these ratios
decreased with high SWNT loadings (0.5 and 1.0 wt%) at both temperatures.
That indicates that hydrogel containing low concentration of SWNT
demonstrated no considerable differences in the electronic properties, while 0.5
and 1 wt% SWNT can be significantly improved by observation of the decrease
of Rs/t ratios (Figure 3.14). It is seen that these changes of hydrogels with high
SWNT loadings (0.5 and 1.0 wt%) are related to the change of volume,
equilibrium swelling ratios and equilibrium water contents as discussed in
section 3.3.2 and 3.3.3, respectively.
In addition, the decreasing of distance between SWNTs and distance of mesh
electrodes during shrinking of the gels is also affected to the decrease of
impedance as mentioned above. However, Rs/t ratios of the gels at 40C and
20C are not different at low SWNT concentrations (0, 0.05 and 0.1 wt%).
Whereas, these ratios of 0.5 and 1.0 wt% at 40C are larger than at 20C. It
indicates the conductivity of these gels measuring in 1 mM NaNO3 decreases
with increasing SWNT concentration at temperature above the LCST possibly
due to the PNIPAM wraps the tubes.
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Furthermore, the good dispersion of SWNTs could be enhanced the
improvement of electronic properties of polymer composites as reported by
Blanchet et al. [37]. Therefore, the increase of agglomerates at high SWNTs
loadings is possibly dropped to the conductivity of the gels.
Figure 3.14 Comparisons of the change of Rs (at 100 kHz) over thickness (t) for
SWNT/PNIPAM composite hydrogels with various SWNT concentrations which
were performed in 1 mM NaNO3 at below 20C ( ) and above 40C ( ) the LCST.
3.4 Conclusions
The
poly
(N-isopropylacrylamide)
/
single-walled
carbon
nanotubes
(PNIPAM/SWNT) hydrogel composites as shrinkable electronic conductors
studied here were successfully synthesised with various SWNT loadings: 0,
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0.05, 0.1, 0.5 and 1.0 wt% through free radical polymerisation. The SWNTs were
dispersed with SDBS as a surfactant under sonication prior to mixing with
monomer, crosslinker, initiator and accelerator. The hydrogel containing very
small amounts of SWNT (0.05 wt%) has high ability to absorb water which can
swell to a great extent. However, the ability of hydrogels to swell in water
decreased with further increasing SWNT loading. Thus, the swelling ratio of the
composite gels depends mainly on the crosslinking density. For the
temperature-response studied, the onset of the lower critical solution
temperature (LCST) was measured to be 30C. All hydrogels except 0.5 and 1.0
wt%, reached the equilibrium thickness at 40C, with 0.5 and 1.0 wt% reaching
equilibrium at 45C. Above the LCST, the volume of the hydrogels has changed
as a function of increasing SWNTs loadings. The extreme volume change of
hydrogels with high SWNT loadings (0.5 and 1.0 wt%) are related to low
equilibrium swelling ratios and equilibrium water contents due to SWNTs
decreasing the extent of chemical crosslinking.
The changes of the mechanical properties of the gels are observed
corresponding to polymer structure. The level of compression strength,
breaking strain and compressive modulus increased as function increasing
SWNT concentrations into the hydrogel matrix. A significant improvement in
compressive mechanical properties is due to SWNTs efficiently reinforced
hydrogel composites. The changes of the mechanical properties of the gels are
observed corresponding to the large change in swelling ratio. However, the
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hydrogel containing low concentration of SWNT demonstrated no considerable
differences in the mechanical properties, while 0.5 and 1.0 wt% SWNT can be
significantly improved the mechanical properties.
The electronic properties of SWNT/NIPAM thermally sensitive composites were
quantified by EIS plots below and above the LCST. 1 mM NaNO3 was selected to
characterise the impedance of composite hydrogel instead of pure water. The
composite hydrogels have been thermally cycled between two temperatures
through the heating-cooling cycles. The impedance change of the shrinkable
hydrogel composites was not only affected by the volume change of the gel but
also by the distance of electrodes and temperature of electrolyte. The resistance
of hydrogels decreased as a function of increasing SWNT concentration. While,
the extreme resistance changes of hydrogels with high SWNT loadings (0.5 and
1.0 wt%) are related to volume changes, low equilibrium swelling ratios and
equilibrium water contents. This is because the distance between SWNTs and
distance of electrodes are decreased with shrinking of thermally sensitive
hydrogels at different temperatures [13, 16]. The gel formed between the
stainless steel meshes makes a good connection with the electrodes leading to
the appropriate system in the electrical and thermal property measurements.
154
CHAPTER 3
Shrinkable Electronic Conductor:
Carbon nanotube/Hydrogel Composites
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1183-1185.
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Zhong, Q., et al., Switching kinetics of thin thermo-responsive hydrogel
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157
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Chapter 4
Thermally sensitive injectable hydrogels
158
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Table of Contents
4 Thermally sensitive injectable hydrogels .................................................................................... 160
4.1
Introduction............................................................................................................................ 160
4.2
Experimental .......................................................................................................................... 164
4.2.1
Chemical and Materials ................................................................................................ 164
4.2.2
Preparation of PNIPAM/Clay hydrogel composites ......................................... 164
4.2.3
Preparation of SWNT/PNIPAM/ Clay hydrogel composites ......................... 166
4.2.4
Fourier Transform Infrared Spectroscopy ........................................................... 166
4.2.5
Swelling properties ........................................................................................................ 167
4.2.6
Extrudable testing........................................................................................................... 168
4.2.7
Rheological analysis ....................................................................................................... 170
4.2.7.1
Oscillatory frequency sweeps.......................................................................... 170
4.2.7.2
Creep recovery....................................................................................................... 171
4.2.7.3
Shear yield experiment ...................................................................................... 178
4.2.7.4
LCST transition-dynamic mechanical measurement ............................. 179
4.3
Results and discussions ..................................................................................................... 179
4.3.1
Appearance and network structure of the hydrogel composites................ 179
4.3.2
Swelling properties ........................................................................................................ 183
4.3.3
Extrudable testing of hydrogels ................................................................................ 184
4.3.3.1
Effect of monomer content. .............................................................................. 186
4.3.3.2
Effect of clay content. .......................................................................................... 186
4.3.3.3
Effect of initiator content. ................................................................................. 187
4.3.3.4
Effect of accelerator content. ........................................................................... 187
4.3.4 Rheological properties of PNIPAM/Clay hydrogels .......................................... 188
4.3.4.1
Oscillatory frequency sweeps.......................................................................... 188
4.3.4.2
Creep-recovery test ............................................................................................. 192
4.3.4.3
Shear yield experiment ...................................................................................... 207
4.3.4.4
Comparison of modulus measured from different approaches ........ 209
4.3.4.5
LCST transition-dynamic mechanical measurement ............................. 211
4.4
Conclusions ............................................................................................................................. 214
4.5
References ............................................................................................................................... 217
159
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
4 THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.1 Introduction
Pelton and Chibante first prepared responsive hydrogels based on
N-isopropylacrylamide (NIPAM) and N,N’-methylenebis (acrylamide) (MBA) as
a chemical crosslinker in 1986 [1]. Since then these hydrogels have attracted
considerable attention due to their enormous potential in various applications,
including drug delivery, catalysis, chemical separation and optical devices [2-6].
However, chemically crosslinked PNIPAM hydrogel has some significant
limitations; such as structural inhomogeneity at high concentration of
crosslinker [7] and mechanically weak and brittle properties [7-9]. Thus, most
of the potential applications of these gels have been abandoned [8]. In order to
improve the mechanical properties of PNIPAM hydrogels, clay nanoparticles
were employed as physical crosslinkers to strengthen the polymer network [8,
10, 11]. Haraguchi et al. [8, 12-15], Oppermann et al. [16] and Zhang et al. [17]
recently
reported
isopropylacrylamide)
a
novel
route
hydrogels
to
which
the
preparation
exhibit
superior
of
poly(N-
mechanical
performance using aqueous clay (Laponite) as a physical crosslinker replacing
the traditional chemical cross linkers. In addition, the equilibrium swelling ratio
and
de-swelling
(shrinking)
rate
with
temperature
changes
of
poly(NIPAM/clay) gels have also improved by the application of laponite as
physical crosslinker [15, 17-19].
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
It is known that in water Laponite, a synthetic hectorite clay formed disk-like
particles about 1 nm thick and 25 nm in diameter (Figure 4.1) with negative
surface charge density stabilizing dispersions in water [18-23].
Figure 4.1 Laponite (a synthetic hectorite clay) [Mg5.34Li0.66Si8O20(OH)4]Na0.66
with particle size (single platelet) 20-30 nm   1 nm thick and cation charge
capacity = 1.04 mequiv/g.
Among hydrogels, thermally sensitive injectable hydrogels are highly desirable
in clinical applications. Injectable hydrogels can be introduced into the body via
a syringe or tube and can avoid the need for open surgery [24-26]. Here, the
injectable hydrogel can be defined as a material which can be extruded and
forms a hydrogel immediately after injection. Applications include injectable
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
constructs in vivo tissue formation. Over the past decade, various
thermosensitive and injectable PNIPAM hydrogels have been developed and
employed in a variety of settings [26-30]. Wang et al. synthesized a
temperature-sensitive poly (NIPAM-co-CSA) hydrogels by the copolymerization
of acrylic acid-derivatized Chitosan (CSA) and NIPAM in aqueous solution [27].
Tan,
et
al.
reported
that
an
aminated
hyaluronic
acid-g-poly(N-
isopropylacrylamide) (AHA-g-PNIPAM) injectable hydrogel was synthesized by
coupling carboxylic end-capped PNIPAM (PNIPAM-COOH) to AHA through
amide bond linkages [29]. Chen and Cheng reported that a Chitosan-graftpoly(N-isopropylacrylamide) injectable hydrogel has been synthesized by
grafting PNIPAM-COOH with a single carboxy end group onto chitosan through
amide bond linkages for cultivation of Chondrocytes and Meniscus cells [28]. Ha
et al. prepared the PNIPAAm-grafted hyaluronic acid copolymer as
temperature-sensitive injectable gels for use in controlled drug delivery
applications [30]. Triblock copolymers have been widely studied by many
researchers in injectable cell delivery systems [31-34]. Triblock copolymers
such as PEO-PPO-PEO (Pluronic), PLGA-PEG-PLGA, PEG-PLLA-PEG, PCL-PEGPCL, PCLA-PEG-PCLA and PEG-PCL-PEG were synthesized for use in injectable
themosensitive biodegradable polymers exhibiting sol-gel transitions by
increasing of temperature [31-34]. These injectable hydrogels have been
successfully applied in cell therapy, tissue regeneration, and wound healing due
to their biocompatibility and long persistence in the gel form in vivo.
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
Herein, the preparation of injectable thermoreversible clay-based hydrogels
(PNIPAM/Clay and SWNT PNIPAM/Clay) by free radical polymerisation
technique [12-15, 17, 35] as reviewed in Chapter 1, is presented. The injectable
hydrogel was prepared using initial aqueous solutions consisting of monomer
(NIPAM), physical crosslinker (inorganic clay, Laponite XLG), initiator (KPS)
and accelerator (TEMED). The concentration of monomer, crosslinker, initiator
and accelerator were varied systematically during the hydrogel preparation to
study which parameters are important for the transition from solid-like to
liquid-like behaviour. The resulting hydrogels were characterised in terms of
swelling, rheological and mechanical properties. A controlled stress rheometer
was used to determine the creep and recovery behaviour of the hydrogels. The
creep-recovery results were fitted using a generalised Kelvin chain model. The
chemical structure of injectable thermally sensitive hydrogels was confirmed by
FTIR spectroscopy. The phase transition behaviour of the PNIPAM/clay and
SWNT PNIPAM/clay composite hydrogels were studied by the change of elastic
modulus of the gels as a function of temperature.
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.2 Experimental
4.2.1 Chemical and Materials
Synthetic hectorite “Laponite XLG” ([Mg5.34Li0.66Si8O20(OH)4]Na0.66, layer size =
20-30 nm   1 nm, cation exchange capacity = 104 mequiv/100 g) was
obtained from Rockwood Ltd., Germany. SWNTs were obtained from CNI
(Batch#: P0900), USA and used without any further treatment. Sodium
dodecylbenzenesulfonate (SDBS, Sigma-Aldrich), N-isopropyl acrylamide (97%,
NIPAM, Sigma-Aldrich, Batch#: MKBB3307), N,N'-methylene-bis-acrylamide (≥
98.0%, MBA, C7H10N2O2, FW= 154.2, Sigma), Potassium persulfate (KPS, K2S2O8,
FW=270.32, Sigma-Aldrich), Ammonium persulfate (APS, (NH4)2S2O8, FW=
228.19, Ajax) and, N,N,N’,N’ -Tetramethyl-Ethylenediamine (≥ 99.0%, TEMED,
C6H16N2, MW=116.21 g/mol Sigma) were used as surfactant, monomer,
crosslinker, initiator and accelerator, respectively. All reagents were used
without further purification. All solutions were prepared with deionised Milli-Q
water (18.2 MΩ).
4.2.2 Preparation of PNIPAM/Clay hydrogel composites
The synthetic procedure for nanocomposite hydrogels is the same as that
reported previously [8, 12]. Nanocomposite hydrogels were prepared using
chemical free radical polymerisation. 1.0 g NIPAM and 0.33 g clay were mixed in
9.5 ml of Milli-Q water, stirred for 30 min and bath sonicated for 30 min until
164
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
transparent. 8.0 μl of TEMED accelerator was added to the mixture and bubbled
under N2 gas in an ice bath for 10 min to remove O2 before adding 0.5 ml of
water containing of 0.01 g KPS initiator. The mixture was immediately suck it up
into a syringe (5 ml) and then sealed with parafilm. The mixture was also
injected into a rubber gasket mould (thickness = 2.5 mm) sandwiched between
2 glass slides as shown in Figure 4.2. In-situ free radical polymerisation was
achieved by removing the samples from an ice bath, and warming to room
temperature (~ 22 C). Polymerisation was carried out at room temperature for
24 h. The concentrations of monomer (10, 8, 6, 4, 2 wt%), clay (3.3, 1.6, 1.0, 0.8,
0.4 wt%), initiator (0.1, 0.2, 0.3 wt%) and accelerator (8, 32, 64, 128, 256 μl)
reagents were varied to study which parameters are important for the
transition occurring from solid-like to liquid-like behaviour.
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CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.2 Hydrogel formed in a rubber gasket mould (thickness = 2.5 mm)
sandwiched between 2 glass slides.
4.2.3 Preparation of SWNT/PNIPAM/ Clay hydrogel composites
In a typical experiment, 1.0 g (1 wt%) SDBS with different SWNT concentrations
as follows: 0, 0.005 g (0.05 wt%) and 0.01 g (0.1 wt%) SWNT were added to 5
mL of Milli-Q water. The mixture was dispersed by ultrasonic horn sonicator
(Sonics and Materials 500 Watt Vibra cell) operated at 30%, 1 s ON 1 s OFF for
30 min. The SWNT dispersion was analysed using an optical microscope. Then,
the dissolved solution of 0.8 g (8.0 wt%) NIPAM in 5 ml Milli-Q water was
added to the mixture which is continually horn sonicated. Subsequently, 0.1 g
(1.0 wt%) of Laponite and was dispersed in 10 ml mixture under vigorous
stirring for at least 30 min. Then, 0.02g (0.2 wt%) APS was added under
continual stirring for another 10 min. The mixture was then degassed by
bubbling nitrogen gas whilst in an ice bath for 10 min to remove O2 prior to
adding 20 l of TEMED. The mixture was immediately suck it up into syringes
and then sealed with parafilm. Polymerisation was carried out at room
temperature (~ 22 C) for 24 h.
4.2.4 Fourier Transform Infrared Spectroscopy
The chemical structure of hydrogel using inorganic clay as a crosslinker was
analysed using a FTIR spectrometer (IRPrestige-21, Shimadzu, Japan) over the
166
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
range of 500-4000 cm-1. The dried hydrogel is ground with 10 µg of KBr and
pressed to form transparent discs for qualitative determinations. The spectrum
was taken in % transmittance mode.
4.2.5 Swelling properties
The hydrogels were kept in Milli-Q water for the determination of their swelling
properties. For the swelling measurements, the hydrogel samples were cut into
cube shaped specimens of approximately 0.5 cm in length and immersed in
Milli-Q water at room temperature for at least 1 week in order to reach the
equilibrium degree of swelling. The hydrogel-swelling behaviour was studied
using the gravimetric method. The completely swollen gels were weighed
accurately after removing the excess water on the hydrogel surface with a
tissue paper. The hydrogel samples were then dried in an oven at 60C for 24 h
prior to weighing again. The equilibrium-swelling ratio (ESR) was calculated as
follows:
ESR 
Me  Md
Md
(4.1)
where Me and Md are the weights of water present in the swollen hydrogels at
equilibrium swelling and dried conditions, respectively. All measurements were
made in triplicate.
167
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.2.6 Extrudable testing
Hydrogels were extruded using a custom-built syringe printer (Figure 4.3 A).
This consists of a gas pressure controller (EFD Ultimus I, under Nitrogen)
connected to a syringe assembly (3 ml syringe with luer lock tips in 3 sizes,
Figure 4.3 B) strapped to the vertical axis of a computer numerically controlled
(CNC) three dimensional stage (Sherline 8020 CNC 8-Direction) at room
temperature. The pressure of N2 required for extrusion of the hydrogels with
tips in three diameter sizes was measured (Figure 4.3 C-E). In addition, the
hydrogels formed in 5 ml syringes were used for the extrudable testing using a
custom-built syringe printer and syringe compressing by hand.
168
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
(B)
(C)
(D)
(E)
Figure 4.3 (A) Schematic illustration of extrudable testing using a custom-built
syringe printer. (B) Different diameters of the tips for extrudable testing of the
hydrogels. (C-E) The hydrogels were extruded from 2.5, 1.5 and 0.5 mm of
diameter of the tips, respectively.
169
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.2.7 Rheological analysis
Rheological measurements were carried out on as-prepared hydrogels with a
rheometer (AR-G2, TA Instruments, New Castle, Del., U.S.A.) using a parallel
plate geometry of diameter of 12 mm. Hydrogels were used as-prepared in
order to retain the water/polymer ratio of that during synthesis. All rheological
measurements were performed at 25 ± 0.1C controlled by a Peltier plate. The
sample temperature was internally controlled by a Peltier system (−20 to 200C
with an accuracy of ± 0.1C) attached to a water circulation unit. A platinum
resistance thermometer sensor positioned at the centre of the plate ensured
temperature control and measurement.
4.2.7.1 Oscillatory frequency sweeps
An oscillatory frequency sweep test is a useful technique as it enables the
viscoelastic properties of a sample to be determined as a function of timescale.
Several parameters can be obtained, such as the storage (elastic) modulus (G'),
the viscous (loss) modulus (G"), phase shift angle () and the complex viscosity
(η*) in one test. The phase shift angle at a particular frequency will indicate
whether the structured material appears to be an elastic solid or a viscous
liquid. For each test, the hydrogel samples which formed as a sheet were
carefully loaded onto the centre of measuring geometry. The shape of the asprepared samples was cut as cylindrical specimens with 12 mm in diameter and
2.5 mm in thickness. The hydrogels were placed between parallel plates with a
170
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
roughened surface (12 mm-crosshatch diameter). The width of the gap between
top and bottom plates was controlled at 1000 μm for all measurements.
Oscillatory frequency sweep tests with a controlled strain of  = 0.01 rad were
performed over the range of angular frequency from 1 to 100 rad/s in triplicate.
The values of complex shear modulus |G*|, dynamic viscosity |*|, storage
modulus G’, loss modulus G” and the phase shift angle , were determined as a
function of frequency. The formula G*() = ()/() = G’ + iG” represents the
relationship between G*, G’ and G”. Storage modulus (G’) represents the elastic
property of a material whereas the viscous property is characterised by the loss
modulus (G”). The absolute magnitude of complex shear modulus |G*|,
calculated from (G’2 + G”2)1/2, represents the shear stiffness of the material. The
phase shift angle is calculated from the ratio of loss and storage modulus (G”/G’
= tan ()), and the dynamic viscosity |*| is derived from |*| = |G*|/. For a
pure elastic material, the phase shift angle should equal 0 while the phase shift
angle equal 90 for pure viscous fluid. If the characteristic of the material is
viscoelastic, the phase shift angle should be between 0 and 90.
4.2.7.2 Creep recovery
A creep recovery measurement is a useful rheological technique for
determining the type of behaviour of viscoelastic materials using controlled
stress. The measurements were carried out by monitoring changes in strain of
171
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
the hydrogel samples over a period of 300 s at a fixed shear stress and another
300 s immediately after the shear stress was removed (Figure 4.4).
Figure 4.4 Creep - recovery experiment where a constant stress () is applied
to the hydrogels (creep part) for a period of time. Afterwards the applied stress
is removed (recovery part).
4.2.7.2.1 Fitting creep-recovery curves using Kelvin or Voigt model
A model of linear vicoelasticity of material which exhibit both viscous and
elastic behaviour is built up by considering combinations of the linear elastic
spring and the linear viscous dashpot [36]. A model of a Maxwell model
represented by the connection of spring and dashpot in series, while a Voigt
model represented by two elements connected in parallel as illustrated in Table
4.1.
172
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Table 4.1 Creep-recovery response of linear spring, linear dashpot, Maxwell
model and Voigt model.
Spring
Dashpot
Maxwell element
Kelvin or Voigt
and model
elements
and model


Strain

Strain
t1
t2
Strain
t1
t2
Strain
t1
t2
t1
t2
The Voigt model is also known as the Kelvin model. The elements, spring and
dashpot, are in parallel arrangement (Table 4.1) meaning the strain in each
element is uniform.
(4.2)
173
CHAPTER 4
where
THERMALLY SENSITIVE INJECTABLE HYDROGELS
is strain in spring and
is strain in dashpot.
An overall stress of  from each component is
(4.3)
where
is stress in spring, and
is stress in dashpot.
The polymer behaves as a linear viscoelastic material. Thus, the deformation of
the polymer can be described by a combination of Hook’s law for elastic
component and Newton’s law for viscous component.
A linear elastic material is described by Hook’s law as

or
(4.4)
where E is the elastic modulus.
Newton’s law is used to describe linear viscous behaviour through the Equation
4.5.

where  is the viscosity and
(4.5)
is the strain rate.
174
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
The individual stresses
and
are for the spring and the dashpot,
respectively.


and
(4.6)
Putting these expressions into Equation 4.3 and rearranging gives

(4.7)

The usefulness or otherwise of the model can again be determined by
examining its response to particular types of loading. The Voigt or Kelvin model
is particularly useful in describing the behaviour during creep where the stress
is held constant at
 . Equation 4.7 then becomes

(4.8)

This simple differential Equation has the solution
*
(
175

)+
(4.9)
CHAPTER 4
The constant ratio 
THERMALLY SENSITIVE INJECTABLE HYDROGELS
can be replaced by 0, the relaxation time and so the
variation of strain with time for a Voigt model undergoing creep loading is given
by
*
(  )+
(4.10)
The Maxwell and Kelvin models are the simplest (two-element) models. More
realistic material responses can be modelled using more elements. The
presence of the isolated spring or dashpot could be accurately described the
response of real materials.
4.2.7.2.2 Fitting creep-recovery curves using generalized Voigt chain
A complex viscoelastic rheological model can be constructed by using more
elements. The generalised Voigt chain consists of N Voigt elements connected in
series as shown in Figure 4.5. An isolated spring is included to model
instantaneous response for solid behaviour, while the presence or absence of an
isolated dashpot will result in fluid behaviour, respectively. The presence or
absence of an isolated spring and/or dashpot is material dependent. The
generalised Voigt chain can model instantaneous response, delayed elasticity
with a range of retardation times, stress relaxation with various relaxation
times, and fluid flow. The generalised Voigt model with N = 1 (or four-element
model) is represented by Equation 4.11 for fitting creep curve and Equation
4.12 and 4.13 for fitting recovery curve.
176
CHAPTER 4

𝐸
THERMALLY SENSITIVE INJECTABLE HYDROGELS
1
N
𝐸1
𝐸N


Figure 4.5 Generalised viscoelastic model (Generalised Voigt chain)
Fitting creep curve (N=1 or a four-element model)
Elastic
Viscoelastic
(
177
( ))
Viscosity

(4.11)
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Fitting recovery curve (N=1 or a four-element model)
(
(
{
))

(
(
))

}
(4.12)
(
(
)
( ))

(4.13)
4.2.7.3 Shear yield experiment
Rheological measurements were performed on PNIPAM hydrogel composites
applying shear strain at constant shear rate to determine the effects of clay
loading on shear strength and shear modulus at the breaking shear strain. The
PNIPAM hydrogel composites containing different clay concentrations were
placed between parallel plates with a roughened surface (12 mm-crosshatch
diameter). For each test, PNIPAM/clay gels (0.8, 1.6 and 3.3 wt% clay) were
placed at the centre of the rheometer plate and compressed at a speed 500
µm/s at 25C until the gap between top and bottom plates reached 1000 μm.
Shear yield experiment was then performed with constant shear rate of 1
or
controlled angular velocity at 1.733 rad/s, sampling 5000 points at the same
temperature for 3 min. Shear stress and strain data were collected until the gels
went to the breaking point.
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4.2.7.4 LCST transition-dynamic mechanical measurement
Temperature ramp step with a controlled stress was performed over the range
of angular frequency from 1 to 31.62 rad/s with a heating rate of 0.5 C/min. to
determine the phase transition of the gel composites. For each test, the hydrogel
samples, formed as sheet (PNIPAM/clay) or formed in the syringe (SWNT
PNIPAM/clay) were carefully loaded onto the centre of measuring geometry.
The shape of the as-prepared sheet samples was cut as cylindrical with
diameter 12 mm. The hydrogels were placed between parallel plates with a
roughened surface (12 mm-crosshatch diameter). The width of the gap between
top and bottom plates was controlled at 1000 μm for all measurements. Finally,
a properly loaded sample was allowed to age for a couple minutes to complete
temperature equilibration before ramping the temperature from 20 to 50C.
4.3 Results and discussions
4.3.1 Appearance and network structure of the hydrogel composites
In the case of shape appearance, high-purity grade Laponite XLG
([Mg5.34Li0.66Si8O20(OH)4]Na0.66) clay is white powder. Chemical compositions of
Laponite XLG are SiO2 59.5%, MgO 27.5%, Na2O 2.8%, LiO2 0.8%, loss on
ignition 8.2% [37]. In this study, Laponite clay was employed as a physical
crosslinker. The synthetic clay suspension was homogeneous and transparent,
179
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
suggesting that clay was exfoliated. The PNIPAM/clay and SWNT/PNIPAM/clay
hydrogel
composites
were
successfully
synthesised
by
free
radical
polymerisation at room temperature (Figure 4.6).
(A)
(B)
(C)
Figure 4.6 Images of (A) PNIPAM/clay formed as sheet and (B)
SWNT/PNIPAM/clay extruded from syringe (C) SWNT/PNIPAM/clay gel in a
glass beaker containing water.
The FTIR spectra (Figure 4.7) from dried PNIPAM/clay hydrogels ground into
KBr matrix show not only the presence of typical bands of PNIPAM (3300 cm-1,
N-H stretching; 2973 and 2934 cm-1 C-H stretching; 1650 cm-1, amide-I band
assigned to C=O stretching;1549 cm-1, amide-II band belonged to N-H in-plane
bending vibration), but also clay (1005 and 653 cm-1, Si-O stretching) as
reported by Haraguchi et. al. [12, 14]. However, it is difficult to prove the
existence of specific clay-polymer interactions probably due to the strong
hydrogen bonding of PNIPAM in the dried state which prevents observation of
hydrogen bonding with clay in the dried PNIPAM/clay gel [14]. There is no
distinctive differences were observed in absorptions of PNIPA, only the
existence of clay in the gel could be confirmed (1073, 1005, 652, and 443 cm-1).
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CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
Absorbance (a.u.)
(B)
(C)
Figure 4.7
FTIR spectra of (A) clay and (C) PNIPAM/Clay hydrogel were
analysed by using a KBr tablet containing clay and PNIPAM/Clay gel powders.
FTIR spectra of (B) chemically crosslinked PNIPAM/MBA hydrogel film which
was coated on gold coated Mylar by induced free radical polymerisation are
included for comparison (as shown in section 2.3.4). Spectra have been offset
for clarity where the intensity has been normalised.
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CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
The hydrogels in this study could withstand higher levels of elongation prior to
failure whilst, tensile modulus and tensile strength are an order of magnitude
lower than the gel as reported by Haraguchi et al. [12]. Clearly, the hydrogels in
this study are very different to those described by Haraguchi et al., and , exhibit
a much lower effective crosslink density.
(A)
(B)
Figure 4.8 Tensile test of 3.3 wt% clay of PNIPAM/Clay hydrogel sheet
(thickness =4.88 mm, width = 2.48 mm, length = 9.8 mm). (A) Image of the gel
composite before breaking point and (B) stress-strain curve which shows
breaking strain of 1254%, tensile strength at break of 6.4 kPa and tensile
modulus of 8.2 kPa.
182
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.3.2 Swelling properties
The equilibrium-swelling ratio (ESR) of PNIPAM/Clay composite hydrogel
decreased as a function of increasing clay concentration, indicating that the
water absorption of the gels decrease with further increasing of clay loading
(Figure 4.9). As the clay content increased, the apparent crosslinking density of
hydrogel increased, which led to a decrease of the swelling properties of the
hydrogels as reported by Haraguchi et al., Zhu et al. and Xia, et al. [8, 18, 38]. It
can be interpreted that the swelling ratio of the composite gels depends mainly
on the clay concentration. Also, it was reported that in the case of PNIPAM/Namontmorillonite layered silicates composite gels (Na-MLS), Na-MLS are easily
ionized and evenly distributed in the PNIPAM gel which enhanced the
hydrophilicity of PNIPAM gel and made it swell more, at low clay concentrations
[38]. However, at high clay concentration, the strong binding of sodium cations
to the negatively charged associates enhanced these counter-ions are not free
and cannot contribute to the osmotic pressure resulting in the shrinkage of the
gel.
183
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.9 Equilibrium-swelling ratio (ESR) as a function of clay concentration
of PNIPAM/Clay composite hydrogels.
4.3.3 Extrudable testing of hydrogels
The hydrogel can be extruded as seen in (Figure 4.3 C-E). Table 4.2 shows the
results of PNIPAM/Clay hydrogels containing various chemical contents
prepared and extrudable tested using a custom-built syringe printer with tips in
three diameter sizes at room temperature as mentioned in section 4.2.6.
184
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Table 4.2 Preparation and extrudable testing of composite hydrogels.
Extrudable testing
Varied
Trans
By hand
(syringe
extrude the gel from a syringe
5ml)
fitted with variable diameter
chemical
NIPAM
Clay
KPS
TEMED
parency
contents
(wt%)
(wt%)
(wt%)
(µl)
observe
d by eye
Monomer
Clay
Initiator
Accelerator
#Pressure
(psi) required to
size needle tips.
2.5
2.5
1.5
0.5
mm
Mm
mm
mm
10
3.3
0.1
8





8
3.3
0.1
8





6
3.3
0.1
8





4
3.3
0.1
8





2
3.3
0.1
8





10
3.3
0.1
8





10
1.6
0.1
8





10
1.0
0.1
8


30
58

10
0.8
0.1
8


15
45

10
0.4
0.1
8


5
20
58
10
3.3
0.1
8





10
3.3
0.2
8





10
3.3
0.3
8





10
3.3
0.4
8





10
3.3
0.1
8





10
3.3
0.1
16





10
3.3
0.1
32





10
3.3
0.1
64





10
3.3
0.1
128





10
3.3
0.1
256


15
20
58
# This test was performed using a custom-built syringe printer with different
tip diameters
185
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
As mentioned in section 4.1, to determine which parameter is important for the
transition occurring from solid-like to liquid-like behaviour, the extrudable
testing was conducted after the synthesis of the nanocomposite hydrogels as
shown in Table 4.2.
4.3.3.1 Effect of monomer content.
Hydrogels were successfully formed and were transparent for all monomer
concentrations however none of these hydrogels could be extruded
successfully, either by hand or using the custom extrusion system. It is
concluded that varying monomer content with a clay concentration at 3.3 wt%
will not lead to solid-like to liquid-like behaviour.
4.3.3.2 Effect of clay content.
All nanocomposite hydrogels were transparent. It was generally observed that
the extrudable testing of PNIPAM/Clay hydrogels strongly depends on clay
content. 1.6 wt% clay hydrogel can be only extruded from syringe by hand,
while 0.4 to 1.0 wt% clay can be extruded both from syringe by hand
compression and by the custom-built syringe printer. It can be seen that 1.0
wt% required higher pressure than 0.8 wt%, indicated that 0.8 wt% is more
viscous than 1.0 wt%. The 0.4 wt% clay hydrogel can be extruded with all tip
diameters using a custom-built syringe printer and syringe compressing by
hand, indicating that 0.4 wt% clay hydrogel is the most fluid like. The
extrudable testing indicates that the hydrogels can take on fluid properties
under applied stress.
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CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.3.3.3 Effect of initiator content.
The hydrogel changed from transparent to translucent at 0.2 wt% KPS. Thus,
increasing the concentration of initiators resulted in increase heterogeneity of
the hydrogel observed by turbidity. As increase of initiator or radicals increased
the rate of initiation reaction which can be affected to different chain length
caused the difference of polymer density. With increasing initiator
concentration, the average polymer chain length should decrease when
termination by coupling is dominant [39]. Even at high KPS concentrations,
none of hydrogels could be extruded. It can be concluded that initiator content
is not the parameter for going from solid-like to liquid-like behaviour with these
chemical compositions.
4.3.3.4 Effect of accelerator content.
Since TEMED is the co-initiator or starter to induce the decomposition of the
initiator. Hydrogels were successfully formed and transparent for all
concentrations of accelerator. All hydrogels with less than 256 µl of accelerator
are unable to be extruded. At very high 256 µl TEMED concentrations, the gel
can be extruded. This may be due to the rate of polymerization initiation being
very fast which results in the polymer chains length being decreased. In
addition, termination reactions may occur in free radical polymerisation by
coupling or by disproportion or by both mechanisms which involve the reaction
between two growing chain ends.
187
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.3.4 Rheological properties of PNIPAM/Clay hydrogels
4.3.4.1 Oscillatory frequency sweeps
As mentioned in 4.3.3, to determine which parameters are important for the
transition from solid-like to liquid-like behaviour, the extrudable testing of
PNIPAM/Clay
hydrogels
strongly
depends
on
clay
concentration.
Concentrations of 3.3, 1.6, 1.0, 0.8 and 0.4 wt% clay were selected for
determining the rheological properties of PNIPAM/Clay hydrogel composites.
Oscillatory frequency sweep tests with a controlled strain of  = 0.01 rad were
performed over the range of angular frequency from 1 to 100 rad/s to study the
relationship between the elastic modulus (G’) and the viscous modulus (G”) of
as-prepared PNIPAM/Clay composite hydrogels various clay contents. Figure
4.10 shows that G’ and G” increases as a function of the angular frequency. The
increase of G’ at low frequency regions indicates an improvement in elastic
behaviour corresponding to an increase in clay loading. This increase in G’ of
various clay concentrations is less at high frequencies. It can be interpreted that
the elastic modulus of PNIPAM/clay hydrogels did not significantly increase as a
function of clay concentration at high frequency regions.
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CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.10 Elastic (G’, filled symbols) and viscous (G”, open symbols) modulus
as a function of angular frequency of as-prepared PNIPAM/Clay hydrogel
composites various clay contents. Oscillatory frequency sweep test with a
controlled strain of  = 0.01 rad was performed at 25 °C and over the range of
angular frequency from 1 to 100 rad/s in triplicate.
It is seen that the phase shift angles () of all samples are between 0 and 90
(Figure 4.11), indicating that the characteristic of hydrogel composites is
viscoelastic. Furthermore, the  of 0.4 wt% clay is larger than the higher clay
concentrations indicating that its viscosity under constant stress is less than
others; in other words the 0.4 wt% clay turns to more fluid-like than elastic-like
(Figure 4.11). It indicates that the decrease of clay loading or crosslink
concentration affects the rheological properties. However, the values of elastic
189
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
modulus G’ are always larger than the loss modulus G”, suggesting the present
hydrogels display a predominantly elastic-like behaviour over the frequencies
studied.
Figure 4.11 Elastic (G’), viscous (G”) modulus and phase shift angle (𝜹) at 10
rad/s as a function of clay concentration of as-prepared PNIPAM/Clay
composite hydrogels. Oscillatory frequency sweep test with a controlled strain
of  = 0.01 rad was performed over the range of angular frequency from 1 to
100 rad/s in triplicate.
In addition, the elastic modulus (G’) and the viscous modulus (G”) of asprepared SWNT/PNIPAM/Clay composite hydrogels increased as function of
clay loading (Figure 4.12). However, the moduli decreased with increasing
initiator concentration as shown in Figure 4.13 and indicates that clay content
190
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
or crosslinking density of the composite gels significantly increases the shear
modulus. While, the increase of initiator decreased the shear modulus due to
concentration of initiators are the effectors of the rate polymerization. With
increasing initiator concentration, the average polymer chain length decreased
[310].
Figure 4.12 Elastic (G’), viscous (G”) modulus and phase shift angle ( ) at 10
rad/s as a function of clay concentration of as-prepared SWNT(0.1
wt%)/PNIPAM/Clay hydrogel composites. Oscillatory frequency sweep test
with a controlled strain of  = 0.01 rad was performed over the range of angular
frequency from 1 to 100 rad/s in triplicate.
191
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.13 Elastic (G’), viscous (G”) modulus and phase shift angle (𝜹) at 10
rad/s
as
a
function
of
initiator
APS
of
as-prepared
SWNT(0.1
wt%)/PNIPAM/Clay hydrogel composites. Oscillatory frequency sweep test
with a controlled strain of  = 0.01 rad was performed over the range of angular
frequency from 1 to 100 rad/s in triplicate. The lines are included to guide the
eye only.
4.3.4.2 Creep-recovery test
The time-dependent viscoelastic properties of the hydrogels were investigated
by creep-recovery experiments. While a constant load is applied, time and
dimensional change are recorded and plotted to give a creep-recovery curve as
illustrated in Figure 4.14. It shows typical creep-recovery curves of as-prepared
nanocomposite hydrogels with various clay concentrations. A creep experiment
where constant stress is applied to samples for 300 s and afterwards the
192
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
applied stress is removed. Here, the values of creep strain are shown as a
function of creep time between 0 and 300 s. Whilst between 300 s and 600 s,
recovery strain is represented. Each creep curve is characterised by the
instantaneous creep strain corresponding to an initial elastic response followed
by a time-dependent creep region related to a viscoelastic response.
Figure 4.14 Creep – recovery curves of PNIPAM/Clay hydrogels with various
contents of clay (wt%). Creep and recovery strains are shown as a function of
creep (0-300 s) and recovery times (300-600 s) under constant stress of 20 Pa.
193
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
4.3.4.2.1 Fitting creep-recovery curves of hydrogel composite with various
SWNT concentrations (as prepared in section 4.2.3)
The behaviour of viscoelastic hydrogel composites with various SWNT
concentrations was also determined using a creep-recovery measurement
performed at various controlled stresses (1, 5, 10, 20 and 50 Pa). The changes in
strain were monitored out over a period of 300 s and another 300 s
immediately after the shear stress was removed. The effect of applied various
constant stresses are illustrated in Figure 4.15, Figure 4.17 and Figure 4.18.
These five curves in each figure are the strains measured at five different stress
levels. It displays that the creep-recovery strain increased as a function of
increasing constant stresses. The resulting creep strains linearly increased over
its full range of time (at 300 s) as a function of applied various constant
stresses. It indicates that the hydrogel composites containing 0, 0.05 and 0.1
wt% SWNTs behave as linear viscoelastic materials. Furthermore, the addition
of SWNT and clay in PNIPAM hydrogel does not affect the linear viscoelastic
properties of the hydrogels.
194
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
(B)
Figure 4.15 (A) Creep–recovery curves of SWNT/PNIPAM/Clay hydrogels with
0.1 wt% SWNT. Creep and recovery strains are shown as a function of creep (0300 s) and recovery times (300-600 s) under various constant stresses and (B)
creep strain at 300 s as a function of applied various constant stresses.
195
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
The creep-recovery curves of the hydrogel have been investigated with the
generalised Voigt chain model (four-element model) giving the closet fit to the
creep and recovery data as displayed in section 4.3.4.2.2. Thus, the effect of
applied various constant stresses on the creep-recovery curves was then
studied using the four-element model to investigate rheological issues. Figure
4.16 shows that the elastic modulus (E0), viscoelastic modulus (E1), retardation
time () and viscosity () from both creep and recovery results do not
significantly change as a function of various applies constant stresses. This
indicates that the hydrogel composite properties were not changed with
applied various constant stresses.
196
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
(E)
(B)
(F)
(C)
(G)
(D)
(H)
Figure 4.16 Relationship between elastic modulus (E0), viscoelastic modulus
(E1), retardation time (), viscosity () and applied various constant stresses of
SWNT/PNIPAM/Clay hydrogels with 0.1 wt% clay resulting from (A-D) creep
and (E-H) recovery curve fitting by using the generalised Voigt chain (a fourelement model).
197
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
(B)
Figure 4.17 (A) Creep–recovery curves of SWNT/PNIPAM/Clay hydrogels with
0.05 wt% SWNT. Creep and recovery strains are shown as a function of creep
(0-300 s) and recovery times (300-600 s) under various constant stresses and
(B) creep strain at 300 s as a function of applied various constant stresses.
198
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(A)
(B)
Figure 4.18 (A) Creep–recovery curves of PNIPAM/Clay hydrogels. Creep and
recovery strains are shown as a function of creep (0-300 s) and recovery times
(300-600 s) under various constant stresses and (B) creep strain at 300 s as a
function of applied various constant stresses.
199
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
To study the effect of SWNT concentration on the rheological behaviour, the
creep-recovery tests were performed at controlled stress 20 Pa as presented in
Figure 4.19 and Figure 4.20. It displays that the creep-recovery strain decreased
as a function of increasing concentration of SWNT at the constant stress. While,
the viscosity increased due to the addition of SWNTs. It indicates that the elastic
modulus of the hydrogel composites containing SWNTs is significantly
improved as a function of increasing SWNT concentrations. Thus, SWNTs
efficiently reinforced hydrogel composites as reported by Xiao and co-workers
[289].
Figure
4.19
Creep–recovery
curves
of
SWNT/PNIPAM/Clay
hydrogel
composites contained 0, 0.05 and 0.1 wt% SWNTs. Creep and recovery strains
200
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
are shown as a function of creep (0-300 s) and recovery times (300-600 s)
under a constant stress 20 Pa.
(A)
(E)
(B)
(F)
(C)
(G)
(D)
(H)
Figure 4.20 Relationship between elastic modulus (E0), viscoelastic modulus
(E1), retardation time (), viscosity () and SWNT/PNIPAM/Clay hydrogels
contained various SWNT concentrations (0, 0.05 and 0.1 wt%) under a constant
201
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
stress 20 Pa resulting from (A-D) creep and (E-H) recovery curves fitting by
using the generalised Voigt chain (a four-element model).
4.3.4.2.2 Fitting creep-recovery curves of hydrogel composite with various
clay concentrations (as prepared in section 4.2.2)
A Voigt unit is used as a simple model which consists of a Hookean spring and
Newtonian dashpot as shown in Table 4.1. A model of linear vicoelasticity of
material which exhibit both viscous and elastic behaviour is built up by
considering combinations of the linear elastic spring and the linear viscous
dashpot [41]. A model of a Maxwell model represented by the connection of
spring and dashpot in series, while a Voigt model represented by two elements
connected in parallel as illustrated in Table 4.1. The hydrogel have been
investigated creep and recovery data using Voigt models and their equations as
illustrated in Table 4.3.
Table 4.3 Three Voigt models and their equations
Voigt model
Equation
(
(
))
two-element model
(
(
))
(
(
))
three-element
model
four-element
model

202
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.21 shows the result of a creep experiment which carried out by
monitoring changes in strain of the 0.4 wt% clay hydrogel (A) over a period of
300 s at a fixed shear stress of 20 Pa and then (B) another 300 s immediately
after the shear stress was removed. It is seen that the two-element models
fitting creep and recovery curves follows the creep and recovery data only
initial or elastic part. This indicates this model can explain only elastic
behaviour. However, three-element model fitting follows the creep and
recovery data of elastic and viscoelastic part except the viscous flow. The
hydrogel have been investigated creep and recovery data using four-element
model gave the closet fit to the creep and recovery data. Therefore, a fourelement model could verify the precision of the time-dependent viscoelastic
properties of the hydrogels.
203
THERMALLY SENSITIVE INJECTABLE HYDROGELS
(mm/mm)
(mm/mm)
CHAPTER 4
204
CHAPTER 4
THERMALLY SENSITIVE INJECTABLE HYDROGELS
Figure 4.21 (A) Creep and (B) recovery curve fitting of 0.4 wt% clay hydrogel by
using a two-element, a three-element and a four-element models and their
equations as illustrated in Table 4.3.
Note: Four-element model was used a linear model to fit curves from 200 to 300
s by excluding data from 0 to 200 s. Then, the slope from this fit was used as the
viscosity value and all data points less than or equal to time 2 s was excluded.
Typical creep-recovery curves were employed to investigate rheological issues
related to different clay loadings of composite hydrogels. Elasticity,
vicoelasticity and viscosity of the gels were compared using the results obtained
from creep-recovery curve fitting using the generalized Voigt chain (a fourelement model) as shown in Figure 4.22. It is seen that the elastic modulus (E0)
and viscoelastic modulus (E1) and viscosity () decreased as a function of
decreasing clay concentration, while the retardation times () is observed to be
constant for various clay loadings. It indicates the composite hydrogel with low
concentration of clay has dropped in elastic modulus, viscoelastic modulus and
viscosity probably due to less clay platelets having less interaction at
crosslinking points in the gel. It is implied that crosslinks are mobile at low clay
platelets concentration. However, the results are related to the high swelling
ratio of the composite hydrogels containing low clay concentration as shown in
section 4.3.2. Therefore, clay loading is important for the transition occurring
from solid-like to liquid-like behaviour which confirm the results obtained from
extrudable (section 4.3.3) and oscillatory frequency sweep (section 4.3.4.1)
tests.
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(A)
(E)
(B)
(F)
(C)
(G)
(D)
(H)
Figure 4.22 Relationship between elastic modulus (E0) and viscoelastic
modulus (E1), retardation time () and viscosity ()) and clay contents of
PNIPAM/clay hydrogels resulting from (A-D) creep and (E-H) recovery curve
fitting by using the generalised Voigt chain (a four-element model).
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4.3.4.3 Shear yield experiment
Rheological measurements were performed on PNIPAM hydrogel composites
applying shear strain at constant shear rate (Figure 4.23) to determine the
effects of clay loading on shear strength and shear modulus at the breaking
shear strain.
Figure 4.23 Diagram of change where a constant shear rate is applied.
Figure 4.24 and Table 4.4 show shear strength and shear modulus of PNIPAM
hydrogel composite increased as a function of increasing clay concentration as
shown in oscillatory frequency sweeps and creep-recovery tests. With
increasing clay concentration, breaking shear strain decreased. It indicates the
improvement in shear strength and modulus corresponds to the clay loading.
The breaking shear strain decreased with increasing clay concentration which
is similar results of the change in elongation at break as reported by Haraguchi
et al. [12]. In addition, the changes of the mechanical properties of the gels are
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observed to correspond to the change in swelling ratio (Figure 4.9) as reported
by Kristi [42].
Figure 4.24 Shear yield experiments by applying shear strain at constant shear
rate of 0.8, 1.6 and 3.3 wt% clay contained in PNIPAM/Clay hydrogel
composites.
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Table 4.4 Shear strength, breaking shear strain and shear modulus of PNIPAM
hydrogel composites containing different clay concentrations applying shear
strain at constant shear rate measurements.
Clay concentration
Shear strength
Breaking shear
Shear modulus
(wt%)
(Pa)
strain
(Pa)
0.8
1403.3±179.0
218.3±12.0
6.2±0.8
1.6
5394.3±293.1
128.3±19.7
51.7±2.7
3.3
17875.0±1873
119.9±4.1
168.1±8.3
4.3.4.4 Comparison of modulus measured from different approaches
Figure 4.25 shows that the modulus results at 1 rad/s measured from
oscillatory frequency sweep test with a controlled strain of 0.01 rad are similar
to the results from creep (at constant stress of 20 Pa) and recovery tests at each
clay concentration. While moduli measured from tensile test and continuous
flow test at constant shear rate at 1
or controlled angular velocity at 1.733
rad/s are drastically smaller than the results from those approaches. However,
the modulus of PNIPAM/clay hydrogels as a function of clay concentration
which measured from all approaches increased as a function of clay
concentration.
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Figure 4.25 Comparison of modulus measured from different approaches of
PNIPAM/clay hydrogels as a function of clay concentration. Shear modulus from
tensile test converted by Equation: s
,
a Poisson’s ratio of 0.5 is assumed (constant volume deformation).
The tensile test of 3.3 wt% clay of PNIPAM hydrogel composite was only
performed to compare with the results of the gel which synthesised by the same
procedure and same chemical composition as reported by Haraguchi et al. [12].
Hydrogels were used as-prepared in order to retain the water/polymer ratio of
that in the initial gel samples on both tests. A stress-strain curve of 3.3 wt% clay
in this work (Figure 4.8) shows breaking strain, tensile strength and tensile
modulus are 1254%, 6.4 kPa and 0.82 kPa, respectively. Whilst, Haraguchi et al.
presented that the breaking strain, tensile strength and tensile modulus are
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1035%, 88 kPa and 7.6 kPa, respectively. Cleary, the modulus measured by
tensile test and shear yield experiment is not the initial elastic term, as strain is
too large. However, the gel from their work is stronger than the gel in this work
due to the tensile strength is significantly higher. These may be due to the
nitrogen gas was bubbled through the distilled water in order to remove O2for
more than 3 hours prior to use as reported by Haraguchi et al. [12] but it was
skipped this step in this study. However, O2 was only removed by bubbling
under N2 gas for 10 min before adding an initiator in this study. In addition, the
gel used to characterise mechanical properties was formed as a sheet in rubber
gasket mould sandwiched between 2 glass slides as shown in Figure 4.2 which
may be not properly prevent oxygen getting in and inhibiting polymerisation.
As known that molecular oxygen inhibits polymerisation by reacting with the
free radical
ions [312] which could affect the gel that is weaker. It is also
feasible that the laponite is not properly exfoliated.
4.3.4.5 LCST transition-dynamic mechanical measurement
The LCST transition-dynamic mechanical behaviour of the PNIPAM/clay and
SWNT/PNIPAM/clay composite hydrogels were studied. The changes of elastic
modulus of the gels over the range of angular frequency from 1 to 31.62 rad/s
as a function of temperature are illustrated in Figure 4.26. The results show that
elastic moduli slightly increased from 20 C to 35C at various angular
frequencies (shown in Figure 4.26) and rapidly increased around 35C in both
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gels. These may be attributed to the decreasing hydrogen bonding interactions
and increasing hydrophobic interaction between PNIPAM chains and water
molecules at above the LCST temperature. This indicates that the elastic moduli
of PNIPAM/clay and SWNT/PNIPAM/clay composite hydrogels above the LCST
are higher than below the LCST. These results are similar to the change in
elastic modulus of PNIPAM/clay at about 34°C measuring in a compression
mode using dynamic mechanical analyzer as reported by Zhang et al. [19]. In
addition, the LCST of the composite hydrogel is also similar to Sur et al. [44]
reported which was determined by a DSC thermogram showed the LCST
approximately 32-33°C. Furthermore, the transition temperature of PNIPAM
thin film is noticed at around 31-32°C determining from the change in Young’s
modulus as calculated from force-displacement curves by AFM [45].
Furthermore, it was readily observed by eye on the transition in gels from
transparent to opaque, indicative of the LCST. It is seen that SWNTs do not
affected to the LCST of the SWNT/PNIPAM hydrogel composites at the
concentrations studies here. Furthermore, the sharp change in elastic modulus
from this procedure can be used to investigate the LCST of thermally sensitive
hydrogel composites.
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(A)
(B)
Figure 4.26 Phase transition of (A) PNIPAM/clay 1:3.3 wt% and (B)
SWNT/PNIPAM/clay 0.1:0.8:1 wt% hydrogels measuring by temperature ramp
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
step performed between 20 to 50C over the range of angular frequency from 1
to 31.62 rad/s with controlled stresses of  = 5 Pa and 0.6 Pa, respectively.
4.4 Conclusions
PNIPAM/clay and SWNT/PNIPAM/clay hydrogel composites were successfully
synthesised with Laponite XLG ([Mg5.34Li0.66Si8O20(OH)4]Na0.66) clay acting as a
physical crosslinker using free radical polymerisation at room temperature. The
FTIR spectra of PNIPAM exhibit all of the bands expected for PNIPAM such as
amide II, C=O, -CH2-CH3 and CONH2 and for synthetic hectorite Si-O as reported
by Zhang et al. [17, 46]. However, it is difficult to prove the existence of specific
clay-polymer interactions, probably due to the strong hydrogen bonding of
PNIPAM in the dried state which prevents observation of hydrogen bonding
with clay in the dried PNIPAM/clay gel [14]. The swelling properties of obtained
crosslinked nanocomposite hydrogels were evaluated resulting in increased
clay content increasing the crosslinking density of hydrogel which led to a
decrease of the swelling properties of the hydrogels as reported by Zhu et al.
and Xia, et al. [18, 38]. It is seen that the water absorption of the gels decreases
with further increase of clay loading. Moreover, the PNIPAM/clay hydrogels
exhibited high ductility with the elongation at break more than 1000% as
shown in Figure 4.8. Furthermore, concentration of monomer, crosslinker,
initiator and accelerator were varied during the hydrogel preparation to study
which parameter influences the solid to like transition of the hydrogel using the
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custom extrusion system. The results show that varying monomer, initiator and
accelerator contents with a clay concentration at 3.3 wt% were not lead to
solid-like to liquid-like behaviour. It was observed that extrudable testing of
PNIPAM/Clay hydrogels strongly depends on clay content due to the gels can be
extruded from the syringe with varying clay contents. Thus, the clay
concentration is important parameter for the transition occurring from solidlike to liquid-like behaviour.
It was found that the hydrogel composites feature a transition from solid-like to
liquid-like behaviour as these gels can be extruded due to the temporary nature
of physical crosslinks. Furthermore, the elastic modulus, viscoelastic modulus
and viscosity of the composite hydrogels with low concentration of clay
decrease due to less interaction at crosslinking points in the gels, illustrating the
mobility of crosslinks. It can be concluded that physical crosslinks are mobile at
low clay concentration.
Characterisation of hydrogels in terms of rheological and mechanical properties
was presented. The results from oscillatory frequency sweeps, creep-recovery
test and shear yield experiment applying shear strain at constant shear rate
confirm that clay contents or crosslinking density of the composite gels is
mainly effect the rheological and mechanical properties. An increase in elastic
behaviour corresponds to the clay loading. Furthermore, the decrease of clay
loading or crosslink concentration effects the rheological properties with the gel
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becoming more fluid-like than elastic-like. In addition, the elastic modulus of
the hydrogel composites containing SWNTs is significantly improved as a
function of increasing SWNT concentrations. SWNTs efficiently reinforced the
hydrogel composites as reported by Xiao et al. [40]. The study of the phase
transition behaviour of the composite hydrogels was presented through the
change of elastic modulus of the gels as a function of temperature. The results
show that elastic modulus rapidly increased around 35C. Furthermore, it was
readily observed by eye the change from transparent to opaque associated with
phase transition in gels indicative of the LCST. It is seen that SWNTs do not
affected the LCST of the SWNT/PNIPAM hydrogel composites at the
concentrations studies here.
216
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THERMALLY SENSITIVE INJECTABLE HYDROGELS
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Okay, O. and W. Oppermann, Polyacrylamide−Clay Nanocomposite
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Struble, L.J. and M.A. Schultz, Using creep and recovery to study flow
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Mongondry, P., T. Nicolai, and J.-F.o. Tassin, Influence of pyrophosphate or
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Tan, H. and K.G. Marra, Injectable, Biodegradable Hydrogels for Tissue
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Wang, J., et al., Cell adhesion and accelerated detachment on the surface of
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1631-1636.
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5178-5185.
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220
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SUMMARY, CONCLUSIONS AND
RECOMMENDATIONS FOR FUTURE WORK
Chapter 5
Summary, Conclusions and
Recommendations for future work
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RECOMMENDATIONS FOR FUTURE WORK
Table of Contents
5 SUMMARY, Conclusions and Recommendations for future work ..................... 223
5.1
Summary and conclusions ................................................................................. 223
5.2
Recommendations for future work ................................................................ 228
5.3
References ................................................................................................................ 229
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RECOMMENDATIONS FOR FUTURE WORK
5 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS FOR
FUTURE WORK
5.1 Summary and conclusions
The aim of this thesis was to synthesize thermally sensitive electrically
conducting hydrogels. The strategies to achieve this focused on the fabrication
of PNIPAM hydrogels containing SWNTs which are expected to be promising
materials as novel structures with new properties. SWNTs were incorporated
into PNIPAM hydrogels to form hydrogel-based hybrid structures using both
chemical and physical crosslinkers. SWNTs were added in an attempt to
improve conductive properties as well as to function as mechanical fillers.
Chapter 2 Gel Encapsulation of Carbon Nanotube Electrodes
SWNT Buckypaper was fabricated using a vacuum-filtration method. PNIPAM
hydrogel encapsulation of Buckypaper electrode was successfully synthesised
by electrochemically induced free radical polymerisation technique using an
eDAQ chart system. The formation of PNIPAM on gold coated Mylar, RVC and
Buckypaper electrodes was confirmed by several observations. First, cyclic
voltammetry was performed before and after polymerisation. The results show
that the reduction peak resulting from decomposition of potassium persulfate
greatly decreases during polymerisation. A smaller charging current after
polymerisation confirms the formation of PNIPAM due to the effective
reduction in the rate of diffusion of the initiator to the electrode surface or the
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RECOMMENDATIONS FOR FUTURE WORK
reduction the electrochemically active surface area of the electrodes. Second,
the FTIR spectra of PNIPAM exhibit all of the bands expected for PNIPAM such
as amide II, C=O, -CH2-CH3 and CONH2. Furthermore, the PNIPAM hydrogel that
formed within and/or adjacent to the electrode surfaces could be readily
observed by eye and in more detail using scanning electron microscopy (SEM).
Moreover, cyclic voltammograms performed after polymerisation in the
presence of potassium ferricyanide showed an impeded diffusion of ions
through the hydrogel coatings comparing with bare electrodes.
A cyclic voltammogram (-0.4 to 0.6V, 50mV/s) recorded in potassium
ferricyanide showed a change in hydrogel permeability for gold coated Mylar
and RVC coated with PNIPAM gel. This change was not observed in the
Buckypaper coated with PNIPAM gel due to the high contribution from the
capacitive charging current that may mask electrochemical redox signals from a
potassium ferricyanide. However, the electrochemical response signal of
potassium ferricyanide clearly exhibits redox peaks as observed at Buckypaper
at low scan rates due to the Faradic current. It is seen that the Faradic current
can be observed when the background capacitive current decreased at lower
scan rates. A PNIPAM hydrogel coated on gold coated Mylar underwent a
transition when varying the temperature of the electrolyte in an aqueous
solution containing sulfate with the LCST transition occurring between 10C
and 20C. The change of peak current in CV at the temperature recorded is
lower than the reported the LCST at ~32C in aqueous solutions [1, 2].
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This may due to the addition of salts (
) from the supporting electrolyte (0.4
M K2SO4) to the hydrogel. The effect of salt on the LCST of PNIPAM coated on
gold Mylar was studied by UV-vis determining the change in absorbance and
shows a decrease in LCST of the gel as a function of increasing salt (
)
concentration in the hydrogel. This change of the LCST is similar to the results
from cyclic voltammograms taken after polymerization in the presence of
potassium ferricyanide (5mM in 0.4 M aqueous potassium sulfate). The LCSTs
of PNIPAM were set between 15-20C and 10-20C which was determining the
change in absorbance and CV, respectively. Importantly, electrochemically
induced free radical polymerisation can be used to deposit smart coatings on
electrode which regulate permeability of reactants based on temperature.
Chapter 3 Shrinkable Electronic Conductor: Carbon nanotube/Hydrogel
Composites
PNIPAM/SWNT composite hydrogels as heat shrinkable electronic conductors
were successfully prepared by free radical polymerisation with various SWNT
loadings: 0, 0.05, 0.1, 0.5 and 1.0 wt%. The ability of hydrogels to swell in water
decreased with increasing SWNT loading. The onset of the lower critical
solution temperature (LCST) was measured to be 30C. All hydrogels except 1.0
wt%, reached the swelling equilibrium thickness at 40C, with 1.0 wt% reaching
swelling equilibrium at 45C. Above the LCST, the extreme volume change of
hydrogels with high SWNT loadings (0.5 and 1.0 wt%) are related to low
equilibrium swelling ratios and equilibrium water contents.
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Furthermore, the changes of the mechanical properties of the gels observed
correspond to the large change in swelling ratio. The increase of compressive
modulus of the composite gels also depends mainly on the increase of SWNT
concentration, indicating that a significant improvement in compressive
mechanical properties due to SWNTs efficiently reinforced the hydrogel
composites. However, hydrogels containing low concentration of SWNT
demonstrated no considerable differences in the mechanical properties, while
addition of 0.5 and 1.0 wt% SWNT can be significantly improved the mechanical
properties.
The electronic properties of SWNT/NIPAM thermally sensitive composites were
quantified by EIS plots below and above the LCST. The composite hydrogels
have been thermally cycled between 20C and 40C through the heating-cooling
cycles. The impedance decreased while the gel shrank when it was warmed
through its LCST, and then reversibly increased while the gel expanded and
swelled when it was cooled below the LCST. The ratios of solution resistance
per thickness (Rs/t) of hydrogels at 100 kHz were not different at low SWNT
concentrations (0, 0.05 and 0.1 wt%). While, the Rs/t ratios decreased with high
SWNT loadings (0.5 and 1.0 wt%) at both temperatures below and above the
LCST. This indicates that hydrogels containing low concentration of SWNT
demonstrated no considerable differences in the electronic properties, while 0.5
and 1 wt% SWNT can be significantly improved.
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Chapter 4 Thermally Sensitive Injectable Conducting Hydrogels
PNIPAM/clay and SWNT/PNIPAM/clay hydrogel composites were successfully
synthesised adding Laponite XLG ([Mg5.34Li0.66Si8O20(OH)4]Na0.66) clay as
physical crosslinker without the need of a chemical crosslinker by free radical
polymerisation at room temperature. The FTIR spectra of PNIPAM/clay,
PNIPAM/MBA and clay confirm that the interaction between polymer and
inorganic clay occurred. The FTIR spectra of PNIPAM exhibit all of the bands
expected for PNIPAM such as amide II, C=O, -CH2-CH3 and CONH2 and for
synthetic hectorite Si-O as reported by Haraguchi et al. [3, 4]. The swelling ratio
of the composite gels depends mainly on clay loadings. The swelling ratio
decreased with increasing clay loading. PNIPAM/clay hydrogels containing
various chemical contents were prepared and extrudable tested using a custombuilt syringe printer with tips in three diameter sizes. It was observed that the
extrudable testing of PNIPAM/clay hydrogels strongly depends on clay content.
Thus, clay concentration is the most important parameter of PNIPAM/clay gel
for going from solid-like to liquid-like behaviour with these chemical
compositions in this study. Moreover, the elastic modulus significantly
improved as a function of increasing clay contents as resulted from rheological
oscillatory frequency sweeps, creep-recovery and shear yield experiment
applying shear strain at constant shear rate tests. In addition, the elastic
modulus of the hydrogel composites containing SWNTs significantly improved
with increasing SWNT concentrations. SWNTs efficiently reinforced hydrogel
composites as reported by Xiao et al. [5]. The LCST transition-dynamic
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SUMMARY, CONCLUSIONS AND
RECOMMENDATIONS FOR FUTURE WORK
mechanical behaviour of the PNIPAM/clay and SWNT/PNIPAM/clay composite
hydrogels were studied through the changes of elastic modulus of the gels over
the range of angular frequency from 1 to 31.62 rad/s as a function of
temperature. The results show that elastic moduli rapidly increased around
35C to 40C in both composite gels. It is seen that SWNTs did not affect the
LCST of the composite hydrogels at the concentration studies here. These
approaches can be used to form thermally sensitive injectable hydrogels which
are highly desirable in clinical applications.
5.2 Recommendations for future work
PNIPAM hydrogel encapsulation of Buckypaper electrode was successfully
synthesised by electrochemically induced free radical polymerisation technique
using an eDAQ chart system. The nanotube Buckypaper composite as applied to
free-standing electrode could be modified as an electrode platform for thermal
sensing applications. In addition, the single-walled carbon nanotubes (SWNTs)embedded composite gels could be used as a scaffold for adsorption of small
molecule. SWNTs could be used as a molecular container in gel for drug delivery
application. Furthermore, it is seen that the thermally sensitive injectable
hydrogel which contained low clay concentration with and without SWNTs can
be extruded as a fibre. Thus, pre-crosslinked hydrogel prior to injection can be
developed a thermally sensitive non-conducting/conducting fibre. The fibres
can be used as a temperature-triggered intelligent devices with applications in
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CHAPTER 5
SUMMARY, CONCLUSIONS AND
RECOMMENDATIONS FOR FUTURE WORK
areas such as drug delivery, sensors, actuators, photonics, and optics devices [68].
5.3 References
1.
2.
3.
4.
5.
6.
7.
8.
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Schild, H.G., Poly(N-isopropylacrylamide): experiment, theory and
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Haraguchi, K., T. Takehisa, and S. Fan, Effects of Clay Content on the
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Haraguchi, K., et al., Mechanism of Forming Organic/Inorganic Network
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Xiao, Y., L. He, and J. Che, An effective approach for the fabrication of
reinforced composite hydrogel engineered with SWNTs, polypyrrole and
PEGDA hydrogel. Journal of Materials Chemistry. 22(16): p. 8076-8082.
Zhang, J.-T., et al., Temperature Sensitive Poly[N-isopropylacrylamide-co(acryloyl β-cyclodextrin)] for Improved Drug Release. Macromolecular
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Liu, Y. and Y. Cui, Thermosensitive soy protein/poly(nisopropylacrylamide) interpenetrating polymer network hydrogels for
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Wang, J., et al., Fast responsive and morphologically robust thermoresponsive hydrogel nanofibres from poly(N-isopropylacrylamide) and
POSS crosslinker. Soft Matter. 7(9): p. 4364-4369.
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