Honors Thesis Proposal
Responsive Materials and Devices Based on
Hierarchical Self-Assembly of Functional Hybrid
Nanofibers
Supervised by
Minko, Sergiy; Chaired Professor, D.Sc; Chemistry Department
Tokarev, Ihor; Research Associate, Ph.D.; Chemistry Department
Anna Paola Soliani
Chemical Engineering B.S.
Mathematics
Honors Program
Clarkson University 2008
November 5, 2006
A. Introduction:
The main goal of this research is to develop new nanomanufacturing techniques
capable of linking a wide range of materials of different scales and properties to yield
various multifunctional materials (e.g., responsive coatings and drug delivery systems)
and active miniature devices (e.g., sensors and microactuators). More specifically, the
proposal describes synthesis and hierarchical assembly of 2- and 3-dimensional hybrid
(organic-inorganic) nanostructures combining two distinct building blocks: inorganic
nanoparticles and surface-functionalized high-aspect-ratio hybrid polymeric nanofibers. It
also describes techniques for creating stable suspensions of the nanostructures in liquid media.
The nanofibers have core-sheath morphology with a flexible polymeric core and an organicinorganic composite sheath. The precisely engineered sheath is a multilayered structure consisting
of self-assembled layers of polymers and inorganic particles which represent the first (lowest)
level of the assembly. Such layers introduce a range of important functionalities that are useful
for sensing, regulation of adhesive and wetting properties, stability in liquid dispersions, and
manipulation and directed assembly of the nanofibers into functional devices. The hybrid coresheath nanofibers form the second level of the hierarchical assembly. The sheath and the core of
the nanofibers may consist of several successive segments with different ingredients.1 A class of
hybrid nanofibers incorporating magnetic nanoparticles is of special interest, because the shape
and alignment of the fibers and the properties of these nanostructured materials based on them
can be controlled by an external magnetic field. The hybrid nanofibers will be used as building
blocks for the directed assembly of various functional devices and sensors. These devices
represent the third (highest) hierarchical level of the assembly.
B. Background:
Fabrication of composite nanofibers
Organic-inorganic (hybrid) composite nanofibers are of great interest in materials science due
to the combination of the properties of polymer fibers, such as a high aspect ratio and specific
surface area, light weight, and high flexibility, with the properties found in inorganic materials,
such as high mechanical strength, metal-like conductivity, plasmon wave propagation, and
magnetism.1 Potential applications of ultrathin fibers of this kind include reinforcement of
1
NIRT 2006 Proposal; Tokarev, Ihor; Minko, Sergei.
elastomers and plastics, catalyst supports, filtration membranes, electrodes for lithium batteries,
anisotropic optical materials, sensors, scaffolds for tissue engineering, medical implants, and
supports for protein immobilization. Several methods exist for the fabrication of nanofibers and
for 1D aggregation of inorganic particles using polymers as templates or connectors. For
example, Cheyne et al. describe the formation of nanofibers by spontaneous aggregation of an
amphiphilic polystyrene-b-poly(ethylene oxide) block copolymer and polystyrene-coated CdS
nanoparticles at the air-water interface.1 Biotin-streptavidin chemistry and a magnetic field have
been used to fabricate flexible magnetically active nanofibers.2,3 The technique involved fieldaligned linear chains of streptavidin-coated magnetic latex nanoparticles that were linked together
by biotin-functionalized PEO chains2 and biotinylated DNA3. Recently Minko et al. described a
technique that employs negatively charged Fe2O3 nanoparticles, which were aligned in a magnetic
field and linked together by a cationic polyelectrolyte, poly(2-vinylpyridine).4
Electrospinning is the most common technique for the fabrication of hybrid fibers with
diameters ranging from tens of nanometers to a few microns. The fibers are deposited onto a
collecting electrode as a randomly oriented, non-woven mat. Three different approaches have
been used to fabricate hybrid composite nanofibers by electrospinning.5,6 The nanofibers can be
prepared directly from a solution containing polymer and inorganic nanoparticles (carbon black,
iron oxides, Ag, TiO2, FePt) or/and carbon nanofibers.5,7 Applications, however, have been
limited because of the high viscosity of the solutions used for electrospinning. This problem is
easily solved by using a solution of a polymer and a sol-gel precursor. The precursor was
hydrolyzed in the nanofibers previously elecotrospun, to yield an inorganic gel embedded in the
polymer matrix.5,6 The hybrid composite nanofibers can be converted into organic-phase-free
ceramic nanofibers by high-temperature calcination. In another application, electrospun polymer
nanofibers were directly coated by inorganic materials (TiO2 or SnO2).8
Nielsch et al. reported the fabrication of cobalt/polymer composite nanotubes using an
alumina membrane as a template.9 The membrane pores were wetted by the polymer solution
(polystyrene or poly-L-lactide) and a Co precursor, which formed thin-wall nanotubes after
evaporation of the solvent. The decomposition of the precursor at elevated temperatures led to the
formation of magnetic nanoparticles embedded in the polymer matrix. Freestanding cobalt
nanoparticles/polymer composite nanotubes were obtained upon dissolution of the alumina
membrane. The main advantage of the template method is the availability of a broad variety of
porous materials, which can be potentially used as templates for synthesis of hybrid composite
nanofibers.
Surface functionalization with polymer brushes and preparation of responsive surfaces
Polymer brushes refer to an assembly of polymer chains which are tethered
(usually covalently bounded) by one end to a surface or interface. The brushes are ideal
building blocks for soft nanotechnology and the engineering of surfaces. 2 Tethered
polymer chains that are grafted to a solid substrate by one chain end may be definitely
distinguished from other anchored polymer layers, since the polymer chains form polymer
brushes if a relatively high grafting density is reached. Brush-like layers are formed due to the
excluded volume effect, when the substrate is completely covered with a relatively dense
monolayer of grafted chains stretched as normal to the support. Covalent attachment of
polymer brushes can be achieved by binding pre-synthesized end-functionalized polymer
chains to a suitable substrate (“grafting to” approach). This produces a relatively low
grafting density, and consequently a film with the thickness of about 5-10 nm.3
Polymer brushes demonstrate responsive wetting properties. There are different
types of responsive brushes: homopolymer brushes, polyelectrolyte brushes, and block
copolymer brushes.
For homopolymer brushes the prefactor depends on solvent quality and grafting
density of polymer chains. Besides the chain constitution, the grafting density is the
parameter which affects the response of polymer brushes to change form and function. At
small grafting densities, the response of grafted chains is very similar to that of a bulk
polymer solution. At high grafting densities the collapse is weak and the brush forms a
homogeneous layer which is thinner in a poor solvent than in a good solvent. (Fig.1)
Fig. 1 Planar homopolymer brush: a homogeneous smooth layer of stretched chains in good
solvent (a), pinned micelles (b) and a layer of collapsed chains (c) in poor solvent.
2
Azzaroni, O.; Moya, S., Farhan, T.; Brown, A.;. Huck W.; Macromolecules, 2005, 38, 10192-10199
3
Advincula, R.; Adv. Polym. Sci., 2006, 197, 107–136
The most important aspect of polyelectrolyte brushes is that they have ionizable groups
that can carry electrical charges when immersed in polar solvents. For the dense strong
polyelectrolyte brush (high grafting density) all counterions are trapped inside the brush
(Fig. 2). At very high densities the excluded volume effect may dominate while at
moderate densities the major contribution is given by the electrostatic nature of the
compound. If salt is added salt, nothing happens to the brush unless the ionic strength of
the solution becomes similar to the one inside the brush.
Weak polyelectrolyte brushes demonstrate responsiveness to changes in external
pH and ionic strength. Such brushes expand when the pH decreases. Acidic
polyelectrolyte brushes, instead expand when the pH increases (Fig. 2 c,d). At a high salt
concentration weak polyelectrolyte brushes shrink due to the same mechanism as strong
polyelectrolyte brushes.
Fig.2 Planar polyelectrolyte (PEL) brush with ionizable groups (a) swells when the groups
dissociate a polar solvent (b); the dissociation is pH dependent for weak polyelectrolytes (c);
addition of salt leads to the shrinkage of the PEL brush (d).
Block co-polymer exist when two (or more) chemically different polymers form a
polymer brush with a so-called “block-copolymer architecture”. The responsiveness of
these brushes is determined by the phase segregation but the mechanism depends on
whether the block co-polymer is tethered by the more or the less soluble block. If the
solvent quality is improved (i.e. increased selectivity of solvent) the structures are
transformed to structures are similar to the mixed brushes in selective solvents.4 (Fig. 3)
Fig.3 The block-copolymer brush constituted of more soluble A blocks and less soluble
B blocks.
Surface functionalization using layer-by-layer deposition
Layer-by-layer film can be created from polyelectrolytes that are oppositely
charged. Such polyelectrolyte multilayers have a variety of applications, such as
biosensors and membranes.
5,6
The LbL method is simple and versatile and is used to
construct ultra thin organic films with controlled thickness in the nanometer range.7
Molecularly thin polyelectrolyte multilayer interface films may be prepared by simply
dipping a (molecularly flat) surface charged solid state substrate into polyelectrolyte
solutions. As dipping occurs in alternating sequence into solutions of oppositely charged
polyelectrolytes, multilayer films are grown.(Fig. 4)
4
Minko, Sergiy; “Responsive Polimer Brushes”; Journal of Macromolecular Sciencew, Part C: Polymer
Reviews, 2006, 46:397–420
5
Caruso, F.; Schüler, C.; Langmuir, 2004, 20, 20
6
Mendelshon, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F.; Langmuir, 2000,
16, 5017
7
Chen, H.; Zeng, G.; Wang, Z.; Zhang, X.; Peng, M.; Wu, L.; Tung, C.; Chem. Mater., 2005, 17, 66796685
Fig. 4 Schematics of LbL
C. Current laboratory investigation
Experimental procedure
Fabrication of nanofibers.
Commercial alumina membranes (Nucleopore, Whatman) were used as templates
for the fabrication of epoxy nanofibers. This was a multi-step process. The first step
consisted in coating the alumina membrane’s surface and pores with alternate deposition
of poly(sodium4-styrenesulfonate) PSS and poly(dimethyl-ammoniumchloride PDDA
(Fig. 5).
(a)
(b)
Fig.5 Chemical structure of PSS (a) and of PDDA (b)
This is called layer-by-layer (LbL) method. The coating was fabricated by repeatedly
dipping the membrane in 0.3% wt. PDDA solution, then in DI water, and finally in 0.3%
wt. PSS solution. In order to increase the thickness of the adsorbed layers, NaCl salt
(0.1M concentration) was added to the solutions. Each layer was adsorbed for about 15
min each with a 5min rinse in DI water. The LbL deposition was carried out on a surface
of clean Silicon wafers as a control experiment.
In the second step, the nanofibers’ core was formed by filling the membrane pores
with a commercial two-part epoxy glue. This was achieved by coating a glass slide with a
50%-50% mixture of the two components of the epoxy glue and then depositing the LbLcoated membrane on it. Because the epoxy is sticky and liquid it rises through the pores
of the alumina membrane through capillary action. The epoxy has to be then solidified
and cured for approximately twenty-four hours at room temperature.
Once the epoxy is cured and the LbL coat has attached to the epoxy nanofiber, the
third step can be done. This consists of exposing the LbL-coated nanofibers by removing
part of the alumina membrane. This was done by mechanical polishing using a rotary tool
that had slurry covering it. The sample was then washed with soap and water and exposed
to plasma treatment at high pressure to remove any residual epoxy, slurry, or any other
contamination that could have deposited on the surface during polishing.
In the final step, the membrane template was removed by dissolving it in a 1M NaOH
solution and then filtered so as to collect the hybrid nanofibers attached by one end to the
continuous layer on the solid substrate. The fibers were dispersed with ultrasonic
treatment for approximately an hour.
Characterization: The LbL coating deposition and the effect of NaOH on the LbL,
which is essential, was monitored using Ellipsometry on a silicon substrate multiple
times. The surface of both the alumina membrane and silica substrates and the various
stages of the experimental procedure were analyzed using an Atomic Force Microscope
for qualitative purposes. Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) used to characterize the nanofibers’ morphology. Optical microscopy
was used to observe the behavior of the water dispersed nanofibers in an external
magnetic field.
Results and Discussion
The nanofibers were made of cross-linked polymers by a template method.
Alumina membranes were used as templates. Such templates are characterized by low
polydispersity and are available in a broad variety of pore sizes. The method was used to
prepare either suspensions or substrate-supported arrays of the nanofibers (Fig.6 and Fig.
7).
Fig 6. Schematic of the fabrication of composite nanofibers
Fig.7 The exposed nanofibers
To control interactions between nanofibers and their environment the composite
nanofibers were surface-modified with polymers. Polymer brushes (Fig.8) and
polyelectrolyte multilayers prepared by the LbL deposition method (Fig. 9) were
employed for this aim.
Fig. 8 Polymer brushes method schematics
Fig. 9 Layer by layer method schematics
Surface modification of nanofibers using the LbL technique
The LbL method was successfully used to fabricate polymer/magnetite
nanoparticles hybrid coatings on the surface of the epoxy resin fibers. The only method to
check if the LbL coating actually formed inside the alumina membrane pores and
consequently get attached to the fiber is to remove completely the membrane template.
To do so we allowed the membrane to dissolve in 1M NaOH and then filtered the
solution and the fibers were analyzed with TEM. The LbL shell can be clearly seen on
the TEM image (Figure 10) due to the contrast provided by the magnetite nanoparticles.
The LbL coating is not uniform on the fibers surface. The dissolution of the alumina
membrane in a highly basic NaOH solution (pH 12) is probably responsible for the partial
damage of the LbL coating.
Fig. 10 TEM Image of epoxy resin nanofibers with LbL shell around it fabricated in the pores of an alumina membrane.
The membrane was dissolved in the 1M NaOH solution. The LBL shell contains magnetite nanoparticles providing a sufficient
contrast for its visualization.
Because there are no instruments that show to that the LbL coating has been
formed inside the alumina membrane’s pores, preliminary tests had to be done to show
that a layer exists. This was done by using silicon wafers as model substrates. The
procedure was the same: alternating immersions in PDDA and PSS. After every
immersion, the thickness of the polymer layer was measured using ellipsometry. If the
thickness changes after every immersion, it means that an extra layer has attached and
formed on the surface of the previous one, as required. After completing the LbL layer it
was exposed to the 1 M NaOH solution to check its stability. If the thickness decreases as
after the NaOH treatment, it means that the layer-by-layer is destroyed. Most times it
showed that the LbL coating was not destroyed because the thickness did not change (Fig
11). However, a few times it was noticed that the thickness suddenly decreased to zero,
i.e. the layer detached (see Fig. 12).
Thickness (nm)
4 LbL , NaOH
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
si
pdda
pss
pdda
pss
NaOH
Layers
Fig. 11 Changes in the thickness of the LbL coating as monitored by ellipsometry. The
treatment with the NaOH solution didn't remove the LbL coating.
4 Lbl, NaOH
5
4.5
Thickness (nm)
4
3.5
3
2.5
2
1.5
1
0.5
0
si
pdda
pss
pdda
pss
NaOH
Layers
Fig. 12 The same as in Figure 11 but the treatment in the NaOH solution removed the LbL
layer from the Si substrate.
The previously shown TEM image of the fibers with non-uniform and at times
detached layer-by-layer on them, prove that the NaOH may destabilize the LbL layer.
One possible reason for this could be that charges developed inside the layer that tend to
tear it apart. Another possible explanation is that the layer itself is not destroyed, but just
detaches from the template, which ever the template is.
Surface modification of nanofibers with polymer brushes
Grafting of polymer brushes is an alternative method to surface modify the fibers
produced in the above-described procedure. There are two kinds of polymers to do so that
have been investigated and will keep on investigating in the future: block copolymer
brushes and homopolymer brushes. Homopolymer brushes are either hydrophilic, such as
polyethyleneglycol (PEG) or hydrophobic, such as polydimethylsiloxane (PDMS). Each
of these compounds was applied on the fibers and measured their surface angle using the
contact angle method. For a hydrophilic surface, if a drop of water is applied it will
spread on the surface and by definition its contact angle will be less than 90°. For PEG,
literature shows that PEG should have a contact angle of approximately 24°. Oppositely,
a compound that is hydrophobic will not let the water droplet spread, creating a contact
angle between the drop and the surface greater than 90°. Literature shows that PDMS’s
contact angle should be between 100° and 110°. The results achieved when the fibers
were surfaced modified with PEG and PDMS were outstanding. For PEG, the contact
angle measured was 27.1° (compared to the 24° found in literature) and for PDMS was
104.3° (compared to the avg 105° in literature).
Hydrophilic brushes were formed from poly(ethylene glycol) (PEG) which is
often used in biomaterials for its anti-adhesive properties.8 Hydrophobic surfaces were
prepared by grating of poly(dymethylsiloxane) (PDMS) chains to a surface. To obtain a
charged surfaces brushes from a cationic polylectrolyte, e.g. poly(2-vinyl pyridine)
(P2VP), were synthesized. Brushes from weak polyelectrolytes (like PVP) are known to
demonstrate pH-dependent wetting behavior.4 (Fig.13 and 14)
8
Fujii, H.; Fujii, S.; Togashi, H.; Yoshioka, M; Nakai, K.; Satoh, H.; Sakuma, I; Kenmotsu, O.; Kitabatake,
A., Thrombosis Research, 2000, 100, 519-528
Proposed Future Research
For the future, a lot still has to be done, especially when it comes to surface
modification of the fibers. A lot more experiments have to be done to make sure the
results are actual and not coincidental. The LbL and grafting techniques also have to be
perfected. Although the experimental procedure seems to be correct, it will most
definitely have to be changed so as to improve results. The first step in future research is
to make more stable, uniform LbL coatings. This can be done by using polymers that can
be
cross-linked,
for
example
poly(acrylic
acid)
(PAA)
and
poly(allylamine
hydrochloride) (PAH), which form amide cross-links upon heating above the glass
transition temperature of the polymers. In addition, a procedure for modification with
block copolymer brushes has to be created and tested. A challenging part of the future
work is a step-by-step modification of the nanofibers with polymer brushes (Figure 13).
Various functional nanoparticles (magnetic and fluorescent) can also be
incorporated into the nanofibers. For example, the nanofibers with the magnetic
nanoparticles attached allowed for manipulations of the single nanofibers and actuation
of the nanofibers' arrays with an external magnetic field. So far, only a few experiments
with the magnetic nanoparticles were done. This part of the research is still new and is
what will mostly be focusing on for the future. An interesting application for the
magnetic polymer nanofibers is a coating which wettability can be switched from ultrahydrophobic to hydrophilic state and vice versa by an external magnetic field (see Figure
14).
Fig.13 Schematic of the multi-step surface functionalization of nanofibers
Fig. 14 Schematic of a substrate decorated with an array of polymer-brush functionalized magnetic composite
nanofibers, which switches the wettability from ultra-hydrophobic (a) to hydrophilic (b) under an external magnetic field.
When the field is turned off the hydrophobic behavior is recovered
Here is an estimated timetable of work over the next year or so:
What has to be done
Time Period
Perfect LbL method
January 2007 ~ February 2007
Polymer Brushes
February 2007 ~ June 2007
Surface Modify fibers magnetic
particles
Thesis writing
June 2007 ~ August 2007
Written by September 2007
Submitted by the end of December 2007
D. Conclusions:
Although everything still has to be perfected, it can be seen from preliminary
results that alumina membranes with the pore size of 200 nm are suitable templates for
the fabrication of epoxy resin nanofibers. It was also shown that the LbL approach, not
only is feasibly doable, but it also has great potential once the technique is perfected.
Overall, though, the experimental results and the technique seemed to have given
preliminary outstanding results, which is a great start. It was also shown that surface
modifying the fibers is not only easy, quick and easy, but efficient.