SUPPLEMENTARY MATERIALS
Stretchable conducting gold films prepared with composite
MWNT/PDMS substrates
M U Manzoor1, P Lemoine2, D Dixon2 and J W J Hamilton2, PD Maguire
1
Department of Metallurgy & Materials Engineering, College of Engineering & Emerging
Technologies, University of the Punjab, Lahore, Pakistan
2
NIBEC, University of Ulster at Jordanstown, Shore Road, Newtownabbey, Co. Antrim, BT37 0QB,
Northern Ireland, UK
E-mail:p.lemoine@ulster.ac.uk
Amorphous carbon impurities within MWNT thermally decompose below 365oC [7]. TGA
measurements performed in this study gave a mass loss below 365C < 2%, however the ash content,
or metallic residue was < 0.2 wt.%; probably made of metal oxide and catalyst residue.
a
b
Figure S1. SEM micrographs of the MWNT before (a) and after (b) tip sonication, marker bar = 2μm.
The de-aggregation effect of the tip sonication treatment is clear.
1
Figure S2. Raman spectra of the pure PDMS, 4 and 8 % MWNT composites. The D, G and 2D peaks
are MWNT peaks. The other vibrations are from PDMS; Si-C symmetric stretching (708 cm-1),CH3
symmetric bending (1262 cm-1), CH3 assymmetric bending (1412 cm-1), CH3 symmetric stretching
(2907 cm-1) and CH3 assymmetric stretching (2965 cm-1)
For pristine MWNT, the ID/IG and ID/I2D ratio are 0.03 and 0.18, respectively. After sonication, these
ratios increase to 0.19 and 1.0, respectively, a sign that the sonication process brings new defects
along the tubes. Figure S2 shows the Raman spectra for the different composites, with peak
assignments for both the MWNT and PDMS vibrational groups [10].
2
a
b
Figure S3. Analysis of the Raman spectra of the CNT/MWNT composites (a) ID/IG and ID/I2D ratio.
The dotted lines give the values of these ratios for the pure tip-sonicated MWNT, (b) Normalized
intensity ratio rx/r0 to the intensity of the Si-O-Si stretching vibration at 488 cm-1, where the subscript
x and 0, respectively, correspond to the composite and pure PDMS.
The ID/IG and ID/I2D ratios are generally higher in the composites, with the exception of the 4 %
composite. Generally, high ID/IG and ID/I2D ratio in CNT/polymer composites are associated with
covalent functionalities neutralizing dangling bonds on the CNT outer shells [8] or with damping of
the G vibrations due to polymer intercalation within the bundles [9]. Hence, the trend shown in figure
S3a is consistent with the composite prepared with the 4% CNT fraction having poorly dispersed
CNT.
Changes in the vibrational structure of the PDMS network upon MWNT addition can also be
investigated with Raman spectroscopy. A comparison of Raman intensities between samples is
difficult as they are affected by scattering, focus conditions and composite homogeneity. However,
the ratio of these intensities informs on the relative importance of the vibrational groups and hence
can be used to gauge the occurrence of chemical reactions, as demonstrated for ZnO-PDMS
composite [11]. In the analysis done in figure S3b, the peak intensity for each PDMS vibration, as
determined from figure S2, has been divided by the peak intensity for the Si-O-Si stretching vibration
at 488 cm-1. The rationale for choosing this molecular group is based on the well-established
hydrosylation cure reaction for PDMS; the vinyl groups in the base and the Si-H bonds in the curing
agent react to produce ethylene bridges which crosslinks the PDMS into a 3D network [12]. The
number of Si-O-Si groups in the initial mixture (base + cross-linking agent) is the same as in the
cross-linked PDMS whereas the numbers of C-H and C-Si bonds change upon cross-linking. Hence,
variations in these intensity ratios are good indications of chemical changes resulting from an
incomplete curing of the PDMS network. As the curing agent (CA) and base are stoichiometrically
matched in the Sylgard 184 formulation, any consumption of the reactants by a reaction with MWNT
would affect the final number of crosslinks. These calculated ratios in the composites were then
normalized with respect to that of the pure PDMS using the expression;
rx
r0
=
(I⁄I
)
488 PDMS−MWNT
(I⁄I
)
(3.1)
488 PDMS
3
Figure S3b shows that for 4 % MWNT, there are small variations of intensity ratio (1.15 ± 0.16); all
PDMS vibration amplitudes are similar to that observed in the pure PDMS. At all other % MWNT,
these intensity ratio exhibit large variations (~ 1.0 ± 0.9), the PDMS molecular groups vibrate with
different relative amplitudes with respect to the pure PDMS. In other words, the PDMS in these
composites has different bonding environments than pure PDMS, indicative of a reaction between the
MWNT and the PDMS cure.
4
Derivate weight loss, au
4%
2%
0%
Weight loss, %
100
80
60
40
200
400
600
800
1000 0
200
400
600
800
1000 0
400
600
800
10 %
8%
6%
200
100
Weight loss, %
1000
Derivate weight loss, au
0
80
60
40
0
200
400
600
800
Temperature, °C
1000 0
200
400
600
Temperature, °C
800
1000 0
200
400
600
800
1000
Temperature, °C
Figure S4. TGA traces; weight loss and derivative weight losses for the various MWNT/PDMS
composites.
Figure S4 shows TGA traces for the composite samples, illustrating both the weight loss and weight
loss derivative. There are no weight loss steps observed below 200°C, demonstrating that no trapped
solvent was present in these samples. However, there is a gradual change in the TGA profile, a second
derivative peak appearing upon MWNT addition at higher temperatures. This suggests that the
thermal degradations of PDMS are altered by the presence of the MWNT. This shift to a higher
temperature peak has previously been associated with a thermal cross-linking reaction [13, 14],
implying that the presence of the MWNT reduces the cure yield for PDMS and results in non-crosslinked regions within the PDMS network. It is consistent with the observed longer curing time for the
composites and the result of figure S3b.
5
Figure S5. Raman spectra of the pure curing agent (CA) and a CA/MWNT mixture analysed within
and outside a MWNT aggregate. The peaks at 488 and 2165 cm-1 respectively correspond to Si-O-Si
stretching and Si-H stretching vibrations. The D and G peaks of the MWNT are also shown.
Separate experiments were conducted on (base + MWNT) and (CA + MWNT) mixtures to identify
more precisely the nature of the reaction between MWNT and PDMS. Using the same approach
followed in figure S3, it was found that the Raman intensity ratio of the various PDMS vibrations (i.e.
to the Si-O-Si stretching vibration at 488 cm-1) were not significantly different for the pure base and
base/MWNT mixture; the MWNT did not react with the base. The situation was different for the
CA/MWNT mixture; Raman spectra of pure CA and a CA/MWNT mixture are shown in figure S5.
The Si-H stretch vibration of the CA can be seen at 2165 cm-1. The intensity ratio for the 2165 and
488 cm-1 vibrations is 0.88±0.13 for pure CA. For the CA/MWNT mixture it is 0.26±0.10 within
MWNT aggregates and 0.87±0.11 outside these aggregates. This clearly shows that CA and MWNT
react together.
Further confirmation of this came from TGA analysis. Firstly, base and MWNT were mixed together,
bath sonicated, the CA was added afterwards. TGA analysis of the cured material showed two peaks
of similar heights, as shown for instance for the 8 % sample of figure S4. Secondly, the CA and
MWNT were mixed together, bath sonicated, the base was added afterwards. In this case the TGA
analysis of the cured material showed a larger second peak, as seen for the 6 % sample of figure S4,
indicative of a larger fraction of un-cross-linked component in the material. This is consistent with
some of the curing agent reacting first with the MWNT and therefore being unavailable to crosslink
the base at a later stage.
6
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