Carbon nanotubes based high temperature vulcanized silicone rubber nanocomposite with excellent elasticity and electrical properties

Composites: Part A 66 (2014) 135–141

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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa

Carbon nanotubes based high temperature vulcanized silicone rubber nanocomposite with excellent elasticity and electrical properties Songmin Shang ⇑,1, Lu Gan 1, Marcus Chun-wah Yuen, Shou-xiang Jiang, Nicy Mei Luo Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong

a r t i c l e

i n f o

Article history: Received 5 April 2014 Received in revised form 16 July 2014 Accepted 17 July 2014 Available online 27 July 2014 Keywords: A. Polymer–matrix composites (PMCs) B. Electrical properties B. Elasticity High temperature vulcanized silicone rubber

a b s t r a c t In the present study, we have fabricated a series of high temperature vulcanized silicone rubber (HTVSR)/ carbon nanotubes (CNTs) nanocomposites with different CNT contents. The CNTs were pretreated by the chitosan salt before being incorporated into the HTVSR. The nanocomposites were then characterized in terms of morphological, thermal, mechanical and electrical properties. It was found that the chitosan salt pretreated CNTs dispersed uniformly within the HTVSR matrix, improving the thermal and mechanical properties of the HTVSR. The nanocomposites could remain conductive without losing inherent properties after 100 times of repeated stretching/release cycles by 100%, 200%, and even 300%. Moreover, the nanocomposites had good response to the compressed pressures. The results obtained from this study indicate that the fabricated nanocomposites are potential to be used in many electrical fields such as the conductive elastomer or the pressure sensor. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Silicone rubber, as a well-known elastomer, has been widely applied in various industrial fields due to its excellent properties including nontoxicity, biocompatibility, flexibility, low cost, and ease of fabrication [1,2]. The high temperature vulcanized silicone rubber (HTVSR) is a representative class of the silicone rubber which has been used for more than fifty years [3]. Generally, the HTVSR raw rubbers are gum-like methyl vinyl polysiloxanes with low molecular weight and mechanical strength. The crosslinking among the vinyl along the HTVSR molecular chains at high temperature, normally initiated by some vulcanizing agents, forms a firm network with very high molecular weight [4], which is the reason why vulcanized HTVSR is stretchable and durable. Recently, the conductive elastomers have received tremendous interest because of their various application areas in electronics, sensors and textiles, etc. [5–8], and the HTVSR may be a potential candidate due to its remarkable elasticity. Moreover, since the HTVSR is biological friendly and nontoxic, it could be even used in fabricating the wearable electronic and the artificial skin [9]. However, the HTVSR is an insulating polymer which is not electrically conductive. To make the HTVSR conductive enough for practical use, it is necessary to integrate conductive fillers into ⇑ Corresponding author. Tel.: +852 3400 3085; fax: +852 2773 1432. 1

E-mail address: [email protected] (S. Shang). These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.compositesa.2014.07.014 1359-835X/Ó 2014 Elsevier Ltd. All rights reserved.

the HTVSR matrix. Traditionally used fillers are metals and metal oxides [10,11]. Recent studies have found the carbon nanotube (CNTs) as an ideal conductive filler [12–14], since the CNTs exhibit not only exceptional electrical properties, but also excellent thermal and mechanical properties [15,16]. Thus compared with the traditional fillers, the CNTs are able to enhance both the electrical and the mechanical properties of the elastomers [17–19]. One challenge of applying the CNTs in large scales is the large van der Waals interaction among the CNTs bundles, which hinders the uniform and individual dispersion of the CNTs within the polymers [20,21]. The aggregation of the CNTs may also limit their reinforcing effect to the matrix polymers [22]. One feasible and simple approach to improve the CNTs dispersion is to physically attach some compatibilizing molecules on the surface of the CNTs [23– 25]. These foreign compatibilizers are embedded into the CNTs bundles, reducing the interfacial forces among the CNTs and increasing the dispersion of the CNTs. In our previous study [26], it has been found that the chitosan salt had good capability in increasing the dispersion of the CNTs in the HTVSR. Hence, the chitosan salt was used as the compatibilizer to increase the dispersion the CNTs in this study. In the present study, a series of highly stretchable conductive elastomers were fabricated based on the CNTs filled HTVSR nanocomposites. Considering that mixing the HTVSRs with different vinyl contents together was able to increase the mechanical strength of the final composites [27], an HTVSR hybrid was used as the matrix, in which one HTVSR with higher vinyl content and

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the other with lower vinyl content were blended together. The thermal stability, mechanical strength and electrical resistance of the prepared elastomers as a function of the CNTs content were investigated. Furthermore, the resistance changes of the prepared elastomers during stretching and pressing cycles were also investigated and discussed. The results showed that even being stretched to 100%, 200%, or even 300% of their original length for 100 times, the prepared elastomers did not lose their mechanical and electrical properties. Moreover, the resistance of the elastomers also had a good response to the pressure changes. It is indicated that the prepared HTVSR elastomers are potential to be applied in diverse electrical fields.

2. Experimental 2.1. Materials Methyl vinyl HTVSR with vinyl molar contents of 0.20% (HTVSR0.2) and 0.05 (HTVSR-0.05), and 2,5-bis(tert-butyl peroxy)-2,5dimethyl hexane(DBPMH) were supplied by Shenyang Silicon materials company (China). Both the HTVSR-0.2 and the HTVSR0.05 were methyl-terminated transparent polymers and had a molecular weight of around 600,000 g/mol. CNTs (multi-walled), with diameters between 10 and 20 nm, length of 15 um, and purity over 95%, were purchased from Shenzhen Nanotech Port Co. Ltd. (China). Chitosan (CS), with a degree of deacetylation of 95% and viscosity-average molecular weight of 600,000 g/mol, was supplied by Shandong Chitin Powder Factory (China). Hydrochloric acid (37%) and chloroform were purchased from Accuchem (Canada). Deionized-distilled water (DDW) was used exclusively in this study.

2.2. Preparation of CNTs filled HTVSR elastomers The process of preparing the chitosan salt pretreated CNTs was described in detail in our previous study [24]. Typically, the CS (1 g) and 37% HCl (3 mL) were first added into 100 mL DDW. After the mixture was sonicated and stirred for about 30 min, a clear solution was obtained. The solution was then stirred at 50 °C for 24 h, dried in the vacuum oven at 60 °C for another 24 h, and the yellow colored chitosan salt was obtained. The prepared chitosan salt was then mixed with the CNTs (mass ratio: 1/4) afterwards. The mixture was grounded in a mortar, stirred at 90 °C for 1 h, and the chitosan salt pretreated CNTs were finally obtained. The chitosan salt here acted as a kind of compatibilizer to enhance the dispersion of the CNTs in the HTVSR and built an interaction between the CNTs and the HTVSR. The CNTs filled HTVSR elastomers with different CNTs contents were prepared as follows. An HTVSR hybrid was firstly obtained by mechanically mixing HTVSR-0.2 and HTVSR-0.05 (mass ratio: 45/ 55) together. The hybrid was then dissolved in 50 mL chloroform to form a uniform solution. After a certain amount of the chitosan salt pretreated CNTs were added, the resulted solution was sonicated for 30 min. Chloroform was then evaporated at 40 °C and the residue was dried subsequently at 60 °C for 24 h in a vacuum oven. DBPMH, the curing agent was then mechanically blended with the vacuumed mixture and the resulting compound was vulcanized at 170 °C for 15 min. Finally, the HTVSR/CNTs nanocomposites with CNTs weight contents of 4.0 wt%, 6.0 wt%, 8.0 wt% and 11.0 wt% were obtained. For comparison, the pure HTVSR hybrid was also prepared following the similar procedures. All the HTVSR samples were vulcanized before characterizing and testing.

2.3. Characterization The surface morphology the HTVSR/CNTs nanocomposites were observed by scanning electron microscopy (SEM, JEOL SEM 6490). All the samples were cryogenically fractured and coated with a thin layer of gold before being observed. The thermal stability of the HTVSR and the HTVSR/CNTs nanocomposites was characterized in terms of thermogravimetric analysis (TGA) which was measured by a TGA instrument (Mettler Toledo TGA/DSC 1 Simultaneous Thermal analyser) with the temperature increasing from 25 °C to 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere (with a flow rate of 50 mL/min). The X-ray diffraction (XRD) patterns were recorded with a Rigaku Smartlab XRD instrument using Cu Ka radiation source (1.54 Å). The Instron 5566 universal testing machine was used to test the stress–strain behavior of the HTVSR samples, in which the load cell was 500 N. The test was carried out at room temperature, with a constant extension speed of 20 mm/min and gauge length of 20 mm. All the samples were cut into a 40  5 mm rectangular shape with the thickness of 1 mm before testing. The resistances of the HTVSR/CNTs nanocomposites being bended or twisted at different angles were recorded with a Keithley 2010 digital multimeter. All the samples were cut into a 40  5 mm rectangular shape with the thickness of 1 mm before testing. The two-probe method was used and two conductive silver wires connecting to Keithley 2010 were linked to each end of the samples. The resistance change during the stress–strain test was conducted by the Instron 5566 universal testing machine and recorded by the Keithley 2010. The resistance data of the samples was acquired simultaneously as a function of the applied strain. Two conductive silver wires connecting to Keithley 2010 were linked to each end of the samples before testing. The resistance response of the HTVSR nanocomposites under repeated stretching and pressing was recorded utilizing a Keithley 2010 digital multimeter and an Instron 5566 universal testing machine as well. The two-probe method was used to investigate the resistance of all the samples throughout the test and all the samples were 1 mm in thickness. In the stretching test, the HTVSR/CNTs nanocomposites were cut to a 40  5 mm rectangular-shape sample. The gauge length of the Instron 5566 was set at 20 mm and the stretching/relaxing rate was set at 20 mm/min for 100 times. The HTVSR/CNTs samples were stretched to diverse degrees of their original level and released to original level thereafter repeatedly. Before stretching, two conductive silver wires connecting to Keithley 2010 were linked to each end of the testing part of the samples. During stretching/relaxing, Keithley 2010 recorded the resistance change of the samples simultaneously. In the pressing test, the HTVSR/CNTs nanocomposites were cut to a 20  20 mm square-shape sample, and two conductive silver wires connecting to Keithley 2010 were linked to the center of the top and bottom surfaces of the samples, respectively. With the gauge length set at 1 mm, and the compressing/relaxing speed 1.0 mm/ s, a 20  20 mm square-shape compression stage with load cell of 200 N was used to provide axial pressure. The samples were compressed to 0.5 mm and released to 1 mm in thickness for 50 repeated times. Both the stretching clamps and the compression stage were wrapped with parafilms to prevent testing errors. All the resistance testing experiments were duplicated with an observed deviation of less than 5%. The schematic illustration of the stretching and compressing was shown in Fig. 1.

3. Results and discussion The dispersion state of the CNTs in the HTVSR nanocomposites was investigated first. Fig. 2 shows the typical SEM images of the

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Intensity / a.u.

HTVSR/CNTs 11.0 wt%

HTVSR/CNTs 8.0 wt% HTVSR/CNTs 6.0 wt% HTVSR/CNTs 4.0 wt% pure HTVSR hybrid

CNTs 10

20

30

2θ /

40

o

Fig. 1. Schematic illustration of resistance response of the HTVSR samples in stretching and compressing test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. XRD patterns of the HTVSR and the HTVSR/CNTs nanocomposites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fracture surfaces of the HTVSR/CNTs nanocomposites with different CNTs contents. It has been known [26] that the chitosan salt pretreated CNTs were able to disperse in the HTVSR matrix well since the chitosan salt acted as a compatibilizer to increase the interactions between the CNTs and the HTVSR. This result was further verified by the present study. It could be clearly observed from Fig. 2 that the white dots and lines were the broken CNTs and they uniformly distributed within the polymer matrix in all HTVSR/ CNTs nanocomposites with the contents of the CNTs from 4 wt% to 11 wt%. It is fairly important to achieve a uniform dispersion of the CNTs in the polymers when fabricating the CNTs based conductive elastomers since the individually dispersed CNTs strips could maximize their reinforcing effect to the polymers on the thermal, mechanical and electrical properties [28,29]. The uniform dispersion of the CNTs in the HTVSR matrix could be further investigated by the XRD patterns shown in Fig. 3. After the CNTs were incorporated into the HTVSR, the characteristic peak of the HTVSR (11.97°) did not change and no CNTs peaks (26.04° and 43.10°) were observed.

The typical stress–strain curves obtained from tensile test are illustrated in Fig. 4. The detailed tensile property values are listed in Table 1 as well. As shown, the incorporation of the CNTs significantly increased the tensile properties of the HTVSR. When the CNTs content was below 8.0 wt%, the tensile stress, elongation at break and modulus all increased with the increasing loading amount of the CNTs. When the CNTs content increased to 11.0 wt%, the tensile stress and modulus still increased while the elongation at break decreased, which meant more CNTs made the HTVSR more brittle but tougher. The increasing of the tensile strength of the nanocomposites indicated that chitosan salt treated CNTs and HTVSR had very strong interfacial adhesions, and the CNTs helped to transfer some tensile force when the HTVSR/CNTs nanocomposite was stretched. It is also very interesting to notice that when the CNTs content in the HTVSR reached to 6.0 wt% or higher, the nanocomposites could be stretched to more than 200% of their original length, especially for the HTVSR/CNTs 8.0 wt% nanocomposite whose tensile strain was as high as 440%.

Fig. 2. SEM images of (a) HTVSR/CNTs 4.0 wt%, (b) HTVSR/CNTs 6.0 wt%, (c) HTVSR/CNTs 8.0 wt%, and (d) HTVSR/CNTs 11.0 wt%.

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2.0

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Table 1 Tensile properties of the HTVSR and the HTVSR/CNTs nanocomposites. Sample name

Tensile stress (MPa)

Tensile strain (%)

Modulus (MPa)

Pure HTVSR hybrid HTVSR/CNTs 4.0 wt% HTVSR/CNTs 6.0 wt% HTVSR/CNTs 8.0 wt% HTVSR/CNTs 11.0 wt%

0.28 0.61 0.82 1.59 1.67

86.7 144.8 243.2 440.1 241.0

0.30 0.42 0.55 0.55 0.95

This is extremely important since an excellent elasticity is critical for fabricating the conductive elastomer. Strong interactions between the chitosan salt treated CNTs and the HTVSR are also revealed from the enhanced thermal properties. As shown in Fig. 5, all the HTVSR/CNTs nanocomposites were more thermally stable than the neat HTVSR hybrid. Fig. 6 shows the relation between the CNTs content and the electrical resistance of the nanocomposites. The neat HTVSR is electrically insulative whose resistance is beyond the maximum value of our multimeter (higher than 1015 X [30]). Due to prominent electrical conductivity of the CNTs (as high as 106 S/m) [31], the nanocomposites were conductive when the fraction of the CNTs reached to 4.0 wt%. A sharp decrease of the resistance

100

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Fig. 4. Typical stress–strain curves of the HTVSR and its nanocomposites as a function of CNTs contents. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Weight Loss / %

Upper Limit of the Multimeter

12

1.6

500

600

700

800

Temperature / oC Fig. 5. TGA curves of the HTVSR and the HTVSR/CNTs nanocomposites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. The electrical resistance of the HTVSR/CNTs nanocomposites as a function of CNTs content. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in Fig. 6 indicated that effective percolate conducting passages were not formed within the HTVSR when the CNTs content was lower than 4.0 wt%. With the increase of the CNTs concentration, the resistance of the nanocomposites declined correspondingly to different orders of magnitude. When the CNTs content increased to 8.0 wt% or more, no obvious decease in composite resistance was observed, indicating that the CNTs had formed continuous, percolate conducting passages within the HTVSR matrix with relatively stabilized conductivity. The relative resistance change of all the nanocomposite samples when being bended or twisted at different angles was also investigated and shown in Fig. 7. The recorded resistance values were normalized by the samples initial resistance values R0. It could be also observed that when CNTs loading reached to 8.0 wt% or higher, the R/R0 change was very small. This indicates that there is a firm, continuous CNTs conducting network within the host matrix. It is known that the carbon black (CB) is the traditionally used conductive filler for the polymer composites [32,33]. From the above results, it could be seen that compared with the CB filled polymer composites, the CNTs filled polymer composites exhibit a same level of electrical conductivity with much less loading amount [34]. The changes of resistance of the nanocomposites as a function of the applied strain were then investigated and the results are shown in Fig. 8. The resistance values recorded by the Keithley 2010 were normalized by the samples initial resistance values R0 at zero strain. An increase in the resistances of all the elastomers accompanied by incremental elongation was clearly observed. It could be seen that the resistance of the HTVSR/CNTs 4.0 wt% nanocomposite at break point was nearly 10 times as large of its original value, and the HTVSR/CNTs 6.0 wt% with about 4 times (see Fig. 9). This is because the CNTs were not able to form enough conducting passages within the HTVSR matrix. When these nanocomposites were stretched to given degrees, the connection between the CNTs in the matrix became lower, resulting in a large increase in the resistance values. Conversely, when the CNTs contents in the nanocomposites were 8.0 wt% and 11.0 wt%, the resistance change upon strain was much lower. It could be seen from Fig. 9 that the relative resistance of the HTVSR/CNTs 8.0 wt% was a little higher than that of the HTVSR/CNTs 6.0 wt% at break point, which was because the elongation of the former (440%) was much higher than that of the latter (243%). The results just indicated firmly linked CNTs conducting passages were formed within the HTVSR that even at an elongation higher than 400%, the nanocomposite was still

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Fig. 10. The relative resistance changes of the HTVSR/CNTs 11.0 wt% nanocomposite during repeated strain cycling between (a) 0% and 100%, and (b) 0% and 200%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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( o)

Fig. 7. The relative resistance of the HTVSR/CNTs nanocomposites at different bending/twisting angles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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10 41

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Tensile Strain /%

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CNTs content / wt% Fig. 9. The relative resistance of the HTVSR/CNTs nanocomposites at different strains.

electrically conductive. Furthermore, the dependence of the R/R0 on strain was nearly linear (Fig. 8) for the HTVSR/CNTs 11.0 wt% nanocomposite, which again demonstrated the formation of the continuous CNTs conducting passages in the HTVSR.

As discussed before [26], the presence of the chitosan salt played the key role for making the HTVSR/CNTs nanocomposites conductive under high strain. As the compatibilizer, the chitosan salt acted as a bridge to build a strong connection between the CNTs and the HTVSR. The chitosan salt on the one hand reduced the Van der Vaals forces between the CNTs bundles, and on the other hand formed strong hydrogen bonding with the HTVSR. As a result, the CNTs dispersed uniformly within the HTVSR, establishing a well-bound network. The network made the elastomers electrically conductive when being stretched, even at high strains. The reversibility of the electrical resistance of the elastomers to different extents of strains was then investigated. The HTVSR/CNTs 8.0 wt% and the HTVSR/CNTs 11.0 wt% nanocomposites were chosen to apply the test. The HTVSR/CNTs 11.0 wt% was stretched to 100% and 200% elongations, and released to normal state for 100 times. The HTVSR/CNTs 8.0 wt% was stretched to 100%, 200% and 300% elongations and released to normal state for 100 times. The results were shown in Figs. 10 and 11. The resistance values were normalized by the samples initial resistance values R0 at zero strain. Compared with the CB, the CNTs are more able to form stably connected conducting passages within the polymers [35]. Thus it was seen that both the HTVSR/CNTs 8.0 wt% and the HTVSR/CNTs 11.0 wt% nanocomposites represented good conductivity. Even being repeatedly stretched and released for many cycles, the nanocomposites remained conductive and did not lose their mechanical strength and electrical conductivity. However, two elastomers showed different resistance changes upon multiple stretching cycles. For the HTVSR/CNTs 11.0 wt%, since the CNTs concentration is higher, the conducting passages within the HTVSR were harder

S. Shang et al. / Composites: Part A 66 (2014) 135–141

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-150

Fig. 12. The relative resistance changes of the HTVSR/CNTs 8.0 wt% and the HTVSR/ CNTs 11.0 wt% nanocomposites during repeated pressing cycling with the compressed ratio from 0% to 50%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0

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conductivity and mechanical strength even it was stretched to 300% of its original length and released for 100 times. The resistance variation of the HTVSR/CNTs 11.0 wt% and the HTVSR/CNTs 8.0 wt% nanocomposites under repeated compressing and release cycles was then tested, with the results shown in Fig. 12. It was seen that the resistance values of both nanocomposites increased with the gradually loaded pressure and decreased when pressure was unloaded, which was consistent with some previous works [36–40]. Both elastomers showed relatively repeatable and stable resistance developments with the periodically changed pressures, in which the resistance change of the HTVSR/ CNTs 8.0 wt% was much more obvious. This was also ascribed to the conducting CNTs network concentration in the HTVSR matrix. With more CNTs loaded, the conducting CNTs network was much stronger in the HTVSR/CNTs 11.0 wt%, and not easy to break or deform when the pressure was applied. It was also observed that the electrical resistance of both samples was able to reverse to their original values even after 50 cycles, which further indicated that enough conducting CNTs passages were formed in the HTVSR matrix. The overall results above indicate that the prepared HTVSR/CNTs nanocomposites are able to be the conductive elastomer with various application potentials.

600

1.0 1

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41

91

100

Cycles Fig. 11. The relative resistance changes of the HTVSR/CNTs 8.0 wt% nanocomposite during repeated strain cycling between (a) 0% and 100%, (b) 0% and 200%, and (c) 0% and 300%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

to be deformed. Thus the relative strain dependent resistance change was not obvious after 100 times of stretching/releasing. The phenomenon was even obvious when the HTVSR/CNTs 11.0 wt% was repeatedly strained with the elongation of 100%, in which the relative resistance change was relatively stable and repeatable. Comparatively, the resistance changes of the HTVSR/CNTs 8.0 wt% were much more obvious during extension/relaxation cycles, since fewer CNTs conducting passages were easier to be deformed during stretching. However, the nanocomposite still kept electrically conductive even after 100 cycles. Owning excellent properties, the HTVSR/CNTs 8.0 wt% still possessed remarkable

4. Conclusion In summary, a series of the CNTs filled HTVSR elastic nanocomposites were fabricated. With the help of the chitosan salt, the CNTs dispersed well within the HTVSR matrix. The mechanical, thermal and electrical properties were then investigated. The results showed that homogeneous CNTs dispersion enhanced the thermal and mechanical properties of the HTVSR. More importantly, the prepared nanocomposites showed excellent electrical conductivity. Even being stretched to 100%, 200%, or as long as 300% for 100 times, the nanocomposites remained conductive and did not lose their inherent properties. Furthermore, the nanocomposites showed good response and reversibility to repeating pressure cycles. Thus the HTVSR/CNTs nanocomposites prepared in this study have various application potentials in the field of conductive elastomer or pressure sensor. Acknowledgements We gratefully acknowledge the grant (ECS 532712) from the Research Grants Council of Hong Kong, and Mr Lu Gan would like

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