Improvement of carbon nanotubes dispersion by chitosan salt and its application in silicone rubber

Composites Science and Technology 86 (2013) 129–134

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Improvement of carbon nanotubes dispersion by chitosan salt and its application in silicone rubber Songmin Shang ⇑, Lu Gan, Marcus Chun-wah Yuen 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 1 June 2013 Received in revised form 8 July 2013 Accepted 10 July 2013 Available online 24 July 2013 Keywords: A. Carbon nanotubes A. Polymer–matrix composites (PMCs) A. Nano composites

a b s t r a c t We have prepared chitosan hydrochloric acid salt and investigated its assistance in improving the dispersion of carbon nanotubes in different solvents and silicone rubber in this study. It was found that the dispersions of carbon nanotubes in some solvents improved to some extent. Long-time stable dispersions were achieved in water and chloroform, even after being placed for 3 months and a following centrifuging (10,000 rpm for 10 min). It was also found that treated carbon nanotubes could be dispersed in the silicone rubber homogeneously based on SEM and XRD analysis. The incorporation of carbon nanotubes enhanced the thermal stability of the silicone rubber. They also significantly improved the mechanical properties of the silicone rubber matrix. From the FTIR spectra, it was believed that chitosan hydrochloric acid salt adsorbed on the surface of the carbon nanotube and then interacted with silicone rubber matrix, resulting in the enhancement of dispersion of carbon nanotubes. From this study, chitosan hydrochloric acid salt was an easy available material which could be used as the treating agent to enhance the dispersion of carbon nanotubes in solvents and polymer matrixes, with improved thermal and mechanical properties in the resulting nanocomposites. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Ever since the discovery of carbon nanotubes (CNTs) [1], they have received tremendous interest in the field of chemistry, physics, biology, and environment [2,3], due to their distinct properties. Especially in material science and engineering, it has been found that the CNTs even with very low concentration can significantly improve the mechanical, thermal and electrical properties of the final prepared nanocomposites [4–6]. However, one major bottleneck for the practical application of CNTs is the poor dispersibility in solvents and/or polymer matrixes [7,8], which is mainly resulted from the strong Van der Waals interaction among them and their high aspect ratio [9]. Thus, aggregation of CNTs is more probably observed in the polymer matrix or solvents rather than well-dispersed individuals. Generally, using some pre-treatment methods, physically and/ or chemically, could help improve the dispersion of nanoscaled materials [10–13], in which the noncovalent pretreatment is regarded as the most promising method. By adsorbing or mixing with certain amount of polymers, electrolyte, surfactants, and biomolecules [14–17], CNTs have much better dispersibility, with little loss of their inherent properties. Chitosan (CS) is a natural linear polysaccharide which is derived from chitin(the second abundant ⇑ Corresponding author. Tel.: +852 3400 3085; fax: +852 2773 1432. E-mail address: [email protected] (S. Shang). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.07.010

natural biopolymer) by N-deacetylation [18]. Carrying amino and hydroxyl groups along with its backbone, chitosan and its derivatives showed characteristic surface activity and dispersive capacity, which have been applied in emulsification, surface absorbing, drug loading, and dispersion [19,20]. Owing to the amino group in its structure, chitosan showed as a pH responsive polymer [21,22]. Under acidic condition (pH < 6), the amino groups of chitosan were protonated and positively charged; at the same time, protonated chitosan was soluble in water and acted as a kind of polyelectrolyte. This property makes chitosan salt as a potential dispersant of CNTs to improve their dispersion in aqueous and organic solvents and polymer matrix. Although there are several studies investigating the dispersion of CNTs in chitosan aqueous solution at low pH value, few studies have examined the dispersion of CNTs pretreated by chitosan salts in organic solvents and in polymer matrixes in detail. In addition, although chitosan is easily available and has the potential to serve as a treating agent, chitosan salts themselves are scarcely separated out to be used as the treating agent for nanoscale materials. In this study, chitosan hydrochloric acid salt was prepared and used to pretreat the multi walled CNTs (MWCNTs). The dispersion performance of the pretreated CNTs in different solvents was investigated. Some solvents were confirmed in which chitosan hydrochloric acid salt treated CNTs could homogeneously and stably dispersed. The dispersion behavior of pretreated CNTs in polymer matrix was also investigated and methylvinyl silicone rubber

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(SR) was chosen as the polymer matrix to disperse CNTs because of its excellent elasticity and wide use. Results showed that with the adsorption of chitosan hydrochloric acid salt on the surfaces, CNTs could be dispersed uniformly in the final fabricated nanocomposites. In addition, the incorporation of CNTs improved the thermal stability and mechanical properties of the silicone rubber composites. These results indicated that chitosan salt was capable as a pretreatment agent for nanoscaled fillers (CNTs and graphene) to enhance their dispersion in solvents, as well as polymer matrixes with improved mechanical or thermal properties. 2. Experimental 2.1. Materials Chitosan (CS) was supplied by Shandong Chitin Powder Factory (China), which had a degree of deacetylation of 95% and viscosityaverage molecular weight of 600,000 g/mol. MWCNTs were purchased from Shenzhen Nanotech Port Co., Ltd. (China), which had diameters between 10 and 20 nm, length of 15 lm, and purity over 95%. Methyl vinyl silicone rubber (SR) and 2,5-bis(tert-butyl peroxy)-2,5-dimethyl hexane (DBPMH) were supplied by Shenyang Silicon materials company (China). The SR was a kind of transparent rubber with a molecular weight of ca. 6.0  105 g/mol and 0.15% vinyl group (n/n). Other reagents and solvents were all of analytical grade and used without further purification. Deionized-distilled water (DDW) was used exclusively in this study. 2.2. Preparation of chitosan hydrochloric acid salt (CSCl) CSCl was prepared by chitosan and hydrochloric acid in aqueous solution. The CS (1 g) and 10% HCl (10 mL) were added into 100 ml DDW. Then the mixture was sonicated for 0.5 h and stirred for another 10 min. A clear and transparent solution could be got. The solution was stirred at 50 °C and blown with nitrogen for 24 h to obtain a kind of white gel. Then the gel was put into the vacuum oven at 60 °C for another 24 h and the CSCl appearing as light-yellow solid was finally obtained. 2.3. Preparation of SR/MWCNTs nanocomposite In our previous work, MWCNTs were pretreated with ionic liquid(1-butyl-3-methylimidazolium chloride), and this pretreatment could help MWCNTs disperse well enough in polyurethane matrix [16]. The pretreatment of MWCNTs by CSCl in this study followed the previous procedures. MWCNTs were first ground with CSCl (the weight ratio was MWCNTs: CSCl = 4:1) in a quartz mortar and pestle to produce a uniform mixture. The mixture was then stirred gently without any solvent at 90 °C for 1 h to further allow the adsorption of CSCl on the surface of MWCNTs. Finally, the CSCl treated MWCNTs were prepared. The pretreated MWCNTs (with 0.8 g MWCNTs loading) were sonicated in 50 mL tetrahydrofuran (THF) for 1 h to obtain a homodisperse solution. At the same time, SR (10 g) was dissolved in 100 mL THF to obtain uniform solution. Then, the dispersed MWCNTs solution was transferred to SR solution, and the mixture was sonicated at room temperature for another 1 h. THF was evaporated at 50 °C with continuous stirring and the residual mixture was subsequently dried in vacuum at 60 °C for 24 h. Then the curing agent, DBPMH (0.1 g), was mechanically blended with the mixture using a porcelain mortar with pestle and the resulting compound was vulcanized at 170 °C for 15 min. Finally, the SR/ MWCNTs (8.0 wt%) nanocomposite was obtained. For comparison, pure SR, SR/MWCNTs (4.0 wt%) nanocomposite were also prepared, as well as SR/MWCNTs (4.0 wt%) composite in which MWCNTs

were not pretreated. All SR and SR/MWNCTs were vulcanized before testing and characterization. 2.4. Preparation of MWCNTs dispersions To examine the dispersion performance of CSCl treated MWCNTs in different solvents, a certain amount of pretreated MWCNTs (with 0.1 g MWCNTs loading) were added into 20 mL solvent and sonicated for 1 h. Under such way, the concentration of MWCNTs in all solvents was 5 mg/mL. The solvents used to investigate the dispersion of CSCl treated MWCNTs were: n-hexane, toluene, iso-propanol (i-propanol), dichloromethane (CH2Cl2), chloroform (CHCl3), tetrahydrofuran (THF), ethanol (C2H5OH), ethyl acetate (EA), acetone, N,N0 -dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and distilled water. 2.5. Characterization Fourier transform infrared (FTIR) spectrometry (Perkin Elmer 100 spectrophotometer) was used to characterize the CSCl, CSCl treated MWCNTs and silicone rubber nanocomposites at wavenumbers ranging from 4000 to 450 cm1 with a resolution of 4 cm1 and 16 scans. All samples were grounded and dispersed in anhydrous KBr pellets before measurement. The dispersions of CSCl treated MWCNTs in solvents were characterized by UV–Vis absorption spectroscopy (Biochrom Libra S35 UV/Vis Spectrophotometer). The thermogravimetric analysis of the nanocomposites 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. All the measurement were carried out under nitrogen atmosphere (with a flow rate of 50 mL/min). The melting behaviors of the nanocomposites were tested by differential scanning calorimetry (DSC) using a Perkin Elmer Pyris 1 DSC analyzer under nitrogen atmosphere. Samples were heated from 60 °C to 0 °C, maintained at 0 °C for 10 min, and cooled to 60 °C, and heated to 0 °C again. In all heating and cooling cases, the rate was set at 10 °C/min. The morphology of cryogenic fractured surface of the nanocomposite and pretreated MWCNTs were observed by scanning electron microscopy (SEM, JEOL SEM 6490). The samples were coated with a thin layer of gold before observation. The X-ray diffraction (XRD, Rigaku Smartlab XRD) was carried out using Cu Ka radiation source (1.54 Å). The mechanical properties of the nanocomposite were measured by a universal tensile testing machine (Instron 5566) with a 500 N cell at room temperature. The samples were cut into a 50  10 mm rectangular shape with the thickness of

1 Chitosan 2 Chitosan Salt

1

Transmittance /%

130

2

1422 1595 2877 1413 1625 1525

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers / cm

Fig. 1. Fourier transform infrared (FTIR) spectra for pristine chitosan and chitosan hydrochloric acid salt.

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131

Fig. 2. Photographs of 5 mg/mL CSCl treated MWCNTs dispersions in 12 solvents. The photographs were taken just after sonication (top) and 3 months later (bottom).

1 mm. The extension rate was 10 mm/min and the gauge length was 20 mm.

3. Results and discussion

Absorbance / a.u.

The structure of chitosan hydrochloric acid salt was confirmed by FTIR spectroscopy as shown in Fig. 1. After acidification, the band at 1595 cm1 (NAH bending vibration) separated to two bands at 1625 cm1 and 1525 cm1 respectively, which represented the bending vibration of NAH in ammonium salt. At the same time, the peak at 1095 cm1 which represented the CAN stretching vibration shifted to 1076 cm1 (CAN stretching vibration in ammonium salt). The wide band from 3000 to 3600 cm1 is the characteristic absorption of OAH stretching vibration in chitosan and CSCl. FTIR spectroscopy provided the evidence of the formation of CSCl. The dispersions of CSCl treated MWCNTs in different solvents were examined. Fig. 2 shows the photographs of dispersion of treated MWCNTs in all twelve solvents, which were taken just after sonication and 3 months later, respectively. It can be seen from Fig. 2 that dispersion in most solvents was visually homogeneous

1.3 (a) 1.2 1.1 1.0 1 0.9 0.8 0.7 2 0.6 0.5 3 0.4 0.3 0.2 0.1 0.0 300 400

1 H2O 2 CHCl3 3 THF 4 Ethanol 5 CH2Cl2 6 DMSO

7 Acetone 8 Hexane 9 Toluene 10 DMF 11 EA 12 iPropanol

4-12

500

600

700

800

900

Wavelength / nm (b)

(c)

Fig. 3. (a) UV–Vis spectra of 5 mg/mL CSCl treated MWCNTs dispersions in 12 solvents (3 months after sonication); (b) photograph of diluted MWCNTs dispersions in water and chloroform and (c) MWCNTs dispersions after centrifuging test in (1) water, (2) chloroform, (3) tetrahedrofuran and (4) ethanol.

with dark black appearance just after sonication, excluding those in n-hexane, DMF and toluene. However, besides the above three unstable dispersions, MWCNTs precipitated quickly in iso-propanol (i-propanol) and ethyl acetate (EA) only a few days after sonication. In the case of dichloromethane (CH2Cl2), acetone and dimethyl sulfoxide (DMSO), the dispersions displayed a little longer stable phase up to about 1 month while precipitated to different extent afterwards. Meanwhile, MWCNTs dispersions in water, tetrahedrofuran (THF), chloroform (CHCl3) and ethanol (C2H5OH) were observed to demonstrate much better stability without any visible precipitation even lasting for 3 months. The UV–Vis spectra of MWCNTs dispersions in different solvents after 3 months of preparation were given in Fig. 3a. The dispersions were diluted in the same ratio before measurement. It is accepted that the absorbance is proportional to the concentration of CNTs dissolved in the suspension. It could be seen from Fig. 3a that the dispersions of water and chloroform had the most intense absorption, which corresponded well with the best dispersive stability observed from Fig. 2, and the diluted dispersions of these two solvents were virtually like clear solutions (Fig. 3b). It could be also seen from the UV–Vis spectra that the absorbance of tetrahydrofuran and ethanol dispersions were nearly the same as those of the dispersions with poor stability, indicating relatively poorer stability compared with water and chloroform dispersions. This was confirmed by the centrifuging test (10,000 rpm, 10 min), after which water and chloroform dispersions still kept stable while MWCNTs precipitated completely in tetrahydrofuran and ethanol (Fig. 2c). It could be concluded from the photograph and UV–Vis absorption spectroscopy that CSCl treated MWCNTs could form dispersions with long-term stability in water and chloroform and relatively short-term stability in some solvents like ethanol, tetrahydrofuran, etc. CSCl treated MWCNTs were used to prepare silicone rubber nanocomposite afterwards. The dispersion of CSCl treated MWCNTs in silicone rubber was studied first. SEM and XRD results of MWCNTs and SR/MWCNTs nanocomposites were shown in Fig. 4. It could be seen from Fig. 4a that MWCNTs were not damaged after being treated by CSCl. From Fig. 4b and c, it could be seen that CSCl treated MWCNTs were well dispersed in silicone rubber matrix without agglomeration. Moreover, from XRD patterns (Fig. 4d), the characteristic diffraction peaks of MWCNTs were observed at 25.6° and 43.3°, and pure silicone rubber showed a diffraction peak at 9.2° and 44.6°. While as to the nanocomposites, the peaks were almost the same as the pure silicone rubber, which indicated that MWCNTs did not change the structure of silicone rubber and were uniformly dispersed in silicone rubber matrix. TGA was used to study the effect of MWCNTs to the thermal properties of silicone rubber. TGA and the corresponding

132

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(a)

Weight Loss / %

100 4

80 60

2

40 1 1 Pure Silicone Rubber 2 SR/4% CNTs 3 SR/8% CNTs 4 SR/4% CNTs(No Pretreat)

20 0 0

100

200

300

400

3

500

600

700

800

T / oC

1st Devirative / %/oC

0.0 -0.2

o

(b) 482 C

-0.6

-1.0 -1.2

o

463 C 1 Pure Silicone Rubber 2 SR/4% CNTs 3 SR/8% CNTs 4 SR/4% CNTs(No Pretreat)

1

o

469 C

-1.4 -1.6 -100

4 o

-0.4

-0.8

528 C

o

532 C

0

2

3 o

545 C

100 200 300 400 500 600 700 800 o

T/ C

Fig. 5. (a) TGA curves and (b) corresponding DTG curve of pure silicone rubber and SR/MWCNTs nanocomposites.

(d)

MWC CNT Ts 1 MW 2 Pur P re Silico one Ru R bb berr 3 SR SR/8 wtt% CNT Ts 4 SR SR/4 wtt% CNT Ts

Intensity/a.u.

1

2 3 4 10

20

30

2 /

40

50

60

o

Fig. 4. SEM images of (a) CSCl treated MWCNTs; (b) fracture surface of SR/8 wt% MWCNTs (10,000) and (c) SR/8 wt% MWCNTs nanocomposites (30,000), and (d) XRD patterns of SR/MWCNTs nanocomposites.

differential thermogravimetric (DTG) curves of pure silicone rubber and SR/MWCNTs nanocomposites were shown in Fig. 5. It could be very clearly seen from DTG curves (Fig. 5b) that both pure silicone rubber and the nanocomposites incorporated with CSCl treated MWCNTs presented a one-step weight loss process.

Compared with that of pure silicone rubber, TGA curves of the nanocomposites shifted to a higher temperature. When the mass content of MWCNTs in silicone rubber increased to 8%, the temperature of the maximum degradation rate for the nanocomposites could increase 70 °C higher than the pure silicone rubber, which indicated that the incorporation of CSCl treated MWCNTs could improve the thermal stability of the silicone rubber. Conversely, if MWCNTs were not pretreated by CSCl, they were not able to uniformly disperse in the polymer matrix when being incorporated into SR. This might affect the crosslinking of the silicone rubber during vulcanizing process. Consequently, the thermal stability of the final composite was also affected. Consistent with TGA and DTG curves, when non-pretreated MWCNTs were added into SR, the composite showed a three-step weight loss process and the thermal stability did not improve as well. This phenomenon indicated that the non-pretreated MWCNTs damaged the internal structure of SR to some extent. The homogeneous dispersion of MWCNTs in silicone rubber and improved thermal stability of it might be ascribed to the adsorption of the CSCl to the surface of MWCNTs and the following interactions between CSCl and silicone rubber. Fig. 6a showed the FTIR spectra of pristine MWCNTs and CSCl treated MWCNTs. It could be clearly observed that the surface of pristine MWCNTs contained a lot of carboxyl groups (1710 cm1, C@O stretching vibration). However, after MWCNTs were treated with CSCl, the peaks in 1710 cm1 almost disappeared, and the band between 3000 and 3600 cm1 became broader, indicating more AOH groups appeared on the surface of MWCNTs. This was an evidence that CSCl were able to adsorb on the surface of MWCNTs, and this was because of the good interactions between carboxylic acid (ACOOH) on the MWCNTs and ammonium cations (NHþ 3 ) of the chitosan, as well as the cation-p interaction between NHþ 3 and MWCNT surface. This adsorption weakened the C@O peak in MWCNTs, and the hydroxyl groups of the chitosan broadened the AOH peak.

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(a) Transmittance /%

1

Table 1 Tensile properties of pristine silicone rubber and SR/MWCNTs nanocomposites.

1 Pure CNTs 2 CS-Treated CNTs

2

1710

3433

3428

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumbers / cm

(b)

1

Transmittance /%

3436 2 3427

3

3422 1 Pure Silicone Rubber 2 SR/4 wt% CNTs 3 SR/8 wt% CNTs

4000

3500

3000

2500

2000

1500

1000

500

Wavenumbers / cm-1

(c)

Fig. 6. Fourier transform infrared (FTIR) spectra for (a) pristine MWCNTs and CSCl treated MWCNTs, (b) SR/MWCNTs composites, and (c) Schematic illustration of interactions between MWCNTs, CSCl and silicone rubber.

Fig. 6b showed the FTIR spectra of SR/MWCNTs nanocomposites with different content. It could be seen that the OAH stretching vibration band at around 3100–3500 cm1 shifted a little when

Samples

Tensile stress (MPa)

Tensile strain (%)

Modulus (MPa)

Pristine silicone rubber SR/4 wt% CNTs composite SR/8 wt% CNTs composite SR/4 wt% CNTs (No Pretreatment)

0.06 0.25 0.35 0.20

36 85 141 78

0.25 0.45 0.56 0.43

MWCNTs was incorporated into SR. For traditional reinforcement fillers like silica, it was widely believed that [23] silicone molecules absorbed silica surface of through hydrogen bonding between hydroxyl groups on silica surface and the oxygen in the silicone rubber chain. Silicone molecules could not easily adsorb on pure MWCNTs surface due to their strong internal Van der Vaal force and absence of AOH groups along their backbones. In our system, CSCl acted as a bridge between MWCNTs and SR matrix, in which the adsorption of it on the surface of MWCNTs decreased the internal force and hence increased MWCNTs dispersion in solvents; furthermore, CSCl treated MWCNTs owned abundant AOH groups which were able to form good hydrogen bonding interactions with SR matrix. The shift in FT-IR spectra just indicated that the hydroxyl groups of the chitosan which adsorbed on the surface of MWCNTs might have interactions with silicone rubber (Fig. 6c). The interactions made CSCl treated MWCNTs disperse in silicone rubber better and enhanced the thermal stability correspondingly. This also explained why the main peaks of MWCNTs disappeared in SR nanocomposites in XRD patterns. Well interactions gave rise to the mechanical properties of the nanocomposites as well. A typical strain–stress test was done and the results were shown in Fig. 7 and Table 1. It is well known that silicone rubber itself has little tensile strength without the help of reinforcement fillers. This property could be easily seen from Fig. 7 and Table 1, in which pure silicone rubber could bear little tensile force. However, with a small portion of MWCNTs filled into SR, the tensile stress, elongation, and modulus of the SR nanocomposites all increased significantly. What is more, when more MWCNTs were incorporated into the silicone rubber, the mechanical properties of the final nanocomposite increased more remarkably. It could be also seen that untreated MWCNTs could give rise to the mechanical properties as well. However, poor interactions limited their reinforcing ability to SR, compared with CSCl treated MWCNTs. This was also because of the well interactions between CSCl treated MWCNTs and the SR matrix, when more MWCNTs were distributed within SR, they could interact with more silicone rubber molecule chains, and after SR was vulcanized, a firmer network in the final composites was form. This network ultimately enhanced the mechanical properties of the nanocomposites.

0.40

4. Conclusions

Tensile Stress /Mpa

0.35 3

0.30 0.25

2

0.20 4

0.15 1Pure silicone rubber 2SR/4 wt% CNTs composite 3SR/8 wt% CNTs composite 4SR/4 wt% CNTs (No Pretreat)

0.10 1

0.05 0.00

0

20

40

60

80

100

120

140

160

Tensile Strain / % Fig. 7. Typical strain–stress behaviors of pristine silicone rubber and SR/MWCNTs nanocomposites.

In summary, the chitosan hydrochloric acid salt (CSCl) was prepared and used to improve the dispersion of MWCNTs in both solvents and silicone rubber. With the help of CSCl, the dispersions of MWCNTs in different solvents could achieve varying degrees of improvement. In water and chloroform, uniform and long-time stable dispersion of MWCNTs could be obtained. The dispersion of CSCl treated MWCNTs in silicone rubber (SR) was further studied. The SEM and XRD results showed that MWCNTs could be homogeneously dispersed in the polymer matrix without influencing the structure of the SR matrix. CSCl was found adsorbing on the surface of the MWCNTs and improve the dispersion of MWCNTs through interactions between it and the SR. These interactions ulti-

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mately improved the thermal stability and mechanical properties of the final SR/MWCNTs nanocomposites. Acknowledgements We gratefully acknowledge the grant (Project No. 572312) from the Research Grants Council of Hong Kong, and Mr. Lu Gan would like to thank the Research Committee of The Hong Kong Polytechnic University for providing him a scholarship. References [1] Iijima S. Helical microtubulers of graphitic carbon. Nature 1991;354(6348):56–8. [2] Guan TJ, Yao MS. Use of carbon nanotube filter in removing bioaerosols. J Aerosol Sci 2010;41(6):611–20. [3] Shang SM, Zeng W, Tao XM. Investigation on the electrical response behaviors of multiwalled carbon nanotube/polyurethane composite in organic solvent vapors. Sens Actuator B – Chem 2012;166:330–7. [4] Shang SM, Li L, Yang XM, Wei YY. Polymethylmethacrylate–carbon nanotubes composites prepared by microemulsion polymerization for gas sensor. Compos Sci Technol 2009;69(7–8):1156–9. [5] Kim KH, Jo WH. Synthesis of polythiophene-graft-PMMA and its role as compatibilizer for poly(styrene-co-acrylonitrile)/MWCNT nanocomposites. Macromolecules 2007;40(10):3708–13. [6] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube-polymer composites. Carbon 2006;44(9):1624–52. [7] Sahoo NG, Cheng HKF, Li L, Chan SH, Judeh Z, Zhao JH. Specific functionalization of carbon nanotubes for advanced polymer nanocomposites. Adv Funct Mater 2009;19(24):3962–71. [8] Clayton LM, Sikder AK, Kumar A, Cinke M, Meyyappan M, Gerasimov TG, et al. Transparent poly(methyl methacrylate)/single-walled carbon nanotube (PMMA/SWNT) composite films with increased dielectric constants. Adv Funct Mater 2005;15(1):101–6. [9] Girifalco LA, Hodak M, Lee RS. Carbon nanotubes, buckyballs, ropes, and a universal graphitic potential. Phys Rev B 2000;62(19):13104–10.

[10] Chatterjee T, Yurekli K, Hadjiev VG, Krishnamoorti R. Single-walled carbon nanotube dispersions in poly(ethylene oxide). Adv Funct Mater 2005;15(11):1832–8. [11] Xu Y, Shang SM, Huang J. Crystallization behavior of poly (trimethylene terephthalate)-poly (ethylene glycol) segmented copolyesters/multi-walled carbon nanotube nanocomposites. Polym Testing 2010;29(8):1007–13. [12] Bellayer S, Gilman JW, Eidelman N, Bourbigot S, Flambard X, Fox DM, et al. Preparation of homogeneously dispersed multiwalled carbon nanotube/ polystyrene nanocomposites via melt extrusion using trialkyl imidazolium compatibilizer. Adv Funct Mater 2005;15(6):910–6. [13] Wang RX, Tao XM, Wang Y, Wang GF, Shang SM. Microstructures and electrical conductance of silver nanocrystalline thin films on flexible polymer substrates. Surf Coatings Technol 2010;204(8):1206–10. [14] Ortiz-Acevedo A, Xie H, Zorbas V, Sampson WM, Dalton AB, Baughman RH, et al. Diameter-selective solubilization of single-walled carbon nanotubes by reversible cyclic peptides. J Am Chem Soc 2005;127(26):9512–7. [15] Dror Y, Pyckhout-Hintzen W, Cohen Y. Conformation of polymers dispersing single-walled carbon nanotubes in water: a small-angle neutron scattering study. Macromolecules 2005;38(18):7828–36. [16] Shang SM, Zeng W, Tao XM. High stretchable MWNTs/polyurethane conductive nanocomposites. J Mater Chem 2011;21(20):7274–80. [17] Patel N, Egorov SA. Dispersing nanotubes with surfactants: a microscopic statistical mechanical analysis. J Am Chem Soc 2005;127(41):14124–5. [18] Yang XM, Tu YF, Li L, Shang SM, Tao XM. Well-dispersed chitosan/graphene oxide nanocomposites. ACS Appl Mater Interfaces 2010;2(6):1707–13. [19] Poon YF, Bin Zhu Y, Shen JY, Chan-Park MB, Ng SC. Cytocompatible hydrogels based on photocrosslinkable methacrylated O-carboxymethylchitosan with tunable charge: synthesis and characterization. Adv Funct Mater 2007;17(13):2139–50. [20] Elsabee MZ, Morsi RE, Al-Sabagh AM. Surface active properties of chitosan and its derivatives. Colloid Surf B – Biointerfaces 2009;74(1):1–16. [21] Yan LY, Poon YF, Chan-Park MB, Chen Y, Zhang Q. Individually dispersing single-walled carbon nanotubes with novel neutral pH water-soluble chitosan derivatives. J Phys Chem C 2008;112(20):7579–87. [22] Liu H, Wang CY, Zou SW, Wei ZJ, Tong Z. Simple, reversible emulsion system switched by pH on the basis of chitosan without any hydrophobic modification. Langmuir 2012;28(30):11017–24. [23] Cohenaddad JP, Roby C, Sauviat M. Characterization of chain binding ro filler in silicone silica systems. Polymer 1985;26(8):1231–3.