Noncovalent dispersion of multi-walled carbon nanotubes with poly(tert-butyl methacrylate) modified hyperbranched polyethylene for flexible conductive films

Abstract

Carbon nanotubes (CNTs) have been extensively utilized in flexible electronics. However, further applications of CNTs are limited by their poor solubility in solvents. To overcome this obstacle, poly(tert-butyl methacrylate) modified hyperbranched polyethylene (HBPE-g-PtBMA) was employed. The HBPE core was designed to endow HBPE-g-PtBMA with a hyperbranched structure while the methyl groups of poly(tert-butyl methacrylate) provide CH–π interactions between HBPE-g-PtBMA and CNTs. Besides, the relative high melting endotherm of PtBMA segments enabled HBPE-g-PtBMA with feasibility to fabricate practical HBPE-g-PtBMA/MWCNTs conductive films. The bundled MWCNTs were found to be dispersed efficiently by HBPE-g-PtBMA into individual tubes with a maximum concentration of 455 mg L⁻¹. Furthermore, fabrication of a conductive film by spin-coating the stable MWCNTs dispersion onto the PET substrate was explored. The conductive film was found to have good conductivity (13.14 S cm⁻¹) and flexibility, which might have potential applications in flexible electronics.

---

Carbon nanotubes (CNTs) have been extensively utilized in flexible electronics (e.g. conductive films,^1–3 supercapacitors^4–6 and batteries⁷ etc.), due to their excellent electronic and mechanical properties.⁸ However, the poor solubility and difficult manipulation of CNTs in solvents attributed to the van der Waals interactions between nanotubes have imposed great limitations to their further application.⁸ To tackle this problem, either covalent attachment of functional groups onto the CNTs surfaces or noncovalent wrap of the CNTs with various functional molecules have been adopted.⁹ In recent years, the noncovalent methods have attracted much interest because of the feasibility of grafting various functionalities onto the CNTs surface without disturbing their π-conjugated skeleton.¹⁰

Noncovalent modification of CNTs has been mainly focused on specially designed polymers, which exhibit π–π stacking interactions with CNTs. Generally, the polymers with either conjugated side groups (e.g. benzene,¹¹ ferrocene,^12,13 pyrene^14,15 and porphyrin¹⁶ etc.) or conjugated main chains (e.g. poly(3-alkylthiophene),¹⁷ poly(m-phenylene ethynylene)¹⁸ and poly(9,9-dioctylfluorenyl-2,7-diyl)¹⁹ etc.) were employed to disperse CNTs. To synthesize these polymers, however, specially designed monomers and/or sophisticated polymerization techniques were usually required, which limited their large-scale industrial applications. Besides, the solubility of CNTs in organic solvents achieved by the non-covalent type dispersants was generally low.^18,20,21 Thus, synthesis of polymers without specific functionality but with excellent CNTs dispersibility was preferred.²²

Recently, Ye et al. revealed that hyperbranched polyethylene (HBPE) could efficiently disperse CNTs in low-boiling-point organic solvent (e.g. chloroform and tetrahydrofuran) based on the nonspecific CH–π interactions.²³ HBPE, which was synthesized from the commercial abundant ethylene, was well-known for its highly branched architecture (about 100 branches/carbon) and abundant terminal methyl groups. Thus, although without no specific functionality, and the CH–π interaction was much weaker compared to the π–π stacking interactions, HBPE still showed excellent CNTs dispersibility in chloroform. The chain topology, molecular weight and terminal groups of HBPE were found to have significant influence on the solubility of CNTs.²² Besides, dispersion of CNTs in polymer matrix (e.g. ethylene–octene copolymer)^24,25 and preparation of electrified composite films were also explored.²⁶ However, although HBPE showed good CNTs dispersibility, there were still some obstacles to overcome before applying it to practical materials. On the one hand, the choices of polymers to prepare HBPE-containing composites are limited mainly because of the low compatibility of HBPE with polar polymers. On the other hand, HBPE is an amorphous material with a low melting endotherm (−33 °C) contributed by its unique hyperbranched structure.²⁷ Therefore, HBPE was hardly utilized in preparing practical HBPE/CNTs composite materials. In this account, we noted that modified HBPE containing polar segments had better miscibility and thermal properties.²⁸ Thus, the modified HBPE might be a potential alternative to HBPE in the case of preparing practical HBPE-based materials/devices. So far, various functional monomers have been used to synthesize modified HBPE.²⁹ In our pervious report, the ferrocene-containing polymer³⁰ and poly(methacrylic acid)³¹ were utilized to modify HBPE to disperse CNTs in chloroform (CHCl₃) and water respectively. However, in those cases, the concentration of CNTs achieved was still low. Considering that the CH–π interactions between the terminal methyl groups of HBPE and the aromatic rings of CNTs were the key to disperse CNTs by HBPE, hence, tert-butyl methacrylate bearing four methyl groups was chose to synthesize the poly(tert-butyl methacrylate) modified HBPE (HBPE-g-PtBMA) (Fig. 1). The HBPE core was designed to endow HBPE-g-PtBMA with hyperbranched structure while the methyl groups of poly(tert-butyl methacrylate) provided CH–π interactions between HBPE-g-PtBMA and CNTs. Besides, the relative high melting endotherm of poly(tert-butyl methacrylate) enabled HBPE-g-PtBMA with feasibility to fabricate practical HBPE-g-PtBMA/CNTs materials/devices. The dispersing efficiency of the MWCNTs was evaluated and fabrication of HBPE-g-PtBMA/MWCNTs/PET conductive film by spin-coating the stable MWCNTs dispersion onto the polyethylene terephthalate (PET) substrate was also explored. Notably, the conductive film was proved to have good conductivity and flexibility.

Fig. 1 Schematic synthetic procedure of HBPE-Br via chain walking copolymerization and HBPE-g-PtBMA via atom transfer radical polymerization.

HBPE-g-PtBMA (M_n = 145.1 kg mol⁻¹, PDI = 2.8) was synthesized according to the literature.³¹ Then, pristine MWCNTs without additional pretreatment were dispersed by HBPE-g-PtBMA in CHCl₃ via ultrasonication at 35 °C for 1.0 h. Before ultrasonication, the MWCNTs were existed as precipitate in all the cases (Fig. 2A). It seemed that the MWCNTs were dispersed in CHCl₃ after ultrasonication (Fig. 2B). But, actually, if no HBPE-g-PtBMA was added, the MWCNTs were still in bundle. Thus, when the MWCNTs dispersion was filtered through glass wool (0.1 g) via syringe (5.0 mL), the filtrate of pristine MWCNTs was almost colorless and transparent. In contrast, the HBPE-g-PtBMA/MWCNTs dispersions with different mass ratio of HBPE-g-PtBMA/MWCNTs were black and opaque (Fig. 2C) due to the existence of MWCNTs in CHCl₃.

Fig. 2 Photos of MWCNTs dispersions with different mass ratio of HBPE-g-PtBMA/MWCNTs. (a) 0, (b) 0.25, (c) 0.5, (d) 1.0, (e) 2.0 and (f) 5.0. (A) Before ultrasonication, (B) after ultrasonication and (C) after being filtered through glass wool.

The concentration of HBPE-g-PtBMA/MWCNTs dispersion with different mass ratio of HBPE-g-PtBMA/MWCNTs was calculated via combination of TGA and UV-vis technique (Fig. S2†). As shown in Fig. 3a, the concentration of MWCNTs increased as the mass ratio of HBPE-g-PtBMA/MWCNTs increased from 0 to 2.0. However, further increase of the mass ratio resulted in a decrease of MWCNTs concentration. This phenomena might be explained as follow.²³ When the mass ratio was less than 2.0, increasing the amount of HBPE-g-PtBMA contributed to more polymer being wrapped onto the MWCNTs surface, which resulted in higher solubility of MWCNTs. At mass ratio of 2.0, the MWCNTs were almost totally covered by HBPE-g-PtBMA (Fig. S3†). Thus, the maximum concentration (455 mg L⁻¹) of MWCNTs was achieved. Further increase of the polymer content, however, the effect of HBPE-g-PtBMA wrap on MWCNTs solubility became weaker, which might decrease the solubility of MWCNTs. Notably, the maximum concentration (455 mg L⁻¹) of MWCNTs achieved by HBPE-g-PtBMA was comparable to the values obtained by non-covalent method using other specially designed polymers^{11,13,15,18,20–22} (Fig. 3b).

Fig. 3 (a) Concentration of MWCNTs in HBPE-g-PtBMA/MWCNTs dispersions with mass ratio of HBPE-g-PtBMA/MWCNTs varied from 0 to 5.0. The corresponding concentrations of HBPE-g-PtBMA in the HBPE-g-PtBMA/MWCNTs dispersions were 0, 0.25, 0.5, 1.0, 2.0 and 5.0 mg mL⁻¹ respectively. (b) Comparison of the concentration of CNTs in organic solvent using different noncovalent-type dispersants.

The dispersed MWCNTs were further characterized by TEM technique. Obvious CNTs clusters were observed in the pristine MWCNTs dispersion (Fig. 4a) due to the van der Waals interactions between them.⁸ However, majority of the MWCNTs were dispersed into individual nanotubes in the HBPE-g-PtBMA/MWCNTs dispersion (Fig. 4b–f). Notably, homogeneously distributed individual MWCNTs were observed when the mass ratio was above 1.0 (Fig. 4d–f). Besides, according to the magnified TEM image of HBPE-g-PtBMA/MWCNTs dispersion (Fig. S3†), the MWCNTs could be wrapped totally by HBPE-g-PtBMA which contributed to the solubility of MWCNTs. Based on these results, we concluded that HBPE-g-PtBMA was an efficient dispersant for noncovalent dispersion of MWCNTs in CHCl₃.

Fig. 4 TEM images of pristine MWCNTs (a) and MWCNTs dispersed by HBPE-g-PtBMA with different mass ratio of HBPE-g-PtBMA/MWCNTs. 0.25 (b), 0.5 (c), 1.0 (d), 2.0 (e) and 5.0 (f).

The HBPE-g-PtBMA/MWCNTs dispersion was then utilized to prepare conductive film via spin-coating method (Fig. 5). The stable HBPE-g-PtBMA/MWCNTs dispersion was firstly prepared via ultrasonication method. Then, it was spin-coated onto the PET substrate followed by pasting two pieces of copper foils at the end of the film. The surface morphology of the conductive film was characterized by SEM technique. As shown in Fig. S4a and b,† the MWCNTs were homogeneously distributed in the polymer matrix and the continuous MWCNTs networks were also observed, which agreed well with the TEM results.

Fig. 5 Schematic illustration of dispersing MWCNTs in CHCl₃ employing HBPE-g-PtBMA and preparing the conductive film by spin-coating method.

The conductivity of HBPE-g-PtBMA/MWCNTs/PET conductive film was studied by four-probe conductivity meter. The voltages measured in the conductive film was plotted against the current applied (Fig. 6a) and the slop of the linear fitting was utilized to calculate the conductivity of the conductive film. The thickness of the conductive film was measured to be 0.46 μm by eddy current thickness meter. Therefore, according to eqn (S1),† the conductivity of the conductive film was calculated to be 13.14 S cm⁻¹, which was comparable to the previously reported values^32–38 (Fig. 6b). Furthermore, the flexible properties of the conductive film were explored by measuring its relative resistance changes using a universal resistance meter (Fig. S5†) after certain times of bend–release cycles (150°, time interval = 1.0 s). As shown in Fig. 6c, the resistance of the conductive film remained almost unchanged after 200 cycles. However, further increase of the cycle index resulted in an increase of its resistance. About 30% increase of the resistance was observed after 1000 cycles. These results suggested that the conductive film had good flexible property, which might find its potential application in flexible electronics. In order to prove this idea, a simple circuit containing battery (3 V), diode and the conductive film was designed (Fig. 7a and b). As shown in Fig. 7e and f, the conductive film maintained its conductivity even after being bent at about 150°.

Fig. 6 (a) Linear fitting curve of the measured voltages in the conductive film against its corresponding current. (b) Comparison of the conductivity of different CNTs/polymer composite materials. *The conductivity of pure MWCNTs is about 1000–100 000 S cm⁻¹. (c) Relative resistance changes after different times of bend–release cycles.

Fig. 7 (a) Photo of real circuit without conductive film. (b) Schematic conductive film-containing circuit. (c and d) Photos of real circuit with conductive film in normal state. (e and f) Photos of real circuit with conductive film in bent state.

In summary, the poly(tert-butyl methacrylate) modified hyperbranched polyethylene (HBPE-g-PtBMA) was designed to disperse MWCNTs in CHCl₃ and fabricate HBPE-g-PtBMA/MWCNTs/PET conductive film. The HBPE-g-PtBMA was proved to be an efficient noncovalent dispersant for MWCNTs as the maximum concentration (455 mg L⁻¹) of MWCNTs in CHCl₃ was comparable to other noncovalent type dispersant and the bundled MWCNTs was found to be dispersed efficiently into individual tubes. The HBPE-g-PtBMA/MWCNTs/PET conductive film was subsequently fabricated and proved to have good conductivity (13.14 S cm⁻¹) and flexibility, which might find its potential application in flexible electronics.

Acknowledgements

Financial supports from the International Science and Technology Cooperation Project of Ministry of Science and Technology of China (2009DFR40640), the Science and Technology Program of Zhejiang Province (2013C24001 and 2013C31146), the Science and Technology Innovation Team of Ningbo (2011B82002) are gratefully acknowledged.

References

1. C. Feng, K. Liu, J. Wu, L. Liu, J. Cheng, Y. Zhang, Y. Sun, Q. Li, S. Fan and K. Jiang, Adv. Funct. Mater., 2010, 20, 885–891.
2. P. Wan, X. Wen, C. Sun, B. K. Chandran, H. Zhang, X. Sun and X. Chen, Small, 2015, 11, 5409–5415.
3. Y. Wang, H. Yang, H. Geng, Z. Zhang, E. Ding, Y. Meng, Z. Luo, J. Wang, X. Su and S. Da, J. Mater. Chem. C, 2015, 3, 3796–3802.
4. Y. Chen, L. Du, P. Yang, P. Sun, X. Yu and W. Mai, J. Power Sources, 2015, 287, 68–74.
5. F. M. Guo, R. Q. Xu, X. Cui, X. B. Zang, L. Zhang, Q. Chen, K. L. Wang and J. Q. Wei, RSC Adv., 2015, 5, 89188–89194.
6. B. C. Kim, J. Hong, G. G. Wallace and H. S. Park, Adv. Energy Mater., 2015, 5, 1500959.
7. L. Li, Z. P. Wu, H. Sun, D. Chen, J. Gao, S. Suresh, P. Chow, C. V. Singh and N. Koratkar, ACS Nano, 2015, 9, 11342–11350.
8. D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006, 106, 1105–1136.
9. M. Adeli, R. Soleyman, Z. Beiranvand and F. Madani, Chem. Soc. Rev., 2013, 42, 5231–5256.
10. P. Bilalis, D. Katsigiannopoulos, A. Avgeropoulos and G. Sakellariou, RSC Adv., 2014, 4, 2911–2934.
11. W. Zhao, Y. Liu, Q. Feng, X. Xie, X. Wang and X. Ye, J. Appl. Polym. Sci., 2008, 109, 3525–3532.
12. X. W. Mao, G. C. Rutledge and T. A. Hatton, Langmuir, 2013, 29, 9626–9634.
13. Z. Deng, H. Yu, L. Wang and X. Zhai, J. Organomet. Chem., 2015, 791, 274–278.
14. Y. Yan, S. Yang, J. Cui, L. Jakisch, P. Poetschke and B. Voit, Polym. Int., 2011, 60, 1425–1433.
15. Y. Yan, J. Cui, P. Poetschke and B. Voit, Carbon, 2010, 48, 2603–2612.
16. D. M. Guldi, H. Taieb, G. M. A. Rahman, N. Tagmatarchis and M. Prato, Adv. Mater., 2005, 17, 871–875.
17. J. Zou, L. Liu, H. Chen, S. I. Khondaker, R. D. McCullough, Q. Huo and L. Zhai, Adv. Mater., 2008, 20, 2055–2060.
18. Z. Zhang, Y. Che, R. A. Smaldone, M. Xu, B. R. Bunes, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2010, 132, 14113–14117.
19. A. Nish, J. Hwang, J. Doig and R. J. Nicholas, Nat. Nanotechnol., 2007, 2, 640–646.
20. T. Umeyama, K. Kawabata, N. Tezuka, Y. Matano, Y. Miyato, K. Matsushige, M. Tsujimoto, S. Isoda, M. Takano and H. Imahori, Chem. Commun., 2010, 46, 5969–5971.
21. C. S. Lovell, K. E. Wise, J. Kim, P. T. Lillehei, J. S. Harrison and C. Park, Polymer, 2009, 50, 1925–1932.
22. L. Xu, Z. Ye, S. Siemann and Z. Gu, Polymer, 2014, 55, 3120–3129.
23. L. Xu, Z. Ye, Q. Cui and Z. Gu, Macromol. Chem. Phys., 2009, 210, 2194–2202.
24. K. Petrie, A. Docoslis, S. Vasic, M. Kontopoulou, S. Morgan and Z. Ye, Carbon, 2011, 49, 3378–3382.
25. O. Osazuwa, K. Petrie, M. Kontopoulou, P. Xiang, Z. Ye and A. Docoslis, Compos. Sci. Technol., 2012, 73, 27–33.
26. O. Osazuwa, M. Kontopoulou, P. Xiang, Z. Ye and A. Docoslis, Carbon, 2014, 72, 89–99.
27. K. Zhang, J. Wang, R. Subramanian, Z. Ye, H. Lu and Q. Yu, Macromol. Rapid Commun., 2007, 28, 2185–2191.
28. Y. Zhao, L. Wang, A. Xiao and H. Yu, Prog. Polym. Sci., 2010, 35, 1195–1216.
29. Y. Chen, L. Wang, H. Yu, Y. Zhao, R. Sun, G. Jing, J. Huang, H. Khalid, N. M. Abbasi and M. Akram, Prog. Polym. Sci., 2015, 45, 23–43.
30. Z. Deng, L. Wang, H. Yu, X. Zhai, Y. Chen, A. Zain ul and N. M. Abbasi, RSC Adv., 2016, 6, 29663–29668.
31. Z. Deng, L. Wang, H. Yu, X. Zhai and Y. Chen, RSC Adv., 2016, 6, 27682–27689.
32. J. Luan, A. Zhang, Y. Zheng and L. Sun, Composites, Part A, 2012, 43, 1032–1037.
33. D. L. Carroll, R. Czerw and S. Webster, Synth. Met., 2005, 155, 694–697.
34. I. A. Tchmutin, A. T. Ponomarenko, E. P. Krinichnaya, G. I. Kozub and O. N. Efimov, Carbon, 2003, 41, 1391–1395.
35. B. Fugetsu, E. Sano, M. Sunada, Y. Sambongi, T. Shibuya, X. Wang and T. Hiraki, Carbon, 2008, 46, 1256–1258.
36. M. A. Moradi, K. L. Angoitia, S. van Berkel, K. Gnanasekaran, H. Friedrich, J. P. A. Heuts, P. van der Schoot and A. M. van Herk, Langmuir, 2015, 31, 11982–11988.
37. H. M. Xiao, W. D. Zhang, N. Li, G. W. Huang, Y. Liu and S. Y. Fu, J. Polym. Sci., Part A: Polym. Chem., 2015, 53, 1575–1585.
38. P. C. Ma, N. A. Siddiqui, G. Marom and J. K. Kim, Composites, Part A, 2010, 41, 1345–1367.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14757g

---