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Boron Nitride Nanotube/Polystyrene Composites
Zhi CY*
Department of Physics and Materials Science, City University of Hong Kong, China

ABSTRACT
We report mechanical reinforcement and thermal conductivity improvement of polystyrene by boron nitride nanotubes. The boron nitride nanotubes were synthesized by a chemical vapor deposition method at high temperature. It was demonstrated clearly that the boron nitride nanotubes synthesized can effectively improve both mechanical properties and thermal conductivity of polystyrene.

INTRODUCTION
Since the discovery of carbon nanotubes (CNTs) and the full range of their unique mechanical, thermal and electrical properties, significant efforts have been made to fabricate advanced CNT composite materials which possess a good combination of those. Many polymers, such as polyacrylonitrile, polystyrene (PS), polyvinylalcohol etc., have been used to fabricate CNT reinforced composites [1]. Tensile strength and modulus enhancements have continuously been reported.
A boron nitride nanotube (BNNT) has comparable mechanical properties with a CNT [2]. The theoretical estimation of the elastic modulus of a BNNT gives ~500 to 850 GPa [3], approximately 0.8 times of that of CNT. In many other respects, BNNT possesses advantageous properties as compared with a standard CNT. For example, BNNT is chemically inert and structurally stable. The thermal conductivity of BNNTs has been predicted to be even higher than that of CNTs [4]. In addition, opposed to CNTs, BNNTs do not absorb visible light due to a wide band gap, which reveal an electrically insulating property of BNNTs. These factors should make BNNTs primarily useful as additives in composites to achieve both high thermal conductivity and high insulation [5]. However, the awaiting breakthrough on BNNT/polymer composites was hampered because it is very difficult to fabricate BNNTs in large quantities [6-8]. Here we report the fabrication of PS/BNNT composite and its mechanical reinforcement, as well as thermal conductivity improvement based on highly pure BNNTs grown by a chemical vapor deposition method using metal oxide and boron powder as reactants (BOCVD) [9].
EXPERIMENTAL
The detailed procedure for BNNTs' growth is described elsewhere. The purified BNNTs possess very high purity, as shown in (Figure 1), the scanning electron microscopy (SEM) image of BNNTs synthesized. In order to fabricate a composite, firstly, the BNNTs were mixed with solvents, typically chloroform and N, N-dimethylformamide (DMF). Then, the mixture was sonicated over 200 min to disperse BNNTs. PS was dissolved in the mixture via magnetic stirring. The mixture was then cast into a glass plate and the solvent was completely evaporated under film heating to 60ºC and annealing over 4 hours [10,11].
The composite films were dismounted from the glass plate for ex situ mechanical tensile tests on a RTC-1225A apparatus. Thermal conductivity measurements were performed on a Hot Disk thermal constant analyzer.
Figure 1 SEM image of BNNTs synthesized by BOCVD method.

Figure 1

SEM image of BNNTs synthesized by BOCVD method.

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Figure 2 Thermal conductivity of pure PS and PS/BNNT composite.

Figure 2

Thermal conductivity of pure PS and PS/BNNT composite.

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RESULTS AND DISCUSSION
In contrast to CNT composites, BNNTs/polymer composites possess good transparency. Mechanical reinforcement was achieved with the BNNTs with very high Young's modulus. For the films made with DMF as a solvent, an elastic modulus increase of 7% and 20% was obtained when a 1 wt.% and 3 wt.% BNNTs fractions were used, respectively. It is further proposed here that some better results may be obtained using covalently functionalized BNNTs. In fact, the functionalization can improve the dispersibility of BNNTs without harmful influence on PS. Similar technique is widely utilized for the CNT/polymer composites [12]. A simple calculation predicts that if the external tensile load can be transmitted from matrix to BNNTs ideally, 110% modulus increase can be achieved with 3 wt.% BNNTs fraction, which indicates that BNNT is a very promising material for polymer mechanical enhancement.
The thermal conductivity of PS can also be improved by using BNNT as additive, as shown in (Figure 2). With 5 wt.% BNNTs fraction, the thermal conductivity of PS can be improved from 0.19 to 0.58 W/mK. The fabrication of PS/BNNT composites with higher BNNT concentration for thermal conductivity measurement is still underway.
CONCLUSION
PS/BNNT composite was fabricated for mechanical and thermal property measurements. It was revealed that 1 wt.% BNNT fraction may result in 21% increase in elastic modulus of PS. The thermal conductivity can be tripled with a 5 wt.% BNNT fraction. Our results indicate that BNNT is a very promising material for polymeric composites

References
  1. Andrews R, Weisenberger MC. Carbon nanotube polymer composites.Current Opinion in Solid State & Materials Science. 2004; 8: 31.
  2. Zhi CY, BandoY, Tang CC, Golberg D. Boron nitride nanotubes. Mat Sci Eng R. 2010; 70: 92.
  3. Suryavanshi AP, Yu MF, Wen JG, Tang CC, Bando Y. Elastic modulus and resonance behavior of boron nitride nanotubes. Appl Phys Lett. 2004; 84: 2527.
  4. Wong CP, Bollampally RS. Thermal conductivity, elastic modulus, and coefficient of thermal expansion of polymer composites filled with ceramic particles for electronic packaging. Journal of Applied Polymer Science. 1999; 74: 3396.
  5. Zhi CY, BandoY, Terao T, Tang CC, Kuwahara H, Golberg D. Towards Thermoconductive, Electrically Insulating Polymeric Composites with Boron Nitride Nanotubes as Fillers. Adv Funct Mater. 2009; 19: 1857.
  6. Zhi CY, BandoY, Tang CC, Golberg D. Effective precursor for high yield synthesis of pure BN nanotubes. Solid State Commun. 2005; 135: 67.
  7. Tang CC, Ding XX, Huang XT, Gan ZW, Qi SR, Liu W, et al. Effective growth of boron nitride nanotubes. Chem Phys Lett. 2002; 356: 254.
  8. Ying C, Jin Z, Campbell SJ, Le Caer G. Boron nitride nanotubes:pronounced resistance to oxidation. Appl Phys Lett. 2004; 84: 2430.
  9. Zhi CY, BandoY, Tang CC, Huang Q, Golberg D. Boron nitride nanotubes:functionalization and composites. J Mater Chem. 2008; 18: 3900.
  10. O'Connell MJ, Boul P, Ericson LM, Huffman C, Wang YH, Haroz E, et al. Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping. Chem Phys Lett. 2001; 342: 265.
  11. Zhi CY, BandoY, Tang CC, Honda S, Kuwahara H, Golberg D. Boron nitride nanotubes/polystyrene composites. J Mater Res. 2006; 21:2794.
  12. Berber S, KwonYK, Tomanek D. Unusually high thermal conductivity of carbon nanotubes. Physical Review Letters. 2000; 84: 4613.

Cite this article: Zhi CY (2013) Boron Nitride Nanotube/Polystyrene Composites. JSM Nanotechnol Nanomed 1(1): 1005.
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