Evidence of multi-walled carbon nanotube fragmentation induced by sonication during nanotube encapsulation via bulk-suspension polymerization E.A. Zaragoza-Contreras *, E.D. Lozano-Rodrı ´guez, M. Roma ´ n-Aguirre, W. Antunez-Flores, C.A. Herna ´ ndez-Escobar, Sergio G. Flores-Gallardo, A. Aguilar-Elguezabal Centro de Investigacio ´n en Materiales Avanzados, S.C., Laboratorio Nacional de Nanotecnologı´a, Miguel de Cervantes No. 120, C.P. 31109, Complejo Industrial Chihuahua, Chihuahua, Chih., Mexico 1. Introduction Carbon nanotubes (CNTs) are of great interest due to their potential applications in different fields of science and technology. They offer a combination of mechanical, electrical, and thermal properties that no other material has displayed before (Coleman et al., 2006). The integration of CNT/polymer composites has been focused on the improvement of mechanical and electrical proper- ties of the matrix (Ramanathan et al., 2005; Frankland et al., 2003; Eitan et al., 2006); however, in order to take advantages of such properties CNTs have to be both compatible and intimately dispersed within the polymer host. Nevertheless, due to strong van der Waals forces, CNTs have a great tendency to self-aggregate (Hill et al., 2002; Park et al., 2005). Several strategies have been employed to improve both compatibility and dispersion of CNTs into polymer matrixes (Sandler et al., 2003; Xie et al., 2005; Shen et al., 2005; Lee et al., 2006). Several studies have shown that in situ polymerization is a reliable method for obtaining CNT/polymer composites (Funck and Kaminsky, 2007; Zhao et al., 2005), with the advantage of improving interfacial interaction to impart compatibility and to maximize CNT dispersion. In particular, the techniques of polymerization in dispersed media have been successfully applied to encapsulate nanomaterials into polymer matrixes; with the most successful being miniemulsion polymerization, which provides proper conditions to integrate either organic or inorganic nanoparticles with the polymer matrix (Tiarks et al., 2001; Lo ´ pez- Martinez et al., 2007; Zhang et al., 2005; Erdem et al., 2000). Nevertheless, considering the typical aspect ratios of CNTs (length/ diameter) encapsulation by miniemulsion polymerization seems to be quite difficult; it would be like trying to introduce a baseball bat into a baseball ball. Reports have shown that in composites obtained via miniemulsion polymerization, the interaction between CNTs and polymer particles has been basically the adhesion of the polymer particles along the surface of the CNTs (Park et al., 2005; Vandesvorst et al., 2006; Yu et al., 2005; Ham et al., 2005). Therefore, miniemulsion polymerization or even conventional emulsion polymerization seems to be not the proper techniques to encapsulate CNTs; thus, some other options should be explored. In this study the synthesis and the characterization of multi- walled carbon nanotube/polystyrene composites, via scanning electron microscopy and other techniques, is reported. In situ bulk- suspension polymerization assisted by sonication was used as the technique of synthesis, since the typical diameter of particles obtained through this technique is appropriate to perform an actual encapsulation of multi-walled carbon nanotubes (MWCNTs) into the matrix of polystyrene (PS). Micron 40 (2009) 621–627 ARTICLE INFO Article history: Received 17 September 2008 Received in revised form 12 February 2009 Accepted 12 February 2009 Keywords: Carbon nanotubes Nanocomposite Suspension polymerization Encapsulation ABSTRACT The synthesis of multi-walled carbon nanotube/polystyrene composites, with nanotube concentrations of 0.04, 0.08 and 0.16 wt%, was carried out by in situ bulk-suspension polymerization with the assistance of sonication. By using this method both encapsulation and exfoliation of the nanotubes into the polymer host were achieved. Evidence of significant nanotube fragmentation was found by scanning electron microscopy; the cause of such fragmentation was attributed to the induction of strong cavitation due to the application of ultrasound during the synthesis. Infrared spectroscopy showed no evidence of the formation of covalent bonds between the nanotubes and the polystyrene during the process of synthesis. The thermal stability was not improved by the inclusion of the nanotubes, it was attributed to the low nanotube concentrations; however, composites glass transition temperature showed improvements. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +52 614 439 4811; fax: +52 614 439 1130. E-mail address: armando.zaragoza@cimav.edu.mx (E.A. Zaragoza-Contreras). Contents lists available at ScienceDirect Micron journal homepage: www.elsevier.com/locate/micron 0968-4328/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2009.02.007