The reinforcement role of carbon nanotubes in epoxy composites with different matrix stiffness Lijie Ci, JinBo Bai * Laboratory of Mechanics of Soils, Structures and Materials, CNRS UMR 8579, Ecole Centrale Paris, 92295 Cha ˆtenay-Malabry, France Received 11 August 2004; received in revised form 3 May 2005; accepted 16 May 2005 Available online 11 July 2005 Abstract This paper systemically evaluates the different reinforcement roles of carbon nanotubes in those nanocomposites with different matrix stiffness while the curing process is controlled. Both mechanical test and microscope observation indicated that such rein- forcement would gradually reduce while increasing the stiffness of matrix. However in the soft and ductile composites, carbon nano- tubes show a significant reinforcement without fracture strain decreasing. The interface interaction is poor between carbon nanotubes and matrix in the stiff composite, and therefore, they have little contribution to the mechanical properties of composite. This research may help to propose a further positive solution for designing and fabricating carbon nanotube–epoxy nanocomposites. Ó 2005 Published by Elsevier Ltd. Keywords: A: Polymer-matrix composites (PMCs); B: Curing; B: Mechanical properties; B: Microstructure 1. Introduction Since carbon nanotubes [1] have proven their novel mechanical and electrical properties [2], researches on carbon nanotube/polymer nanocomposites have become a hot spot in the field [2,3]. Some previous investigations have revealed that the introduction of carbon nanotubes as a structural element in polymer matrix can improve their mechanical properties [4–6], and as conductive filler, they also can modify the electrical conductive property of polymers [7]. Therefore, the introduction of carbon nanotubes into polymers may improve their applications in the fields of reinforcing composites, elec- tronic devices and more. The recent researches have extensively focused on the epoxy resin-based carbon nanotube composites because of their wide applications in the electronics, aeronautics and astronautics industries [7–10]. How- ever, the reinforcement role of carbon nanotubes in epoxy-based composites seems divergent. Some early research results just showed a barely stronger or weak- er carbon nanotube–epoxy composites than the net epoxy [8,11,12]. Enhanced strength was also observed from several recent reports [10,13–15]. Carefully com- paring these controversial results indicates that the stiffness of epoxy matrix seems to be a key point. Car- bon nanotubes in a soft matrix seemed to easily take a significant role in the mechanical reinforcement [13– 15]. However, there is no systemic investigation on the effect of the stiffness of matrix in the carbon nano- tube nanocomposites. In this paper, we systemically evaluate the effect of the stiffness of epoxy matrix to the reinforcement role of carbon nanotubes by control- ling the curing process. Mechanical test and micro- scope observation indicated that the reinforcement role of carbon nanotubes would gradually reduce while increasing the stiffness of matrix. 0266-3538/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2005.05.020 * Corresponding author. Tel.: +33 141131316; fax: +33 141131430. E-mail address: baijinbo@mssmat.ecp.fr (J. Bai). Composites Science and Technology 66 (2006) 599–603 COMPOSITES SCIENCE AND TECHNOLOGY www.elsevier.com/locate/compscitech