In this study, two types of multiwalled carbon nanotubes (MCNTs) are used for the preparation of polystyrene (PS)/MCNT composites. For each type of MCNT, two surface modifications, i.e., grafted with PS brushes (MCNTs-PS) and grafted with polyamidoamine (PAMAM) dendrimers and subsequent PS brushes (MCNTs-PAMAM-PS), are applied. The effects of aspect ratio and surface modification of the MCNTs on their dispersion in the PS matrix, as well as the dynamic mechanical properties and supercritical carbon dioxide (scCO(2)) foaming of the composites, are investigated. The results indicate that the MCNTs-PS have a more uniform dispersion and a higher dynamic mechanical property reinforcement compared to MCNTs-NH2 and MCNTs-PAMAM-PS. For the heterogeneous nucleation of MCNTs in PS scCO(2) microcellular foaming, long MCNTs show a small nucleation effect, while the nucleation effect of the short MCNTs is much more significant. Furthermore, the introduction of PAMAM dendrimers on the surface of MCNTs can improve the heterogeneous nucleation efficiency. (C) 2013 Elsevier B.V. All rights reserved.