The role of clay network on macromolecular chain mobility and relaxation in isotactic polypropylene/organoclay nanocomposites Ke Wang a , Si Liang a , Jinni Deng a , Hong Yang a , Qin Zhang a , Qiang Fu a, * , Xia Dong b , Dujin Wang b, * , Charles C. Han b a Department of Polymer Science and Materials, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, PR China b Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received 13 June 2006; received in revised form 24 July 2006; accepted 24 July 2006 Available online 28 August 2006 Abstract It is well known that a so-called ‘‘three-dimensional filler network structure’’ will be constructed in the polymer/layered silicate nanocompo- sites when the content of layered clay reaches a threshold value, at which the silicate sheets are incapable of freely rotating, due to physical jam- ming and connecting of the nanodispersed layered silicate. In this article, the effect of such clay network on the mobility and relaxation of macromolecular chains in isotactic polypropylene(iPP)/organoclay nanocomposites was investigated in detail with a combination of DMTA, DSC, TGA, TEM, rheometry and melt flow index measurements. The main aim is to establish a relationship between the mesoscopic filler net- work structure and the macroscopic properties of the polymer nanocomposites, particularly to explore the role of the clay network on the mobility and relaxation of macromolecular chains. It was found that the nanodispersed clay tactoids and layers play less important or dominant roles on the mobility of iPP chains depending on the formation of percolating filler network. The turning point of macroscopic properties appeared at 1 wt% organoclay content. Before this point, the effect of organoclay can be negligible, and the increase of chain mobility was ascribed to the decrease of molecular weight of polymer chains, as commonly occurs during dynamic melt processing; after this point, however, a reduced mobility of chains and a retarded chain relaxation were observed and attributed to the formation of a mesoscopic filler network. The essential features of such a meso- scopic organoclay network were estimated and discussed on the basis of stress relaxation and structural reversion measurements. A schematic model was proposed to describe the different relaxation and motion behaviors of macromolecular chains in the unfilled polymer and the filled hybrids with partial and percolated organoclay networks, respectively. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: iPP/organoclay nanocomposite; Clay network; Chain mobility 1. Introduction Recently, structureeproperty relationships in polymer/lay- ered silicate nanocomposites (PLSNs) have become of interest to academia and industry [1e4]. In many previous reports, the improved properties of PLSNs, such as higher mechanical strength, enhancement of modulus, high thermal stability, low gas barrier property, increased electronic conductivity and ex- cellent optical transparency, were commonly attributed to the structural characteristics, including the intercalated/exfoliated microstructures [5] and the good compatibility or strong inter- action of layered silicate with basal polymer [1,6e9]. On the basis of investigations of the intrinsic structures of PLSNs, it was proposed that a so-called three-dimensional filler network structure will form when the content of layered clay reaches a threshold value, at which the silicate sheets are incapable of freely rotating, due to physical jamming and connecting of the nanoscale dispersed fillers [10]. The network is comprised of randomly oriented clay tactoids with locally correlated * Corresponding authors. Tel.: þ86 28 85460953; fax: þ86 28 85405402 (Q.F.); tel.: þ86 10 82618533; fax: þ86 10 82612857 (D.W.). E-mail addresses: qiangfu@scu.edu.cn (Q. Fu), djwang@iccas.ac.cn (D. Wang). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.07.067 Polymer 47 (2006) 7131e7144 www.elsevier.com/locate/polymer