Rheological behavior of new melt compounded copolyamide nanocomposites

In this paper the rheological behavior of new polyamide-based nanocomposites produced by melt compounding using three different silicate loadings and screw speeds was investigated. The thermoplastic matrices selected were a polyamide 6 and its statistical copolymer having partially aromatic structure, whereas the clay was a commercial organo-modified montmorillonite. Hybrid systems were prepared by means of a laboratory-scale twin screw extruder and were submitted to rheological and structural investigations. The rheological experiments (dynamic frequency sweep, steady rate sweep and stress relaxation tests) were performed to evaluate the effect of both system composition (kind of matrix and clay content) and extrusion rate on the flow behavior of the nanocomposites. Rheology, that is highly sensitive to the nanoscale structure of the materials, put out a pseudo-solid like flow behavior at long times in the hybrids with silicate content higher than 6 wt% and produced with high extrusion rate; this response was related to the formation of an extended structural network across the polymer matrix due to strong polymer-silicate interactions that slow the relaxation times of the macromolecules. Corresponding to this behavior, TEM micrographs have shown a quite uniform dispersion of clay particles on micron-scale and a fair level of silicate exfoliation on nanoscale with a macroscopic preferential orientation of the layers in samples. The rheological measurements also reveal that this flow response is more marked for nanocomposites based on the copolyamide matrix, suggesting that this resin may have a higher silicate affinity respect to polyamide 6 homopolymer. (C) 2004 Elsevier Ltd. All rights reserved.