AN EXPERIMENTAL INVESTIGATION OF THE RELATIONSHIP BETWEEN MOLECULAR STRUCTURE AND LENGTH SCALES IN INELASTIC DEFORMATION OF AN AMORPHOUS THERMOPLASTIC

摘要

Experimental observations have indicated that the presence of strain gradients has an influence on the inelastic behaviour of polymers as well as in other materials such as ceramics and metals. A brief review is presented on experimental results showing strain-gradient effects in both thermoplastic and thermoset polymers. The present study has experimentally quantified length scale effects in inelastic deformations of the polymer material polystyrene with respect to the molecular length. The experimental technique that has been used is nano-indentation to various depths with a Berkovich indenter. The hardness has been calculated with the method by Oliver and Pharr, and also by direct measurements of the indented area from atomic force microscopy. The experiments showed that the length scale effects in inelastic deformations exist in polystyrene at ambient conditions. The direct method gave a smaller hardness than the Oliver-PhaiT method. It was also showed that the length scale parameter according to Nix and Gao increases with increasing molecular weight. For high molecular weights above a critical value of entanglement, there was no pertinent increase in the length scale parameter. The length scale parameter for strain-gradient plasticity has a size of around 0.1μm for polystyrene.