Residual stresses in rapidly quenched various amorphous polymers

As large polymeric parts are quenched rapidly, the surface cools more quickly than the core, creating thermal stresses across the sample thickness. Because the stresses in the hotter portions relax quickly, the net effect of this complicated thermal and mechanical history is to produce residual stresses (manifested as residual birefringences) in the final sample. The present work has to do with the study of residual birefringence profiles in three amorphous polymers; polystyrene (PS), polymethylmethacrylate (PMMA) and polycarbonate (PC).

The analysis of these profiles was done with the simplified theory of Aggarwala and Saibel, which is a special case of the more general theory due to Lee, Rogers and Woo. In this theory one must have a single stress optical coefficient (or strain optical coefficient) for the particular material in question. While there are extensive rheo-optical data for PS, there are only limited data for PMMA and PC. The necessary coefficients were obtained by making independent measurements of the stress optical coefficients at various temperatures and by using elastic moduli taken from the literature. The optical coefficients vary sharply with temperature in the neighborhood of the glass transition temperature (Tg). It was found empirically if one uses a coefficient somewhat in the neighborhood of Tg, a reasonable approximation of the observed profiles can be predicted, at least at the higher initial sample temperatures for PMMA and PC, and at the lower initial temperature for PS.