The high temperature (T > T_g) amorphous transition in atactic polystyrene

Abstract

Abstract We first summarize available experimental evidence indicating the existence of a second‐order transition or relaxation region in atactic polystyrene at 60 to 100°C. above T g . It increases from 137°C. at a molecular weight of 3000 to 192°C. at a molecular weight of 392,000. It appears in such diverse measurements as thermal expansion, heat capacity, x‐ray studies, deformation‐temperature plots, dynamic mechanical loss, stress relaxation, melt viscosity, and DTA. It may not appear if the test method is a very slow one, presumably because it merges with T g . We ascribe this phenomenon to an ability of the entire polymer chain to act as a unit because of the available free volume and enhanced chain mobility at elevated temperatures. This is consistent with the fact that this transition shows up in melt viscosity studies and is completely suppressed by crosslinking. Since this represents a change from one liquid state to another, we designate this transition as T l,l .