Amorphous structure heat: Temperature dependence of heats of solution for polystyrene in toluene and ethylbenzene

When a polymer is dissolved in a solvent, the heat measured is a sum of a polymer-solvent interaction term and a term related to the structure that existed in the solid polymer relative to its amorphous liquid state. This latter contribution, termed the “residual” heat, can have an endothermic contribution due to the fusion of crystalline regions and an exothermic contribution due to the disruption of structure in noncrystalline amorphous regions. For atactic polystyrene between 30 and 110°C, it is shown that the “residual” heat is exothermic, decreases linearly with temperature differences below Tg, and extrapolates to zero in the vicinity of Tg. The existence of an exothermic heat above Tg is probably related to a 160°C transition in polystyrene. This “residual” heat was further observed to be independent of the pressure at which the polystyrene was glassified.