Excess Specific Heats in Miscible Binary Blends with Specific Interactions

Abstract

Excess specific heats of different binary blends involving different levels of relatively strong specific interactions have been determined from specific heats of the pure components and those of the blends. A heat-flux (Calvet) calorimeter and a step-by-step methodology have been used. Blends of poly(hydroxy ether of bisphenol A) (phenoxy resin) with polyesters and polyethers, where specific interactions are supposed to play a role in miscibility, and blends with stronger hydrogen-bond interactions, such as poly(vinyl phenol)/poly(methyl methacrylate) (PVPh/PMMA), have been included in the study. Only the experimental excess specific heat for the PVPh/PMMA blend is negative, whereas those relative to phenoxy blends are all positive. These results have been analyzed on the basis of an association model specifically designed for polymer blends in which miscibility is mainly caused by hydrogen bonding. The model predicts, in all cases, a continuous decrease of the enthalpy of mixing with temperature, i.e., a negative value of ΔCp. Equation of state effects have been included in the theoretical simulations, giving modified trends of the enthalpy of mixing with temperature in a more reasonable agreement with the experimental results.