Enthalpy Relaxations and Concentration Fluctuations in Blends of Polystyrene and Poly(oxy-2,6-dimethyl-1,4-phenylene)

A series of enthalpy relaxation measurements were carried out for the pure polymers polystyrene (PS) and poly(oxy-2,6-dimethyl-1,4-phenylene) (PPE) and for homogeneous blends thereof. The data were analyzed using Moynihan's four-parameter approach. For the pure components the best fit parameter values for the simple cooling/heating experiments differ somewhat from those for the annealing experiments at least partly due to thermal lag. The amount of enthalpy relaxation during annealing of the blends turned out to be considerably lower than for the corresponding homopolymers. Moreover, the annealing experiments on the blends could not be fit satisfactorily with the Moynihan model. The first observation agrees with a similar result found by Cowie and Ferguson for blends of PS and poly(vinyl methyl ether). Since this effect is not present for a number of polymer blends involving polymers with comparable glass transition temperatures, it seems to be related to the large difference in glass transition temperatures of the blend components. The presence of concentration fluctuations, with a corresponding range of Tg values, is the most obvious explanation for both observations.