Composition-dependent glass transition temperature in mixtures: Evaluation of configurational entropy models*

The composition dependence of the glass transition temperature (Tg) in mixtures remains an important unsolved problem. Here, it is revisited using three model systems: a series of oligomeric and polymeric cyanurates, blends of oligomeric and polymeric α-methyl styrene, and molecular mixtures of itraconazole and posaconazole. We evaluate several entropy-based models to determine the theoretical Tg as a function of molecular composition and compare the results against the experimental data. The assumption that the configurational entropy is invariant at the Tg is tested, where the change in configurational entropy is assumed to be given by the integral of ΔCpdlnT, where ΔCp is the temperature-dependent change in the heat capacity at Tg. We find that, although the temperature-dependent heat capacities in both liquid and glassy states are nearly independent of composition for several of the systems studied (i.e., they are nearly ideal mixtures), the composition dependence of Tg is not well described by simply adding the changes in the mass-weighted configurational entropy of the components on going from the Tg in the pure state to that of the blend. The implication is that either configurational entropy is not invariant at Tg or that it cannot be obtained from the integral of ΔCpdlnT.