T_g and T_g breadth of poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene miscible polymer blends characterized by differential scanning calorimetry, ellipsometry, and fluorescence spectroscopy

The glass transition temperature (T_g) behavior of bulk, miscible poly(2,6-dimethyl-1,4-phenyleneoxide)/polystyrene (PPO/PS) blends was studied by differential scanning calorimetry (DSC), ellipsometry, and fluorescence. Previous studies have shown that two underlying component T_gs can be quantified by fluorescence in blends for which DSC fails to do so, either because the difference in homopolymers T_gs is too small or one component is at trace levels. However, all methods show only a single glass transition in PPO/PS blends, indicating that the existence of two effective T_gs due to self-concentration effects is not a universal phenomenon in miscible polymer blends with a large T_g contrast. A T_g-broadening effect is quantified in PPO/PS blends by both DSC and ellipsometry, but the glass transition breadth is relatively small, smaller even than for some homopolymers. The PPO/PS blends show a maximum T_g breadth at ∼85 wt% PPO by DSC but at ∼50 wt% PPO by ellipsometry. Thus, the details of T_g breadth for a multicomponent material can differ significantly between techniques, even when each technique evaluates T_g breadth on a basis grounded in fundamental quantities (temperature dependence of heat capacity or of thermal expansivity). While fragility is useful in understanding perturbations to T_g of well-dispersed, trace levels of one polymer by its blend partner in immiscible blends, fragility is not significantly correlated with T_g perturbations in the form of T_g breadth in strongly miscible PPO/PS blends with coupled dynamics, e.g., neat PPO has a high fragility relative to some blend compositions but a much smaller T_g breadth.