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

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The glass transition temperature (Tg) 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 Tgs can be quantified by fluorescence in blends for which DSC fails to do so, either because the difference in homopolymers Tgs 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 Tgs due to self-concentration effects is not a universal phenomenon in miscible polymer blends with a large Tg contrast. A Tg-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 Tg breadth at ∼85 wt% PPO by DSC but at ∼50 wt% PPO by ellipsometry. Thus, the details of Tg breadth for a multicomponent material can differ significantly between techniques, even when each technique evaluates Tg 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 Tg of well-dispersed, trace levels of one polymer by its blend partner in immiscible blends, fragility is not significantly correlated with Tg perturbations in the form of Tg 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 Tg breadth.