Direct detection of effective glass transitions in miscible polymer blends by temperature-modulated differential scanning calorimetry

Individual effective glass transitions of components in miscible blends of polystyrene/poly-(vinyl methyl ether), poly(o-chlorostyrene)/poly(vinyl methyl ether), and poly(styrene-block-n-butyl acrylate)/polystyrene were detected by temperature-modulated differential scanning calorimetry. These blends apparently showed broad single glass transitions in their heat capacity (C-p) curves. This result has been widely accepted as a criterion of miscible mixing of both components. However, the derivative C, curves of some blends showed bimodal peaks. This indicates the coexistence of two relaxations in the miscible blend. Moreover, the ratio of the two peaks was related to the compositions of the blends. The temperatures at the peaks of the derivative Cp curve were taken to be effective glass transition temperatures (T-g,T-A and T-g,T-B) of the components in the blend. T-g,T-A and T-g,T-B were much different from the average glass transition temperature (T-g,T-av) at which the endothermic shift was half in the C-p curve. On the other hand, T-g,T-av and T-g,T-A of homopolymers and poly(styrene-random-n-butyl acrylates were almost coincident with each other. T-g,T-A and T-g,T-B of the blends were in excellent agreement with the effective glass transition temperatures predicted by the model, taking into account the self-concentration effect.