The glass transition in athermal poly(α-methyl styrene)/ oligomer blends

The glass transition behavior in athermal blends of poly(a-methyl styrene) (PaMS) and its hexamer is investigated using differential scanning calorimetry (DSC). The results, along with previous data on similar blends of PaMS/pentamer, are analyzed in the context of the Lodge-McLeish self-concentration model. A methodology is described to partition the calorimetric transition to obtain effective T_gs for each component of the blend. The dependences of these effective T_gs on overall blend composition are described by the Lodge-McLeish model, although the self-concentration effect is less than expected based on the Kuhn length. The length scales of the cooperatively rearranging regions for the two components in the blends are also calculated adapting Donth's fluctuation model to the partitioned DSC transitions and are found to be similar for the two components and show a slight decrease at intermediate concentrations. The kinetics associated with the glass temperature, T_g, is examined by studying the cooling rate dependence of Tg for the pure components and the blends, as well as by examining the enthalpy overshoots in the heating DSC scans. It is observed that the cooling rate dependence of Tg in PaMS/hexamer blends at intermediate concentrations is similar to that of the hexamer, indicating that the kinetics of the glass transition for blends is dominated by the high mobility oligomeric component. Moreover, compared to the pure materials, the PaMS/ hexamer blends exhibit a considerably depressed enthalpy overshoot, presumably resulting from their broader relaxation time distribution.