(742b) Oligomer-Polymer Blends: Rheological Evidence against Dynamic Broadening

There is much work currently being performed to examine the broadening of the glass transition in miscible polymer blends. Here we examine the behavior of a blend of poly(Α-methyl styrene) with its hexamer at concentrations from 0 to 100% by mass. While the calorimetric response, as measured by differential scanning calorimetry (DSC) seems broadened (temperature broadening) the rheological response as represented by G' and G" in the segmental relaxation regime, shows time concentration superposition of the responses. This latter is direct evidence that the segmental dynamics of the blend do not change with concentration, except in absolute time scale but not in shape. Rather, it is necessary to look elsewhere for the explanation. Importantly, the result is contrary to expectations based on dynamical heterogeneity of the response behavior. We examined the response within the framework of classical models of structural recovery and found that the only possible explanation for the apparent contradiction between the temperature broadening and the lack of broadening by the direct dynamical measurements in the segmental relaxation regime lies in the structural recovery parameter x, which is often referred to as the nonlinearity parameter. If x varies as a function of concentration, which we are currently testing as a hypothesis, then the implication is that the effect of structure (fictive temperature or departure from equilibrium) on the relaxation behavior varies as a function of concentration. We discuss the significance of this finding.