Segmental dynamics of individual species in a miscible polymer blend

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The segmental motion of each species in polyisoprene/poly(vinylethylene) (PI/PVE) miscible blends is studied at three different compositions using two-dimensional deuteron exchange NMR. The individual species exhibit widely different mean mobilities and broad mobility distributions near the glass transition of each blend. As PVE content increases, both the difference in mean mobilities between the two species and the width of mobility distribution for both components increase. Such enhancement of these two types of dynamic heterogeneity with PVE content appears to produce the anomalous broadening of the glass transition. The mean reorientational correlation times of each component can differ by two orders of magnitude under identical conditions. The mean correlation times and the monomeric friction coefficients can be described by a common temperature dependence, which differs for individual species. This difference can be described in terms of distinct effective glass transition temperatures, [...], for the two species. The separation between the two effective glass transition temperatures increases with PVE content, consistent with more pronounced thermorheological complexity of blends rich in PVE. The individual [...] also exhibit different compositional dependence from that of the calorimetric [...] of blend. This behavior can give rise to the complex compositional dependence of individual mobilities, apparent when mobilities are compared at the same [...], with respect to the calorimetric [...] of blend. Origins of these two types of dynamic heterogeneity are examined further by using a simple model that takes into account effect of random compositional variations. Although compositional variations can give rise to the observed width of mobility distribution and its compositional dependence, the observed difference in mean mobilities cannot be explained at the same time. This suggests that the observed dynamic heterogeneities can be explained only by including two distinct contributions: local compositional variations in the blend and intrinsic difference in chain mobilities. Distinct dynamic constraints between the two species can arise from their structural differences. In light of phenomenological models of cooperative local dynamics, distinct mobilities can arise from the difference in critical size of free volume or in cooperativity