COMPOSITIONAL DEPENDENCE OF SEGMENTAL DYNAMICS IN A MISCIBLE POLYMER BLEND

The segmental motion of each species in polyisoprene/poly(vinylethylen e) (PI/PVE) miscible blends is studied at three different compositions using two-dimensional deuteron exchange NMR (2D H-2 NMR). The individ ual species exhibit widely different mean mobilities and broad mobilit y distributions near the glass transition of each blend. As the PVE co ntent increases, both the difference in mean mobilities between the tw o species and the width of the mobility distribution for both componen ts increase. The change in these two types of dynamic heterogeneity wi th PVE content appears to produce the anomalous broadening of the glas s transition. The mean reorientational correlation times of each compo nent can differe by 2 orders of magnitude under identical conditions. This difference can be described in terms of distinct effective glass transition temperatures, T(g), for the two species. The separation be tween the two effective glass transition temperatures increases almost monotonically with PVE content, consistent with the more pronounced t hermorheological complexity of blends rich in PVE. The individual T(g) 's also exhibit a different compositional dependence from that of the calorimetric T(g) of the blend observed by differential scanning calo rimetry (DSC). This behavior can give rise to the complex compositiona l dependence of individual mobilities, apparent when the mobilities ar e compared at the same T - T(g) with respect to the DSC T(g) of the bl end.