Component Chain Dynamics in a Miscible Blend of Low-M Poly(p-t-butyl styrene) and Polyisoprene

Dielectric and viscoelastic behavior was examined for a 8/2 (wt/wt) blend of a cis-polyisoprene (PI; M = 19.9×103) and a low molecular weight (M) poly(p-t-butyl styrene) (PtBS; M = 16.4×103). In general, PtBS and PI exhibit the LCST-type phase behavior, but the blend examined was in the miscible region at temperatures examined, T = 30-70 °C. The dielectric loss ε" of the blend at low frequencies was exclusively attributable to the large-scale motion of the PI chains having type-A dipoles. The ε" data obeyed the time-temperature superposition, suggesting that all PI chains in the blend felt the same frictional environment. This result was in contrast to the failure of the superposition previously observed for the same PI chains in a 8/2 miscible blend with a higher-M PtBS (M = 69.5×103). This difference was attributed to a difference of the lifetime of the dynamic heterogeneity due to the concentration fluctuation of PtBS. In the blend with high-M PtBS, the terminal relaxation of PtBS was slower than that of PI and thus the concentration fluctuation was effectively frozen and the PtBS chains behaved as non-uniformly distributed frictional objects during the terminal relaxation of the PI chains. Then all PI chains could not feel the same segmental friction, which naturally resulted in the failure of the time temperature superposition. In contrast, in the blend with low-M PtBS examined in this study, the terminal relaxation was slower for PI than for PtBS. For this case, PtBS behaved as uniformly distributed frictional objects during the terminal relaxation of PI thereby allowing the PI chains to obey the superposition.