Entanglement Dynamics in Miscible Polyisoprene / Poly(p-{\it tert}-butyl styrene) Blends

Viscoelastic and dielectric behavior was examined for well entangled, miscible blends of high-$M$ {\it cis}-polyisoprene (PI) and poly(p-{\it tert}-butyl styrene) (PtBS). The dielectric data of the blends, reflecting the global motion of the PI chains having the type-A dipoles, indicated that PI and PtBS were the fast and slow components therein. At high temperatures $T$, the blends exhibited two-step entanglement plateau. The high frequency ($\omega$) plateau height was well described by a simple mixing rule of the entanglement length based on the number fraction of the Kuhn segments. At low $T$, the blend exhibited the Rouse-like power-law behavior of storage and loss moduli, $G' = G'' \sim \omega^{0.5}$, in the range of $\omega$ where the high-$\omega$ plateau was supposed to emerge. This lack of the high-$\omega$ plateau was attributed to retardation of the Rouse equilibration of the PI chain over the entanglement length due to the hindrance from the slow PtBS chains: The PI and PtBS chains were equilibrated cooperatively, and the retardation due to PtBS shortened the plateau for PI to a width not resolved experimentally. A simple model for this cooperative equilibration formulated on the basis of the dielectric data described the viscoelastic data surprisingly well.