Viscoelastic and Dielectric Behavior of a Polyisoprene/Poly(4-tert-butyl styrene) Miscible Blend.

Linear viscoelastic and dielectric measurements were conducted for a blend of polyisoprene (PI, M19.9× 103) and poly(4-tert-butyl styrene) (PtBS, M69.5× 103) with a PI/PtBS composition of 8/2 (w/w). In general, PI and PtBS exhibit the lower-critical-solution-temperature (LCST) type phase behavior. At temperatures examined, T≤ 70 °C, our PI/PtBS blend was in a statically homogeneous state. The PI chain has the so-called type-A dipoles parallel along the backbone, and its large-scale (global) motion activates prominent dielectric relaxation, while the PtBS chain has no type-A dipoles and its global motion is dielectrically inert. In fact, at angular frequencies between 101s-1and 105s-1and at T≤ 70 °C, the dielectric signal of the blend was exclusively attributed to the PI chains therein. The time−temperature superposition failed for the dielectric loss ‘ ‘ of the PI chains, despite the fact that the blend was statically homogeneous. This result suggested that the frictional environment for the global motion was not the same for all PI chains. Namely, the PtBS chains relaxed more slowly than PI (as revealed from comparison of Gand ‘ ‘ data) and their dynamic concentration fluctuation was frozen in the time scale of PI relaxation to give a spatially nonuniform frictional environment for the PI chains. The magnitude of this frictional nonuniformity changed with Tthereby leading to the failure of the time−temperature superposition for PI. In contrast, the superposition worked excellently for the viscoelastic modulus Gof the PtBS chains (obtained by subtracting the PI contribution from the blend modulus). This result suggested that the PI chains relaxing faster than PtBS erased the heterogeneity in the time scale of the PtBS relaxation to provide all PtBS chains with the same frictional environment thereby allowing this relaxation to obey the superposition. [ABSTRACT FROM AUTHOR]