DIELECTRIC NORMAL MODE PROCESS OF BIFURCATED LINEAR CIS-POLYISOPRENE

We examined the dielectric normal mode process of a particular type of linear cis-polyisoprenes (PI), referred to as bifurcated linear PI, which are composed of two linear PI arms connected in a head-to-head fashion and have dipoles along the chain contour inverted once at the center. Their behavior was compared with that of regular linear PIs without dipole inversion, and also with that of six-arm star PIs with dipoles diverging from the center to the arm ends. The relaxation time tau-n of bifurcated linear PIs was shorter by a factor of congruent-to 4 than that of regular linear PIs with the same molecular weight, but their relaxation-mode distributions were almost the same, regardless of whether they were in the nonentangled or entangled regimes. These results are consistent with the results of normal-mode analyses on the basis of the Rouse, reptation, and/or configuration-dependent constraint release models. The behavior of six-arm star PIs was significantly different from that of bifurcated linear PIs. In the nonentangled regime, the tau-n of a six-arm star chain was nearly the same as that of a bifurcated linear chain with the same arm molecular weight M(p). However, the relaxation-mode distribution was significantly broader for the former, presumably because of a strong influence of the arm-length distribution on the Rouse-Ham eigenmodes for star chains. In the entangled regime, the tau-n was longer and the mode distribution was much broader for a six-arm star chain than for a bifurcated linear chain of the same M(p). These results suggest a difference in the dynamics of six-arm star chains in entangled and nonentangled regimes.