Dielectric relaxation of cis-polyisoprene chains in oligo- and polybutadiene matrices: Matrix effects on mode distribution and relaxation time

The dielectric behavior of dilute cis-polyisoprenes (PI) chains was examined in matrices of oligo- and polybutadiene (B) chains of various molecular weights M(B). The PI chains had type-a dipoles parallel along their contour so that their global motion was dielectrically detected. Specifically, the matrix effects were examined for the dielectric mode distribution as well as for the longest and second longest relaxation times, tau(1,epsilon) and tau(2,epsilon). tau(1,epsilon) and tau(2,epsilon) were determined from dielectric loss (epsilon '') peak frequencies for PI chains without and with (symmetrical) inversion of the type-A dipoles. These relaxation times were compared at an isofrictional state: For this purpose, the relaxation times for the PI chains in short matrices of M(B) < 9K (having excess free volume) were corrected with factors zeta(infinity)/zeta with zeta being the segmental friction for PI in those matrices and zeta(infinity) being the zeta value in long matrices of M(B) > 9K. As done by Colby et al. (Macromolecules 1987, 20, 2226), those zeta-correction factors were determined from standard WLF analysis for the time-temperature shift factor a(T). For long PI chains (M(I) congruent to 48K) with and without dipole inversion, the relaxation times hardly depended on M(B) in the nonentangling matrices but increased with M(B) in entangling matrices (in a constraint release regime). On the other hand, tau(1,epsilon) of a short PI chain (M(I) congruent to 6K) was found to be independent of M(B) in both entangling and nonentangling matrices. More importantly, for both long and short PI chains, the dielectric mode distribution was found to be broadened with increasing M(B). This change in the mode distribution was not due to changes in the entanglement effects, the excluded volume interaction, and the hydrodynamic interaction. Instead, coupling of the motion of the PI and matrix chains appeared to have led to the mode broadening. The broadening was completed in a rather narrow crossover zone of M(I) and M(B): In that zone, a ratio of tau(1,epsilon) to the longest (viscoelastic) relaxation time of the matrix, tau(1,G), was not constant but strongly dependent on tau(1,G). This fact indicated that the coupling of PI and matrices was not of simple viscoelastic nature, for which the tau(1,epsilon)/tau(1,G) ratio should be essentially constant in the crossover zone.