Dielectric Relaxation of <i>cis</i>-Polyisoprene Chains in Oligo- and Polybutadiene Matrices: Matrix Effects on Mode Distribution and Relaxation Time

Abstract

The dielectric behavior of dilute cis-polyisoprenes (PI) chains was examined in matrices of oligo- and polybutadiene (B) chains of various molecular weights MB. 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, τ1,ε and τ2,ε. τ1,ε and τ2,ε were determined from dielectric loss (ε‘‘) 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 MB < 9K (having excess free volume) were corrected with factors ζ∞/ζ, with ζ being the segmental friction for PI in those matrices and ζ∞ being the ζ value in long matrices of MB > 9K. As done by Colby et al. (Macromolecules 1987, 20, 2226), those ζ-correction factors were determined from standard WLF analysis for the time−temperature shift factor aT. For long PI chains (MI ≅ 48K) with and without dipole inversion, the relaxation times hardly depended on MB in the nonentangling matrices but increased with MB in entangling matrices (in a constraint release regime). On the other hand, τ1,ε of a short PI chain (MI ≅ 6K) was found to be independent of MB 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 MB. 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 MI and MB: In that zone, a ratio of τ1,ε to the longest (viscoelastic) relaxation time of the matrix, τ1,G, was not constant but strongly dependent on τ1,G. This fact indicated that the coupling of PI and matrices was not of simple viscoelastic nature, for which the τ1,ε/τ1,G ratio should be essentially constant in the crossover zone.