Dielectric relaxation in concentrated solutions of cis-polyisoprene. Part 1.—Effect of entanglement on the normal-mode process

Abstract

Dielectric measurements were carried out on concentrated toluene solutions of narrow molecular-weight distribution cis-polyisoprenes (cis-PI) with molecular weight of 1.6 × 10³ to 5 × 10⁴. Two loss maxima were observed in the temperature range 200–400 K. The low-frequency process was assigned to the normal mode process due to the fluctuation of the end-to-end distance of the cis-PI molecules; and the high-frequency process to the segmental mode process related to the glass transition of the polymer. The molecular-weight dependence of the relaxation time τ_n for the normal mode process at a fixed concentration was found to be similar to the viscoelastic relaxations: below the characteristic molecular weight M_c, τ_n∝M² but in the range above M_c, τ_n∝M^4.3. The M_c varied in proportion to the inverse of concentration. The high-frequency segmental mode process exhibited the relaxation time τ_s almost independent of the molecular weight. The time–temperature superposition principle was applied to construct master curves of the dielectric loss factor ε″ over a wide frequency range. The ε″ curves broadened with increasing molecular weight and concentration in the high frequency side of the curves. However, the half width of the loss curves was almost independent of the molecular weight and concentration.