Comparison of Dielectric and Viscoelastic Relaxation Behavior of Polyisoprene Solutions: Coherence in Subchain Motion

Abstract

Viscoelastic relaxation of polymer chains at long time scales is induced by their global motion. This motion also induces dielectric relaxation if the chains have dipoles parallel along their contour. However, the motion is differently reflected in the viscoelastic and dielectric quantities : The former reflects orientational anisotropy of stress-generating units in the chains (hereafter referred to as subchains) at respective times, while the latter reflects orientational correlation of the subchains at two separate times. Comparison of those two quantities enables us to specify details in the chain motion. In particular, for two extreme cases of incoherent and coherent subchain motion in each chain at short time scales, viscoelastic moduli G * are explicitly calculated from relaxation times τ p and eigenfunctions f p for a local correlation function that represents the orientational correlation of the subchains. G incoh * and G coh * calculated for respective cases are quite different, and comparison with the G * data specifies the coherence of the subchain motion. On the basis of these backgrounds, G * were measured for solutions of a monodisperse cis-polyisoprene (PI-49 ; M = 48.8k) in oligobutadiene (OB-0.7, M = 0.7k) and compared with G incoh * and G coh * . (Dielectrically determined τ p and f p were available for the PI-49 solutions.) G incoh ' agreed with the G' data when the PI concentration c was less than the entanglement concentration c e , but for c > c e significant deviation was observed. These results mean that the subchain motion is incoherent for nonentangled chains but some degree of coherence emerges for entangled chains. However, some incoherence still survived for the entangled chains even in a well-entangled state (M/M e ≃ 10), as evidenced from differences between the G' data and G coh '. These findings were utilized to discuss the observed changes of the G * data with c and further to re-examine conventional interpretation for those changes in terms of the Zimm/Rouse/reptation dynamics.