Rheo-Dielectric Behavior of Oligostyrene and Polyisoprene (FUNDAMENTAL MATERIAL PROPERTIES-Molecular Rheology)

Abstract

Flow effects were examined for dielectric behavior of oligostyrene (OS; M = 950) and cis-polyisoprene (PI; M = 8200). OS-1 has monomeric dipoles perpendicular to its backbone and the terminal dielectric relaxation results from the segmental motion, while for PI-8 having parallel dipoles this relaxation reflects the global chain motion. The dielectric loss e” and viscosity h were measured for OS and PI at T well above respective Tg under steady shear flow at rates g << 1/t1, with t1 being the linear viscoelastic terminal relaxation time. The e” and h of PI were independent of g, as usually expected under such slow flow. In contrast, OS exhibited acceleration of the dielectric relaxation and the shear-thinning of h at g << 1/t1. This thinning was related to flow-induced changes in some sort of dynamic structure, probably a cooperative domain structure, and the dielectric change detected acceleration of the segmental motion due to this structural change.