COMPARISON OF DIELECTRIC AND VISCOELASTIC RELAXATION SPECTRA OF POLYISOPRENE

Measurements of complex dielectric constant ϵ*, shear viscosity η0, and complex shear modulus G* were carried out on narrow molecular weight distribution cis-polyisoprenes (cis-PI) to determine their dielectric relaxation, g(τ), and viscoelastic relaxation, Η(τ), spectra. For each sample the spectra were compared over a wide range of time scale covering the wedge-type to the box-type region of the viscoelastic spectrum. The viscoelastic relaxation time, τV, agreed with the dielectric normal-mode relaxation time, τn, while the relaxation time at the maximum of the wedge-type spectrum agreed with the dielectric seg-mental-mode relaxation time, τs. In the wedge-type region, the slope of the log Η(τ) vs log τ plot changed at the time scale of the transition from the Rouse-like motions to the segmental motions as τ decreased. For unentangled cis-PIs with a molecular weight, Mw, less than the characteristic molecular weight, Mc, the observed τn agreed with the longest Rouse relaxation time calculated with η0. For entangled cis-PIs with Mw > Mc, the shape of the spectra Η(τ) and g(τ) differed in the terminal region. © 1990, American Chemical Society. All rights reserved.