Segmental relaxation and molecular structure in polybutadienes and polyisoprene

The glass transition dispersion in the mechanical spectrum was measured for polybutadienes of various vinyl content and for polyisoprene. The breadths of spectra were found to parallel the concentration of pendant vinyl groups on the main chain. Consistent with results on many other materials, the shape of the spectra were well described by the Kohlrausch-Williams-Watts function; consequently, the segmental relaxation behavior can be specified by the dependence of the fractional exponent on molecular structure. It was determined that the spectral shapes correlate with the temperature dependence of the time-tem-perature shift factors. Specifically, segmental relaxations that are more intermolecularly cooperative exhibit larger shift factors, indicating a relationship between time and temperature dependencies. The spectral bandwidths were not associated with structural diversity within the polymer backbone. The behavior can be interpreted using the coupling model of relaxation. This model identifies the origin of the Kohlrausch form for the spectral dispersion and provides a plausible physical picture. The model's prediction of a correspondence between the spectralshape and the temperature dependence of the shift factors is corroborated. Introduction in the transition zone only at longer times. Near their glass transition temperature, polymers exhibit a dispersion in their mechanical response. Independently