Dynamic rotational isomeric state approach for segmental motions of cis-polyisoprene in the bulk state

Dynamic rotational isomeric state formalism is used to compute dipolar correlation functions for cis-polyisoprene in the bulk state. Transitions between rotational isomeric states take place through coupled motion of triplets of neighboring bonds in a given repeat unit, as follows from the study of intramolecular conformational energetics in the polymer. The intermolecular effect on local chain dynamics is included in two stages. First the relaxation in a homogeneous environment is treated through adoption of a local effective frictional resistance increasing with the size of the kinetic segment. Secondly, free-volume or density fluctuations of the medium are approximated by a bistate environment. The frequency distribution of relaxational modes is found to broaden with increasing number of bonds cooperatively participating in the segmental mode process. From a comparison with recent dielectric measurements of bulk cis-polyisoprene, the experimentally observed Kohlrausch-Williams-Watts (KWW) exponent of 0.39 is attributed to the cooperative relaxation of a kinetic segment of about three repeat units in a fluctuating environment.