Intramolecular Contributions to Stretched-Exponential Relaxation Behavior in Polymers

Abstract

Theoretical and experimental evidence for the occurrence of multiple relaxation regimes in polymers far above the glass transition temperature is presented and interpreted in terms of the Kohlrausch- Williams-Watts (KWW) stretched exponential expression. Intramolecular effects responsible for the observation of stretched exponential decay of correlation functions are discussed in relation to four different theoretical models: The kinetic Ising model of Glauber, the Shore-Zwanzig model of linear fluctuating dipoles, the Hall-Helfand model of bistable oscillators, and the dynamic rotational isomeric state model. A decrease in the KWW exponent is predicted with increasing intramolecular coupling. Chain connectivity appears as a major factor underlying the stretched exponential behavior observed in an intermediate time regime covering a time span of 2-3 orders of magnitude. In the short time limit, a single exponential decay of correlations is approached in all models. At long times, on the other hand, different KWW exponents are observed depending on the degree of intramolecular cooperativity. Results of depolarized Rayleigh scattering measurements by Tandem Fabry-Perot interferometry on poly(methylphenylsi1oxane) at high temperatures are observed to be insensitive to variations in temperature and concentration, which lends support to the notion that intramolecular interactions play a fundamental role in ascribing the apparent KWW behavior.