Molecular friction coefficients in polymers and their temperature dependence

Abstract

Analysis of the temperature dependence of viscoelastic and dielectric relaxation processes in several individual polymers by the equation of Williams, Landel, and Ferry provides values of the fractional free volume at the glass transition temperature and its thermal expansion coefficient which are rather similar in different systems and of reasonable magnitude, with one exception (poly-n-butyl methacrylate). The friction coefficient per monomer unit atT g , derived from viscoelastic data, varies from 103 to 106 dyn-sec/cm; it is believed to depend on the magnitude of intermolecular forces as well as the disposition of local free volume in the packing of chain segments, which will be affected by detailed geometry even though the average fractional free volume atT g is apparently not much affected. The friction coefficient of a foreign molecule of comparable size to a chain monomer unit, derived from diffusion measurements, is similar in magnitude to that of a chain unit for polyisobutylene 96° aboveT g ; but is very much smaller than that of chain unit for polyvinyl acetate only 12° aboveT g . The temperature dependences of these quantities are similarly related. It is suggested that a foreign molecule is provided with local excess free volume which is relatively unimportant when the average free volume is considerable (well aboveT g ) but greatly reduces the friction coefficient when the average free volume is small (nearT g ).