Rate-determining factors for enthalpy relaxation of glassy polymers. Molecular weight

In order to obtain information concerning the effect of molecular weight on the molecular mobility involved in the relaxation processes associated with the nonequilibrium thermodynamic state of glassy polymers, enthalpy-relaxation studies have been undertaken on glassy atactic polystyrenes of various molecular weights, ranging from 2.0×103 to 811×103. As was found in previous studies of enthalpy relaxation in organic glasses, the glass transition temperature Tg is the principal rate-determining factor. From a more detailed analysis of the data, however, it is evident that the relaxation processes at corresponding temperature intervals below Tg are a function of the molecular weight. The relaxation rate decreases somewhat with increasing molecular weight and approaches a limiting value at molecular weights in the vicinity of 50×103, the critical molecular-weight range in the molecular-weight dependence of glass transformation. For Aroclor 5460, a nonpolymeric material of lower molecular weight but having a Tg comparable to that of atactic polystyrene of molecular weight 2.0×103, a slightly faster relaxation rate was observed.