Polymer drying, 12: kinetic evidence for synergistic interaction between polymer relaxation and desorption from polystyrene in a glassy state

The results observed in two additional time studies, which monitored evaporation of polystyrene—carbon tetrachloride systems from saturation to virtual dryness, verify the observation made earlier that the population distribution of molecules entrapped in a glassy state (i.e., fi vs i) reflects sensitively the mode in which that system underwent transition from its rubbery state. In the cases for which CCl4 is the volatile component, however, this transition occurs so rapidly (usually within 10 min) that it is difficult to reproduce the exact pathway through the transition interval, and consequently the fi vs i distribution is affected accordingly, especially the fraction (f1) for the population having the fastest decay rate (i = 1). In all such cases, however, the fraction having the slowest decay rate (kn = ca 10−6 for i = n) is usually greater than fn = 0.25, i.e., it represents about one fourth of the composition αg = 0.3 to 0.4) that marks the start of the glassy state. The escape of these entrapped molecules serves to alleviate accumulation of internal stresses caused by “polymer relaxation” in attaining the thermodynamically (meta)stable end state. Since the rate-constant for decay of the i = n population is less than that for polymer relaxation, the effect of the latter on the former is manifested in the form of stepwise perturbations that are superimposed on the normal logarithmic decrease with time of the number of residual entrapped molecules. This begins only after elimination of the populations having the faster decay rates (i < 6), typically after about two months at room temperature. Thus, the frequency of these stepwise perturbations is a rough measure of the rate of polymer relaxation relative to that for decay of the i = n population.