The effect of molecular weight and orientation on the sorption of n‐pentane by glassy polystyrene

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Abstract
The effects of polymer molecular weight, molecular weight distribution, and orientation on the rate of relaxation‐controlled sorption of n‐pentane by glassy polystyrene were studied. The sorption follows Case II kinetics but for films which sorb slowly the sorption rate increases at relatively long times until sorption is sharply terminated. This rate increase may be explained by the development of dispersed microvoids within the unrelaxed film core. Overshoot of the equilibrium n‐pentane content occurs in sorption experiments in which accelerated sorption is pronounced.The sorption rate is independent of polymer molecular weight and molecular weight distribution per se over a broad wrange of these parameters. Essentially identical vapor sorption kinetics were observed for well annealed polystyrene films of different molecular weights and distributions. Conversely, for vapor sorption by uniaxially oriented films and for liquid sorption by partially annealed films, high molecular weight film (1,880,000) exhibits greater sorption rates than low molecular weight film (ca. 200,000). These differences in rate are not due to molecular weight differences per se, but are a consequence of the dissimilar response of free volume and strain development for films of different molecular weight prepared with a given time‐temperature‐strain history.Crazing of carefully annealed polystyrene films occurs during desorption of n‐pentane from partially saturated films. The depth of craze penetration reflects the point of advance of the discontinuous Case II sorption boundary.