Anomalous transport of hydrocarbons in polystyrene

The transport kinetics and equilibrium concentrations of n‐pentane at high penetrant activities in cast, annealed polystyrene were determined and compared with similar measurements in biaxially‐oriented polystyrene. The rate of Case II (relaxation‐controlled) sorption in biaxially‐oriented polystyrene is three to four times faster than the sorption rate in cast, annealed polystyrene. The Case II sorption process in biaxially‐oriented polystyrene is more highly temperature dependent than in cast, annealed film. The higher activation energies coupled with the larger relaxation‐controlled sorption rates in biaxially‐oriented polystyrene imply the involvement of larger polymer segments in the rate controlling polymer relaxations. The sorption in cast, annealed polystyrene was a position‐dependent relaxation controlled transport process; in contrast the sorption in biaxially oriented polystyrene, albeit relaxation‐controlled, was not position dependent. The position dependence of the Case II sorption appears to be a consequence of the presence of residual benzene in the film which accelerates the rate‐determining relaxations. Desorption measurements at very low penetrant activities were quite similar for both biaxially oriented and cast, annealed polystyrene. The desorption kinetics were Fickian and were only a weak function of polymer orientation at these low activities.