Diffusion of allyl chloride in polyvinyl acetate. Part II. The transient state of permeation

The permeation rate of allyl chloride through polyvinyl acetate films has been studied as a function of time during the transient period preceding the establishment of steady states of permeation. It was found that two distinct processes could be distinguished. When the ingoing pressure of allyl chloride was changed from a value at which a steady state had been established to a new value, there was a gradual change in the permeation rate to a new steady value. Depending upon the initial and final pressures this change required from several hours to several weeks. When the initial ingoing pressure was small or zero there was a large and immediate increase in the permeation rate when the ingoing pressure was increased. This rapid rate decreased over periods which ranged from about three hours at low ingoing pressures to a few minutes for higher ones, and died away before the more gradual increase in rate had begun to take effect. Using the equations of Frisch, the time lags for transition between pairs of steady states have been calculated, from previously determined sorption and diffusion data, and compared with the observed values. The calculated values were usually smaller than the observed ones. The discrepancy was most serious at low vapor concentrations and decreased with increasing concentration. For concentrations greater than 1 × 10 −3 mole cm. −3 there was a close agreement. For comparable concentrations, the discrepancy was less marked for decreasing than for increasing ingoing pressure and was also less for thin membranes than for thicker ones. These observations are consistent with the theory that the swelling of the membrane which accompanies the sorption of vapor gives rise to elastic stresses which slowly decay by viscous movements within the polymer. The initial jump in permeation rate proved more difficult to study. As far as could be determined it occurred instantaneously. When plotted against logarithmic time, S‐shaped curves were obtained very similar in appearance to elastic stress decay curves observed for polymers and predicted theoretically for a series of Maxwell units with a wide distribution of relaxation times. This suggests that the high permeability may arise from lateral strains set up when an instantaneous swelling takes place at the ingoing face of the membrane on the admission of more vapor. The instantaneous stress then decays by elastic motions of segments of polymer molecules, over a period determined by the temperature and initial sweller concentration, and so reduces the high permeation rate. For initially unswollen polyvinyl acetate at 40° the time for half‐decay of the permeation rate was agout 25 minutes for a membrane 32 μ thick. It was found that the peak permeation rate increased as successive experiments were performed with the same membrane. This suggested that permeation through a membrane clamped in a diffusion cell produced orientation parallel to the membrane surface. It has not yet been possible to confirm this.