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

Abstract

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.

Gas-Wall Partitioning of Oxygenated Organic Compounds: Measurements, Structure–Activity Relationships, and Correlation with Gas Chromatographic Retention Factor

“…One interpretation of these observations is that gas-wall partitioning involves rapid uptake and equilibration of an organic compound on a timescale of »10 to 100 min due to perturbation and subsequent relaxation of the Teflon polymer structure, followed by much slower continuous uptake on a timescale of »10 to 100 h and greater that occurs as the polymers slowly relax under stress caused by absorbed organic compounds. This mechanism is similar to one proposed by Meares (1958) to explain results of experiments in which first rapid transient and then slow continuous changes in the permeability of allyl chloride in polyvinyl acetate films were observed on timescales similar to these.…”

Gas-Wall Partitioning of Organic Compounds in a Teflon Film Chamber and Potential Effects on Reaction Product and Aerosol Yield Measurements

“…Our results can only be explained by non-Fickian diffusion, which has been observed in many polymer systems (Frisch 1980). The mechanism proposed here has some similarities to one suggested by Meares (1958) to explain his observations of transient permeability of allyl chloride in polyvinyl acetate films. The basic premise is that when large OC molecules penetrate the Teflon film they distort the polymer structure, creating holes that enhance local permeability (OC diffusivity).…”

Organic vapour transport in glassy perfluoropolymer membranes: A simple semi-quantitative approach to analyze clustering phenomena by time lag measurements

“…The free volume approach provides additional understanding of the segmental motions of polymers and, consequently, of the diffusion process inside a polymeric matrix [14,15]. In the case of glassy polymers, the diffusion coefficients for penetrant-polymer mixtures were found to be history (thermal and polymer concentration) dependent for particular systems [16].…”