Diffusion of gases and vapors through polyvinyl chloride‐polyvinyl acetate copolymer films I. Glass transition effect

The specific volume of a polyvinyl chloride‐polyvinyl acetate copolymer was studied as a function of temperature. It was found that two second‐order transition temperatures exist, one at 30°C. and the other at 77°C. The lower transition temperature has not been reported previously. Diffusion coefficients for several gases of varying van der Waals diameters were measured at several different temperatures and the activation energies for diffusion calculated. It was found that the activation energies were exponential functions of the gas diameters. For the smaller‐sized gases, plots of log diffusion coefficient against 1/ T unexpectedly did not change slope at either of the two transition temperatures. On the other hand, the transport processes for carbon dioxide and water vapor were affected by the glass transition temperatures of the copolymer. For the former the slope changed at the lower transition temperature of 30°C. In contrast, the water vapor diffusion coefficients increased normally with temperature until the second transition temperature of 77°C. was reached. At this point the diffusion coefficients exhibited a marked increase with temperature. An explanation for the anomalously high diffusion values for water vapor is offered.