Diffusion of water vapor in polymers

Abstract

AbstractStudies have been made by a sorption‐desorption procedure on the diffusion of water vapor at relative humidities of from 30 to 85% into the polymers, polyvinyl acetate cellulose acetate (37.9% acetyl), cellulose nitrate (12.0% N), 6–10 nylon, and polyvinyl alcohol. Diffusion of water in polyvinyl acetate obeys Fick's law at temperatures of from 30 to 50°C. and also at 22°C., which is below the second‐order transition. The energy of activation for diffusion of water into polyvinyl acetate is 15 kcal. per mole and at 40°C. the diffusion coefficient is 7 × 10−6 cm.2/min. In contrast to the results with organic vapors, the diffusion coefficient for water is found to be independent of concentration of water in the polyvinyl acetate. Similar behavior is found for diffusion of water at 40° into cellulose acetate, cellulose nitrate, and 6–10 nylon. The chief difference is that for these three polymers the diffusion departs mildly from Fick's law at the later stages of some of the sorption and desorption experiments. This generally uncomplicated diffusion of water in these three polymers contrasts with the markedly anomalous diffusion found for organic vapors such as methanol and acetone into cellulose acetate and nitrate. With polyvinyl alcohol the diffusion of water is distinctly anomalous and closely resembles the non‐Fickian diffusion of organic vapors in cellulose acetate and nitrate. One consequence is that the rate of sorption and desorption of water in polyvinyl alcohol is considerably slower than in the other polymers and is also concentration dependent.