Permeation, Diffusion and Solution of Gases in Films of Ethylene-Vinyl Acetate Copolymers and Their Hydrolyzed Films

Abstract

Ethylen-vinyl acetate copolyrners (EVAc) containing 11.2 and 13.8 mol % of vinyl acetate unit were used throught this work. After purifying by a sequence of dissolutions in toluene and precipitations with methanol, the EVAc films having uniform thickness of 60 a to 160 a were prepared from toluene solutionsby evaporating the solvent on a glass plate at room temperature. The hydrolyzed films (EVAl) were obtained by suspending the EVAc films in a methanol solution of sodium methoxide at 0-400C Partially hydrolyzed films which consist of three layers (EVAl-EVAc-EVAl) were also prepared from the EVAc films by adjusting the reaction conditions of hydrolysis.
The transmission of helium, hydrogen, carbon dioxide, oxygen, argon and nitrogen was studied in the films of EVAc, EVAI and EVAl-EVAc-EVAl. By using a time-lag method, the permeability coefficients, thediffusion coefficients and solubility coefficients were measured at 10-.-60'C. The data of the EVAl films were compared with those obtained in the EVAc films. Both the permeability coefficients and the diffusion coefficients of the EVAL films at constant temperature were much lower than those of the EVAc films. For example, the permeation and diffusion results for carbon dioxide, oxygen, argon andnitrogen in the EVAl film derived from the EVAc film containing 13.8 mol % of vinyl acetate were about 1/50 of the values in the EVAc film. However, the apparent energies of activation for the permeation and diffusion in the EVAl film were almost eqaul to those of the EVAc film and polyethylene. No increase in crystallinity was observed upon hydrolysis, so the reduction of permeation and diffusion was ascribed to the effect of chemical composition changes, namely to the immovability of polymer chains due to the formation of hydrogen bonding. Barrie, et al. reported that PID', solubility coefficients (S') of gases in laminated films, which were obtained from the apparent permeability coefficients (P) and the apparent diffusion coefficients (D') were different from the solubility coefficients(S) which were obtained from an equation of the equilibrium solution17. However, the solubility coefficients (S') of helium and nitrogen in the three layered film, EVAl-EVAc-EVAl, were found to be almost equal to the solubility coefficients(S). This was assumed to be due to the similarity of the solubility coefficientsof these gases and the large difference of their permeability coefficients in the EVAc film and the EVAl film.