Study of transport of small molecules through ethylene- co-vinyl acetate copolymers films. Part A: Water molecules

Abstract

The transport of liquid water through poly(ethylene- co-vinyl acetate) (EVA) films with 4.5%, 19%, 33%, 50% and 70% w/w VA contents, is studied by permeation measurements. A low density polyethylene LDPE, used as reference, is also studied. The diffusion of water is determined from transient permeation. For a better understanding of the role played by the structure and the surface of these copolymers whose crystallinity varies with the presence of VA groups, Differential Scanning Calorimetry and contact angle (surface free energy) measurements are performed. It is found for LDPE and 4.5% w/w VA membranes that the water diffusion coefficient decreases with increasing water concentration, while for 19% VA a constant diffusion coefficient is obtained. For membranes with 33, 50 and 70% w/w VA a diffusion coefficient which increases exponentially with the local water concentration is obtained. These results are explained by the formation at low VA content of small water clusters, clusters evidenced by means of IR spectrometry. On the other hand, the plasticization effect (concentration-dependent diffusion coefficient) is observed for VA content greater than 19% wt. VA is explained by the increase of the amorphous phase ratio, which allows an increase of the number of interactions between polar groups (carboxyl units in the statistical copolymer) and water molecules