Water diffusion and sorption-induced swelling as a function of temperature and ethylene content in ethylene-vinyl alcohol copolymers as determined by attenuated total reflection Fourier transform infrared spectroscopy

Attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy has been advantageously used to carry out a simultaneous study of the effect of temperature on sorption, diffusion, swelling rate, and swelling rate factor of ethylene-vinyl alcohol (EVOH) cast films with three different ethylene contents (29, 38, and 44 mol % of ethylene). While the sorption and swelling levels at equilibrium did not appear to be affected by temperature in the temperature range studied, the effect of increasing ethylene content was seen to largely decrease the sorption-induced swelling. It should be noted that all samples showed significant levels of swelling (similar to 60% in the copolymer with lowest ethylene content), suggesting that films obtained by solution-casting generate polymer morphologies that are far more prone to uptake water than typical melt-extruded ones. It was also observed that increasing the ethylene content led to a reduction of the "effective" D value, while raising the temperature increased diffusion and swelling rate factor. The activation energies obtained for the diffusion of water were relatively low and similar to the typical energy barrier required to break hydrogen bonding interactions, suggesting that water molecules diffuse very easily across the film due to its high chemical affinity with the polymer matrix.