Effect of a low-pressure plasma treatment on water vapor diffusivity and permeability of poly(ethylene-co-vinyl alcohol) and polyethylene films

Poly(ethylene-co-vinyl alcohol) and polyethylene films have been modified by CF4 microwave plasma. The influence of vinyl alcohol comonomer contents on the properties of these materials under CF4 microwave plasma has been studied by means of surface energies, atomic force microscopy (AFM), SIMS and calorimetric measurements (DSC). The CF4 plasma treatment improved the hydrophobicity and the roughness of the film surfaces while the bulk phase in the films were hardly changed. Pervaporation measurements were carried out to characterize the effects of these plasma treatments on water vapor transport through the films. Fluorination of the film surfaces, obtained by the CF4 plasma treatment allows reducing water permeability of the films. The barrier effect to water in plasma treated layers of EVOH was improved significantly, especially with the sample containing 71 mol% of vinyl alcohol comonomer unit and for which an increase of crystallinity was observed. Such a surface modification with fluorinated groups leads to decrease in the water uptake at the film surface at the feed side so that the plasticization effect of the hydrophobic films by the sorbed water molecules was decreased during the water vapor permeation.