Effect of ageing atmospheres on the changes in surface free energies of oxygen plasma-treated polyethylene films

Abstract

The changes in the surface properties of oxygen plasma-treated polyethylene films during ageing in various atmospheres (water, dry nitrogen gas, and hexane) were studied from the viewpoint of the interaction of the surface functional groups formed on the films and the ageing media. The XPS (x-ray photoelectron spectroscopy) and the SSIMS (static secondary ion mass spectrometry) spectra indicated the formation of polar groups containing oxygen such as C=O on the film surface. The changes in the critical surface tension (γC) of the film with ageing time were largely affected by the ageing atmospheres: the γC value of the film aged in water increased, and those of the films aged in nitrogen gas and hexane decreased with an increase in ageing time. These different tendencies among the ageing media could be understood reasonably with examining the surface free energy ratios (the total energy, γStot, the dispersion force component, γSd/γStot, the polar component, γSp/γStot, the hydrogen bonding component, γSh/γStot) of the films. The ageing in water of which γL is large gave the films with higher γSp/γStot values, suggeting that the overturn and/or the orientation of the polar groups toward the water phase occurred so as to minimize the discrepancy of the surface free energy between the polymer surface and water. On the other hand, the ageing in nitrogen gas and hexane media of which γL are small gave the films with lower γSp/γStot and γSh/γStot values, suggesting the overturn and/or the orientation of the polar groups into the bulk polymer.