Effects of ultraviolet radiation on polymer surfaces

Abstract Supported films of six hydrophobic polymers, three of medium surface energy and three of low surface energy, were exposed to the unfiltered radiation of a mercury lamp. Changes in their contact angle behavior were monitored as a function of exposure time. None of the polymers showed any change in contact angle when irradiated in vacuum, indicating that all changes were due to atmospheric photooxidation. Wavelengths longer than 305 nm were ineffective on five of the six polymers and only slightly effective on the sixth. Wavelengths longer than 340 nm were ineffective in this sixth polymer, poly(methyl vinyl ketone). From the water, methylene iodide, and octane-water contact angles, changes in the dispersion and polar components of the surface-free energy were calculated. The calculations were made by both the Kaelble and the Fowkes-Hamilton methods. In all cases the surface-free energy ( γ s ) and its polar component ( γ s p ) increased. The dispersion component ( γ s d ) of γ s decreased for the three medium energy surfaces but increased for the three low energy surfaces. There is no indication nor any reason to conclude, however, that this difference is general. The Kaelble and the Fowkes-Hamilton methods gave parallel results that differed only quantitatively. The difference between them was greater for the surfaces of higher initial free energy.