VACUUM-ULTRAVIOLET INDUCED OXIDATION OF THE POLYMERS POLYETHYLENE AND POLYPROPYLENE

The emission from low-pressure microwave plasmas in the vacuum-ultraviolet (VUV) region (lambda < 200 nm) was investigated in order to use these plasmas as light sources for the study of the VUV photochemistry of polyethylene (PE) and polypropylene (PP) as part of the study of plasma-polymer interaction. These polymers, immersed in low-pressure oxygen, were exposed to radiation with wavelengths down to 112 nm, the cutoff of magnesium fluoride used as a window to separate the polymer specimen from the plasma light source. Total oxygen incorporation in the surface [0], and the formation of hydroxyl, carbonyl, and carboxyl groups were measured using XPS in combination with chemical. derivatizations, particularly their dependence upon the radiation spectrum and the oxygen pressure around the sample. In most experiments the surface oxygen concentration [O] attained a constant value that appears to be related to the initial oxidation rate; this suggests a competition between oxygen incorporation and chain scission reactions, followed by the removal of volatile oxidation products. PE is usually oxidized to a higher level than PP, the latter appearing to be more susceptible to reaction with atomic oxygen than PE. A general initiation mechanism for the VUV experiments is proposed that allows us to explain the observed differences in behavior between PE and PP, and the results obtained under different irradiation conditions. The nature of oxidation products is in both cases very similar to what is observed after direct plasma treatment of the polymers. We conclude that short wavelength radiation contributes very appreciably to the observed surface modification effects during plasma treatment of PE and PP. (C) 1995 John Wiley and Sons, Inc.