Surface engineering of polymers
Ultra-violet photons and non-equilibrium plasmas have been used as modification methods to treat polymer surfaces. The photoreactivity of polystyrene with oxygen and nitrous oxide was found to be linked to the photochemistry occurring at the polymer/gas interface. In situ mass spectrometry studies of the photolysis and photo- oxidation of polystyrene enabled possible mechanisms of these reactions to be determined, and perhaps proved for the first time the presence of intermediate phenyl radicals. Photo-oxidised and plasma oxidised polystyrene surfaces were compared using valence band and core level XPS, and provided another insight into polystyrene surface oxidation processes. The more reactive experimental conditions during plasma treatment were found to produce a less oxidised surface than during photo-oxidation. These observations were attributed to the sputtering characteristics of a plasma environment. Polyethylene and polystyrene were oxidised by plasma treatment and the aging of the resulting surfaces in air was studied by core level XPS, which showed a gradual loss of surface oxidation after treatment. Valence band XPS of the aged polyethylene surface revealed a uniquely selective surface structure. Oxidised polyethylene rearranged upon aging to give a polypropylene-type structure. The relative importance of substrate/gaseous molecule photo- reactivity was again highlighted by comparing chromophoric (polystyrene)/nonchromophoric (polyethylene) polymers with chromophoric (hexamethyldisilane)/nonchromophoric (tetramethylsilane) species. Greatest reaction occured when both chromophoric alternatives were used. Dehydrochlorination of polyvinylchloride was found to depend upon the surrounding atmosphere, vacuum conditions giving the greatest degree of modification. This reaction was monitored by in situ mass spectrometry, and indicated that the rate of dehydrochlorination depended upon the initiation mechanism of dehydrochlorination.