Oxidation of ultra high molecular weight polyethylene (UHMWPE) part 1: Interpretation of the chemiluminescence curve recorded during thermal oxidation

The chemiluminescence curve of polyethylene differs from that of polypropylene even though the oxidation behaviour is similar. The CL curve of PE normally exhibits a double sigmoidal behaviour, whereas PP shows a single sigmoid, and its luminescence intensity is also much lower. It was found, by investigating the build-up of carbonyls and hydroperoxides by means of FTIR, that the first peak coincides with a maximum in hydroperoxide concentration and the second with the build-up of carbonyls. The intensity of the first peak is enhanced by doping the PE with DPA (9,10-diphenylanthracene), which is a chemiluminescence activator, but unaffected by doping the PE with DBA (9,10-dibromoanthracene), which is an energy acceptor. The intensity of the first peak also depends on the presence of carbonyls in the sample. From these observations it is concluded that the CL from PE is a type of activated chemiluminescence, which originates from hydroperoxide decomposition, with carbonyls acting as activator.