Electron Spin Resonance Study of Polyethylene Irradiated with Ultraviolet Light

Abstract

ESR Spectrum of polyethylene irradiated with ultraviolet light at −196°C was composed of a superposition of two six line spectra, whose total widths were about 155 and 110 gauss, respectively. Free radicals responsible for these two spectra are –CH2–CH–CH2– and –CH2–CH2. The latter radicals were very reactive and abstracted hydrogen atoms from other parts of polymers to produce former radicals. During the warm-up process, a sharp singlet spectrum appeared at about −125°C, which was assigned to acyl radicals . On further warming allylic free radicals were observed at the temperatures above nearly −30°C. An increase of signal intensity was observed during the warm up process.The primary process of photo-induced reactions in UV-irradiated polyethylene at −196°C was thought to be a Norrish-type I reaction of the carbonyl group, and a production mechanism of free radicals is proposed.The absorption spectrum of polyethylene film in O2 showed greater absorbance than that in N2, which could be attributed to the formation of charge-transfer complexes of polyethylene with oxygen molecules. It was also shown that the charge-transfer complexes participated in radical formation. Photo-induced conversions of acyl radicals and peroxy radicals were also reported.