The purpose of this study is to investigate the thermo-oxidative degradation behavior of polypropylene (PP) by comparing three types of pristine PP granules (consisting of homopolymer, random copolymer, and block copolymer) with their corresponding oxidized analogues. These analogues were intensely oxidized under oxygen at 90 °C for 1000 h by using the electron spin resonance (ESR) spin trapping method that can detect short-lived radical intermediates during the degradation. The degrees of oxidation could be evaluated by chemiluminescence (CL) intensity, which was related to the concentration of hydroperoxide groups generated in the PP chain. In the pristine PP samples, a small amount of hydroperoxides were found to be formed unintentionally, and their homolysis produces alkoxy radicals, RO•, which then undergo β-scission to yield chain-end aldehydes or chain-end ketones. These oxidation products continue to take part in homolysis to produce their respective carbonyl and carbon radicals. On the other hand, in the oxidized PP granules, because of their much higher hydroperoxide concentration, the two-stage cage reaction and the bimolecular decomposition of hydroperoxides are energetically favorable. Carbonyl compounds are formed in both reactions, which are then homolyzed to form the carbonyl radical species, •C(O)-. PP homopolymer produced the largest amount of carbonyl radical spin adduct; thus, it was found that the homopolymer is most sensitive to oxygen attack, and the presence of ethylene units in copolymers enhances the oxidation resistance of PP copolymers.
Keywords: chemiluminescence; electron spin resonance (ESR); polypropylene (PP); radical intermediates; spin trapping method; supercritical carbon dioxide (scCO2); thermo-oxidative degradation.