An ESR Study of Oxidation in Uniaxially Deformed Polybutadiene and Polyisoprene

Abstract

Abstract The ESR spectra obtained from diene rubbers following mechanical deformation at cryogenic temperatures in a commercial purity nitrogen atmosphere are composite spectra arising from two or more radicals. The two radicals are thought to be an allyl radical resulting from main-chain rupture and a peroxy radical. The majority of the peroxy radicals observed in the composite spectra are formed as a result of oxygen in the test environment reacting with the allyl radicals formed shortly as chain rupture occurs. However, it appears that a small proportion of the peroxy radicals arise from oxygen incorporated into the rubber network during the processing stage before mechanical testing commences. Only if oxygen is rigorously excluded at all stages of processing and testing is the true spectrum of the primary allyl radical observed following mechanical degradation, showing that the chain rupture site is at the weakest bond, between the α- methylene groups. Oxygenated radical species incorporated during processing can be removed by heat treatment, provided the heat treatment is carried out in an oxygen-free atmosphere. The radical stability vs. temperature tests have shown that the peroxy radical decays at a temperature approximately 20 K lower than the allyl radical, which in turn decays rapidly in the vicinity of the Tg of the rubber. It appears that the anomalous increase in radical concentration near Tg, particularly noticeable in the case of mechanically deformed polymers, possibly arises from stored energy release on warming assisted by the presence of oxygen, oxygen being the major contributor to the process. However, the anomalous effect appears to be complex and other factors may contribute to it. It is generally concluded that the ESR technique offers a sensitive method of observing the oxidation of polybutadiene and other diene rubbers.