The free radicals forming during the mechanical destruction of polyethylene and polypropylene

Abstract

THE free radicals created by irradiation and the mechanical destruction of polyethylene (PE) or polypropylene (PP) had been studied by electron paramagnetic resonance (1-6]. Two types of radical were detected during the low-temperature mechanical destruction of linear PE, i.e. those with a free valency at the end, --CH2--CH 2 (I) [6], and in the centre of the polymer chain, --CH2--CH--CH ~ (II) [5]. The radicals of type II also formed during the radiolysis of PE [1]. The structure of the radicals in irradiated PP is the subject of lively discussion [2-4]. Those produced by the mechanical destruction of PP were not investigated. The work reported here deals with the analysis of the EPR spectra of free radicals in the decomposition products of PP; their structure was established and their mutual reaction (in PP and PE) examined at low temperatures. The measurements were made using samples of isotactic and atactic PP, and of linear PE (less than 0.5% branching), as kindly supplied by the laboratory of N.M. Chirkov, Institute of Chemical Physics, U.S.S.R. Academy of Sciences. The fraction of ataetic PP was obtained by extraction from a bulk polymer; the polymer samples were washed free from catalyst and purified by repreeipitation. PP and PE were mechanically dispersed in a micro blender made of glass at 80-100°K under vacuum, or in an inert gas atmosphere. The EPR spectra were recorded on radio spectrometer EPR-2, IKhF at 77°K. The apparatus and method were described in greater detail elsewhere [7]. Figure la shows the spectrum after the dispersion of isotactic PP for a short time. The signal consists of 8 components of hyperfine structure (HFS) with an average split of 23:~1 oersted. A temperature elevation to 140°K produced an irreversible change of the signal into one with 6 components with the same split (Fig. lb); the radical concentration fell during this change by not more than 20%. A similar 6-component signal was obtained after longer dispersal of PP. The EPR spectra of isoand atactic PP samples were identical. The radical of PP decayed without change of the EPR spectrum at 290°K. The signal of the allyl radicals remained at this temperature in the spectrum of irradiated PP [2].