Photo-induced radical formation in ethylene-propylene copolymer by electron spin resonance spectroscopy* R. P. Singh, R. Mani and S. Sivaram t Polymer Chemistry Division, National Chemical Laboratory, Pune 411008. India and J. Lacoste and J. Lemaire Laboratoire de Photochimie, Moleculaire et Macromoleculaire, URA CNRS 433, Universit@Blaise Pascal et ENS Chimie de Clermont-Ferrand, 63177A ubire Cedex, France (Received 7 June 1993) The photodegradation of heterophasic ethylene-propylene copolymers and their homopolymers has been investigated by the study of hydroperoxide formation, and by electron spin resonance (e.s.r.) and infra-red spectroscopy. On irradiation of polymer films in air at 60°C with a light of wavelength longer than 300 nm, polymer hydroperoxides are formed. The behaviour of peroxyl radicals generated in the samples on irradiation of polymer hydroperoxides with ultraviolet light at - 50°C in vacuum has been identified and compared with a combination of e.s.r, and derivatization reactions. From Fourier transform infra-red spectroscopy the structure of peroxyl radicals formed is identified. (Keywords: radical formation; ethylene-propylene copolymer; electron spin resonance) INTRODUCTION Procedure Ethylene-propylene (E-P) copolymer is an outstanding The methods of purification, fractionation and sample material widely used in various industrial fields. Such preparation (,-~ 100~tm thick film) have already been copolymers may have a range of properties, from stiff reported 3. The copolymer EPQ 30R was fractionated as thermoplastics to soft elastomers, depending upon the fractions A and B (EPQ 30R-A and EPQ 30R-B). The composition of the polymer. Several authors 1'1 have molar percentages of ethylene in the copolymer samples attempted to interpret free radicals produced in poly- EPQ 30R, EPQ 30R-A and EPQ 30R-B are 15.1, 7.7 and olefins upon radiolysis; most of the studies have employed 40.9, respectively. All samples were irradiated in a Pyrex very low temperatures in an attempt to freeze out the e.s.r, tube at ambient humidity in a SEPAP 12/24 subsequent reactions of the initial radicals. However, the photoirradiation chamber for 140h at 60°C and at exact mechanism ofphotodegradation and radical forma- 2>~290nm. The details of this equipment are given tion in E-P copolymers has not been fully elucidated, elsewhere 3'4. The preirradiated film (0.012g) was sealed Both the chemical composition of the copolymer as well under high vacuum in a Pyrex e.s.r, tube and inserted in as conditions of photodegradation are likely to affect the the e.s.r, cavity for irradiation from a 1000 W xenon arc photodegradation processes, at -50°C. The xenon arc emission was filtered with In this paper, we report our results on the effect of u.v. 305 nm quartz filter to eliminate wavelengths below light on the photodegradation of E-P copolymer and the 305 nm. The distance between the xenon source and the homopolymers of ethylene and propylene by the study sample was set at 15cm. of hydroperoxide formation and electron spin resonance Macromolecular hydroperoxides formed on the photo- (e.s.r.) on free radicals produced after irradiation under oxidized samples during SEPAP irradiation (140h)were vacuum of preoxidized films, with a view to elucidating titrated by iodometry s (refluxing in 10:1 isopropanol/ the mechanism of photodegradation and identifying the acetic acid with excess sodium iodide). The blank reagents macroperoxyl radicals, were handled in the same way. 2ROOH + 21 - ~ ROH + 12 EXPERIMENTAL 12 + I- ~ 13 Materials Any I3 formed was measured spectrophotometrically at Commercialsamples of isotacticpolypropylene(i-PP); 355nm (extinction coefficient of I3 assumed to be IPCL, Baroda, India), linear low density polyethylene 25 0001 tool- x cm- 1). (LLDPE; Du Pont, Canada, Sclair)and heterophasic The e.s.r, measurements were performed with a ethylene-propylene copolymer (EPQ 30R; Himont Italia) Bruker Electron Paramagnetic Resonance 200 instrument were used in this study, equipped with liquid nitrogen temperature-controlled * NCL Commun. No. 5701 device. Spectra were measured at 12 mW power when peroxyl tTo whom correspondence should be addressed signals did not saturate. After e.s.r, measurements, 0032.3861/94/07/1382-04 © 1994 Butterworth-Heinemann Ltd 1382 POLYMER Volume 35 Number7 1994