The radiation degradation of polymers

Abstract

The effects of radiation on poly(olefin sulphone)s, polyethylenes, and polystyrene have been examined, particularly with reference to the parameters of polymer structure and irradiation environment.A series of olefins have been reacted with sulphur dioxide to form copolymers with their 1:1 alternating structure confirmed by infrared, 13C n.m.r. and 1H n.m.r. spectral analysis in conjunction with elemental analysis. The molecular weight distribution was investigated by GPC, light scattering, viscosity and osmometry experiments. The volatile products of radiolysis have been identified and quantitatively studied for irradiations at 0-150°C and for 0-10 Mrad doses. The polymers degraded rapidly into the original monomers - G(total gas) = 10 to 10,000 depending on olefin structure and temperature of irradiation, G(olefin)/G(SO2) rises from 0,1-0,5 at 0°C to approach unity at high temperatures. The intermediates, free radical and cationic, have been analysed by (1) studying the effects of scavengers on the radiation yields and (2) e.s.r. spectral studies. A general mechanism has been proposed which includes (A) free radical and cationic depropagation routes after C-S bond scission, (B) olefin homopolymerization initiated at cationic sites in the irradiated polymer and (C) isomerization of olefin during radiolysis via the cationic intermediate (e.g. significant yields of 2-methyl-2-butene from poly(3-methyl-1-butene sulphone) radiolysis). The influence of configurational and steric interactions - most important at low temperatures - have also been considered. Studies of (1) the thermal degradation of the copolymers and (2) the radiolysis of dialkyl sulphones as low molecular weight analogs of the copolymers, have also been completed for comparison with copolymer radiolysis results.Polyethylene is a generic name given to a variety of polymers in which gradations in crystallinity, density and related properties depend primarily on the frequency of short chain branches. Ethane and butane are the chief low molecular weight hydrocarbons produced upon irradiation of low density, polyethylene (LDPE) in either the solid or liquid phase. Comparison of these radiolysis results with structural information from 13C n.m.r. and infrared spectra lead to the interpretation that the 13C n.m.r. spectra indicate the branch distribution, i.e. butyl branches dominate in the present LDPEs. The distribution of radiolysis products were then explained by (a) cleavage at the tertiary carbon atom with branch detection efficiencies proposed from α-olefin-ethylene copolymer study, (b) fragmentation of main chain ends with product distribution similar to that obtained from linear hydrocarbons and (c) some fragmentation of branches. Following directly on from the polyethylene work studies of a similar nature were undertaken on (1) α-olefin-ethylene copolymers and (2) a series of reduced poly(vinyl chloride)s. Random copolymers of α-olefins and ethylene contain isolated alkyl branches, if only small percentages of α-olefins are used, attached to a poly-methylene backbone. Therefore, they were useful as model and calibration compounds for LDPE radiolysis. Poly(vinyl chloride) - PVC - is one of many polymers which are known to have structural variations from simple monomer repetition. The radiolysis technique demonstrated the presence of methyl branches with evidence also for butyl branches, although butyl branches have not been considered significant in 13C n.m.r. studies. The method was shown to be extremely sensitive to small quantities of residual reductant either occluded in or reacted with the polymer and to residual chlorine.The changes in the radiation chemistry of polystyrene with alterations in the irradiation environment (presence of air or high temperatures) have been considered with the major aspects being (1) the need to consider the whole molecular weight distribution as well as single molecular weight averages, (2) the correlation of the changes in physical properties produced by irradiation with alterations in the molecular weight distribution, (3) the of air on the radiation-induced changes in mechanical and molecular properties, and (4) the temperature dependence of G(crosslinking) and G(scission) between 20 and 150°C. Scission and crosslinking yields, G(S) and G(X), were determined as a function of depth by GPC of layers removed progressively from the surface. G(S) decreased and G(X) increased with depth, in accord with the concepts of increased scission by reaction with oxygen and a diffusion effect. As the temperature of irradiation (in vacuo) increases the products of C-C bond scission prefer to undergo disproportionation instead of recombination, so that above 80-100°C the G(S)/G(X) ratio became greater than one. The increased mobility of the chains at the higher temperatures reduces the cage effect and therefore the recombination rate. The crosslinking reactions were not greatly affected by increasing temperatures since G(X) rose only slowly from 0.035 at 30°C to 0.04 at 80-90°C above which G(X) slowly decreased to 0.027 at 150°C, whereas G(S) = 0.01 (30°C) rising to 0.075 (150°C).Throughout these studies a novel ampoule analysis technique has been utilized which allows efficient volatile product analysis from radiolysis of small (5-20 mg) polymer samples.