Photodegradation of methacrylate/acrylate copolymers
Although the effects of u.v. radiation on poly(methyl methacrylate) and poly(methyl acrylate) are superficially different, nevertheless there are obvious fundamental similarities and this copolymer system was chosen for study in order to observe the influence of one type of chain unit on the degradation reactions of the other. Rates of chain scission and crosslinking were measured at room temperature on copolymer films covering the whole composition range using molecular weight and sol gel analysis measurements. Crosslinking is associated with sequences of two or more methyl acrylate units but chain scission passes through a minimum at a methyl methacrylate content in the region of 40 to 50 per cent. Crosslinking is inhibited in solution and differences in the relationship between chain scission and composition compared with degradation in film can be accounted for in terms of the relative mobilities in film and solution of the primarily formed radicals. The environment in which these radicals find themselves has also a strong influence on the pattern of volatile degradation products. Relationships between these reactions and the thermal and photothermal degradation which occurs in acrylate/methacrylate systems are described and discussed. The various contributions to this symposium have demonstrated the diversity of interest which exists in the interaction of visible and ultra-violet radiation with polymeric systems. It has also been recognized, almost since polymers first became commercially significant, that their resistance to this type of radiation is a very large factor in determining their useful life. Thus the commercial importance of the degradative effects of radiation and the comparatively more recent interest in the more fundamental interaction of radiation with matter have together stimulated interest in the photodegradation of polymers especially during the past five to ten years. During this period a great deal of progress has been made in understanding the mechanism of photodegradation of a number of homopolymers and the position in 1967 was admirably reviewed by Dr Fox' who is contributing to this symposium in other directions. Since that time significant contributions have been made, particularly in Belgium by Dr Geuskens, Dr David and their collaborators24. In my laboratory during the past few years we have attempted to contribute to a more fundamental understanding of photodegradation processes by