The Mechanochemical Modification of High Polymers

Degradation of natural rubber during mastication has been shown to proceed via two alternative mechanisms, oxidative scission at high temperatures and mechanical scission at lower temperatures. The low temperature process, cold mastication, has received the greater attention. The energy supplied to the extended rubber chains during mechanical deformation is sufficient to cause homolytic scission into polymeric free radicals. The degradation of high polymers by a rupture process via mechanical scission has been shown to occur during the cold mastication of synthetic elastomers and during the mechanical working of high molecular weight vinyl and acrylic polymers in the visco-elastic state. The application of shearing forces to certain polymers in the brittle glass state has provided evidence for both homolytic scission into polymeric free radicals and heterolytic scission into polymeric ions. Polymeric radicals, produced by mechanical chain scission, have been used as initiators of vinyl polymerization to give block copolymers of an essentially linear character. Thus the block copolymerizations of methyl methacrylate, styrene, vinyl acetate, acrylonitrile, and ethyl acrylate have been initiated by mechanically shearing natural rubber, polymethyl methacrylate, polystyrene, polyvinyl acetate, polyethylene, polyvinyl chloride and polyvinyl formal during the process of extrusion of the polymer plasticized to a viscoelastic state with the monomer. Many other polymer-monomer systems have yielded block copolymers by cold mastication. Cold mastication of elastomer blends, such as natural rubber and neoprene, also leads to block copolymer formation by both combinative and hydrogen abstractive processes between the different species of elastomer radicals present. If two polymers are completely compatible so that one continuous phase is present in the blend, and if the polymeric constituents have a common viscoelastic temperature range, then mechanical working during extrusion or internal mixing can lead to block copolymer formation. If the tendency of the polymeric radicals formed by mechanical rupture is to recombine rather than to disproportionate, then the chances of block copolymer formation are increased. The presence of sites for hydrogen or halogen abstraction upon one of the polymer constituents is also an aid to grafted block copolymer formation. Thus polyvinyl chloride-neoprene blends give grafted block copolymers on extrusion or internal mixing and polyethylene-polyvinyl acetate blends block copolymerize when masticated in the absence of oxygen. Block copolymerization is largely controlled by the viscoelastic properties of the systems chosen.