Thermal degradation behaviour of acrylic salt polymers and ionomers

Abstract

The thermal degradation behaviour of polymers and copolymers containing acrylic and methacrylic acid salt units has received much less attention than that devoted to the corresponding ester polymers from these acids. Whether present in a homopolymer or copolymer chain, however, these ionic salt units can have a major effect on thermal behaviour. The mode of thermal breakdown of the parent acid homopolymers is briefly considered. As expected, dehydration to form anhydride structures is the first effect of progressive rise in temperature, followed at higher temperatures by fragmentation of the modified chains. The degradation of the salts from these structures is found to depend both on the acid type (whether acrylic or methacrylic) and the nature of the metal ion involved. The behaviour of salt polymers based on alkali metals, alkaline-earth metals, zinc, lead and some transition metals is considered. Some similarities to the behaviour of the acrylic ester polymers can be found, but there are also major differences due to the presence of the salt structure. Among the factors influencing the type and quantity of products formed are the volatility of the salt monomer and the stability of the corresponding metal carbonate. In the cases of the divalent metal salts, some similarities to the classical pyrolysis of low molecular weight carboxylate salts can be seen. The most important commercial ionomers are copolymers of ethylene with small amounts of salt units. The description 'ionomer' can also be broadly applied to other copolymers containing both salt monomers and covalently bonded monomer units. The degradation of such systems is considered for the cases of ionomers of ethylene with sodium and with zinc methacrylate, styrene with zinc acrylate, and methyl methacrylate with alkali metal methacrylates. Patterns of behaviour for these various materials with ionic salt units have now been established and mechanisms are proposed to account for the various products found.