The crystallinity of ionomers

Ionomers are a recently developed class of copolymers consisting of a hydrocarbon repeat unit such as ethylene or butadiene and an ionized carboxylic acid group such as ionized acrylic or methacrylic acid. In this paper measurements of crystallinity in poly(ethylene-methacrylic acid) ionomers from DSC endotherms and polymer densities are shown to be in good agreement. The effect of annealing on the DSC endotherms of acid copolymers and ionomers and on the small-angle x-ray scattering of a poly(ethylene-methacrylic acid) ionomer is presented. It is shown that annealing results in the appearance of a secondary endotherm and a shift of the crystalline x-ray scattering to smaller angles. The data are interpreted in terms of an ionomer crystallinity model in which the hydrocarbon portion of the ionomer crystallizes into folded-chain lamellae.