Synthesis, structure and properties of ionic thermoplastic elastomers based on maleated ethylene/propylene copolymers

Ionic thermoplastic elastomers with dicarboxylate functionalities were obtained via neutralization of high-molecular-weight hydrolyzed maleated EPM rubber (MAn-g-EPM) with potassium (K+) and zinc (Zn2+) acetate. The lower reactivity of the second of two adjacent carboxylic acid groups after neutralization of the first group prevents neutralization beyond 50% with monovalent K+ cations, while the remaining positive charge for divalent Zn2+ cations enables further neutralization. Small-angle X-ray scattering (SAXS) showed the presence of microphase-separated domains already for the MAn-g-EPM precursor as well as for the K- and Zn-ionomers. The type of cation has an important influence on the final morphology and properties. K-ionomers display a better phase separation than the corresponding Zn-ionomers. The SAXS data strongly suggest the presence of small, isolated multiplets, randomly dispersed within the matrix, next to the microphase-separated domains. This contribution is incorporated in the classical Yarusso-Cooper model together with the polydispersity of the domain size, and a very good fitting of the SAXS patterns is obtained. Neutralization causes the ionic network to persist up to higher temperatures, while the overall network density remains unaffected. The tensile properties and elasticity improve with increasing degree of neutralization (DN), showing significantly better properties for the K-ionomers than for the Zn-ionomers.