Morphology of Neutralized Low Molecular Weight Maleated Ethylene−Propylene Copolymers (MAn-g-EPM) As Investigated by Small-Angle X-ray Scattering

Abstract

The thermoreversible nature of the ionic associations in aggregates makes ionomers with a low glass transition temperature interesting candidates for thermoplastic elastomers. In this paper, a low-Tg ionomer based on low molecular weight ethylene−propylene copolymers modified by maleic anhydride (MAn-g-EPM) is introduced, and the morphology of a series of ionomers, neutralized with Cs+, Na+, K+, Li+, Zn2+, Ba2+, and Mg2+, was investigated with small-angle X-ray scattering (SAXS). To determine the size and composition of the ionic aggregates, the observed SAXS peak was interpreted with the help of the Yarusso−Cooper model, which describes the ionomer morphology by spherical aggregates of the ionic species with a high electron density surrounded by a layer with a restricted mobility. The results from the ionomer precursors suggest that there is a critical concentration above which aggregation of the polar groups in the apolar matrix occurs. Upon increasing degree of neutralization, the average dimension of the aggregate remains almost constant, while the restricted mobility layer increases. The size of the aggregates is much larger in comparison to other ionomer systems, and as a consequence, the number of acid groups within an aggregate is much larger. For low degrees of neutralization, it was shown that the aggregates contain a large fraction of EPM fragments of at least 60 vol %. For the ionomers neutralized with divalent cations, it was observed that the morphology changes drastically beyond degrees of neutralization of 50%; the number of the aggregates decreases, and the size increases. This was explained with the help of the coordination mechanism.