Effect of cation local structure on the physical properties of sulphonated polyurethane ionomers based on toluene diisocyanate

A series of sulphonated polyurethane ionomers based on poly(tetramethylene oxide) of molecular weight 1000, neutralized with Ca²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Cs⁺ and Eu³⁺ were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The degree of local order shown by EXAFS correlates inversely with the strain at which stress crystallization begins, indicating that the cohesiveness of the ionic aggregates controls the large-strain behaviour of the materials by suppressing ion-hopping. Upon hydration, the materials lose their mechanical strength. The local structure in the Ca²⁺-neutralized material is insensitive to hydration, while that in the Ni²⁺-neutralized material changes dramatically.