Infrared and XAFS study on internal structural change of ion aggregate in a zinc salt of poly(ethylene-co-methacrylic acid) ionomer on water absorption

The internal structural change of an ion aggregate in a zinc salt of poly(ethylene-co-methacrylic acid) ionomer on the water absorption-desorption process was investigated by infrared and XAFS spectroscopies. In the infrared carboxylate antisymmetric stretch region, a prominent spectral change was found. In a wet state, the singlet at 1584 cm(-1) appeared, while in a dry state the triplet at 1537, 1554, and 1625 cm(-1) was observed. The spectral change was reversible with respect to the water content. From the XAFS analysis, the Zn-O distance and the coordination number were evaluated as 1.95 Angstrom and 4, respectively, in both the wet and dry states. The coordination form of the carboxylate groups around the zinc atom was considered as the bridging bidentate form in the two states. The large infrared spectral change was ascribed to a little reversible geometrical distortion of the tetrahedral coordination structure in the ion aggregate.