EFFECTS OF WATER SORPTION ON THE STRUCTURE AND PROPERTIES OF ETHYLENE IONOMERS

Effects of water sorption on the structure and properties of poly(ethylene-co-methacrylic acid)-based ionomers were investigated by various physicochemical techniques (FTIR, differential scanning calorimetry(DSC), thermogravimetry, X-ray scattering, and dilatometry). It is revealed that water molecules are absorbed preferentially at the COONa ion pairs in both the amorphous and ionic cluster regions, and that three water molecules per one sodium ion form the primary hydration shell. In the more hydrated samples, the excess water molecules are found to locate just around the primary hydration shell. It was found by DSC that both the transition temperature and enthalphy change for the transition near 330 K decrease with increasing hydration. However, this transition was observed even in the fully hydrated samples. These results are well explained by the order-disorder transition model of ionic clusters proposed previously. It is concluded that the ionic clusters consist of the COONa ion pairs and a small portion of the polyethylene backbones.