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SYNOPSISThermal properties by DSC, stiffness, melt viscosity, tensile properties, and dynamic mechanical properties were measured for the Na+, K', Mg2+, Zn2+, Cu2+, Mn2+, and Co2+ salts of poly (ethylene-co-methacrylic acid) (EMAA) . The changes in the structure and properties with increasing neutralization are larger in the alkaline and alkaline earth metal salts than in the transition metal salts. The stiffness shows a maximum at 33% neutralization in both the alkaline and alkaline earth metal salts, while no maxima are found up to 60% neutralization in the transition metal salts. The microphase separation of salt group aggregates is observed in both the alkaline and alkaline earth metal salts, but is not seen in the transition metal salts. These differences were attributed to both the stronger ionic interactions and the larger number of carboxyl groups associated with the alkaline and alkaline earth metal salts in the ordered structure of ionic salt groups (ionic crystallites). The mechanical properties measured at low strain, such as stiffness and yield stress, strongly depend on the degree of the crystalline order of the ionic crystallites. The high-strain properties, such as tensile strength and elongation at break, depend on the strength of the ionic interactions and the valence of the cation.

The microphase structure of noncrystalline poly(ethylene-co-13.3 mol % methacrylic acid) (E-0.133MAA) ionomers was investigated by using infrared (IR) spectroscopic, X-ray scattering, differential scanning calorimetric (DSC), and dielectric measurements. The noncrystallinity was confirmed by small-angle X-ray scattering (SAXS) and DSC studies, which has enabled a quantitative analysis of the SAXS ionic peak associated with ionic aggregates without being perturbed by the polyethylene lamellae peak. In 60% neutralized Na ionomer, it was revealed that almost 100% of MAA side groups including unneutralized COOH are incorporated into the ionic aggregates with an average ionic core radius (R 1) of ∼6 Å. The dielectric relaxation studies showed that the ionic aggregates form a microphase-separated ionic cluster. Analysis of dielectric strengths indicated the most (∼90%) of the COONa groups are present in the ionic cluster. On the other hand, in the 60% neutralized Zn ionomer, both SAXS and dielectric studies indicated that the ionic aggregates with R 1 ∼ 4 Å are almost isolated and dispersed in the matrix; the formation of ionic cluster was not recognized. Similarly to partly crystalline E-MAA ionomers, all noncrystalline E-0.133MAA ionomers exhibited an endothermic peak at 320−330 K (labeled T i) on the first heating, depending on the aging time at room temperature. Several factors that would be critical for the DSC T i peak were discussed. It was concluded that the DSC T i peak is certainly associated with changes of the state of ionic aggregate region.

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.