Ionic and chain interdiffusion and interfacial strength development in ionomers of poly(ethylene-co-methacrylic acid)

Diffusion coefficients of sodium and zinc ions have been measured in bilayers of commercial, partially neutralized poly(ethylene-co-methacrylic acid). The diffusion behavior shows a break at the melting temperature of the resin, suggesting that chain backbone motion is intimately related to ionic transport. The ionic diffusion coefficients are orders of magnitude greater than those of the chain backbone, which suggests that metal ion pairs are transported along the contour of the polymer network via exchange with protons of neighboring acid groups. Interfacial strength development in welded films of ionomers is primarily determined by the melting behavior of the bulk resin. Ionic diffusion rates do not correlate with the strength development behavior. This observation, along with melting and chain center of mass diffusion information, suggests that strength development is dependent on chain interdiffusion like that of many other thermoplastics.