The role of excess acid groups in the dynamics of ethylene-methacrylic acid ionomer melts

Ethylene-methacrylic acid (E/MAA) ionomers are commonly neutralized to substoichiometric levels, leaving unneutralized ("excess") carboxylic acid groups along the chain. Previous observations indicate that these excess acid groups interact preferentially with the ionic units and produce a reduction in viscosity. Here, we employ a combination of melt rheometry and cation diffusion experiments to elucidate the role of these excess acid groups in the chain and ion dynamics of E/MAA ionomers. We find that the excess acid groups act not by introducing a separate relaxation mechanism, but by accelerating the rate of ion hopping in these materials-a "plasticization" of the ionic interactions-which in turn speeds up chain relaxation proportionately. At low ion contents, the functional dependence of viscosity vs ion content is profoundly altered by excess acid groups, due to the competing effects of viscosity increase through interacid hydrogen bonding and viscosity reduction through plasticization of the ionic associations. A substantial dependence of the ion-hopping rate on the neutralization level is noted and quantified, with the ionic group diffusivity decreasing as neutralization level is increased. Finally, diffusion experiments pairing partially neutralized and unneutralized E/MAA ionomers show an additional contribution to the diffusion rate from a gradient in neutralization level, beyond that provided by the concentration gradient.