Ion hopping in ethylene-methacrylic acid ionomer melts as probed by rheometry and cation diffusion measurements

The dynamics of chain and ionic group motion were investigated in a series of ethylene-methacrylic acid (E/MAA) ionomers of varying cation type and neutralization level, all formed from the same E/MAA copolymer. Low-shear-rate theological measurements were utilized to determine the terminal relaxation time (t_d) of the polymer chains. The much faster ion-hopping time (τ) was measured through the use of cation diffusion studies, in which a finite slab of one ionomer diffuses into a matrix of a second ionomer. For the Mg/Ca, Mg/Li, Na/Ca, and Na/Li diffusion couples studied, the diffusion coefficient follows inversely with melt viscosity, though τ extracted from the diffusion coefficients depends more weakly on cation type than does t_d obtained from melt rheometry. For the highly neutralized E/MAA ionomers, τ is 4 orders of magnitude shorter than t_d, but the two relaxation mechanisms have a similar dependence upon temperature, in agreement with current theories of ionomer dynamics.