Modifying the Structure and Dynamics of Ionomers through Counterion Sterics

The role of counterion sterics on the structure and dynamics of a low glass transition temperature, amorphous poly(isoprene-ran-styrenesulfonate) copolymer was investigated using a series of symmetric, tetraalkylammonium counterions with methyl (TMA), ethyl (TEA), propyl (TPA), and butyl (TBA) pendent groups, in addition to a sodium (Na) control. A detailed analysis of the aggregate structure was achieved by fitting the copolymers' X-ray scattering profiles with a modified hard sphere model. Increasing the counterion sterics from Na to TEA resulted in slight changes to the aggregates with some ionic groups present in the isoprene matrix. For the more hindered TPA and TBA counterions, considerable disruption of the structure occurred. Using solid-state NMR, dynamic mechanical analysis, and rheology, the effect of the counterion sterics on the copolymer's dynamics was determined. The T-1p, relaxation of the copolymers revealed a rigid isoprene fraction, associated with the ion clusters, and a mobile isoprene matrix fraction. Copolymers with larger counterions exhibited an increase in the dynamic moduli at high frequency and a decrease in the dynamic moduli at lower frequencies in addition to possessing faster molecular dynamics. These two observations are attributed to an increased incorporation of ionic groups into the isoprene matrix and screening of the dipole-dipole interactions.