Synthesis and melt dynamics of model sulfonated ionomers

We investigate the dynamics of model ionomers through linear viscoelastic measurements on well-defined sulfonated styrene-ethylene-butene (SEB) ionomers of varying molecular weights and functionalization levels. Random styrene-butadiene copolymers with low polydispersities were synthesized via anionic polymerization, from which ionomers were prepared by selective hydrogenation of the butadiene units, followed by partial sulfonation of the styrene units and subsequent neutralization. The low glass transition temperature of the SEB backbone provides access to both the rubbery and terminal flow regions in lightly functionalized materials. All ionomers show multiple relaxation mechanisms and do not obey time-temperature superposition. We identify the faster relaxations with motions of the outermost portions of the polymer chains, which contain no ionic groups. Rapid relaxation of these chain segments leads to a plateau modulus for the ionomers lower than that for highly entangled unfunctionalized SEB, despite the ionic associations. The slower relaxations correspond to the ion-hopping process and to the terminal relaxation time of the polymer chain; these two relaxations overlap for the low number of ionic groups per chain explored. The terminal relaxation time shows only a weak dependence on molecular weight but an extremely strong dependence on sulfonation level, reflecting a strong dependence of the ion-hopping time on the extent of functionalization.