Joshua S. Enokida
Ionomers, random copolymers with a low fraction of ionic groups, possess distinctive ion cluster structures due to the strong electrostatic interactions. These ion clusters act as physical crosslinks which improve the polymer’s mechanical toughness and increase its melt viscosity. The strength of these interactions can be adjusted by exchanging the counter ion. Bulkier ions provide weaker associations of charged groups due to steric screening which allows for ion cluster dissociation and flow at lower temperatures. In this study, poly(isoprene-ran-dimethyloctylammonium styrenesulfonate) (P(I-ran-DMOASS)) copolymers were synthesized via nitroxide-mediate polymerization. High molecular weight copolymers with dimethyloctylammonium styrenesulfonate (DMOASS) compositions ranging between 0 and 40 mol% (30 – 77 wt%) were obtained. The thermal and viscoelastic characterization revealed distinct behaviors between the low and high DMOASS content. At low DMOASS content, the small angle X-ray scattering (SAXS) profiles showed an asymmetric scattering peak in the high q regime, indicating the presence of ion cluster structures. These clusters introduced additional elasticity to the copolymer’s linear viscoelastic response. For the high DMOASS content copolymers, SAXS revealed a smaller, more regular structure associated with the backbone-backbone spacing between elongated polymer chains. This resulted in restricted flow behavior and the disappearance of a definitive plateau modulus despite the high molecular weight of the polymers. A discontinuity in the glass transition temperatures marked the transition between the high and low DMOASS content copolymers. Further characterization using dynamic mechanical analysis (DMA) revealed two distinct Tg’s at intermediate contents. Based on these characteristic responses, three structural regimes are postulated: ion clusters, continuous ionic phase, and the coexistence of the two.