Progression of the Morphology in Random Ionomers Containing Bulky Ammonium Counterions

Poly(isoprene-ran-dimethyloctylammonium styrenesulfonate) (P(I-ran-DMOASS)) copolymers with high molecular weights and dimethyloctylammonium styrenesulfonate (DMOASS) compositions ranging between 8 and 40 mol % (30 and 77 wt %) were synthesized via nitroxide-mediated polymerization. Thermal and viscoelastic characterization revealed distinct behaviors for the low (30–51 wt %) and high (56–77 wt %) DMOASS content copolymers. Three structural regimes were identified: ion clusters (30 wt % DMOASS), continuous ionic phase (56–77 wt % DMOASS), and the coexistence of the two (42–51 wt % DMOASS). As DMOASS content increased, small-angle X-ray scattering revealed a gradual transition from the characteristic ion cluster structure to a smaller, more regular backbone–backbone structure associated with a continuous ionic phase. The ion clusters acted as physical cross-links and introduced additional elasticity into the low DMOASS content copolymer, while the continuous ionic phase showed restricted flow behavior and the disappearance of a definitive plateau modulus. Dynamic mechanical analysis revealed two distinct Tg’s at intermediate DMOASS content, indicating the coexistence of both structures.