Microstructure of ionomers based on sulfonated block copolymers of polystyrene and poly(ethylene-alt-propylene)

Block copolymer ionomers were prepared by lightly sulfonating the polystyre ne (PS) blocks of diblock and triblock copolymers of PS and poly(ethylene-a lt-propylene) (PEP), followed by neutralization of the sulfonic acid groups with either sodium or zinc. The block copolymers contained between 10 and 50 wt % PS, and as the PS content increased, the mesophase texture varied f rom PS spheres to hexagonal-close-packed (hcp) PS cylinders to alternating PS and PEP lamellae. The sulfonation level was varied from 1 to 17 mol % of the styrene repeat units, which provided a range of functionalization (N-S ) of between 2 and 9 sulfonate groups per molecule. The microstructure was analyzed by small-angle X-ray scattering (SAXS) and dynamic mechanical ther mal analysis (DMTA). An ion-rich microphase formed within the PS microdomai ns, and that hindered the development of a well-ordered block texture. Sulf onation of a diblock copolymer containing ca. 20 wt % PS resulted in a tran sition of the block microphase texture from a hcp to lamellar. The order-to -disorder transition temperature, T-ODT, Of the block copolymer was relativ ely insensitive to the choice of the cation, and T-ODT increased as N-S inc reased.