Microstructure of Ionomers based on Sulfonated Block Copolymers of Polystyrene and Poly(ethylene-alt-propylene)
Block copolymer ionomers were prepared by lightly sulfonating the polystyrene (PS) blocks of diblock and triblock copolymers of PS and poly(ethylene-alt-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 from PS spheres to hexagonalclose-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 (NS) of between 2 and 9 sulfonate groups per molecule. The microstructure was analyzed by small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). An ion-rich microphase formed within the PS microdomains, and that hindered the development of a well-ordered block texture. Sulfonation of a diblock copolymer containing ca. 20 wt % PS resulted in a transition of the block microphase texture from a hcp to lamellar. The order-to-disorder transition temperature, TODT, of the block copolymer was relatively insensitive to the choice of the cation, and TODT increased as NS increased.