Morphology and Transport Properties of Sulfonated Poly(styrene-isobutylene-styrene) with Counter Ion Substitution

In this study, the transport properties of poly(styrene-isobutylene-styrene) (SIBS)  were determined as function of sulfonation level and counter ion substitution for gas sensors and fuel cell applications. Sulfonated SIBS was neutralized with with +2 and +1 cations: Mg⁺², Ca⁺², Ba⁺², Mn⁺², Cu⁺², K⁺¹; to create highly selective polymer-metal nanocomposite membranes. Absorption limitations and their effect on the membrane transport were investigated for sulfonated and cross-linked membranes through methanol (MeOH) swelling and permeability experiments. Results show that MeOH permeability increases with sulfonation level until a maximum suggesting an optimum ion exchange capacity (IEC) level, while counter ion substitution decreases permeability. Morphologies studies using Small-angle X-ray scattering (SAXS) were performed to determine size and shape of nanostructured channels. Although increasing sulfonation levels influences morphology towards a more ordered structure (i.e., spherical, hexagonal cylinders, lamellar), increasing sulfonation beyond a certain level creates an amourphous morphology perhaps with additional interconnections in the 3-D network. To complement the studies, selectivity (i.e., proton conductivity/methanol permeability) of sulfonated membranes was determined and compared to Nafion® 117.