Transport Properties of Sulfonated Poly(styrene-b-isobutylene-b-styrene) Triblock Copolymers at High Ion-Exchange Capacities

Abstract

Transport properties of sulfonated poly(styrene-b-isobutylene-b-styrene) (S-SIBS) triblock copolymers were examined as a function of ion-exchange capacity (IEC), specifically at high IECs (up to ∼2 mequiv/g). The proton conductivity of S-SIBS was ∼1 order of magnitude higher than sulfonated polystyrene at similar IECs and 3-fold higher than Nafion 117 at an IEC of 2 mequiv/g. However, all polymers in this study possessed similar selectivities (i.e., proton conductivity/methanol permeability) regardless of chemistry or morphology. Small-angle X-ray scattering reveals a periodic-to-nonperiodic transition in S-SIBS with an anisotropic lamellar morphology oriented in the plane of the membrane at IECs ranging from 0.5 to 1 mequiv/g and an isotropic cocontinuous morphology at IECs ranging from 1.1 to 2 mequiv/g. This morphological transition coincides with a discontinuity in the IEC-dependent transport properties. In addition, S-SIBS transport properties were measured after solution casting from 15 different solvents at a constant IEC (0.8 mequiv/g). Transport properties varied by almost 3 orders of magnitude when comparing S-SIBS solution cast from toluene to a toluene/ethanol mixture. X-ray scattering results show morphological differences with solvent choice. This study demonstrates the significant impact of morphology on transport properties in ionic block copolymers.