Ionic conduction in nanostructured membranes based on polymerized protic ionic liquids

Polymerized ionic-liquids (PILs) are promising materials whose ionic properties can be tuned based on their chemistry. By incorporating PILs into block copolymer (BCP) structures, it is possible to provide complementary functionality (i.e., structural stability) and transport tunability to ion-conducting materials. In this study, we describe the self-assembly and conductivity of novel poly(styrene-block-histamine methacrylamide) diblock copolymers (PS-b-PHMA) and the resulting PS-b-PIL derivatives obtained after treatment with trifluoroacetic acid (TFA). These materials self-assemble into ordered BCP structures with tunable domain sizes as demonstrated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). PS-b-PHMA membranes show conductivities up to 2 × 10^-4 S/cm at room temperature, which increase by an order of magnitude in the presence of acid. In addition, both PHMA- and PIL-based membranes exhibit lower water uptake (λ = 4-6 and 8-10, respectively) in comparison with most proton conducting membranes reported elsewhere. The low water content in these membranes translates into a stronger effect of morphology on transport behavior, resulting in a measurable increase in ion conductivity as a function of conducting channel size.