Effect of the preparation method on the morphology and proton conductivity of membranes based on sulfonated ABA triblock copolymers

Abstract

Sulfonated ABA triblock copolymers were synthesized based on ionomers with a hydrophilic block such as sulfonated poly(ether ether sulfone) (sPEES) and hydrophobic blocks such as poly(methyl methacrylate) (PMMA) and poly(pentafluorostyrene) (PPFS). The formation of these copolymers was evidenced by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H NMR), and gel permeation chromatography (GPC) analyses. The copolymers were used to prepare membranes by two approaches: direct self-assembly and infiltration of the polymers into anodized aluminum oxide (AAO) templates. The effect of the copolymer chemical structure (fluorinated and non-fluorinated) and preparation method on the properties of the membranes, such as morphology, thermal stability, water uptake, ion exchange capacity (IEC), and proton conductivity, was evaluated. Both methods used to obtain membranes from ABA copolymers synthesized were successful. Morphological characterization showed that the fluorinated membranes prepared by self-assembly exhibited phase separation. This phase-segregated morphology of partially fluorinated block copolymer structures and high density of sulfonic acid groups had a positive impact on proton exchange capacity and proton conductivity. Moreover, methanol permeability was improved in comparison with Nafion® 117. Otherwise, the morphology of AAO membranes pores filled by infiltration was verified, and the proton conductivity was low due to an increase in the tortuosity which affected proton transport. The present study shows that the membranes prepared by self-assembly of sulfonated ABA block copolymers presented the best properties to be applied as proton exchange membranes.