Ionomer solution to film solidification dependence upon solvent type and its impact upon morphology and ion transport

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Ionomer chain and solvent molecule interactions during solution-casting, solvent evaporation, and solidification impacts chain entanglements, spatial arrangement of functional groups, microstructure development, morphology, and physical properties. These concepts were investigated using poly(t-butylstyrene-b-ethylene-alt-propylene-b-sulfonatedstyrene-b-ethylene-alt-propylene-b-t-butylstyrene) with a fixed ion-exchange capacity of 1.0, and estimated solubility parameter of 21.8 (J/cm3)1/2. Films were solution-cast using an equal volume cyclohexane:heptane mixture (C:H), chloroform (CHCl3), and tetrahydrofuran (THF). Subsequent film structures were evaluated using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), FT-IR, and electrochemical impedance spectroscopy. A commercially supplied film had sulfonated domains randomly distributed throughout it, and its initial proton conductivity was 11.8mS/cm. A film created by solution-casting from C:H had a morphology containing randomly distributed sulfonated domains. This random morphology became more ordered with a lamella-like morphology when solution-cast using THF. The film produced from CHCl3 had a morphology that was between random and ordered. Film morphology differences were attributed to a poorer solvent system that inhibited chain solvation. The solution-cast film’s proton conductivity was 1.0mS/cm for a random morphology, and 15.3mS/cm with a lamella-like structure. Increasing the ionomer-THF solution-casting temperature to 40°C produced a film with a 103% increase in conductivity (31.2mS/cm). This led to a water uptake change from 29wt% to 80wt%. Cycling a THF solution-cast film from its dry to a wet state revealed that a lamella-like morphology would maintain its conductivity, but the commercial film’s conductivity decreased from 11.8mS/cm to 0.98mS/cm. Ionomer film properties were found to be dependent upon solvent quality and processing.