Interaction, structure and tensile property of swollen Nafion® membranes. (20th January 2021) - Shi, Ce
Abstract: Commercial Nafion® membranes are fabricated from an ionomer with strongly hydrophobic polytetrafluoroethylene (PTFE) backbone and strongly hydrophilic pendant sulfonic group. Understanding the difference in the phase separated structures for them in dry and solvent-swollen states is crucial for their optimal application and recycling. Structure changes in solvent swollen membranes were studied by Small angle X-ray scattering (SAXS), their thermodynamic origins from polymer-solvent interactions, solvent uptake and tensile properties were explored. Dry membranes have tightly packed backbone and crystallite, acting as the matrix that afford strong mechanic strength in tensile tests. Solvents that can swell both hydrophobic matrix and ionic clusters lead to high level of solvent uptake, evidenced from larger correlation lengths for both polymer matrix and ionic clusters, and the swollen membranes become soft and flexible. Semi-quantitative relationships to correlate polymer-solvent interactions, the length and distribution of structural domains, solvent uptake and tensile properties were presented. Dimethylacetamide as the primary solvent and 40% isopropanol-water as the binary solvent, both have intermediate difference in Hansen solubility with the PTFE backbone, can permeate into hydrophobic matrix and ionic clusters, resulting in the most swollen Nafion membranes. Based on correlation matrix analysis, we also found that solvents with larger molecule volume will benefit the elongation of swollen membranes. Graphical abstract: Image 1 Highlights: Two Scaling exponents for the correlation length of ionic clusters against the weight percentage of ionomer were found. The two Scaling exponents are -0.77 for variant solvents at room temperature, and -1.7 for vapor at 80 o C. Binary solvents with synergistic effect have around three times uptake of that for primary solvent. Solvents that can permeate into hydrophobic matrix and solvate packed PTFE backbone cause decrease in Young's modulus. Solvents with larger molecular volume benefit the elongation of swollen membranes.