Effect of Ionomer–Solvent Interactions in PFSA Dispersions: Dispersion Morphology

Alcohol–water solvent systems are commonly used to disperse perfluorosulfonic acid ionomers (PFSAs) for the fabrication of proton-exchange membranes, catalyst layers, and thin films. The profound effect of PFSA chemical structure, concentration, and solvent composition on the colloidal morphology of PFSA dispersions is investigated using small-angle X-ray scattering (SAXS). Five different PFSA chemical structures and three different binary alcohol–water solvent systems are utilized for relevance to industrial processing parameters. Although evidence for a cylindrical PFSA aggregate morphology is shown, the strong scattering maximum frequently observed in scattering patterns of semidilute PFSA dispersions is demonstrated to prevent the quantification of aggregate length. A semiempirical small-angle scattering model is introduced to fit the dispersion scattering patterns over a wide q-range and quantify aggregate dimensions on length scales smaller than the average interaggregate spacing. A thermodynamic model based on the self-assembly of cylindrical micelles is shown to describe aggregate dimensions. The surface area per side chain, σ, calculated from this model is observed to increase with increasing alcohol concentration in the solvent, while the aggregate radius and average number of chains per aggregate both decrease. These observations suggest that increases in σ may represent an increase in the hydrophobic component of the PFSA aggregates to the solvent due to improved compatibility between the PFSA and solvent. The PFSA–solvent interactions are studied in more detail in a second publication of the present study.