Viscoelasticity Enhancement and Shear Thickening of Perfluorinated Sulfonic Acid Ionomer Dispersions in Water-Alcohol Solvent Mixtures

We investigate the effect of dispersion media composition, water-to-isopropanol (IPA) ratio, on the structure and rheology of perfluorinated sulfonic acid (PFSA) ionomers using short-side-chain PFSA (EW ~725 g/mol) as a model ionomer. In the semidilute unentangled regime, their viscosity scaling remains unaffected by the IPA% in the solvent mixture. The dispersions display a similar polyelectrolyte-like scaling n0 ~c0.5 for all IPA%, indicating the role of electrostatic interactions, particularly intrachain electrostatic repulsions, resulting in an extended conformation at dilute concentrations like in salt-free polyelectrolytes. This is supported by the small-angle X-ray scattering results revealing a rod-like structure at low concentrations for all IPA%. In contrast, in the semidilute entangled and concentrated regimes, as the IPA% is increased, their viscosity scaling becomes increasingly stronger, increasingly deviating from salt-free polyelectrolyte solutions. Such behavior is accompanied by strong viscoelastic nonlinearities, particularly shear-thickening and strain-stiffening nonlinearities with maximum intensities at intermediate IPA%. These trends suggest IPA is inducing ionomer association at higher concentrations via ion-dipolar interactions due to its lower polarity. These findings showing a dramatic effect of solvent composition on the aggregated structure and rheology of ionomer dispersions, particularly at higher concentrations, have significant implications for both optimization of their morphology and processing during membrane casting.