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 sulfonicacid (PFSA) ionomers using short-side-chain PFSA (EW & SIM; 725g/mol) as a model ionomer. In the semidilute unentangled regime, theirviscosity scaling remains unaffected by the IPA% in the solvent mixture.The dispersions display a similar polyelectrolyte-like scaling & eta;(0) & SIM; c (0.5) for all IPA%, indicatingthe role of electrostatic interactions, particularly intrachain electrostaticrepulsions, resulting in an extended conformation at dilute concentrationslike in salt-free polyelectrolytes. This is supported by the small-angleX-ray scattering results revealing a rod-like structure at low concentrationsfor all IPA%. In contrast, in the semidilute entangled and concentratedregimes, as the IPA% is increased, their viscosity scaling becomesincreasingly stronger, increasingly deviating from salt-free polyelectrolytesolutions. Such behavior is accompanied by strong viscoelastic nonlinearities,particularly shear-thickening and strain-stiffening nonlinearitieswith maximum intensities at intermediate IPA%. These trends suggestIPA is inducing ionomer association at higher concentrations via ion-dipolarinteractions due to its lower polarity. These findings showing a dramaticeffect of solvent composition on the aggregated structure and rheologyof ionomer dispersions, particularly at higher concentrations, havesignificant implications for both optimization of their morphologyand processing during membrane casting.