Microstructures and Rheological Properties of Short-Side-Chain Perfluorosulfonic Acid in Water/2-Propanol

The viscosity and viscoelasticity of polyelectrolyte solutions with a single electrostatic interaction have been carefully studied experimentally and theoretically. Despite some theoretical models describe experimental results well, the influence of multiple interactions (electrostatic and hydrophobic) on rheological scaling is not yet fully resolved. Herein, we systematically study the microstructures and rheological properties of short-side-chain perfluorosulfonic acid (S-PFSA), the most promising candidate of a proton exchange membrane composed of a hydrophobic backbone with hydrophilic side-chains, in water/2-propanol. Small-angle X-ray scattering confirms that semiflexible S-PFSA colloidal particles with a length of ~38 nm and a diameter of 1-1.3 nm are formed, and the concentration dependence of the correlation length (ξ) obeys the power law ξ~c^-0.5 consistent with the prediction of Dobrynin et al. By combining macrorheology with diffusing wave spectroscopy microrheology, the semidilute unentangled, semidilute entangled, and concentrated regimes corresponding to the scaling relationships η_sp~c^0.5, η_sp~c^1.5, and η_sp~c^4.1 are determined. The linear viscoelasticity indicates that the entanglement concentration (c_e) obtained from the dependence of η_sp on the polymer concentration is underestimated owing to hydrophobic interaction. The true entanglement concentration (c_te) is obtained by extrapolating the plateau modulus (G_e) to the terminal modulus (G_t). Furthermore, G_e and the plateau width, τ_r/τ_e (τ_r and τ_e denote reptation time and Rouse time), scale as G_e~c^2.4 and τ_r/τ_e~c^4.2, suggesting that S-PFSA dispersions behave like neutral polymer solutions in the concentrated regime. This work provides mechanistic insight into the rheological behavior of an S-PFSA dispersion, enabling quantitative control over the flow properties in the process of solution coating.