Structure and Rheology of Poly(vinylidene difluoride-co-hexafluoropropylene) in an Ionic Liquid: The Solvent Behaves as a Weak Cross-Linker through Ion–Dipole Interaction

Abstract

Mixtures of poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP) and ionic liquids (IL) are known to form flexible, self-healing, and ion-conductive materials that are often referred to as polymer–gel electrolytes. The ion–dipole interaction occurs between PVDF-HFP and the cation; that is, the IL acts not only as a solvent but also as a transient cross-linker to form the network structure, and the density of the binding site (C–F dipole) along the polymer chain is quite high. While the practical importance of this type of polymer/IL has become evident, the fundamental solution properties of these types of polymers have not been clearly understood. Here, we studied the structure and viscoelastic properties of the PVDF-HFP/IL system using infrared spectroscopy, wide- and small-angle X-ray scattering (WAXS/SAXS), and steady-state and oscillatory shear rheology. The polymer network structure was found to maintain the geometrically similar figure ξ ∼ ϕ1/3, where ξ and ϕ denote the correlation length (length of the network strand) and volume fraction of the polymer, respectively. In addition, with an increase in ϕ, the larger-scale structure changed from a mass fractal to a surface fractal morphology at the overlap concentration. The dynamics of this polymer network system is presumed to be governed by the relaxation through repeated association–dissociation processes with the cation. The specific viscosity followed ηsp ∼ ϕ8 in the concentrated solution of PVDF-HFP. The viscometry results indicated that the apparent binding energy increased with ϕ; that is, the C–F dipoles were cooperatively bound to each other via the cations.