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 ζ ∼φ1/3, where ζ 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∼φ8in 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.