Crystallinity and morphology of PVdF–HFP-based gel electrolytes

The structural behaviour of a random copolymer consisting of polyvinylidene fluoride (PVdF) and hexafluoropropylene (HFP), when mixed with organic solvents and a lithium salt, has been investigated. In its neat form, the copolymer exhibits a semicrystalline matrix, where the PVdF constituent crystallises partially. The crystalline phase appears mostly as the non-polar (trans–gauche–trans–gauche′) phase II. The relative crystallinity of the PVdF domains remains constant on addition of a plasticiser, propylene carbonate (PC) or triethylene glycol dimethyl ether (TG), but is greatly decreased on addition of the salts lithium bis(trifluoromethane sulfone) imide, Li(TFSI) or lithium tetrafluoroborate, LiBF4. This is shown by the results from both wide-angle (WAXS) and small-angle (SAXS) X-ray scattering and FTIR-spectroscopy. Surprisingly, the addition of such a polar medium as the salt, and to a lesser extent the plasticisers, also causes a change in the structure of the crystalline phase to a more polar phase III (intermediate polar conformation). The thermal stability of the gels and the large influence the salt addition has on the material is also shown by the results from differential scanning calorimetry (DSC) measurements.