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 sol vents and a lithium salt, has been investigated. In its neat form, the copo lymer exhibits a semicrystalline matrix, where the PVdF constituent crystal lises partially. The crystalline phase appears mostly as the non-polar (tra ns-gauche-trans-gauche') phase II. The relative crystallinity of the PVdF d omains 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 bot h wide-angle (WAXS) and small-angle (SAXS) X-ray scattering and FTIR-spectr oscopy. 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 conf ormation). The thermal stability of the gels and the large influence the sa lt addition has on the material is also shown by the results from different ial scanning calorimetry (DSC) measurements. (C) 2000 Elsevier Science Ltd. All rights reserved.