Effect of evaporation temperature on the crystalline properties of solution-cast films of poly(vinylidene fluoride)s

The crystalline properties of poly(vinylidene fluoride) (PVdF) and its copolymer films, prepared from the solvent (N-methyl-2-pyrrolidone) evaporation at different temperatures and subsequent slow cooling to ambient temperature, were investigated by using polarized optical microscopy, differential scanning calorimetry, wide-angle X-ray diffractometry, and Fourier-transform infrared spectroscopy. The results can provide helpful data for determining the optimal processing conditions of PVdFs as the polymer binder materials in making the electrodes of rechargeable lithium batteries. The morphology analysis gives useful information that the residual solvent remaining after the evaporation shows distinguishable amounts with respect to the temperature regions dividing by the crystallization (T_c) and melting (T_m) points of original PVdF samples. It is also proved that smallest spherulitic state coexisting with dominant α- and minor γ-phase crystals, simultaneously showing the lowest heat of fusion (e.g., the lowest crystallinity), can be obtained when the solvent is evaporated at a temperature between T_c and T_m. Letting the minor γ-phase crystals exist by controlling the evaporation temperature like this can be one of the best drying (evaporation) conditions of PVdF-containing slurry in lithium rechargeable battery system.