Preparation of Poly(vinylidene fluoride)/P123 Blend Microporous Membranes via Thermally Induced Phase Separation Process

The thermally induced phase separation (TIPS) process was employed to prepare poly(vinylidene fluoride)/polyethylene oxide-co-polypropylene oxide-co- polyethylene oxide (PVDF/PEO-PPO-PEO, or PVDF/P123) blend microporous membranes using sulfolane as the diluent. The effect of P123 content on the phase diagram of the PVDF/P123/sulfolane system was analyzed. The effects of P123 weight fraction and cooling rate on the cross-sectional morphology, crystallinity, crystal structure, thermal stability and porous structure of the resulting membranes were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and mercury porosimetry, respectively. The mechanical properties of the membranes were evaluated by tensile tests. It was found that solid-liquid phase separation occurred in the PVDF/P123/sulfolane system. The addition of P123 decreased the dynamic crystallization temperature of the PVDF/P123/sulfolane system. SEM revealed that neither increasing the P123 weight fraction nor decreasing the cooling rate would lead to macroscopic phase separation between P123 and PVDF. P123 weight fraction and cooling rate had some influence on the crystallinity, porous structure and mechanical properties, but no influence on the polymer crystal structure of the membranes. P123 weight fraction influenced the thermal stability of the final membranes but the cooling rate did not. The influences of the change in morphology, crystallinity, etc on the property of membranes for lithium ion battery, such as ionic conductivity, were also investigated.