Effects of annealing time on physical and mechanical properties of PVDF microporous membranes by a melt extrusion‐stretching process

Polyvinylidene fluoride‐based separators for secondary battery have attracted enormous attention both in academia and in industries during the last decade. In this study, a facile, cost‐effective, and environmentally friendly melt extrusion‐stretching (MES) process was applied to prepare high performance PVDF microporous membranes for separator applications. In order to achieve a balance between the formation of row‐nucleated lamellar structure and sufficient lamellae separation during stretching, a blend of high‐ and low‐molecular‐weight PVDF grades was chosen as raw material. Meanwhile, the effects of annealing time on physical and mechanical properties of PVDF cast films and membranes were systematically investigated, demonstrating that 6 h @145°C annealing treatment can lead to a preferable crystalline structure for the following stretching process. Therefore, PVDF membrane with high porosity, high electrolyte uptake ratio, and superior tensile strength was obtained by this MES process. Interestingly, the increase of β phase content by stretching was not significant for this PVDF membrane, revealing that the formation of micropores/microfibrils in the stretching process would lead to stress relaxation and less crystalline structure conversion.