Influence of uniaxial cold stretching followed by uniaxial hot stretching conditions on crystal transformation and microstructure in extrusion cast and annealed polyvinylidene fluoride porous membranes

Polyvinylidene fluoride (PVDF) film samples were prepared by extrusion casting followed by annealing. Then, the samples were uniaxially cold stretched at room temperature, followed by uniaxial hot stretching. The total uniaxial stretching ratio of the samples after cold and hot stretching steps was kept constant at 100%. The influence of hot-stretching temperatures and cold and hot stretching ratios on the crystal transformation and pore structure was investigated using wide-angle X-ray diffraction(WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The results showed that the stretching-induced β-phase content did not depend on hot stretching temperature and cold and hot stretching ratios. Apparent connecting bridge structure between the separated lamellae was observed at 130℃ stretching temperature and 40% to 60% or 50% to 50% cold and hot stretching ratio. Compared with that induced by cold stretching alone, more connecting bridges were formed and higher β-phase concentration was created by cold stretching followed by hot stretching. During hot stretching, some imperfect chains from main lamellae were stretched and converted to part connecting bridges, and some other imperfect chains were transformed to the β-phase. But, the β-phase content during hot stretching was not related to the extent of connecting structure.