Polypropylene microporous membranes were prepared by melt-extrusion and cold-stretching (MSCS) method.The structure changes of microporous membrane during cold stretching

heat setting and hot stretching were studied in suit to reduce the effect of other factors on cold-stretching samples

such as natural retraction induced by cold stretch and the need of cool detection after heat setting.The influences of cold-stretching ratio on micro-lamellae and micropore structure of polypropylene microporous membranes were investigated by in situ small angle X-ray scattering (SAXS)

scanning electron microscope (SEM) and differential scanning calorimeter (DSC).The results show that different cold-stretching ratios lead to the different hole formation mechanism.Long-period SAXS signals of the meridian direction at the cold-stretching ratio 15% indicate a slight change of macroscopic strain.Thus the corresponding pore formation mechanism is that cold stretching induces micro defects while hot stretching transforms them into micropores.At the cold stretching ratio 30%

small lamellae transform into unstable bridges and part of the bridges break after heat setting.Besides

noticeable signals of streak emerge in the vertical direction.When the cold-stretching ratio is increased to 40%

signals in stretching direction demonstrate that the growth rate of actual lamellar long period is lower than that of macroscopic strain.This phenomenon can be attributed to that partial skeleton lamellae are destroyed by stretching and transformed into bridging crystal fibers.Macro statistics and micro calculations show that the larger the stretching radio

the shorter the length of bridges and the smaller the aperture.Thus the S30 is the most appropriate cold-stretching radio to prepare microporous membranes.