In-situ Small Angle X-ray Scattering Study On The Polypropylene Microporous Membranes Prepared By The Melt-stretching Method

Polypropylene microporous membranes were prepared by melt-spinning and cold-stretching method. The structure changes of microporous membrane during cold stretching, heat setting and hot stretch were studied by in-situ small angle X-ray scattering,scanning electron microscope and differential scanning calorimeter. The home-made uniaxial tensile device with constant width, was used in this research. The device has mini size and light weight. It can combine with X-ray diffraction for in-situ detection, collect stress-strain date and heat setting. And even it can simulate rapid stretching of industrial production in laboratory.In order to study annealing effect on the microporous membrane structure and performance, the influence of annealing to the mechanical properties during stretching, the microstructure and the final structure and performance was studied. Results shows that annealing in this process can help to thicken lamellar, increase lamellar amount, which made the hard elastomer more neat and easier to deformation. To the contrary, lamellar in samples without annealing was more easier to slip or crash. Therefore, annealing can heal some defects in prefabricated film reduce residual stress and strain and then improve the structure and properties of microporous membrane.For the study of stretching at room temperature, the uniaxial tensile device with constant width, combined with small Angle X-ray scattering measurement, was used to collect the signals after stretching, which can avoid the effects of retraction influence of membranes and reflect influence of stretching to cavitation correctly. The results shows that the larger the stretching speed, the harder the amorphous between lamellar was destroyed. When stretched to 15% at room temperature, long period from small Angle X-ray scattering signals shows little difference, and cavitation signals did not show up. But after hot stretched, the cavitation signals were found. It suggests that cold stretching yields defects and hot stretching creates pores. 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. Then, small Angle X-ray scattering resulted in 40% cold stretched samples,showing that the growth rate of actual lamellar long period was smaller than macroscopic strain, which declare that some bone lamellar had been stretched into holes and transformed into fibre crystal.Calculated by one dimensional correlation function and combined with data statistics of the images from scanning electron microscope, we studies the thickness of the slice of lamellar stack, long period and lamellar thickness and their variation in their respectivescale. The results shows that the separated bone lamellar stack during cold stretching and heat setting, was further separated during hot stretching, and the bridges connecting lamellar stack become longer because of stretching effects. With the increase of hot stretching ratio,the weaker bridges broke. Broken chains and stretched chains made up the bridges structure.At the same time, some chains in slipped, rotated, and broken lamellar stack was drawn into bridges and some bridges gathered into a bundle to be stronger. When stretched more than200% at high temperature, many bridges began to fracture and some pores closed. Besides,when the temperature was lower, it may weaken the influence of heat setting’s crystallization effect. 135℃ hot drawing temperature can maintain basic bridge length and lamellar stack.Therefore, it can be used in industrial processing for its cheap cost and is the proper condition for microporous films processing. However, much higher temperature made some bridges broken and might cause pores to close and then lower porosity.