Study of structure and properties of polypropylene microporous membrane by hot stretching

The polypropylene (PP) microporous membrane is prepared by only hot stretching of annealed PP film under different stretching rate and temperature. The structure and properties are compared with those by cold and hot stretching. The results show that apparent connecting bridges are observed on the surface of final microporous membrane only by hot stretching. With increasing stretching rate and decreasing stretching temperature, the pore size decreases and the air permeability becomes worse. The membrane only by hot stretching under 50 mm/min shows better lamellae separation, whereas under 10 mm/min the membrane gives lowest Gurley value. Compared with cold and hot stretching under the same stretching rate and whole stretching ratio, the membrane stretched only by hot stretching shows longer connecting bridges length, higher porosity and lower Gurley value. During only hot stretching, first the tie chains in the amorphous region are stretched. Then the combination of crystalline main lamellae and crystalline part formed during annealing is stretched and converted to connecting bridges. Through the control of stretching temperature and rate, PP microporous membrane with better permeability property can be obtained by only hot stretching, where room-temperature stretching is not needed. The obtained membrane can be used in Li-ion battery field as a separator, where the air permeability is an important technical parameter to characterize the Li-ion penetration ability through the separator.