Influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane

AbstractThe influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane was investigated using small‐angle X‐ray scattering, temperature‐modulated differential scanning calorimetry and scanning electron microscopy. It is found that with increasing annealing temperature from 105 to 145 °C, the main lamella melting peak combines with that from connecting bridges and a uniform pore arrangement is obtained in the microporous membrane. The annealed lamella thickness is increased and lamellar structure is improved, due to the occurrence of melting and recrystallization during annealing. At the same time, more secondary crystals are formed. The melting and recrystallization and secondary crystals contribute to the appearance of an annealing peak in the differential scanning calorimetry curve of annealed film. During the following cold and hot stretching, the secondary crystals disappear and convert to initial connecting bridges. The improved lamellar structure can support the scaffold of pore structure, resulting in a uniform connecting bridge arrangement. But further increasing the temperature to 150 °C degrades the initial lamellar structure, leading to a decrease of pore arrangement in the stretched microporous membrane. Annealing leads to the difference of lamellar structure: the initial lamellar structure is improved and some weak secondary crystals are formed in the amorphous region. © 2014 Society of Chemical Industry