The 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.

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Two polypropylene resins with approximate weight‐average and z‐average molecular weight but differing in low molecular weight tail content were used for the fabrication of microporous membrane based on melt‐stretching mechanism. The precursor film with row‐nucleated crystalline structure, annealed film and cold‐stretched film were prepared and characterized. It was found that the existence of the low molecular weight tail resulted in higher lamellar thickness in precursor film and more perfect grown crystalline part during annealing. During stretching under room temperature, within the plastic region in stress–strain curve, these grown parts were pulled out and converted to bridges connecting separated lamellae. Higher content of connecting bridges existed in the cold‐stretching film with low molecular weight tail. In addition, the polypropylene resin with low molecular weight tail showed more pronounced stain‐hardening behavior in stress–strain curve, indicating crystallization occurrence of stretched tie chains, resulting in more stable connecting bridges and less deterioration to formed pore structures. Therefore, it can be seen that for polypropylene resin with certain high molecular weight chains, the existence of low molecular weight tail is important to the formation and stabilization of pore structure during the fabrication of microporous membrane by stretching. POLYM. ENG. SCI., 53:2594–2602, 2013. © 2013 Society of Plastics Engineers

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The changes of plastic plateau in the stress-strain curves of annealed polypropylene (PP) films during stretching under room temperature were followed and the corresponding melting properties and microstructure were characterized by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). It was found that during stretching the plastic plateau disappeared progressively with the increase of drawing ratio. At the same time, the endotherm plateau in DSC curves also disappeared progressively. The presence of the plastic plateau was attributed to the stretching of unstable crystalline part which was formed by tie chains around initial row-nucleated lamellae structure during annealing. During stretching, the unstable part was stretched and converted to bridges connecting separated lamellae. There was direct relationship between the disappearance of plastic plateau and pore formation.