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The correlation between the lower temperature melting plateau endotherm and the stretching-induced pore formation in annealed polypropylene films

The room temperature stretching process of annealed polypropylene hard elastomer films was analyzed and the relationship between the low-temperature endotherm plateau in the differential scanning calorimetry curves generated during annealing and the stretching-induced pore formation was studied. It was found that compared with that of precursor film, an endotherm plateau and some diffraction phenomena around 2θ angles of 13.0 degrees and 18.4 degrees were observed after annealing. During stretching, the plateau disappeared progressively and these diffraction phenomena also vanished. At the same time, the pores and bridges connecting lamellae were formed. The endotherm plateau came from the crystallization of tie chains around initial row-nucleated lamellar structures. During pore creation stage, the grown crystals were first drawn and converted to bridges connecting lamellae. Further stretching into the strain-hardening stage led to the deformation of the row-nucleated lamellar structures. The experimental results indicated that there was a direct relationship between the endotherm plateau and pore formation during the fabrication of microporous membrane based on melt stretching mechanism.