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Four β‐nucleated polypropylene samples with increasing die draw ratio (DDR) were prepared to modify lamellae arrangement. The DSC, SEM, and 2D‐XRD results show that all four cast films had similar crystallinity, high contents of β‐crystal but lowering stability of β‐lamellae with ascending DDR. Meanwhile, the anisotropy of β‐lamellae distribution strengthens gently and the stacked lamellae structure perpendicular to the machine direction (MD) predominates dramatically. Tensile testing at 25 °C and 90 °C were conducted along MD and transverse direction (TD), respectively. The markedly expanding difference of deformation indicates the anisotropy highlighted significantly. Additionally, when the samples stretched along MD, a more homogeneous deformation occurs with ascending anisotropy, which is completely opposite to the β‐lamellae stability. But samples deformed more heterogeneous when stretched along TD. The characterization of morphological evolutions during stretching shows that the stacked lamellae debonds uniformly and abundant microvoids formed when the sample stretched along MD with higher anisotropy, resulting in evenly dispersion of stress, consequently making a more uniform distribution of defects and a better isotropic deformation. Moreover, the microfibrils and defects distributed uniformly within higher orientation sample after longitudinal stretching stretched along MD, leading to the dramatic improvement of pore size distribution of the membrane after biaxial stretching. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1745–1759

In order to improve the -lamellae distribution and properties of -isotactic polypropylene ( -iPP) membranes, amounts of 5 and 10% active nano-CaCO 3 were added into -iPP. Differential scanning calorimetry, X-ray diffraction and scanning electron microscopy results show that nano-CaCO 3 does not reduce the content of -crystals, but the thickness, lamellae thickness distribution and stability of -lamellae decrease apparently. Tensile testing was conducted at 25 and 90 ∘ C. The results manifest that the second yield point, which has a strong negative correlation with lamellae thickness distribution, is delayed monotonically with addition of nano-CaCO 3 when stretched at 25 ∘ C, indicating that nano-CaCO 3 could narrow effectively the lamellae thickness distribution of -iPP. Furthermore, when stretched at 90 ∘ C, the subdued yield peak, retarded necking-down phenomenon and enhanced strain-hardening modulus demonstrate that the deformation stability improves gradually with introduction of nano-CaCO 3 , which is completely opposite to the trend for -lamellae stability. Through further detailed characterization of morphological evolutions during stretching, we found that interfacial debonding between nano-CaCO 3 and -iPP is triggered and abundant microviods can be formed, which can retard the rotation and slip of -lamellae and make the -transformation slow down in the initial stage of stretching, consequently leading to better isotropic deformation. Moreover, nano-CaCO 3 could efficiently restrain the formation of coarse fibrils, leading to more uniform pore size distribution within the biaxial stretching microporous membrane. However, excessive nano-CaCO 3 (10%) would cause aggregation within the -iPP cast film and finally result in larger pores and poor pore distribution in the membrane.

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