Investigation of deformation and pore formation in isotactic polypropylene containing active nano‐CaCO 3

In order to improve the β ‐lamellae distribution and properties of β ‐isotactic polypropylene ( β ‐iPP) membranes, amounts of 5 and 10% active nano‐CaCO 3 were added into β ‐iPP. Differential scanning calorimetry, X‐ray diffraction and scanning electron microscopy results show that nano‐CaCO 3 does not reduce the content of β ‐crystals, but the thickness, lamellae thickness distribution and stability of β ‐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 β ‐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 β ‐lamellae stability. Through further detailed characterization of morphological evolutions during stretching, we found that interfacial debonding between nano‐CaCO 3 and β ‐iPP is triggered and abundant microviods can be formed, which can retard the rotation and slip of β ‐lamellae and make the β – α 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 β ‐iPP cast film and finally result in larger pores and poor pore distribution in the membrane. © 2017 Society of Chemical Industry