Microporous films were obtained from an extruded sheet of isotactic polypropylene (PP) containing N,N’-dicyclohexyl-2,6-naphthalene dicarboxamide as a crystal nucleating agent under suitable hot-stretching conditions. The crystalline structure before and after the stretching operations was evaluated by two-dimensional wide-angle X-ray diffraction, and it was found that the original sheet contains a large amount of β-form crystals oriented to the transversal direction (TD) by the extrusion at 200 °C, which is lower than the dissolution temperature of the nucleating agent in PP. During stretching of the sheet, α-form crystals oriented to the stretching direction appeared by the transformation from β- to α-form crystals. Moreover, numerous microvoids were generated by stretching around at 100 °C to the machine or flow direction (MD) of the sheet, although the microvoids were prolonged. The sequential TD stretching, however, expanded the area of voids, leading to a microporous film with a pronounced porosity.
