Stretching Isotactic Polypropylene: From “cross-β” to Crosshatches, from γ Form to α Form

Abstract

A method for the detailed analysis of the possible crystalline structures and preferred orientations of crystals that develop during stretching metallocene-made isotactic polypropylene (iPP) samples is presented. The analysis is illustrated in the case of various iPP samples characterized by a different distribution of stereodefects along the chain, generated by different catalysts. The method is based on the calculation of schematic X-ray fiber diffraction patterns of model structures and the comparison between calculated and experimental diffraction patterns recorded during stretching. The analyzed samples, initially crystallized in the γ form or in disordered modifications intermediate between the α and γ forms, undergo structural and morphological transformations during stretching. At low deformations, crystals of the γ form are oriented with chain axes directed nearly perpendicular to the stretching direction. The γ form gradually transforms into the α form, with chain axes parallel to the stretching direction, with increasing deformation. At high deformations, when the sample is almost totally crystallized in the α form, the crosshatched morphology, characterized by the presence of daughter lamellae epitaxially grown on mother lamellae, develops. The origin of the nonstandard orientation of crystals of the γ form with chain axes perpendicular to the fiber axis (perpendicular chain axis orientation or “cross-β”) is investigated. This kind of preferred crystal orientation involves lamellae of the γ form frozen in strained regions of the polymer matrix. Its development may be ascribed to the peculiar architecture of the γ form, consisting in a nearly perpendicular arrangement of the chain axes in the unit cell, and to the elongated shape of the γ lamellae in the direction normal, rather than parallel, to the chain axes. At low draw ratios, the chain axes, which emerge at the crystal boundaries according to two mutually perpendicular directions, act as anchors for the crystals of the γ form in the perpendicular chain axis orientation. At high draw ratios, these crystals transform into the α form and assume the orientation with chain axes parallel to the stretching direction as in the standard fiber morphology. The relationships between structure and morphology on one hand and mechanical properties of the analyzed iPP samples on the other hand are also discussed.