Shear-induced smectic ordering and crystallisation of isotactic polypropylene

Abstract

Shear-induced smectic ordering and crystallisation of isotactic polypropylene (iPP) has been studied with in-situ small- and wide-angle X-ray scattering. Shear-induced smectic bundles with a periodicity of about 4 nm have been observed at temperatures below as well as above the melting point. This applies to iPP of different molecular weight and from different sources. An increase in the average molecular weight leads to a larger periodicity of the smectic layers. The smectic layers assemble in a fibrillar morphology with a length and a width up to 200 and 10 microm, respectively. After crystallisation, the smectic bundles show upon heating higher melting temperatures than their crystalline counterparts. In agreement with this behaviour, the correlation length along the smectic layer normal is of the order of tens of nanometers, much larger than the crystal thickness. We present an anisotropic drop model of smectic domains forming a conserved system in which the smectic layers can rotate. On the basis of this model we can explain the relative orientation of the smectic layers, the crystalline lamellae and the long axis of the drop, as well as the reversibility of the smectic periodicity during cooling and heating. In the supercooled melt, the smectic ordering is followed by crystallisation; during this process crystals grow epitaxially on the surface of the smectic bundles. This leads to a new picture of the shish-kebab structure in which smectic bundles rather than extended-chain crystals play the role of the shish. The crystallisation process of the smectic regions themselves indicates that the 'mesophase' previously reported in fast-quenched iPP, is a metastable state formed during the transition from the high-temperature smectic phase to a crystal. Moreover smectic domains rather than alpha crystallites form the nuclei for crystallisation of the beta phase. The high-temperature smectic phase presents an ideal model system to study the coupling between density and conformational ordering under shear flow. Our results support a nucleation and growth process for polymer crystallisation, in which smectic bundles or other mesophases are the primary nuclei.