An unusual promotion of γ-crystals in metallocene-made isotactic polypropylene from orientational relaxation and favorable temperature window induced by shear

Combined effects of shear flow and non-isothermal processing, which were akin to the practical industrial processing, on the crystallization of metallocene-made isotactic polypropylene (m-iPP) were investigated by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The simultaneous appearance of α- and γ-crystals was observed in quiescent samples, while shear flow led to the occurrence of α-crystals prior to γ-crystals at the initial stage of crystallization. Although the local alignment of chain segments promoted by shear flow facilitated the α-crystals, the relaxation of oriented chains was experimentally confirmed to boost the formation of γ-crystals. Interestingly, the resultant γ-crystal content was higher in the sheared samples than the quiescent counterparts at a given cooling rate and further increased with the decrease of cooling rates. Such a discrepancy was not only related to shear-induced orientation and relaxation, but also was accredited to the advanced crystallization temperature window, which was favorable for the γ-crystal growth. It was quite different from the isothermal crystallization situation, where the formation of γ-crystals was depressed in presence of shear flow. The dynamic competition between α-crystal and γ-crystal formation under the effects of shear during the non-isothermal crystallization was discussed. Our work is of valuable reference to tailor the crystalline structure of m-iPP during practical processing and hence to acquire the desired performance of products.