On the structure and nucleation mechanism in nucleated isotactic polypropylene crystallized under high pressure

Nucleating agents used for the monoclinic alpha-form of isotactic polypropylene (PP), commonly occurring under atmospheric pressure, are able to nucleate its crystallization in the orthorhombic gamma-form under high pressure. Nucleation of gamma-lamellae is possible through epitaxy involving (001)gamma crystallographic plane on substrates nucleating the alpha-form through epitaxy engaging (010)alpha plane because of equivalency of both planes. Another possibility is nucleation of alpha-lamellae through epitaxy involving either (010)alpha or (110)alpha plane, and formation of the gamma-phase through gamma/alpha epitaxy. The study focuses on the structure and mechanism of nucleation of high pressure crystallization of PP with selected nucleating agents: poly(tetrafluoroethylene) particles, commercial Hyperform HPN-20E and ADK Stab NA11UH. The nucleated PP was isothermally crystallized in the gamma-form under pressure of 200 and 300 MPa and studied ex-situ by WARD, PLM and SEM. Analysis of lamellar structure provided insight into the nucleation mechanism of high pressure crystallization of PP. In all the materials the alpha-lamellae were nucleated first and served as seeds for the gamma-form. (C) 2018 Elsevier Ltd. All rights reserved.