Flow and Pressure Jointly Induced Ultrahigh Melting Temperature Spherulites with Oriented Thick Lamellae in Isotactic Polypropylene

Abstract

In practical processing, polymer melts generally experience flow and pressure fields simultaneously, but their flow-induced crystallization behavior under pressure was barely investigated. For this reason, we provided an insight into the crystallization behavior and crystalline morphology and structure of isotactic polypropylene (iPP) obtained under the combination of flow (2.5–22.5 s–1) and high pressure (200 MPa) by using self-designed pressurizing and shearing device. Unprecedented iPP spherulites were observed, which are composed of oriented thick lamellae (18 nm) with ultrahigh melting temperature (179.5 °C). Such spherulitic crystals with lamellae aligning perpendicular to flow have never been reported. All samples have a double melting peak behavior including an additional low temperature peak (165–169 °C) apart from the ultrahigh melting peak. The in situ synchrotron X-ray measurements show that the parents of cross-hatched structure are responsible for the ultrahigh melting temperature and the daughters give rise to the low temperature melting peak because the parents crystallized much earlier and had a higher growth rate than the daughters. Moreover, the parents are all in α-form while the daughters consist of α- and γ-forms. Our results afford a new method to obtain thick lamellae in a relatively short time.