Influence of Molecular Weight on the Nucleation and Growth of Different Crystal Forms in Isotactic Polypropylene: <i>In Situ</i> Synchrotron Microfocus X-Ray Scattering Combined with Fast-Scanning Chip Calorimetry Investigations

Abstract

The molecular weight (Mw)-dependent formation of mesophase and the α phase of isotactic polypropylene (iPP) was explored by utilizing in situ synchrotron microfocus X-ray scattering measurements combined with a portable fast-scanning chip calorimetry (FSC) technique. A phase diagram illustrating the relationship between the lamellar long spacing (dac) and crystallization as well as melting temperatures of three iPP samples with different molecular weights suggests that a common crystallization line was shared by the mesophase of all three samples, whereas three different crystallization lines were observed for these iPP samples. Furthermore, unlike in the case of the α phase where the melting lines were significantly different from their crystallization lines, the melting line of the mesophase was found to overlap with its crystallization line. The results indicate that the crystallization of the α phase satisfied the multistage crystallization model in which an intermediate layer is suggested to be generated prior to the formation of native crystallites. The Mw-independent formation of the mesophase demonstrated that the nucleus size did not rely on the Mw of iPP because the final sizes of the mesophase were close to the initial nuclei sizes. The larger difference of the lamellar long spacing presented in the α phase of iPP with different Mw indicated that the growth from nuclei to crystallites was significantly influenced by the Mw.