Estimations of the Thicknesses of Smectic Layer and Rigid Amorphous Layer in Mesomorphic Isotactic Polypropylene with Synchrotron Small-Angle and Wide-Angle X-Ray Scattering

Abstract

Despite extensive research, the stabilization mechanism of mesomorphic isotactic polypropylene (iPP) at room temperature has not been elucidated completely. To address this issue, nanoscale structural information is indispensable. In this study, mesomorphic-α phase transition is studied with synchrotron wide-angle diffraction and small-angle X-ray scattering. It is found that during phase transition, the mobile amorphous content remained unchanged, while rigid amorphous fraction (RAF) decreased constantly, accompanied by an increase in long period. Assuming that the increase in long period is induced by the extension of the rigid amorphous coil (RAC), the thickness of RAF (R_RAF) is estimated, which is found to be 2.2 nm. This value is greater than the length of b axis in α crystal cell, which can be the reason that folded-chain iPP clusters in mesomorphic iPP cannot approach closer to form α crystal at room temperature. Besides, with R_RAF, the thickness of smectic layer is estimated, which is found to be 0.9 nm. At this thickness, bulk-free energy cannot compensate for the folded-surface free energy, which can be the other reason that mesomorphic iPP chains cannot transit into α crystal at room temperature. The information obtained in this study is favorable for understanding the stabilization mechanism of mesomorphic iPP.