1. School of Materials Science and Engineering, Tianjin University, Tianjin, P. R. China;2. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P. R. China;3. Department of Chemistry, School of Science, Tianjin University, Tianjin, P. R. China
Abstract:
The conformation of a helical type polymer chain in the crystalline phase prefers the state with lower energy. The molecular simulation of the melting behaviors of α and β isotactic poly(propylene) (iPP) crystals with ultrafast heating shows that the ratio of right‐handed and left‐handed helix stays at 50% in α‐iPP and changes from 100% to 50% in β‐iPP before and after melting. Till date, the formation of β form crystal in iPP has not been understood well together with the mechanism of the crystallization process. One of the approaches to produce β‐iPP is to shear the melt. The influence of shear flow on the crystallization behavior of iPP was investigated via ex situ and in situ X‐ray measurements. It was found that preceding or simultaneously initiated formation of β‐iPP together with α‐iPP occurs under proper shear conditions. The β‐rich iPP sample could no longer form β crystal at 135 °C after heating to the β‐melting temperature with the melting of β‐iPP and the retaining of oriented α‐iPP, which is contrary to the accepted formation process of β‐iPP. A mechanism was proposed to understand the shear‐induced β crystal in iPP, considering the mesophase and stereocomplex type helical conformation of the polymer chains.
