Mechanistic Insights into the Shear-Inducedβ-Form Crystal Formation of iPP

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.