Influences of molecular weight on the non-isothermal crystallization and melting behavior of β-nucleated isotactic polypropylene with different melt structures

Abstract

Previous studies reported that by tuning the fusion temperature T f (i.e., controlling the melt structure), the β-nucleation efficiency of β-nucleated isotactic polypropylene (β-iPP) can be greatly enhanced, which was called ordered structure effect (OSE). The aim of this study is to investigate the roles of melt structures and molecular weight in non-isothermal crystallization behavior of β-iPP. Five samples with different molecular weights were prepared, and their melt structures were controlled using differential scanning calorimetry (DSC) by tuning T f (T f = 200 or 170 °C represent the case of without or with OSE, respectively). Results revealed that for all the samples, the lower the PP molecular weight, the lower the crystallization peak temperature T c, and the higher the activation energy ΔE, but the occurrence of OSE behavior (i.e., T f = 170 °C) can elevate T c, reduces ΔE and encourages crystallization. The decrease of PP molecular weight decreases β-phase proportion no matter OSE occurs or not; moreover, for PP with high molecular weight, OSE behavior not only enhances the β-phase proportion, but also increases the thermal stability of the β-phase; however, for samples with low molecular weight, OSE behavior enhances the β-phase proportion in a larger extent, and decreases the thermal stability of the β-phase. This study provided important understandings in the effect of OSE, cooling rate and PP molecular weight the β-crystallization of iPP.