The influence of melt status and beta-nucleation agent distribution on the crystallization of isotactic polypropylene

Although investigated extensively in past decades, the factors affecting beta-crystallization in beta-nucleating agent/iPP composites have not been identified completely. In this study, beta-crystallization in mesomorphic melt and free melt with 0.05 wt% WBG-II was investigated at various melting and crystallization temperatures, employing polarized optical microscopy, scanning electron microscopy, differential scanning calorimetry, and synchrotron small-angle and wide-angle X-ray diffraction. It was found that beta-nucleation efficiency and thermodynamic stability of beta-crystals depended strongly on the melting temperature. Higher beta-nucleation efficiency and thermodynamic stability can be obtained only when the melting temperature was below 195 degrees C, where the beta-nucleating agent (beta-NA) was well dispersed in the melt. Further, ordered structures in the mesomorphic structured melt can further promote the formation of more perfect beta-crystals. It is thought that beta-NA distribution and ordered structures in the polymer melt are the two key factors affecting the formation of beta-crystals. The former affects the number of beta-nucleating sites accessible for iPP chains, while the latter affects the growth of beta-crystals. Only when beta-NA is well dispersed and the iPP melt exists massive chiral intrachain clusters similar to beta-crystals, can perfect beta-crystals be induced fully.