Fluctuation-assisted nucleation in the phase separation/crystallization of iPP/OBC blends

This work mainly focused on the nucleation behavior in iPP/OBC (isotactic polypropylene/polyolefin block copolymers) blends with two distinct OBCs. The influence of composition and molecular structure of the OBC component on the crystallization kinetics of the blends was investigated systematically with the aim to better understand the interplay between the two coupled phase transitions in the blends: macrophase separation and crystallization. The isothermal crystallization kinetics showed component and composition dependence in iPP/OBC blends. All the blends in the studied range have enhanced nucleation ability of iPP than the pure iPP under identical conditions. Furthermore, the distinct macrophase separation morphology resulting from the different compatibility between the various OBCs and iPP caused remarkable diversity between the blends: the nuclei density is qualitatively higher (or the nucleation rate is qualitatively faster) in the more compatible blends, and this enhancement of nucleation can be depressed by imposing a macrophase separation process before crystallization. The crystal nuclei from the phase separated matrix were preferentially formed at the interface of the phase domains, and then grew toward and into the iPP-rich phase. It is postulated that the increased nuclei density and/or nucleation rate followed the fluctuation-assisted nucleation mechanism: the enhanced concentration fluctuation at the interfacial area created by the spinodal decomposition played an important role in the nucleation behavior of iPP/OBC blends. The decreased interface areas with increased domain sizes after deeper phase separation, coupled with a more depressed concentration fluctuation, are responsible for lower nuclei density after long time annealing for phase separation.