Nucleation and crystallization of isotactic poly(propylene) droplets in an immiscible polystyrene matrix

Abstract

In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1–2 μm), the iPP crystallizes in a fractionated fashion at temperatures between 104 and 42°C. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i. e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sufficient amount of a compatibilizer is used to obtain very small particle sizes and more homogeneous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exhibits an onset at 51°C and peaks at 46°C. Wide-angle X-ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α-phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity-free droplets that nucleate homogeneously at temperatures close to 51°C.