Heterogeneous Nucleation and Self-Nucleation of Isotactic Polypropylene Microdroplets in Immiscible Blends: From Nucleation to Growth-Dominated Crystallization

Nucleating agents (NAs) are widely used for speeding up processing and tuning the final optical and mechanical properties. Despite their industrial importance, the search for highly efficient NAs is still mainly empirical. In this work, the heterogeneous nucleation process of isotactic polypropylene (i-PP) droplets containing nucleating agents (i.e., sodium benzoate (SB), (4,6-di-tert-butylphenyl)phosphate (NA-11), quinacridone quinone (QQ)) dispersed in an immiscible polystyrene matrix has been studied by differential scanning calorimetry (DSC) using an isothermal step crystallization protocol. When semicrystalline polymers are confined into isolated micro- or nanodomains, nucleation can kinetically control the overall crystallization. Differently, in a bulk polymer, the contributions of the nucleation and growth steps to the overall crystallization rate are commonly of similar importance. In the studied system, it is shown for the first time that the crystallization of nucleated i-PP droplets exhibits a first-order kinetics, with characteristic times much larger than those required for the crystals to grow and occupy the whole microdomain volume. Therefore, the kinetics of heterogeneous nucleation can be directly assessed. On the contrary, when neat i-PP droplets are self-nucleated, the crystallization kinetics shows a sigmoidal trend, with times comparable to those for the crystal space filling. This indicates the absence of any primary nucleation barrier for ideally self-nucleated PP melts, or at least that this nucleation barrier is negligible in comparison to the one for secondary nucleation/crystal growth. For the first time, it is evidenced that, as long as nucleation is the rate-determining step in the overall crystallization kinetics, a first-order kinetics can be obtained for both homo- or heterogeneously nucleated droplets in immiscible blends. A novel way to calculate an "intrinsic"nucleation efficiency, based on the derived free-energy barriers of the different NAs, is proposed. This fundamental knowledge of heterogeneous nucleation can serve as a tool for a more rational search for new nucleating agents and can provide a method to unambiguously identify the origin of multiple-crystallization exotherms in fractionated crystallization of immiscible blends.