Flow-induced crystallization of isotactic polypropylene: Modeling formation of multiple crystal phases and morphologies

- A model for flow-induced crystallization of isotactic polypropylene in multiple crystal phases and morphologies.
- All parameters but one (for flow induced β−phase crystals) are priori fixed.
- The model is validated with experimental results relevant for processing conditions.
- It is shown that γ crystals can nucleate directly on α-phase shish.

A modeling framework is presented to describe flow-induced crystallization of isotactic polypropylene at elevated pressures in multiple crystal phases and morphologies. By combining two models for flow induced crystallization developed in previous work, all parameters but one are a priori fixed. In the present work only one additional parameter was introduced, determining the portion of spherulites nucleated by flow that form β-crystals.Model calculations show good agreement with experimental data for crystal volume fractions of all phases over a very wide range of flow-conditions, with shear rates varying from 0 to 200 s−1, pressures varying from 100 to 1200 bar and shear temperatures from 130 to 180 °C. Moreover, the model provides a tool to investigate two open questions regarding crystallization of isotactic polypropylene. First, it is shown that flow-induced formation of β-phase can be accurately described by assigning a fixed portion of flow-induced spherulites to the β-phase. Due to the high growth rate of β-phase compared to the α and γ-phases, although over a relatively narrow temperature range, only a seemingly small portion of 0.2% of all flow-induced nuclei becoming β-spherulites is enough to explain the experimentally observed volume fractions of β up to 20%. Secondly, it is shown that experimentally found volume fractions of γ-phase at high shear rates and pressures can only be matched if γ-crystals can directly nucleate on highly oriented flow-induced crystallites (so-called shish).