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

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 beta-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 0-200 s−1, pressures varying from 100-1200 bar and shear temperatures from 130-180 ◦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 beta-phase can be accurately described by assigning a fixed portion of flow-induced spherulites to the beta-phase. Due to the high growth rate of beta-phase compared to the alpha- and
gamma-phases, although over a relatively narrow temperature range, only a seemingly small portion of 0.2% of all flow-induced nuclei becoming beta-spherulites is enough to explain the experimentally observed volume fractions of up to 20%. Secondly, it is shown that experimentally found volume fractions of gamma-phase at high shear rates and pressures can only be matched if gamma-crystals can directly nucleate on highly oriented flow-induced crystallites (so-called shish).