Modelling of morphology development towards spherulites and shish-kebabs: Application to isothermal flow-induced crystallization experiments on isotactic polypropylene

The final properties of a plastic object are related to the structures developed during the process adopted for part production. The structures development is induced by the complex thermomechanical history experienced by the macromolecules during the process. In particular, for isotactic polypropylene it has been observed that flow field influences the spherulitic crystallization and also induces the formation, when strong flow fields are present, of fibrillar structures composed of a core (the shishes) oriented along the flow direction, and lamellae that grow in the transverse direction (the kebabs). In this work, a model able to describe the morphology development during the polymer crystallization is proposed. This model accounts for the morphologies, shish-kebabs and spherulites, which can develop during the crystallization of the alpha-phase of the considered iPP, the effect of flow on crystallization kinetics and the dependence of the relevant material properties on the crystallinity. Two competing kinetics are considered, one for spherulites and another one for the shish-kebabs. In order to validate the model predictions, isothermal flow induced crystallization experiments were conducted. The analysis of the flow induced crystallization tests adopting the proposed model correctly reproduces the formation of fibrillar and spherulitic structures and detects the shearing condition at which fibrillar morphology overcomes the spherulitic morphology.