Multimorphological Crystallization of Shish-Kebab Structures in Isotactic Polypropylene: Quantitative Modeling of Parent-Daughter Crystallization Kinetics

A model is presented for the crystallization kinetics of flow-induced shish-kebab structures in isotactic polypropylene. The model accounts for two phenomena that affect the crystal growth of kebab structures. First, the temperature of the material increases due to latent heat release. Second, polymer chains are deformed in such a way that growth of kebabs (with their c-axis in flow direction) is promoted. Furthermore, we incorporate crystallization kinetics of the daughter morphology which nucleate on the (010) lateral surface of kebabs. The model is validated with in-situ wide-angle X-ray diffraction data from an in-house-developed extensional flow device and a modified multipass rheometer used to apply a flow pulse in a slit flow geometry. Excellent agreement is found between the model and experimental data, in terms of crystallization kinetics as well as parent/daughter ratio. The well-known Avrami model, in which it is assumed that crystal growth rate is constant, is widely used to analyze (quasi-)isothermal flow-induced crystallization experiments. We show that, because the growth rate of kebabs is strongly time dependent due to temperature and orientation effects, this analysis fails for experiments like the ones presented in this study. This is manifested in noninteger Avrami coefficients typically smaller than 2, which have been observed in a number of different studies. The current model explains these observations.