Analysis of flow induced crystallization through molecular stretch

- The effect of flow is lumped into a parameter representative of the molecular stretch.
- A correlation between the stretch parameter and the equilibrium melting temperature.
- The morphology evolution during crystallization of an i-PP has been modeled.
- The model nicely compare with experimental results of the final morphology of samples.

In this work, specific experiments on an isotactic polypropylene are carried out, aiming to investigate the flow induced crystallization and the final morphology. The viscoelastic nature of the polymer is described by a non-linear Maxwell model applied to the conformation tensor. Shear stress evolutions, recorded during step shear isothermal experiments, are satisfactory described considering the molecular stretch, i.e. the difference between the two main eigenvalues of the conformation tensor. In the general model, the effect of temperature, pressure, and crystallinity are taken into account. Furthermore, a modeling framework is proposed to describe flow-induced crystallization of isotactic polypropylene. The spherulitic growth rate is analyzed on the basis of a flow dependent equilibrium melting temperature, using the molecular stretch as the key parameter. A phenomenological correlation of the nucleation rate with growth rate is observed. By combining the morphological models, both for nucleation and growth rate, for flow induced crystallization is possible to explain the effect of shear rate and shearing times in different experimental results, and potentially in the simulation of polymer processing.

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