Modeling and validation of viscoelastic stress induced nucleation and crystallization

A model for the description of the combined process of quiescent and flow induced crystallization of polymers was developed based on earlier work of Schneider et al. [1] and Eder et al [2].. They used the shear rate as the relevant parameter that drives the flow induced crystallization. In this study viscoelastic models (Leonov [3], extended PomPom [4]) are added from which the resulting recoverable strain, expressed by the elastic Finger tensor or the related viscoelastic stress with the highest relaxation time, is now used as the driving force for flow induced crystallization. This idea is supported by experimental results of Vleeshouwers et.al., indicating that there is a pronounced influence of molar mass on shear induced crystallization. It was postulated that the influence of deformation on the crystallization process is governed by the high-end tail of the molecular mass distribution, which is characterized by its largest relaxation times. As nucleation sites are considered to act as physical cross-links that cause local branching of the molecules, the maximum rheological relaxation time is coupled with the number of nucleation sites. The interplay between rheology and flow induced crystallization can therefore be rather complex. The model is implemented in VIp, a FEM-code for the numerical simulation of the injection molding process. For the validation of the models well defined flow experiments are used which allow for processing conditions, including reversed flow. For this purpose the Multi-Pass-Rheometer is used. A wide range of experimental methods, including WAXD, SAXS, optical microscopy, birefringence and FTIR, is used to monitor the structure during and after flow. Application of the model is not only found in macroscopic processes such as injection molding, but also in the analysis of hard particle filled polymer melts.