Modelling and validation of viscoelastic stress induced nucleation and crystallization

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Abstract

Crystallization of polymers is kinetically controlled and the motion involved refers to the transport ofmolecules from the disordered liquid phase to the ordered solid phase. The crystallization process can besubdivided into three stages. (a) nucleation, (b) growth, and (c) perfecting. Perfecting is the process ofimprovement of the interior crystalline structure of the crystalline regions, also referred to as secondarycrystallization.Dependent on the thermomechanical history experienced during flow, the number and type (point-like:not su±cient deformation, thread-like: su±cient deformation) of formed nuclei will be di®erent as willthe final crystalline structure. In both cases lamellar growth takes place which results in a spheruliticalstructures for the point-like nulei and in shish-kebab structures for the thread-like nuclei. For example,the absence of shear in the centre of an injection moulded product results in a spherulitical structure,while in the highly strained regions near the cavity walls a shish-kebab structure can be present. Thisinternal structure determines the final product properties.A model for the description of the combined process of quiescent and flow induced crystallization ofpolymers is presented. The model is based on the work of Schneider et al. and Eder et al. where theshear rate was taken as the relevant parameter that drives the flow induced crystallization. A viscoelasticmodel (Leonov, extended PomPom) is added from which the resulting recoverable strain (expressed by theelastic Finger tensor or the related viscoelastic stress) with the highest relaxation time is now used as thedriving force for flow induced crystallization. This is supported by experimental results (Vleeshouwers),indicating that there is a pronounced influence of molar mass on shear induced crystallization. It waspostulated that the influence of deformation on the crystallization process is governed by the high-end tailof the molecular mass distribution, which is characterized by its largest relaxation times. As nucleationsites are considered to act as physical crosslinks, the maximum rheological relaxation time is coupledwith the number of nucleation sites. The interplay between rheology and flow induced crystallization cantherefore be rather complex.The model is implemented in VIp, a FEM-code for the numerical simulation of the injection mouldingprocess. Comparison with experimental results shows good agreement.
Original languageEnglish
Title of host publicationProceedings of the EUROMAT 2004
Place of PublicationSwitzerland, Lausanne
PagesCD-Rom
Publication statusPublished - 2003

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