Continuum mechanical representation of the process-dependent caloric and thermomechanical behaviour of semicrystalline polymers

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Due to their versatility, the mechanical, physicochemical and thermodynamic understanding and the representation of semi-crystalline polymers are for many years in the centre of research projects. Semi-crystalline polymers exhibit spatially separated regions in which the molecules form amorphous and crystalline phases. In between, there is often an interphase. It usually possesses a higher glass transition temperature than the mobile amorphous phase and is therefore referred to as a rigid amorphous phase. The current research project deals with the phenomenological and micromechanical modelling of the process-dependent behaviour of semi-crystalline polymers under thermal and volumetric loads and has the following main objectives: 1) Modelling of crystallization and melting of a semi-crystalline model polymer as function of temperature and pressure on the basis of a consistent experimental database. To this end, the methods of phenomenological continuum thermomechanics with internal variables are applied. The constitutive model is formulated for three spatial dimensions and specialized to represent the volumetric behaviour. In order to understand the local mechanisms at the phase boundaries and to check or improve the assumptions made in the framework of the phenomenological model, such as the existence of three phases or the mathematical structure of the evolution equation for the degree of crystallization, a method of homogenisation will be developed that can describe the mesoscopic properties of the material on the basis of the microscopic behaviour of the individual phases. 2) Micro-mechanical modeling of melting and crystallization. The kinetics of the crystallization should be the basis of a simplified micro-structure model (based on rank-1 and rank-2-laminates with constitutively modelled interface) which allows a quantitative understanding of the crystallization, starting from a spatially resolved model with mobile phase boundaries. The two constitutive models to be developed provide first blocks are within a complex modeling strategy which goes beyond this project. Experimental data of a model polymer form an essential basis for the constitutive representation with respect to calorimetry and thermomechanics. For the model development and the identification of the influence of the mechanical and thermal loading histories on the crystallization and melting behavior, an appropriate model polymer is to investigate experimentally. As many material parameters as possible (for example, the mechanical and caloric material parameters of the amorphous and crystalline phases) are used for the phenomenological and micromechanical model. This is possible because both models exhibit similar physical properties.