Dislocations in Semicrystalline Polymers

Although semicrystalline polymers have become increasingly important for technological applications in the last decades, the micromechanical processes on an atomistic scale occurring in plastic deformation are still a matter of dispute. As an example, this concers the question whether dislocations in the crystalline phase play a role for the macroscopic strength or not. Based on a new dedicated X-ray diffraction method (Multiple X-ray Line Profile Analysis, MXPA) which allows the verification and the quantification of dislocations in semicrystalline polymers, the present project intends to clarify the following questions related to this problem: a) Do dislocations affect the mechanical properties? b) If dislocations do play a role, what are the mechanisms of dislocation generation and/or interaction and what are the specific consequences for the mechanical properties and the microstructure behind? c) In case dislocations do not play a role for the plasticity of the polymer, what are the alternative mechanisms governing the strength, as e.g. adiabatic melting and/or shear banding? To answer these questions, this project aims at a systematic experimental program including the verification of the dislocations, the quantitative determination of their density and distribution for different types of semicrystalline polymers, and also at the investigation of other microstructural parameters as a function of plastic deformation. Among other experiments, the use of in-situ MXPA during plastic deformation will provide direct evidence for the context between the evolution of the microstructure and that of macroscopic strength. Using the experimental findings, a theoretical framework for the deformation and the strength of semicrystalline polymers will be developed which builds on existing models but also considers dislocation compatible activation parameters. These modified models must be carefully tested with respect to the experiments, and compared with tests comprising other inherent deformation mechanisms, as adiabatic melting and/or shear banding.

The aim of the present project was to clarify the micromechanics of plastic deformation in a number of semicrystalline polymers. For the first time it was shown that so called dislocations - i.e. line shaped defects in the crystalline phase play a distinct role and affect the mechanical strength significantly. For this purpose the so called X-ray Line Profile Analysis (XPA) has been applied which not only can measure the densities of dislocations in semicrystalline polymers but also allows for a principal check of their presence. Polymers are materials built from long-chain molecules. Semicrystalline polymers consist of a crystalline and an amorphous -glassy- part. The most important plastics in terms of application, such as polyethylene (PE), polypropylene (PP), fall into this category. Since the crystalline phase is usually much harder than the amorphous one, it greatly governs the strength of the whole material. The systematic studies performed within this project revealed that dislocations are indeed formed during plastic deformation in many of the materials investigated, although to different extents; at any case, they play a considerable role for the strength of semicrystalline polymers. The ability to generate and mobilize dislocation depends on the polymer, its chemical constitution and the arrangement of the polymer chains within the crystals. For instance in polypropylene a crystal phase with crossed chains,the gamma-phase suppresses the formation of deformation induced dislocations but favours the formation of geometrically necessary dislocations in order to reduce internal stresses. For the important polymer polypropylene the generation and annihilation could be associated to a specific relaxation mechanism of the macromolecular chains, indicating that conformational defects in the crystal markedly affect the formation of dislocations. On the other hand also the rigidity of the amorphous phase triggers the formation of dislocations in polymeric materials as it was shown or the first time by a critical experiment within the present project. A dislocation model was developed aiming at simulating the strength of a semicrystalline polymer as a function of the deformation rate, the temperature and the microstructure of the material. Good agreement with experimental data for several different semicrystalline polymer types is obtained which justifies the relevance of the model with respect to the number and movement of dislocations.