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Zare Ghomsheh M.,University of Vienna | Spieckermann F.,University of Vienna | Polt G.,University of Vienna | Wilhelm H.,University of Vienna | And 2 more authors.
Polymer International | Year: 2015

Systematic studies were conducted of discrete deformation processes occurring during nanoindentation creep of polyethylene with respect to load and loading rate, indicating dislocation-mediated plasticity. Nanoindentation creep experiments on polyethylene were carried out in order to investigate dislocation-based plastic deformation mechanisms. Similarly to that reported in a recent paper (Li J and Ngan AHW, Scr Mater 62:488-491 (2010)), discrete deformation processes occur during nanoindentation creep tests which again seem to arise from the break-off of dislocation avalanches. That interpretation is supported from systematic studies of the effect of variations of the loading rate and of the applied load on the number and the height of bursts. © 2015 Society of Chemical Industry. Source

Spieckermann F.,University of Vienna | Wilhelm H.,University of Vienna | Wilhelm H.,Laboratory of Polymer Engineering LKT TGM | Schafler E.,University of Vienna | And 3 more authors.
Journal of Physics: Conference Series | Year: 2010

The evolution of the microstructure during compressive deformation of the biodegradable polymer poly(3-hydroxybutyrate) (P3HB) was investigated in-situ via X-ray diffraction using synchrotron radiation. Flow curves were measured in-situ together with X-ray profiles for several degrees of deformation. The profiles were analysed using Multi-Reflection X-ray Line Profile Analysis (MXPA) adapted by the authors for semicrystalline polymers providing lamella thickness, crystallinity, and the presence and density of dislocations as a function of the deformation. In contrast to previous investigations in α crystallised isotactic polypropylene (α-iPP), P3HB does not exhibit a deformation induced increase of the dislocation density which suggests mechanisms other than dislocations to be involved in plastic deformation of P3HB. © 2010 IOP Publishing Ltd. Source

Spieckermann F.,University of Vienna | Wilhelm H.,University of Vienna | Wilhelm H.,Laboratory of Polymer Engineering LKT TGM | Kerber M.,University of Vienna | And 5 more authors.
Polymer | Year: 2010

X-ray line profile analysis was used to determine the size distribution of the crystalline lamellae in isotactic polypropylene (iPP) assuming a log-normal size distribution. A comparison with the size distribution as determined by differential scanning calorimetry (DSC) yields an excellent agreement of both methods. It is noted that the agreement depends strongly on whether linear lattice defects, particularly dislocations are taken into account in the X-ray analysis. This is especially true for deformed iPP with a high number of deformation induced dislocations. It was also found that for a multimodal distribution of lamella thickness in the DSC experiment as induced by the introduction of titanium dioxide nanoparticles as filler material the lamella thickness distribution from X-ray profile analysis is still in good agreement with DSC although the model used was only monomodal. © 2010 Elsevier Ltd. Source

Polt G.,University of Vienna | Spieckermann F.,University of Vienna | Wilhelm H.,University of Vienna | Wilhelm H.,Laboratory of Polymer Engineering LKT TGM | And 4 more authors.
Mechanics of Materials | Year: 2013

Samples of γ-iPP were prepared by crystallization at elevated pressures using a specially designed pressure chamber. The γ-phase was subsequently investigated by dedicated in-situ X-ray diffraction experiments during deformation using synchrotron radiation. Parameters such as the crystallinity, the density of dislocations and the coherently scattering domain size (CSD-size) which in polymers corresponds to the lamella size, have been evaluated as a function of the strain by means of the multi-reflection X-ray line profile analysis (MXPA). Compared to the results for the α-phase, those for the γ-phase reveal an enhanced strength and a strongly reduced evolution of the dislocation density. The latter is explained in terms of a model comprising the formation of misfit dislocations. © 2013 Published by Elsevier Ltd. Source

Spieckermann F.,University of Vienna | Spieckermann F.,Albert Ludwigs University of Freiburg | Wilhelm H.,University of Vienna | Wilhelm H.,Laboratory of Polymer Engineering LKT TGM | And 3 more authors.
Polymer (United Kingdom) | Year: 2014

The aim of the paper was to use recent experimental dislocation data for the development and testing of a dislocation mediated strength model based on that introduced by Scogna and Register for semicrystalline polymers. It is shown that the model can successfully describe measured and evaluated data of high density polyethylene (PE-HD) on the flow stress as function of strain rate and temperature. Similar coincidences of model fit with experimental yield stress data from literature for polypropylene and polyethylene-ethylenemethacrylic acid co-polymers (E/MAA) were found which suggests prevailing of dislocation mediated plasticity mechanisms also in these materials. It also turned out that two parameters of the model-namely the dislocation density and the lamella size-are not determined by the molecular chemistry but by the conditions of processing and/or sample preparation. Therefore the model allows for a reliable estimation of the dependence of the yield stress on these conditions. © 2013 Elsevier Ltd. All rights reserved. Source

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