2019, Schneider, Konrad, Schwartzkopf, Matthias

Vulcanized natural rubber (unfilled and filled with 20 phr carbon black) is strained. We suppress the macroscopic formation of fiber symmetry by choosing strip-shaped samples ("pure-shear geometry") and investigate the orientation of the resulting crystallites by two-dimensional wide-angle X-ray diffraction (WAXD), additionally rotating the sample tape about the straining direction. Indications of a directed reinforcing effect of the strain-induced crystallization (SIC) in the thin strip are found. In the filled material fewer crystallites are oriented and the orientation distribution of the oriented crystallites is less perfect. The results confirm, that it is important for the evaluation of crystallinity under deformation to check, whether fiber symmetry can be assumed. This has consequences in particular on the quantitative interpretation of space-resolved scanning experiments in the vicinity of crack tips. Furthermore it raises the question, whether there is an asymmetric reinforcing effect of the SIC in the vicinity of crack tips inside natural rubber. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

2020, Stribeck, Almut, Schneider, Konrad, Eling, Berend, Pöselt, Elmar

Strain steps are applied to elastomers in a pneumatic relaxometer and monitored by small-angle X-ray scattering (SAXS). The relaxometer provides a rise time of 13 ms for strain pulses of step height ?e = ±1 in strain. The basic character of the 2D SAXS frames is examined and corresponding invariants Q(t) are analyzed. Three thermoplastic polyurethanes (TPU) of hardness 85 Shore A with different soft segments are studied both unannealed and annealed. The first response of all materials is a fast morphology conversion which finishes within tmc =250 ms. Because it has been untraceable, it is characterized by a settling stroke Q(tmc) - Q(0). The second response is a slow morphology adjustment process which complies with logarithmic relaxation. It is characterized by a relaxation rate DQ = Q(10 t)/Q(t) - 1. Comparison indicates that the nanoscopic morphology relaxation processes appear to have little direct relation to the macroscopic stress relaxation curves. The materials differ with respect to hard-domain morphology stability and morphology recovery. Most unstable is the morphology of the annealed polyether-based material. It forms nanofibrillary entities when strained. © 2020 The Authors. Published by Wiley-VCH GmbH

2019, Oberer, Jürgen, Schneider, Konrad, Majschak, Jens-Peter

The tensile testing of amorphous polyethylene terephthalate is observed until failure by IR thermography and optical strain measurement. The deformation can be subdivided in six deformation phases: elastic deformation, neck formation with a localized sharp temperature rise, neck propagation, which is also known as cold-drawing, with heat generation in a transition zone, crack initialization with local heating, crack growth, and rupture. These deformation phases are showing different mechanical and thermal reactions to the deformation. The initial and drawn samples are studied with differential scanning calorimetry. Alongside heating due to the dissipation of mechanical energy, latent heat due to strain-induced crystallization was detected. While the material is cold-drawn, a high dependence on the crosshead speed is found for the heat generation as well as the draw ratio, mechanical response, and morphological changes due to orientation and crystallization. For cold-drawing, a thermomechanical model is introduced, which is based on the first law of thermodynamics and reproduces the temperature distribution along the sample.