Öffentliche Seminare

Crystallization is often initiated at interfaces. Understanding the physical process underlying interface-induced crystallization is of fundamental interest and is relevant for many material applications. Interface-induced crystallization of liquids can occur either by heterogeneous nucleation or by the equilibrium phenomenon of prefreezing. First, we present a combined theoretical and experimental study of the effect of substrate-material interactions on the thermodynamics of prefreezing [1-3] and on the kinetics of heterogeneous nucleation in model polymers on various substrates [4]. Second, the knowledge gained about interface-induced crystallization is used to elucidate the role of interfaces for crystal orientation in films of conjugated polymers, which is important for device performance. Using polythiophenes as model conjugated polymers, we show that different crystal orientations can be formed at the interfaces to a substrate and vacuum as a result of two competing interfacial interactions. Our results demonstrate that increasing the polarity of polythiophene side chains influences the interactions at the interfaces, resulting in a change of crystal orientations [5]. Thus, we disclose the crucial role of interfacial interactions for crystallization kinetics, thin film morphology, and control of molecular orientation in films of model and semiconducting polymers. [mehr]

Block copolymers are known for their elaborate microphase separating capabilities. Due to the many tuning parameters a predictive modelling approach is required. Molecular dynamics for such large system sizes is expensive. On the other hand, self-consistent field theory is capable of simulating much larger systems, but its mean-field based approximation only becomes correct when the chain density becomes unrealistically high. As a result some experimental effects such as a first order order-to-disorder phase transition are not reproduced. Field-theoretic simulations, where fields are not constrained to their mean field value but allowed to fluctuate, bridge the gap between these methods. I will give an introduction into this method, with some of the associated phenomena such as the ultraviolet divergence, and successful applications in symmetric block copolymers and block copolymer-homopolymer blends. [mehr]

Innovative solar system missions must become increasingly innovative and elaborate since "the low-hanging fruits have already been picked." Solar sails, which are propelled solely by solar radiation pressure, are among the key technologies for the future exploration of the solar system because they make missions possible that would otherwise be infeasible due to their immense propellant requirements. The optimization of solar sail trajectories, however, is a difficult task. In the talk, a method is presented that is based on machine learning, fusing artificial neural networks and evolutionary algorithms. Such optimization methods may also be applied for subsurface ice melting probes, as they are required to explore Jupiter's and Saturn's icy moons, which may harbor life in the oceans beneath their thick ice crusts. Such ice melting probes have been developed at FH Aachen and successfully tested in Antarctic ice. It will be interesting to discuss whether those methods can also be applied in polymer research. [mehr]

Recent advancements in the understanding of the dynamics of soft matter is presented, concentrating on polymer melts with some outlook on semiconducting polymers and lipid membranes in solution. Traditionally, techniques like rheology are very popular, as these enable high-throughput experiments that help connect model materials with applications. Despite substantial progress, even the simplest model materials are not entirely understood, at least when it comes to a simultaneous modeling of experimental results from different techniques. This indicates that there is still a lack of information that prevents holistic understanding. This presentation concentrates on augmented analysis of recent experimental results on bottlebrush polymer melts, semiconducting polymers in solutions, and lipid membranes. Using the advantage of length- and time-scale dependent information of neutron spectroscopy, we distinguish different processes in polymer melts, nanocomposites, bottlebrushes, semiconducting polymers, and lipid membranes. As the results point to a generic picture, the procedures used appear to be a promising path to further elevate fundamental understanding of polymers, including confined chains and architectures of increasing complexity. [mehr]