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From Structure Property to Structure Process Property Relations in Soft Matter – a Computational Physics Approach

Final Report Summary - MOLPROCOMP (From Structure Property to Structure Process Property Relations in Soft Matter – a Computational Physics Approach)

All materials are the result of a specific process and can hardly be understood without taking this into account. Physically speaking they are usually not in thermodynamic equilibrium, i.e. their properties are ‘history dependent’ and systems are stuck in a metastable minimum of the free energy landscape. The project focuses on the molecular basis of the relation between material properties and the way materials are produced/created. Though known for a long time and practically employed for ages (e.g. in processing of metals), a systematic approach to connect material properties to the formation process based on molecular level insight into these processes is still missing. This is partially connected to a lack in theoretical understanding and methodology as well as to experimental difficulties. Here soft matter offers unique opportunities. Soft matter, especially polymers, can be easily manipulated in a controlled manner and experimental progress allows unique molecular level insight. New simulation methods as well as some new experimental approaches have been developed to investigate case studies to demonstrate the general relevance of molecular processes in three different problem fields. We study macromolecular systems (mostly highly entangled and non-entangled non-equilibrium polymer melts, but also proteins and active systems), crystal growth in the presence of impurities/additives as idealized very first models for bio-mineralization and precursors of layered structures in organic electronics. First several computer simulation methods developed by the PI and his coworkers had to be adjusted and continuously improved. Most notably this holds for two approaches. The adaptive resolution simulation method (AdResS) allows for true open system molecular dynamics simulations. The second concerns the creation of well-defined starting states for melts of very long polymers (bulk and films) and the continued improvement and optimization of our Open Source package ESPResSo++. In parallel, an experimental activity on non-equilibrium polymer melts was started, which directly connects to the simulations. Samples of very long glassy polymers with and without entanglements have been prepared by computer simulation and in experiment, resulting in interesting low viscosity, or viscoelastically inhomogeneous but chemically homogeneous, or nanoporous systems. Synthesis dependent para and meta sequences along conjugated polymers led to rather different precursors for graphene nanoribbons. Adaptive resolutions methods were used to demonstrate how to drive systems externally in a controlled way and led to a simple direct way to take advantage of the Jarzinski equality. This allows to easily study response of systems subject to external driving.

Results point to interesting new metastable structures in highly entangled, mechanically deformed polymer melts, the modification and actually acceleration of crystal growth upon adding solutes and its dependence on solvation properties, and the relation of bond ‘defects’ (para vs meta in conjugated polymers) and conjugation defects in adsorbed polymeric semiconductors.