Macromolecular Systems: Selected Projects
Nonequilibrium Processes involving Single Macromolecules
In a biological context (e.g. DNA from a virus infects a cell by penetrating it through a pore of the cell membrane) it is of interest to study the dynamics of the conformational changes of single macromolecules under the action of external driving forces. Using coarse-grained models, phenomena such as the "escape transitions" of polymers or "forced translocation" of polymers through membranes are simulated. Even in the "thermodynamic limit" of infinite chain length counter-intuitive results such as inequivalence between different ensembles of statistical thermodynamics are found. This research is done in collaboration with A. M. Skvortsov (St. Petersburg), L. Klushin (American Univ. of Beirut) and A. Bhattacharya (Univ. Central Florida, Orlando). For more information, please contact Kurt Binder and/or Hsiao-Ping Hsu.
Interplay of Electrostatic and Hydrodynamic Interactions in Complex Fluids
The project aims at the development of new efficient simulation methods for investigating electrostatic and hydrodynamic effects in complex fluids. Examples are the dynamics of charged macromolecules in solution under the influence of external fields and/or in confined geometry, as well as the (hydro)dynamics of interfaces in phase separating fluids. Examples of physical problems of interest are (i) the electrophoresis of charged polyelectrolytes or colloids in microchannels with different geometries and different wall properties, (ii) the dielectrophoresis of polyelectrolytes or colloids in an alternating electric fields, and (iii) multiphase flow. We collaborate with C. Holm (Uni Stuttgart) and B. Dünweg (MPI-P Mainz), and we are funded in part by the VW foundation and in part by the SFB TR6. For more information, please contact Friederike Schmid.
Crystalline or Liquid-Crystalline Order in Polymeric Systems
A single polymer chain in a poor solvent may collapse into a dense fluid globule, but it may instead also crystallize: By extensive simulations with the Wang-Landau algorithm we have shown that the crystal is favorable if the range of the attractive interactions between the monomers exceeds the range of the repulsions only slightly. These findings may be useful to understand scenarios for protein crystallization. The resulting structure is also modified when an attractive substrate surface is present, and/or when one considers a semiflexible rather than a flexible polymer: then liquid-crystal-line ordering comes into play. Single chains then may collapse forming torodial or plate-like strucutures, or lamellae attached to walls. Multichain systems, or semi-flexible polymers, however, are found to undergo isotropic to nematic transitions, similar to systems of hard rods. In the presence of confinement into thin films by hard walls, "capillary normalization" (i.e. wall-induced nematic order) is found. This research is carried out in collaboration with V. A. Ivanov (Moscow State University), J. Luettmer-Strathmann (The University of Akron), M. P. Taylor (Hiram College), and W. Paul (Martin Luther Universität Halle.) For more information, please contact Kurt Binder.
Bottle-Brush Polymers: From Tunable Stiffness to Intramolecular Phase Separation
A "bottle-brush" polymer consists of a long (flexible or semiflexible) backbone macromolecule, to which (at high grafting density) flexible side chains are anchored. Varying the length of these side chains, the stiffness of the resulting cylindrical object can be controlled. Monte Carlo simulations help to understand the interplay of the various length scales that describe the multiscale structure of these complex macromolecules, and contribute to the understanding of experiments perfomed at the Max Planck Institute for Polymer Research and the Institute for Physical Chemistry at the University of Mainz. Under poor solvent conditions, pearl-necklace type structures form, and if two types (A.B) of side chains are grafted, structures such a "Janus dumbbells" or "Janus cylinders" are predicted. This research is carried out in the framework of the collaborative research SFB 625 at Mainz, and in collaboration with W. Paul (University of Halle). For more information, please contact Kurt Binder and/or Hsiao-Ping Hsu.
Polymer Brushes on Flat and Curved Substrates
A "polymer brush" is created if flexible macromolecules are anchored (via special endgroups) on a (otherwise repulsive) substrate at high grafting density, so the chains stretch away from the substrate to avoid that monomers sit on top of each other. We simulate such systems by Molecular Dynamics methods, both to elucidate their static structure, but also the nonequilibrium dynamic response as well as the interpenetration between two brush-coated nano-particles when two brushes are sheared against each other is studied. We interact on this topic with J. Klein (Weizmann Institute, Israel) and with T. Kreer and J. Baschnagel (Institute Charles Sadron, Strasbourg) and with A. Milchev (Bulgarian Academy of Sciences, Sofia) and S. Egorov (University of Virginia). For more information, please contact Kurt Binder and/or Federica Loverso.
The Interface between Graphite and Polybutadiene Melts: A Model System for a Polymer-Solid Contact
In the framework of the priority program SPP 1369 "polymer-solid contacts" of the German Research Foundation (DFG) we simulate an atomistically realistic model for fluid polybutadiene confinded in a slit pore between two graphite walls, and study over which scale the walls modity structure and dynamics of the polymers, and characterize the resulting nanoscale "interphase" layers. These simulations require huge computer ressources and are done in collaboration with W. Paul of the University of Halle using the massively parallel JUGENE supercomputer at Jülich. For more information, please contact Kurt Binder and/or Peter Virnau.
Self-Assembling Block Copolymers
Melts of one or more kinds of polymers have been demonstrated over the last few decades to exhibit a wealth of diverse phases whose geometric properties make them interesting systems not only for condensed matter research, but for industrial applications, as well. Specifically, block copolymers made of chemically incompatible monomers (say, A and B) exhibit microphase separation, thus forming regular nanoscale patterns of varying complexity. Our current research focusses on structure formation in rod-coil block copolymers, which have possible applications, e.g., for photovoltaics. Moreover, we are interested in the effect of crosslinking for the stabilization of ordered structures. For more information, please contact Friederike Schmid.
Hybrid Field-based Simulation Methods for Polymers
The so-called 'self-consistent field' (SCF) theory is one of the most successful density functional theories fo inhomogeneous polymer systems, which allows to calculate the local structure of dense blends at an almost quantitative level (see review article). We use dynamic self-consistent field theory to study the kinetics of structure formation in solutions containing amphiphilic block copolymers. Furthermore, we develop new hybrid simulation schemes for such systems, combining particle- and field-based representations as well as different kinetic descriptions (diffusive Langevin and hydrodynamic Lattice-Boltzmann fields) in a consistent way. The projects are funded by the VW foundation and by the DFG, and carried out in collaboration with Agur Sevink (Leiden, the Netherlands), Andrei Zvelindovsky (Preston, UK), and Alexander Boeker (Aachen), as well as with Wolfgang Paul (Halle). For more information, please contact Friederike Schmid or Hans Behringer.