Macromolecular Systems

Self-Assembling Block Copolymers and Polymer Brushes

Melts of one or more kinds of polymers 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. In solvent, they self-assemble to nanoparticles or vesicles which can be used, e.g., as nanocontainers.

Among other, we study the influence of curvature on structure formation and pattern orientation in thin films and membranes, and we are interested in the effect of crosslinking for the stabilization of ordered structures. Furthermore, we use dynamic self-consistent field theory to study the kinetics of structure formation, e.g., in solutions containing amphiphilic block copolymers.

Another important application for polymers is to attach them to surfaces, thus modifying the surface properties. We are interested in the effect of polydispersity on the structure of such ''polymer brushes'', and on strategies to design smart surfaces that can be used as sensors and switches. 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 develop new hybrid simulation schemes for such systems that combine particle- and field-based representations of polymers, thus allowing to treat large parts of a system at the field level and zoom into certain areas in space with adaptive resolutions. Furthermore, we develop methods to combine different kinetic descriptions of polymeric fluids (diffusive Langevin and hydrodynamic Lattice-Boltzmann fields) in a consistent way. For more information, please contact
Friederike Schmid .

Knots in Polymers, Proteins and DNA

Although globular homopolymers are typically highly knotted, less than one in a hundred protein structures contain a knot ( http://knots.mit.edu ). Nevertheless, intriguing counter-examples exist, like the most complicated protein knot, which was discovered recently during a diploma thesis in our group (see figure on the left). Apart from analyzing biological data, we perform Monte Carlo simulations of simplified protein and DNA models to learn more about entanglements in viral DNA, chromatin and proteins. On this topic, we collaborate with theory groups at MIT and an experimental group at the MPI for Polymer Research. If you are interested in interdisciplinary investigations at the frontier of physics, mathematics and molecular biology, please contact Peter Virnau.

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 Universit├Ąt Halle.) For more Information, please contact Kurt Binder .