Research activities

In our research group we attempt to study theoretically and by means of computer simulation the physical properties of polymer and related soft matter systems. More specifically, our activities are regrouped around four main research axes:

1. Polymer solutions and melts: We study the thermodynamic, conformational and dynamical properties of (mainly monodisperse) polymeric liquids in the bulk, close to surfaces and in confined geometries. Recent research focuses on small, but systematic deviations to Flory's ideality hypothesis formulated half a century ago for dense polymer solutions and melts. While most of our current work is dedicated to neutral linear chains we have also investigated the effects of long range interactions in charged polymers and in topologically constrainted rings. The "activated reptation" proposal formulated by A.N. Semenov attempts to incorporate consistently the density fluctuation effects into the standard theory of polymer reptation. Ongoing molecular dynamics simulations of polymers in ultrathin films are trying to put to a test some of these predictions. More ...


2. Polymer self-assembly: We investigate here, e.g., "equilibrium polymers" an important example of "open aggregation" where (linear) polymerization takes place under condition of chemical equilibrium between the polymers and their respective monomers. Examples for this large class of systems include liquid sulfur and selenium, giant surfactant micelles, supramolecular aggregates of dyes, dipolar colloids and protein filaments. Equilibrium polymers differ from conventional "quenched" or "dead" polymers in that they can break and recombine. This has profound consequences for their rheological behaviour and the way they may be treated in computer simulations. A more recent project is dedicated to the description of the linear and non-linear rheological response of "bridged equilibrium polymers" formed by transient networks of giant micelles linked together by telechelic polymers (in comparison to the similar system of bridged microemulsions studied by the soft-matter research group in Montpellier). More ...


3. Polymer glasses: Due to the irregular configuration of the chains, polymers have an intrinsic difficulty to crystallize. Many polymers rather form disordered solids, i.e., glasses, below a characteristic temperature, the glass transition temperature T_g. The glass transition and the properties of the glassy state have therefore received continual scientific attention throughout the last decades. We contribute to this research by computer simulations of coarse-grained polymer models. Our current research interests involve the structural relaxation of supercooled polymer melts above T_g (quantitative comparison with mode-coupling theory, study of dynamic heterogeneities), elasto-plastic properties of polymer glasses, dynamics of tracers in polymer matrices and of polymer-solvent mixtures, deviations from bulk behavior in thin polymer films (e.g. shift of T_g with film thickness h; see figure). More ...


4. Polymer crystallization: Polymeric solids cannot only be glassy, but semicrystalline if the microstruture of the chains is sufficiently regular to allow ordered structures to form. Semicrystalline polymers contain both amorphous and crystalline regions. The crystalline regions consist of lamellar sheets (see figure) in which the polymers are folded back and forth so that sections of chains can align parallel to each other. On larger length scales, the sheets twist and branch as they grow outward from a nucleus into spherulitic structures. This hierachy of morphological features, ranging from the lamellar ordering of the chains (~10 nm) to the macroscopic packing of the spherulites (100 μm and larger), reflects the complexity of the underlying crystallization process which is not yet fully understood. We study the polymer crystallization process and the morphology of the resulting structures in the bulk and close to solid substrates, mainly by computer simulations of specifically designed models which are derived by controlled coarse-graining techniques from realistic polymer models. More ...

Both static and dynamical issues (close and far from equilibrium) are considered theoretically (classical mean-field, scaling and field theoretical approaches) and by means of computer simulations (various Monte Carlo and molecular dynamics schemes). The interaction between theory (A. Johner, I. Nyrkova, A.N. Semenov) and simulation (J. Baschnagel, H. Meyer, J.P. Wittmer, O. Benzerara, J. Farago) subgroups is very close. Generally, we consider simple and strongly coarse-grained classical model hamiltonians (characterized by only a few effective parameters) to explain generic properties of thermodynamic phases and kinetic pathways between equilibrium states. Our research is generally directed on the generic behavior ("universality") and orders of magnitude rather than spitting out specific numbers.

In addition to the above main research axes, we are interested in related problems found in other complex fluids and soft matter systems: general theory of closed and open association, membranes, glass and jamming transition, elasto-plasticity of amorphous solids and granular matter.