Dr. Andreas Heuer
Max-Planck-Institut für Polymerforschung
Postfach 3148
D-55021 Mainz, Germany
e-mail: heuer@mpip-mainz.mpg.de
Tel.: * 49(0)6131/379-124
Fax.: *49(0)6131/379-100
Theoretical physics: Static and dynamic properties of glasses on a microscopic scale
Scientific career:
Andreas Heuer studied physics at the universities of Dortmund and Heidelberg. In 1991 he gained his Ph.D. with the work on translational and rotational tunneling in the group of Prof. U. Haeberlen, Heidelberg. From 1992 to 1993 he was a Feodor-Lynen Fellow of the Alexander v. Humboldt-Stiftung with Prof. R. J. Silbey, MIT, Cambridge, working on the low-tempe-rature anomalies of glasses, mainly via computer simulations. In 1993 he joined the staff at the MPI for Polymer Research in the group of Prof. Spiess.
Scientific research:
Experimental methods like NMR are sensitive to properties on a microscopic level. In our group we elucidate a variety of microscopic properties from a theoretical point of view . One general aspect concerns the underlying nature of the non-exponential relaxation around the glass transition. A longstanding discussion in literature has been whether the dynamics is heterogeneous or homogeneous. This question can be tackled by analyzing four-point correlation functions which can be determined by multidimensional NMR. It is possible to introduce the general concept of a rate memory and an orientational memory in order to capture their essential information [1]. They allow to quantify, on the one hand, the degree of dynamical heterogeneity of the non-exponential relaxation and, on the other hand, the degree of fluctuations of the dynamical heterogeneities with time. Via simulations these new concepts are also applied to a variety of model systems like the fluctuation bond model, which is a polymer model on a lattice, and the Fredrickson model. Of present interest is the question how static heterogeneities which can be found, e.g., in polymer blends, influence the glass transition and the dynamic heterogeneities.
Local properties of glasses are directly visible via its low-temperature anomalies. One example is the strong increase of the specific heat of glasses beyond the Debye-contribution which is typically related to the occurrence of local bistable modes. The phenomenological soft potential model captures most of the low-temperature properties. However, the exact nature of the bistable modes is still under debate. For a model glass an algorithm has been developed which allows a systematic search for adjacent local potential energy minima and hence to find a microcopic basis for the explanation of the low-temperature properties. The phenomenological parameters could be determined from first principles[2-4]. Presently this work is extended to include all minima of the energy surface in order to see how the bistable modes are embedded in the total energy surface with the ultimate goal to capture some properties of the glass transition when coming from low temperatures.
Specific NMR experiments are sensitive to the chemical shift distribution which can be related to the conformational statistics of polymers via application of quantum chemistry calculations [5]. Presently we develop an algorithm which allows to obtain the conformational statistics in an optimized way and which can be applied even for very dense polymers like PIB or PMMA.
Representative Publications: