Plenarvorträge - DPG-Tagungen

Chemische Physik und Polymerphysik Montag
K. Günther-Schade, D. Hofmann, D. Fritsch, T. Strunskus, F. Faupel,
Macromolecules, 33, 2242 (2000).
CPP 4.4 Mo 12:00 H 38
Local Modes in liquid Deuterium Fluoride — •Demmel Franz 1 ,
Gerhard Heusel 2 , Isabella Waldner 2 , Markus Kreitmeir 2 ,
and Helmut Bertagnolli 2 — 1 ILL, 38042 Grenoble, France —
2 Universitaet Stuttgart, Physikalische Chemie, Stuttgart
Hydrogen bonded liquids are certainly one of the most complex, but
also one of the most important liquids. Structure and dynamics is strongly
determined by the hydrogen bonds. Water, the most prominent model
system, shows a 3D network of bonds and has been studied for a long
time. In contrast to water liquid hydrogen fluoride is connected in onedimensional
hydrogen bonded chains. A MD simulation of liquid HF has
predicted an acoustic-like mode and a further optic-like mode in the spectra
of the longitudinal current correlation functions [1]. For the interpretation
of the optic mode a collective movement of HF molecules against
themselves was suggested. We performed an inelastic neutron scattering
experiment at the ILL, Grenoble, on liquid deuterium fluoride at two
temperatures. The corrected spectra show evidences for an optic mode
in a certain region of momentum transfer vectors as predicted by the
MD simulation. These indications weaken or even vanish at the higher
temperature near the triple point.
[1] D. Bertolini, G. Sutmann, A. Tani, R. Vallauri, PRL 81, (1998)
2080
CPP 5 Computational Physics
CPP 4.5 Mo 12:15 H 38
Lösung von Reaktions-Diffusionsgleichungen mit Hilfe der finiten
Elemente Methode — •Thomas Pletl, Michael Schulz und
Peter Reineker — Abteilung Theoretische Physik, Universität Ulm,
Albert-Einstein-Allee 11, D-89069 Ulm
Chemische Reaktionen im Festkörper oder Glas werden häufig mittels
eines Satzes von gekoppelten Reaktions-Diffusionsgleichungen beschrieben.
Ein Beispiel hierfür sind Gläser, die zunächst mit Ag + angereichert
werden, welche dann mittels H2 zu atomaren Silber reduziert werden. Die
Ag-Atome neigen sofort zur Clusterbildung und lagern sich in dünnen
Bändern innerhalb des Glases ab (sog. Liesegang-Strukturen). Damit
erhält man wohldefinierte Profile für den Brechungsindex dieser Gläser,
welche daher auch Anwendungen in der optischen Industrie finden.
Um Parametervariationen bei der Produktion solcher Gläser zu vermeiden,
müssen theoretische Modelle entwickelt werden. Bei der Lösung der
diesen Modellen zugrunde liegenden Reaktions-Diffusionsgleichungen zusammen
mit der Fokker-Planck-Gleichung, welche den Wachstumsprozeß
der Silbercluster beschreibt, kommt ein neu entwickeltes finite Elemente
(FE) Programm zum Einsatz. Erste Ergebnisse für die Konzentrationen
der beteiligten Atome und Ionen werden vorgestellt. Unser Programm
läßt sich auf beliebige Transport-Diffusions-Reaktions-Systeme erweitern,
die aufgrund der oft sehr speziellen gekoppelten Gleichungen mit handelsüblichen
FE-Programmen nicht berechenbar sind.
Zeit: Montag 10:00–11:15 Raum: H 39
CPP 5.1 Mo 10:00 H 39
Numerical Studies of Tethered Chains at Adsorbing Surfaces —
•Radu Descas 1 , Jens-Uwe Sommer 2 , and Alexander Blumen 1 —
1 Theoretische Polymerphysik, Univesität Freiburg — 2 Institut de Chemie
des Surfaces et Interfaces, Mulhouse Cedex
We present extensive Monte Carlo simulations of tethered chains on
adsorbing surfaces, considering the dilute case in good solvents, and analyze
our results using scaling arguments. We focus on the mean number
of chain contacts with the adsorbing wall, on the chain extension (the
radius of gyration) perpendicular and parallel to the adsorbing surface,
on the probability distribution of the free end and on the density profile
for all monomers. For the cross-over from non-adsorbed to adsorbed
behavior we obtain best results using a cross-over exponent of 0.59. We
also investigate the dynamical scaling behavior at the critical point of
adsorption, considering the end-to-end correlation function and the correlation
function of adsorbed monomers at the wall. We find that the
dynamic scaling exponent a (which describes the relaxation time of the
chain as a function of its length) is the same at the adsorption threshold
as that of free chains. Moreover, we find that tethered chains relax
quicker perpendicularly to the wall than parallel to it.
CPP 5.2 Mo 10:15 H 39
Viscoelastic Relaxation of Copolymeric Structures — •Cristian
Satmarel 1 , Alexander Blumen 1 , Andrei Gurtovenko 2 , and
Christian von Ferber 1 — 1 Theoretische Polymerphysik, Universitaet
Freiburg, Herman-Herder Str. 3, D-79104, Freiburg, Germany —
2 Institute of Macromolecular Compounds, Russian Academy of Sciences,
Bolshoi Prospect 31, V.O., St. Petersbug, 199004, Russia
We study theoretically the viscoelastic relaxation of copolymeric structures
(alternating copolymers chain, cross-linked alternating copolymers
chain and tree-like structures) in the framework of generalized Gaussian
structures, which are extensions of the Rouse model to arbitrary geometries.
The heterogeneity of these systems is attained by considering beads
which differ from each other due to different friction coefficients with
the solvent. For all the structures we were able to examine the complex
(shear) modulus using its real and imaginary components. These show
a multitude of features, which depend mainly on the difference in the
mobilities of the beads. In addition, for tree-like structures of arbitrary
functionality and number of generations, and with alternating arrangements
of the monomers, we developed an analytical method to determine
the eigenfrequencies.
CPP 5.3 Mo 10:30 H 39
Statistical Properties of Off-Lattice Heteropolymers —
•Michael Bachmann, Wolfhard Janke, and Handan Arkin —
Institut für Theoretische Physik, Universität Leipzig, Augustusplatz
10/11, 04109 Leipzig, Germany
We apply a multicanonical algorithm to variants of the AB model [1,2]
being an off-lattice model for heteropolymers. Heteropolymers are considered
as chains of hydrophobic (A) and hydrophilic (B) monomers only.
Into the energy function enter the bending energy and a Lennard-Joneslike
potential between nonbonded monomers, where short-range repulsion
and long-range attraction compete. Contacts between hydrophobic
monomers are favoured, thereby assuming that the global energy minimum
state of proteins is characterized by a compact hydrophobic core
screened from the solvent by a shell of hydrophilic residues. We calculate
thermodynamic quantities for known sequences by means of a modified
AB model [2] and identify the temperatures, where conformational
pseudo transitions are expected. Since the multicanonical algorithm allows
for an accurate sampling of the low-temperature region, we also
obtain good estimates for the global energy minimum. Therefore we apply
our algorithm to sequences where minimum energies are quoted [3]
and compare with lowest-energy states found by minimizing procedures.
[1] F. H. Stillinger, T. Head-Gordon, and C. L. Hirshfeld, Phys. Rev. E
48, 1469 (1993).
[2] A. Irbäck, C. Peterson, F. Potthast, and O. Sommelius, J. Chem.
Phys. 107, 273 (1997).
[3] H.-P. Hsu, V. Mehra, and P. Grassberger, e-print: cond-mat/0302545.
CPP 5.4 Mo 10:45 H 39
Collapse of Long Lattice Polymers — •Thomas Vogel, Michael
Bachmann, and Wolfhard Janke — Institut für Theoretische
Physik, Universität Leipzig, Augustusplatz 10/11, 04109 Leipzig,
Germany
Using the nPERM algorithm[1], we have investigated long polymer
chains on the simple cubic lattice in three and four dimensions. At the
so-called θ-point, there is a second-order phase transition from a globular
state to a coil-like state predicted from mean-field theory. Surprisingly, for
finite chains, in four dimensions this transition looks first-order-like[2].
We investigate this pseudo-first-order transition by means of thermodynamic
quantities such as the end-to-end distance and the radius of
gyration.
We will also report some details about the behaviour of the PERM algorithm
we found by analysing energy distributions near the critical temperature
and how to avoid strong correlations between growing chains.
[1] P. Grassberger, Phys. Rev. E 56 (1997) 3682; H.-P. Hsu, V. Mehra,