Liquid-crystal phase transitions

in suspensions  of plate-like particles

 

G.J. Vroege and H. Lekkerkerker

 

Van 't Hoff Laboratory for Physical and Colloid Chemistry

Debye Research Institute, Utrecht University

H.R. Kruytgebouw, N-704, Padualaan 8, 3584 CH Utrecht

The Netherlands

 

We study the liquid-crystalline phase behavior of suspensions of hard colloidal platelets, in particular, the relationship between their size polydispersity and the  stability of a nematic, columnar and smectic phase. A first-order nematic-to-columnar transition is observed for suspensions up to 25% polydispersity in platelet diameter.

     

The observed tolerance of polydispersity in the columnar phase, being a two-dimensional crystal, seems remarkable in light of current predictions for the terminal  polydispersity for hard-sphere crystallization and hard-rod smectic ordering.


 

 

Simulation studies of solvent diffusion

and the glass transition in polymers

 

Philip L. Taylor

 

Dept. of Physics, Case Western Reserve University,

 Cleveland  OH 44106-7079 (USA)

 

The glass transition and non-Fickian diffusion are two related problems in polymer physics that can be studied through atomistic molecular dynamics simulations.  The mean squared deviations of atoms, monomers, and molecules from their initial positions can be analyzed by means of a technique that separates the  effects of diffusive motion from the underlying vibrational motion. 

     

One can interpret diffusive motion in syndiotactic poly(methyl methacrylate) to find a novel power-law variation with time, with an  exponent that varies continuously from 0.5 below the glass transition  temperature to unity at high temperatures.  The  diffusion of methanol into PMMA is interesting in that nonlinear  effects dominate those of Fickian diffusion.  Entry of the solvent  into the PMMA is accompanied by a swelling of the polymer matrix.


 

 

Hybrid Molecular Dynamics / Lattice Boltzmann Approach

to Polymer Solution Dynamics

 

Burkhard Duenweg

 

Max-Planck-Institut  für Polymerforschung

Ackermannweg 10, D-55128 Mainz, Germany

 

We establish a new efficient method for simulating polymer-solvent systems which combines a lattice Boltzmann approach for the fluid with a continuum molecular dynamics (MD) model for the polymer chains. The two parts are coupled by a simple dissipative point-particle force, and the system is driven by Langevin stochastic forces added to both the fluid and the polymers. Extensive tests of the new method for the case of a single chain in good solvent are performed. The dynamic and static properties predicted by simple scaling arguments (which are explained in the talk) are validated. In this context, the influence of the finite size of the simulation box is discussed. Usually the finite size corrections scale as 1/L, where L denotes the linear dimension of the box. This is a result of the Coulomb-like long-range nature of hydrodynamic interactions. However, the decay rate of internal polymer relaxation modes is only subject to an 1/L^3 finite size effect, corresponding to an effective dipolar interaction. Furthermore, the mapping to an existing MD simulation of the same system is done so that all physical input values for the new method can be derived from pure MD simulation. Both methods can thus be compared quantitatively. The main advantage of the new approach compared to a particle method is the fact that the solvent is completely structureless, such that the polymer system can be relaxed into thermal equilibrium without surrounding solvent. Moreover, it turns out that the lattice spacing provides a convenient lower length scale cutoff to regularize the hydrodynamics, thus naturally accounting for the fact that in nature there are no point particles with finite friction. The method is then applied to a semidilute system of chains of length N = 1000, which would not be accessible to a pure particle method. We observe the crossover from Zimm dynamics at short length and time scales to Rouse dynamics at long length and time scales. The dynamic crossover length is proportional to the static screening length, as predicted by de Gennes. At short times, the dynamics turns out to be Zimm-like even for the long-wavelength modes. This suggests the simple picture of Zimm blobs which can move freely up to their own size, while later they feel constraints and offer frictional resistance to the flow. This finding shows that the screening of hydrodynamic interactions is an intrinsically time-dependent dynamic phenomenon, and therefore invalidates any theoretical description which is based on a screened hydrodynamic interaction depending only on distance.

 References:

1. P. Ahlrichs and B. Duenweg, Journal of Chemical Physics 111, 8225 (1999).

2. P. Ahlrichs, R. Everaers, and B. Duenweg, Physical Review E 64, 040501

   (R) (2001).


 

 

COMPUTER SIMULATION OF

LIQUID CRYSTAL INTERFACES

 

Luis F. Rull

 

Departamento de Fìsica Atòmica, Molecular y Nuclear,

Area de Fìsica Teòrica, Universidad de Sevilla,

Aptdo. 1065, Sevilla 41080, Spain.

 

A computer simulation study of a solid-vapour interface of the Gay-Berne fluid will be presented. The anisotropic parameter k’ has  been chosen such that the bulk system presents a vapour-isotropic-nematic  triple point (k’ = 1.5, k= 3). Two isotherms, above and below  this triple point, are evaluated in the Monte Carlo simulations.

     

The sytem was simulated in a box with periodic boundary conditions in x and y directions while in the z direction, one of the side of the simulation box was specified as a planar Yukawa potential and, in order to avoid cappillary condensation effects, a single hard wall was used in the another one. Starting from the subcritical vapour and increasing the chemical potential and/or the pressure,  the density and order parameter profiles are obtained during the simulations.


 

 

Simulations and modeling

of defect dynamics in nematics

 

S. Žumer

 

Physics Department, University of Ljubljana, Jadranska 19,

SI-1000 Ljubljana, Slovenia

 

Advances in modelling and simulations of defect annihilation in nematic liquid crystals are presented. On the phenomenological side the effect of the backflow is taken into account. Generalized nematogenic hydrodynamics for the tensor order parameter is used. In particular annihilation of the pair of two line defects is analyzed.  On the microscopic side the Brownian dynamics is used to follow coarsening of a nematic after an isotropic-nematic quench of a nematogenic material. Particular attention is focused on the scaling behavior.


 

 

Phase diagram of discotic liquid crystals

in bulk and in confined geometry.

 

L. Bellier-Castella+*, D. Caprion+ and J.-P. Ryckaert+

 

(+) Physique des polymères, Université Libre de Bruxelles

(*) Laboratoire de physique des matériaux, Université de Lyon.

 

The columnar phase in discotic liquid crystals is of high technological importance. It is therefore useful to know the appropriate range of intermolecular parameters which lead to the formation of phases with well defined parallel columns (2D order) while avoiding at the same time a full crystallisation of the system.

     

Another question is to which extend a flat surface can promote a suitable orientation of these columns and perhaps enhance their stability. This largely depends upon the nature of the wall-discotic molecule interaction which favours the orientation of the discs either parallel or perpendicular to the surface.

     

New NPT Monte Carlo simulations of discotic liquid crystals in bulk and in confined geometry will be presented and  discussed  in this talk along the  lines summarized above.


 

 

Computer Simulations of

 Real and Virtual Liquid Crystals

 

Claudio Zannoni

 

Università di Bologna, Dipartimento di Chimica Fisica ed Inorganica,

Viale Risorgimento 4, 40136 Bologna, ITALY.

 

While atomistic Molecular Dynamics (MD) simulations are widely and successfully used in many fields of chemistry and biochemistry, there are not many examples of their reliability when applied to liquid crystals problems, in particular for the prediction of nematic-isotropic transition temperatures [1].  Here we present a computer simulation study of the odd-even effect in liquid crystals [2].  We have investigated the temperature dependence of the orientational order of the first three homologues of the series of phenylalkyl-4-(4'-cyanobenzylidene) aminocinnamates, modelled with full atomistic details with a suitably modified AMBER-like force field description, using MD simulations [3]. It is well known [4] that those molecules present a large "odd-even" effect, i.e. a large alternation in properties and in mesophase transition temperatures when varying the parity of the number of their alkyl groups.

This effect is well reproduced in our simulation results, and the transition temperatures are in relatively good agreement with the experiment.  We have a performed  conformational analysis, trying to explain the physical reasons of the odd-even effect for these molecules. We present here detailed results and a comparison between the homologues and more generally we try to assess the potentialities of atomistic simulations in the field at this point in time.

Beside this simulation of real liquid crystals, we discuss simpler, molecular resolution models useful to try and facilitate the search for good candidates for yielding mesophases with specific properties of interest (such as ferroelectricity). We present a brief summary of recent developments for systems of particles interacting with model potentials based on the Gay-Berne (GB) molecular level interactions (see [5] for a review), and discuss, e.g, formation of smectic C phases from dipolar systems [6] and the modelling of non-centrosymmetric molecules for the simulation non chiral ferroelectric nematics [7].

     

[1] see, e.g. contributions by M. Wilson, M. Glaser, P. Procacci in P. Pasini and C. Zannoni, eds., Advances in the computer simulations of liquid crystals, Kluwer, Dordrecht, 2000.

[2] R. Berardi, L. Muccioli and C. Zannoni, to be published (2002)

[3] P. Procacci, E. Paci, T. Darden and M. Marchi, J.Comp.Chem., 18, 1848 (1997)

[4] G.W.Gray in The Molecular Physics of Liquid Crystals, G.R. Luckhurst and G.W. Gray editors, Academic Press (1979).

[5] C. Zannoni,  J. Mater. Chem., 11, 2637 (2001

[6] R. Berardi, S. Orlandi and C. Zannoni,  to be published (2002)

[7] R. Berardi, M. Ricci and C. Zannoni, Chem. Phys. Chem., 2 (2001), 443 


 

 

Molecular Simulations

of the Liquid Crystalline Phases

of Fan-Shaped Molecules

 

A.G. Vanakaras, D.J. Photinos

 

Department of Materials Science, University of Patras, Patras 26500, Greece.

 

Recent computer simulation studies [1] have shown that idealised fan-shaped molecules can form layered fluid phases exhibiting the symmetry characteristics of the usual smectic-A phase but with strongly correlated rotations about the fan axes of neighboring molecules. The physical properties distinguishing these novel phases from the known mesophases are investigated in the present work using molecular theory and NPT Monte Carlo simulations.

     

The relevant order parameters are defined and evaluated for model systems. The molecular structure requirements for the stability of the ordered fluid phases are identified for representative architectures of fan, propeller and cage molecules. Shape non-convexity, allowing interdigitation-driven orientational and rotational correlations, is the key feature of these architectures.

     

[1] A.G. Vanakaras, D.J. Photinos, Chem. Phys. Lett., 341 (2001) 129.


 

 

Atomistic simulation of smectics

 

Matt Glaser

 

Department of Physics and

Ferroelectric Liquid Crystal Materials Research Center

University of Colorado

Boulder, CO 80304

 

Smectic liquid crystals constitute an extraordinarily rich and diverse family of low-symmetry fluids, comprising tilted and untilted 2d fluid and hexatic phases, ferroelectric, antiferroelectric, and ferrielectric phases, twist grain boundary phases, and a bewildering variety of ‘banana’ phases, including spontaneously chiral phases of nonchiral molecules.

     

Atomistic simulation is a powerful (albeit under-utilized) tool for confronting the ‘self-assembly problem’ in the context of smectics, i.e., for understanding how chemical structure encodes macroscopic properties and phase behavior. We describe atomistic simulation studies aimed at exploring the molecular origins of tilt and clinicity in tilted smectics and explaining the unusual temperature dependence of layer spacing and tilt in partially fluorinated ‘de Vries’ smectic materials.


 

 

Simulation of LC polymers and dendrimers

 

Mark R. Wilson

 

Department of Chemistry, University of Durham South Road,

 Durham DH1 3LE, U. K.

 

This paper describes results from the study of a number of simplified models for liquid crystalline systems in which molecular flexibility has been taken into account. These include a liquid crystal dimer molecule, a low-molecular weight mesogen with two flexible tails, two liquid crystal polymers and a LC dendrimer in a liquid crystalline solvent.

     

In the majority of studies a hybrid model is employed, which combines     spherical Lennard-Jones sites, to represent flexible chains (e.g. alkyl tails, polymer backbones, flexible spacers in polymers and dendrimers) and anisotropic Gay-Berne sites to describe the mesogenic moieties. Parallel molecular dynamics simulations are used to study the phase behaviour of the models, and to study the structure and dynamics of the molecules in the mesophases that form. In the case of the liquid crystalline dendrimer system, we also consider a fully atomistic Monte Carlo study for a single dendrimer molecule, and use this to derive a coarse-grained model that can be used for the simulation of bulk phases formed by LC dendrimers.


 

 

Liquid-Crystalline Phase Behavior

of a Colloidal Rod-Plate Mixture

 

G.J. Vroege and H. Lekkerkerker

 

Van 't Hoff Laboratory for Physical and Colloid Chemistry

Debye Research Institute, Utrecht University

H.R. Kruytgebouw, N-704, Padualaan 8, 3584 CH Utrecht

The Netherlands

 

The phase behavior of rod-plate mixtures was investigated using model systems containing unambiguously rod-and plate-shaped colloids. We find that the  theoretically disputed biaxial nematic phase is unstable with respect to demixing into  an isotropic and two uniaxial nematic phases.

     

The phase behavior at very high densities is exceptionally rich and includes the coexistence of up to four different liquid crystalline phases, which stem from the coupling between the employed  particle shapes and polydispersity.


 

 

LIQUID-VAPOR AND LIQUID-SOLID COEXISTENCE

IN A DLVO POTENTIAL MODELIZATION OF

GLOBULAR PROTEIN SOLUTIONS

 

C.Caccamo , M.C.Abramo, D.Costa and G.Pellicane,

 

Istituto Nazionale FIsica della Materia and  Dipartimento di Fisica,

University of Messina, Messina, Italy.

 

We report Gibbs Ensemble Monte Carlo calculations of liquid-vapor coexistence lines, and perturbation theory calculations of the liquid-solid coexistence lines, for a DLVO model with potential parameters appropriate to describe globular protein solutions of ionic strength ranging from 0.5 to 1.2 molar concentration of the added salt. Similar calculations are also reported for a very narrow  square-well potential suited to mimic the protein-protein interaction in the investigated solutions.

     

The different model predictions are compared with the available experimental data for the phase diagram of lysozime in  a H2O+NaCl solution. Preliminar results indicate that the DLVO  modelization is at least qualitatively accurate in reproducing the phase diagram of the system under study.


 

 

Coarse graining polymers as soft colloids

 

A.A. Louis

 

 Dept of Theoretical Chemistry,    Cambridge University

Lensfield Rd, Cambridge CB2 1EW, United Kingdom

 

Coarse-graining methods are crucial to deriving tractable statistical mechanical treatments of soft matter systems, where a large number of different length and time-scales may coexist.  In this talk I discuss an explicit example, where polymers are coarse-grained to single “soft colloids”, interacting with a pair potential.  The resulting large speedups in simulation times allow us to calculate the phase-diagrams of polymer-colloid mixtures, where we find good agreement with recent experiments.  This coarse-graining technique also leads to new insights, including a novel “mean-field fluid” paradigm, and highlights some of the subtleties arising when density dependent pair potentials are used to model materials.   

  

references

 

Beware of density dependent pair potentials, http://www.arxiv.org/abs/cond-mat/0205110

Coarse-graining polymers as soft colloids, Physica A 306, 251 (2002)


 

 

From basic interactions to phase diagrams:

 solutions of rod-like biomolecules

 

Hartmut Löwen

 

Institut für Theoretische Physik II,

Heinrich-Heine-Universität Düsseldorf

Universitätsstrasse 1, D-40225 Düsseldorf, Germany

 

One fundamental problem of statistical mechanics is to predict the phase behaviour on the basis of the microscopic interactions. This is a formidable task, in particular for many-body systems of rod-like particles which exhibit a variety of liquid crystalline phases.

     

Two cases of charged rod-like biological macromolecules are discussed in detail which lead to a rich phase behaviour, namely columnar aggregates of DNA and suspensions of tobacco mosaic virus particles.


 

 

Phase behavior and structure of model liquid crystals

and of fluids of Janus particles

 

Siegfried Hess

 

Institut f. Theoretische Physik, Techn. Univ. Berlin, PN 7-1,

Hardenbergstr. 36, D - 10623 Berlin

(S.Hess@physik.tu-berlin.de)

 

In the first part of the lecture, results of anlytical [1] and of Monte Carlo [2] calculations are presented for the pressure and the phase behavior of a model liquid crystal.  Isotropic, nematic and smectic phases are studied, structural properties are investigated for the bulk system and for the fluid in restricted geometries, in particular between flat orienting walls.

     

The second part of the lecture deals with the thermo-physical properties of a model fluid composed of Janus particles which have two different faces where like ones (different ones) have an extra attractive (repulsive) interaction. Data from Monte Carlo simulations for the pressure and the phase behavior are compared with analytical calculations [3].

     

1 S. Hess and Bin Su, Z. Naturforsch. 54a (1999) 559;

2 H. Steuer, M. Schoen, and S. Hess, to be published;

3 T. Erdmann, M. Kroeger, and S. Hess, to be published.


 

 

Simulation and theory

of inhomogeneous  liquid crystals

 

Mike Allen

 

Centre for Scientific Computing & Department of Physics

University of Warwick, Coventry CV4 7AL, UK

 

There are various theoretical models of liquid crystals: some assume that the free energy simply depends on gradients of the director field, some incorporate additional variation of the degree of ordering, and others attempt to write the free energy as a functional of the single-particle density.

Near defects and surfaces, significant spatial variations of the degree of ordering occur and some or all of the assumptions in these theories may break down. 

     

Molecular simulations are capable of giving detailed structural information in these situations. This talk will focus on some specific examples: the nematic-isotropic interface, suspensions of spherical and non-spherical colloidal particles, and disclination defects in confined geometry.


 

 

Theory and Simulation Studies of the

Elusive Polar and Biaxial Phases

 

G. Jackson

 

Department of Chemical Engineering and Chemical Technology

Imperial College of Science, Technology and Medicine

Prince Consort Road, London SW7 2BY United Kingdom

 

A fine balance between anisotropic repulsive and dispersive forces, and polar interactions is crucial in determining the phase behaviour of liquid crystal materials.  Two areas have attracted a great deal of interest and controversy in recent years: 1) the existence of polar nematic (ferroelectric phases) in thermotropic mesogens; and 2) the possibility of biaxial nematic phases in mixtures of prolate and oblate molecules. In our theoretical studies [1] of polar fluid phases formed by model systems, the anisotropic interactions can be treated using the well known Gay-Berne potential, which exhibits nematic, and smectic liquid crystalline phases, as well as the more common vapour and isotropic liquid  phases. In the first part of the presentation, the effect of including a central longitudinal point dipole on the phase behavior of a Gay-Berne model is studied using a simple density functional theory. The Gay-Berne potential is divided into repulsive and attractive terms according to the scheme of Weeks, Chandler, and Andersen (WCA) perturbation theory. The contribution of the repulsive interactions to the free energy is approximated by the Parsons-Lee theory of the hard Gaussian overlap potential.  The attractive term is taken as a perturbation; in order to keep the expressions as manageable as possible, a simple but ad. hoc. mean-field approximation is used, which incorporates the main properties of the first-order perturbation theory. The dipolar part of the potential is treated at the level of a second-order virial theory. The depolarization effects at the boundaries of the sample, which arise due to the long-range nature of the dipolar forces, are avoided by performing the calculations in an ellipsoidal sample of infinite aspect ratio using the technique of Groh and Dietrich. The phase coexistence between orientational disordered and ordered fluid phases is considered in detail. We focus on the possibility of finding a stable ferroelectric fluid phase which such an approach. The stability of the different phases is examined by changing the molecular elongation, the anisotropy parameter of the attractive interaction, and the strength of the dipole interaction. We compare our results to existing Monte Carlo simulation data where available. The theory is found to provide a good quantitative description of the dipolar Gay-Berne model for small values of the dipole moment. The second part of the presentation will focus on the stability or otherwise of the biaxial state for simple models of prolate and oblate molecules. The phase diagram of binary mixture of hard rod and plate molecules has been investigated by numerical minimization of a free energy functional of Onsager and Parsons. Both the rods and plates are represented by cylinders. The effect of various approximations are discussed: the use of the second Legendre representation of the excluded volume (L2 model); and the incorporation of the lower order terms of the excluded volume (end effects). The subtle competition between the orientational entropy, translational (packing entropy) and the entropy of mixing is very sensitive to the approximation that is used. At first the mixture is chosen to be symmetric at the level of the second virial theory (so that the phase behavior of the two pure components is identical), and the particles are examined in the Onsager limit (infinite aspect ratios) [2]. The effect of the unlike rod-plate excluded volumes on the phase behaviour is examined in detail. Different conclusions are reached about the stability of the biaxial phase depending on the approximation that is used. In the case of the full solution (Parsons free energy functional incorporating end effects) [3] we show that  the  biaxial nematic is unstable relative to demixing even for the symmetric mixture of the very long rod and very flat plates, where the Parsons-Lee theory becomes identical with the Onsager theory. Only two types of phase transitions are observed: isotropic-nematic phase coexistence and demixing transition involving either two isotropic or two nematic phases. The stability of the nematic region is found to be very sensitive to the aspect ratios: for moderate aspect ratios of the rods, the destabilization of the nematic phase is observed over the isotropic phase, while for large aspect ratios the tendency is the opposite. Our results are in good agreement with the results of simulation studies [4]. Finally, mixtures in which the volume of the plate is orders of magnitude larger that the volume of the rod are considered, so that an equivalence can be made where the plates are colloidal particles while the rods play the role of a depleting agent. A combined analysis of the isotropic-nematic bifurcation transition and spinodal demixing is carried out to determine the geometrical requirements for the stabilization of a demixing transition involving two isotropic phases. Global phase diagrams are presented in which the boundaries of isotropic phase demixing are indicated as functions of the molecular parameters [5]. The stability analysis indicates that for certain aspect ratios, the isotropic-nematic phase equilibria always preempts the demixing of the isotropic phase, irrespective of the diameters of the particles. When isotropic-isotropic demixing is found, there is an upper bound at large size ratios (Asakura and Oosawa limit), and a lower bound at small size ratios (Onsager limit) beyond which the system exhibits a miscible isotropic phase. It is very gratifying to find both of these limits within a single theoretical framework. We draw comparisons between the predicted regions of stability for isotropic demixing and recent experimental observations.

 

[1] S. Varga, I. Szalai, J. Liszi, and G. Jackson, J. Chem. Phys., 116, 9107 (2002).

[2] S. Varga, A. Galindo, and G. Jackson, Phys. Rev. E, 66, 0117XX (2002).

[3] S. Varga, A. Galindo, and G. Jackson, in preparation (2002).

[4] A. Galindo, G. Jackson, and D. J. Photinos, Chem. Phys. Lett., 325, 631 (2000).

[5] S. Varga, A. Galindo, and G. Jackson, J. Chem. Phys., submitted (2002)


 

 

A Monte Carlo study of the chiral columnar

organisations of chiral discotic mesogens

 

Roberto Berardi, Marco Cecchini, and Claudio Zannoni,

 

Dipartimento di Chimica Fisica e Inorganica,

Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.

 

Chiral columnar mesophases1-4 have been of particular interest at least since tilted chiral phases were found to be ferroelectric and switchable.1,2 From the theory and modelling point of view the relation between a molecular structure with a certain chirality5 and the type of mesophases formed is certainly not clear.  This is partly due to the complexity of the molecular structures that can give chiral columns and to the difficulty in identifying a precise relation between molecular features and liquid crystal properties.  Here we examine the different types of chiral columns that can be obtained by self assembly of chiral discotic molecules. 

     

We introduce to this end a simple two-site chiral model molecule formed by two suitably oriented interpenetrating Gay–Berne squashed ellipsoids similar to those that have proved very useful in the modelling of various liquid crystals.6 We then perform Monte Carlo computer simulations of various sets of these particles for different chiralities.  At low temperatures we find discotic mesophases formed by overall chiral columns and we analyse the results in terms of suitably defined observables and chiral correlation functions.

     

We observe a coupling between molecular tilt and twist between pair of molecules within the same columnar structure.  For our model system, the column chirality is not originating from a regular chiral arrangement of particles but it seems to be mainly due to one–particle high–chirality defects separated by low chirality domains.

     

[1] H. Bock and W. Helfrich, Liq.  Cryst., 1992, 12, 697; Liq.  Cryst., 1995, 18, 707.

[2] G. Sherowsky and X.H. Chen, J. Mat.  Chem., 1995, 5, 417; Liq.  Cryst., 1998, 24, 157.

[3] K. Praefcke, A. Eckert and D. Blunk, Liq.  Cryst., 1997, 2, 113.

[4] J. Barberá, A. Elduque, R. Giménez, F.J. Lahoz, J.A. López, L.A. Oro and J.L. Serrano, Inorg.  Chem., 1998, 37, 2960.

[5] A. Ferrarini and P.L. Nordio, J. Chem.  Soc.  Perkin Trans.  2, 1998, 455.

[6] C. Zannoni, J. Mater.  Chem., 2001, 11, 2637.


 

 

Wetting of polymer solutions:

 Monte Carlo Simulations

and Self-Consistent Field theory

 

Marcus Müller

 

Institut für  Physik,WA331,Jo.Gutenberg Universität

55099 Mainz, Germany

 

We investigate surface and interface properties of a coarse grained bead-spring polymer model by grandcanonical Monte Carlo simulations and self-consistent field theory. Both approaches are compared qualitatively: we find good agreement for the structure of the polymers at the surface/interface, but only qualitative agreement for the packing effects of the density at surfaces.

     

The surface and interface free energies are measured in the simulations and the wetting transition is located via the Young equation. The wetting transition is of strong first order, the drying transition is a weak first order or second order transition. Alternative methods for locating the wetting transition in the Monte Carlo simulations are discussed.

     

The wetting properties can be tuned by grafting a brush of identical chains on the surface. The complex wetting behavior as a function of the grafting density and the attractive strength (Hamaker constant) of the wall are explored by self-consistent field calculations. The system exhibits first order wetting transitions at low and high grafting densities and critical wetting transitions at intermediate overlap of the grafted chains.


 

 

Potential of mean force in dimeric proteins:

an MD simulation with constraints

 

M. Ferrario

 

Dipartimento di Fisica, Universita' degli Studi di Modena e Reggio Emilia,

Via G. Campi 213/A, 41100 MODENA, Italy.

 

Using holonomic constraints the potential of mean force of a dimeric protein is calculated as a function of monomer separation. Some aspects of the application  of the method of constraints will be reviewed in the contexts of extended (non-hamiltonian) dynamics and parametric free energy calculations.


 

 

Liquid Crystals at Surfaces

 

F. Schmid

 

Fakultät für Physik, Universität Bielefeld

Universitätsstraße 25, D-33615 Bielefeld

Germany

 

Fluids of soft ellipsoids are studied by computer simulations in the bulk and in the vicinity of surfaces. Our work aims at a deeper understanding of the relationship between the local structure of liquid crystals and mesoscopically or macroscopically relevant material properties such as elasticity and surface anchoring.

     

The talk shall be divided into two parts.

     

First, we discuss the local liquid structure of the bulk and present results for one of the central characteristic quantities, the direct correlation function (DCF). Inserting this information into an appropriate density functional, one can calculate the elastic constants in the nematic phase, and the density and order parameter profiles at surfaces. Predictions of the theory are compared with simulation results.

     

In the second part, we present computer simulations of a more complex situation: Surface anchoring on grafted liquid crystalline polymer brushes. The system is designed such that the orienting force of the substrate competes with that of stretched chains. We show that the grafting density can be used to tune the anchoring angle, and discuss the microscopic mechanisms which contribute to the anchoring.