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Monte Carlo simulations of lattice spin models are a powerful method for the investigation of confined nematic liquid crystals and allow for a study of the molecular organization and thermodynamics of these systems. Investigations of models of polymer-dispersed liquid crystals are reviewed devoting particular attention to the calculation of deuterium NMR spectra from the simulation data.

We review our Monte Carlo studies of molecular ordering in nematic liquid crystals with dispersed polymer networks. Starting from the aligning effect of a single fiber, we study different network topographies and investigate regular and random arrays of straight and distorted polymer fibrils. We analyze the aligning ability of rough fibrils, external field-induced switching, and pretransitional ordering. The simulation output is used to calculate selected experimental observables: ^2H NMR spectra, capacitance, and intensity of transmitted polarized light.

This article describes some of the progress made towards the simulation of liquid crystalline polymers and dendrimers within our laboratory. We describe the use of hybrid models, where a mixture of spherical and nonspherical potentials can be linked together to form model macromolecules. Results are presented for hybrid models of a side-chain and a main chain liquid crystal polymer, which have been studied by molecular dynamics simulation. Preliminary results are also presented from a modelling study of a third generation carbosilane liquid crystalline den-drimer. These involve molecular dynamics studies of single molecules in a solvent using a hybrid Gay-Berne/Lennard-Jones model; and studies of the bulk phases of the dendrimer using a coarse-grained hybrid spherocylinder/Lennard-Jones model. We also review briefly some of the progress made with other models for liquid crystals and polymers, point to the problems still faced and some of the current developments designed to overcome them.

Monte Carlo simulations have been recently performed for model liquids of dimers and trimers consisting of rigid groups connected by semiflexible spacers. Though highly idealized, the models take into account the three principal factors responsible for the onset of nematic order in oligomers and polymers of this kind, i.e. the anisometry of the rigid groups, the anisotropy of their attractive interactions and the intrinsic conformational properties of the molecules under study. In a first set of simulations, the conformation of model trimers has been approximately regulated to mimic idealized systems of rigid groups separated by (CH_2)_n spacers with n odd or even. The simulated systems show reversible isotropic/nematic phase transitions at well defined temperatures, with odd-even oscillations in good agreement with experiments. The transitions are coupled with a conformational selection favoring extended conformations in the nematic liquids. Simulations of model oligomers with conformational properties approximating those of a well characterized series of mesogenic oligoesters are currently underway.

Polymers containing randomly distributed spherical filler particles have been simulated by Monte Carlo methods for various particle sizes (4 to 28 times the transverse diameter of the polymer chains) and partial volumes of filler (10% to 50%). The polymer/filler interface consists of densely packed and partly ordered shells of polymer units of thickness nearly twice the diameter of the units. A number of parameters characterizing the molecular arrangements in these systems have been analyzed, leading to a general picture in which the chains are considered to be sequences of interface, bridge and loop segments. The results can be approximately predicted on a quantitative level using a few simple rules. It is also shown that phantom chains can be utilized in the simulations, provided that the interaction energy between chains and filler is modified in order to counterbalance the intrinsic tendency of the chain segments to avoid the filler surfaces. This makes possible to study systems that cannot be simulated at full density (i.e. systems with long chains, and/or with large particles and small filling density).

We discuss a Dissipative Particle Dynamics (DPD) approach to simulate oligomers and polymeric chains with nematic mesophases. Definition of mesogenic units are discussed, based either on semi-rigid units with standard DPD beads interacting only via soft repulsive potentials and linked by harmonic springs or on corrected DPD potentials including an orienting term between adjacent couples of beads. In the first case oriented phases are presented for systems made of single free semi-rigid units and, as an example of main-chain flexible liquid crystal polymer, by three linked semi-rigid units. In the second case an example of switching system is discussed, in the presence of an external potential.

We present in this contribution results from Molecular Dynamics (MD) simulations of a chemically realistic model of 1,4-polybutadiene (PB). The work we will discuss exemplifies the physical questions one can address with these types of simulations. We will specifically compare the results of the computer simulations with nuclear magnetic resonance (NMR) experiments, neutron scattering experiments and dielectric data. These comparisons will show how important it is to understand the torsional dynamics of polymers in the melt to be able to explain the experimental findings. We will then introduce a freely rotating chain (FRC) model where all torsion potentials have been switched off and show the influence of this procedure on the qualitative properties of local dynamics through comparison with the chemically realistic (CRC) model.

We present Monte Carlo simulations on the phase behavior of semiflexible macromolecules. For a single chain this question is of biophysical interest given the fact that long and stiff DNA chains are typically folded up into very tight compartments. So one can ask the question how the state diagram of a semiflexible chain differs from the coilglobule behavior of a flexible macromolecule. Another effect connected with rigidity of the chains is their tendency to aggregate and form nematically ordered structures. As a consequence one has two competing phase transitions: a gas-liquid and an isotropic-nematic transition potentially giving rise to a complicated phase diagram.

Macromolecular dynamics at the scale of a few chain bonds is largely controlled by the ”internal viscosity” effect if the energy barriers hindering the skeletal rotations are sufficiently large. In an extensive spin-echo neutron scattering analysis, Richter and co-workers ( Macromolecules , (2001), 34 , 1281) investigated by spin-echo neutron scattering the dynamic properties of polyisobutylene (PIB) and polydimethylsulfoxide (PDMS) in toluene solution, the latter polymer being currently assumed to have very small rotational barriers. Analysis of the data according to a theory proposed by one of us (G.A.) enabled them to obtain realistic values both of the rotational barrier around C-C bonds (≈ 3kcal/mol) and of the natural frequency of the rotational jumps for PIB. — A problem related to chain internal viscosity concerns the iso - and syndio tactic forms of polystyrene (respectively i-PS and s-PS). After a careful conformational analysis it is shown that i-PS has very large effective energy barriers due to interactions between phenyl rings. This effect is compounded with that of the intrinsic rotational barrier and helps explaining the kinetic difficulty to crystallise of i-PS as compared with s-PS.

We review here recent atomistic simulations of the adsorption of some protein fragments on a hydrophobic graphite surface. Fragments of unlike secondary structure containing either α-helices or β-sheets and of unlike hydropathy were taken into account. The simulations were carried out with simple energy minimizations to describe the initial ad- sorption on a bare surface, and with molecular dynamics runs to study the final and most stable adsorption geometry in a dielectric medium. Large conformational changes were found, involving complete denaturation of the fragments and a large spreading on the surface, with some degree of bidimensional ordering. The kinetics of surface spreading is also briefly reported. Finally, the statistical hydration of the isolated and adsorbed fragments was also investigated by explicitly accounting for the solvent.