Simulation of bulk phases formed by polyphilic liquid crystal dendrimers

A coarse-grained simulation model for a third generation liquid crystalline dendrimer (LCDr) is presented. It allows, for the rst time, for a successful molecular simulation study of a relation between the shape of a polyphilic macromolecular mesogen and the symmetry of a macroscopic phase. The model dendrimer consists of a soft central sphere and 32 grafted chains each terminated by a mesogen group. The mesogenic pair interactions are modelled by the recently proposed soft core spherocylinder model of Lintuvuori and Wilson [J. Chem. Phys, 128, 044906, (2008)]. Coarse-grained (CG) molecular dynamics (MD) simulations are performed on a melt of 100 molecules in the anisotropic-isobaric ensemble. The model LCDr shows conformational bistability, with both rod-like and disc-like conformations stable at lower temperatures. Each conformation can be induced by an external aligning eld of appropriate symmetry that acts on the mesogens (uniaxial for rod-like and planar for disc-like), leading to formation of a monodomain smectic A (SmA) or a columnar (Col) phase, respectively. Both phases are stable for approximately the same temperature range and both exhibit a sharp transition to an isotropic cubic-like phase upon heating. We observe a very strong coupling between the conformation of the LCDr and the symmetry of a bulk phase, as suggested previously by theory. The study reveals rich potential in terms of the application of this form of CG modelling to the study of molecular self-assembly of liquid crystalline macromolecules.Розроблено огрублену модель рідкокристалічного дендримера (РКД) 3-ї генерації, завдяки чому вперше в літературі вивчено взаємозв'язок між формою рідкокристалічної макромолекули та симетрією макроскопічної фази за допомогою молекулярної динаміки. Модель складається з м'якої центральної сфери, до якої приєднано 32 ланцюжки, кожен із яких закінчується мезогенною групою. Останні взаємодіють за допомогою недавно запропонованого анізотропного потенціалу з м'якою серцевиною Лінтувуорі та Вілсона [J. Chem. Phys, 128, 044906, (2008)]. За допомогою методу огрубленої молекулярної динаміки змодельовано розплав 100 молекул РКД в анізотропно-ізобаричному ансамблі. Запропонована модель РКД продемонструвала конформаційну бістабільність, за якої як паличкоподібна, так і дископодібна конформації виявляються стабільними за низьких температур. Кожна з цих 2-х конформацій може ініціюватись зовнішнім полем із відповідною симетрією, в результаті в розплаві спонтанно формуються або монодоменна смектична фаза (із паличкоподібних конформацій, ініційованих одновісним полем), або стовпцева фаза (із дископодібних конформацій, ініційованих планарним полем). Обидві фази є стабільними приблизно в тому ж температурному інтервалі та у процесі нагрівання переходять в ізотропну кубічну фазу через яскраво виражений фазовий перехід. Спостережено сильний зв'язок між конформацією РКД і симетрією об'ємної фази, на що вказано раніше в теоретичних роботах. Дослідження виявляє великий потенціал в застосуванні огрубленого моделювання до опису просторової самоорганізації рідкокристалічних макромолекул

A domain decomposition molecular dynamics program for the simulation of flexible molecules of spherically-symmetrical and nonspherical sites. II. Extension to NVT and NPT ensembles

We describe the algorithms for NVT and NPT-ensemble simulations developed within the parallel molecular dynamics program GBMOLDD. This program uses the domain decomposition algorithm and is targeted at large-scale simulations of molecular systems (particularly polymers and liquid crystals) composed of both spherically-symmetric and nonspherical sites. The nonspherical sites can be described either by a Gay-Berne potential or by soft repulsive spherocylinders. The molecules can be of arbitrary topology and the intramolecular forces are described via standard force fields. We tested the stability of both leap-frog and velocity-Verlet integrators on two "real-life" systems-a nematic liquid crystal phase of 1944 one-site Gay-Berne molecules and on 512 flexible liquid-crystalline dimers. In both cases the algorithm demonstrates good stability over the typical simulation times required for new phase formation and/or molecular relaxation processes. (C) 2002 Elsevier Science B.V. All rights reserved

Rotational viscosities of Gay-Berne mesogens

Rotational viscosities gamma(1) are calculated for three Gay-Berne models for a wide range of state points in the nematic phase. There was a strong density dependence in the results, with gamma(1) increasing with increasing density. Away from the clearing point, the temperature dependence of gamma(1) was described by simple Arrhenius-like behaviour. A comparison of the values of gamma(1) and the Arrhenius activation energies with real mesogens pointed to a number of problems with the Gay-Berne potential, when used as a model for real mesogenic systems

Simulation of bulk phases formed by polyphilic liquid crystal dendrimers

A coarse-grained simulation model for a third generation liquid crystalline dendrimer (LCDr) is presented. It allows, for the first time, for a successful molecular simulation study of a relation between the shape of a polyphilic macromolecular mesogen and the symmetry of a macroscopic phase. The model dendrimer consists of a soft central sphere and 32 grafted chains each terminated by a mesogen group. The mesogenic pair interactions are modelled by the recently proposed soft core spherocylinder model of Lintuvuori and Wilson [J. Chem. Phys, 128, 044906, (2008)]. Coarse-grained (CG) molecular dynamics (MD) simulations are performed on a melt of 100 molecules in the anisotropic-isobaric ensemble. The model LCDr shows conformational bistability, with both rod-like and disc-like conformations stable at lower temperatures. Each conformation can be induced by an external aligning field of appropriate symmetry that acts on the mesogens (uniaxial for rod-like and planar for disc-like), leading to formation of a monodomain smectic A (SmA) or a columnar (Col) phase, respectively. Both phases are stable for approximately the same temperature range and both exhibit a sharp transition to an isotropic cubic-like phase upon heating. We observe a very strong coupling between the conformation of the LCDr and the symmetry of a bulk phase, as suggested previously by theory. The study reveals rich potential in terms of the application of this form of CG modelling to the study of molecular self-assembly of liquid crystalline macromolecules

Solvation force between tethered polyelectrolyte layers. A density functional approach

We use a version of the density functional theory to study the solvation force between two plates modified with a tethered layer of chains. The chains are built of tangentially jointed charged spherical segments. The plates are immersed in an electrolyte solution that involves cations, anions and solvent molecules. The latter molecules are modelled as hard spheres. We study the dependence of the solvation force and the structure of chains and of solute molecules on the grafting density, length of chains, architecture of the chains and on concentration of the solute