Relation between the grafting density of liquid crystal macromolecule and the symmetry of self-assembled bulk phase: coarse-grained molecular dynamics study

I consider a generic coarse-grained model suitable for the study of bulk self-assembly of liquid crystal (LC) macromolecules. The cases include LC dendrimers, gold nanoparticles modified by polymer chains with terminating LC groups and others. The study is focused on the relation between a number of grafted chains, $N_{\rm ch}$, and the symmetry of the self-assembled bulk phases. Simple space-filling arguments are used first to estimate stability intervals for a rod-like, disc-like and spherulitic conformations in terms of $N_{\rm ch}$. These are followed by coarse-grained molecular dynamics simulations for both spontaneous and aided self-assembly of LC macromolecules into bulk phases. In spontaneous self-assembly runs, essential coexistence of rod-like and disc-like conformations is observed (via analysis of the histograms for the molecular asphericity) in a broad interval of $N_{\rm ch}$, which prevents the formation of defect-free structures. The use of uniaxial and planar aiding fields is found to improve self-assembly into monodomain phases by promoting conformations of respective symmetry. Strong shape-phase relation, observed experimentally, is also indicated by the simulations by the coincidence of the stability intervals for the respective conformations with those for the bulk phases.Comment: 12 pages, 8 figure

Novel morphologies for laterally decorated metaparticles: Molecular dynamics simulation

We consider a mesoscale model for nano-sized metaparticles (MPs) composed of a central sphere decorated by polymer chains with laterally attached spherocylinder. The latter mimics the mesogenic (e.g., cyanobiphenyl) group. Molecular dynamics simulations of $100$ MPs reveal the existence of two novel morphologies: $\textrm{uCol}_\mathrm{h}$ (hexagonal columnar arrangement of MPs with strong uniaxial order of mesogens collinear to the columns axis) and $\mathrm{wCol}_\mathrm{h}$ [the same arrangement of MPs but with weak or no liquid crystalline (LC) order]. Collinearity of the LC director and the columnar axis in $\textrm{uCol}_\mathrm{h}$ morphology indicates its potentially different opto-mechanical response to an external perturbation as compared to the columnar phase for the terminally attached mesogens. Preliminary analysis of the structures of both phases is performed by studying the order parameters and by visualisation of the MPs arrangements. Different mechanisms for the mesogens reorientation are pointed out for the cases of their terminal and lateral attachment.Comment: 6 pages, 5 figure

On the properties of a single OPLS-UA model curcumin molecule in water, methanol and dimethyl sulfoxide. Molecular dynamics computer simulation results

The properties of model solutions consisting of a solute --- single curcumin molecule in water, methanol and dimethyl sulfoxide solvents have been studied using molecular dynamics (MD) computer simulations in the isobaric-isothermal ensemble. The united atom OPLS force field (OPLS-UA) model for curcumin molecule proposed by us recently [J. Mol. Liq., 2016, 223, 707] in combination with the SPC/E water, and the OPLS-UA type models for methanol and dimethyl sulfoxide have been applied. We have described changes of the internal structure of the solute molecule induced by different solvent media in very detail. The pair distribution functions between particular fragments of a solute molecule with solvent particles have been analyzed. Statistical features of the hydrogen bonding between different species were explored. Finally, we have obtained a self-diffusion coefficient of curcumin molecules in three model solvents.Comment: 20 pages, 17 figures, 4 table

Pressure-driven flow of oligomeric fluid in nano-channel with complex structure. A dissipative particle dynamics study

We develop a simulational methodology allowing for simulation of the pressure-driven flow in the pore with flat and polymer-modified walls. Our approach is based on dissipative particle dynamics and we combine earlier ideas of fluid-like walls and reverse flow. As a test case we consider the oligomer flow through the pore with flat walls and demonstrate good thermostatting qualities of the proposed method. We found the inhomogeneities in both oligomer shape and alignment across the pore leading to a non-parabolic velocity profiles. The method is subsequently applied to a nano-channel decorated with a polymer brush stripes arranged perpendicularly to the flow direction. At certain threshold value of a flow force we find a pillar-to-lamellar morphological transition, which leads to the brush enveloping the pore wall by a relatively smooth layer. At higher flow rates, the flow of oligomer has similar properties as in the case of flat walls, but for the narrower effective pore size. We observe stretching and aligning of the polymer molecules along the flow near the pore walls.Comment: 14 pages, 12 figure

Shape characteristics of the aggregates formed by amphiphilic stars in water: dissipative particle dynamics study

We study the effect of the molecular architecture of amphiphilic star polymers on the shape of aggregates they form in water. Both solute and solvent are considered at a coarse-grained level by means of dissipative particle dynamics simulations. Four different molecular architectures are considered: the miktoarm star, two different diblock stars and a group of linear diblock copolymers, all of the same composition and molecular weight. Aggregation is started from a closely packed bunch of $N_{\text a}$ molecules immersed into water. In most cases, a single aggregate is observed as a result of equilibration, and its shape characteristics are studied depending on the aggregation number $N_{\text a}$. Four types of aggregate shape are observed: spherical, rod-like and disc-like micelle and a spherical vesicle. We estimate "phase boundaries" between these shapes depending on the molecular architecture. Sharp transitions between aspherical micelle and a vesicle are found in most cases. The pretransition region shows large amplitude oscillations of the shape characteristics with the oscillation frequency strongly dependent on the molecular architecture.Comment: 10 pages, 7 figure

Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study

We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azobenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field

Exploring fluid-solid interfaces with Stefan Sokołowski

This special issue of "Condensed Matter Physics" is a collection of works prepared to mark the 65th birthday of Prof. Stefan Sokołowski, our good friend, co-worker and member of the Editorial Board of this journal. By presenting this issue we would like to add our contribution towards the celebration of his scientific achievements and valuable contributions to the science. The majority of the works in the issue deal with the description of fluid-solid interfaces by using modern statistical mechanics methods - the principal line of Stefan's research. However, some contributions from the related areas authored by his close friends are included as well

Aided- and self-assembly of the liquid crystalline nanoparticles in bulk and in solution: computer simulation studies

We review computer simulation studies of aided and self-assembly of nanoparticles that are decorated with liquid crystalline (mesogenic) ligands, termed hereafter as liquid crystalline nanoparticles (LCNPs). In bulk, LCNPs self-assemble into ordered morphologies, typically displaying a polydomain structure. We demonstrate that a range of monodomain morphologies can be grown by changing the density of ligands and employing external fields of specific symmetry which act on mesogens. It is also demonstrated that the speed of self-assembly of LCNPs with chromophoric (e.g. azobenzene) mesogens can be increased by applying illumination at a certain wavelength and polarization. Another case study covers the formation of an interconnected macromolecular network in a solution of the LCNPs in a polar solvent. Here, the network structure depends strongly on the decoration pattern of the LCNPs. Finally, the adsorption of LCNPs on a compatible liquid crystalline brush is discussed with a focus on the prerequisites and optimal conditions for this phenomenon. The review demonstrates the ability of classical particle-based models to produce a molecular-based understanding of the structure and interactions of LCNPs, and also to reproduce a wide set of physicochemical phenomena related to LCNP aided- and self-assembly processes