Surface-Induced Ordering on Model Liquid Crystalline Dendrimers (original) (raw)
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Homogeneous alignment of liquid crystalline dendrimers confined in a slit-pore. A simulation study
Journal of Physics: Condensed Matter, 2016
In this work we present results from NPT (isobaric-isothermal) Monte Carlo Simulation studies of Liquid Crystalline Dendrimer (LCDr) systems confined in a slit-pore made of two parallel flat walls. We investigate the substrate induced conformational and alignment properties of the system at different thermodynamic state points under uniform (unidirectional) anchoring condition. Tractable coarse grained force fields to model both monomer-monomer and monomer-substrate interaction potentials have been used from our previous work. In this anchoring condition, at lower pressure almost all the monomers are anchored to the substrates and mesogens are perfectly aligned with the aligning direction. This alignment is not uniformly transmitted to the bulk region as the pressure grows, instead, it decays with distance from the surface to the bulk region. Due to this reason, the global orintational order parameter decreases with increasing pressure (density). In the neighborhood (2 − 3 mesogenic diameter) of upper and lower walls, mesogenic units form smectic A like structure whose layers are separated by layers of spherical beads. In this region individual LCDrs possess a rod like shape.
2015
In this work we present results from NPT (isobaric-isothermal) Monte Carlo Simulation studies of Liquid Crystalline Dendrimer (LCDr) systems confined in a slit-pore made of two parallel flat walls. We investigate the substrate induced conformational and alignment properties of the system at different thermodynamic state points under uniform (unidirectional) anchoring condition. Tractable coarse grained force fields to model both monomer-monomer and monomer-substrate interaction potentials have been used from our previous work. In this anchoring condition, at lower pressure almost all the monomers are anchored to the substrates and mesogens are perfectly aligned with the aligning direction. This alignment is not uniformly transmitted to the bulk region as the pressure grows, instead, it decays with distance from the surface to the bulk region. Due to this reason, the global orintational order parameter decreases with increasing pressure (density). In the neighborhood (2− 3 mesogenic ...
Order and frustration in liquid-crystalline dendrimers
The European Physical Journal E, 2013
X-ray diffraction has been used to elucidate the structure and phase behavior of several liquidcrystalline dendrimers with a different surface topology of the terminal chains. This includes secondgeneration liquid-crystalline block and statistical dendrimers with mixed aliphatic and mesogenic terminal groups as well as homo-dendrimers of several generations containing only mesogenic end groups. The homodendrimers of generation one to four display a monolayer smectic phase, while the fifth generation shows a more ordered columnar phase. The block-dendrimer of the second generation has a bilayer smectic phase. The precise structure of the lamellar ordering has been determined by X-ray reflectivity from thin films on a substrate. The second-generation statistical dendrimer does not show any mesogenic phase. The observed phase behavior is discussed in terms of the frustration due to competition between the stiff geometry of the dendritic matrix and the close-packing conditions of the terminal chains.
Single liquid crystalline dendrimer under smectic-like mean field
Liquid Crystals, 2017
Monte Carlo Simulation studies of different generation liquid crystalline dendrimers (LCDrs) (G n D 3) under smectic-like mean field have been presented. Mean field-induced configurational and alignment properties of several LCDrs at different field strengths and layer spacing values were investigated. Tractable coarse-grained force fields to model both monomer-monomer and monomer-field interaction potentials have been formulated. Particularly, the effect of mean field on the shape, size and orientation of LCDrs was addressed. Depending on the generation of an LCDr, strength of the field and the layer spacing, on average spherically symmetric LCDrs were changed to either rod-or disc-like shapes. At a relatively strong field, smaller-generation LCDrs stretched to their maximum size at bigger values of layer spacing, whereas larger-generation LCDrs compressed to attain short-cylinder-/disc-like shape at smaller values of layer spacing. In addition to stretching and compression, sub-molecular partitioning has also been observed. Two layers of mesogenic units separated by populations of spherical beads were formed along the field direction. Those layers of mesogens are oriented along the field direction with intensity of orientability highly depending on the field strength.
Computer simulations of a liquid crystalline dendrimer in liquid crystalline solvents
The Journal of Chemical Physics, 2003
Molecular dynamics simulations have been carried out to study the structure of a model liquid crystalline dendrimer ͑LCDr͒ in solution. A simplified model is used for a third generation carbosilane LCDr in which united atom Lennard-Jones sites are used to represent all heavy atoms in the dendrimer with the exception of the terminal mesogenic groups, which are represented by Gay-Berne potentials. The model dendrimer is immersed in a mesogenic solvent composed of Gay-Berne particles, which can form nematic and smectic-A phases in addition to the isotropic liquid. Markedly different behavior results from simulations in the different phases, with the dendrimer changing shape from spherical to rodlike in moving from isotropic to nematic solvents. In the smectic-A phase the terminal mesogenic units are able to occupy five separate smectic layers. The change in structure of the dendrimer is mediated by conformational changes in the flexible chains, which link the terminal mesogenic moieties to the dendrimer core.
Soft Matter, 2005
Coarse-grained simulations are described in which the behaviour of a system of model liquid crystalline dendrimer molecules is studied in both liquid and smectic-A liquid crystalline phases. The model system is based on a third generation carbosilane dendrimer, which is functionalised at the surface by short polymeric chains terminated in mesogenic units. The design of the coarsegrained model is based on initial Monte Carlo studies of a single carbosilane molecule at an atomistic level, which yield structural data. The coarse-grained dendrimer is represented in terms of a combination of spherical sites representing the dendrimer core and polymer chains, and spherocylinders representing the mesogenic groups. A strong coupling is seen between internal molecular structure and molecular environment, with individual dendrimer molecules undergoing a remarkable transition from spherical to rod-shaped at the isotropic-smectic phase transition. The driving force for mesophase formation is provided by nanoscale microphase separation of mesogens and the dendrimer core.
Simulation of bulk phases formed by polyphilic liquid crystal dendrimers
Condensed Matter Physics, 2010
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 (Sm A) 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.
Atomic Force Microscopy Study of Structural Organization of Carbosilane Liquid Crystalline Dendrimer
Langmuir, 2000
Structural organization of a carbosilane liquid crystalline dendrimer of fifth generation with cyanobiphenyl mesogenic groups in the 25-130°C range was examined by atomic force microscopy (AFM). Studies were performed on the dendrimer films of different thicknesses, from single macromolecular layers to films several micrometers thick. Packing motifs of spherical macromolecules (5.5 nm in diameter) were revealed in submicrometer AFM images of the dendrimer films. Rectangular and hexagonal lattices were found in the single layers on different substrates. With an increase of film thickness, first smectic layers are formed, and then domains consisting of aggregates of flat-on lying and edge-on standing smectic layers were found. In thick films, the domains exhibit layered structures of different thicknesses and with different orientations. AFM images recorded at different temperatures demonstrated height changes of different domains caused by anisotropic changes of lattices of the smectic layers.
Direct Visualization of Individual Cylindrical and Spherical Supramolecular Dendrimers
Science, 1997
Electron microscopy methods have been used to visualize individual spherical and cylindrical supramolecular dendrimers, providing definitive confirmation of the structures suggested by previous x-ray diffraction analysis that assumed a microsegregated model. These dendrimers are self-assembled, self-organized, and aligned spontaneously and simultaneously in hexagonal columnar or cubic thermotropic liquid-crystal phases with high uniformity. Homeotropic and planar ordering of the hexagonal columnar liquid crystal was precisely controlled by a variety of surfaces. The stiffness of these cylinders was evaluated by examining their planar texture and its defects.