All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals

The nature of the nanoscale structural organization in modulated nematic phases formed by molecules having a nonlinear molecular architecture is a central issue in contemporary liquid crystal research. Nevertheless, the elucidation of the molecular organization is incomplete and poorly understood. One attempt to explain nanoscale phenomena merely “shrinks down” established macroscopic continuum elasticity modeling. That explanation initially (and mistakenly) identified the low temperature nematic phase (NX), first observed in symmetric mesogenic dimers of the CB-n-CB series with an odd number of methylene spacers (n), as a twist–bend nematic (NTB). We show that the NX is unrelated to any of the elastic deformations (bend, splay, twist) stipulated by the continuum elasticity theory of nematics. Results from molecular theory and computer simulations are used to illuminate the local symmetry and physical origins of the nanoscale modulations in the NX phase, a spontaneously chiral and locally polar nematic. We emphasize and contrast the differences between the NX and theoretically conceivable nematics exhibiting spontaneous modulations of the elastic modes by presenting a coherent formulation of one-dimensionally modulated nematics based on the Frank–Oseen elasticity theory. The conditions for the appearance of nematic phases presenting true elastic modulations of the twist–bend, splay–bend, etc., combinations are discussed and shown to clearly exclude identifications with the nanoscale-modulated nematics observed experimentally, e.g., the NX phase. The latter modulation derives from packing constraints associated with nonlinear molecules—a chiral, locally-polar structural organization indicative of a new type of nematic phase.

A Review on Synthesis, Properties, of Liquid Crystal Dendrimers

This review provides brief information concerning with the dendrimer. The supramolecular organization of selected examples of liquid-crystalline dendrimers within lamellar, columnar and nematic phases is reported. It is shown that tuning of the mesomorphic structure can be achieved by an appropriate molecular design depending upon the chemical nature of the terminal mesogenic groups, dendritic core and dendrimer generation. The pseudospherical shape of a dendrimer arises from its structure, which consists of an internal region (the core) which is connected to repeating units constituting a radial branching pattern.

Flows of Linear Polymer Solutions and Other Suspensions of Rod-like Particles: Anisotropic Micropolar-Fluid Theory Approach

We formulate equations governing flows of suspensions of rod-like particles. Such suspensions include linear polymer solutions, FD-virus, and worm-like micelles. To take into account the particles that form and their rotation, we treat the suspension as a Cosserat continuum and apply the theory of micropolar fluids. Anisotropy of suspensions is determined through the inclusion of the microinertia tensor in the rheological constitutive equations. We check that the model is consistent with the basic principles of thermodynamics. In addition to anisotropy, the theory also captures gradient banding instability, coexistence of isotropic and nematic phases, sustained temporal oscillations of macroscopic viscosity, shear thinning and hysteresis. For the flow between two planes, we also establish that the total flow rate depends not only on the pressure gradient, but on the history of its variation as well.

Synthesis of Some New Bis- 2, 3-Dihydroquinazolin-4(1H)-one Derivatives from Dapsone Drug and Investigating their Liquid Crystalline Properties and Antibacterial Activities

During this paper, new Schiff's base derivatives [F1-F5] were prepared through the reaction of dapsone drug with different substituted benzaldehyde compounds. Schiff's bases were then converted into 2,3-dihyroquinazolin-4(1H)-one derivatives [F6-F10] through the reaction with 2-amino benzoic acid in ethanol. The synthesized compounds were identified by their physical properties using UV-Vis, FT-IR and 1H-NMR analyses. The liquid crystalline properties of some prepared compounds revealed smectic and nematic phases. Using two separate bacterial species, Pseudomonas aeruginosa (Gram -ve) and Staphylococcus aureus (Gram +ve), the antimicrobial activities of certain synthesized compounds [F1,F2,F7,F8] were investigated using the agar diffusion process. The findings revealed that some of the assayed compounds had antimicrobial activities against the bacteria tested.

Shear orientation of nematic phases of clay nanosheets: processing of barrier coatings

When suspensions are exposed to shear forces, the particles may form ordered structures depending on their shapes, concentrations, and the material. For some processes, e.g., for wet-film coating, it is important to know how fast these structures form in shear fields and for how long the structures persist when the shear is relaxed. To obtain information on the particle structure formation and the decay time, the effective viscosity of nematic suspensions of Na-hectorite nanosheets was investigated by rheology employing a cone-plate measurement geometry. The necessary time for the formation textured nematic films could be deduced by carrying out effective viscosity measurements at constant time steps. Information could also be obtained on the lifetime of the platelet textures when shear is relaxed. All this information was employed to identify geometrical requirements for slot dies to produce barrier liners with nanosheet layers oriented parallel to PET substrates. Thereby, we obtained green and simple coatings that are in line with state-of-the-art high-performance materials such as metalized plastic foils in terms of oxygen barrier properties.

Mesomorphic and Thermal Behavior of Symmetric Bent-Core Liquid Crystal Compounds Derived from Resorcinol and Isophthalic Acid

Bent-core liquid crystals (BCLC) have been widely studied as a result of their unusual polar and chiral properties. Similar to calamitic and discotic molecules, BCLC molecules also exhibit nematic phases, besides other higher order mesophases. The aim of this work is to comparatively analyze the mesomorphic behavior of some bent-core 1,3-disubstituted benzene core compounds derived from resorcinol and isophthalic acid. Thus, the two classes of compounds differ in the nature of the orientation of the ester bond between the benzene central core and the two branches attached to the core. The mesomorphic behavior was elucidated by polarized light optical microscopy and differential calorimetry. Given the relatively high isotropic points of the compounds, confirmation of the thermal stability in the domains manifesting liquid crystalline properties was performed by thermogravimetric studies. The theoretical explanation of the difference in mesomorphic behavior for the two classes was based on molecular modeling studies.

Effect of the degree of protonation of amino acid dopants in Chirality amplification in DSCG Lyotropic Nematic phases.

Disodium Cromoglycate (DSCG), a lyotropic liquid crystal in water, is shown to be an amplifier of chirality when doped with small chiral molecules. Here, we study the behaviour of the DSCG nematic phase in the presence of three aminoacids with different degrees of protonation: L-Alanine, L-Arginine·HCl and L-Arginine. The results demonstrate that the sign of the helicity of the doped nematic phase (i.e., a cholesteric phase) depends on the sign of the Helical Twisting Power of the dopant.

On the molecular origins of the ferroelectric splay nematic phase

Nematic liquid crystals have been known for more than a century, but it was not until the 60s–70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular structure enables denser packing of the molecules when they exhibit polar order, which shows that reduction of excluded volume is in the origin of the polar nematic phase. Additionally, we propose that molecular dynamics simulations are potent tools for molecular design in order to predict, identify and design materials showing the polar nematic phase and its precursor nematic phases.