Cholesteric Mesogen Research Articles

Rheological characteristics of different carbon nanoparticles in cholesteric mesogen dispersions as lubricant coolant additives

The effect of two types of carbon nanoparticles, i.e., shungite carbon (SC) and multiwalled carbon nanotubes (MWCNTs), on the mesomorphic and rheological properties of cholesteric mesogen in the concentration range of 0.005–0.04 wt % has been reported. The main physical parameters for tribological applications, such as phase transition temperature, viscosity, elastic modulus G', loss modulus G'', and complex viscosity η*, have been studied. The preference of using SC as a lubricating coolant additive component has been shown in the comparison with MWCNTs.

Mesophase Behaviors, Mesomorphic and Optical Properties of Side-Chain Liquid Crystalline Polymers Bearing Cholesteric and Nematic Mesogens

A series of side-chain liquid crystalline polymers (LCPs) have been prepared. Their chemical structures and properties have been fully investigated. Homopolymers P1 and P7 show a nematic phase and a smectic A phase, respectively, and copolymers P2-P6 reveal a cholesteric phase. The reflection wavelengths of P2-P6 exhibit blue shift. P2-P6 display selective reflection at temperatures below the glass transition on first cooling cycle, which cannot be observed on the first heating. The isotropic phase transition temperatures and glass transition temperatures of the polymers increase with the increase of cholesteric mesogens in the polymer systems.

Structure–color mechanism of iridescent cellulose nanocrystal films

Chirality, hydrogen bond interaction and surface charge repulsion among sulfate CNCs have vital impact on the formation of a cholesteric mesogen in a suspension or solidified film. Therefore, changing surface charge density through mechanical disintegration is an effect way to tune the chiroptical properties of iridescent CNC films.

Supramolecular self-assembly of a novel hydrogen-bonded cholesteric liquid crystal exhibiting macromolecular behaviour

The mesomorphic, thermoptic and glass-forming properties of 4-[6-((cholesteryloxy) carbonyl)oxy hexyloxy] benzoic acid (Ch-BA) have been investigated as a novel supramolecular hydrogen-bonded cholesteric mesogen. Fourier transform infrared and 1H nuclear magnetic resonance studies have confirmed the chemical structure and the hydrogen-bond formation between the mesogens. According to polarising optical microscope observations, the compound exhibited smectic and chiral nematic phases. Differential scanning calorimetry indicated an unexpected glass transition (T g) around 32°C and a liquid crystalline region between 32 and 122°C, in which the cholesteric phase appeared at 80°C. As a result of the glass formation, samples of Ch-BA which were rapidly cooled below the T g were found to preserve the long-range ordering of the liquid crystalline state and retained the iridescent colours of the cholesteric phase. These results led to the conclusion that the formation of identical dimers by intermolecular hydrogen-bonding of the terminal carboxylic acids accompanying the lateral packing of the rigid cores, built a trimeric arrangement and this was responsible for the macromolecular behaviour of Ch-BA, despite its relatively simple structure and low molecular weight.

Liquid crystalline and thermo-optical properties of cyclic siloxane tetramers containing cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate

A series of cyclic siloxane tetramers covering the whole composition range were synthesised using cyclotetrasiloxane, cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate, and the effects of the cholesteryl-based mesogenic units on liquid crystalline and thermo-optical properties were studied. The presence of a cholesteric mesogen was observed to widen the mesogenic temperature range and tended to induce smectic A phases in the tetramers containing a higher mole fraction of cholestryl-4-allyloxy-benzoate (X chol). The selective reflection of cholesteric tetramers shifted to shorter wavelengths with increasing X chol and temperature. Blue phases can be easily observed for those tetramers containing more than 50.0 mol% cholesteric mesogen. The shorter pitch sample showed the wider blue phases range. The blue phases range width increased from 2.8°C to 18.5°C as X chol increased from 0.5 to 1.0.

Fabrication and characterization of gratings based on thermal shrinkable reactive cholesteric mesogens

Gratings were successfully fabricated by photolithography by using a particular reactive cholesteric liquid crystal at two different temperatures. The reflection band of the grating was found to be wider than that of a film having a single helical order, because the grating exhibited a binary helical order. Moreover, the surface topography of the grating was regulated by using photomasks with different pitches. The optical reflection spectrum of this grating structure spans two reflection spectra of cholesteric liquid crystal films that were prepared by UV curing at two heating temperatures. Experimental results reveal that the method proposed in this study can be used to produce broadband circular polarizing films.

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.

Novel supramolecular hydrogen-bonded cholesteric mesogens: liquid crystalline, thermoptical and glass-forming properties

Liquid crystalline, thermoptical and glass-forming properties of a series of novel supramolecular hydrogen bonded cholesteric mesogens have been described. These assemblies were built via intermolecular hydrogen bonding between cholesteric esters containing pyridine groups and cholesterol containing carboxylic acid moieties. All hydrogen-bonded complexes exhibit well defined cholesteric phases over a range of temperatures depending upon the number of carbon atoms present in their alkyl chains. The formation of pure LC materials on 1 ∶ 1 complexation could be confirmed from the phase diagrams of the binary mixtures and infrared absorption spectra. By rapid cooling of the hydrogen bonded assemblies from their cholesteric temperature to 0 °C glassy liquid crystalline films maintaining helical molecular order were obtained. Different reflection colors of the cholesteric phase were selectively and repeatedly fixed by changing the starting temperature of rapid cooling.

A Review of the Status of Polymerization in Thermotropic Liquid Crystal Media and Liquid Crystalline Monomers

Abstract Thermotropic liquid crystals offer uniquely ordered media for intermolecular reactions such as polymerization. Unlike the recently investigated topochemical polymerizations where molecules are rigidly constrained on lattice points [l], the liquid crystalline state permits full two-dimensional (nematic and cholesteric) and one-dimensional (smectic) movement. In addition, various degrees of translational freedom are attainable depending on the class of mesogen. Such freedom of choice, plus the ability to orient nematic and cholesteric mesogens on a molecular scale in an electric or magnetic field or on certain surfaces, makes the liquid crystal state an attractive polymerization medium.