Liquid Crystal Composites Research Articles

Main features of application of liquid crystal composites for the near-wall flow diagnostics

In the work the special aspects of panoramic temperature and/or shear stress diagnostics by liquid crystals are considered. Some metrological characteristics of two types of LC composites on the basis of cholesterics and nematics are discussed. The temperature and shear stress hue sensitivity are analyzed. The examples of application are presented.

Self-healing properties of liquid crystalline composite based on borosiloxane

A liquid crystalline composite (LC-composite) based on synthesized borosiloxane (BS) and 5 wt. % liquid crystal (LC) 4-cyano-4’-N-heptylbiphenyl was obtained and investigated. The microstructure of the obtained composite was characterized by the method of polarization optical microscopy (POM). It is shown that the LC in the composite is contained as a separate phase, which is not miscible with BS. The phase is represented by spherical micro droplets with sizes from 1 to 50 microns. The introduction of the LC into the BS matrix leads to the clouding of the initially transparent BS. This phenomenon is associated with the difference in the refractive indices of the LC and BS, which leads to light scattering on the micro droplets of the LC component. It was established that the BS matrix and the LC composite have self-healing properties after surface damage. The time of self-healing of surface damage at room temperature and normal conditions was about 1.5 - 2 hours. The obtained LC composites may be interesting as promising materials for flexible multilayer biocompatible electronics and electro-optical devices with self-healing properties.

Tuning of Photonic band gap via combined effect of electric and optical fields in a blue phase liquid crystal composite

ABSTRACT Blue phase liquid crystals are soft 3D photonic crystals in which the liquid crystal molecules self-assemble to form a cubic structure with lattice spacing of a few hundred nanometers resulting in selective reflection of colours in the visible spectrum. The corresponding wavelength or the ‘photonic band gap’ can be tuned using various external stimuli such as thermal, electric, magnetic and optical fields. Here, we report efficient tuning of photonic band gap by utilising the combination of electric and optical fields in a blue phase liquid crystalline system. The studies indicate that the chirality of the medium has a direct bearing on the direction of the wavelength shift and the extent of the photonic band gap tunability. More importantly, the synergistic effect of the two fields helps in reversible tuning of the band gap.

Magnetic segregation effect in liquid crystals doped with carbon nanotubes.

We study the orientational transitions in a suspension of carbon nanotubes in a nematic liquid crystal induced by an external magnetic field. The case of a finite orientational anchoring of liquid crystal molecules at the surface of doped carbon nanotubes is considered. It is shown that in a magnetic field the initial homogeneous planar texture of the liquid crystal–carbon nanotubes mixture is disturbed in a threshold manner (Fréedericksz transition). The orientational and concentration distributions of the suspension are studied for different values of the magnetic field strength and segregation intensity of the impurity subsystem. The optical phase lag between ordinary and extraordinary rays of light transmitted through a layer of a liquid crystal composite is calculated. The possibility of changing the nature of the Fréedericksz transition from second order to first order is shown. This tricritical behavior is related to the redistribution of the carbon nanotubes (segregation effect) inside the layer.

Fast copolymer network liquid crystals for tunable birefringence colors.

A fast-switching, tunable color filter was found in a copolymer network liquid crystal (LC), which was in situ generated in a conventional LC test cell with parallel aligned glass plates and investigated with polarized light. Polarization filters were used to convert the tunable optical phase retardance of the test cells to birefringence colors as is always possible in a LC test cell with carefully adjusted cell gap and effective birefringence. The cell gap of the samples could be adjusted to a value of 9 μm, which is not easily possible in a polymer LC composite without creating defects. In these samples, the typical pastel colors seen frequently in birefringent samples could be avoided. The transmittance spectra were recorded and converted to CIE 1931 color coordinates, which showed that the colors seen had a reasonable distance to the white point. The electro-optic switching times of the samples were investigated: Fast responses of t on +t off <5 ms were found, which is an impressive speed for tunable birefringence colors in LCs and LC composites. Upon increasing addressing voltages, a blueshift of the peak seen in the transmittance spectra was observed. The samples consisted of copolymer network LC, generated from a reactive mixture with mesogenic monomer and nonmesogenic comonomer. The tunable color was seen selectively in samples with dodecyl acrylate as comonomer. The experiments show how even a straightforward electro-optic experiment still can result in unexpended findings, which may expand the use of LC composites in nondisplay applications. The polymer morphology in samples with a larger cell gap was investigated with scanning electron microscopy, and interdefect distances of ≈40 μm were found. The appearance of defects in test cells with a cell gap of 9 μm could be avoided because the cell gap was much smaller than the measured interdefect distances in test cells with a larger cell gap.

Photo- and electrical-responsive liquid crystal smart dimmer for augmented reality displays.

A dual-stimuli polarizer-free dye-doped liquid crystal (LC) dimmer is demonstrated. The LC composition consists of photo-stable chiral agent, photosensitive azobenzene, and dichroic dye in a nematic host with positive dielectric anisotropy. Upon UV exposure, the LC directors and dye molecules turn from initially vertical alignment (high transmittance state) to twisted fingerprint structure (low transmittance state). The reversal process is accelerated by combining a longitudinal electric field to unwind the LC directors from twisted fingerprint to homeotropic state, and a red light to transform the cis azobenzene back to trans. This device can be used as a smart dimmer to enhance the ambient contrast ratio for augmented reality displays.

Novel easy to fabricate liquid crystal composite with potential for electrically or thermally controlled transparency windows

Switchable liquid crystal (LC) composites are a unique and attractive class of functional materials due to their extensive use in various applications including smart and privacy windows. Demand for developing smart windows with good switchable performance has steadily increasing in the past decades due to their importance in energy saving. Herein, we present the use of novel and highly active switchable LC composite material-octadecanol-doped LC-prepared via a facile, low-cost, and scalable process, for thermally or electrically controlled transparency windows. A systematic study of the switchable behavior reveals the formation of a reversible molecular arrangement between the LC and the octadecanol, which allows control of the transparency through scattering modulation of the device by voltage or temperature. The devices fabricated by sandwiching the LC composite material between two ITO-covered glass slides present switchable performance with high potential for cost-effective utilization in various applications, such as light shutters, smart or privacy windows.

Influence of azo dye concentration on dielectric response in polymer dispersed liquid crystal composites

In this paper, we have investigated dielectric properties of azo dye methyl red (MR) doped polymer dispersed liquid crystal (PDLC) composites. Norland optical adhesive (NOA 65) and nematic liquid crystal (E63) materials were used to construction of PDLC composite. In order to observe the effect of MR concentration, PDLC was dispersed with 1, 3 and 5% wt/wt MR. Dielectric properties have been investigated using the dielectric spectroscopy method in the frequency range of 500 Hz to 10 MHz at room temperature. Dielectric constant (ε′ and ε″), dielectric anisotropy (Δε′), relaxation frequency (fR), relaxation time (τ), threshold voltage (Vth) and also splay elastic constant (K11) have been determined for the pure PDLC and its doped version PDLC + 1% MR, PDLC + 3% MR and PDLC + 5% MR composites by using experimental data. Dispersal of increasing MR concentration caused τ, Vth, K11 are decreased whereas fR is increased for the investigated samples. Results show that different MR concentrations significantly affect the dielectric properties of PDLC composites and make it suitable to design new based on PDLC device applications.

Nematic Liquid Crystal Composite Materials for DC and RF Switching

Liquid Crystals (LCs) are widely used in display devices, electro-optic modulators, and optical switches. A field-induced electrical conductivity modulation in pure liquid crystals is very low which makes it less preferable for direct current (DC) and radio-frequency (RF) switching applications. According to the literature, a conductivity enhancement is possible by nanoparticle doping. Considering this aspect, we reviewed published works focused on an electric field-induced conductivity modulation in carbon nanotube-doped liquid crystal composites (LC-CNT composites). A two to four order of magnitude switching in electrical conductivity is observed by several groups. Both in-plane and out-of-plane device configurations are used. In plane configurations are preferable for micro-device fabrication. In this review article, we discussed published works reporting the elastic and molecular interaction of a carbon nanotube (CNT) with LC molecules, temperature and CNT concentration effects on electrical conductivity, local heating, and phase transition behavior during switching. Reversibility and switching speed are the two most important performance parameters of a switching device. It was found that dual frequency nematic liquid crystals (DFNLC) show a faster switching with a good reversibility, but the switching ratio is only two order of magnitudes. A better way to ensure reversibility with a large switching magnitude is to use two pairs of in-plane electrodes in a cross configuration. For completeness and comparison purposes, we briefly reviewed other nanoparticle- (i.e., Au and Ag) doped LC composite’s conductivity behavior as well. Finally, based on the reported works reviewed in this article on field induced conductivity modulation, we proposed a novel idea of RF switching by LC composite materials. To support the idea, we simulated an LC composite-based RF device considering a simple analytical model. Our RF analysis suggests that a device made with an LC-CNT composite could show an acceptable performance. Several technological challenges needed to be addressed for a physical realization and are also discussed briefly.

Polymer (PEO)-liquid crystal (LC E8) composites: The effect from the LC inclusion

The effect from the inclusion of nematic liquid crystal (LC) E8 into the polymer poly(ethylene oxide) (PEO) was inspected by studying of structural and electrical properties of PEO-E8 composites at weight percentage of E8 ranging from 10 % to 50 %. Flexible PEO-E8 films with a thickness of 0.1 mm were structurally characterized by X-ray diffraction and X-ray photo-electron spectroscopy. The results obtained by our analyses indicate that a polymer-LC intermolecular complex is formed by inclusion of E8 LC molecules in the PEO host at a certain their concentration level. The structural properties of PEO-E8 composites were correlated with their electro-conducting properties as depending on the E8 LC concentration. By the amount of the LC fraction one can achieve a controlled modification of the structural and electro-conducting properties of the PEO-E8 material. As compared to PEO, the inclusion of E8 LC in the PEO polymer matrix can lead to a considerably enhanced electrical conductivity of PEO-E8 composites.

Impact of co-doping concentration in copolymer network liquid crystals

ABSTRACTThe impact of varying the co-doping concentration of a mesogenic and a non-mesogenic monomer in the reactive mixtures used to create a copolymer network LCs was investigated. Use of copolymer has been found to improve the response properties in the obtained liquid crystal composites. The polymer network in the studied copolymer network LCs was examined by scanning electron microscopy and the response times in various samples were investigated. Samples were prepared with various reactive mixtures, each of which had a constant concentration of mesogenic monomer, various concentrations of non-mesogenic monomer, and the same amount of photoinitiator. These reactive mixtures were filled in home assembled test cells with planar alignment and then exposed to UV light. With increasing concentration of the non-mesogenic monomer, the response properties of the resulting copolymer network LC were improved. Usually, if the overall polymer content in a polymer network LC is increased, the threshold voltage is also increased. However, both threshold voltages and response times were lowered and the response properties were thus improved in the studied copolymer network LCs. This unexpected behavior could be traced back to inducing a grainy polymer morphology of the copolymer network by using a non-mesogenic monomer.

Effective permittivity of liquid crystal nanodispersions and composites with different pore structure

ABSTRACTThe volume filling fraction dependence of the effective permittivity of the nematic liquid crystal 4-n-pentyl-4’-cyanobiphenyl embedded in different porous membranes and dispersed with aerosil nanoparticles was determined using broadband dielectric spectroscopy in the frequency range from 106 to 109 Hz. The experimental data were analyzed and compared with some existing theories based on the effective medium approximation and their modifications. The obtained effective permittivities as a function of the volume filling fraction lie between the lower limits of the Wiener and Hashin–Shtrikman bounds. The observed shift of the experimental points reflects the changes in the structure of the investigated composites.

Electrically switchable light transmittance of epoxy-mercaptan polymer/nematic liquid crystal composites with controllable microstructures

In this paper, we report the design, preparation and properties of epoxy-mercaptan polymer/nematic liquid crystal (LC) composites with electrically switchable optical transmittance. It is found that the polymer microstructure can be regulated from porous structure to microspheres or beads by appropriately modulating the curing temperature, the molecular structure of mercaptan, as well as the compositions of epoxy, mercaptan and nematic LC host. Under an external electric field, the thin films based on such composites can switch from opaque to transparent. The electro-optical performance is found to be highly dependent on the polymer microstructures and the optimum characteristics can be achieved in the composite with a microstructure of smaller microspheres. The possible mechanism for the formation of polymer microspheres and the effects of the microstructures on the electro-optical characteristics of the composites are also discussed. This work is expected to provide an efficient way to control the microstructure of epoxy/mercaptan polymer and develop the composites with switchable optical transmittances for both civil and military applications.

Multiple bio-responsive polymer dispersed liquid crystal composites for sensing applications

Polymer dispersed liquid crystal (PDLC) composites were prepared by the encapsulation of cholesteryl acetate (L-ChAc) in polyvinyl alcohol boric acid (PVAB) by microemulsion technique. The supramolecular and morphological peculiarities of the resulted composites were analyzed by polarized optical microscopy, fluorescence microscopy and scanning electron microscopy respectively. The thermal behavior of the systems was evaluated by differential scanning calorimetry and the surface characteristics in terms of wettability and surface free energy were calculated based on contact angle measurements. Circular dichroism was further used to evidence the chiroptical properties of the studied PDLCs and to have an insight on the potential interactions between the components. The ability of the composites to act as sensors for different blood analytes was preliminary tested by POM. The results indicated that PVAB facilitated a uniform distribution of the cholesteric liquid crystal as micrometric droplets with narrow polydispersity and planar anchoring. The obtained PDLCs proved moderate wettability and they showed a selective responsiveness for L or D sugars, amino acids and DNA. All these results indicated the new polymer dispersed liquid crystal systems as promising materials for biosensors building.

Panoramic Temperature, Pressure, and Shear Stress Sensors Based on Liquid Crystal Composites

Thermo- and mechano-optical characteristics are described for three types of composites based on liquid crystals and a luminophore in a polymer matrix. These composites are designed for the panoramic diagnostics of temperature, surface friction, and surface pressure. The studies are performed by optical spectroscopy, colorimetry, and thermometry. Examples of using these composites as sensors in an aerodynamic experiment are given.

Stability of Laser-Patterned Electrode Compositions

Electrodes can be created with maskless metallization of Cu tracks ablated with laser direct structuring on the surface of a Cu–liquid-crystal polymer composite. The composition of these electrodes may not be stable in a corrosive environment, such as an electrolyte, which is needed to use these structures as conductometric sensors. We have evaluated five different electrode compositions in a low corrosive environment and found electroless palladium–immersion gold to be the most stable. The other electrode compositions have pinholes in the metal layers enabling metal migration and local galvanic corrosion. We also investigate the effect of CO2 snow jet cleaning on the electrodes and its relationship with the electrode composition stability. The CO2 snow jet cleaning leads to smoother electrode surfaces. A smooth electrode surface will have fewer imperfections and therefore less metal migration and galvanic corrosion.

Polyethylene oxide (PEO) – Liquid crystal (E8) composite electrolyte membranes: Microstructural, electrical conductivity and dielectric studies

A series of flexible and free standing composite membranes based on polyethylene oxide (PEO) and the nematic liquid crystal (LC) E8 were prepared using solution casting technique. The effect of the LC concentration on the structural modifications of PEO/E8 LC composites was investigated by Fourier transform IR spectroscopy, X-ray photoelectron spectroscopy (XPS), micro-Raman and differential scanning calorimetry (DSC). The electrochemical impedance measurements were performed in the applied frequency range of 0.1 Hz – 2 MHz. The dielectric properties of the PEO/E8 LC membranes were studied upon variation of the LC concentration and results were analyzed by Kohlrausch – Williams – Watts and Havriliak – Negami formalisms. These investigations pointed out that LC doped polymeric membranes display remarkable features that could be interested for their use as novel composite electrolytes for rechargeable metal ions batteries.

Synthesis, molecular and crystal structure of spirocyclopropyl derivatives of lupane series and their ability to induce cholesteric mesophase in nematic solvents

New spirocyclopropyl derivatives of lupane series were synthesised and their spatial structure was determined by X-ray analysis. These compounds may be used as chiral dopants for cholesteric liquid-crystal compositions which are applied in the surface stabilized cholesteric texture and polymer stabilized cholesteric texture displays with low power consumption. The relationship between stereostructure of the synthesised substances and their ability to induce cholesteric mesophase in different nematic solvents were examined. The highest values of the helical twisting power |β| (213,8 ± 1,1) and (230,5 ± 4,3) mkm-1mol⋅pats-1 in the 4′-pentyl-1,1′-biphenyl-4-carbоnitrile as a nematic solvent were showed by the (1S,2S)-1-(1-phenyl-3-(4-tolyl)-1H-pyrazole-4-yl)-2,2′-spirocyclopropylallobetulone and (1S,2S)-1-(3-(4-methoxyphenyl)-1-phenyl-1H-pyrazole-4-yl)-2,2′-spirocyclopropylallobetulone correspondingly.

Investigation of thermodynamic properties of 4-decyloxybiphenyl-4′-carboxylic acid liquid crystal and preparation of polymer dispersed liquid crystal composite

In this study, firstly, retention behavior of solvents on the liquid crystalline material 4-decyloxybiphenyl-4′-carboxylic acid (DBCA) has been studied by inverse gas chromatography (IGC) technique. Liquid crystal (LC) selectivity was investigated using the acetate isomers by the IGC method at temperatures between 333.2 K and 543.2 K. Later on, the interactions of LC with n-octane, n-nonane, n-decane, undecane, dodecane, tridecane, n-propyl benzene, isopropyl benzene, ethylbenzene, chlorobenzene and toluene between 518.2 K and 543.2 K were examined. LC-solvent interaction parameters for Flory-Huggins theory, χ12∞, and equation of state theory, χ12∗, effective exchange energy parameters, Xeff, the weight fraction activity coefficent, Ω1∞, the partial molar heat of mixing at infinite dilution of the solvent, ∆H1∞, molar heat of vaporization of solvent, ∆HV and the partial molar heat of sorption of the solvent, ∆H1, s were obtained.Secondly, polymer dispersed liquid crystal (PDLC) composite film in composition 20/80 poly(methyl methacrylate) (PMMA):DBCA by weight was synthesized by solvent-induced phase separation method. The mesomorphic behavior of DBCA and PDLC composite has been investigated by differential scanning calorimetry (DSC) and optical polarizing microscopy (PM).

Giant enhancement of photoluminescence and tertiary emission in a chiral nematic by matching photonic band gap and excitation wavelength

We demonstrate that chiral nematic liquid crystal compositions incorporating fluorophores, wherein the photonic band gap (PBG) of the liquid crystal is matched with the excitation wavelength of the fluorophore results in (a) significant enhancement in the overall photoluminescence of the system and (b) eliciting secondary and tertiary emission such that a single low wavelength illumination can give emissions at multiple and highly desirable Red, Green and Blue wavelengths. The light-trapping strategy of matching the PBG with the excitation wavelength instead of the previously attempted protocol of matching with the emission wavelength is shown to be superior in terms of the magnitude of emission and consequently, the quantum yield of the device. Further, we show that by using an applied electric field, three different emission levels can be achieved, two of them stable even when the field is switched off; the magnitude of the field provides a powerful handle to control the emission level with an overall change of a factor of 2.The observed PL features are explained using radiative energy transfer mechanism and the short-range ordering of the fluorophores, as obtained from X-ray diffraction experiments.