Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACT

The structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.     

An electrically controlled light-scattering device based on liquid crystal/polymer microsphere composites

ABSTRACT

There is a widely applied prospect of electrical controlled liquid crystal (LC) light-scattering device. Numerous electrical controlled LC light-scattering technologies have been studied, but each technology has its own shortcomings, such as high driving voltage, high hysteresis, complex electrode structure, and serious heating. In this work, the composite of LC and polymer microspheres are used to fabricate light-scattering devices. This device is operated by the vertical electric field and does not require complex preparation process. LC/polymer microsphere composite has the advantages of low driving voltage and zero hysteresis. The role of microspheres in the composites is to change the size and density of a refractive-index-mismatched micro-domain. The effects of the ratio, particle size, and refractive index of microspheres on the optical characteristics of a composite are studied. The normal directional light transmittance at the transparent state and light-scattering state decreases with an increasing weight ratio of microspheres. The particle size of microspheres has negligible influence on the electro-optical properties of composites when the weight ratio of microspheres is small. The LCs doped with Polymethylsilsesquioxane (PMSQ) microspheres or polymethyl methacrylate (PMMA) microspheres are compared, and the mismatched refractive index and density of micro-domain show the influence on the electro-optical properties of the composites.     

Dielectric and electro-optical properties of a nematic liquid crystalline material with gold nanoparticles

ABSTRACT

We have prepared the composites of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and gold nanoparticles (GNPs). Thermodynamic, electro-optical and dielectric properties have been investigated. Effect of dispersion of GNPs on various electro-optical and display parameters of host liquid crystalline material have been studied. Physical parameters such as threshold voltage, dielectric anisotropy and splay elastic constant have altered for composite systems. Due to the dispersion of GNPs, nematic to isotropic transition temperature is significantly increased. Relaxation frequency corresponding to flip-flop motion of the 6CHBT molecules about their short axes has increased due to the presence of GNPs.     

Dielectric properties of two high birefringence liquid crystal mixtures in the Sub-THz band

ABSTRACT

In order to produce liquid crystal (LC) with high birefringence (Δn) in the THz band, eight LC monomers were designed and synthesized. We formulate two mixtures (LC S200-2 and E7-2) by adding eight LC monomers to two commercial LCs (S200 and E7), respectively. The dielectric constants of the mixture LCs were measured using the frequency selective surface (FSS) in the region from 90 to 140 GHz. We compare the simulated results with the experimental measurements and determine the dielectric constants of the LCs. The dielectric anisotropy (Δε) of the nematic S200-2 and E7-2 LC mixtures are 16.4% and 16.3% higher than that of the commercial S200 and E7 LCs, respectively. The results show the frequency tunability of the two LC mixtures is greater than that of the commercial LCs.     

Effects of silver nanoparticle doping on the electro-optical properties of polymer stabilized liquid crystal devices

ABSTRACT

Polymer stabilised liquid crystal (PSLC) devices are widely used in various smart light modulation occasions. Their electro-optical properties can still be improved to address future challenges. It is well known that doping liquid crystal (LC) materials with nanoparticles can change the material’s electro-optical performance. In this study, silver nanoparticles (AgNPs) with size about 15–20 nm and surfactant were doped into PSLC devices. The effects of AgNPs doping on the PSLC electro-optical performances were studied. The result shows that AgNPs and surfactant doping can increase the dielectric anisotropy of LC mixture. PSLC devices with AgNPs doping have lower driving voltage and response time than un-doped PSLC devices. Therefore, doping of a specific concentration of AgNPs resulted in PSLC devices with improved electro-optical performance.     

Dielectric and electro-optical properties of Mn12-acetate and ferroelectric liquid crystal composite

The impact of single-molecule magnet (SMM), Mn₁₂-acetate, on the dielectric properties of ferroelectric liquid crystal (FLC) has been investigated by dielectric spectroscopy and electro-optical techniques in a dual electrode sample cell (DESC). The temperature-dependent dielectric studies on Mn₁₂-acetate/FLC composite have revealed the enhancement in the ferroelectric (smectic C*) to paraelectric (smectic A*) phase transition temperature by 3.5°C. The relaxation process corresponding to Goldstone mode in the ferroelectric phase of the composite is found to be slower compared to pure FLC sample. The electrical response for an input triangular wave shows the existence of one extra polarisation peak in Mn₁₂-acetate/FLC composite which is ascribed to the induced dipole moment in Mn₁₂-acetate molecule. The electro-optical texture of Mn₁₂-acetate/FLC composite revealed that the incorporation of SMM in FLC significantly improve the memory effect.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

New hydrogen-bonded comblike polyimides with mesogenic side chains for stable orientation and fast switching of liquid crystals

ABSTRACT

New supramolecular comblike polyimides with mesogenic side chains for stable homeotropic orientation and fast electro-optical switching of liquid crystals (LCs) have been prepared through selective intermolecular hydrogen bonding between 4-(4-heptylphenyl)benzoic acid (4HPB) and host polyimide. A low concentration of 4HPB as the mesogenic guest molecule was hydrogen-bonded to polyimide backbone leading to the self-assembled comblike polyimide with enhanced homeotropic orientation properties. The electro-optical characteristics of the LC device containing hydrogen-bonded comblike polyimide exhibited better performance than those of LC cell with conventional polyimide. Because the conventional covalent approach for preparation of polyimides requires considerable synthetic efforts to achieve new functionality in polyimide materials, the proposed noncovalent method is a simple one and highly cost effective. Our controlled methodology should find wide application for the fabrication of functional alignment materials requiring high orientation ability.     

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.     

The effect of polymerizable surfactants on morphology and electro-optical of PDLCs films based on epoxy acrylate system

Polymer dispersed liquid crystals (PDLCs) have been extensively studied for various excellent electro-optical applications. The anchoring interaction of liquid crystals (LCs) molecules on the surface of the polymer cavity surrounding an LCs droplet has a crucial effect on the electro-optical performance of the PDLCs. The effect of polymerizable surfactants on the electro-optical properties of PDLCs films was studied in detail. The active double bonds were polymerized with prepolymer to stabilize the performance of polymer matrix. The experimental results showed that polymerizable surfactants could effectively reduce the driving voltage. The speed of polymerization was monitored by real-time transmittance. The electro-optical properties of PDLC films were measured by Polarimeter (PerkinElmer Model 341). The driving electric field was reduced from 3.9 V/μm to about 2.8 V/μm for doping undec-10-enoic acid at curing temperature 80?°C. The surfactants containing polymerizable functional groups, polarity, and alkyl chain weakened the surface anchoring between LCs droplets and polymer interface. The morphologies of PDLCs films were also investigated by polarizing optical microscopy (POM) and Fourier transform infrared (FTIR) images. The LC droplets were encapsulated by polymerizable surfactant according to FTIR images.     

Synthesis and properties of allyloxy-based tolane liquid crystals with high negative dielectric anisotropy

A series of tolane liquid crystals (LCs) containing 2,3-difluorophenyl and allyloxy terminal groups have been synthesised via multi-step reactions. Their thermotropic mesophases, birefringences and dielectric anisotropy properties are discussed by comparison with the non-fluorinated analog. The results show that the allyloxy-based tolane LC 3TOV reveals a high value of the birefringence (0.29), negative dielectric anisotropy (?4.44) and a broad nematic mesophase with a low melting point and high clearing point. The effects of the terminal alkyl chains, the lateral fluoro substituents, the allyloxy terminal group and alkyne bridge on the mesomorphic and physical properties were also discussed. Meanwhile, Density Functional Theory (DFT) calculations of molecular conformation and polarisability were used to correlate the experimental findings.     

Unusual electro-optical behaviour of the nematic polyacrylate

ABSTRACT

High sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties.

The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated.     

Impact of silica nanoparticles dispersion on the dielectric and electro-optical properties and absorption spectra of host ferroelectric liquid crystal

The present work concerns with the investigation of the effect of dispersion of Silica (SiO₂) nanoparticles (NPs) in host ferroelectric liquid crystal (FLC) KCFLC10S on the dielectric and electro-optical properties and ultraviolet-visible (UV-VIS) absorption spectra of the pristine and dispersed systems. We have found that the dispersion of SiO₂ NPs in the host FLC strongly influences the various properties of dispersed systems. No evidence of aggregates and clumps in the dispersed system has been observed. Due to SiO₂ NPs dispersion, a rapid decrease in dielectric permittivity ε’, increase in conductivity σ with frequency, increase in spontaneous polarisation P_s and decrease in switching time with bias voltage have been observed. Based on the absorption spectra, we have also made an attempt to link the electro-optical and dielectric response with the mechanism of FLC–NPs interactions.     

Enhanced electro-optical properties of a nematic liquid crystals in presence of BaTiO3 nanoparticles

Composites of nematic liquid crystals (NLCs) and ferroelectric barium titanate (BaTiO₃) nanoparticles (NPs) have been prepared. The alignments of NPs in the host medium have been demonstrated. Effect of NPs doping on various display parameters of NLCs, namely, threshold voltage, dielectric anisotropy and splay elastic constant has been studied using electro-optical and dielectric studies. The nematic ordering of host supports alignment of NPs parallel to the director which consequently improves electro-optical parameters in the composite system. The dielectric and electro-optic properties of LC–NPs composites have been discussed in frame of conventional theories of NLCs.     

Polar smectic phases in quasi-bent-core liquid crystals with hockey-stick-shaped mesogen: synthesis and crossover mesomorphism

ABSTRACT

New hockey-stick-shaped liquid crystals (HSLCs) have been designed and synthesised. By extreme shortening of one ‘arm’ in the HSLCs, we have constructed the pronounced overall hockey-stick-shaped molecular conformation of these materials, and called the pseudo-rod-like (PRL) molecules with hockey-stick-shaped mesogen. The chemical structures were identified using spectroscopy and elemental analysis. The mesomorphic behaviour was investigated by polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), and via electro-optical experiments. As a result, in spite of the PRL molecules, the various mesomorphic polymorphism, the interdigitated bi-layered structure and macroscopic polarity were found. Similar to up-to-date HSLCs, the quasi-bent-core LCs showed a transition from the synclinic SmC phase (SmC_s) to anticlinic SmC (SmC_a) phase, and exhibited SmA-type (N_CybA) and SmC-type (N_CybC) cybotactic nematic phases. At the same time, similar to practical rod-like LCs, the PRL molecules showed good surface alignment behaviour under planar boundary conditions.     

Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACT

This article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article.     

Dielectric and electro-optical properties of a photosensitive liquid crystal

The temperature dependencies of the dielectric and electro-optical properties of a pure photo-ferroelectric liquid crystal have been investigated, in the chiral smectic C (SmC*) phase, with and without ultraviolet (UV) illumination. The SmC* dielectric Goldstone mode characteristics, the spontaneous polarization, the tilt angle and the rotational viscosity are affected by UV irradiation. Under increasing UV light intensities the dielectric strength Δ?_G increases whereas the relaxation frequency f_G , the spontaneous polarization P _s, the electro-optical rotational viscosity γ_eo and the tilt angle θ decrease. The twist elastic constant K ₃ and the Goldstone mode rotational viscosity γ_G of the studied compound are deduced from dielectric experimental results. Good agreement was found between both viscosities. The observed dielectric behaviours are controlled by the elastic constant and the rotational viscosity variations.     

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACT

In this paper, SiO₂ nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO₂ nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO₂ nanoparticles on the electro-optical properties of PDLC ?lms are studied. It is established that SiO₂ nanoparticles affect the microstructure of PDLC ?lms signi?cantly because of the formed agglomerates of SiO₂ nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO₂ nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO₂ nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.     

A study on the polymer structures and electro-optical properties of epoxy-mercaptan-based polymer dispersed liquid crystal films

ABSTRACT

In this paper, polymer dispersed liquid crystal (PDLC) films based on epoxy-mercaptan system were prepared by thermal-initiated polymerization. The effects of the liquid crystal (LC) content, the proportion and the functionality of epoxy monomers on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It was found that the morphologies of the polymer matrix can be altered from polymer meshes to polymer balls by increasing the LC content as well as the functionality of epoxy monomers. Accordingly, the electro-optical properties could be regulated by the morphologies of polymer networks. Especially, the as-made PDLC films with homogeneous porous structures exhibited the optimal electro-optical properties. Consequently, this work offers a meaningful approach to control the microstructures and optimize the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.     

Polymer encapsulated and stabilised blue-phase liquid crystal droplets

Polymer-encapsulated–polymer-stabilised blue-phase liquid crystals (LCs) are investigated. Encapsulated droplets are formed in a polyvinyl alcohol solution by emulsification, and blue-phase (BP) LCs in the droplets are stabilised via the polymerisation of reactive monomers to extend the BP temperature range. Polymer stabilised droplets are found to cause the expansion of the BP temperature range from 53°C to below 0°C. The effects of composition on droplet formation and the electro-optical behaviour and morphological properties of these droplets are reported.