Investigation of dielectric and electro-optical properties of nematic liquid crystal with the suspension of biowaste-based porous carbon nanoparticles

ABSTRACT

The effect of biowaste porous carbon nanoparticles (PCNPs) on the dielectric and electro-optical properties of nematic liquid crystal (LC) mixture (1823A) of 4-(4-alkyl-cyclohexyl) benzene isothiocyanates and 4-(4-alkyl-cyclohexyl) biphenyl isothiocyanates has been studied. The dielectric permittivity of nematic LC has been increased with the dispersion of carbon NPs. The dielectric anisotropy has been calculated and found to be decreased with the dispersion of PCNPs into the pure nematic LC. The response time and birefringence have been also observed with the variation of temperature, frequency as well as the concentrations of carbon NPs. After the dispersion of PCNPs achieved better birefringence and faster response in the dispersed system, which is the significant application in display devices. Threshold voltage splay elastic coefficient and rotational viscosity have been calculated for both pure and NPs dispersed nematic system. Its value is increased with the dispersion of NPs. Additionally, photoluminescence and figure of merit have investigated as a comparative study of nematic matrix as well the dispersed system. The experimental results have been found to have good agreement with the theoretical data of nematic LC. An effort has been made to explain these experimental results on the basis of interaction between nematic molecules and carbon NPs.     

Dispersion and orientation of single-walled carbon nanotubes in a chromonic liquid crystal

A post-synthesis alignment of individual single-walled carbon nanotubes (SWCNTs) is desirable for translating their unique anisotropic properties to a macroscopic scale. Here, we demonstrate excellent dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal. The methods to obtain stable suspension are described, and order parameters of the liquid crystal matrix and of the nanotubes are measured independently.     

Orientational control of liquid crystal molecules via carbon nanotubes and dichroic dye in polymer dispersed liquid crystal

Polymer dispersed liquid crystals (PDLCs) using nematic liquid crystal and photo-curable polymer (NOA 65) were prepared by polymerisation-induced phase separation technique, in equal ratio (1:1) of polymer and liquid crystal (LC). We demonstrate that doping of small amount (0.125%, wt./wt.) of multiwall carbon nanotubes (CNTs) and orange azo dichroic dye in PDLC generously controlled the molecular orientation, dynamics of LC in droplet and size of droplets. The effects of multiwall CNTs and dye on PDLCs were studied in terms of transition temperature, droplet morphology, transmittance characteristic, contrast ratio and response time. The results exhibited that the values of the threshold electric fields were reduced from 8 V/µm (pure PDLC) to 1.18 and 1.72 V/µm, doped with multiwall CNTs and dye, respectively. The CNTs-doped PDLC shows faster switching response as compared with pure PDLC and dye-doped PDLC. However, dye-doped PDLC shows much higher contrast among all PDLC samples. Further, the results also illustrate that the birefringence value of LC in PDLCs was changed with doping of CNTs and dye.     

The physical properties of alkene-terminated liquid crystal molecules/E8 mixture and the electro-optical properties as they doped in polymer-dispersed liquid crystal systems

Effects of the content of fluorinated alkene-terminated liquid crystal (LC) molecules on the physical properties of the fluorinated alkene-terminated LC/E8 mixture were studied. The morphology and electro-optical properties as they doped in polymer-dispersed liquid crystal (PDLC) films were investigated. The detailed discussion of the obtained results is given. As a result, comparing with the physical properties of the series of LC mixtures with the same content of the analogous fully saturated compounds doped with E8, we find that the birefringence is significantly larger for the LC mixture with the alkene-terminated materials. Both fluorinated alkene-terminated LC molecules and the analogous fully saturated compounds doped with E8 reduce the driving voltage of PDLC films. Moreover, PDLC films with the fluorinated alkene-terminated LC molecules possessed higher contrast ratio and faster response time than that of the PDLC films prepared by adding the same mass fraction of the analogous fully saturated compounds. Thus, the ability to manipulate physical properties of LC mixture and electro-optical properties of PDLC films by changing the LC molecular structures may have future relevance for new LC structures design and applications of PDLC films.     

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.     

Effects of multiwalled carbon nanotube on holographic polymer dispersed liquid crystal

The morphology and electro‐optical performance of holographic polymer dispersed liquid crystal (HPDLC) were investigated with the addition of multiwalled carbon nanotube (CNT), both pristine and chemically modified one (CNT‐C?C). With the addition of CNT, the diffraction efficiency increased and showed a maximum at 0.6% while nucleation was delayed due to the increased mixture viscosity. Film was driven only with CNT due to the induced local electric field of polymer to overcome the threshold resistance. Among the two types of CNT, chemically modified one gave finer CNT dispersion, lower mixture viscosity, larger liquid crystal (LC) droplet, higher diffraction efficiency, and shorter response time while the pristine CNT decreased the driving voltage. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and properties of reactive liquid crystal monomers and side-chain liquid crystalline polymers

Two nematic liquid crystal (LC) monomers containing double bonds in the side chain were designed and synthesised. Length of the side groups varied from 1 to 2 methylene units. The side-chain polymers were synthesised by hydrosilylation reaction. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared, elemental analysis and nuclear magnetic resonance spectroscopy. The thermal phase behaviour of the monomers and polymers were investigated by differential scanning calorimetry and polar optical microscopy coupled with hot stage. The LC monomers showed only one nematic mesophase in the cooling process. And, the two polymers exhibit an enantiotroppic nematic mesophase either in the heating or in the cooling process.     

Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (C ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density D was shown to be essentially non-linear and a minimum at certain concentration of CNTs, C ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced trans–cis–trans isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.     

Enhancement of electrical conductivity,director relaxation frequency and slope of electro-optical characteristics in the composites of single-walled carbon nanotubes and a strongly polar nematic liquid crystal

In the present work, single-walled carbon nanotubes (SWCNTs) were dispersed in a room temperature nematic liquid crystal 4-pentyl-4′-cyanobiphenyl at the concentration of 0.02 and 0.05 wt%. Differential scanning calorimetry and temperature-dependent dielectric studies suggest decrease in clearing temperature of the composite materials as compared to the pure material. Ionic conductivity increases by two orders of magnitude due to the dispersion of such a low concentration (0.05 wt%) of SWCNTs. Dielectric studies also show that the presence of the SWCNTs decreases the effective longitudinal as well as transverse components of the dielectric permittivity. For homeotropic aligned samples, a relaxation mechanism has been detected in the lower MHz region both for the pure as well as dispersed samples. Presence of SWCNTs increases the relaxation frequency corresponding to flip-flop motion of molecules around their short axes. From frequency-dependent dielectric studies, important dielectric parameters such as relaxation frequency, dielectric strength and distribution parameters have been determined. Electro-optical experiments show that the threshold voltage decreases and the steepness of the transmission voltage curve improves due to the dispersion of SWCNTs.     

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.     

Effects of the fluorinated liquid crystal molecules on the electro-optical properties of polymer dispersed liquid crystal films

The different fluorinated liquid crystal (LC) molecules doped to E8 were used as LC component to prepare polymer dispersed liquid crystal (PDLC) films. The mass fraction of the LC mixture is fixed 50.0 wt%. Results indicate that doping 8.0 wt% fluorinated LC molecule ME3CP to E8 significantly reduced the driving voltage of the PDLC films, and the driving voltage reduced with the rise of mass fraction of ME3CP. Besides, the terminal flexible chain length of the fluorinated LC molecule influenced the LC mixture properties based on E8, such as the dielectric anisotropy, birefringence and viscosity of the LC mixture, and the morphology and the electro-optical properties of PDLC films were controlled not only by the physical properties of the LC mixture, but also by the terminal flexible chain length of the fluorinated LC molecule .     

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.     

Mesomorphic phase transitions and electro-optical properties of benzyl fluoroalkyl thioether liquid crystalline compounds

Four series of liquid crystalline (LC) compounds containing benzyl perfluoroalkyl thioether groups (BFT), 4-n-alkoxyphenylbenzyl perfluoroalkyl thioethers (nO-BBFT-FmF), 4-n-alkoxyphenylbenzyl perfluoroisoalkyl thioethers (nO-BBFT-FmIF), 4-n-propylcyclohexylbenzyl perfluoroalkyl thioethers (3-C1BFT-FmF) and 4-ethylcyclohexyl-3'-fluorobenzyl perfluoroalkyl thioethers (2-C1FBFT-FmF), were synthesized and their mesomorphic phase transitions and electro-optical properties investigated. nO-BBFT-FmF and nO-BBFT-FmIF have SmA, SmB and CrE phases, while 3-C1BFT-FmF and 2-C1FBFT-FmF showed a SmB phase. It was found that the appearance of a LC phase and the thermal stability were closely connected with the chemical structures of the rigid core part and the terminal groups. The electrooptical properties were investigated for BFT using ZLI-1132 as nematic solvent. Some of the BFT compounds have a good solubility of 15 wt % in ZLI-1132 and can be used as a component for lowering the refractive anisotropy (Delta n) of the host ZLI-1132 solvent.     

Intra-cell ionic properties of twisted nematic liquid crystal cells

Under 1-kHz and low-frequency driving, we report our investigation of intra-cell ionic properties of two twisted nematic (TN) liquid crystal (LC) cells made of same LC mixture but different polyimide (PI) materials for LC alignments. A heterodyne interferometry system was used to measure the phase retardations of the TN cells versus applied voltages at 1 kHz. We also measured the phase retardations and currents of the TN cells versus time by applying a mid-grayscale voltage of 1.3 V at 0.1 Hz to the cells. Related to the above-measured data, we have developed equations to characterise the field-driven transports of mobile charge carriers within the PI films independently from that within the LC medium of same TN cell to obtain time-dependent in-cell voltages of mobile charge carries across the LC mixture and across two PI-alignment films, respectively. Our experimental methods can be used to optimise LC mixtures and PI materials for low-refresh-rate thin-film-transistor-driven liquid crystal displays (LCDs) for low power and provide bases for further investigations on mobile-charge-carrier generation and transport within thin in-cell PI-alignment films as well as within the LC mixture of same LC cell.     

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.     

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).     

Enhanced dielectric and electro-optical properties of a newly synthesised ferroelectric liquid crystal material by doping gold nanoparticle-decorated multiwalled carbon nanotubes

In this article, a newly synthesised ferroelectric liquid crystal (FLC) material, namely LAHS 22, has been characterised. The characterisation of the FLC material has been performed using dielectric relaxation spectroscopy, differential scanning calorimetry and polarisation optical microscopy. We observed an enhancement in the dielectric and electro-optical properties of the FLC material by incorporating gold nanoparticles (GNPs)-decorated multiwalled carbon nanotubes (MWCNTs). The GNPs-decorated MWCNTs cause an increment in dielectric dispersion (up to kHz), absorption, spontaneous polarisation and rotational viscosity of the FLC material. The pure and GNPs-decorated MWCNTs doped FLC cells were analysed by means of various dielectric spectroscopic and optical measurements. The observed enhancement in the dielectric and electro-optical properties of the FLC material has also been studied with concentration of GNPs-decorated MWCNTs in FLC material. The GNPs-decorated MWCNTs/FLC composites are not only of fundamental importance, but also useful materials for device applications such as liquid crystal displays and memory devices.     

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4?-[1,ω-alkanediylbis(thio)]bis-[1,1?-biphenyl]-4-carbonitriles (CBSnSCB), and 4?-({ω-[(4?-cyano[1,1?-biphenyl]-4-yl)oxy]alkyl}thio)[1,1?-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 3⁴-{6-[(4?-cyano[1,1?-biphenyl]-4-yl)thio]-hexyl}[1¹,2¹:2⁴,3¹-terphenyl]-1⁴-carbonitrile (CT6SCB), is also reported and shows enantiotropic N_TB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-N_TB transition corresponds to 4 longitudinal molecular distances.     

Effect of terminal aryl rings of liquid crystal trimers on liquid crystalline properties

Liquid crystalline properties of five series of biphenyl derivatives connected with p-nitrophenyl, phenyl, 2-naphthyl, 9-phenathryl and bromo as the terminal groups via flexible spacers were studied. All the compounds exhibited nematic phase. The nematic–isotropic or isotropic–nematic transition properties resulted in an odd–even effect as the length of the spacers was varied, in which the even members exhibited higher values, for the compounds with p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, while the transitional properties of the compounds substituted with bromo atom as the terminal group instead of the aryl groups did not show such odd–even effect. These results suggest that the aryl rings, such as p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, play a role of mesogen while the terminal bromo atom does not. The effect of the terminal aryl rings on the liquid crystalline properties will be reported.