Liquid Crystal Transition Temperatures Research Articles

Computation Legendre moments using image analysis technique

Abstract In this paper, phase transition temperatures of nematic liquid crystals: 4-n alkoxy benzoic acid (nOBA, where n = 4 and 6) are determined from the Legendre moments of liquid crystal textures. Here, the image analysis technique in conjunction with polarizing optical microscope is used to compute the Legendre moments from the textures of liquid crystals. As a function of temperature, Liquid crystal textures are recorded from the arthroscopic mode of optical microscope. Changes in textural features of the sample with respect to temperature bring the variations in intensity values of liquid crystal textures. The brightness image that is generated from the liquid crystal textural patterns separates the information about fluctuations in intensity. In this approach, Legendre moments are computed from the brightness images of the liquid crystal textures and are plotted as a function of temperature. Significant variations in the brightness Legendre moment curve provide insight into the liquid crystal transition temperatures. Results obtained from the present methodology are compared with gray color textures data and the experimental technique of differential scanning calorimetry and are in good agreement with each other. This method is an objective and quantitative technique whereas POM method is qualitative and subjective.

Determination of thermodynamic properties and phase transition temperatures of phenylbenzoate-based calamitic liquid crystals by inverse gas chromatography method

ABSTRACT In this study, Phenylbenzoate-based calamitic liquid crystals (LCs) were synthesized and characterized, and then the thermodynamic properties and phase transition temperatures were investigated by inverse gas chromatography (IGC) at infinite dilution. The phase transition temperatures of the LCs were determined in the range of 40–170°C by the IGC method. Finally, in the thermodynamic equilibrium region (the range of 160–200°C), Flory-Huggins theory and the equation of state theory with the LC-solvent interaction parameters, the effective exchange energy parameters, the weight fraction activity coefficient, and also the partial molar heat of mixing at infinite dilution of solvent, the molar heat of vaporization of solvent, and the partial molar heat of sorption of solvent were calculated.

The ferroelectric nematic phase: on the role of lateral alkyloxy chains

ABSTRACT The synthesis and characterisation of the 4-({[4’-nitro-(1,1’-biphenyl)−4-yl]oxy}carbonyl)phenyl 2-alkoxy-4-methoxybenzoates are reported: the 1O(m-On)PEPEBNO2 series. These materials include an additional phenyl ring compared to the extensively studied ferroelectric nematogen RM734, added to increase the liquid crystal transition temperatures. All members of the series exhibited both conventional nematic (N) phase and ferroelectric nematic (NF) phases; TNI and TNFN both decrease on increasing n, the length of the lateral alkoxy chain. The properties of this four-ring series are compared with analogous three-ring laterally substituted variants of RM734; the addition of the extra phenyl ring has a significantly more pronounced effect on the value of TNI than on TNFN. The increase in TNI may be attributed to the enhanced structural anisotropy and more favourable intermolecular interactions arising from the insertion of the phenyl ring, whereas the much weaker effect on TNFN may reflect a change in the shape of the molecule. We also report two materials with three-ring structures, the 4’-nitro-(1,1’-biphenyl)-4-yl 2-alkoxy-4-methoxybenzoates: 1O(m-On)PEBNO2 (n = 1 and 2). The removal of the ester linkage between RM734 and 1O(m-O1)PEPNO2 extinguishes the NF phase and this appears to be consistent with a model in which the molecules are described in terms of a longitudinal surface charge density wave.

Textural, electro-optical, dielectric and fluorescence studies of citrate buffer stabilized gold nanoparticles doped in polymer-dispersed liquid crystals composites

ABSTRACT This work demonstrates the effect of citrate buffer stabilised gold nanoparticles (AuNPs) on the morphological, optical and dielectric properties of polymer-dispersed liquid crystal (PDLC) composites prepared by polymerisation-induced phase separation method. The effect of voltage and temperature on liquid crystal (LC) droplet configurations, transmission, permittivity, conductivity and transition temperatures are studied. The relative transmission and contrast were improved by 22% and 54%, respectively, after the dispersion of AuNPs in PDLC composites as compared to undoped PDLC. The threshold voltage (Vth) of the doped composite was reduced by 13.34% as compared to the undoped composite. The increasing concentration of AuNPs has a minor effect on the (Vth) and transmission as well. Moreover, the low concentration of AuNPs also has better optical responses. The size of LC droplets was found in the range of 35–53 µm and almost independent of voltage and temperature. Beyond threshold voltage, maltese type LC droplet occurred. The photoluminescence spectra reveal that the intensity of AuNPs (1.5 µl) doped sample is higher and a blue shift is seen at 3.0 µl AuNPs-doped sample than others. With increasing AuNPs concentration, the energy bandgap also increases and is reported in the range of 3.4 eV at a higher concentration of AuNPs.

Optical and dielectric analysis of ZnO nanorods doped polymer dispersed liquid crystal and ethanol gas sensing investigation

ABSTRACT The spectra and dielectric properties of polymer-dispersed liquid crystal films doped with different concentrations of ZnO nanorods were studied. The scanning electron microscopy images show that doping ZnO nanorod concentration can increase the size of liquid crystal droplets to microscale. The absorption spectra of the films also show that the edge of the absorption band of the composite system moves towards high energy with the increase of the concentration of ZnO nanorods. On the other hand, the investigation results show that the dielectric constant and loss increase with the increase of doping concentration from 4 Hz-1 MHz. Further analysis of the Cole–Cole diagram fitted by Havriliak-Negami equation shows that there are three relaxation processes. The improvement of dielectric strength and relaxation frequency of nanocomposites was observed at low frequencies of 100 Hz. A threefold reduce in relaxation time for the doped sample (0.5 wt.%) was obtained compared with that of pure sample without doping. In addition, the studies show that the temperature has a great influence on the activation energy of materials, which have a slight increase (~5°C) in transition temperature of liquid crystals. Practically, in ethanol gas sensing experiment, the results show the value of permittivity increases and impedance decreases.

Symmetrical and asymmetrical liquid crystal dimers: synthesis, characterisation and mesomorphic behaviour

ABSTRACTA series of new asymmetrical liquid crystal dimers, RnO–S2O–ORn (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the molecule and bearing different lengths of terminal alkoxy chains were synthesised in order to study the effects of the length of the terminal alkoxy chains on mesomorphic properties. As well as, five symmetrical and asymmetrical dimers linked by butylene spacers bearing different linkages to the core and various terminal chains were also synthesized in order to study the effect of the nature of the spacer and terminal groups. The structures of the synthesised dimers were confirmed by physico-chemical techniques, i.e. FTIR, NMR and mass spectra. Differential scanning calorimetry and polarising optical microscopy verified the liquid crystal behaviour transition temperatures. The isotropic transition temperatures of the dimers RnO–S2O–ORn (n = 2–10) decreased with increasing length of alkoxy chain. Structural effects on the mesomorphic and physical properties were investigated in terms of alteration of carboxylate and thioester groups linking the spacer. The mesomorphic investigation reveals that all the dimers formed an enantiotropic Nematic phase except for dimer HO2SH which is not a liquid crystal.

Prediction of the transition temperature of bent-core liquid crystals using fuzzy “digital thermometer” model based on artificial neural networks

A dataset containing transition temperature values for 243 bent-core liquid crystal (LC) compounds was used to develop quantitative structure–property relationship (QSPR) models using only 2D molecular descriptors and general regression neural network (GRNN). Beside a standard analogue GRNN model, another GRNN model with fuzzy digital response was created with the aim to estimate the prediction error for each compound. Two approaches for the selection of most relevant subset of descriptors, namely the partial mutual information (PMI) and self-organizing maps combined with chi square ranking, were also compared. The best results were obtained using analogue GRNN model based on PMI selected subset (R2=0.91), with the mean absolute error (MAE) lower in comparison with previously published corresponding QSPR models. The digital PMI-GRNN model enabled distinction between high and low accurate predictions, i.e. ones with absolute error higher than mean absolute error (MAE) and others with absolute error ≤ MAE, with the accuracy of 81%.

Influence of flexible chain, polar substitution and hydrogen bonding on phase stability in Shiff based (4)PyBD(4I)BrA-nOBA series of liquid crystals

ABSTRACTShiff based Linear Hydrogen Bonded Liquid Crystalline homologues, viz., (4)PyBD(4I)BrA-nOBAs for n = 2,3,4,5,6,7,8,10,11 and 12 are synthesized. 1H-NMR and 13C-NMR studies confirm the formation of HBLC and IR confirms the linear hydrogen bonding. Liquid crystal phase abundance, transition temperatures, enthalpy and order of transition are investigated by Polarized Optical Microscopy textural studies and Differential Scanning Calorimetry. Nematic, Smectic-A, -D, -C, Bhexatic, -BCrystal, -F, -I and –G phases are identified. Large LC phase variances, viz., NABhexFG and NACFG are exhibited by intermediate chain length with n = 5 and 6. A-C transition exhibited unique II-order nature. II-order nature for G-Solid transition explained. Phase diagram drawn with flexible chain length infers INA, and ACF and CIF MultiCritical Points and Lifshitz behaviour. NA TriCiritical Point predicted in [3+7] binary. Influences of chain length, linear HB and end polar(-Br) substitution for LC phase stability are is discussed. Results of Phase stability are discussed in the wake of the body of the data and reports in other LCs. DSC enthalpy suggests for utility of intermediate and higher homologues for device savvy tilted Smectic LC phases.

Liquid crystal composites doped with inorganic nanoparticles for recording of polarization holographic gratings

The properties of polarization holographic gratings formed in liquid crystal polymer composites doped with inorganic nanoparticles (SiO2, ZnO, Al2O3, and Y2O3) are considered. The effect of these additives on the phase transition temperature of liquid crystals of polarization gratings, diffraction efficiency, and driving voltage is demonstrated.

Mesogenic Polyelectrolyte Gels Absorb Organic Solvents and Liquid Crystalline Molecules.

In this paper, mesogenic polyelectrolyte gels (MPEgels) tethering mesogenic groups on the side chains were synthesized from a mesogenic monomer and ionic monomer via a conventional radical polymerization process. The obtained MPEgels absorbed various organic solvents in a wide range of dielectric constants from chloroform (ε = 7.6) to DMSO (ε = 46.5). The electrostatic repulsion among the polymer chains and the osmotic pressure between the interior and exterior of the MPEgel is responsible for the high swelling ability, revealed by the common ion effect using tetra(n-hexyl)ammonium tetra(3,5-bis(trifluoromethyl)phenylborate (THATFPB). The obtained MPEgels could also absorb liquid crystalline molecules such as 4-cyano-4’-pentylbiphenyl (5CB), analogously caused by the above-mentioned polyelectrolyte characteristic. The MPEgels exhibited liquid crystal transition temperature (TNI) on differential scanning calorimetry (DSC) measurement, and the increase of the ionic group content lowered TNI. The MPEgels absorbing liquid crystalline molecules exhibited differing TNI, dependent on the compatibility of the mesogenic group on the side chain to the liquid crystalline molecule.

A GMDH-type neural network with multi-filter feature selection for the prediction of transition temperatures of bent-core liquid crystals

The QSPR study on transition temperatures of five-ring bent-core LCs was performed using GMDH-type neural networks. A novel multi-filter approach, which combines chi square ranking, v-WSH and GMDH algorithm was used for the selection of descriptors.

The Effect of the Liquid-Crystalline Order on Releasing Dye Molecules from Liquid-Crystalline Nano-Emulsions

We fabricated dye-containing liquid-crystalline nano-emulsions (LCNEs) dispersed in water, and studied the controllability of releasingdye molecules from the coreof LCNEs into water. We estimated the concentration of the dye molecules held in the core of LCNEs from transmission spectrum. As a result, it was revealed that the dye molecules are released from the core of LCNEs below the characteristic release-temperature on cooling. Moreover, the depolarized static light scattering measurement was performed to estimate the Isotropic-Nematic(I-N) phase transition temperature of low molecular weight liquid crystals in the core of LCNEs. By controlling mixing ratio of binary liquid crystalline mixture, we tuned theI-N phase transition temperature. The release-temperature completely corresponds to the I-N phase transition temperature in the coreof LCNEswithin the whole mixing ratio. Thus, we confirmed that the emergence of the liquid-crystalline order in the core of LCNEs plays a principal role in releasing the dye molecules from LCNEs.

Synthesis and Liquid Crystal Behavior of a Novel Ester Homologous Series

A novel ester homologous series of 4-[4’-n-alkoxy cinnamoyloxy] benzyl benzoates has been synthesized. The series consists of 12 homologues. Liquid crystal properties commence from the sixth member to the last member of the series with the exhibition of an enantiotropic nematic phase without the exhibition of any smectogenic mesophase. The remaining homologues do not exhibit liquid crystal behavior. The texture of the nematic phase is of the threaded or Schlieren type. The solid-nematic or isotropic transition curve adopts a zigzag path and the nematic-isotropic transition curve steeply rises and then falls in the phase diagram and behaves in normal manner. An odd–even effect is absent in the nematic-isotropic transition curve with an alteration of transition temperatures. The average thermal stability for the nematic mesophase is 153.1°C, and the nematogenic temperature ranges vary from 12°C to 50°C. Analytical data confirm the molecular structures of the homologues. The Liquid Crystal properties and transition temperatures were observed through an optical polarizing microscope, equipped with a heating stage. The mesomorphic characteristics of the novel ester series are compared with other, known and structurally similar series. The novel series is predominantly nematogenic with the absence of any smectogenic character, with relatively short mesophase temperature ranges and of the middle ordered melting type.

QSPR study on nematic transition temperatures of thermotropic liquid crystals based on DFT-calculated descriptors

The individual correlations between nematic transition temperatures and the molecular descriptors derived from density functional theory (DFT) method are investigated. Significant quantitative structure–property relationships (QSPRs) for TN of a set of 42 theromtropic liquid crystal were established through principle component analysis (PCA) and multiple linear regression (MLR). A promising six-descriptor linear model with good statistical fitting and predicting capability were developed with the help of DFT. The model is based on EHOMO, ELUMO, μ, S, W and X. The reliability of this model is clear from its correlation coefficient (R2 = 0.95) and cross-validation coefficient (R2CV = 0.93).

Aromatic thermotropic polyesters based on 2,5-furandicarboxylic acid and vanillic acid

This paper addresses a route to synthesize bio-based polymers with an aromatic backbone having a liquid crystalline (LC) phase in the molten state. The LC phase is employed to achieve uniaxial orientation during processing required in e.g. fiber spinning. For this purpose 2,5-furandicarboxylic acid (2,5-FDCA) and O-acetylvanillic acid (AVA), obtained from natural resources, are used as monomers. Similar to the 2,6-hydroxynapthoic acid used to perturb the crystalline packing of poly(oxybenzoate) in the Vectran® series, these bio-based monomers are used to lower the crystal to liquid crystal transition temperature. Considering that the poly(oxybenzoate) can also be obtained from natural resources, the adopted route provides the unique possibility to synthesize bio-based polymers that can be used for high performance applications. To obtain the desired polymers, a synthetic route is developed to overcome the thermal instability of the 2,5-FDCA monomer. Experimental techniques, such as optical microscopy, FTIR spectroscopy, DSC, and TGA are employed to follow the polymerization, phase transitions and evaluate thermal stability of the synthesized polymers.

Synthesis and Mesomorphic Properties of Novel Ethylene Derivatives

A novel homologous series, α-4-[-4′-n-alkoxy cinnamoyloxy] benzoyl-β-4″-methoxy phenyl ethylenes, has been synthesized, which consists of 12 homologues. The pentyl to tetradecyl homologues are enantiotropically mesomorphic, and the rest of the homologues are nonmesomorphic. The pentyl to octyl derivatives exhibit smectogenic character in addition to nematogenic character, but the decyl to tetradecyl homologues exhibit only nematogenic character without the exhibition of a smectic phase. A phase diagram of three transition curves shows a normal phase behavior. Analytical and spectral data confirm the molecular structure of the homologues. The optical microscopic textures are threaded or Schlieren for the nematic phase and the normal smectic A type, respectively. Liquid crystal properties and transition temperatures were determined using a hot stage polarizing microscope. The smectic and nematic thermal stabilities are 159.7°C and 174.1°C, respectively. Thus, the present novel series is predominantly nematogenic and partly smectogenic in nature and is compared with other structurally similar homologous series.

Image Analysis Studies for Phase Transitions of Ferroelectric Liquid Crystals

Phase transition temperatures of ferroelectric liquid crystals ((S)-(-)-2-methylbutyl 4′-(4″-n-alkanoyloxybenzoyloxy) biphenyl-4-carboxylates (where n=16 and 18)) are studied through two techniques of image analysis. One is a statistical method, applied to compute the statistical parameters from the textures of each sample and the other, computation of Legendre moments being applied as image moment analysis, both of which are considered as a function of temperature. The textures of the samples are recorded with the polarizing optical microscope (POM) attached to the hot stage and high resolution camera. The phase transition temperatures of samples are inferred by the abrupt changes in the computed parameter values. The results obtained from the present methodology are in good agreement with those published in earlier literature done by the different techniques, like differential scanning calorimetry (DSC).

Effect of nematic liquid crystals on optical properties of solvent induced phase separated PDLC composite films

In the present article the effect of two different types of nematic liquid crystals on electro-optical and thermo-optical properties of polymer dispersed liquid crystal films are discussed. Composite films composed of polymer poly(methyl methacrylate) and two different nematic liquid crystals E7 and E8 were prepared by solvent induced phase separation technique. Scanning electron microscopy and polarizing optical microscopic results showed that liquid crystal phase is continuously embedded in a sponge-like polymer matrix. The domains of E7 were found slightly larger and more elongated than E8 under the same conditions of preparation. Variation of optical transmission properties of the composite films under the conditions of an externally applied alternating electric field (0-300 Vp-p, 50-1000 Hz) over a wide temperature range was studied using He-Ne laser (wavelength 632.8 nm) as a light source. The experimental results showed that the E7-polymer film has better electro-optical as well as thermo-optical properties as compared to the E8-polymer system. The performances of both the composites improved significantly with increasing temperature. It was found that the physical properties such as viscosity, refractive index and transition temperature of liquid crystals play a major role on the properties and behavior of the polymer dispersed liquid crystal composite films. Keywords: Optical materials, Electro-optical, Liquid crystal, PDLC, Hysteresis.

Morphological studies of blends of conjugated polymers and acceptor molecules using langevin dynamics simulations

AbstractWe use coarse‐grained Langevin dynamics simulations of blends of generic conjugated polymers and acceptor molecules to show how architecture (e.g., side chains, backbone flexibility of oligomers) and the pair‐wise interactions between the constituents of the blend affect morphology and phase transition. Alkyl side chains on the conjugated oligomer backbones shift the liquid crystal (LC) transition temperature from that of bare conjugated backbones and the direction of the shift depends on backbone–backbone interactions. Rigid backbones and constrained side chains cause a layer‐by‐layer morphology of conjugated polymers and amorphous acceptors, whereas flexible backbones and unconstrained side chains facilitate highly ordered acceptor arrangement. Strong backbone–backbone attraction shifts LC transition to higher temperatures than weak backbone–backbone attraction, and strong acceptor–acceptor attraction increases acceptor aggregation. Pure macro‐phase separated domains form when all pair‐wise interactions in the blend are strongly attractive, whereas interconnected domains form at intermediate acceptor–acceptor attraction and strong polymer–polymer attractions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Novel approach to study liquid crystal phase transitions using Legendre moments

A novel approach is proposed to investigate the phase transitions of cholesteric liquid crystals using the Legendre moments. The textures of cholesteric liquid crystals (cholesteryl butyrate, cholesteryl n-valerate, cholesteryl decanoate, and cholesteryl myristate) are captured as a function of temperature using high-resolution camera attached to the arthroscopic mode of polarizing optical microscope with hot stage. A recurrence formula is used to compute the Legendre moments of the liquid crystal textures based on the Legendre polynomial using MATLAB software. The abrupt change in the values of Legendre moments as a function of temperature gives the phase transitions of liquid crystals. The investigated transition temperatures of cholesteric liquid crystals are compared with other techniques.