Nematic-isotropic Phase Transition Temperature Research Articles

Thermo- and electro-optical properties of photonic liquid crystal fibers doped with gold nanoparticles.

Thermo- and electro-optical properties of a photonic liquid crystal fiber (PLCF) enhanced by the use of dopants have been investigated. A 6CHBT nematic liquid crystal was doped with four different concentrations of gold nanoparticles (NPs), 0.1, 0.3, 0.5 and 1.0 wt %, for direct comparison of the influence of the dopant on the properties of the PLCF. The thermo-optical effects of the liquid crystal doped with gold NPs were compared in three setups, an LC cell, a microcapillary and within the PLCF, to determine if the observed responses to external factors are caused by the properties of the infiltration material or due to the setup configuration. The results obtained indicated that with increasing NP doping a significant reduction of the rise time under an external electric field occurs with a simultaneous decrease in the nematic–isotropic phase transition temperature, thus improving the thermo- and electro-optical properties of the PLCF.

Nanoparticles-enhanced photonic liquid crystal fibers

Optical properties of photonic crystal fibers (PCFs) as well as micro-capillaries infiltrated with a 6CHBT liquid crystal (LC) doped with spherical silver (Ag) nanoparticles (NPs) have been studied. Three different concentrations of Ag NPs dopants were taken into consideration and their presence in the 6CHBT LC was found to modify the spectral properties of the infiltrated PCF. A significant improvement of switching times was observed for the NPs-doped LC-infiltrated PCF. Moreover, the Ag NPs-doped 6CHBT LC inside both micro-capillaries and PCFs was found to reduce the nematic-isotropic phase transition temperature.

Polarization singularities in the self-focusing of an elliptically polarized laser beam in an isotropic phase of nematic liquid crystal close to the temperature of phase transition

ABSTRACTNumerical simulations of initially homogenously polarized Gaussian light beam self-focusing in isotropic phase of nematic liquid crystal, the temperature of which is close to the temperature of nematic-isotropic phase transition, were carried out. Here, we show that light polarization singularities (C-lines) of both polarization handednesses are nucleated near these transversal cross-sections of the beam, in which local extrema of peak intensities of corresponding circularly polarized components of the beam are attained. These lines are closed loops, surrounding the beam axis. They appear in wide range of beam and nonlinear medium parameters. Minor fluctuations of beam polarization and amplitude profiles do not destroy these singularities.

Shift of the nematic–isotropic phase transition temperature in a thin layer of a metallomesogenic complex

The effect of boundaries on the nematic–isotropic phase transition temperature in a melt of a metallomesogenic complex was studied for the first time. This was done by comparison of the electro-optical constant of the isotropic phase with the dielectric and optical anisotropy of the nematic phase on the basis of the Landau–de Gennes theory. In a real experiment, the two liquid phases (nematic and isotropic ones) coexist in a range of several degrees around the transition. According to polarization microscopy data, the phase transition temperature decreases by more than 10°C as the metallomesogen layer thickness is reduced from 200 to 5 μm.

Electric behaviour of a Schiff's base liquid crystal compound doped with a low concentration of BaTiO3 nanoparticles

We report here a low frequency (1Hz–10MHz) dielectric study of a Schiff base liquid crystal compound, N-(4-n-Pentyloxybenzalidene) 4′-n-Pentylaniline (in short 5O.5) dispersed with Barium titanate (BaTiO3) nanoparticles (0.4%). The compound 5O.5 exhibit N, SmA, SmC, SmF and SmG phases. The isotropic-nematic phase transition temperature and the dielectric anisotropy (∆ε) have been determined. The compound in pure state exhibits negative dielectric anisotropy. The effect of the ferroelectric nanoparticles (FNPs) on dielectric permittivity and dielectric anisotropy as a function of temperature as well as frequency has been discussed. It has been found that due to permanent dipole moment of FNPs, the majority of ions in isotropic and nematic phases gets trapped, consequently Maxwell-Wagner-Sillars (MWS) effect has diminished. The conductivity of the dispersed samples was found to decrease. Comparative dielectric relaxation behavior of the liquid crystal molecules of pure and BaTiO3 dispersed samples are discussed in nematic, SmA and SmC phases of the compound.

Critical Slowing of Density Fluctuations Approaching the Isotropic-Nematic Transition in Liquid Crystals: 2D IR Measurements and Mode Coupling Theory.

Two-dimensional infrared (2D IR) data are presented for a vibrational probe in three nematogens: 4-cyano-4'-pentylbiphenyl, 4-cyano-4'-octylbiphenyl, and 4-(trans-4-amylcyclohexyl)-benzonitrile. The spectral diffusion time constants in all three liquids in the isotropic phase are proportional to [T*/(T - T*)](1/2), where T* is 0.5-1 K below the isotropic-nematic phase transition temperature (TNI). Rescaling to a reduced temperature shows that the decays of the frequency-frequency correlation function (FFCF) for all three nematogens fall on the same curve, suggesting a universal dynamic behavior of nematogens above TNI. Spectral diffusion is complete before significant orientational relaxation in the liquid, as measured by optically heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy, and before any significant orientational randomization of the probe measured by polarization selective IR pump-probe experiments. To interpret the OHD-OKE and FFCF data, we constructed a mode coupling theory (MCT) schematic model for the relationships among three correlation functions: ϕ1, a correlator for large wave vector density fluctuations; ϕ2, the orientational correlation function whose time derivative is the observable in the OHD-OKE experiment; and ϕ3, the FFCF for the 2D IR experiment. The equations for ϕ1 and ϕ2 match those in the previous MCT schematic model for nematogens, and ϕ3 is coupled to the first two correlators in a straightforward manner. Resulting models fit the data very well. Across liquid crystals, the temperature dependences of the coupling constants show consistent, nonmonotonic behavior. A remarkable change in coupling occurs at ∼5 K above TNI, precisely where the rate of spectral diffusion in 5CB was observed to deviate from that of a similar nonmesogenic liquid.

Screening out the non-Arrhenius behaviour of nematic-isotropic transition by room temperature ionic liquid.

Differential Scanning Calorimetry (DSC) and optical polarization microscopy of a mixture of the liquid crystalline material (N-(4-methoxybenzylidene)-4-butylaniline, MBBA) and a Fe-based room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate ([Emim](+) [FeCl4](-), EMIF) indicate a decrease in the nematic-isotropic (N-I) phase transition temperature (T(NI)) with an increase in EMIF concentration, explained by a proposed model of Coulomb "screening" of MBBA quadrupoles by the EMIF ions along with ionic "self screening." DSC studies of EMIF-MBBA and pure EMIF and comparison with pure MBBA results show that the major transitions in pure EMIF have Arrhenius behaviour, but more importantly the previously found convex Arrhenius behaviour of the pristine MBBA [K. Dan et al., Europhys. Lett. 108, 36007 (2014)] becomes Arrhenius in the mixture, indicating a conversion of the entropic N-I activation barrier to an enthalpic one. In presence of EMIF, a drastic decrease in the intensity of out-of-plane distortions of benzene rings in MBBA is found from Fourier transform infrared spectroscopy, consistent with significant reduction in the conformational states of MBBA. This suppression of large amplitude motion is again consistent with a Coulomb screening and gives a molecular basis for the entropic-to-enthalpic conversion of the N-I activation barrier.

Entropic screening preserves non-equilibrium nature of nematic phase while enthalpic screening destroys it.

The present manuscript describes the role of entropic and enthalpic forces mediated by organic non-polar (hexane) and polar (methanol) solvents on the bulk and microscopic phase transition of a well known nematic liquid crystalline material MBBA (N-(4-methoxybenzylidene)-4-butylaniline) through Differential Scanning calorimetry (DSC), UV-Visible (UV-Vis), and Fourier Transform Infrared (FTIR) spectroscopy. DSC study indicates continuous linear decreases in both nematic-isotropic (N-I) phase transition temperature and enthalpy of MBBA in presence of hexane while both these parameters show a saturation after an initial decay in methanol. These distinct transitional behaviours were explained in terms of the "depletion force" model for entropic screening in hexane and "screening-self-screening" model for methanol. Heating rate dependent DSC studies find that non-Arrhenius behaviour, characteristic of pristine MBBA and a manifestation of non-equilibrium nature [Dan et al., J. Chem. Phys. 143, 094501 (2015)], is preserved in presence of entropic screening in the hexane solution, while it changes to Arrhenius behaviour (signifying equilibrium behaviour) in presence of enthalpic screening in methanol solution. FTIR spectra show similar dependence on the solvent induced screening in the intensities of the imine (-C = N) stretch and the out-of-plane distortion vibrations of the benzene rings of MBBA with hexane and methanol as in DSC, further establishing our entropic and enthalpic screening models. UV-Vis spectra of the electronic transitions in MBBA as a function of temperature also exhibit different dependences of intensities on the solvent induced screening, and an exponential decrease is observed in presence of hexane while methanol completely changes the nature of interaction to follow a linear dependence.

The ferroelectricity effect of nanoparticles on thermodynamics and electro-optics of novel cyanobiphenyl eutectic binary mixture liquid crystals

The phase transition of pentylcyanobiphenyl (5CB) 55% and heptylcyanobipheny (7CB) 45% (E5CN7) doped with TiO2 and Fe3O4 was investigated using differential scanning calorimetry (DSC) and polarized optical microscopy (POM) techniques. Based on the obtained results, noticeable differences were indicated in nematic–isotropic phase transition temperature and thermodynamic properties including enthalpy and entropy changes between the pure and doped compounds. The mentioned phenomena were attributed to the spherical structure of nanoparticles while the LC texture showed completely different results which were attributed to the formation of Schlieren textures by TiO2. Furthermore, electronic properties of LCs doped with TiO2 and Fe3O4 were monitored using impedance spectroscopy. The impedance results showed that Fe3O4 NP effect on LC semiconducting properties is stronger compared to TiO2. Doping of LCs with NPs forms the colloidal LCs with negative anisotropic properties and nanoparticles change the local ordering of liquid crystals. The diffusion coefficient calculations showed that the insertion of NPs reduces the diffusion of LCs toward the electrode surface. The dielectric constant of mentioned systems was studied as well.

Magnetically induced shift of the isotropic–nematic phase transition temperature in a mixture of bent-core and calamitic liquid crystals doped with magnetic particles

We have investigated the influence of doping with spherical magnetic nanoparticles on the mixture of a bent-core and a calamitic liquid crystal. Results showed a reduction of the critical field of the magnetic Fréedericksz transition by more than a factor of two after the doping. Moreover, we give for the first time experimental evidence of the theoretically predicted magnetically induced negative shift of the isotropic to nematic phase transition temperature.

Effect of an electric field applied during the injection procedure on the pretilt angle of a nematic liquid crystal

We examined the effect of an electric field applied during the injection procedure on the polar pretilt angle of a nematic liquid crystal (LC). The pretilt angle of the sample injected at 25°C gradually increased as the electric field was increased. On the other hand, the pretilt angle of a sample injected at 90°C (which is above the nematic-isotropic phase transition temperature of LC) showed a sudden increase in the presence of the electric field and also increased with a greater electric field. We think the alignment layer might be swollen with LC molecules, and the rotation of the immersed LC molecules by the electric field induces a deformation of the alignment layer. These results imply LC and the alignment layer were coupled, and their cooperation had an influence on determining the bulk pretilt angle.

Transferability of Coarse-Grained Force Field for nCB Liquid Crystal Systems

In this paper, the transferability of the coarse-grained (CG) force field originally developed for the liquid crystal (LC) molecule 5CB ( Zhang et al. J. Phys. Chem. B 2012 , 116 , 2075 - 2089 ) was investigated by its homologues 6CB and 8CB molecules. Note that, to construct the 5CB CG force field, we combined the structure-based and thermodynamic quantities-based methods and at the same time attempted to use several fragment molecular systems to derive the CG nonbonded interaction parameters. The resultant 5CB CG force field exhibits a good transferability to some extent. For example, not only the experimental densities, the local packing of atom groups, and the antiparallel arrangements of nearest neighboring molecules, but also the unique LC mesophases as well as the nematic-isotropic phase transition temperatures of 6CB and 8CB were reproduced. Meanwhile, the limitations of this 5CB CG force field were also observed, such as the phase transition from nematic to smectic was postponed to the lower temperature and the resulting smectic phase structure is single-layer-like instead of partially interdigitated bilayer-like as observed in underlying atomistic model. Apparently, more attention should be paid when applying a CG force field to the state point which is quite different from which the force field is explicitly parametrized for. The origin of the above limitations can be potentially traced back to the inherent simplifications and some approximations often adopted in the creation process of CG force field, for example, choosing symmetric CG potentials which do not explicitly include electrostatic interactions and are parametrized by reproducing the target properties of the specific nematic 5CB phase at 300 K and 1 atm, as well as using soft nonbonded potential and excluding torsion barriers. Moreover, although by construction this CG force field could inevitably incorporate both thermodynamic and local structural information on the nematic 5CB phase, the anisotropic diffusion coefficient ratios for different LC phases in both 6CB and 8CB systems are reproduced well. All these findings suggest that the multiproperty parametrization route together with fragment-based method provides a new approach to maximize the possibility to simultaneously reproduce multiple physical properties of a given molecule or related molecules with similar chemical structures at other state points.

Unusual temperature dependence of the splay elastic constant of a rodlike nematic liquid crystal doped with a highly kinked bent-core molecule

We report an unusual temperature dependence of the elastic constants of a rodlike nematic liquid crystal (RLC) mixed with a highly kinked bent-core liquid crystal (BLC). On cooling through the nematic phase, the splay elastic constant (K(11)) of the RLC-BLC mixture increased below the nematic-isotropic phase transition temperature, but started to decrease midway through the nematic phase. The decrease of K(11) was more prominent with a greater concentration of BLC. On the other hand, the bend elastic constant (K(33)) of the RLC-BLC mixture monotonically increased through the nematic phase with decreasing temperature.

Anisotropic Colloidal Micromuscles from Liquid Crystal Elastomers

Monodomain liquid crystal elastomers (LCEs) are new materials uniquely suitable for artificial muscles, as they undergo large reversible uniaxial shape changes, with strains of 20-500% and stresses of 10-100 kPa, falling exactly into the dynamic range of a muscle. LCEs exhibit little to no fatigue over thousands of actuation cycles. Their practical use has been limited, however, owing to the difficulty of synthesizing components, achieving consistent alignment during cross-linking across the whole material and often a high nematic-isotropic phase transition temperature. The most widely studied method for LC alignment involves mechanical stretching of the material during one of two cross-linking steps, which makes fabrication difficult to control and lends itself mainly to samples that can be easily grasped (with sizes of the order of mm). In this article, we describe a method of adapting the LCE synthesis to microscale objects, achieving monodomain alignment with a single cross-linking step, and lowering the cycling temperature. LCE precursor droplets are embedded in and then stretched in a polymer matrix at high temperature. Confinement of the uniaxially stretched droplets maintains the alignment achieved during stretching and allows us to eliminate one of the cross-linking steps and the variability associated with it. Adding a comonomer during the polymerization leads to lowering of the nematic-to-isotropic transition temperature (58 °C), significantly expanding the range of potential applications for these micromuscles. We demonstrate reversible thermal switching of the micromuscles in line with the largest strain changes observed for side-chain LCEs and a differential scanning calorimetry characterization of the material phase transitions. The method demonstrates the parallel fabrication of many microscale actuators and is amenable to further scale-up and manufacturing.

Theory of liquid crystalline micelles

A theory is introduced to describe self-assembly of liquid crystalline AB diblock copolymers, consisting of a homopolymer (A) and a side-chain liquid crystalline polymer (B). We derive the free energy of the liquid crystalline micellar solutions and examine the equilibrium solution properties: critical micelle concentration (CMC), nematic-isotropic phase transition (NIT) of the rigid side-chains inside the micelle core, and phase separations. It is shown that there is a critical micelle size below which the NIT becomes continuous due to a packing effect. We also find re-entrant micellizations near the NIT temperature. The phase diagrams, including binodal, spinodal, CMC, and NIT curves are also examined on the temperature-concentration plane.

Ordering of the 7CB liquid crystal induced by nanoscale confinement and boundary lubrication

In this study, the thermodynamic properties of nanoscale-thick 4-heptyl-4′-cyanobiphenyl (7CB) liquid crystals (LCs) confined in carbon nanotubes were investigated using differential scanning calorimetric methods. Both the isotropic-nematic phase transition temperature and the enthalpy increase decrease as the nanoscale confining size decreases, indicating the ordering effect of the 7CB molecules under nanoscale confinement. An average-order parameter <S> is calculated to quantitatively identify the ordering effect of LCs by utilising a simple expression of the Landau–de Gennes (LdG) free energy. The average-ordering parameter <S> of the 7CB LC can be 0.7 within a confinement thickness of ∼25 nm and above 0.5 within a film thickness of up to 100 nm. When lubricating the 7CB LC between a steel ball and chromium-coated glass disk at low speeds, a stable boundary layer with a thickness of 10 nm is formed that is independent of the entrainment speed. At higher speeds, the thickness of the 7CB lubrication film ranges from several tens of nanometres to 100 nm, resulting in a stable velocity-independent friction coefficient of approximately 0.03. The ordered microstructure of the 7CB molecules under confinement provides the underlay mechanism of its boundary lubrication property.

Orientation in Nematic Liquid Crystals Doped with Orange Dyes and Effect of Carbon Nanoparticles

Some properties of nematic liquid crystal E7 doped with two disperse orange dyes used together and effect of addition of carbon nanoparticles (single walled carbon nanotube or fullerene C60) on them were studied. Two dyes (disperse orange 11 and 13) having high solubility and order parameter were used as co-dopants. A notable increase in order parameter was obtained comparing to that of liquid crystal doped with single dye. When carbon nanoparticles were used as dopant, a decrease in order parameter was observed at low temperatures while it increased at high temperatures. When applied voltage changed, the order parameter abruptly increased in its threshold value and saturated in higher voltages as expected. An appreciable change in textures was not observed with addition of dopants. This addition gave rise to an increase in nematic-isotropic phase transition temperatures compared with that of pure liquid crystal.

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic–isotropic phase transition temperature ( T NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.

Synthesis and Physico-Chemical Properties for 2-Oxochromen-6-yl 4-(4-Alkoxyphenylazo)benzoates

This paper describes the preparation and thermal properties for homologous series of 2-oxochromen-6-yl 4-(4-alkoxyphenylazo)benzoates (compounds 1-n). Compounds 1-n show nematic phase exclusively, where the nematic–isotropic phase transition temperatures are between 293°C for 1–4 and 256°C for 1–8. Interestingly, compound 1–8 exhibits smectic A and re-entrant nematic phases in addition to the nematic one. The smectic A phase are characterized using a small angle X-ray diffraction study, indicating the smectic A phase has a partially bilayer molecular arrangement.

Direct Measurement of Surface-Induced Orientational Order Parameter Profile above the Nematic-Isotropic Phase Transition Temperature

The spatial and temperature dependence of the surface-induced orientational order parameter S(z,T) was determined in the isotropic phase. An optical fiber was immersed in a thin liquid crystal layer and the retardation was measured as a function of the fiber's height above the surface, from which the model-independent S(z,T) was deduced with resolution <or=2 nm. It was found that (i) S(z=0) <or=0.12 close to the nematic transition temperature, (ii) the susceptibility is mean-field-like, and (iii) S(z,T) deviates significantly from exponential spatial decay. The results are discussed in terms of a nonlocal potential.