Nematic Phase Transition Temperature Research Articles

Investigation of thermodynamical, electro-optical and dielectric properties of nematic liquid crystals dispersed with low-concentration barium titanate nanoparticles

ABSTRACT A liquid crystals nanoparticles composite was prepared by dispersing barium titanate nanoparticles (BT-NPs) into a multi-component liquid crystalline material having wide range room temperature nematic phase. The thermodynamic, optical textures, electro-optical and dielectric properties of the composite sample were studied along with pristine materials. Isotropic to nematic phase transition temperature was enhanced for nano-composites as observed from thermodynamic, optical textures and dielectric investigations. Effect of BT-NPs dispersion on various display parameters of nematic liquid crystals, namely, threshold voltage, dielectric anisotropy, DC conductivity and splay elastic constant has been explored. A relaxation mode in kHz frequency range has been observed for pure and nano-composites samples, which varies with temperature and follows Arrhenius behaviour. DC conductivity enhances approximately fivefold for BT-NPs dispersed samples. The host liquid crystalline material has nematic ordering which supports alignment of BT-NPs parallel to the nematic director which consequently improves thermal stability, dielectric and electro-optical parameters of the nano-composite system.

Structure and optical properties of twist-bend nematic liquid crystals doped with chiral dopants.

Twist-bend nematic liquid crystals (N_{TB} LCs), although consisting of achiral molecules, possess a spontaneous conic helix. They have been intensively studied and utilized in many applications in recent years. Herein we add chiral molecules to N_{TB} LCs and study their effects on the structure of the conic helix. We observe that the system is in the regular chiral nematic phase at high temperature and is still in the twist-bend nematic phase at low temperature. The addition of the chiral molecules does not induce a twist of the conic helical axis. The main effect of the chiral molecules is increasing the cone angle of the conic helix. We show that the structural chirality parameters in the chiral nematic phase and the twist-bend chiral nematic phase can be calculated from the same intrinsic chirality parameter, which only depends on the molecular structure and concentrations of the chiral molecule. We also observe a pretransitional phenomenon that the helical pitch of the chiral nematic phase increases dramatically when temperature is decreased toward the chiral nematic to twist-bend nematic phase transition temperature.

Thermodynamic and spectroscopic characterization of a weakly polar liquid crystalline compound dispersed with polyvinyl pyrrolidone capped gold nanoparticles

In the present study a deliberate attempt has been made to examine the consequences of dispersion of gold nanoparticles in an azoxybenzene-based nematic liquid crystal, namely 4,4′- di pentyl azoxy benzene. Investigations were made on gold nanoparticles doped nematic liquid crystals analysing differential scanning calorimetry, polarized light microscopy, ultraviolet–visible absorption spectroscopy, photoluminescence and Fourier-transform infrared spectroscopy (FTIR). Gold nanoparticles of different concentrations 0.25, 0.5, and 1.0 wt% were dispersed in pure 4,4′- di pentyl azoxy benzene, and it was observed in the experiment that no significant change occurs in the crystal to nematic phase (Cr-Ne) transition temperature with the increase in gold nanoparticles concentration. However, the nematic to isotropic (Ne-Iso) transition temperature decreases significantly with the increase in gold nanoparticles concentration. This study reports around 76% increase in ultraviolet–visible absorbance and 46% enhancement of photoluminescence intensity with the suitable amount of gold nanoparticles dispersion in 4,4′- di pentyl azoxy benzene. The surface plasmonic resonance (SPR) and re-excitation of 4,4′- di pentyl azoxy benzene molecule due to collision with gold nanoparticles has been accredited to the escalation in photoluminescence intensities.

A promising "metastable" liquid crystal stationary phase for gas chromatography

The liquid crystal state is an ordered physical state between a solid and a liquid. Previous research, in gas chromatography, proved that it provides a geometric selectivity, which allows the separation of geometric position isomers and cis-trans isomers that are difficult to separate on conventional gas chromatography stationary phases (polydimethyl siloxane derived and polyethylene glycol stationary phases). However, their use was generally very limited by the rather high temperature at which they must be operated, normally above the solid-liquid crystal transition temperature. In the present study we are interested in a new synthesized material, 1,4- bis (4-bromohexyloxy benzoate) phenyl (BHOBP). The first characterizations of BHOBP were carried out by thermogravimetric analysis, hot-stage optical microscopy and differential scanning calorimetry to control the thermal stability of the BHOBP as well as the nematic texture of the mesophase highlighted in a well-defined temperature range (120 °C-200 °C). When heated, the solid compound led to a stable liquid crystal state. Its cooling has revealed "a new metastable physical state, which is the supercooled liquid crystal phase". After these first characterizations, the new material was used as a stationary phase for gas chromatography. The BHOBP was deposited in a capillary column by the dynamic method. The inverse gas chromatography study of the column revealed a solid-stable nematic phase transition temperature, in agreement with the first characterization methods. The stable liquid crystal phase showed good resolutions in the analysis of some geometric isomers of low volatility as PAHs. The presence of the supercooled liquid crystal state in the chromatographic column has also been confirmed. This new metastable state is particularly interesting because it enlarged the scope of this material by improving the resolution of several mixtures. Thus, the separation of highly volatile mixtures of geometric isomers (e.g. cis and trans-decalin) was achieved only through this metastable mesophase confirming its unique selectivity. The metastable liquid crystal, used at 80 °C, has also exhibited an original behavior by its stability after several weeks of use at the same temperature, maintaining constant retention factors and selectivity.

Normal red blood cells\u2019 shape stabilized by membrane\u2019s in-plane ordering

Red blood cells (RBCs) are present in almost all vertebrates and their main function is to transport oxygen to the body tissues. RBCs’ shape plays a significant role in their functionality. In almost all mammals in normal conditions, RBCs adopt a disk-like (discocyte) shape, which optimizes their flow properties in vessels and capillaries. Experimentally measured values of the reduced volume (v) of stable discocyte shapes range in a relatively broad window between v ~ 0.58 and 0.8. However, these observations are not supported by existing theoretical membrane-shape models, which predict that discocytic RBC shape is stable only in a very narrow interval of v values, ranging between v ~ 0.59 and 0.65. In this study, we demonstrate that this interval is broadened if a membrane’s in-plane ordering is taken into account. We model RBC structures by using a hybrid Helfrich-Landau mesoscopic approach. We show that an extrinsic (deviatoric) curvature free energy term stabilizes the RBC discocyte shapes. In particular, we show on symmetry grounds that the role of extrinsic curvature is anomalously increased just below the nematic in-plane order-disorder phase transition temperature.

Effect of ZnO nanoparticles dispersed in liquid crystalline p-n-propoxy/propyl benzoic acids and mixtures – optical studies

In the present work, characterization studies are carried out on liquid crystalline (LC) p-n-propoxy benzoic acid (3oba), p-n-propyl benzoic acid (3ba) and the mixture of these two benzoic of acids dispersed with low weight concentrations of ZnO nanoparticles (nps). The existence of ZnO nps in above LC compounds are studied by scanning electron microscope and X-ray diffraction techniques. The LC phase transition temperatures of the compounds with the dispersion of ZnO nps are measured using polarizing microscope (POM) and differential scanning calorimeter (DSC). The nematic phase transition temperatures of 3ba, 3oba and mixture of 3ba and 3oba obtained from POM and DSC found to decrease with increase in the dispersion of various weight concentrations of ZnO nps. The birefringence studies are carried out for these compounds at different wavelengths (400, 500, 570, and 635 nm) with the dispersion of ZnO nps with increasing in weight concentrations (1, 1.5, 2 and 2.5 wt%) using modified spectrometer and the orientational order parameter ‘S’ of corresponding LC nanocomposites is measured from different optical methods viz., Kuczynski internal field model, Vuks isotropic model and effective geometry model . Among the three models the order parameter found to increase due to the strong alignment of LC molecules with the ZnO nps. The refractive indices measured at different wavelengths are fitted with two and three coefficient Cauchy’s model. For high birefringence LCs three coefficient Cauchy’s model is favorable than two coefficient model and vice versa. The results obtained are compared and discussed.

Strain tuning and anisotropic spin correlations in iron-based systems

Nematic order in the iron-based superconductors is closely tied to a lattice distortion and a structural transition from tetragonal to orthorhombic symmetry. External stress of the appropriate symmetry acts as a conjugate field of the nematic order parameter and can be utilized to detwin nematic domains but also smears an otherwise sharp nematic transition. On the other hand, applying stress in proper symmetry channels allows one to tune the nematic phase transition. Recent experiments analyzed the stress-induced changes of the nematic and magnetic phase transition temperature. Here we show that the observed trends can be understood in terms of spin-induced nematicity. The strain sensitivity is shown to be a fluctuation effect. The strong sensitivity to antisymmetric strain is a consequence of the anisotropic nature of the magnetic excitation spectrum. The formalism presented here can be naturally generalized to determine the strain-sensitivity of vestigial phases related to other magnetic states that have been observed in the iron-based systems, such as e.g.\ the spin-charge density wave and the spin-vortex crystals.

Threshold voltage decrease in a thermotropic nematic liquid crystal doped with graphene oxide flakes.

We report a threshold voltage decrease in a nematic liquid crystal compound, 4-cyano-4′-pentylbiphenyl (5CB), doped with graphene oxide (GO) flakes at a concentration of 0.05–0.3 wt %. The threshold voltage decrease was observed at the same concentration in electro-optic and dielectric spectroscopy measurements. The effect is related to the disrupted planar alignment due to the strong π–π stacking between the 5CB’s benzene rings and the graphene oxide’s structure. Additionally, we present the GO concentration dependence on the isotropic–nematic phase transition temperature, electric anisotropy, splay elastic constant, switch-on time, and switch-off time. The shape and dimensions of the GO flakes were studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The influence of the GO concentration on the physical properties and switching process in the presence of the electric field was discussed.

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.

Test of Halperin-Lubensky-Ma crossover function at theN–Sm−Atransition in liquid crystal binary mixtures via high-resolution birefringence measurements

We report optical birefringence data for a series of mixtures of the liquid crystals octylcyanobiphenyl (8CB) and decylcyanobiphenyl (10CB). Nematic order parameter S data in the nematic and smectic A phases have been derived from phase angle changes obtained in temperature scans with a rotating analyzer method. These S values have been used to arrive at values for possible entropy discontinuities at the smectic A to nematic phase transition temperature T_{NA}. The 10CB mole fraction dependence of the obtained entropy discontinuities could be well fitted with a crossover function consistent with the mean-field free-energy expression with a nonzero cubic term arising from the coupling between the smectic-A order parameter and the orientational order parameter director fluctuations in the Halperin-Lubensky-Ma theory. The obtained results are in good agreement with existing results from adiabatic scanning calorimetry. By exploiting the fact that the temperature derivative of the order parameter S(T) near T_{NA} exhibits the same power law divergence as the specific heat capacity, we have extracted the effective critical exponent α values for the compositions under study. The critical exponent α has been observed to reach the tricritical value α_{TCP}=0.5 for the 10CB mole fraction of x=0.330.

Thermal and optical study of semiconducting CNTs-doped nematic liquid crystalline material

We report the thermal and spectroscopic analysis of the carbon nanotubes (CNTs)-doped nematic liquid crystal (NLC) material. The CNTs have been oriented in the p-ethoxybenzylidene p-butylaniline NLC. The thermal study of the CNTs doped nematic mixtures shows a significant decrease in the isotropic to nematic phase transition temperature. However higher doping concentration of CNTs has led to the further increase in transition temperature. The UV-Visible spectroscopy has been attempted on the CNTs/NLC mixtures at room temperature. The investigated NLC present one absorption band corresponding to π–π* electronic transition. A red shift of λmax with the increasing concentration of CNTs in the mixture has been observed. The band gap of NLC has been found to decrease after the doping of CNTs. The absorbance was measured for the UV light, polarized parallel and perpendicular to the LC director in the planar aligned cell.

Chemically and thermally stable, emissive carbon dots as viable alternatives to semiconductor quantum dots for emissive nematic liquid crystal–nanoparticle mixtures with lower threshold voltage

ABSTRACTDispersions of chemically and thermally robust carbon dots (2.5 ± 0.5 nm in core diameter) were prepared and investigated by polarised optical microscopy, electro-optic measurements including dynamic tests and numerical simulations as well as fluorescence confocal microscopy. The carbon dots were prepared by a straightforward thermal decomposition method from citric acid and hexadecylamine, and they show typical excitation wavelength-dependent photoluminescence behaviour. All dispersions, ranging from 0.5 to 5.0 wt.%, showed lower values for isotropic–nematic phase transition temperature and broader isotropic–nematic biphasic temperature intervals with increasing carbon dot content in comparison to the neat material. Doping of the nematic host with the carbon dots resulted in lower values for the apparent threshold voltage and the elastic constants, but higher values for the rotational viscosity. At 2.5 wt.% and higher, carbon dots residing at the confining interfaces in planar cells induce an increasing initial pre-tilt of up to 8° at lower temperatures. Fluorescence confocal microscopy confirmed this, where the luminescence of the carbon dots permitted visualisation of the distribution of the carbon dots in the bulk with a noticeable, in some cases even pattern-like, segregation to the confining interfaces.

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.

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy.

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

Dielectric and electro-optic studies of a bimesogenic liquid crystal composed of bent-core and calamitic units.

A bimesogen, BR1, composed of a bent-core and calamitic unit, linked laterally via a flexible spacer is investigated by dielectric and electro-optic techniques. X-ray results show the presence of clusters in the nematic phase, and the cluster size is of the order of the thickness of a single layer. The splitting of the small-angle scattering Δχ/2 is about 50°, which indicates SmC like clusters with a significant tilt of the molecules in the quasilayers. The sign reversal of the dielectric anisotropy Δε' is observed as a function of frequency; the behavior is rather similar to that exhibited by the conventional dual frequency nematics, composed of a calamitic mesogen, with the exception that it occurs at much lower frequencies in this material. Interestingly, as the bimesogen enters its nematic phase, the average permittivity decreases as the temperature is lowered. This indicates the onset of antiparallel association of some of the dipoles in the system, and this type of association is much more prominent in BR1 in comparison to other bent-core liquid crystalline systems composed of the same bisbenzoate core unit. The analysis of the dielectric spectra using the Maier-Meier model confirms the onset of an antiparallel correlation of dipoles occurring at the isotropic to nematic phase transition temperature. Additionally these results support a model of the cluster where the transverse dipole moments in the neighboring layers are antiparalleled to each other.

Thermotropic Liquid Crystalline Dimers with Varying Alkoxy Terminal Chain Length

We have design and synthesized of homologous Series I dimers and to study the effect of molecular structure variation and change in alkoxy terminal chain length (n = 6, 7, 8) on mesomorphic properties of liquid crystals. Explore further understanding of the structure-property relationship of liquid crystal homologous Seriesl dimers having varying alkoxy terminal chain length. It was found that the decrease in crystalline phase, smectic C and nematic phase transition temperatures with increasing terminal alkoxy chain length. The molecular structures of the Seriesl dimer were characterized by Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy. Their mesomorphic properties were investigated by differential scannning calorimetry (DSC) and on the hot-stage of a polarizing microscope (POM).

Length Scales and Structural Dynamics in Nematogen Pseudonematic Domains Measured with 2D IR Vibrational Echoes and Optical Kerr Effect Experiments

Nematogen liquids in the isotropic phase are macroscopically homogeneous but on multinanometer length scales have pseudonematic domains with correlation lengths that grow as the isotropic to nematic phase transition temperature (TNI) is approached from above. Orientational relaxation of nematogens in the isotropic phase manifests as two fast power laws and a slow exponential decay when measured by optical heterodyne detected optical Kerr effect (OHD-OKE) experiments. The long time exponential relaxation is associated with complete randomization of pseudonematic domains. We examine the effect of local orientational correlation on spectral diffusion (structural evolution) experienced by a vibrational probe molecule within the pseudonematic domains of 4-cyano-4'-pentylbiphenyl (5CB) using two-dimensional infrared (2D IR) vibrational echo spectroscopy. The addition of low concentration 4-pentyl-4'-thiocyanobiphenyl (5SCB) as a long-lived vibrational probe to 5CB is shown to lower TNI of the sample slightly, but the fast power law dynamics and exponential decays observed by OHD-OKE spectroscopy are unchanged. We compare the complete orientational relaxation and spectral diffusion for samples of 5SCB in 5CB to 5SCB in 4-pentylbiphenyl (5B) at four temperatures above TNI. 5B has a molecular structure similar to 5CB but is not a nematogen. At all but the lowest temperature, the spectral diffusion in 5CB and 5B is described well as a triexponential decay with very similar time constants. The results demonstrate that the presence of local orientational order at temperatures well above TNI does not affect the spectral diffusion (structural evolution) within pseudonematic domains when the correlation lengths are short. However, when the temperature of the sample is held very close to TNI, the spectral diffusion in 5CB slows dramatically while that in 5B does not. It is only as the correlation length becomes very long that its presence impacts the spectral diffusion (structural fluctuations) sensed by the vibrational probes located in pseudonematic domains. The orientational relaxation is modeled with schematic mode coupling theory (MCT). Fitting with MCT provides density and orientational correlation functions. The density correlation decays are similar for 5B and 5CB, but the orientational correlation decays are much slower for 5CB. Additionally, the time dependence of the spectral diffusion in 5CB is strikingly similar to that of the density correlation function decay, while the orientational correlation function decay is far too slow to contribute to the spectral diffusion. Therefore, density fluctuations are likely the source of spectral diffusion at temperatures at least 5 K above TNI.

Biaxiality-induced magnetic field effects in bent-core nematics: Molecular-field and Landau theory

Nematic liquid crystals composed of bent-core molecules exhibit unusual properties, including an enhanced Cotton-Mouton effect and an increasing isotropic (paranematic)-nematic phase transition temperature as a function of magnetic field. These systems are thought to be good candidate biaxial liquid crystals. Prompted by these experiments, we investigate theoretically the effect of molecular biaxiality on magnetic-field-induced phenomena for nematic liquid crystals, using both molecular field and Landau theory. The geometric mean approximation is used in order to specify the degree of molecular biaxiality using a single parameter. We reproduce experimental field-induced phenomena and predict also an experimentally accessible magnetic critical point. The Cotton-Mouton effect and temperature dependence of the paranematic-nematic phase transition are more pronounced with increased molecular biaxiality. We compare our theoretical approaches and make contact with recent relevant experimental results on bent-core molecular systems.

Impact of graphene incorporation on the orientational order of graphene/liquid crystal composites

2H NMR spectroscopy was used to characterize changes in the liquid crystal (LC) phase behavior and orientational order resulting from the dispersion of graphene nanoplatelets within the nematic 4-cyano-4′-n-pentyl-biphenyl (5CB) liquid crystal. The graphene/5CB composites revealed a decreased isotropic to nematic phase transition temperature (TIN) and a reduction in LC orientational order, with increasing graphene volume fraction. In contrast, the graphene/benzene/5CB composites revealed both an increased TIN and an increase in the LC orientational order with graphene addition. These changes result from anisotropic surface interactions between the graphene nanoplatelets and LC, and are discussed in terms of the mean-field theory for nanoparticle doped LCs.

High-resolution birefringence investigation of octylcyanobiphenyl (8CB): An upper bound on the discontinuity at the smectic-Ato nematic phase transition

We report optical birefringence data by two different methods with high temperature resolution for octylcyanobiphenyl (8CB) near the smectic-A to nematic (Sm-A-N) phase transition temperature T(AN). Within the resolution of our experiments, we find that the Sm-A-N phase transition is continuous. For a possible discontinuity in the orientational order parameter S(T) at T(AN), we arrive at an upper limit of 0.0002, which is substantially smaller than other estimates in literature, but consistent with the value of 0.00008 derived from the upper limit of the latent heat from high-resolution adiabatic scanning calorimetry (ASC), which is itself consistent with the Halperin-Lubensky-Ma theory. The temperature derivative of the order parameter exhibits a power law divergence with a critical exponent that is consistent with the value α = 0.31 ± 0.03 for the specific heat capacity obtained by ASC.