Nematic Phase Transition Research Articles

Hydrophobic gold nanoparticles via silane conjugation: chemically and thermally robust nanoparticles as dopants for nematic liquid crystals

We examine for the first time how chemically and thermally stable gold nanoparticles (NPs), prepared by a silane conjugation approach, affect both the thermal and the electro-optical properties of a nematic liquid crystal (LC), when doped at concentrations ranging from 0.25 to 7.5 wt%. We find that the octadecylsilane-conjugated gold NPs stabilize both the enantiotropic nematic and the monotropic smectic-A phases of the LC host with a maximum stabilization of 2(°)C for the nematic and 3.5(°)C for the smectic-A phases for the mixture containing 1 wt% of the silanized particles. The same mixture shows the lowest values for the Fréedericksz transition threshold voltage and the highest value for the dielectric anisotropy. Generally, all NP-containing mixtures, except mixtures with NP concentrations exceeding 5 wt%, reduce the threshold voltage, increase the dielectric anisotropy and reduce both rise and decay time; the latter particularly at temperatures at least 10(°)C below the isotropic-nematic phase transition on cooling.

Nematic order and phase transition dynamics under intense electric fields

Electrically induced order variations, occurring in the microseconds timescale, in nematic liquid crystals are framed within the Landau-de Gennes Q -tensor theory. We provide a numerical model implemented with a moving mesh finite element method and describe the order dynamics in a symmetric π-cell subjected to very strong electric fields. Our simulations put in evidence bulk order reconstruction delocalisation and transient order lowering phenomena induced by intense applied electric fields, as well as first and second order characteristics of the splay – bend transition.

Diphenyltriacetylenes: novel nematic liquid crystal materials and analysis of their nematic phase-transition and birefringence behaviours

Improvements in birefringence properties of LC materials are actively being pursued because of the increase in their applications in optical devices. 1,6-Diphenyl-1,3,5-hexatriyne, i.e., diphenyltriacetylenes (DPTAs; number of acetylene units, n = 3) are expected to have wide nematic mesophases and exhibit extremely high birefringence properties because of the high aspect ratio of their conjugated structures. Therefore, we synthesised DPTA derivatives as novel, highly birefringent nematic liquid crystal materials. The derivatives exhibit stable enantiotropic nematic phases with longer alkoxy carbon chain lengths (i.e., with dodecyloxy groups, m = 12). High birefringence values of Δn = 0.46 at 550 nm and 140 °C and a high extraordinary refractive index (ne) of over 2.0 were exhibited for triyne DPTA–OC6. Furthermore, we investigated the effect of the acetylene bond on the nematic transition behaviours and refractive index parameters of the conjugated oligoyne or polyyne compounds with two terminal aromatic rings. An increasing number of acetylene units (n = 1–3), i.e., extended conjugation, led to wider nematic phases and higher birefringence; the increment in Δn per acetylene unit was estimated to be approximately 0.14.

Crossover From Under to Overdamped Dynamics in Free-Standing Smectic Films Close to the Smectic-A-Nematic Second Order Phase Transition

The hydrodynamic properties of free-standing smectic films close to a second order smectic-A (SmA) — nematic (N) phase transition are investigated. By using an extended McMillan model, the profiles of the smectic elastic constants are computed at different temperatures. We show that the amplitude of smectic fluctuations has a strong dependence on the film temperature. By considering the critical behavior of viscoelastic coefficients of thin films, we show that the typical underdamped relaxation of the smectic order is replaced by an overdamped behavior close to the bulk transition temperature.

Towards understanding the empty liquid of colloidal platelets: vapour–liquid phase coexistence of square-well oblate ellipsoids

We study the phase behaviour of hard oblate ellipsoids with superimposed short-ranged oblate-shaped square-well attractions using the replica exchange Monte Carlo method and a simple van der Waals type perturbation theory. By fixing the square-well range (λ = 0.25σ‖), we examine the effect of varying the oblates aspect ratio, k = σ⊥/σ‖, on the density profiles, vapour–liquid and isotropic–nematic phase transitions. We observe that the formation of a liquid phase with vanishing density, an empty liquid, is possible with increasing aspect ratio. Surprisingly, the increasing shape anisotropy does not shift the stability region of the nematic phase to low densities, while the oblate-shaped square-well attraction favours disordered states. For k > 1.5, the critical temperature and packing fraction monotonically decrease with k, while the isotropic–nematic phase boundary moves towards higher packing fractions with decreasing temperature for all k. Simulations show that oblates preferably orientate their faces parallel to the vapour–liquid interface when located at the liquid side of the interface region and perpendicular when located at the vapour side.

Orientational order in a glass of charged platelets with a concentration gradient

Colloidal dispersions of anisometric particles can display dynamical arrest and ordering involving both translational and rotational degrees of freedom. We show that orientational order can develop in glassy colloidal dispersions of charged platelets when a concentration gradient is imposed through solvent evaporation. Our model system of Laponite (LRD) platelets in deionized water has been extensively studied for its ergodic to non-ergodic transitions, and the existence of an underlying isotropic–nematic phase transition has been a subject of debate. We use small-angle X-ray scattering, dynamical light scattering and birefringence to show that the orientational order we observe does not result from an underlying, uniquely determined equilibrium state with orientational order, but from plastic deformation of the colloidal glass.

Detailed mechanism for the orthogonal polarization switching of gold nanorod plasmons

In this work, we describe an electro-optic material capable of orthogonally switching the polarization of the localized surface plasmon resonance scattering of single gold nanorods, independent of their orientation. Liquid crystal samples are prepared in a sandwich configuration with electrodes arranged so that an applied voltage induces alignment-switching of the liquid crystal molecules covering individual gold nanorods. Due to the birefringence of the nematic liquid crystal, the reorientation in the nematic director alignment causes a change in the output polarization of the scattered light. We propose the underlying mechanism to be based on a homogeneous nematic to twisted nematic phase transition and provide support for it via Jones calculus by modelling the effect of ideally aligned homogeneous nematic and twisted nematic phases on polarized light transmitted through the sample. In the model, we include the effects of sample thickness and surface plasmon resonance wavelength, expressed in terms of the phase retardation, χ, on the observed output polarization. We find four distinctively different trends for the output polarization as a function of the incident polarization as χ is varied. Two of these cases provide reproducible orthogonal polarization switching of the surface plasmon resonance while maintaining a high degree of polarization. These results are verified experimentally with liquid crystal cells of different thicknesses. The deviation of the experimental samples from ideal behaviour can be explained by the inherent variations in the surface plasmon resonance maximum and local cell thickness.

3-Dimensional structural control of silica microparticles using photoresponsive materials

ABSTRACTWe demonstrate the three-dimensional arrangement of silica microparticles in body-centered cubic lattice structure using both a relief pattern formed in the film of the azopolymer and silica micro particles dispersed in a nematic liquid crystal. In the method, firstly, the square lattice relief structures with the spacing of 2000 nm and the depth of 400 nm were formed on the azopolymer film by twice exposure to two beam interference patterns with coherent light of 488 nm in wavelength from Ar ion laser. Next, a nematic liquid crystal (5CB) containing 2000 nm diam. silica microparticles, with the surface modified with silane coupling reagent giving rise to homeotropic texture, was dropped on the relief structure on the azopolymer. The two dimensional square lattice structures of the micropaticles were formed spontaneously in an isotropic to nematic phase transition. Finally, the microparticles were three-dimensionally arranged by using optical tweezers. The microparticles were successfully stacked in a four-step pyramid structure with body centered tetragonal lattice structure.

Micellar shape anisotropy and optical indicatrix in reentrant isotropic—nematic phase transitions

In this work, we will show that the concept of nematic order parameter and a generalization of the concept of optical indicatrix, largely used in optics of anisotropic crystals, can be joined to give a new geometrical representation to the anisotropies found in lyotropic liquid crystals. Our theoretical approach will be illustrated using refractive index data of reentrant isotropic-calamitic (discotic) nematic phase transitions of potassium laurate, decanol, and heavy water mixtures.

Effect of Field Strength on Electrooptical Properties of Low-Molecular Weight and Polymer Liquid Crystals

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with side mesogenic groups was investigated. It was established that the electric field induces the isotropic—nematic phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was found that the relaxation time τ of the orientational order parameter is proportional to the square of electric field strength. For comb-shaped polymer liquid crystals, the time of transition from the nematic to the isotropic phase is several orders longer then that for low-molecular liquid crystal.

Isotropic–nematic phase separation and demixing in mixtures of spherical nanoparticles with length‐polydisperse nanorods

AbstractAn extension of Onsager theory is developed to simulate isotropic–nematic phase separation in a mixture of spheres with length‐polydisperse system of rods. This work is motivated by recent experimental data on nanorod liquid crystals. Prior theoretical investigations indicate that both polydispersity and the presence of spheres should increase the biphasic–nematic phase transition, that is, the nematic cloud point. Results indicate that the phase diagrams undergo drastic changes depending upon both particle geometry and rod length polydispersity. The key geometric factor is the ratio between the sphere diameter and the rod diameter. In general, length fractionation is enhanced by the addition of spheres, which may be experimentally advantageous for separating short nanorods from a polydisperse population. Simulation results also indicate that the nematic cloud and shadow curves may cross one another because of the scarcity of spheres in the shadow phase. In general, these results do indicate that the nematic cloud point increases as a function of sphere loading; however, in certain areas of phase space, this relationship is nonmonotonic such that the nematic cloud point may actually decrease with the addition of spheres. This work has application to a wide range of nanoparticle systems, including mixtures of spherical nanoparticles with nanorods or nanotubes. Additionally, a number of nonspherical particles and structures may behave as spheres, including crumpled graphene and tightly coiled polymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Design, synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering

Design, synthesis and characterization of seven linear hydrogen bonded liquid crystal complexes derived from mesogenic p–n-decyloxy benzoic acid and p–n-alkyl benzoic acids designated as 10OØn (where n varied from ethyl to octyl) are reported. FTIR studies confirm the hydrogen bond formation in all these complexes. The phase transition temperatures and their corresponding enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. POM and DSC data are further utilized for the construction of 10OØn phase diagram. Two Odd–even effects have been evinced, one in enthalpy values and the other in corresponding transition temperatures across the isotropic to nematic phase transition. An interesting result is the observation of re-entrant smectic ordering, designated as smectic CR in three higher ordered mesogens. A new smectic ordering, smectic X, has been observed which is sandwiched between traditional smectic C and re-entrant smectic CR. Magnitudes of optical tilt angle in smectic C, smectic X and smectic CR are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic CR are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic CR phases have been analyzed.

Fermi-Surface Truncation from Thermal Nematic Fluctuations

We analyze how thermal fluctuations near a finite temperature nematic phase transition affect the spectral function A(k,ω) for single-electron excitations in a two-dimensional metal. Perturbation theory yields a splitting of the quasiparticle peak with a d-wave form factor, reminiscent of a pseudogap. We present a resummation of contributions to all orders in the Gaussian fluctuation regime. Instead of a splitting, the resulting spectral function exhibits a pronounced broadening of the quasiparticle peak, which varies strongly around the Fermi surface and vanishes upon approaching the Brillouin-zone diagonal. The Fermi surface obtained from a Brillouin-zone plot of A(k,0) seems truncated to Fermi arcs.

Study of Field Induced Transition (FiT) and Analysis of Crystallization Kinetics in the Nematic Phase of an Interhydrogen Bonded Nanoliquid Crytsal

An interhydrogen bonded liquid crystal with complementary hydrogen bonding between succenic acid (SA) and pentyloxy benzoic acid (5OBA) referred as SA + 5OBA has been synthesized and characterized. Multiwalled carbon nanotubes (MWCNT) are dispersed in SA + 5OBA and the resultant complex is referred as SA + 5OBA+MWCNT. Both complexes exhibit liquid crystallinity with the presence of nematic phase. FTIR and NMR studies confirms the formation of the interhydrogen bonds. Transition temperatures and enthalpy values are obtained by DSC studies. Considerable hyteresis in dielectric permittivity has been observed in SA + 5OBA + MWCNT, which enable it to be used in device applications. An interesting observation in SA + 5OB A + MWCNT complexes is the field induced transition (FiT) in nematic phase, which is studied by conductance, permittivity, and helicoidal structure deformations. This complex can be used in light modulation applications. The liquid crystalline behavior together with the rate of crystallization in nematic phase of pure and MWCNT dispersed hydrogen bonded complex are discussed in relation to the kinetophase (which occurs prior to the crystallization). The molecular mechanism and dimensionality in the crystal growth are computed from the Avrami equation. The characteristic crystallization time (t*) at each crystallization temperature is deduced from the individual plots of log t and ΔH. The influence of MWCNT on crystallization kinetics in the nematic phase of an interhydrogen bonded liquid crystal is discussed.

An optical conoscopy study of a reentrant discotic nematic—Biaxial nematic phase transition

A fundamental phase diagram with lyotropic nematic liquid crystals was reported by Yu and Saupe a long time ago. Two uniaxial (calamitic—NC and discotic—ND) and one biaxial nematic (NB) phases were determined by these authors. The phase diagram also predicted the existence of the ND−NB−ND phase transitions. Along the years, this phase diagram has been studied by several authors and yet these phase transitions have not been observed. In this work, we present a study of this phase sequence through optical conoscopy. The occurrence of these phase transitions is here investigated and optically characterized through conoscopic image.

Simulation of scattering and phase behavior around the isotropic–nematic transition of discotic particles

The detailed study of the isotropic–nematic phase transition in a system of discotic particles of aspect ratios L/D⩽0.1 presented here is relevant to a broad range of colloidal suspensions of chemically modified clay particles. Using Monte Carlo simulation techniques the equation of state, radial distribution functions, structure factors and normalized scattering intensities are calculated for each phase. The results are interpreted and related to previously reported free energy calculations [Fartaria and Sweatman, Chem. Phys. Lett. 478 (2009) 150], suggesting a nearly continuous isotropic–nematic transition for lower aspect ratios. Given this behavior we examined the structural information for each phase to determine how experimental scattering data might be used to distinguish the two phases. The radial distribution functions in each phase depend strongly on aspect ratio, and for larger aspect ratios a dramatic increase in the local ordering of discotic particles (represented here as cut-spheres) is observed just before the phase transition. However, this nearest-neighbor ordering seen in g(r) around r/D=0.1 would hardly be discernible in experimental scattering data subject to usual statistical errors. The structure factors and scattering intensities were calculated for L/D=0.1, 0.04 and 0.01 for the isotropic and nematic phases at and away from the isotropic–nematic transition. While the isotropic–nematic phase transition can be detected from the height and shape of the first scattering peak around 7QD for larger aspect ratios, this feature becomes much less discriminatory with decreasing aspect ratio. Instead, scattering intensities at low scattering vector amplitudes (Q→0) can be used for detection of the phase transition at low aspect ratios. These results provide useful insight to guide interpretation of X-ray and light scattering measurements for colloidal dispersions of thin platelets undergoing isotropic–nematic transitions.

ASYMPTOTICS OF MINIMIZERS OF VARIATIONAL PROBLEMS IN A MULTI-CONNECTED DOMAIN ASSOCIATED WITH THE THEORY OF LIQUID CRYSTALS

We consider the asymptotic behavior of minimizers of the Ericksen functional with the Dirichlet boundary condition in a general domain. We examine the asymptotic behavior of minimizers of the functional as some of the elastic coefficients tend to the infinity, according to the de Gennes predictions with respect to nematic smectic-A phase transition.

The structures and conformations of mesogenic molecules in the pre-transitional region of the isotropic phase: 5OCB and MBBA and their mixtures

A nuclear magnetic resonance spectrometer operating with a magnetic field of 23.49 T, corresponding to 1000 MHz for protons, has been used to study mesogenic molecules N-(4-methoxybenzylidene)-4-butylaniline (MBBA) and 4-pentyloxy-4′-cyanobiphenyl (5OCB) and their approximately equimolar mixture at temperatures above the isotropic to nematic phase transition. In this strong magnetic field, B 0, the molecules acquire a strong partial alignment whose magnitude depends on , and also on the anisotropic intermolecular interactions, which grow in strength as the isotropic to nematic phase transition is approached. The well-resolved spectra obtained are analysed to obtain magnetic field induced dipolar couplings between proton (1H), and between 1H and carbon nuclei, which are used to investigate the structure, conformational distribution and orientational order in both the pure compounds and in their mixtures.

Volume phase transitions of biaxial nematic elastomers

We present a mean-field theory to describe biaxial nematic phases of side-chain liquid crystalline elastomers. Novel biaxial nematic phases are theoretically predicted in a side-chain liquid crystalline polymer and gel, where side chains (mesogens) and rigid-backbone chains favor mutually perpendicular orientations. We calculate uniaxial and biaxial orientational order parameters and examine deformations of the gel and stable biaxial nematic phases of the liquid crystalline gel dissolved in isotropic solvents. We predict first-order uniaxial-biaxial nematic phase transitions of the gel and the volume of the gel is discontinuously changed at the phase transition temperature.

Uniaxial to biaxial nematic phase transition in a bent-core thermotropic liquid crystal by polarising microscopy

Polarising optical microscopy (POM) shows evidence for a reversible uniaxial nematic (NU) to biaxial nematic (NB) phase transition in a quiescent melt of bent-core mesogens (BCM). A second superimposed Schlieren texture, attributed to a second director field, grows into the NU phase on lowering the temperature and disappears again on heating, indicating a fully reversible NB to NU phase transition. This POM result together with recent X-ray data provides a new signature for the NB phase in this class of BCMs, and offers a new method for assessment of new biaxial nematic phases.