Pressure variation of reentrant transition temperature in liquid crystals

High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T_AN exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (γ, λ_eff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling λ_eff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through γ and λ_eff which justifies the experimental pressure dependence in the reentrant transition temperature . The pressure dependence of γ and λ_eff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.     

Effect of shape anisotropy on the phase diagram of the Gay-Berne fluid

We have used the density functional theory to study the effect of molecular elongation on the isotropic-nematic, isotropic-smectic A and nematic-smectic A phase transitions of a fluid of molecules interacting via the Gay-Berne intermolecular potential. We have considered a range of length-to-width parameter 3.0 ⩽ x₀ ⩽ 4.0 in steps of 0.2 at different densities and temperatures. Pair correlation functions needed as input information in density functional theory are calculated using the Percus-Yevick integral equation theory. Within the small range of elongation, the phase diagram shows significant changes. The fluid at low temperature is found to freeze directly from isotropic to smectic A phase for all the values of x₀ considered by us on increasing the density while the nematic phase stabilizes in between isotropic and smectic A phases only at high temperatures and densities. Both isotropic-nematic and nematic-smectic A transition density and pressure are found to decrease as we increase x₀. The phase diagram obtained is compared with computer simulation result of the same model potential and is found to be in good qualitative agreement.     

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Δn) and the dielectric constant (epsilon_⊥) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T_PN) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T_PN, indicating the presence of polarised domains. Our measurements show that ΔT_PN(= T_PN(E)-T_NI(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E². It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.     

NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline

Proton spin-lattice relaxation times,T ₁, have been measured in the smectic phases, S _G ² , S _G ¹ and S_A, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the S _G ¹ and S _G ² phases,T ₁ has been measured at 15 and 40 MHz. The (S _G ¹ →S _G ² ) and (S _G ² →S _G ¹ ) transitions are clearly seen as discontinuities inT ₁. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the S _G ¹ phase have been fitted to a model in which anisotropic rotational diffusion of the end hydrocarbon chains as also that of the rigid part of the molecule are considered. In the nematic phase, at 351 K, the observed behaviour ofT ₁, measured in the frequency range, 5 to 40 MHz, agrees well with the theoretical predictions of Uklejaet al and Freed, who take into account long range collective order fluctuations and local reorientations. Using these theories, the correlation time for short range reorientations has been calculated from our results to be 4.3 × 10⁻¹⁰ and 1.1 × 10⁻⁹ s respectively.     

Ultrasonic attenuation near the cholesteric → smectic A transition

Ultrasonic attenuation at 2 MHz has been measured through the cholesteric → smectic A transition in cholestryl nonanoate. Very near the transition, 0.1 < (T ? T_c) < 0.6°C, the excess attenuation varies as (T ? T_c)ⁿ, where n for four separate runs varies between 0.30 and 0.34. These values are very close to the exponent of 0.33 predicted using a scaling laws and observed for the divergence of the twist viscosity in the nematic phase as the smectic A transition is approached.     

X-ray investigation of dialkyldimethylammonium bromides

Abstract

A Series of dialkyldimethylammonium bromides (DDAB), (C _n H_2n+1)zN⁺(CH₃)₂Br^?with n = 10, 12, 14, 16 and 18 were investigated using X-ray diffraction. Measurements were performed by the photographic Guinier method as well as by diffractometer on samples aligned on a glass substrate. For a sample with n=12, X-ray high-pressure measurements were also performed which allowed us to confirm the destabilization under pressure of the phase existing on heating above the first transition point. For all the studied compounds, except for that with n =16, strong hysteresis and rich polymorphism were observed. A smectic T phase - high-temperature liquid crystal was confirmed for compounds with n= 16 and 18. All the remaining alkylammonⁱum salts have smectic an E high-temperature liquid crystal phase.     

Design, synthesis and characterization of a linear hydrogen bonded homologous series

A linear hydrogen bonded liquid crystalline homologous series has been synthesized and characterized. Hydrogen bond is formed between p-n-dodecyloxy benzoic acid and various p-n-alkyl benzoic acids whose alkyl chain vary from octyl to ethyl. Synthesized complexes are characterized by FTIR, ¹H NMR and ¹³C NMR studies for inferring the formation of hydrogen bonds. Polarizing Optical Microscopy (POM) and DSC studies reveal various mesophases and their corresponding transition temperatures along with respective enthalpy values. All the seven synthesized complexes exhibit rich liquid crystalline mesomorphism. A new phase namely smectic X has been observed in five of the complexes with a narrow thermal range. This phase has been characterized by optical textural, DSC, tilt angle and helicoidal pitch studies. Smectic X is sandwiched between traditional smectic C and re-entrant smectic C (designated as C_R) phases. Homeotropic transition in nematic phase is observed in all the mesogens and thus these materials can be used as thermally controlled optical shutters. Tilt angle in smectic C, smectic X and smectic C_R phases have been experimentally elucidated for all the mesogens.     

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids (n indicates the homologue number). The homologues with 7?≤?n?≤?13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originating from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.     

Reexamining the phase diagram of the Gay—Berne fluid

This work reexamines and updates earlier investigations on the phase behaviour of the Gay-Berne liquid crystal model, concentrating on the effect of varying temperature. Constant volume and constant pressure Monte Carlo simulations are combined for systems consisting of N = 500 molecules along different isotherms over the reduced temperature range 0.60 ≤ T ≤ 1.25. As in previous simulation studies of the model, the study identifies nematic and smectic B phases on compressing the isotropic fluid, the particular phase sequence depending on temperature. The nematic phase is found to be stable with respect to the isotropic phase for reduced temperatures T ≥ 0.75. In the temperature range 0.75 ≤ T ≤ 1.25, the phase boundaries of the isotropic-nematic transition are obtained by computing the Helmholtz free energy of both phases from thermodynamic integration. From the simulation data, the relative volume change at the isotropic-nematic transition is about 2%, and this value appears to be rather insensitive to changes in temperature. On compressing the nematic phase, the Gay-Berne fluid undergoes a strong first-order transition to the smectic B phase. This transition is studied by using constant pressure simulation, and the coexistence properties are estimated from the limits of mechanical stability of the nematic phase. Larger relative volume changes are found at the transition than those suggested by previous studies, with typical values increasing up to 10.5% as the temperature is decreased. The results are consistent with the existence of strong coupling between nematic and smectic order parameters. For temperatures T ≤ 0.70 the nematic phase is no longer stable, and the phase sequence isotropic-smectic B is observed. Therefore, the Gay-Berne model exhibits an isotropic-nematic-smectic B triple point. Extrapolating the present simulation data, this triple point is located approximately at reduced temperature T_INB ? 0.70 and reduced pressure P_INB ? 1.825.     

Smectic A order parameter measurements and critical scattering near the smectic A↔nematic phase transition in CBOOA

The temperature dependence of the order parameter for the smectic A phase of CBOOA is determined by measuring, with elastic coherent neutron scattering, the intensity of the Bragg reflection from the smectic layers. Within the experimental accuracy, the transition smectic A? nematic appears continuous in this system. However, a fit with (T_c-T)^2β yields β = 0.18 ± 0.01. Critical scattering is also observed in both phases. It can be described with an anisotropic Ornstein-Zernike law. The correlation length perpendicular to the smectic layers behaves like (T-T_c)^?v, with an exponent v = 0.47 ± 0.06. In contrast, the lateral extension of the layers seems to increase uncritically.     

Effect of aerosil dispersions on the nematic-to-isotropic interface

The effect of silica aerosils on the kinetics of the first-order nematic-isotropic (NI) phase transition is phenomenologically described in the framework of the time-dependent Landau-Ginzburg equation. A steady-state solution to the equation is presented such that the NI interface may propagate with a solitary-like wave profile under constant quenching. The results provide a plausible basis for the interpretation of the dynamical effects of quenched disorder in the liquid-crystal systems, caused by randomly interconnected porous media, such as aerosils. In the low silica aerosil ρ_s ( ≤0.1 g/cm^3) regime, the calculated values of the interface velocity v(T,ρ_s), the interface thickness κ(T,ρ_s), and the critical radius of a spherical nucleus of new nematic phase in a bulk isotropic environment, composed of polar molecules, such as 4-n-octyl- 4^′- cyanobiphenyl and 4-n-heptyl- 4^′- cyanobiphenyl shows that the effect of silica aerosils on the kinetics is reflected in a shifting of the set of temperature-dependent curves to lower temperature values.-1     

The effects of a laser field on phase transitions in liquid crystals

We investigate theoretically some phase transitions in liquid crystals in the presence of a laser beam. We found, in non-absorbing nematics, a laser-induced one-way transition from a paranematic to a nematic phase. In absorbing nematics we found, in addition to this transition, a one-way transition from a nematic to a paranematic phase with increasing laser intensity. Further, we found a reentrant nematic or a reentrant paranematic via paranematic or nematic phase respectively. In the case of smectic A, laser absorption results in a coupling between the positional and orientational orders. As a result, the smectic A to nematic transition can change from second order to first order and the smectic C to smectic A transition can become first-order in the field of a laser.     

Dynamic critical scattering near the smectic-A—nematic phase transition in CBOOA

High resolution neutron quasielastic scattering has been applied to a study on the critical dynamics near the smectic-A—nematic phase transition in CBOOA. A line narrowing is observed for a certain value of the momentum transfer. We interpret this narrowing as a critical slowing down of the smectic order parameter fluctuations near the transition temperature T_SN. The relaxation times vary between 10^-8 and 10^-7 sec for a temperature interval of 2 K below T_SN.     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

Phase transition behavior of hydrogen bonded liquid crystal (6BA)<Subscript>2</Subscript>-(BPy)<Subscript><Emphasis Type="Bold">x</Emphasis></Subscript> as studied by <Superscript>2</Superscript>H NMR

The thermal properties and the orientational order of hydrogen-bonded liquid crystals (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3 (6BA: 4-n-hexylbenzoicacid, BPy: 4,4’-bipyridine) were investigated by DSC and ²H NMR. On cooling, isotropic liquid - liquid crystal phase transition temperatures were T _C= 409 and 405 K for (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3, respectively. Thermal anomalies in the liquid crystal phase were observed at T _LC1= 402 and T _LC2= 375 K for (6BA)₂-(BPy)_0.4 and at T _LC1= 398 and T _LC2= 375 K for (6BA)₂-(BPy)_0.3. For (6BA)₂-(BPy)_0.4, only the smectic component was created above T _LC1. In addition, the nematic component was created below T _LC1. The nematic component gradually changed to the smectic component with decreasing temperature and only the smectic component was observed below T _LC2. For (6BA)₂-(BPy)_0.3, only the nematic component was created above T _LC2. The phase transition from the nematic phase to the smectic phase took place at around T _LC2.     

A simple molecular theory for photo-induced smectic phase

The response of liquid crystals to light is very important for applications of liquid crystals in display and memory devices. Recently experiments have been carried out on liquid crystals doped with photoactive azo compounds. It is seen that UV rays incident on such systems can lower the nematic isotropic transition temperature T _NI . Also, in some mixtures, a photo-induced smectic phase is observed. This is attributed to the change in the trans (longer) isomer to cis (shorter) isomer of the photoactive dopant. We have earlier developed a molecular mean-field model assuming the medium to consist of inter-converting anti-parallel and parallel pairs to explain the molecular origin of “two lengths”. The model was used to explain double re-entrance, the effect of electric field on T _NI , etc. This model is modified to include the change of trans to cis isomer which is equivalent to an increase of fraction of parallel (shorter) pairs. The calculated phase diagram with respect to incident UV radiation energy shows an induced smectic phase. This is in qualitative agreement with experimental trends.     

The McMillan parameter as an effective field variable for the nematic–smectic tricritical point in NO.M compounds

The dependence of the coupling of orientational and translational order parameters on the nematic-smectic A tricritical point (TCP), observed through birefringence experiments in the pure compounds and in the binary mixtures in the homologous series of N(p-n-alkyloxybenzylidene)p-n-alkylanilines, results in a mean field tricritical exponent. The McMillan parameter, M = T _NA/T _NI (where T _NA and T _NI are the smectic A—nematic (AN) and the nematic—isotropic (NI) phase transition temperatures respectively), is used as an effective and relevant field variable in analysing the TCP. The observed linear dependence of the jump in the orientational order parameter with new density variables in the tricritical region is found to be consistent with the results obtained from both the Landau mean field and the decoupled mean field models. The width of the mean field tricritical region is discussed in terms of the McMillan parameter. Further, the McMillan parameter is observed to be characteristic of chemical composition at the NA tricritical point.     

Density studies across the smectic G-nematic and nematic-lsotropic transitions

The densities of N(p-n-pentyloxy benzylidene) p-ethylaniline and N(p-n-hexyloxy benzylidene) p-ethylaniline are measured as a function of temperature from the isotropic liquid to the smectic G phase. Both the compounds exhibit enantiotropic smectic G and nematic liquid crystalline phases. The changes in density across the phase transformations and the thermal expansion coefficient confirm the order of the transitions as of first order. The particular importance of the smectic G to nematic transformation is apparent from the density jump across the transition. An estimate of the pressure dependence of the isotropic-nematic transition temperature is found to be in reasonable accord with the literature data.     

Synthesis and phase transition studies on non-symmetric liquid crystal dimers: N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-benzylidene)-4-chloroanilines

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)benzylidene)-4-chloroaniline containing benzothiazole and benzylideneimine units connected by a flexible alkyl spacer, –(CH₂) _n –, with n ranging from 4 to 12 in even parity have been prepared. All five members of this homologous series exhibit an enantiotropic nematic phase. The compounds with greater n of 8–12 exhibit both nematic and smectic phases upon cooling. A notable feature among this series is that for the member with n?=?10, the smectic–nematic transition is also present. The nematic–isotropic transition temperatures and associated entropy changes with respect to all compounds in this series exhibit a dramatic dependence on the length of the flexible spacer. A comparison of the transitional properties of this series with those of α-(4-benzylidenechloroaniline-4′-oxy)-ω-[4-(thiophene-2-carboxyl)benzylideneaniline-4′-oxy]alkanes reveals that replacing benzothiazole moiety at one side of the flexible alkyl spacer reduces the nematic–isotropic transition temperature.     

X-rays investigations of the smectic A1 - smectic A2 transition

We confirm by X-rays studies on aligned samples the existence in a binary system of a smectic A - smectic A transition. We find that the smectic layer periodicity is doubled from the higher phase S_A1 to the lower one S_A2. The observation of diffuse scattering planes in nematic and S_A1 indicates the existence of pretransitional S_A2 effects and we suggest an analogy between the S_A1–S_A2 transition and the classical para-antiferroelectric transition in crystals.