smectic-A smectic-C Research Articles

Critical behavior at the isotropic to nematic, nematic to smectic-A and smectic-A to smectic-C phase transitions in a pyrimidine liquid crystal compound

High-resolution optical birefringence (∆n) measurement of a pyrimidine liquid crystal compound having nematic, smectic-A and smectic-C phases are reported. The high-resolution ∆n data are rather successful in assessing the critical anomaly at different phase transitions in the investigated compound with a reasonably good accuracy. The critical exponent β, describing the limiting behavior of the nematic order parameter close to the isotropic–nematic (I–N) phase transition, is found to be in good agreement with the tricritical hypothesis. The critical behavior at the nematic–smectic-A (N–Sm-A) and the smectic-A–smectic-C (Sm-A–Sm-C) phase transitions has been explored with the aid of a differential quotient extracted from the ∆n values. The yielded effective critical exponent α′ is appeared to be nearly tricritical in nature for the N–Sm-A phase transition. For the Sm-A–Sm-C phase transition, α′ exhibits a weak dependence on the fit range and assumes tricritical value for large temperature range considered, which again is found to be diminished slightly with reduction in the temperature range. Related critical amplitude quotient and corrections-to-scaling quotient are found to display deviations from the theoretical models. Such behavior signals the appearance of a non-Landau character for the orthogonal to tilted smectic phase transition in the investigated compound.

Tuning the mesomorphic properties of carbosilane-terminated and conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole liquid crystals as binary mixtures with bimodal length distributions

The phase diagrams of binary mixtures of smectic C (SmC) mesogens – the carbosilane-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl)-1,3,4-thiadiazole (QL13-6) and one of nine conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole mesogens (QL14-m/n) – with a molecular length ratio QL13-6/QL14-m/n ranging from 1.24 to 1.46 show the induction of a smectic A (SmA) phase in the mole fraction range 0.1 ≤ xQL13-6 ≤ 0.6. The results show that the temperature range of the induced SmA phase is remarkably sensitive to the molecular length ratio, and that this approach to tuning the mesomorphic properties of SmA–SmC liquid crystal mixtures may provide a powerful new tool in the formulation of mixtures for SSFLC and electroclinic applications.

Smectic-A–smectic-C–smectic-C⁎ Lifshitz point in mixtures of chiral and achiral smectic liquid crystals

A phenomenological theory to describe the smectic-A–smectic-C transition, smectic-A–smectic-C⁎ transition and smectic-C–smectic-C⁎ transition in liquid mixtures is proposed. The influence of the concentration of the chiral liquid crystal on these phase transitions is discussed by varying the coupling between various order parameters. The phase diagrams of the thermodynamic parameters, wave vector, helix pitch, tilt angle and polarization are studied and a possible smectic-A–smectic-C–smectic-C⁎ Lifshitz point is predicted. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a Lifshitz point is observed in the phase diagram. Calculations based on this model agree qualitatively with experiment.

Multicriticality in smectic liquid crystals

A phenomenological model permitting a unified description of different types of smectic liquid crystalline phases including the hexatic-B phase has been developed. The model describes five different liquid crystalline phases: nematic (N), smectic-A (SmA), smectic-C (SmC), hexatic-B (HexB) and smectic-E (SmE). We observe various multicritical points among different smectic phases. General arguments are presented for the topologies of the phase diagrams in the vicinity of the SmA–SmC–HexB and N–SmA–SmC bicritical points and in the vicinity of the SmA–HexB–SmE, SmA–SmC–SmE and SmC–HexB–SmE triple points. The existence of the SmA–HexB, SmA–SmC and SmC–HexB tricritical points near the SmA–HexB–SmE, SmA–SmC–SmE and SmC–HexB–SmE triple points is discussed. Calculations based on this model agree qualitatively with the experiments.

Carbon nanotube-induced chirality in an achiral liquid crystal

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.

Static and dynamic critical behaviours in de Vries liquid crystals

Measurements of dielectric dispersion have been carried out on two liquid crystal compounds 8422[2F3] and 8O23[7F8-], showing non-layer-shrinkage (NLS) behaviour through the smectic-A (Sm-A)–smectic-C (Sm-C) transition. The critical exponent γ for the susceptibility determined from the data lies in the range 1.9–2.0 for 8422[2F3] and 1.1–1.2 for 8O23[7F8-]. While the former agrees with the general trend reported so far for this group of liquid crystals, the latter result is distinct from them, the γ value being definitely smaller. These observations indicate that the Sm-A–Sm-C transition with NLS behaviour can exhibit quasi two-dimensional Ising critical behaviour under certain conditions. The critical exponent Δ for the relaxation frequency has also been obtained as Δ = 2.0–2.1 for 8422[2F3] and 1.2–1.3 for 8O23[7F8-]. The results support the general expectation Δ > γ rather than the conventional behaviour Δ = γ.

Investigation of the Smectic-A–Smectic-C* Transition in Liquid Crystals by Adiabatic Scanning Calorimetry

We performed Adiabatic Scanning Calorimetry (ASC) measurements on two ferroelectric liquid crystals: S-(-)-4-(2′ -methylbutyloxy)phenyl 4-n-octyloxybenzoate and S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4′-carboxylate. Our experiments showed that no latent heat was present for the smectic-A – smectic-C* (AC*) transition to within the experimental resolution. Therefore the AC* transition in both samples can be classified as a continuous one. By fitting the specific heat data we showed that this phase transition can be described by an extended mean-field model.

Influence of dispersed aerosil particles on the collective dynamic modes in a ferroelectric liquid crystal with polarization sign reversal

Broadband dielectric spectroscopy was applied to study the influence of the dispersed hydrophilic aerosil particle densities on the second-order smectic-A–smectic-C* phase transition and polarization sign reversal in the ferroelectric liquid crystal S-(−)-2-methylbutyl 4- n-nonanoyloxybiphenyl-4′-carboxylate. With increasing aerosil concentration the dielectric strength of the soft and the Goldstone mode considerably decreases and the complete disappearance of the Goldstone mode at ρ s = 0.20 g/cm 3 occurs along with significant broadening of the soft mode. The characteristic frequency of the Goldstone mode slightly increases with increasing density of the silica particles. The frequency degeneracy taking place in the smectic-A–smectic-C* phase transition is removed in the presence of aerosil and an increase in the frequency gap is observed. In a point of polarization sign reversal a rapid drop of the dielectric strength and the disappearance of the Goldstone mode was detected for bulk and confined ferroelectric liquid crystal. However, with increasing concentration of aerosil particles the dielectric strength of the Goldstone mode decreases as a function of temperature stronger below than above the sign reversal point. The characteristic frequency of the Goldstone mode below the point of polarization sign reversal decreases with increasing aerosil concentration.

Low Frequency Dielectric Relaxation, Spontaneous Polarization, Optical Tilt Angle and Response Time Investigations in a Flourinated Ferroelectric Liquid Crystal, N125F2(R*)

Experimental investigations of thermal microscopy, low frequency dielectric relaxation, spontaneous polarization P s (T), optical tilt angle θ(T) and response times τ s (T) are carried out in a fluorinated ferroelectric liquid crystal, N125F2(R*) exhibiting smectic-A and smectic-C* phases. Dielectric loss in the smectic-A phase exhibits single Debye's relaxation in the MHz region. The smectic-C* phase exhibits two relaxations, viz., a Goldstone mode at ∼500 Hz and another at a much higher frequency, ∼5 MHz. The Arrhenius shift of Smectic-A relaxation frequency (f R ) gives an activation energy of 1.5 eV. The influence of temperature and applied voltage on the smectic-C* Goldstone mode relaxation is studied. In the smectic-C* high frequency relaxation mode, the relaxation frequency (relevant to the tilt) is found to increase with decreasing temperature. The temperature variation of the reciprocal of the dielectric strength (1/Δϵ s ) above the smectic-A—smectic-C* phase transition qualitatively supports the Curie-Weiss law. Temperature variation of primary (tilt) and secondary (P s ) order parameters are presented. The temperature variation of τ s in the smectic-C* phase studied through square wave technique is presented. The influence of fluorine atoms on the rigid core part of the present ferroelectric liquid crystal on the physical properties exhibited in it's smectic-C* phase is discussed.

Surface Transitions, Critical Adsorption, and Spontaneous Polarization Inversion in Freely Suspended Liquid-Crystal Films

A summary is given of recent results of ellipsometric measurements on freely suspended films, between two and several hundred molecular layers thick, of 2-methylbutyl 4-alkanoyl-oxybiphenyl-4′-carboxylate liquid-crystal compounds showing smectic-A and ferroelectric smectic-C phases. The results concern in particular: the sign inversion of the spontaneous polarization in thin films, the thickness divergence of the surface-induced smectic-C order on approaching the bulk smectic-A - smectic-C transition in very thick films, and the behavior of the smectic-A - smectic-C surface transition in thin and thick films. Special attention is paid to new observations the origins of which are still to clarify.

Thin and ultra thin smectic C* films: An X-ray reflectivity study

Abstract We report on X-ray reflectivity studies of thin films of the commercial ferroelectic smectic C mixtures (FELIX 008, Hoechst), prepared by spin coating on various substrates. Upon casting, the films ordered spontaneously with smectic layers parallel to the substrates. This alignment is observed for many different substrates suggesting that it is induced by interactions at the film-air interface. The smectic C phase is maintained even in films of a few smectic layers. However, the smectic-A—smectic-C phase transition temperature, Tc, and the smectic layer spacing are affected by the interfaces and finite size effects. We demonstrate an interesting dynamical effect immediatly after the quenching and the solvent evaporation. The force between the interfaces decay as 1/d3 (d being the film thickness) suggesting a van der Waals (v.d.W) type of force. However, its magnitude and probably the sign are inconsistent with conventional models.

Physical Properties of Schiff's Base Esters:n-Alkyl 4-[4′-(4″-n-octyloxybenzoyloxy)benzylidene] aminobenzoates

Abstract A homologous series, n-Alkyl 4-[4′-(4″-n-octyloxybenzoyloxy)benzylidene] aminobenzoates, exhibiting nematic (N), smectic A (SA) and smectic C (SC) trimorphism has been prepared. The liquid crystal behavior of the series is compared with that of a corresponding series, n-Alkyl 4-[4′-(4″-n-octyloxybenzoyloxy)-3-methoxy benzylidene] aminobenzoates. The effect of a lateral methoxy substitution on mesomorphic properties of the series is discussed. Physical properties, such as, transition temperatures, transition heats, optical textures, densities, refractive indices and the orientational order parameter are studied. The SA–SC phase transition is found to be second order. The SC phase appears to show a temperature dependent tilt angle. A binary phase diagram of one of the members of the series with terephthalylidene-bis-(4-n-butylaniline) (TBBA) is presented. It serves as an exception to the miscibility rule and also gives an induced smectic B phase.

The existence of a tricritical point in the smectic A-smectic C transition line

In the light of a recent x-ray study on the smectic layer spacing in the vicinity of the smectic A-smectic C (SmA-SmC) transition of the TBBA (terephthal-bis-(4n)-butylaniline) homologous series, a Landau theory with coupled order parameters is proposed to provide a unique explanation of the x-ray data. It is found that as the length of the alkyl-tails increases the SmA-SmC transition change from being second-order to being first-order with a tricritical point in between.