Critical Behaviour of the Polarization, Tilt Angle, Electric Susceptibility and the Specific Heat Close to the SmA- Ferroelectric SmC (SmC*) Phase Transitions

We study here the tilt angle and the temperature shifts as a function of concentration for the AC* phase transition in a binary mixture, using our mean field model with the biquadratic P2θ2 coupling — and also with the bilinear Pθ and P2θ2 couplings. By expanding the free energy in terms of the tilt angle and polarization, the tilt angle and the temperature shift are evaluated by using the coefficients given in the free energy expansion. By employing a concentration-dependent coefficient, the tilt angle and the temperature shift are calculated as a function of concentration of 10.O.4 for the SmAC* transition in a binary mixture of C7 and 10.O.4. Our calculated values of the tilt angle and the temperature shifts decrease as the concentration of 10.O.4 increases, as confirmed experimentally for the AC* transition in this binary mixture. This indicates that our mean field models studied here are satisfactory to explain the observed behavior of the AC* transition of the binary mixture of C7 and 10.O.4.

The most important goal of our research is to show the influence of the ‘guest’ (bent-core mesogen, 1,3-phenyldicarboxylatebis{4-[(4-octylbenzoyl)sulphanyl]phenyl} [IFOS8], banana-shaped liquid crystal [BLC]) on the ‘host’ (calamitic liquid crystal [CLC], (S)-(+)-1-methylheptyloxybiphenyl-(4-n-octylphenyl)thiobenzoate [MHOBS8]), on the stability and the destabilization of the antiferroelectric B2 and the ferroelectric smectic C* (SmC*) phases, and change of the temperature ranges of other phases in the binary liquid crystal mixtures. This work is focused on polymorphism of three new binary liquid crystal mixtures, exhibiting a ‘guest–host’ (guest liquid crystal–host liquid crystal [GH-LC]) effect. MHOBS8 has, among others, a ferroelectric SmC* phase, and IFOS8 assumes the B2 phase with antiferroelectric properties. The observed properties of the mixtures, such as variation of the phase transition temperatures, spontaneous polarization, tilt angle and switching time, are characteristic of a ‘guest–host’ mixture. The influence of BLC on the character of the interactions within the CLC host is discussed, with particular attention paid to electro-optical properties of the GH-LC mixtures.

Temperature dependence of magnetic and thermal properties of chiral HyFe and HyMn close to phase transitions by using the Landau mean field model

We present a systematic experimental dataset on the temperature dependence of specific heat capacity in a binary mixture of the second and seventh homologous series of 5-alkyloxy-2-(4-nonyloxy-phenyl) pyrimidine(PhP) liquid crystal compound. These binary mixtures exhibit nematic, smectic-A, and smectic-C phases within a concentration range of xPhP1 = 0 to 0.45. The liquid crystalline phases are structurally characterized using synchrotron X-ray diffraction. We determine the apparent molecular length in the nematic phase, smectic layer spacing, average distance between the long axes of molecules, correlation length, and orientational order parameters (<P2> and <P4>) as functions of temperature. The tilt angle in the SmC phase is inferred from the layer spacing data. To examine the critical behavior near the nematic to smectic A(NA) and smectic A to the smectic C (AC) phase transitions, we evaluate the critical exponents: α from specific heat capacity, β from the fitting of the temperature-dependent tilt angle, and ν‖, ν⊥from the temperature-dependent longitudinal (ξ‖) and transverse (ξ⊥) correlation lengths. Modulated Differential Scanning Calorimetry (MDSC) measurements indicate the absence of phase shift, latent heat and imaginary specific heat capacity, suggesting that the AC transitions are second-order for all binary mixtures. The results obtained from heat capacity reveal that both the AC and NA transitions exhibit non-universal behaviours with effective exponents lying between the tricritical and 3D-XY values and follow nearly identical curve with decreasing width of the Sm-A and N phases. The Josephson hyperscaling relation is verified for both the NA and AC transitions in different mixtures. Moreover, knowing the heat capacity critical exponent α and the order parameter critical exponent β, the susceptibility critical exponent γ for the AC transition can be estimated from Rushbrooke equality α+2β+γ=2, with γ values ranging from 1.015 to 1.313, indicating the system's crossover character and apparently validating the Rushbrooke equality.

Information on the properties of liquid crystals (LC) can be derived by using atomic or molecular probes and measuring their NMR spectra. It has emerged that xenon-129 (due to its large polarizable electron cloud; spin ½) is very sensitive to various changes in its local environment. In LCs, this means sensitivity to temperature, density, phase transitions, phase structure, diamagnetic susceptibility, orientational and positional order parameters, and tilt angle. Information about electric field gradients in LCs may be obtained through quadrupolar xenon-131 (spin 3/2). Xenon-131 may also distinguish between uniaxial and biaxial LC phases via the second-order quadrupole shift. This article describes experiments and related theories of 129/131Xe NMR in liquid crystalline samples. Keywords: liquid crystals; xenon; phase transitions; order parameters; quadrupole coupling; second-order quadrupole shift; tilt angle

Orientationally ordered soft matter is exceptionally responsive to a variety of excitations. That’s the basis for its great range of applications.

Review: 18 refs.

A homologous series of liquid-crystalline compounds with an azo-group in the molecular core have been synthesized. A tendency for the layer shrinkage at the SmA–SmC* phase transition to decrease with decreasing alkyl chain length (n) was observed, with one homologue exhibiting almost no change in layer spacing. All studied compounds exhibited the photoferroelectric effect, namely a lowering of the spontaneous polarization and tilt angle in the SmC* phase under illumination by visible light. This effect is stronger for homologues with lower n and the time constant of this process, connected with the trans–cisisomerization of the azo group, follows the Arrhenius law. In addition to these effects, a lowering of the phase-transition temperature to the ferroelectric phase by a few Kelvin also occurs. Under sufficiently high light intensity, the ferroelectric SmC* phase completely disappears. After the light is switched off, the spontaneous quantities are restored to their original values.

We report the dielectric relaxation behavior of a room temperature ferroelectric liquid crystalline material (code name Felix-SCE9) which undergoes paraelectric SmA--ferroelectric SmC* phase transition around T C = 64 - 0.1°C. All measurements have been performed using thin (∼10 w m) indium-tin-oxide coated cell in the frequency range 10 Hz to 1 MHz. The influence of the bias field, applied parallel to the smectic layers, on the parameters characterizing the Goldstone mode in the ferroelectric SmC* phase has been analyzed. Relaxation frequency of the Goldstone mode increases with increasing bias field. Dielectric increments ( j l ) and relaxation frequencies ( x ) of the para and ferroelectric phases are estimated from the fits of the Cole-Cole function of the dielectric spectrum. Thermal variations of dielectric increment and relaxation times suggested a first order SmC*-SmA transition (Tc ∼ 64°C) in Felix-SCE9. Experimental frequency and temperature dependent dielectric constant data follow the generalized Landau theoretical model.

New lactic acid derivatives exhibit paraelectric SmA and TGBA phases, ferroelectric SmC* and TGBC phases and antiferroelectric phases, the TGB and antiferroelectric phases being unusually broad. The polymorphism depends on the number of lactate units in the chiral chain and on the length of the non-chiral chain. The dielectric spectroscopy revealed the characteristic modes in all these phases. In phases with the dipolar order the spontaneous polarization and the tilt angle have been determined.

(S)‐2‐[1‐(2‐Ethylbutoxy)]propanol was designed and synthesized as a chiral building block for the preparation of chiral liquid crystals. Its derivatives, the (S)‐2‐[1‐(2‐ethylbutoxy)]propyl 4‐[4‐(4‐alkoxyphenyl)phenoyloxy]benzoates, EPmPPB (m = 8–12), were prepared and their mesomorphic phases investigated. The results showed that the chiral materials displayed enantiotropic SmA* and SmC* phases, while those with shorter alkyl chains (m = 8–10) displayed an additional unidentified SmX* phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro‐optical response for the materials in the ferroelectric SmC* phase were measured. The electro‐optic responses of the materials in polyimide film‐coated, homogeneously aligned cell exhibit thresholdless, V‐shaped switching in the ferroelectric phase.

Macroscopic properties like tilt angle, pitch, spontaneous polarisation and response times of the mixture of DOBAMBC and SmC have been studied over the ferroelectric range of temperature. The mixture shows a similar temperature dependence as that of DOBAMBC. The tilt angle, spontaneous polarisation and switching speed decrease in the mixture whereas the pitch increases. The effect of adding SmC to SmC* is to dilute its chirality and to control the properties like tilt angle, pitch etc. This behaviour has been attributed to the 'diluting' effect of adding a non-chiral compound (SmC) in a chiral smectic liquid crystal compound DOBAMBC (SmC*). The operating conditions for better electro-optical response are relatively easier to achieve from the device point of view. The operating temperature range increases and appears relatively at lower temperature without much effect on the switching characteristics of the display.

A chlorinated liquid crystalline compound, exhibiting paraelectric SmA, ferroelectric SmC∗, and antiferroelectric SmCA∗ phases as well as sub-phase alpha, has been studied by means of electrooptic and DSC calorimetry methods. The sub-phase alpha has been observed by reversal current method. To this end the response currents have been measured vs. temperature, electric flied strength and frequency of the driving voltage. It has been found that the reversal current exhibits one peak in the ferroelectric SmC∗ phase whereas the antiferroelectric phase shows two separated peaks. In the case of the sub-phase alpha (SmC∗α) a characteristic shape of the response current in the form of a doublet has been observed in a narrow temperature range between the SmA∗ and SmC∗ phases. Spontaneous polarisation and tilt angle have been evaluated as a function of temperature. The electrooptic measurements have been carried out on a single mono-domain grown under a strong electric field during slow cooling from the paraelectric SmA∗ to antiferroelectric phase.

The (E–T) phase diagram of the chiral smectic liquid crystal C7F2 has been established by varying the two thermodynamic parameters: temperature (T) and electric field (E). In its bulk state without field, the studied compound exhibits only the SmA, and the phases. These ground phases are transformed into the ferroelectric SmC* phase under high field with an intermediate ferrielectric phase appearing at lower field in the temperature range around the to phase transition.

Two models exist for the orientational distribution of the long molecular axes in smectic-A liquid crystals: the conventional unimodal distribution and the "cone-shaped" de Vries distribution. The de Vries hypothesis provides a plausible picture of how, at a molecular level, a first-order Sm-A to Sm-C* transition may occur, especially if there is little or no concomitant shrinkage of the layer spacing. This work investigates two materials with such transitions: C7 and TSiKN65. The azimuthal distribution of in-layer directors is probed using IR and polarized Raman spectroscopy, which allows us to obtain orientational order parameters. In C7, we observe a discontinuous change in the order parameter, the magnitude of which is small compared with the corresponding change in the in-layer director tilt angle Theta . Assuming that the smectic-A liquid crystal is of the de Vries type, we calculate the Theta required to reproduce the apparent order parameter <P2>app, obtained from IR, by using the true order parameter <P2>, obtained from polarized Raman scattering. The results indicate that, for C7, the tilt angle so calculated is much smaller than that in the Sm-C* angle and hence de Vries behavior may not be the appropriate explanation in this case. Conversely, we find that TSiKN65 shows a different behavior to C7, which can be explained in terms of the de Vries concept. Thus, we conclude that either type of distribution may exist in Sm-A phases which undergo a first-order transition to the Sm-C* phase. We also discuss the changes in the smectic layer spacing and the orientational order parameters across the Sm-A-Sm-C* phase transition, together with changes in birefringence with applied electric field.