Optical activity measurements in the smectic blue phases

Optical activity measurements have been performed on the smectic blue phases ( BP _Sm), which are a new kind of chiral liquid crystal. BP _Sm exhibit both three-dimensional orientational order, like the classical blue phases, and smectic positional order. Thus, they can be viewed as the three-dimensional counterpart of the twist grain boundary phases. A comparison with the optical activity of the classical blue phases is carried out, and an estimation of the BP _Sm lattice parameter is reported for the first time. Received 25 May 2001     

Synthesis,characterization and mesomorphic properties of new liquid-crystalline compounds involving ester–chalcone linkages

We report the synthesis and evaluation of thermal behaviour of two novel series of chalcone-based liquid-crystalline compounds. The flexibility in these systems is provided by attaching straight alkoxy chain at one end. All the mesogens have been characterized by spectroscopic methods such as ¹H-NMR, FTIR, mass spectra, UV/Vis, and also elemental analysis. Their thermal behaviour was evaluated mainly by polarizing optical microscopic observation and DSC studies. It has been observed that the majority of the compounds display nematic and/or smectic mesophases. In both the series, compounds having C₃ to C₇ tail display only enantiotropic nematic phase, whereas C₈ and C₁₀ homologues exhibit enantiotropic smectic C and nematic phases. The higher homologues with C₁₂, C₁₄ and C₁₆ tails show the smectic C phase only.     

Divergence du coefficient de viscosite γ1 mesure en phase nematique au voisinage d'une transition smectique a-nematique

The thermal variation of the twist viscosity coefficient γ₁ has been investigated in the nematic phase of two materials with a smectic A phase. The twist coefficient γ₁ shows a critical behaviour in (T-T_c)^?ν, but the value of ν is very dependant of the method used to analyse the experimental results.     

New investigation on the tetragonal liquid-crystalline phase or SmQ

Chiral liquid crystals, for which twist and smectic order strongly compete, can exhibit complex architectures. This is the case of the SmQ phase which shows a high degree of 3D order (tetragonal or hexagonal) as well as a liquid signature. We have combined X-ray scattering and optical observations on a new compound exhibiting a SmQ phase to discriminate between the two models proposed by A.M. Levelut et al. (J. Phys. II 7, 981 (1997)) at the molecular level for the phase with the I4₁22 space group.Received: 9 March 2004, Published online: 10 November 2004PACS: 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures - 61.30.Mp Blue phases and other defect-phases     

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.     

Theoretical investigation of antiferroelectric (SmCA) subphases by hydrodynamical approach

We provide a hydrodynamical approach utilizing time dependent Landau-Ginzburg model (L-G) and the Cahn-Hilliard model (C-H) to investigate antiferroelectric liquid crystals (AFLCs) exhibiting different chiral phases between paraelectric smectic A (SmA^?) phase and antiferroelectric smectic C_A^? phase (SmC_A^?). Introducing conserved and non-conserved order parameters in C-H and L-G models, we have predicted the appearance of a chiral smectic C (SmC^?) phase and a ferrielectric SmC_FI1^? phase (three layers SmC_A^?) in an antiferroelectric phase sequence. The three layers periodicity for SmC_FI1^? phase is studied in detail with a non-uniform layer interactions among smectic layers with strong experimental support. Finally, we provide some theoretical basis for the non-uniformity of our proposed layer interactions.     

Structural investigations on smectic blue phases

Smectic blue phases ( BP(Sm)) are thermotropic liquid crystalline phases which exhibit both three-dimensional orientational order, such as classical blue phases, and smectic positional order. BP(Sm) appear as the three-dimensional counterpart of twist grain boundary phases. X-ray scattering and optical polarizing microscopy provide information on the hexagonal and cubic symmetries of these new phases.     

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T^*_AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q_ij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.     

Smectic-A edge dislocations in a very thin cell

We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d _s) and intrinsic (d₀) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure with dislocations is formed when the condition d ₀/( d ₀/d _s - 1) ∼ 2 is fulfilled, where is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous and takes place when d ₀/(L d ₀/d _s - 1) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10^-6 J/m ² and d ₀/d _s - 1∼ 5 ^. 10^-4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point. Received 21 February 2002     

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.     

Lamellar versus isotropic structures in dilute phases of fluid membranes

In recent years, considerable attention has been focused on dilute phases of fluid membranes in surfactant systems. At the present time, the structure of two different phases have been well characterized. The lamellar phase (L_α) shows long range smectic order and consists of a regular stack of parallel bilayers. At high dilution, its long range order has been shown to be stabilized by the steric interaction between adjacent undulating membranes. The other phase (L₃) is isotropic and shows no long range positional order. Scattering patterns and transport properties strongly suggest that its structure consists of a randomly multiconnected bilayer separating two equivalent subvolumes of solvent. We here discuss the relative stability of these two structures in connection with the elastic properties of the amphiphilic membrane. A general scaling law for the free energy of the L₃ phase as function of the degree of dilution is proposed and is checked against experimental measurements of some of its static and dynamic physical properties.     

The bulk twist action on the single local monodomains of the smectic C liquid crystal

The angular dependence of the surface anchoring energies of the smectic C with temperature independent tilt angle (type C₁) on two surfaces, SiO _x /ITO/glass and holographic diffraction grating, were measured using a twist-cell method. The smectic C monodomains rotation, starting at a critical imposed bulk twist, revealed a surface anchoring breaking. The surface anchoring strengths of the SiO _x /ITO/glass and holographic grating were evaluated. A model for the bulk twist influence on the smectic C monodomain growth was suggested.     

Optical and thermal studies on viscous lamellar smectic phases in binary mixture of DTAC and ortho-phosphoric acid

The binary mixture of two non-mesogenic compounds, namely, dodecyl trimethylammonium chloride (DTAC) and ortho-phosphoric acid (H₃PO₄) exhibits very interesting liquid crystalline smectic phases at large range of concentrations and temperature. The mixture with lower and higher concentrations of DTAC exhibits SmA, SmD, SmB and SmE phases, sequentially when the specimen is cooled from its isotropic phase. Different liquid crystalline phases observed in the mixture were studied using optical microscopic techniques. The temperature variations of optical anisotropy and electrical conductivity have also been discussed. Helfrich potential and elastic moduli have also been estimated in the smectic phase using the Helfrich model.     

Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that ₆ is two-dimensional whereas the positional order in the crystallineS _J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.     

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.     

Phase transition to anticlinic texture in free-standing smectic c films

Experimental data on the optical reflectance of free-standing smectic C films were analyzed within the framework of a phenomenological Landau approach. At a certain temperature T _0N (determined from experimental data), which exceeds the known temperature T _c of the volume phase transition from smectic A to smectic C state, a surface phase transition takes place whereby molecules in the surface layer become sloped relative to the normal of the smectic layers. The transition temperatures T _0N ^s,a for N-layer films possessing synclinic (symmetric) and anticlinic (antisymmetric) textures of the order parameter (tilt angle θ) were determined. A comparison of the theoretical and experimental data allowed all parameters of the model to be determined (including critical indices of the correlation length and the surface order parameter). Three possible models of the transition from the state with transverse polarization (perpendicular to the molecular tilt plane) to the state with longitudinal polarization (parallel to this plane) are analyzed. The transition takes place at low (°–°) values of the order parameter θ in the middle layer of the film.     

Smectic order parameters via liquid crystal NMR spectroscopy: Application to a partial bilayer smectic A phase

Solute molecules were dissolved in the liquid crystal 4-cyano-4??-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes?? and solvent??s orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent??s positional order parameters and solutes?? positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4?C0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.      

Thermal and optical characterization of a novel series of supramolecular liquid crystals

Supramolecular hydrogen bonded liquid crystal (SMHBLC) homologous series is synthesized and characterized. Hydrogen bond is formed between p-n-alkyloxy benzoic acids (nOBA, where n=7 to 12) and Iodo substituted benzoic acid (I_mBA) respectively. The isolated homologs are characterized by various techniques like Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), ¹H Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). From the POM and DSC studies phase diagram has been constructed and discussed. A new smectic ordering, labeled as smectic R, has been characterized which exhibits a ribbon like texture. This phase is observed in a complex pertaining to the undecyloxy carbon number. Tilt angle in smectic R phase and in traditional smectic C of all the complexes have been experimentally deduced and the results are fitted and so obtained β value concurrences with the Mean field theory predicted value.     

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 C_R 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 C_R. Magnitudes of optical tilt angle in smectic C, smectic X and smectic C_R are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic C_R are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic C_R phases have been analyzed.