First Truly Ferroelectric Liquid Crystal

The first faroelectric liquid crystalline phase (in 8SI* (ΔS-(+)-(4-(2′methylbuty1) phenyl-4′-n-octylbiphenyl-4 carboxylate) is reported and its physical properties are discussed. The close connection between the phase sequences of 8SI* and its racemate are established. Both compounds show a number of smectic phases above the newly discovered ferroelectric phase and its analogue in the racemate. These phases (CI and G') are tilted in the racemate 8SI and tilted with a helicoidal structure in 8SI*. In the truly ferroelectric phase, X, the director of the molecules can be oriented in a bistable way in an electric field Furthermore when the field is turned off the director orientation remains unchanged. This behaviar is fundamentally different from that in bulk smectic phases which support the helix structure in the absence of external forces. A model far ‘racemates’ in higher smectic phases like G, G', H’ etc as well as a the newly discovered ferroelectric phase is presented. The response of cholesteric blue phases of 8SI* to an external field is discussed in an appendix.     

Tilt Angle Determination of a Smectic C Phase by Field-Induced Freedericksz Transition and x ray Investigations

A smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates. When an electric field was applied normal to the director direction a dielectric reorientation (Freedericksz transition) occurred. Dissolving a dichroic dye in the smectic C phase and applying an electric field, a guest host effect similar to nematic phases was observed. From the dielectric reorientation the tilt angle of the smectic C phase was determined at different temperatures. The tilt angles obtained by the electrooptical measurements agree sufficiently with the values measured by x ray investigations.     

A Novel Method for Study of a Smectic Phase Obtained from a Strongly-Deformed Nematic Texture

A simple method for the growth of conventional or reentrant smectic phases from strongly-deformed conventional or reentrant nematics, is proposed. It permits the formation of surface-induced and surface-governed conventional and/or reentrant smectic textures which can be studied from the scientific point of view or can be applied in the laser-addressed thermo-optic smectic liquid crystal storage displays for the creation of static figures. The simple method proposed might be applied for study of many smectics arrising either from conventional or reentrant nematic phases under cooling or heating.     

Field-induced Colour Change of Liquid Crystalline Dyes

Planar oriented nematic layers and also partially oriented smectic C phases of certain diphenyl-1,2,4,5-tetrazine compounds due to their positive dielectric anisotropy can be oriented by an electric field (Freedericksz-transition). The orientation is connected with a distinct change of absorption respectively colour. The mechanism of the effect is discussed. For both liquid crystalline phases the response times and decay times were measured.     

Effects of electrohydrodynamic instability in smectic C liquid crystals

The effects of electrohydrodynamic instability (EHDI) in smectic C liquid crystals in an electric field are studied. The objects of study are oriented layers of para-hydroxybenzoic acid possessing a smectic phase. The observed effects are divided into three groups according to the character of the motion of molecular centers of mass and the orientation of the director and the smectic C liquid crystal layers. The instabilities of the azimuthal and Kapustin-Williams domains are experimentally found. The experimental cell consists of two glass plates with tin dioxide electrodes separated by Mylar spacers of a specified thickness. Various schemes of domain rotation are considered. The EHDI is found to depend on the layer geometry.     

A special shear method of alignment for smectic liquid crystals

A special shear method is presented for the planar alignment of smectic liquid crystals. The shear was applied at the isotropic-smectic A or cholesteric-smectic A phase transition temperature, when the “batonnets” floated freely in the isotropic or cholesteric fluid. This type of shearing provided good alignment in the broad range of sample thickness (up to 100 μm) with monodomain areas of about one square centimeter.     

Dielectric relaxation in nematic,smectic A and C phases in the MHz region

The dielectric properties of four 4-n-alkyloxyphenyl 4-n-alkyloxybenzoates with nematic, smectic C and partly additional S_A phases were investigated. In 2 substances for ϵ′ reproducible values also in the smectic phases resulted allowing to conclude on a good orientation in the samples. In all 4 substances the dielectric relaxation in the MHz region was measured and the respective activation energies were determined. The molecular mobility in the S_C phases is higher than in the S_A phases, however comparable with that of the nematic phases. In all 3 phase types a single relaxation mechanism exists, which is explained as rotation of the molecules around a short axis.     

Phase transition studies of the liquid crystal 4-[4-n-hexyloxybenzylidene amino] azobenzene thermal microscopy and DSC

The phase transition temperatures of the liquid crystal 4-[4-n-hexyloxybenzylideneamino]-azobenzene have been investigated by the thermal microscopy and the Differential Scanning Calorimetric (DSC) studies. These studies confirm that this mesogen exhibits nematic, smectic A and smectic B phases. The transition enthalpies, transition entropies and also the textures exhibited by the different phases are presented.     

Thermal Microscopy and DSC Studies of 4-[4-n-pentyloxybenzylideneamino] Azobenzene

Phase transition studies of the liquid crystal, 4-[4-n-pentyloxybenzylidene–amino]–azobenzene (PBAAB) have been investigated by thermal microscopy and Differential Scanning Calorimetry (DSC) studies. From these studies it is confirmed that the compound, PBAAB exhibits nematic, smectic A and smectic B phases. The textures exhibited by different phases are presented. The enthalpies and entropies at various phase transitions are also reported.     

The Smetic Polymorphism and the Phase Transitions in the Liquid Crystal 4, n-Hexyloxyphenyl-4, n′-Decyloxybenzoate

Three different methods — microscopical, depolarized light scattering and conoscopic have been used for the investigation of the polymorphism of 4, n-hexyloxyphenyl-4, n′-decyloxybenzoate (HOPDOB) at homeotropic orientation. A new textural transition is registered during cooling in smectic C phase connected with a change in the tilt (ω) and azimuthal (φ) angles of the molecular director. At cooling we found smectic E phase and by the conoscopic method we determined this phase as tilted smectic type (E_t) with 24° tilt angle. It is also found that B phase is a normal type smectic liquid crystal — B_N.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

Schlieren textures in smectic liquid crystals

The details of well developed schlieren textures have been observed at smectic C and B modifications. In these textures whole numbered point singularities occur and inversion walls with strong similarities to nematic schlieren textures. It is shown that the occurrence of smectic schlieren textures is due to suppositions derived theoretically by Nehring and Saupe. These suppositions are: tilted structure and plane layers with the layers parallel to the supporting glass slides. In smectic B inversion walls occur which allow to conclude on variable tilt angles of the structure.     

Induced smectic A phase by homeotropic orientation in 4, n-octyloxybenzoic acid (OOBA) at nematic-smectic C phase transition

Using homeotropic and quasi-homeotropic orientation in thin film of 4-n-octyloxybenzoic acid, we indicated intermediate smectic A-like state during the nematic-smectic C phase transition. Pseudoisotropic and focal conic textures are characteristic for homeotropic and quasihomeotropic orientations respectively. The smectic A-like state induced by the boundary conditions exists in a very short (about 0.5 °C) temperature region.     

Linear Double Hydrogen-bonded Thermotropic Liquid Crystals Formed Between Oxaloacetic Acid and p-n- Alkyloxy Benzoic Acids

Double hydrogen-bonded thermotropic liquid crystal complexes (DHBLC) have been isolated from molar ratios of Oxaloacetic acid (OAC) and eight alkyloxy benzoic acids (nBAO) whose carbon number varied from pentyloxy to dodecyloxy. The complexes are referred as OAC+nBAO where n varied from 5 to 12. In each of the synthesized complexes, complementary hydrogen bonds formed are confirmed by FTIR study and the complexes are further investigated by Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) for the construction of phase diagram. Characteristic phases like nematic, smectic C, smectic F, and smectic G are characterized by various textures. From DSC studies, odd–even effect is evinced in transition temperatures across isotropic to nematic phase transition and across smectic F to smectic G phase transition in enthalpy values. The order of all transitions observed in eight complexes is calculated by thermal analysis. The magnitude of optical tilt angle in smectic C is fitted to a power law and the scale of critical exponent is found to concur with the Mean Field theory predicted value.     

Investigation of Ultrasonic Velocity and Related Parameters in Liquid Crystalline Mixtures

Investigations of ultrasonic velocity, specific volume and related parameters were carried out in a mixture of p-n-butoxybenzylidene-p-chloroaniline (BBCA) and p-anisal-p-toluidine (pApT) at eight different concentrations. BBCA exhibits polymorphism with two smectic phases, viz., smectic B and smectic A, while pApT is non-mesomorphic. All the mixtures studied were nematic, the mesomorphism vanishes below 34.3 mole percent of BBCA. Specific volume showed a large increase in the vicinity of the phase transition and the ultrasonic velocity showed an abrupt minimum near the transition. The adiabatic compressibility showed a maximum near the phase transition. It was found that the dip in ultrasonic velocity, change in specific volume and the jump in adiabatic compressibility at the transition decreases with increasing concentration of the non-mesomorphic compound. The parameter, viz., molar sound velocity was also estimated. The coefficient of thermal expansion and adiabatic compressibility were also estimated and they were found to attain very high values near the phase transition. The experimental results are explained on the basis of de Gennes theory of pre-transitional effects generalized by Bendler. The critical exponents are estimated in the isotropic as well as in the nematic phases.     

Liquid crystal–gold nanoparticle composites

Nanoparticles (NPs) have emerged as extremely promising materials to alter and improve the properties of liquid crystals (LCs) used, for example, in device applications. In this paper, we summarise recent work from our lab that aims to provide a fundamental understanding of structure–property and composition–property relationships governing LC–NP interactions, which may point to new directions for major improvements in the efficiency of LCs used in display applications. A variety of LC hosts (phases) doped with surface-functionalised gold NPs have been systematically studied ranging from one-dimensionally ordered nematic over two-dimensionally ordered smectic to three-dimensionally ordered columnar phases. Significant progress with respect to LC–NP interactions was made for NP-doped nematic phases. Here, the observation of an unusual texture for Au NP-doped nematic LCs, that is, the formation of birefringent stripe textures and the induction of homeotropic alignment of the nematic LC similar to chiral finger (or fingerprint) textures, provided the basis for numerous experimental studies using Au NPs with different core sizes and surface functionalities.     

On Packing in Smectics: X-Ray Diffraction Study on the Homologous Series 4, 4′-di-n-alkyl-azoxybenzenes

The smectic layer spacings of the homologous series 4,4′-di-n-aLkyl-azoxybenzenes with alkyl chain length from m=6 to m=10 were determined and a mean layer spacing increment of 0.109 nm per methylen group was found.

The results are discussed in relation to two different models:

(I) The diffuse cone model of A. de Vries. Differences between layer spacing and molecular length are attributed to orientational disorder leading to mean tilt angles up to 18° of the molecules.

(II) The second model differentiates between the directions of the rigid aromatic core and that of the alkyl chains. Orthogonality of the rigid core long axis to the smectic layer planes and therefore tilting of the alkyl chains results in coincidence between calculated layer thickness and measured smectic layer spacings.

X-ray film erposures of oriented samples clearly demonstrate the S_A -type of the phases and confirm a slight odd-even effect of layer spacing increments.     

Liquid crystals,fractals and art

We discuss the aesthetic appeal of liquid crystal textures and take a brief polarising microscopic tour through the wealth of the appearances of nematic, cholesteric, smectic and banana phases. Some examples are given on how these textures may be used in a creative process employing digital image processing to produce ‘computer art’, including images exhibiting self-similarity. A discussion of the ‘art’ of computer-generated mathematical fractals leads us to examples of fractal structures observed in the phase-ordering process of some liquid crystalline phases. This includes a first brief report of dilatation invariance observed for aggregates of a conventional SmC phase and percolation simulations to quantitatively explain the obtained textures. The circle between liquid crystals, fractals and arts is closed by a discussion of self-similarity in modern arts, especially works of the movement known as abstract expressionism. More than 100 paintings of the artists Wassily Kandinsky, Jackson Pollock, Mark Tobey and Franz Kline were analysed, placing an emphasis of the discussion on the ‘drip-paintings’ of Pollock in relation to recent reports.     

2H-NMR Spectral Patterns in Magneto-Aligned Smectic E and G Phases

Unique ²H-NMR spectral patterns can be obtained from biaxially ordered smectic E and G phases, where only one director of the phase is aligned by the magnetic field. Two effects on the shape of the deuterium spectral pattern are demonstrated: (1) The effect of a motionally induced asymmetry parameter in the time averaged deuterium quadrupole interaction, and (2) the effect when the principal z-axis of the time averaged interaction is not parallel to the aligned director. Spectral patterns for the S_G phase of the compound 4-n-pentoxybenzylidene-4-n'-heptylaniline selectively deuterated on one of the aromatic rings (50.7-d ₄) were obtained for both unaligned and magneto-aligned samples. Both of these spectral patterns were consistent with that expected for an asymmetry parameter, n < 0.2. Spectral patterns in the S_G phase of the compound 50.6 (24.7 wt.%) + 90.4-αd ₂-γd ₂ (75.3 wt.%) were studied showing a different behavior with respect to the spectral patterns obtained in 50.7. We believe that a shift in the orientation of the z principal axis relative to the sample c axis has occurred in this S_G phase. Spectral patterns were also recorded for the S_E phase in magneto-aligned samples of the mixture of compounds 4-cyano-4'-octyloxybiphenyl (80CB) with 50 wt.% 40.8-d ₄. These data also show a small asymmetry parameter but, in addition, a distribution in the orientation of the principal z-axis indicating that the time scale of the motion of the long molecular axis is in a regime where it is of the order of the ²H-NMR measurement. Models for the smectic E and G phases based on these observations are discussed.     

Thermal and optical studies on induced smectic phases of inter molecular hydrogen bonded liquid crystals between decyloxy benzoic acid and citric acid

Hydrogen bonded liquid crystal complex (HBLC) is prepared from mesogenic 4-decyloxy benzoic acid (10OBA) and aliphatic nonmesogenic citric acid (CA). Liquid crystal (LC) phases are investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) studies. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR) validate the intermolecular complementary, cyclic type of hydrogen bond (HB) and molecular environment in the designed HBLC complex. Mesomorphic phases like nematic, smectic X (Sm X) and smectic G (Sm G) are characterized by various textures and using different techniques such as POM, DSC and optical tilt angle measurements. Thermal span width and thermal stability factor for the observed phase is calculated. The complexes are prepared in different mole ratio and their corresponding influences on the phase transitions are discussed. Also it is observed that the HB units play a vital role in stabilizing the new Sm X phase. The variation in thermal stability of smectic phases due to the influence of aromatic cores and length of end chain in the different mole ratio of the present HBLC complexes are also discussed. The variation of tilt angle with respect to temperature in the smectic phase has been experimentally calculated and analyzed. The lowered melting and clearing transition temperatures and extended thermal span width in the Sm X phase are also reported.