Smectic F Phases in the Terephthalyidene-Bis-4-n-Alkylanilines

Recently terephthalylidene-bis-4-n-pentyl-aniline (TBPA) was reported to have two types of tilted smectic B (smectic H) phase. However, our study of this material shows that this compound has a smectic F phase and a smectic H phase, and not two H phases as previously thought.     

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.     

Characterization of the Hexatic B and Crystal B phases by Optical Microscopy

Recent investigations concerning the structure of the B Phase have unearthed a variety of packing arrangements of the molecular layers for this semctic modification. Two general structures have been discovered; one in which the B Phase has three-dimensional crystalline order (crystal B), and one that is characterized by short-range in-plane positional ordering of the constituent molecules, and long-range bond orientational order, both within and between the layers (heatic B). In this present study n-hexyl 4′-n-pentyloxybiphenyl-4-carboxylated (650BC) has been synthesized and its smectic phases charaterized. Typically, this ester exhibits an A, B,E, phase sequence, with the B Phase being of the hexactic type. The Synthesis, microscopic textures, and miscibility studise ar reported here. The results show that while the two types of B phase appear co-miscibly by conventional optical mehtods, in fact X-ray diffraction studies show that a transition occurs between the two phases in certain binary mixtures. Thus, the hexactic B and crystal B modification are thermodynamically distint phasaes.     

Induced SA phases in binary systems with EDA complexing

The phase diagrams of 13 binary systems are presented in which electron donor-acceptor (EDA) complexing is clearly detectable. In all binary systems studied the same electron donor compound was used whereas the electron acceptor compounds were members of two homologous series. It was found that the induction of smectic A phases increases with increasing length of the alkyl chain of the acceptor compound and with increasing stability of the complexes.     

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.     

Studies on the Smectic Phase of Some Schiff's Bases With a Terminal Trifluoromethyl Group

Some terminally substituted trifluoromethylbenzylideneanilines have been synthesized with a view to investigate an unidentified smectic phase which exhibits an unusual texture as observed under the polarizing microscope. This phase was first reported by Griffin et al.² for the compound N-(4-n-decyloxybenzylidene)-4'-trifluoromethylaniline. In a comparable compound but with reversed imine linkage the nature of this smectic phase remains unchanged. Also if the terminal n-decyloxy group is replaced by an n-decyl group the nature of the smectic phase remains the same except that it is observed at a considerably lower temperature. We observe that this smectic phase has uniaxial symmetry when studied with a polarizing microscope as well as with NMR techniques; the latter showing an order parameter S = 0.8 for this phase. X-ray diffraction patterns show this phase to be crystal smectic B.     

Filled SA phases in binary liquid crystalline systems of terminal-nonpolar compounds (III). The role of the molecule length of the rod-like component

The phase diagrams of 8 binary systems of liquid crystals are presented whereby one component is a “biswallow tailed” compound and the second component is a member of the homologous series of 4,4′-bis-n-alkyloxy-azoxybenzenes. In all phase diagrams studied induced smectic A phases occur. X-ray investigations give evidence that in the binary systems of short-chain homologues the layer spacing d of the smectic A phase is clearly greater than the average molecular length L̄, whereas in the systems of long-chain homologoues d is found to be somewhat smaller than L̄. The results are interpreted on the basis of a simple packing model.     

Structure parameters in mixed liquid crystals (I). Layer thickness of the smectic mixed phases B in the binary system of ethyl-4-(4-ethoxy-benzylidene-amino)-cinnamate and terephthylidene-bis-(4-n-butyl-aniline)

The thickness of the smectic layers in binary liquid crystalline mixed phases of the phase type B are measured in dependence on the concentration. Both components have different B-phase structures. In the part of the binary system with a smectic polymorphism BA the long molecular axes are perpendicular to the layer planes. If the polymorphism is changed by the appearance of a mixed phase of the type C, i.e. the smectic polymorphism BCA exists, there occurs a transition into the B-phase structures with tilted axes, the tilt angle of which is changed continuously with the concentration.     

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.     

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.     

Density Studies in Polymorphic Liquid Crystal N- (p-n-Heptyloxybenzylidene) p-n-Pentylaniline

Density studies in polymorphic liquid crystal, N-(p-n-heptyloxybenzylidene) p-n-pentyl-aniline is carried out in isotropic liquid, nematic, smectic A, smectic C, smectic B and smectic G phases. The temperature variation of density in all the mesophases confirms that isotropic liquid to nematic, nematic to smectic A, smectic C to smectic B, smectic B to smectic G transitions are of first order while smectic A to smectic C transition is of second order. The density variation across smectic C to smectic B is greater than that of any other transition. Thermal expansion coefficients are also calculated.     

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.     

Discontinuous Dielectric Reorientation in the Smectic C Phase of 3-n-Heptyl-6-[4-n-hexyloxyphenyl]-1,2,4,5-tetrazine

In the nematic phase and smectic C phase of 3-n-Heptyl-6-[4-n-hexyloxyphenyl]- 1,2,4,5-tetrazine the Freedericksz transition was studied. In both phases the threshold voltage and the switching times were measured. In the smectic C phase two thresholds were observed which can be explained by an asymmetric chevron structure.     

Density Studies in Terephthalylidene-bis-p-n-dodecylaniline

The temperature dependence of density in Terephthalylidene-bis-p-n-dodecylaniline (TBAA12) is carried out to investigate the phase transitions, associated volume jumps, order of the transitions and to estimate the pressure dependence of transition temperatures and pretransitional effects. The compound exhibits smectic C, smectic I, smectic F and smectic G phases. The isotropic-smectic C and smectic C-smectic I transitions are found to be first order from the density jumps and the calculated thermal expansion coefficient data also supported the density results. The smectic I-smectic F transition is detectable by a change of slope in density studies, which is a second order transition inferred from thermal expansion coefficient variation with temperature.     

Density and Ultrasonic Velocity Measurements in Hexyloxybenzylidene Phenylazoaniline

The temperature variation of density and ultrasonic velocity of the liquid crystal hexyloxybenzylidene phenylazoaniline are reported. The density across the smectic A—smectic B transition is more predominant than the other transitions. The density variation with temperature and the calculated thermal expansion coefficients suggest that the transitions isotropic liquid—nematic, nematic—smectic A and smectic A—smectic B are of first order. Anomalous behaviour of ultrasonic velocity is observed across the isotropic liquid—nematic transition and prominent dips in velocity are observed at the nematic—smectic A and smectic A—smectic B transitions. The adiabatic compressibility (β _ad ) Rao number (R _a ) and molar compressibility (B) are estimated using the experimental density and ultrasonic velocity.     

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.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

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.     

Phase transition behavior and structure of the thermotropic liquid crystal 6‐{[(4′‐{[(undecyl)carbonyl]oxy}biphenyl‐4yl)carbonyl]oxy}‐1‐hexyne

The phase transition behaviors and corresponding structures of 6‐{[(4′‐{[(undecyl)carbonyl]oxy}biphenyl‐4yl)carbonyl]oxy}‐1‐hexyne (A4EE11) were investigated using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and wide angle X‐ray diffraction (WAXD). In comparison with the published homologues, 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)carbonyl]oxy}‐1‐pentyne (A3EO7) which shows a monotropic smectic A (SmA) phase and a metastable monotropic smectic C (SmC) phase; 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)oxy]carbonyl}‐ 1‐pentyne (A3E'O7) that exhibits three enantiotropic stable liquid crystalline (LC) phases, SmA phase, SmC phase and smectic X (SmX) phase; 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)carbonyl]oxy}‐1‐undecyne (A9EO7) which has a monotropic SmA phase and a metastable crystal phase, A4EE11 integrates the enantiotropy, monotropy and metastability of the LC phases of those three compounds. Upon cooling from isotropic state to room temperature, in the temperature range of 62.0 to 58.5°C, A4EE11 shows an enantiotropic smectic A (SmA) phase with a layer spacing d=32.69Å. Further lowering the temperature, it enters into a metastable monotropic smectic B (SmB) phase with a longer layer spacing d=34.22Å which has a tendency towards crystallization. The metastability of the liquid crystalline phase may associate to the linkage order of the ester bridge between the mesogenic core and the flexible spacer. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Mesomorphic Properties of 2,5-Di-(4-N-Alkyloxyphenyl)Thiazole[5,4-d]Thiazoles

The synthesis and measurement of physical properties using DSC, X-ray and optical methods are described for the 2,5-di-(4-alkyloxyphenyl)thiazolo|5,4-d|thiazoles with the alkoxy chains varying from 3 to 8 carbon atoms. The phases observed were principally nematic and smectic C phases but for seven and eight carbons the appearance of a highly ordered smectic phase was observed.