Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a 'quasi-smectic' structure are observed. The films exhibiting the 'quasi-smectic' structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Volumetric,dielectric, calorimetric and X-ray studies of smectogenic 10PBO8 at atmospheric and elevated pressures

The synthesis and pressure–volume–temperature (PVT), differential thermal analysis (DTA), dielectric and X-ray diffraction data of 2-(4-octylcarbonyloxyphenyl)-5-decylpyrimidine (10PBO8) are presented. The substance exhibits two crystalline and smectic C (SmC) phases on heating and a SmC–monotropic crystalline smectic B (SmB_cr) SmB_cr–crystal sequence of phase transitions on cooling. Above ca. 15 MPa, the SmB_cr phase becomes enantiotropic (reversible polymorphism). The phase behaviour and molecular dynamics in the liquid crystalline phases are analysed and discussed, with the conformational component of the total entropy for the SmC–isotropic liquid transition estimated. We also calculate from the PVT results the potential parameter characterising the steepness of the interaction potential.     

Epitaxial growth alignment of ferroelectric smectic C phases on rubbed polymers

Several of the polymers reported for surface alignment of ferroelectric smectic C phases in the literature are tested with a ferroelectric room temperature mixture. The results for this material and the reported findings in the literature are compared to the known crystal structure of the polymers. It is found that polymers with triclinic or monoclinic crystals give good alignment and bistability for the smectic C phase. Other crystal structures or non-crystalline polymers give a poorer performance. The mechanisms for creating a highly crystalline polymer surface are discussed, and the epitaxial growth of smectic phases on the crystalline surface is shown to be in accord with experiment.     

From monomolecular films to bulk liquid crystals

Molecules forming thermotropic liquid crystal bulk phases, were investigated at the air-water interface. With nematic and smectic substances, compression of the film resulted in an increase of the lateral pressure, provided the molecules had an amphiphilic character. Two types of π-A-diagrams were found for areas below a threshold value: (i) A single coexistence region (constant pressure at different areas) is obtained for molecules forming nematic liquid crystal bulk phases. The original monomolecular film grew in the third dimension to form a nematic liquid crystal at the interface. (ii) The π-A-diagram showed several coexistence regions with constant pressures if the molecules had the capacity to form smectic bulk phases. The “collapses” were at areas in the relation 1 : 1/2 : 1/3 etc. suggesting that smectic layers have been formed. The aqueous subphase is found to make a large contribution to the latent heat. The initial layer (together with the subphase) is found to be of lower symmetry than the upper layer(s). The transition is first order at low temperature (< 30°C) and second order at high temperatures. Evidences are given for a smectic Shubnikov phase where the layer-to-layer material flow is concentrated in flux lines.     

Synthesis of side chain liquid crystalline copolymers and characterization of their polar structure

A series of side chain liquid crystalline copolymers having different spacer lengths, copolymer compositions, and chromophore types were synthesized and characterized both in the bulk and at the gas‐water interface. Liquid crystalline properties were identified by differential scanning calorimetry (DSC), optical microscopy, and X‐ray diffraction (XRD). Copolymer with spacer lengths 4, 5, 10, and 11 showed smectic A (S_A) phases with a bâtonnet texture. The liquid crystalline (LC) phase stabilized as the spacer increased. Copolymers with different compositions were investigated both as monolayers and transferred films. The isotherms suggest nanodomain formation at the gas‐water interface in copolymers with high nitrobiphenyl (NBP) content. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1057–1070, 1999     

Ferroelectric liquid crystalline polymers based on mesogens with halogen-containing terminal groups

A new series of halogen-containing side chain ferroelectric liquid crystal polymers was synthesized. Mesophases were characterized by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and molecular simulation. The behaviour of the liquid crystalline phase was investigated with variation of chiral centres, spacer units and grafted ratios. It was found that the thermal stability and temperature range of the chiral smectic C phase decreased with increasing length of the oligo-oxyethylene spacer, and decreasing mesogenic group content. The bulky substituent attached to the chiral centre reduces molecular packing in smectic liquid crystal phases, which disturbs the orientation of the side chain liquid crystal polymer. Furthermore, the influence of molecular structure on electrooptical properties of FLCPs has been studied by broad band dielectric spectroscopy (from 0.1 to 1 ×10 6 Hz).     

Liquid crystals derived from multi-cationic azamacrocyclic alkylsulphates

Two homologous series of azamacrocyclic n-alkylsulphates were synthesized and characterized. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. At room temperature both series exhibited lamellar crystalline phases. For the tetraazacyclotetradecane alkylsulphate salts a highly ordered smectic phase was observed following their melting. The triazacyclododecane derivatives however melted into disordered smectic A phases, apparently due to the less symmetric polar group which does not favour in-layer organization. Extensive hydrogen bonding was observed in the crystalline phases of both series of compounds as well as above their melting into smectic phases, albeit rather weak in the case of the triaza derivatives.     

STUDY ON CHIRAL LIQUID CRYSTALLINE POLYMERS WITH BIPHENYLENE AND AZOBENZENE AS THE MESOGENS

Chiral liquid crystalline polymers containing biphenylene and azobenzene as the mesogensand S(-)-2-methyl-1-butanol as the chiral end group were synthesized and characterized by DSC,POM and X-ray diffraction. These polymers show crystalline or glassy liquid crystalline phase atroom temperature. Most polymers show smectic A or highly ordered smectic phases abovemelting temperature.     

Liquid crystal properties of a mesogenic polyacetylene, poly(11-[(4'-heptoxy-4-biphenylyl)carbonyloxy]-1-undecyne)

The liquid crystalline properties of a mesogenic poly(1-alkyne) and the corresponding monomer were studied using transmission electron microscopy, X-ray diffraction, polarizing optical microscopy and differential scanning calorimetry. The monomer exhibits a monotropic smectic A phase and a metastable crystalline phase. The rigid polymer backbones do not prevent the mesogenic moieties from packing into smectic A and B phases in the temperature ranges 127.6-74.1°C and 74.1°C-room temperature, respectively, on cooling from the isotropic melt.     

Morphology of side chain liquid crystalline block copolymer thin films and effects of thermal annealing

The self-assembly behavior of siloxane based side chain liquid crystalline block copolymer thin films are investigated via grazing incidence small angle X-ray scattering and atomic force microscopy. The as-spun films displayed polystyrene cylinders perpendicular to the substrate and the cylinders reoriented parallel to the surface after thermal annealing. The morphology observed in the as-spun films is resultant from the orientation of the smectic LC mesophase relative to the substrate. Annealing above both the polystyrene glass transition temperature and the smectic to isotropic transition temperature eliminates the influence of the LC phase, leading to a reorientation of the morphology that minimizes the interfacial energy of the system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3263–3266, 2007     

Bulk optical properties in binary mixtures of antiferroelectric liquid crystal compounds showing V-shaped switching

In order to clarify the origin of the V-shaped switching observed in thin cells ofantiferroelectric/ ferrielectric liquid crystals, bulk properties have been studied by means of helical pitch and conoscope measurements using thick free-standing films of binary mixtures with various mixing ratios. In the temperature range showing V-shaped switching in thin cells, helical structure clearly exists, indicating the existence of ordered phases. Some indistinct phase changes with temperature, coexistence of phases and quasi-continuous phase changes with an applied electric field were observed, suggesting a system with weak inter-layer correlation. By comparing the phase diagrams made using thin homogeneous cells and thick free-standing films, it was found that V-shaped switching occurs in the region where various subphases exist in the bulk. The appearance of many indistinct phases is consistent with the weak interlayer correlation. In this way, it was concluded that the V-shaped switching occurs in tilted smectic layers, in which the tilt direction is weakly correlated along the layer normal.     

Temperature‐Independent Hole Mobility of a Smectic Liquid‐Crystalline Semiconductor based on Band‐Like Conduction

A liquid‐crystalline (LC) phenylterthiophene derivative, which exhibited an ordered smectic phase at room temperature, was purified by vacuum sublimation under a flow of nitrogen. During the sublimation process, thin plates with sizes of 1 mm grew on the surface of the vacuum tube. The crystals exhibited the same X‐ray diffraction patterns as the ordered smectic phase of the LC state that was formed through a conventional recrystallization process by using organic solvents. Because of the removal of chemical impurities, the hole mobility in the ordered smectic phase of the vacuum‐grown thin plates increased to 1.2×10^?1 cm² V^?1 s^?1 at room temperature, whereas that of the LC precipitates was 7×10^?2 cm² V^?1 s^?1. The hole mobility in the ordered smectic phase of the vacuum‐sublimated sample was temperature‐independent between 400 and 220 K. The electric‐field dependence of the hole mobility was also very small within this temperature range. The temperature dependence of hole mobility was well‐described by the Hoesterey–Letson model. The hole‐transport characteristics indicate that band‐like conduction affected by the localized states, rather than a charge‐carrier‐hopping mechanism, is a valid mechanism for hole transport in an ordered smectic phase.     

Biaxial smectic phases in non-linear optical properties and phase behaviour of an oxdiazole liquid crystal

A symmetric, low-molecular-mass liquid crystal based on the oxadiazole ring was synthesized and characterized. The symmetric position of the heterocyclic group in the centre of the liquid crystal, bis(4-hexyloxyphenyl)4,4-(1,3,4-oxadiazole-2,5-diyl)dicarboxylate, yields a very rigid and non-linear mesogenic core. Despite this non-linear structure, a broad liquid crystalline range with a smectic C and a smectic A phase was found. Conoscopic experiments on freely suspended films revealed the existence of two optical axes in the smectic A phase, indicating a phase symmetry anticipated for either a 'McMillan' biaxial smectic phase or a biaxial (achiral) ferroelectric smectic phase.     

Activity coefficients of butanol isomers in liquid crystalline cholesteryl tridecylate according to data of reverse phase gas chromatography and tensiometry

The reverse phase gas chromatography technique was used to determine the activity coefficients of butanol isomers at infinite dilution in the smectic, cholesteric and isotropic phases of cholesteryl tridecylate. Experimental parameters with which the chromatographic process may be viewed as balanced and proceeding within the bulk of the liquid crystal were defined. Alcohol activity coefficients in binary systems with cholesteryl tridecylate determined by the gas chromatographic and tensiometric techniques were found to be in good agreement. A conclusion is reached on the suitability of the reversed-phase gas chromatographic technique for studying smectic and cholesteric liquid crystalline phases.     

Structural phase transition in ultrathin films of disk-shaped molecules

We have used grazing-incidence X-ray diffraction and atomic force microscopy to study Langmuir-Blodgett films of 2-(3-hydroxypropyl)oxy-3,6,7,10,11-pentapentyloxytriphenylene, a disk-shaped molecule which also forms a bulk liquid crystalline columnar phase. Upon heating, we observe a phase transition from a low symmetry ordered crystalline structure to a high temperature liquid crystal. The transition is reversible, with considerable hysteresis.     

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxypropan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and ¹H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.     

Novel diphenylacetylene-based room-temperature liquid crystalline molecules with alkylthio groups,and investigation of the role for terminal alkyl chains in mesogenic incidence and tendency

In this study, we offer a new family of alkylthio-containing diphenylacetylene-based liquid crystalline molecules (the so-called tolanes) showing nematic phases stable down to room temperature and high birefringence, with insights into the role for terminal alkyl chains in mesogenic incidence and tendency. A number of asymmetric tolane homologues having various alkyl chain lengths in the alkylthio and alkyl groups at each p-, p’-position were synthesised, and their phase transition behaviour was investigated by polarising optical microscope observation, differential scanning calorimetry and wide-angle X-ray diffraction measurement. Consequently, several homologues exhibited monotropic nematic or highly ordered smectic (soft crystal) phases stable down to room temperature. It is found that a long alkyl group on the side opposite to an alkylthio group is prerequisite for mesogenic incidence. In addition, a nematogenic homologue exhibited a higher birefringence value of 0.20 compared to that of 0.19 for an alkoxy counterpart.