Optical textures in TGBA mesophases

In this paper, we present the results of polarized optical microscopy observations performed on different series of compounds exhibiting twist grain boundary mesophases (TGB). From all the compounds investigated, it appears there are two main typical optical textures which characterize TGBA mesophases in general. One type of texture is characteristic of a planar helical structure, while the other is a cylindrical (or marginally cone-like) domain texture, resembling the developable domains observed in columnar systems. Moreover, both textures can coexist in the same sample over the whole temperature range of the TGB phases. The detection, at the same temperature, of these two types of textures could then be considered as one of the signatures of the TGBA mesophases.     

Evidence of electroclinic effect in some TGBA mesophases

Two members of a chiral tolane series exhibiting a new TGBA phase and a S*_C ferroelectric phase have been investigated by means of electro-optical measurements. These materials belong to the optically active series of (R)-4-(1-methylheptyloxycarbonyl)-4'-(4'- alkyloxy-3'-fluorobenzoyloxy) tolanes and are the tetra- and octa-decyloxy derivatives. Due to the large pitch values in these TGBA mesophases, complete unwinding of the helix is found to be possible in thick cells. This also allows measurements of the electroclinic properties, such as the electric field induced tilt angle, response time, and the ferroelectric properties in the S*_C phase.     

Optical Bragg reflection from the TGBA phase

We have studied theoretically the reflection band properties of the twist grain boundary A phase (TGBA). A 4 x 4 matrix approach for the electromagnetic wave propagation is used. At normal incidence, simple analytical expressions are found, while at oblique incidence numerical methods have to be used since no analytical solution is available. The chart of stability of the propagating waves and the reflection spectra are given and discussed. No evidence of the incommensurability properties of the TGBA phases can be found optically. However, the analysis of the output polarization within the Bragg reflection band, on thick enough samples, allows us to determine the physical parameters characterizing the TGBA phase. In particular conditions, we can get the rotation Delta Theta between two homogeneous slabs and the total optical phase phi of one slab. When Delta Theta and phi go to zero, the optical properties of the TGBA phase become identical to those of the cholesteric phase. The essential difference appears when Delta Theta and phi are increased; an infinite number of reflection bands forms each Bragg reflection order, due to the periodicity of the TGBA parameters.     

Optical Bragg reflection from the TGBA phase

We have studied theoretically the reflection band properties of the twist grain boundary A phase (TGBA). A 4 x 4 matrix approach for the electromagnetic wave propagation is used. At normal incidence, simple analytical expressions are found, while at oblique incidence numerical methods have to be used since no analytical solution is available. The chart of stability of the propagating waves and the reflection spectra are given and discussed. No evidence of the incommensurability properties of the TGBA phases can be found optically. However, the analysis of the output polarization within the Bragg reflection band, on thick enough samples, allows us to determine the physical parameters characterizing the TGBA phase. In particular conditions, we can get the rotation Delta Theta between two homogeneous slabs and the total optical phase phi of one slab. When Delta Theta and phi go to zero, the optical properties of the TGBA phase become identical to those of the cholesteric phase. The essential difference appears when Delta Theta and phi are increased; an infinite number of reflection bands forms each Bragg reflection order, due to the periodicity of the TGBA parameters.     

Re-entrant and induced mesophases: Mixed systems showing re-entrant TGBA and re-entrant cholesteric phases

We have investigated the behaviour of chiral carbohydrate-based trioxadecalines with a cyano group in mixed systems. The mixture of a trioxadecaline having a Ch phase and a trioxadecaline having a Ch, S_A and a S*_Cphase sequence induces a re-entrant TGB_A and Ch phase. Below the stabilized S*_C phase, a re-entrant Ch phase is observed. In the mixture of the cholesteric trioxadecaline with CCH7, itself only showing a nematic phase, a TGB_A phase and a broad range S_A phase are induced from two non-smectic compounds. Both mixtures show a blue phase with a pitch ranging from the UV to red depending on the molar fraction.     

A review of textures of the TGBA* phase under different anchoring geometries

Polarizing microscope textures of the twist grain boundary A* (TGBA*) phase are reviewed for two different compounds in different geometries with different surface treatments giving monostable planar and homeotropic boundary conditions. The textures are discussed in the light of the helical structure of the TGBA* phase. Depending on the compound, the underlying phase is either SmA* or SmC*, whereas the adjacent phase at higher temperature is cholesteric (N*). Sample preparations in wedge-shaped cells subjected to a slight temperature gradient exhibit TGBA* textures much more typical for the cholesteric than for the ordinary SmA* phase. For instance, Grandjean steps and fingerprint textures are observed for planar and homeotropic boundary conditions, respectively. Preparation of smectic droplets clearly reveals the helical axis of the TGBA* phase to be perpendicular to the helical axis of the helielectric SmC* phase. For thin samples, a suppression of the TGBA* helix leading to a surface-induced structure corresponding to a conventional bulk SmA* phase is observed. Under certain conditions, a cholesteric phase in the vicinity of a twist inversion point may exhibit very similar textures to the TGBA* phase near the transition to the SmA* phase. On exemplified textures similarities are discussed and differences pointed out.     

Optical and structural modeling of disclination lattices in carbonaceous mesophases

An integrated microstructural and optical model for carbonaceous mesophases is developed and used to explain the principles that govern the formation and stability of experimentally observed disclination lattices. The model is able to capture the orientation features of disclination lattices, including the type and location of disclination lines, and the orientation field in the mesophase matrix. The optical model based on reflection polarized optical microscopy is able to replicate all the details observed in actual observations. The typical brush figures have the proper distribution, orientation, and intensity. The computational predictions offer science-based routes to create and control desirable material architectures based on carbonaceous mesophase-carbon fiber composites.     

Chromonic mesophases

Over the last 10 years, there has been a growing acceptance of the concept of chromonic phases and a wider recognition that they form a well-defined family of lyotropic liquid crystalline phases, with a package of properties distinct in almost every aspect, from those of conventional amphiphiles. New chromonogenic compounds have appeared and new technological uses for chromonic systems are being actively explored. Recent promising investigations include the synthesis of a chromonic dye, C.I. Direct Blue 67, which has an N phase of high order parameter and which can be dried down to give well-oriented films of solid. When dried down on a ‘command surface’ of photoaligned substrate this can produce a highly patterned film. The use of chromonic phases in the construction of compensating plates for improving the viewing characteristics of twisted nematic displays has been explored. Although this technology may not be suitable for commercially exploitation in its present form, the success of the devices is significant. It is suggested that current studies of the way in which the temperature range of thermotropic discotic mesophases is enhanced in 1:1 CPI mixtures may well lead to improved formulations for chromonic dyes. It is predicted that the marriage of chromonic phase technology with current biochemical analytical techniques will give rise to a new generation of medical diagnostic tests.     

Metastable mesophases

Mesophases are briefly summarized and defined on a uniform basis, making use of order and mobility parameters. A schematic one-component phase diagram is then reviewed that includes equilibrium as well as nonequilibrium phases. Assuming only five condensed phases (crystal, melt, glass, mesophase, and mesophase glass), fifteen different phase areas exist with simultaneous presence of up to three phases. Examples of all phase areas are documented with results from our laboratory. This scheme can be extended to more components and more than these five phases and produces then many more possible phase areas than can be handled experimentally at the present.     

Diacetylenes and mesophases

During the past five years, over 600 papers have been published on research work carried out with diacetylenes. Far fewer papers can be found on diacetylene mesophase studies, and these are reviewed in this paper. Some new findings by the authors are also presented.     

Thermotropic mesophases in linear polysiloxanes

Flexible linear polysiloxanes with inorganic backbones such as poly(diethylsiloxane) and poly-(dipropylsiloxane) contain no traditional mesogenic groups neither in the main chain nor in side chains and, nevertheless, they are able to form thermotropic mesophases. Recent developments in the study of thermodynamics, kinetics, structure and morphology of these mesophase polymers are considered. The temperature interval of existence of the mesophase is strongly dependent on the length of side radicals and molecular weight. These dependences are examined in detail. X-ray structure and the optical textures seen in the polarizing microscope are discussed. It is shown that large mesophase lamellae represent a very characteristic feature of the morphology of poly(diethylsiloxane). The linear growth rate of the lamellae and the overall calorimetric kinetics of the mesophase formation have been studied at various temperatures and the results obtained have been analyzed using the Avrami equation and the kinetic theory of the linear growth. As a result of the kinetic analysis two-dimensional growth has been suggested in accordance with the microscopic observation. The formation of the mesophase in slightly crosslinked poly(diethylsiloxane) samples can be initiated by stretching. The stress-strain and thermomechanic (deformation calorimetry data) behaviour of such samples is briefly discussed.     

Modelling of microstructure in mesophases

Small-molecule liquid crystals show textures which are readily studied at low magnification in the optical polarizing microscope. In polymeric liquid crystals, however, the textures are often much finer, taxing the microscope's resolution. Nevertheless, studies of microstructure in such polymers have been made and it is apparent that they can differ widely both from small-molecule liquid crystals and, indeed, from polymer to polymer. This paper sets out to account for these variations by exploring the effect on microstructure of the marked differences between the magnitudes of the splay, twist and bend elastic constants which are a characteristic of many liquid crystalline polymers. We report a computer model which simulates the development of microstructure for different ratios of the elastic constants. When these are approximately equal, textures characteristic of small-molecule liquid crystals result, such as those involving escape into the third dimension with the degeneration of line defects into points. When the splay energy is high in comparison with bend and twist, as is the case for many thermotropic liquid crystalline copolyesters, escape does not occur and half integral disclination lines predominate. For simulations involving planar boundary conditions, layered microstructures result, frequently with little matching of the orientation from layer to layer. Within the layers the trajectory of the orienting units is sinuous. This simulated microstructure resembles textures observed in thermotropic copolyesters, studied both in this laboratory and elsewhere. The computer model uses a lattice approach which is similar in some respects to that developed by Lebwohl and Lasher. It should not be thought of as a molecular scale model, however, but rather as one based on assemblies of molecules which share a common director.     

Nucleation of filaments of the TGBA phase in free-standing films

A model of filament nucleation in the twist grain boundary A (TGBA) phase in free-standing smectic films is proposed. It is based on a concept of finite blocks of parallel smectic layers forming a helical structure in the interior of a film. In our model, the blocks resemble twin-like domains in smectic-A liquid crystal. The blocks are mediated to surrounding smectic layers in film either continuously (coherent twin boundary) or discontinuously via dislocation loops wrapping up blocks. Edge components of dislocation loops form incoherent twin boundary. Screw components forming twist grain boundary connect neighbour blocks among themselves. Nucleation of TGBA filament in a film is treated using a simplified model based on isotropic smectic-A elastic theory. Discussion showed that filaments can be nucleated just below the transition temperature from the isotropic to the TGBA phase under slow temperature decrease. The nucleation of dislocation loops is possible due to low compression modulus of material which can be expected just below the transition temperature.     

TGBA phase in a series of propionates containing two stereocenters

A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4'-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGB_A) and smectic C (TGB*_C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGB_A phase behaves as a metastable-like phase mediated between N and S_A phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGB_A and TGB_C phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two different mesophase sequences: N-TGB_A-S_A-S_Cand N-TGB_A-TGB_C. A kind of parquet texture displaying two types of domain with different relative directions of the smectic layer normal was found in the S phase from the materials (m = 9-14) packed in 2 μm homogeneously aligned cells and cooled down from the isotropic liquid without applying an electric field. The magnitudes of spontaneous polarization (P_s) in the S_C and TGB_C phases showed that the P_s values are nearly the same for all compounds at the same temperature below the Curie point. Dielectric measurements revealed no significant occurrence of soft mode switching in the TGB_A phase.     

Flow-induced effects in mixed surfactant mesophases

Spatially resolved small-angle neutron scattering, SANS, has been used to investigate the response of the mixed microstructure of the dialkyl chain cationic and nonionic surfactant mixtures of (2,3-diheptadecyl ester ethoxy-n-propyl-1), 1,1,1-trimethyl ammonium chloride/octadecyl monododecyl ether, DHTAC/C18EO10, and DHTAC/dodecyl monododecadecyl ether, Coco20, over the velocity flow pattern of a crossed-slot elongational flow cell. The two different surfactant mixtures have different relative amounts of lamellar and micellar components, and this results in some differences in the flow-induced response. For the DHTAC/C18EO10, which is predominantly in the form of lamellar fragments, a complex pattern of orientational ordering is observed which reflects the competition between or demixing of the two principal flow directions in the cell.     

Deuterium NMR of the TGBA* phase in chiral liquid crystals

A deuterium NMR (DNMR) study of the TGBA* (twist grain boundary smectic A*) phase in an NMR magnetic field of 9.4 T for the chiral compound 4-[4'-(1-methyl heptyloxy)] biphenyl 4-(10-undecenyloxy) benzoate (11EB1M7) is reported. The deuterium two-dimensional (2D) exchange spectra were observed for the first time in this phase. The present study allows us to learn how the helicoidal structures arrange in an external magnetic field. To derive quantitative kinetic parameters of this novel phase, both 1D and 2D experimental spectra were simulated by means of a jump diffusion model. By comparing the experimental and simulated spectra, an accurate determination of the dynamic parameters in the TGBA* phase was obtained. Furthermore, the twist angle between two neighboring smectic A blocks is found as 26 +/- 10 degrees, which is consistent with the X-ray results for similar chiral liquid crystals. The diffusion constant (D(parallel)) is estimated to be 3.2 x 10(-12) m(2)/s at 379.5 K.     

Water-in-water mesophases for templating inorganics

A water-in-water mesophase that contains only hydrophilic domains is reported for the first time; the mesophase templates highly porous calcium phosphate.     

Novel mesophases in fluorine substituted banana-shaped mesogens

A new homologous series of achiral banana-shaped mesogens ('Dn') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (D6-D8) exhibit a two-dimensional phase 'B1_x' different from a B1 phase with a rectangular lattice. The longer homologues (D9-D14) present a mesophase which displays the defects of the B7 phase of the PIMB-NO₂ compounds. Nevertheless the D9-D14 mesophase is not miscible with the B7 phase, and contrary to B7, exhibits a bistable behaviour ('ferroelectric' type) suggesting at least a B7 variant.