Highly first order silane end-group difluoroterphenyl liquid crystal was added to trifluoroterphenyl host with a nematic to smectic-C phase transition to increase its first order nature, achieve a higher cone angle and reduce its temperature dependence. Increasing concentration of the additive, caused a linear decrease and eventual disappearance of the nematic phase. A smectic-A phase was injected at 35% concentration, making the transition second order. Optical studies of the mixtures showed little dependence of the highest value of cone angle on concentration. The original host showed first order cone angle behavior and was used to fabricate a twisted FLC device.

Dielectric and polarization measurements are carried out for characterizing the subphases in a antiferroelectric liquid crystal with a variety of ferri-, ferro- and antiferroelectric phases. It is found that ferri- phases in between SmCA are somewhat unstable and disordered; the reasons of the instability are given in terms of the ANNI model. A meta-stable field induced ferri phase has been found thus confirming our discovery (Panarin et al Phys. Rev. E 55, 4345, 1997) of such a ferri phase. A surface domain mode having a frequency about a decade higher than that of the Goldstone mode has also been observed for the first time in a antiferroelectric liquid crystal.

Optical Bragg reflections were used to probe phase transitions of four heterocyclic esters that exhibit various frustrated chiral smectic phases. These transitions showed considerable hysteresis and were detected by the dicscontinuities of the Bragg peaks. For one of these esters, applied fields of similar to 10(-6) V/m completely destroyed the Bragg reflections. Smaller fields resulted in peaks of reduced intensity and shifts to longer wavelengths.

The authors have investigated the conformational structure of the ferroelectric liquid crystal compound 4-3-methyl-2-chloropentanoyloxy-4"-hexyloxy-biphenyl also known under the abbreviations 3M2CPHOB and C6 using vibrational (IR and Raman) spectroscopy. The measured spectra exhibit two bands corresponding to the C=O stretching vibration that are separated by 20 cm(-1). In contrast, the molecular structure comprises only one such group. They assigned the two bands to different conformers that coexist in a temperature range between 25 and 65 degrees C covering the entire mesophase of this material. This assignment is strongly confirmed by calculated vibrational spectra based on the density functional theory.

We report X-ray resonant scattering experiments performed on the prototype liquid-crystalline compound MHPOBC doped with a chemical probe containing a resonant atom (selenium). We determined directly for the first time the microscopic 3- and 4-layer structure of the ferrielectric subphases ( SmCFI1* and SmCFI2*) present in MHPOBC. Despite the low fraction of the selenium probe, the resonant signal is strong enough to allow an unambiguous determination of the basic structure of the ferrielectric subphases. These experiments demonstrate that the resonant scattering technique can be extended to liquid crystalline materials without resonant element and may stimulate new studies. A non-resonant Bragg reflection was also found in the SmCFI1* phase in pure MHPOBC, consistent with the 3-layer distorted model, but never detected before.

In this article we describe research into the molecular design features for potential thermotropic biaxial nematogens of low-molar mass. We discuss how we can systematically change the shape of molecular structures in view of this aim of our work. The optical biaxialities of the materials prepared were tested through conoscopic and birefringence measurements. So far none of these materials synthesised and tested in this way have unequivocally exhibited the biaxial nematic phase. An overview of numerous further materials different in molecular shape and in part studied by others is also given.

A detailed study of a novel material exhibiting antiferro-, ferri-, and ferroelectric phases has been undertaken. The tilt angle, spontaneous polarisation and the dynamics of the helical unwinding have been examined as a function of temperature and voltage in all the phases of interest. The material exhibits large tilt angles (similar to 31 degrees) and values of spontaneous polarisation (similar to 100nCcm(-2)) in cases where the helix is totally unwound. The behaviour of tilt angle as a function of applied electric field is examined in the ferri- and antiferro-electric phases. A stepwise increase in tilt angle as a function of applied voltage in the ferri-electric phase is presented, as well as unusual behaviour in the antiferro-electric state. In addition, a preliminary investigation of the change in spontaneous polarisation as a function of pressure is presented. It is shown that the dependence of spontaneous polarisation in the ferroelectric phase is relatively weak.

With a view to obtain a molecular model for the subphases produced by the frustration between ferro- and antiferroelectricity in chiral smectic liquid crystals, we report results on two compounds and observe (i) the staircase character of uniaxial Sm C(*)(alpha) itself in the bulk and (ii) the multipeaked characteristic reflection bands due to the modulated helical structures just above the Sm C(*)(A) temperature range. We suggest the emergence of several uniaxial and biaxial subphases. The results show that both types of subphases can be specified by q(T) = [F] / ( [A] + [F] ) in the zero-order approximation; [A] and [F] are the numbers of antiferroelectric and ferroelectric orderings in the unit cell. We consider the basis of both types of subphases, particularly the description of the short-pitch helical structure of Sm C(*)(alpha), in terms of the molecular models so far proposed and emphasize the important role played by the discrete flexoelectric polarization.

The molecular orientation and the dichroic behavior of the vibrational bands of a homogeneously aligned helical cell containing chiral smectic liquid crystal (R)-(-)-1-methylheptyl 4-(4(')-dodecyloxybiphenyl-4-ylcarbonyloxy)-3-fluorobenzoate are studied at various temperatures as a function of the bias field. These temperatures correspond to the various phase states of the sample at zero field. For those bands that exhibit significant dichroism, the field dependencies of the dichroic parameters (the dichroic ratio and the polarization angle of maximum absorbance) are found to be dependent on temperature, phase state, and helical unwinding. For the SmA* and SmC(*)(alpha) phases, the phenyl band dichroic ratio and the corresponding orientational order parameter are found to be almost independent of the bias field. The temperature dependence of the orientational order for zero field is discussed by taking into account the structures of the phases and the molecular tilt angles. The field dependencies of the phenyl band dichroic parameters for the SmC(*)(A) and SmC(*)(gamma) phases yield results about the distribution of directors in the layers of their unit cells and the state of helical unwinding. The azimuthal orientational distribution function of the carbonyl transition moments with respect to the long molecular axis has been determined. It is found that the degrees of the polar and quadrupolar biasing increase with decrease in temperature and the azimuthal biasing angle for the chiral carbonyl group increases significantly with a reduction in temperature.

Analysis of the reflection spectra of chiral smectic liquid crystal phases is used to provide detailed information on physical parameters, including refractive indices, helicoidal pitch and tilt angle. Numerical models of the antiferroelectric and ferroelectric phases are constructed, based on a 4 × 4 matrix technique, and a downhill simplex algorithm is employed to fit the model to reflection spectra measured from free-standing films of the materials of interest. The temperature dependence of the refractive indices, dispersion, tilt angle and pitch are reported for three different liquid crystalline materials. The accuracy of the fitting method is around 1%, better than the experimental error, and this is confirmed by comparison of the parameters determined by the fitting program with those measured by independent methods.

The electric-field-induced structural rearrangement of smectic layers in the antiferroelectric and ferroelectric phases of three different materials is reported. The materials all have high optical tilt angles (around 30 degrees ), compared with the steric tilt angles deduced from layer spacing measurements (around 18 degrees ). The chevron angles observed in devices agree well with values found for the steric tilt angle across the tilted mesophase range. Electric fields were applied to liquid crystal devices while the smectic layer structures, in both the depth and in the plane of the device, were probed using small angle x-ray scattering. Two separate aspects of the influence of the field on the layer structure were studied. First, the organization of the smectic layers in the antiferroelectric phase is described before, during, and after the application of an electric field of sufficient magnitude to induce a chevron to bookshelf transition. Second, the evolution of the field-induced layer structure change has been investigated as the field was incrementally increased in both the antiferroelectric and ferroelectric phases. It was found that the chevron to bookshelf transition has a distinct threshold in the antiferroelectric phase, but shows low or zero threshold behavior in the ferroelectric phase for all the materials studied.

A C-13-{H-1} NMR spectrum of 2,2'-difluoro-4"-hexoxy-4-pentyl-[1,1': 4',1"]-terphenyl in the nematic phase has been obtained and analysed to yield a set of partially-averaged dipolar couplings, D-ij(CF), which are used to determine the relative conformation of the two fluorinated rings. The probability distribution for the inter-ring angle is found to be peaked at 47.5+/-2.5degrees and 122.5+/-2.5degrees. The integrated, normalised probability that the inter-ring angle is less than 90degrees is 0.43.

The structures of the liquid crystalline chiral subphases exhibited by several materials containing either a selenium or sulphur atom have been investigated using a resonant x-ray scattering technique. This technique provides a unique structural probe for the ferroelectric, ferrielectric, antiferroelectric, and SmC(*)(alpha) phases. An analysis of the scattering features allows the structural models of the different subphases to be distinguished, in addition to providing a measurement of the helical pitch. This paper reports resonant scattering features in the antiferroelectric hexatic phase, the three- and four-layer intermediate phases, the antiferroelectric and ferroelectric phases and the SmC(*)(alpha) phase. The helicoidal pitch has been measured from the scattering peaks in the four-layer intermediate phase as well as in the antiferroelectric and ferroelectric phases. In the SmC(*)(alpha) phase, an investigation into the helical structure has revealed a pitch ranging from 5 to 54 layers in different materials. Further, a strong resonant scattering signal has been observed in mixtures of a selenium containing material with as much as 90% nonresonant material.

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. Measurements of the optical rotatory power (ORP) of these phases have elucidated the quality of this method for finding the phase transitions between several phases and for investigating their helical structure. The optical rotatory power as a function of temperature for seven wavelengths of light in the range 441 to 665 nm was measured for different phases of the AFLC material investigated, (R)-1-methylheptyl 4-(4'-n-dodecyloxybiphenyl-4-yl-carbonyloxy)- 3-fluorobenzoate, with acronym 12OF1M7. The values of the pitch and the optical anisotropy in the plane of the smectic layers for the FiLC (SmC*(F13)) phase and SmC*(A) phase have been calculated from the ORP data. The results of the ORP rule out the Simple clock model for describing the structure of the SmC*(gamma) (SmC*(F11)) and AF (SmC*(F12)) phases. The results for these phases can be explained either in terms of the modified Ising model or the highly distorted clock model. The ORP measurements establish the existence of SmC*(F13) found already from dielectric, polarization and polarized IR spectroscopy.

High-resolution resonant polarized x-ray diffraction experiments near the sulfur K edge have been performed on free-standing liquid crystal films exhibiting the chiral smectic-C*FI2 phase. It is widely accepted that this phase has a four-layer repeat unit, but the internal structure of the repeat unit remains controversial. We report different resolved features of the resonant x-ray diffraction peaks associated with the smectic-C*FI2 phase that unambiguously demonstrate that the four-layer repeat unit is locally biaxial about the layer normal and that the measured angle, describing the biaxiality, is in good agreement with optical measurements.

Resonant x-ray diffraction was carried out at the Se K edge in thick free-standing films of a selenophene liquid crystalline material, revealing detail of the structure of the ferro-, ferri-, and antiferroelectric phases. The ferrielectric phase was shown to have a three-layer superlattice. Moreover, the structure of a lower temperature hexatic phase was established. For the antiferroelectric phase, investigations were also carried out in a planar device configuration. The device allowed resonant scattering experiments to be carried out with and without the application of an electric field and resonant data are compared with electro-optic measurements carried out on the same device.

The layer structure in the antiferroelectric, ferrielectric, and ferroelectric phases of a liquid crystal device is reported, together with its electric field-induced deformation. The field-free chevron angle is comparable to the steric tilt angle, but differs significantly from the optical tilt angle. A sharp field threshold is observed for the chevron to bookshelf transition in the antiferroelectric phase at 1.3 V/μm, while layer deformations occur at much lower fields (0.3 V/μm) in the other subphases. Models are proposed for the layer deformations.

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical polar phases. Measurements of pyroelectric response of these phases as a function of temperature and bias field have elucidated the ability of this method for investigating the nature of antiferroelectric phases and phase transitions under the bias field. The pyroelectric signal as a function of the bias field at fixed temperatures and as a function of temperature for fixed bias fields was measured for different phases of the investigated AFLC material. A theoretical model describing the pyroelectric response in different phases of AFLC is given, and the experimental results are interpreted. The threshold fields for field induced phase transitions are determined. The type of field induced phase transition from the AF phase in particular is found to be dependent on the temperature within its range. The properties of an unusual ferrielectric phase existing between ferrielectric chiral smectic-C (SmC*) and antiferroelectric AF phases are studied in a great detail. The results confirm that this phase is one of the incommensurate phases, predicted by the axial next-nearest neighbor Ising model and Landau model for this temperature region.

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