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The nematic (chromonic N) and isotropic phases formed by aqueous solutions of a perylene dye are investigated by x-ray diffraction and visible light absorption spectroscopy. X-ray diffracti...

The recent interest in lyotropic chromonic liquid crystals (LCLCs) has resulted in significant understanding of the assembly behaviour and liquid crystal properties of these water-based systems. On...

Techniques to achieve planar anchoring of the director of a lyotropic chromonic liquid crystal (LCLC) have been developed only recently. These techniques range from the very weak anchoring achieved by simply scratching the glass with a very fine abrasive to the very strong anchoring achieved with lithographic techniques. A possible alignment technique is to use rubbed polyimide (PI) alignment layers as is routinely done to align thermotropic liquid crystals (TLCs). The anchoring strength of the LCLC disodium cromoglycate (DSCG) on different rubbed PI alignment layers is measured using a /2 twist cell and optical polarisation techniques. Although the anchoring strength is not as large as can be achieved with lithographic techniques, it is larger than scratched glass for one PI. Just as important, rubbed PI provides a much more uniform surface than is possible with scratched glass. Interestingly, addition of a small amount of surfactant to the LCLC, which results in improved alignment, does not incr...

Almost 30 years ago, when the study of chromonic liquid crystals was in its infancy, Alfred Saupe and L.J. Yu investigated disodium cromoglycate using deuteron magnetic resonance spectroscopy. Among their findings were that the quadrupolar splitting of the D2O spectrum increased with decreasing temperature and that the addition of salt to the solution increased the temperature stability of the nematic phase. A good deal of research on chromonic liquid crystals has been reported since that time, and therefore a lot more is known. However, it is also interesting that some of the questions from that time remain unanswered.

Trans–cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo‐optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic–i...

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C* A, and smectic Q. In the other system the phase sequence is smectic C*, smectic C* A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phas...

This communication was stimulated by a meeting of the Sonderforschungsbereich 335, ‘Anisotrope Fluide’ at the Technische Universitat Berlin, in December 1994. At this conference we realized that there are some very strong similarities between the melting processes of blue phases, twist grain boundary phases, and cubic D phases to the isotropic, amorphous liquid. In all three cases the melting appears to be a two stage process with a sharp melting enthalpy followed by a broad diffuse endotherm as the disorganized liquid is formed. In this article we wish to draw attention to similarities in the nature of these melting processes, to suggest a general descriptive model for the breakdown in molecular ordering, and to stimulate a theoretical debate in order to gain an understanding of the role of frustration phenomena in melting processes.

The optical activity and light transmission of the blue phases and isotropic phase of 4″-(2-methylbutylphenyl)-4′-(2-methylbutyl)-4-biphenylcarboxylate (CE2) are investigated under high pressure. The temperature ranges of the first (BP I) and third (BP III) blue phases show only the slightest change, if any, with increasing pressure. This is the first time BP III, a phase which is not understood at all, has been investigated under pressure. In addition, an alternative analysis to the one reported previously is reported which confirms that the pretransitional optical activity in the isotropic phase changes very slightly due to pressure, and even this small change seems to be due to small changes in pitch and index of refraction. Taken together, these observations indicate that high pressure has little effect on both the BP III phase and the transition to the isotropic phase in highly chiral systems, even though the BP III phase differs significantly from the other blue phases and strong short rang...

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.

Optical microscopy in mixtures of di-octylazoxybenzene (8AB) and di-nonylazoxybenzene (9AB) reveals that a smectic A-nematic-isotropic triple point occurs at 71·5±0·5°C and 38±2wt% 8AB. Although this concentration induced triple point is different from the pressure induced triple point known to exist in pure 9AB at elevated pressure, analysis of the data from both the mixing and pressure experiments reveals that the phase transition surfaces in temperature-pressure-concentration space for this system are nearly planar.