K. Betterton

Rod-like mesogens with antipathetic fluorocarbon and hydrocarbon tails

Abstract A perfluorinated (or semi-perfluorinated) chain is affixed to various classical mesogenic moieties in lieu of the usual hydrocarbon tail. The consequences of this modification on the mesogenic behaviour are discussed by comparison with the perhydrogenated homologues. Smectic A phases are highly favoured. Their layer spacings show a trend to bilayer stacking in many cases likely to result from microsegregation among incompatible portions of the molecules.

Collective and molecular processes in a new class of ferroelectric liquid crystals

Abstract Electro-optic and dielectric studies have been performed for a thioester (with a chiral chain incorporating the lactic acid ester moiety), which exhibits a monotropic room temperature ferroelectric Sc* phase. Moreover, there are some hints that this substance also shows the recently discovered helicoidal SA* phase. The dielectric and electro-optic measurements have been carried out using ITO coated glass plates (d = 10 μm). Planar alignment was obtained by means of an AC electric field (v = 1Hz and V p-p = 70V). The tilt angle and spontaneous polarization temperature dependences show that the SA*-Sc* transition is of second order. Soft mode behaviour in the pretransitional region also confirms this result. The monotropic Sc* phase exhibits a rather strong Goldstone mode (GM) with pronounced temperature dependence of the dielectric parameters. The molecular reorientation about the long axis has been found for both principal alignments. The homeotropic effective alignment was obtained by applying a...

The strong temperature dependent goldstone mode relaxation frequency in a broad range SmC* phase

Abstract Broad frequency range dielectric spectroscopy, X-ray diffraction, electro-optical methods and thermal analysis are used to characterize a single component liquid crystal with a monotropic broad range room temperature ferroelectric SmC* phase. The dielectric spectrum shows a Soft mode and a Goldstone mode. Unlike in other ferroelectric liquid crystals the Goldstone mode of this substance has a strongly temperature dependent characteristic frequency. It is proposed that glassy-state fluctuations are responsible for the appreciable increase of activation energy (from 37 to 91 kJ/mol) upon cooling from 318 K (TCA) to 278 K. Beside these modes a Domain mode is detected. Measurements in the high frequency range 1 MHz to 450 GHz show an additional relaxation process.

Synthesis and ferroelectric properties of liquid crystals derived from 4-mercaptobenzoic acid

Abstract A series of new ferroelectric liquid crystals characterized by the presence of a thioester core and an alkoxycarbonyl tail derived from a chiral 2-alkanol has been prepared and evaluated. The synthesis of these materials is straightforward and employs p-mercaptobenzoic acid as a critical intermediate. Some of the new liquid crystals are remarkable due to their large polarizations and rapid electro-optic switching near room temperature in the SSFLC configuration.

Smectic liquid crystals modified by tail and core fluorination

The systematic substitution of fluorine for hydrogen in mesogenic materials has proved to be of great value for the modification of a wide range of physical properties. This is especially true in the case of ferroelectric liquid crystals in which the fluorine has been strategically introduced into the aromatic core (k), at or near the chiral center (Z) or in the achiral tail (n) in the generic liquid crystal (with X equals O, S and Z equals Me, halogen): H/F(CF2)n(CH2)mOC6H(4 - k)FkCOXPhOCOC*HZR. In this system the fluorine can be introduced in any of these three specific locations, or any combinations thereof, with the intent of enhancing performance such as broadening of the ferroelectric Sc* phase temperature range, enhancement of the magnitude of spontaneous polarization Ps and a variety of more subtle yet important modifications such as viscosity and dielectric and optical anisotropy. The ramifications of the incorporation of fluorine on these physical properties are discussed with a variety of examples.

Orientational ordering of ferroelectric thiobenzoate liquid crystals

Abstract The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance 13C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle of c. 45° with respect to the magnetic field, the smectic director aligns parallel to the spinning axis, leading to narrow peaks in the 13C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled 13C signals in the ω2 dimension and first-order Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C[sbnd]H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order paramete...

Remarkable Influence of Fluorination in Smectic and Ferroelectric Liquid Crystals

Abstract Liquid crystals possess a wide variety of interesting and useful properties which are strongly correlated to their structure. In order to modify and enhance certain useful properties we have examined the effects of systematic introduction of fluorine. Especially interesting examples are the influences on the phase range and type including ferroelectric properties and on miscibility leading to phase separation via spinodal decomposition of the anisotropic LC phase.

INFLUENCE OF STRUCTURE ON SMECTIC A-SMECTIC A PHASE SEPARATION

Abstract The critical temperature at the consolute point where two smectic A phases of different compositions phase separate has been measured for binary systems which combine one compound from a newly synthesized series of fluorinated mesogens with one homologue from the commercially available cyanobiphenyl family. Taking advantage of the regular evolution of the layer spacings within a series of homologues, this systematic analysis identifies a clear connection between miscibility and difference in layer spacings of the smectic A phases of the pure compounds. However, additional data obtained on significantly different systems show the non-universal character of this analysis and suggest that factors in addition to this structural difference are relevant to this phenomenon.

Reentrant phenomenon in cyanoaryl alkylthioxybenzoates and cyanobenzoyloxyphenyl alkylthiobenzoates

Abstract Three new series of cyanoaryl 4-alkylthioxybenzoates have been synthesized and characterized. The 4-cyanophenyl series displays only transient mesomorphic properties while the 6-cyano-2-naphthyl and 4-cyanobiphenyl series exhibit the classical nematic and smectic A phases; a reentrant nematic phase is also found in the latter. Two other new series in which the position of the sulphur atom in the central rigid core has been varied were also prepared: the 4–[4′-cyanobenzoyloxy]-phenyl 4-alkylthiobenzoates and 4–[4′-alkylbenzoyloxy]phenyl 4-cyanothiobenzoates. As in the analogous benzoate series, the reentrant phenomenon with the sequence C, SAre, Nre, SA, N, I is observed with the nonyl derivatives. Comparisons between the alkoxybenzoates and the alkylthioxybenzoates are given.

Thermally stable chromophores and polymers for χ(2) applications

Due to the requirements for thermal stability in the fabrication and operation of electro-optic devices, the quest for suitable NLO chromophores and polymers involves more than the search for highly nonlinear materials. We describe here the preparation and characterization of aryl substituted azole derivatives as a class of thermally stable nonlinear chromophores and their incorporation into high temperature thermoplastics as a host-guest array. Criteria for predicting the thermal stability of NLO chromophores are discussed as are the polar order decay properties of various poled polymer systems as a function of temperature.