C. Carboni

Low transition temperature organosiloxane liquid crystals displaying a de Vries smectic A phase

The synthesis, phase properties and electro-optic response of two new chiral low molar mass organosiloxane liquid crystalline materials are presented. The structure of the molecule contains a short (three silicon atoms) siloxy chain attached via a hydrocarbon chain to the non-chiral end of a chiral mesogenic moiety. The materials have low melting points (< -20°C) and broad smectic phase ranges (≈80°C wide) extending to below room temperature. Both materials are electroclinic at room temperature. A 30° tilt of the optic axis can be achieved in fields of the order of 25 V μ;m -1. The data presented indicate that in these materials the smectic phase is a de Vries-type smectic A phase. The electroclinic effect arises because the electric field produces a bias in the distribution of the direction of the tilt.

Low Frequency Dielectric Relaxation, Spontaneous Polarization, Optical Tilt Angle and Response Time Investigations in a Flourinated Ferroelectric Liquid Crystal, N125F2(R*)

Experimental investigations of thermal microscopy, low frequency dielectric relaxation, spontaneous polarization P s (T), optical tilt angle θ(T) and response times τ s (T) are carried out in a fluorinated ferroelectric liquid crystal, N125F2(R*) exhibiting smectic-A and smectic-C* phases. Dielectric loss in the smectic-A phase exhibits single Debyes relaxation in the MHz region. The smectic-C* phase exhibits two relaxations, viz., a Goldstone mode at ∼500 Hz and another at a much higher frequency, ∼5 MHz. The Arrhenius shift of Smectic-A relaxation frequency (f R ) gives an activation energy of 1.5 eV. The influence of temperature and applied voltage on the smectic-C* Goldstone mode relaxation is studied. In the smectic-C* high frequency relaxation mode, the relaxation frequency (relevant to the tilt) is found to increase with decreasing temperature. The temperature variation of the reciprocal of the dielectric strength (1/Δϵ s ) above the smectic-A—smectic-C* phase transition qualitatively supports the Curie-Weiss law. Temperature variation of primary (tilt) and secondary (P s ) order parameters are presented. The temperature variation of τ s in the smectic-C* phase studied through square wave technique is presented. The influence of fluorine atoms on the rigid core part of the present ferroelectric liquid crystal on the physical properties exhibited in its smectic-C* phase is discussed.

Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10 MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf R is found to follow an Arrhenius shift, with an activation energy of 1.7 eV. Temperature variation of the dielectric strength (Δε = ε o − ε∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent αeff = 0.072 indicates weak growth of transitional fluctuations across the IN transition.

Dielectric Response in the Smectic A and Smectic C* Phases of a Ferroelectric Liquid Crystal, 12CN5(R*)

This paper reports the results of the dielectric relaxation studies performed in the Smectic A (SmA) and Smectic C∗ (SmC∗) phases of the ferroelectric liquid crystal, 12CN5(R∗) in the frequency range 5 Hz to 13 MHz. Planar orientation used throughout the study was achieved by rubbed polyamide coating on the surfaces of the dielectric cell. Transition temperatures determined from the derivative of the dielectric permittivity agrees with that obtained using thermal polarizing microscopy. The dielectric spectrum in the SmA phase shows single relaxation, whereas the SmC∗ phase has two relaxations, viz., the low frequency Goldstone mode and the high frequency soft mode. Special attention was paid to the study of the low frequency Goldstone mode relaxation. From the Cole-Cole plots, the temperature dependence of dielectric strength (Δ˜), the relaxation frequency (fR) and the distribution parameter (α) relevant to the Goldstone mode have been estimated. The relaxation frequency follows the Arrhenius relation in the asymptotic regions far from the transition. Results of the bias-dependent dielectric measurements of the Goldstone mode relaxation have also been presented.

Electroclinic effect in low molar mass organosiloxane liquid crystals

Monomesogen low molar mass organosiloxane liquid-crystal molecules consist of a single mesogenic moiety attached via an alkyl chain to a short siloxane moiety. When the material is cooled from the isotropic phase, the siloxane moieties micro-separate from the mesogenic moieties and aggregate in planes that acts as an effective polymer backbone. The material is therefore in a smectic phase with alternating siloxane-rich layers and mesogen-rich layers. With a suitable design of the mesogenic moiety one can achieve a certain degree of decoupling between the mesogenic-rich layers. Such materials can be expected to display de Vries smectic A phases. In this paper we present monomesogen low molar mass organosiloxane liquid-crystal materials that display a wide temperature range de Vries smectic A∗ phase. The field dependence of the electric field-induced tilt of the optic axis and the field dependence of the birefringence have been measured at several temperatures. One of the materials displays a transition from de Vries smectic A∗ to smectic C∗ at low temperatures.

Novel fluorinated ferroelectric organosiloxane liquid crystals

A progress report on the synthesis and characterisation of a series of fluorinated chiral bi-mesogen low molar mass organosiloxane materials is presented. The mesogenic moiety is identical to that in the TSiKN65F mono-mesogen material reported by Naciri et al., which displays a de Vries-type SmA* phase. The parameter varied across the series is the length of the alkyl chain linking the mesogen moiety to the siloxane. The material with 10 carbon atoms in the chains displays a stable smectic phase with ferroelectric response at room temperature, whereas the materials with 6, 7 and 8 carbon atoms tend to crystallize.

A Novel Series of Chiral Fluorinated Organosiloxane Liquid Crystals

The synthesis and characterization of a series of chiral fluorinated low molar mass organosiloxane materials is presented. The mesogenic moiety is similar to that in the TSiKN65F mesogen reported by Naciri et al., which displays a de Vries-type SA* phase. The one parameter varied across the series reported herein is the length of the alkyl chain linking the mesogen moiety to the siloxane tail.

Threshold field for switching the de vries SA * phase in a low molar mass organosiloxane material

The existence of a threshold field for the onset of switching in the organosiloxane material, ETSiKN65, has been investigated. This material displays a de Vries smectic A* type phase, and the presence of the threshold confers an anti-ferroelectric-like response. However, it has been found that the value of the threshold depends on the thickness of the specimen, in fact it vanishes altogether if the specimen is more than 20 μm thick. It is concluded that the threshold is due to a complex anchoring mechanism at the interface with the cell walls.

Observation of bubble domains at the cholesteric to homeotropic-nematic transition in a confined chiral nematic liquid crystal

We report observations on a chiral nematic liquid crystal confined in a plane cell treated for homeotropic alignment. The characteristics of the specimen investigated are such that at room temperature a uniform homeotropic alignment is observed and at higher temperatures the material displays the helical structure. It is observed that the changeover between homeotropic and helical textures does not occur at one single threshold. There are two thresholds, between which there is an intermediate texture where small circular domains are observed. These domains may be regarded as circular analogues of the plane solitons predicted by previous authors.

Bifurcated-tail chiral fluorinated organosiloxane liquid crystalline materials

Abstract The synthesis and characterization of a series of chiral fluorinated low molar mass (bifurcated tail) organosiloxane materials is presented. The mesogenic moiety is similar to that in the TSiKN65F mesogen reported by Naciri et al., which displays a de Vries-type SA* phase. The one parameter varied across the series reported herein is the length of the alkyl chain linking the mesogen moiety to the bifurcated siloxane tail.