Room Temperature Ferroelectric Liquid Crystal Research Articles

Room-temperature ferroelectric nematic liquid crystal showing a large and diverging density.

The ferroelectric nematic phase (NF) is a recently discovered phase of matter in which the orientational order of the conventional nematic liquid crystal state is augmented with polar order. Atomistic simulations suggest that the polar NF phase would be denser than conventional nematics owing to contributions from polar order. Using an oscillating U-tube densitometer, we obtain detailed temperature-dependent density values for a selection of conventional liquid crystals with excellent agreement with earlier reports. Having demonstrated the validity of our method, we then record density as a function of temperature for M5, a novel room-temperature ferroelectric nematic material. We present the first experimental density data for a NF material as well as density data for a nematic that has not previously been reported. We find that the room-temperature NF material shows a large (>1.3 g cm-3) density at all temperatures studied, notably including phases without polar order. An increase in density at phase transitions is observed. The magnitude of the increase for the intermediate-to-ferroelectric nematic (NX-NF) transition is an order of magnitude smaller than the isotropic-nematic (I-N) transition. We then probe potential consequences that may result from an elevated density through measurement of the refractive indices (no and ne). The navg of M5 is compared with 5CB and polar smectic liquid crystals. We observe how the highly polar nature of the system counteracts the effects of an increase in density. With knowledge of experimental density, we are able to derive an approximation that yields the polar order parameter, 〈P1〉, from polarisation measurements. Present results may be typical of ferroelectric nematic materials, potentially guiding material development, and is especially relevant for informing ongoing studies into this emerging class of materials.

Effect of γ-irradiation on various important physical properties of a room temperature ferroelectric liquid crystal mixture and its silver nanocomposite

A room temperature ferroelectric liquid crystal (FLC) mixture and its silver nanocomposite have been irradiated by various doses of γ-radiation from 60Co. Phase transition temperatures, dielectric permittivity, optical tilt angle, spontaneous polarization, rotational viscosity, switching time, and conductivity of the systems have been determined as a function of the irradiation dose and temperature. Phase transition temperatures and optical tilt angles of both the systems decrease significantly, more in the pristine mixture, meaning a decrease of the order parameter with irradiation. Remarkable changes are exhibited in the dielectric permittivity as well as in their molecular dynamics as confirmed by the change in Goldstone and soft mode relaxation frequencies. The conductivity of the irradiated mixture increases considerably compared to its nanocomposite probably due to the production of more ions by the radiolysis process. Spontaneous polarization, rotational viscosity, and switching time are also found to decrease linearly with irradiation dose. At the maximum absorbed dose, switching time decreases by 140 μs in the pristine mixture and 110 μs in the nanocomposite suggesting their more suitability in display applications. The concentration of the chiral dopant also influences the impact of irradiation.

Effect of γ-irradiation on the display parameters of a room temperature ferroelectric liquid crystal mixture

ABSTRACT A room temperature ferroelectric liquid crystal (FLC) mixture has been studied by irradiating with γ-radiation from 60Co source. Important display parameters such as dielectric permittivity, conductivity, spontaneous polarisation, optical tilt angle, rotational viscosity and switching time have been determined as a function of the irradiation dose. Study reveals that transition temperatures involving different phases and optical tilt angle decrease significantly, more at higher doses, which means irradiation decreases the order parameter in different phases. Conductivity increases twelve times probably due to the production of ions or impurities by radiolysis process. Irradiation also affects the dynamics of the molecules manifested by the change in critical frequencies of relaxation modes. Changes in other electro-optic properties suggest that degradation of display quality may not be significant at low γ-radiation exposure but the effect will be severe at high dose.

Effect of silver nanoparticle doping on the physicochemical properties of a room temperature ferroelectric liquid crystal mixture

We have investigated the effect of doping of a small amount of silver nanoparticles (AgNPs) on the physicochemical properties of a room temperature ferroelectric liquid crystal mixture. Polarizing optical microscopy, differential scanning calorimetry, synchrotron X-ray diffraction, dielectric and electro-optic studies showed that the inclusion of AgNPs greatly influences the phase transition temperatures, structural, dielectric, and electro-optic properties. Doping slightly decreases the SmC*-SmA* and SmA*-Iso transition temperatures signifying decreased ordering which is also reflected in reduced tilt angles. The Goldstone mode dielectric increment of the nanocomposite increases twofold but the critical frequency decreases slightly. Spontaneous polarization of the mixture also increases significantly in the presence of AgNPs whereas the conductivity decreases indicating trapping of ions by the nanoparticles. Goldstone mode rotational viscosity decreases and as a result of which the switching time also decreases by 88 μs making the nano doped mixture more suitable for practical applications.

Dielectric Investigations in a Room Temperature Ferroelectric Liquid Crystal

Dielectric relaxations along with other optical and thermal properties of a room temperature ferroelectric liquid crystal (S) -1-[4`-(octyloxy carbonyl) phenyl]-4-[2-chloro-(4-benzamido acetyl) phenyl propionato]-Benzoate is reported in the low frequency range. The mesogen exhibits cholesteric, smectic A* and smectic C* phases identified by textures threaded, focal conic and broken focal conic respectively. It is interesting to note the wide thermal span of smectic C* ranging from 65 °C to -20 °C. A relaxation process in which the relaxation is suppressed by external bias voltage, named as Gold stone mode originated at around 500 Hz is investigated with respect to temperature in the entire thermal span of smectic C* phase and is analyzed by Cole-Cole plots. The relaxation frequency shifted to the lower side as the temperature is decreased indicating Arrhenius behaviour. The distribution parameter α for various relaxation processes is calculated. From the Arrhenius graph the activation energy of the smectic C* phase is elucidated. In addition, the thermal stability of three mesophases namely cholesteric, smectic A* and smectic C* is discussed. The bi-stable switching (ON and OFF states) of the mesogen are recorded. A token of evidence for memory storage is demonstrated through dielectric hysteresis data. Light filtering action in smectic C* phase is discussed.

Wide range room temperature ferroelectric liquid crystal mixture with microsecond order switching

Using four pyrimidine compounds as host matrix and a partially fluorinated biphenylyl as dopant a ferroelectric liquid crystal mixture has been formulated which exhibits SmC* phase over 90° from below ambient down to at least 12°C. X-ray, dielectric and electro-optic studies reveal that the mixture possess almost temperature independent high optical tilt, low driving voltage and switching speed around 100μs, thus the mixture will be suitable for display applications.

Confinement driven effects in a room temperature ferroelectric liquid crystal: X-ray, linear and non-linear dielectric investigations

We present results of X-ray, linear and non-linear dielectric constant measurements on a room temperature ferroelectric liquid crystalline phase in its bulk form and upon confinement in an Anopore membrane. The used material exhibits smectic C* (SmC*) helical pitch of p ≅ 200 nm, which is comparable to the pore dimension of the membrane. X-ray measurements show several interesting results including unusually strong harmonic reflections, substantial broadening of the peak profile, correlation length smaller than the pore diameter, etc. The data suggest that the smectic A–SmC* transition could be very close to a tricritical point in the bulk and moving away from it upon confinement. The dielectric studies show that confinement accelerates the relaxation dynamics of both the soft and the Goldstone modes, although with a reduced strength. The non-linear component of the dielectric constant exhibits qualitatively different thermal behaviour in the bulk and Anopore samples.

Influence of silica nanoparticles on dielectric spectroscopy and polarization switching responses of novel ferroelectric liquid crystals

Silica (SiO2) nanoparticles in different wt/wt% ratios were dispersed in a novel room temperature ferroelectric liquid crystal mixture. Dielectric relaxation spectroscopy (50 Hz–1 MHz) and electro-optic responses were investigated. Dielectric permittivity gradually decreases by about 40% up to 1 kHz with increasing SiO2 concentration in the chiral smectic C phase. However, an increase of up to 1.5% has been observed for all the silica-doped samples above 2 kHz. We noticed an increase in optical transmission with increasing SiO2 concentration from 0.01 to 0.1 wt/wt%.

Induction and Enhancement of Ferroelectric Smectic C* Phase in Multi-Component Room Temperature Mixtures

Six multi-component room-temperature ferroelectric liquid crystal (FLC) mixtures have been formulated and investigated using optical polarizing microscopy, dielectric spectroscopy as well as electro-optic methods. In all the mixtures smectic C* phase is either induced or enhanced. Effect of various dopants on the occurrence and thermal stability of different phases has been discussed. Spontaneous polarizations, measured as a function of temperature by reversal current method, are found to vary between 13–62 nC/cm2 at room temperature in all the mixtures. Different types of dielectric absorption behavior were observed in planar geometry of the FLC cells. Critical frequencies of Goldstone and soft modes have been determined by frequency dependent dielectric study. Rigidity of the core structure, nature of chirality and extent of fluorination of the constituent molecules are found to have pronounced effect on the collective mode relaxation behavior of the mixtures.

EFFECT OF CELL THICKNESS ON THE DIELECTRIC RELAXATION PROCESSES IN FERROELECTRIC LIQUID CRYSTALS

Dielectric relaxation spectroscopy in the frequency range 50 Hz to 1 MHz has been carried out in a room temperature ferroelectric liquid crystal mixture in the SmC*, SmA and N* phases in cells of different thickness. The relaxation frequency fr, distribution parameter α and dielectric strength δ∊ have been evaluated. Goldstone mode, domain mode and soft mode have been observed. It is found that the cell thickness has a significant effect on the dielectric parameters of the ferroelectric liquid crystal material. The results have been discussed.

Effect of polymer viscosity on morphological and electro-optic properties of aligned polymer dispersed ferroelectric liquid crystal composite films

A room temperature ferroelectric liquid crystal mixture was dispersed in UV curable polymers of different viscosity in 30:70 wt/wt ratio. These polymer dispersed ferroelectric liquid crystal (PDFLC) composite films were prepared by polymerization induced phase separation technique. It was found that the polymer viscosity influences the droplet size and the electro-optic properties. The spontaneous polarization of PDFLC decreases with an increase in polymer viscosity. The droplet morphology and electro-optic properties of these materials have been investigated in an aligned configuration.

Ferroelectric Characterization of a Room Temperature Liquid Crystal: 2Cl.BAAP.180-BBP by Low Frequency Dielectric and Optical Methods

The isotropic to nematic(IN), nematic to cholesteric(NCh), cholesteric to smectic-A (ChA) and smectic-A to smectic-C* (AC*) phase transition temperatures exhibited by a room temperature ferroelectric liquid crystal RTFLC, viz., 2Cl.BAAP.180-BBP are determined by the observed anomaly in differential permittivity, d l '/dT. The low frequency dielectric dispersion along with the activation energy are detailed in the N, Ch, SmA and SmC* phases. The Soft mode and Goldstone mode relaxation in the vicinity of AC* phase transition are discussed. The trend of distribution parameter h-values for the off-centred Debye relaxation is discussed. Temperature variation of dilectric strength for the soft mode relaxation is estimated through ferrolectric Curie-Weiss law. Observed temperature variation of Spontaneous Polarization P s (T), Tilt angle θ(T), Response Time s (T), and Effective Torsional viscosity m (T) is found to agree with the Mean field expectation. Relative strength of P s among the reported values is discussed as a function of extended conjugation about the chiral centre.

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for fNRM, δeNRM and distribution parameter αNRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.

Low frequency dielectric modes in the SmC*phases of the ferroelectric liquid crystals

Abstract The low frequency dielectric relaxation has been studied for the SmC* phase of the room-temperature ferroelectric liquid crystal mixture. The dielectric spectra have been measured on the „bookshelf”oriented sample in the frequency range from 100 Hz to 100 kHz with the electrical measuring field being parallel to the layer planes. From experimental results presented in this work one can see for the SmC* phase in the real finite samples it is necessary to be taken into account the deformation of director field within the smectic layer fixed by the boundary conditions.

Measurements of Pitch of a Ferroelectric Liquid Crystal at High Pressures

Abstract We have carried out pitch measurements in the smectic C* phase of a commercially available room temperature ferroelectric liquid crystal, SCE12 (BDH), as a function of pressure. An optical diffraction technique has been employed for the purpose. The diffraction patterns were recorded using a video camera as a 2-d detector. The data is extracted using a frame analysis hardware-software combination. A detailed description of this method is presented. The results show that the application of pressure not only affects the absolute value of the pitch but influences its temperature dependence also. Measurements of spontaneous polarisation also show a similar feature. Possible correlation between the two results is discussed.

Thickness Dependence of Dielectric Properties of a Ferroelectric Liquid Crystal

Abstract A room temperature ferroelectric liquid crystal has been studied by means of dielectric measurements. The results of the studies of the complex electric permittivities for aligned samples of different thicknesses and for the different electric measuring fields are presented. We discuss how dielectric properties depend on boundary conditions, thicknesses of the samples and the magnitude measuring electric fields. The obtained experimental results and the discussion make it possible to determine the spontaneous polarization from dielectric measurements.

Electroclinic effect in finite samples of ferroelectric liquid crystals

Abstract The dispersion of the electroclinic effect of a room temperature ferroelectric liquid crystal benzoic acid 4-octyloxy 4|)2-methyl butyloxy) carbonyl| phenylester has been studied in the vicinity of its ferroelectric phase transition temperature Tc. The relaxation frequencies determined from the dispersion agree with those found from previous dielectric study. The origin of the texture observed on samples in both SC* and SA phases is explained as a consequence of lowering of the smectic layer thickness in finite samples caused by the electroclinic effect.

Behaviour of Ionic Effects on Response Times of Surface Stabilized Ferroelectric Liquid Crystal Devices

The response time measurements of a new room temperature ferroelectric liquid crystal have been done by using the field reversal method. The response time has been found to be frequency dependent and decreases considerably at frequencies above 10 kHz. The temperature dependence of response time showed the presence of ionic effects as is evident from the appearance of the second peak in the current response to an applied bipolar pulse.

Dielectric relaxation in the smectic-A and smectic-C phases of a ferroelectric liquid crystal.

The results of a study of the dielectric properties of a room-temperature ferroelectric liquid crystal in the frequency range from 5 Hz to 13 MHz are presented here. Measurements of complex electric permittivity versus temperature have been taken on aligned samples of different thicknesses with the electric measuring field being parallel or perpendicular to the layer planes. In homogeneously aligned samples two dielectric relaxation regions have been observed in the smectic-${C}^{\mathrm{*}}$ phase, one known as the Goldstone mode and the other showing up in the vicinity of the smectic-${C}^{\mathrm{*}}$\char21{}smectic-${A}^{\mathrm{*}}$ transition called the soft mode. In homeotropically aligned samples the low-frequency molecular relaxation was observed in both smectic phases. Temperature dependences of both dielectric increments and relaxation times suggest that the system studied exhibits a ferroelectric phase transition of the first-order type.

The effect of biasing electric field on the soft mode in the vicinity of the ferroelectric phase transition in liquid crystals

Abstract The complex permittivity of a room-temperature ferroelectric liquid crystal 4-n-octyloxy benzoic acid 4′-[(2-methylbutyloxy)carbonyl]phenyl ester, has been measured in the vicinity of the phase transition in the frequency range 40 Hz-300 kHz. In the para-electric phase the contribution εs of the soft mode to the permittivity and the soft-mode relaxation frequency f s satisfy the Curie-Weiss law. Under a biasing field E, the helicoidal structure is unwound, and εs and f s can then be measured even in the ferroelectric phase. On the other hand, the phase transition is smeared under the influence of E. This smearing results in deviations from the Curie-Weiss law for both εs and f s in the vicinity of the transition. On increasing E, the maximum of the permittivity, εmax, is lowered and shifted to higher temperatures. Both the shift and ε−1 max are proportional to E 2/3. From experimentally found dependences, some constants in the free energy are determined.