Electroclinic Liquid Crystals Research Articles

Wide range room temperature electroclinic liquid crystal mixture with large induced tilt and very small layer contraction

Optical polarizing microscopy, synchrotron X-ray scattering, and electro-optic studies in the SmA* phase of a formulated wide range room temperature liquid crystal mixture indicate a large electric field induced optical tilt up to 23.5° along with a very small contraction of smectic layers. Applied field primarily lifts the zero-field degeneracy of the azimuthal order of the molecules in smectic layers resulting in small layer contraction.

Effect of ZnO nanoparticles on the SmC*-SmA* phase transition temperature in electroclinic liquid crystals

ZnO nanoparticles (NPs), synthesized in an alcoholic medium at room temperature, were added to electroclinic liquid crystal (ELC) materials. The addition of ZnO NPs in ELCs, caused a remarkable shift in SmC*-SmA* phase transition which was investigated from the dielectric and electro-optical measurements. The anchoring of ELC molecules around ZnO NPs creates orientational distortions near the surface, which may give additional ordering to the ELC molecular arrangement. After analyzing collective dielectric relaxation processes of ZnO NP doped ELCs, three distinct loss peaks were observed. The different behavior of ZnO NP doped ELC from pure ELC has been explained by determining the dielectric strength, the distribution parameter and the corresponding relaxation frequency, and so on, and then these results have been compared with the data calculated by using the theoretical model. The effect of ZnO NPs addition on physical parameters, such as spontaneous polarization (Ps) and rotational viscosity (η) has also been observed.

Effect of graphene oxide nanomaterial in electroclinic liquid crystals

The collective dielectric relaxation studies have been carried out on the electroclinic liquid crystals (ELCs) doped with 0.1 to 0.2 wt % of graphene oxide (GO) in the frequency range of 20 Hz to 1 MHz. The GO favors for a good quality vertical alignment without any surface treatment of the substrates. The coupling of GO with indium tin oxide (ITO) substrate and ELC materials affects the molecular ordering and supports the ELC molecules to be aligned along the GO attached to the ITO surface in vertical direction. The vertical alignment can be changed to homogeneous by applying a high bias field to the sample and such converted homogeneous cell shows an additional dielectric relaxation peak in the low frequency side of Goldstone mode in SmC∗ phase due to presence of GO whereas in the pure material no such peak was observed. The frequency separation of both peaks (Goldstone mode and an additional peak) increases with temperature and low frequency peak vanishes near transition temperature.

Field-induced transition from homeotropic to planar geometry in the SmC* phase of an electroclinic liquid crystal

The optical and electrical behavior was investigated of a symmetric liquid crystal (LC) cell: ITO–silane–LC–silane–ITO. The silane layer induces a perfect homeotropic alignment of the molecules of the studied electroclinic liquid crystal (ELC) material, BDH 764E. A field-induced transition from the perfect homeotropic to planar orientation in the chiral smectic C (SmC*) and smectic A (SmA) phases of the ELC was observed. Optical and dielectric studies were performed for both alignment (geometry) modes. The field-induced transition from the homeotropic to planar orientation was studied vis-à-vis the high negative dielectric anisotropy obtained in the studied material. Such an ELC with large negative dielectric anisotropy and perfect homeotropic alignment may have important implications for modern LC display technology.

Correction to Molecular Packing in Electroclinic Liquid Crystal Elastomer Films

Correction to Molecular Packing in Electroclinic Liquid Crystal Elastomer Films

Enormous Change in Tilt Angle and Transition Temperature of Smectic C*–Smectic A Phase in Electroclinic Liquid Crystal Material

Optical and textural studies have been carried out on special class of ferroelectric liquid crystals (FLCs) known as de Vries electroclinic liquid crystals (ELCs) in conjunction with ethanol (C2H5OH). Ethanol shifts the threshold voltage to a low value of 3 V and nearly doubles the optical tilt in the ELC under study. Addition of ethanol shifts the smectic C*–smectic A (Sm C*–Sm A) phase transition temperature remarkably thereby suggesting a strong interaction between ELC and ethanol. The typical randomized structure of de Vries ELCs probably paves way to a possible interaction with ethanol.

Enhancement of ferro-para transition in ethanol doped ferroelectric liquid crystals

The influence of ethanol (C2H5OH) doping in the electroclinic liquid crystal has been investigated using dielectric spectroscopy. A giant shift in ferroelectric (SmC∗) to paraelectric (SmA) phase transition has been observed. After analyzing collective relaxation processes, it is confirmed that Curie–Weiss law is obeyed in the vicinity of SmC∗ to SmA phase transition after doping with ethanol. It has been predicted that the dipole-dipole interaction due to ethanol molecules creates the uniformity in the randomized liquid crystal molecules in paraelectric phase, resulting in the increase in the ferroelectric phase in de Vries electroclinic liquid crystal materials.

Anomalous dependence of response time on the electric field in an electroclinic liquid crystal with large induced tilt and polarization

We investigate the response time of an electroclinic liquid crystal with large induced tilt and polarization (de Vries smectic A∗). The response time is found to increase with the increase in electric field up to a threshold value in contrast with the general case of a decrease with field. This behavior is unusual and contrasts with that observed for a conventional smectic-A∗ phase. The anomalous behavior is explained by the Langevin process of the director reorientation by assuming an increase in the effective dipole moment (μeff) with field. The response time decreases with the increase in temperature in the smectic-A∗ phase. We find the existence of a finite correlation length of a few tens of nm which in both temperature and field dependent.

Molecular Packing in Electroclinic Liquid Crystal Elastomer Films

The structure and molecular packing of an electroclinic liquid crystal elastomer (ELCE) is examined using X-ray diffraction in an unstressed state, under mechanical load, and in the presence of electric fields of varying strength and opposing polarity. A fully three-dimensional pattern of the unstressed elastomer is constructed revealing a detailed look into the molecular packing. Several expected features have been observed in the elastomer film, including well-ordered smectic layering, chevron formation under increasing mechanical load, and molecular reorientation accompanied by layer contraction in the presence of an electric field. Additional intralayer rearrangements related to the molecular switching in the presence of an electric field have also been observed. A model based on periodic pair distribution functions is presented to explain the X-ray scattering observations under each of the tested conditions, starting with the observation that layer-related diffraction bands cannot simply be indexed a...

Memory effect in Sm C* phase of electroclinic liquid crystals

Dielectric, textural, and electro-optical investigations have been carried out on a material exhibiting de Vries smectic A phase transition in sample cells of various thicknesses. Thickness independent memory effect has been observed in this special class of ferroelectric liquid crystals (FLCs) known as de Vries electroclinic liquid crystals. Unlike the FLC materials, surface anchoring energy does not play a role in the memory effect due to the fact that pitch is around 2 micron and basic assumption of SSFLC effect is not valid. The conversion of short-range azimuthal (Φ) ordering of molecules in Sm C* phase into a long-range azimuthal ordering under the influence of field could be one of the possible explanations. The transformation of molecular ordering in Sm C* phase under field may be mediated by dipolar interaction and viscoelastic properties.

Anisotropic actuation in electroclinic liquid crystal elastomers

The macroscopic mechanical response of a freestanding, electroclinic liquid crystal elastomer to an applied electric field is described. Contraction strains and shearing were observed upon e-field application when 60-μm-thick elastomer films were tested normal and parallel to smectic layers, respectively. The anisotropic response observed in the two orthogonal directions with respect to the smectic layers correlate calculated from mechanical studies with the induced tilt of the molecules. The electrostrictive and electroclinic coefficients agree well with the values calculated from optical tilt angle measurements.

A dielectric mode in electroclinic liquid crystals

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt near Tc. Tilt randomization has been considered as an order parameter because in both the phases molecules are tilted but it is disordered in Sm-A phase making its intralayer polarization zero.

Thickness-independent memory effect in ferroelectric liquid crystals

Memory in ferroelectric liquid crystals (FLCs) is well known in the literature where thickness of the cell is less than the pitch value of the material. Here, we report a thickness independent memory in a class of FLCs called the de Vries electroclinic liquid crystals. Thickness independency of memory effect is observed by dielectric spectroscopy and texture observation. The memory observed in SmC* phase of de Vries material is entirely different from conventional FLCs. In the former case, it is the inherent property of the material but in the latter it is dependent on the cell geometry. In de Vries material, it is probably the randomization that is playing a major role.

Is Curie–Weiss law valid in every ferro-to-para transition?

The Curie–Weiss law is obeyed where there is a change in the order parameter and the symmetry of molecules at the transition temperature (Tc) of ferro-to-para phase. The ferroelectric transition is not strictly governed by the Curie–Weiss law. In fact, a number of solid ferroelectrics show deviation from the Curie–Weiss law. It has been shown in the present letter that the de-Vries electroclinic liquid crystals, with a wide smectic A (Sm-A) phase and a high electroclinic effect, do not obey the Curie–Weiss law. The nonlayer shrinkage leads to symmetry conservation that makes the observance of soft mode impossible and hence the deviation.

Layer and molecular orientation in electroclinic liquid crystals: dynamic µ-diffraction

An X-ray beam size was about 3 x 4 µm 2 . The samples were ferroelectric liquid crystals (TK-C101) and measured in the SmA phase. At the high electric field, the compound chevron structure, consisting of the curved vertical chevron and the horizontal chevron, was realized. The molecular orientation measured by the high angle halo-pattern showed the spatially-alternate molecular inclination corresponding to the layer deflection. It was shown that the anchoring effect at the alignment film played an important role for the layer structure. The layer response of the electroclinic effect was similar to that in the high electric field treatment of SmC phase, though the origin of the layer shrinkage was different between them.

Collective dielectric relaxation in smectic A and smectic C⋆ phases near transition temperature of electroclinic liquid crystals

The collective dielectric relaxation processes of electroclinic liquid-crystal materials have been investigated in a thin (3.5μm) planar cell in the frequency range of 1 Hz–10 MHz. Measurement of complex dielectric permittivity versus temperature has been taken on aligned samples filled with different electroclinic liquid-crystal materials. The dielectric data were collected by using the measuring electric field perpendicular to the smectic layer planes. The behavior of the collective dielectric modes has been studied in smectic C*(SmC*) and smectic A(SmA) phases. However, near the transition temperature of SmC*-SmA phase, the dielectric permittivity due to the Goldstone mode and soft mode could not be separated. It has also been observed that the dielectric permittivity increases with temperature before the transition temperature of SmC*-SmA phase under bias field. Temperature dependence of dielectric increments before the transition temperature under bias field and relaxation frequency suggests that the electroclinic liquid crystal shows transitional behavior over a wide temperature range and does not obey the Curie–Weiss law near the transition temperature like in other ferroelectric liquid-crystal materials.

Detection of Continuous Sm C*–Sm A Phase Transition in Electroclinic Liquid Crystals by Dielectric Method

The dielectric relaxation processes near the transition temperature (Tc) of Sm C*–Sm A phase transition in an electroclinic liquid crystal have been investigated in a homogeneously aligned cell of 2.5 µm thickness in the frequency range of 50 Hz to 10 MHz. No abrupt change in phase during the Sm C*–Sm A phase transition has been detected by the dielectic method; therefore no soft mode contribution to the dielectric permittivity near Tc in the Sm C* phase has been found. It has also been observed that the dielectric strength (Δε) near the transition temperature is very high. These two unusual behaviours of electroclinic liquid crystal material have been analyzed by means of theoretical calculations, taking into account the effect of the electric field in generalized Landau-type free energy densities (ΔF). The experimental results in this study agree well with theoretical predictions.

Behaviour of Goldstone and soft mode at Tc in ferroelectric liquid crystals

The dielectric behaviour of an electroclinic ferroelectric liquid crystal has been investigated by dielectric relaxation method in the frequency range of 50 Hz–10 MHz in a thick (6.5 μm) cell. The results have been reported in a planarly aligned sample, where the measuring electric field is perpendicular to the director and parallel to the smectic layer planes. It is observed that in the phase transition region from Sm C *–Sm A, the collective dielectric processes such as Goldstone and soft mode could not be separated. These two dielectric processes are reported and discussed as a function of temperature and bias voltages in Sm C * and Sm A phases and near the phase transition temperature.

Dielectric Relaxation Near the Transition Temperature of Sm C*-Sm A Phase in Electroclinic Liquid Crystal

The dielectric relaxation processes of electroclinic liquid crystal material have been investigated in a thin (2.5 w m) planarly well aligned cells in the frequency range of 50 Hz to 10 MHz. The behaviour of Goldstone and Soft mode which comes due to phase fluctuation and tilt fluctuation of the molecules has been studied at different temperatures and different bias fields in Sm C* and Sm A phases. It has been observed that Goldstone mode appears in Sm C* phase and soft mode in Sm A phase. However, near the transition temperature of Sm C*-Sm A, the dielectric permittivity due to Goldstone mode and soft mode could not be separated. It is concluded and discussed that there is no sharp transition temperature of Sm C*-Sm A phase in electroclinic liquid crystal, due to the continuous decrease in the dielectric permittivity and increase in the relaxation frequency near the transition of Sm C* to Sm A phase.

Field-dependent tilt and birefringence of electroclinic liquid crystals: theory and experiment.

An unresolved issue in the theory of liquid crystals is the molecular basis of the electroclinic effect in the smectic-A phase. Recent x-ray scattering experiments suggest that, in a class of siloxane-containing liquid crystals, an electric field changes a state of disordered molecular tilt in random directions into a state of ordered tilt in one direction. To investigate this issue, we measure the optical tilt and birefringence of these liquid crystals as functions of field and temperature, and we develop a theory for the distribution of molecular orientations under a field. A comparison of theory and experiment confirms that these materials have a disordered distribution of molecular tilt directions that is aligned by an electric field, giving a large electroclinic effect. It also shows that the effective dipole moment, a key parameter in the theory, scales as a power law near the smectic-A--smectic-C transition.