Antiferroelectric Liquid Crystal Research Articles

Technical aspects of dielectric spectroscopy measurements of liquid crystals

AbstractThe dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose, the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in Sm A*, Sm C*, and Sm CA* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all possible relaxations in antiferroelectric liquid crystals (LCs).

Temperature and bias electric field dependence of dielectric parameters of ( S )-(+)-4-(1-methylheptyloxycarbonyl) phenyl-4’-(3-butanoiloxyprop-1-oxy)biphenyl-4’-carboxylate

Temperature and bias electric field-dependent dielectric behaviour of an antiferroelectric liquid crystal compound was investigated in the frequency range 1 Hz to 10 MHz. The investigated compound possessed paraelectric (SmA*) and antiferroelectric () phases, in addition to a monotropic hexatic antiferroelectric smectic () phase. The dielectric study showed the presence of a soft mode in the SmA* phase. In the phase, two weak relaxation modes were observed for a planar orientation of molecules. These two modes were related to antiferroelectric ordering and the helical structure of the phase. Two modes, related to the hexatic order of the phase, were also observed and characterized in terms of bond-orientational order and coupling to the molecular tilt direction. With respect to non-collective modes, rotation around the short axis of molecules was detected in the SmA* and phases for a homeotropic alignment.

Nonlinear Dielectric Spectroscopy of MHPOBC

We apply nonlinear dielectric spectroscopy to an antiferroelectric liquid crystal MHPOBC at various smectic sub-phases. In the SmCα* phase, the third-order nonlinear dielectric spectrum ϵ3* is found to be composed of the contributions from ferroelectric soft mode and soft mode of the SmCα* phase. The obtained spectrum is well ascribable by the theoretical one calculated by the Landau-type of phenomenological theory. In the SmC* phase, ϵ3* shows a complicated form, and is different from one observed in usual ferroelectric liquid crystals. In the SmCFI1*( SmCγ*) phase, ferrielectric Goldstone mode with broad distribution of relaxation times is observed in both ϵ1* and ϵ3*.

Synthesis and Mesomorphic Properties of New Achiral Liquid Crystals with 3-Alkoxy-2-(alkoxymethyl)-1-propoxy Swallow-Tails

New liquid crystalline (biphenylcarbonyloxy)benzoates with an achiral swallow-tail derived from 3-alkoxy-2- (alkoxymethyl)-1-propanol [(ROCH2)2CHCH2OH, R = Me, Et, Pr, Bu] were prepared. These liquid crystals exhibited the phase sequence (I-SmA-SmCalt-(SmCX)-Cr) and showed antiferroelectric-like Smectic C phase (SmCalt) at temperature lower, and temperature range broader than do the compounds containing a branched alkyl group as a swallow-tail. The temperature ranges of antiferroelectric phase were found to be 30-90 oC and crystallization temperatures were 4-60 oC. The binary mixture of an achiral swallow-tailed liquid crystal and a chiral antiferroelectric liquid crystal, (S)-MHPOBC showed antiferroelectric smectic C phase at temperature much lower than the single chiral antiferroelectric liquid crystal does.

Influence of ionic conductivity and interfacial charges on the relaxation dynamics of smectic phases of an antiferroelectric material

The results, reported here, are on the dielectric relaxation spectroscopy and the switching current behavior at the SmC ⁎ and SmC A ⁎ phases of an antiferroelectric liquid crystal (AFLC) material. It has been observed that a large number of ions are generated due to thermal energy in the studied AFLC compound. The mobility of these charges destabilizing the SmC ⁎ phase for 2 μm cell, moreover such charges creates an interaction making non-linear behavior of soft mode in Sm A phase. In 2 μm thin cell we have observed the coexistence of the SmC ⁎ and SmC A ⁎ phases by extending SmC ⁎ phase into certain temperature region of SmC A ⁎ phase. It is clear from the dielectric data that the thin cell helps to promote super-cooled state during cooling. The observed Maxwell-Wagner-Sillars (MWS) mode in SmC ⁎ phase appears at around 100 Hz for 10 μm cell due to the inhomogeonity of thermally generated ions between the layer planes as well as at the electrode surface of cell. In 2 μm cell, the MWS mode is shifted to lower frequency and superimposing with DC loss. It is observed experimentally that a new phase seems to be emerged at super-cooled temperature region in the 2 μm cell that may be due to the confined geometry of thermally generated ions and high surface anchoring force.

Antiferroelectric liquid crystals with induced intermediate polar phases and the effects of doping with carbon nanotubes

By mixing a commercial broad-temperature-range nematic liquid crystal mixture with a single-component antiferroelectric chiral smectic exhibiting two different chiral smectic-C-type phases as only mesophases, we have induced three phases which appear in neither of the two components; the paraelectric SmA∗ phase and the so-called intermediate phases SmCβ∗ and SmCγ∗, antiferroelectric and helielectric in nature, respectively. The generation of the two latter phases in mixtures where one component is an essentially non-chiral nematic is highly unexpected, since these phases are generally linked to high degree of smectic order and/or strong chiral interactions. It is probably made possible through microphase segregation driven by the incompatibility of the fluorinated tail of the smectic component with the non-fluorinated constituents of the nematic mixture. We also doped the nematic with single-wall carbon nanotubes (SWCNTs) before adding it to the smectic at the same concentration, allowing us to study the effect of SWCNTs on antiferroelectric liquid crystals. Although the final SWCNT concentration was very small (0.002wt%) the phase sequence was radically altered, the ordinary SmC∗ phase now being present all the way between SmA∗ and crystallization, while all other variations of smectic-C-type order were suppressed.

Antiferroelectric liquid crystals with amide linking group positioned at chiral tail

A homologous series of chiral liquid crystal compounds, N‐methyl‐N‐pentyl‐(S)‐2‐(6‐(4‐(4‐alkyloxyphenyl)benzoyloxy)‐2‐naphthyl)propionamide, with an amide linkage in a chiral tail was synthesized and their mesomorphic properties studied. All the materials possessed an antiferroelectric smectic C (SmCA*) phase, which was confirmed by observations of microscopic texture, switching current behaviour and electro‐optical responses. The spontaneous polarization, P s, and apparent tilt angle, θ, were also measured. The maximum P s values are in the range of 173–222 nC cm−2, and the maximum θ values are in the range of 26–30°.

Pressure–Temperature Phase Diagram of Antiferroelectric Liquid Crystal

The pressure–temperature phase diagram of an antiferroelectric liquid crystal, 4-(1-methylheptyloxycarbonyl)phenyl-4'-octyloxybiphenyl-4-carboxylate (MHPOBC), was obtained from switching current and dielectric constant measurements. The ferroelectric chiral smectic-C phase disappeared at about 60 MPa and the regions of the chiral smectic-Cα and chiral smectic-Cγ phases widened under pressure.

Anticlinic Antiferroelectric Smectic Liquid Crystal Formed by an Asymmetric Banana‐Shaped Molecule

Anticlinic Antiferroelectric Smectic Liquid Crystal Formed by an Asymmetric Banana‐Shaped Molecule

Phase Transitions in Ferroelectric and Antiferroelectric Liquid Crystals

The research development of ferroelectric and antiferroelectric liquid crystals is reviewed, focusing on our findings. First, their structures are briefly explained and the Landau theories are presented such as the two-layer model and the discrete model. Then, a unique phase having a short pitch helical structure, designated as SmCα*, as along with the electric field effect on it, are discussed on the basis of the discrete model. Lastly, results on the nonlinear dielectric spectroscopy of the soft mode which induces the SmCα* phase are presented.

Influence of the Optical Purity on the Phase Sequence of an Antiferroelectric Liquid Crystal, MHPOBC, under Pressure

The influence of the optical purity on the phase sequence of an antiferroelectric liquid crystal, MHPOBC, under pressure was studied by dielectric and switching current measurements. In the case of R: S = 75:25, the phase sequence is independent of pressure up to 150 MPa. As the pressure is increased, the temperature width of the ferroelectric SmC* phase widens slightly but that of the antiferroelectric SmC A * phase narrows a little. On the other hand, the ferroelectric SmC* phase in the case of R: S = 100:0 disappears at 100 MPa. The ferrielectric SmCγ* phase don't disappear and the temperature width of the phase widens. On the basis of the results under pressure, we discuss the strengths of the ferroelectric and antiferroelectric orderings in MHPOBC with different optical purities.

Unique Pitch Evolution in the Smectic-Cα*Phase

Employing resonant x-ray diffraction, we observed unique pitch evolutions in the smectic-C*(alpha) phase in mixtures of two antiferroelectric liquid crystals. Our results show that the pitch in this phase continuously evolves across 4 layers, contradicting a theoretical model that predicts that the smectic-C*(FI2) phase intervenes in the smectic-C*(alpha) phase. The phase sequences we found can be explained by another model that includes one type of long-range interaction among smectic layers.

Electric field‐induced birefringence, optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator‐based novel set‐up, the electric field‐induced in‐plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in‐plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmCA*(1/2)] compared with that for two layers [SmCA*(0)]. On application of the electric field on the high temperature side of the SmCA*(1/2) phase (80.1–81.5°C), a field‐induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4–80.1°C) the transition takes place to SmC* via the SmCA*(1/3) phase. The in‐plane birefringence exhibits a critical power law dependence for the SmC*–SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free‐standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmCA* phase and below SmCA*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.

Solitary wave propagation in antiferroelectric liquid crystal cells and the quadrupolar term in the interlayer interaction

Planar antiferroelectric liquid crystal (AFLC) cells exhibit propagating solitary waves. By measuring the retardation and the apparent tilt angle, we find that the conventional model for the solitary wave propagation in AFLCs is not generally applicable. The model being proposed here shows that the solitary wave propagation both along and normal to the smectic layer plane can be explained by a double well potential energy governing the relative orientations of the c directors in the adjacent smectic C layers with an energy barrier between the synclinic and the anticlinic states. The quadrupolar term of the interlayer interaction is found to be significantly large in a pure AFLC.

Orthoconic antiferroelectric liquid crystals containing a terphenyl rigid core

Four homologous series of orthoconic (45 degree tilt) antiferroelectric liquid crystals, containing either a terphenyl or fluorosubstituted terphenyl core as mesogenic unit, have been synthesized and their mesomorphic behaviour investigated by the DSC and polarized light microscopy were discussed. X-ray diffraction studies have shown that above the ferroelectric smectic C* phase or antiferroelectric there was a de Vries-type smectic A phase. Some correlations between phase transition temperatures and positions of fluorosubstitution of rigid core are discussed. Temperature dependence of the helical pitch was measured for three different cores.

Orihara-Ishibashi Continuous Model of Antiferroelectric Liquid Crystals

Soon after the discovery of antiferroelectric liquid crystals, Orihara and Ishibashi proposed a theoretical model for these systems. The model allows for three simply modulated structures: the ferroelectric, the antiferroelectric and the structure with ferrielectric properties. In the region where neither of simply modulated phases is stable, the incommensurate phase exists, formed by domains of a simply modulated structure separated by domain walls, where order parameters change rapidly. Although experimental observations ruled out the simply modulated ferrielectric structure as well as the incommensurate structure, the analysis of the model can still be important for similar systems.

Frequency dispersion of dielectric permittivity–voltage hysteresis during field driven ferroelectric-antiferroelectric-ferroelectric transition in antiferroelectric liquid crystal (SmCA*)

Dielectric permittivity at different polarizing states during field driven ferroelectric-ferroelectric (F-F) transition and the dielectric relaxation spectra in the antiferroelectric liquid crystal (AFLC) phase have been studied to elucidate the interaction processes involved during the transition. The unwinding process of antiferroelectric helix has been found to be governed by the polar coupling between the applied field and the antiferroelectric polarization vector followed by nonpolar interaction with the dielectric anisotropy of the tilted molecules. Frequency dependence of the permittivity-voltage hysteresis behavior observed during the F-F transitions is concluded to be due to different relaxation processes involved in the AFLC phase.

Collective and antiferroelectric dielectric modes in a highly tilted three‐ring ester

Broadband dielectric spectroscopy was applied to investigate a newly synthesized antiferroelectric liquid crystal, (S)‐(+)‐4′‐(1‐methylheptyloxycarbonyl)biphenyl‐4‐yl 4‐(6‐heptanoyloxyhex‐1‐oxy)benzoate. Near the SmA–SmC* phase transition the soft and the Goldstone modes are observed. The temperature dependence of the soft mode dielectric strength follows the Cure–Weiss law. In a narrow range of temperature the collective and antiferroelectric relaxation modes coexist as a result of the competition between ferroelectric and antiferroelectric order in the successive smectic layers. In the antiferroelectric phase the low frequency, PL, and high frequency, PH, modes appear, which can be related to the in‐phase and anti‐phase azimuthal angle fluctuations of the director, respectively. The relaxation frequency of the PH and PL modes decreases linearly with decreasing temperature and satisfies the Arrhenius law. Some parameters involved in the extended Landau model of the SmA–SmC* phase transition could be obtained.

Thermodynamic and dielectric studies on antiferroelectric liquid crystal ( S )-4-(1-methylheptyloxycarbonyl)phenyl 4-(6-heptanoyloxyhex-1-oxy)biphenylate

Dielectric relaxation phenomena have been investigated for the antiferroelectric liquid-crystal compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4-(6-heptanoyloxyhex-1-oxy)biphenylate, exhibiting a paraelectric SmA* phase, a ferroelectric SmC* phase, and a wide-temperature-range (∼54–86.0°C) antiferroelectric phase. Dielectric investigations under planar molecular anchoring have been carried out in the frequency range 0.1 Hz to 10 MHz. The paraelectric SmA* phase shows soft mode relaxation related to amplitudons. Its relaxation frequency is highly temperature dependent, and decreases with decrease in temperature. The dielectric strength obeys Curie–Weiss law. The ferroelectric SmC* phase shows a Goldstone mode of relaxation related to the phasons. The relaxation frequency of this mode lies at ∼3.6 kHz, and is almost invariant with temperature. The dielectric strength of the Goldstone mode slowly increases with decrease in temperature. Two characteristic absorption peaks are seen in the phase: the higher-frequency peak corresponds to antiphase fluctuation of the azimuthal angle of directors in the anti-tilted molecular pairs, whereas the lower-frequency peak corresponds to in-phase fluctuation of the azimuthal angle of directors in the anti-tilted molecular pairs.

Theory of two- and three-dimensional bent-core mesophases

An extension of the vector-wave theory of antiferroelectric bent-core liquid crystals to columnar and soft-crystalline mesophases is presented. We enumerate and describe the phases resulting from the condensation of one, two, and three vector waves in the isotropic liquid of bent-core molecules. Beside the one-dimensional B2 and smectic- CP phases, 24 columnar and soft-crystalline orthorhombic, monoclinic, and triclinic phases can be stabilized when two or three waves have nonparallel wave vectors. Their symmetry groups, molecular arrangements, and phase diagrams are worked out and used to identify the structures of the main observed bent-core phases and their various subphases.