Ferroelectric Liquid Crystal DOBAMBC Research Articles

NMR Study of Ferroelectric Liquid Crystal DOBAMBC

Molecular rotational motion in ferroelectric liquid crystal DOBAMBC (p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate) was studied in the smectic A (SmA) and chiral smectic C (SmC*) phases by 13 C-NMR. Spin-lattice relaxation times T 1 of the aromatic carbons at the ortho-positions reflect the rotational motion about the long molecular axis in the liquid crystal phases. Obtained activation energy in the SmC* phase is smaller than that in the SmA phase, indicating that the rotational motion becomes less hindered in the SmC* phase.

Electric field induced domain structure in ferroelectric liquid crystal DOBAMBC

Abstract The influence of an ac electric field on the helical pitch in ferroelectric liquid crystal DOBAMBC has been investigated in a broad range of frequency. The role of the permanent polarization and dielectric anisotropy in the helix unwinding process were studied. The formation of a periodic domain structure under influence of ac electric field was observed. The period of this structure decreases with increasing ac electric field frequency. The possible mechanisms leading to the observed periodic domain structure and its behavior in ac electric field are discussed. The experimental results are compared with predictions of the Dmitrienko-Belyakov and Martinot-Lagarde models.

Phase transitions in ferro-electric liquid crystal dobambc by density

Thermal microscopy, differential scanning calorimetry and dilatometric investigations are carried out on the ferroelectric liquid crystal N(p-n-Decyloxybenzylidene) p-amino 2-methylbutyl cinnamate viz., DOBAMBC which exhibits smectic-A, smectic-C* and monotropic Smectic-H phases. The isotropic-smectic-A (IA), smectic-A-Smectic-C* (AC*) and smectic-C*-Smectic-H (C*H) transitions are found to be first order. Detailed study of pre-transitional effects inferred the nucleation growth analogy to IA transition and a clear first order AC* transition in contrast to predicted second order AC transition. The estimated pressure dependence of transition temperatures using the molar volume and enthalpy data are presented.

Rotational viscosity in ferroelectric liquid crystal dobambc determined by dielectric method

Abstract Spontaneous polarization, tilt, pitch of the helix, and low frequency dielectric dispersion were measured as functions of temperature in the smectic C* phase of the DOBAMBC. It is shown that measured thermodynamic quantities satisfy simple relations, predicted by a phenomenological model for ferroelectric liquid crystals. These relations allow the evaluation of the rotational viscosity, which temperature dependence is of Arrhenius type with an activation energy of 0.51 eV.

Biaxiality of the Ferroelectric Liquid Crystal DOBAMBC in Electric Field

A new electrooptic phenomenon manifesting itself as a field-stimulated change in the optical axes plane is observed in the ferroelectric liquid crystal DOBAMBC. The phenomenon makes an anal-ogon of the thermooptical Mitscherlich effect observed in some solid biaxial crystals. The dependence of the angle between the optical axes on electric field and temperature is determined. It is shown that the existing theories do not explain the observed phenomenon. Eine neue elektrooptische Erscheinung wird beobachtet, die auf der durch das elektrische Feld verursachten änderung der Lage der optischen Achsen in ferroelektrischen Flüssigkristallen be-ruht. Dieses Phänomen ist dem thermooptischen Mitscherlich-Effekt ähnlich, der in einigen festen Kristallen vorkommt. Die Abhängigkeit des Achsenwinkels von der Temperatur und Feldstärke wird untersucht. Die experimentellen Ergebnisse lassen sich mit Hilfe der existierenden Theorien nicht völlig erklären.

NMR evidence of pinning effects in incommensurate insulators

Abstract Magnetic resonance lineshape data provide clear-cut evidence for a pinning of the modulation wave in most incommensurate insulators. Spin-lattice relaxation data fully confirm this conclusion and further show that the impurity induced phason gap is in the 1010−1011 Hz range. An exception to this rule seems to be the organic incommensurate crystal biphenyl and the ferroelectric liquid crystal DOBAMBC. In this last case the phason gap is less than 100 Hz.

Molecular reorientation processes and dynamics of polarization reversal in thin surface stabilized ferroelectric liquid crystal dobambc

Molecular reorientation processes in thin (3 μm) surface stabilized ferroelectric liquid crystal (SSFLC) DOBAMBC have been studied by applying symmetric and asymmetric triangular voltage pulses. For symmetric (-5V-OV-+5 V) triangular pulse a non-linear behaviour and two peaks of polarization reversal current and for asymmetric [(+ or -) 10 V-0 V] pulse four peaks of polarization change near 0 V region have been observed. For symmetric pulse two peaks originate due to composite effect of molecular change initially in the bulk and then at the surface under the influence of depolarization field. The non-linear current response to a linearly varying triangular pulse have been qualitatively explained by considering an intrinsic depolarization field. Dynamics of polarization reversal amongst the two uniform states and also amongst the twisted intermediate state to either of the two uniform states have been studied by applying symmetric and asymmetric square wave pulses. Switching response time corresponding to the change amongst uniform states have an intermediate value between the rise and decay time for the changes from twisted intermediate to uniform state and for the uniform state to twisted intermediate state. The switching amongst the two stable states takes place without actually attaining the twisted intermediate state of FLC.

Colour Switching, Dynamics and Molecular Reorientation Processes in a Very Thin Ferroelectric Liquid Crystal

Symmetric colour switching in unaligned and aligned very thin (<1 µm) samples of ferroelectric liquid crystal DOBAMBC has been observed by applying both polarities of a dc electric field. The polarization-change current developed in a very thin FLC cell has been studied by applying symmetric (-5 V–0 V–+5 V) and asymmetric [(+ or -) 10 V–0 V] triangular wave pulses. The dynamics of colour changes amongst the same colours and amongst the multicoloured 0 V favoured state and a coloured state have been studied by applying low-voltage symmetric and asymmetric square pulses. For a symmetric triangular pulse, a single peak of polarization change and for an asymmetric pulse, two peaks of polarization change near the 0 V region for molecular reorientation amongst the same coloured state have been observed. It has been established that it is essentially the presence of an additional field, called a depolarization field, which causes a single peak of polarization-change current in the case of a symmetric pulse and it is a manifestation of the combined molecular tilt from one coloured state to another without actually attaining the multicoloured state. The switching response time corresponding to a change amongst uniform identical colours has an intermediate value between the rise and decay time for the changes amongst the 0 V favoured multicoloured state and either of the coloured states.

Molecular Reorientation Process and Non-Linear Polarization Reversal Current in Helicoidal Ferroelectric Liquid Crystal DOBAMBC

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Macroscopic properties and electro-optical response of the mixture of ferroelectric liquid crystal DOBAMBC and SmC

Macroscopic properties like tilt angle, pitch, spontaneous polarisation and response times of the mixture of DOBAMBC and SmC have been studied over the ferroelectric range of temperature. The mixture shows a similar temperature dependence as that of DOBAMBC. The tilt angle, spontaneous polarisation and switching speed decrease in the mixture whereas the pitch increases. The effect of adding SmC to SmC* is to dilute its chirality and to control the properties like tilt angle, pitch etc. This behaviour has been attributed to the 'diluting' effect of adding a non-chiral compound (SmC) in a chiral smectic liquid crystal compound DOBAMBC (SmC*). The operating conditions for better electro-optical response are relatively easier to achieve from the device point of view. The operating temperature range increases and appears relatively at lower temperature without much effect on the switching characteristics of the display.

Dielectric Properties due to Interfacial Interaction in Ferroelectric Liquid Crystal

Interfacial effects on the dielectric properties like a coercive field Ec, a residual polarization Pr and D–E hysteresis loop, are studied in ferroelectric liquid crystal DOBAMBC. The strong relation between the pitch Z of the surface disclination (the stripe pattern) and Pr is observed, that is, Pr increases with increase in Z. Ec becomes larger as the thickness of the cell d decreases and tends to be infinite toward d=0. This suggests the existence of the surface layer in DOBAMBC. Moreover no change of Z is observed for the change of the instantaneous field (a triangular wave) of 60 Hz, supporting that the stripe pattern can only repond to the low applied frequency. Z changes however when the effective field intensity is varied. From these results, it can be said that the <E>-dielectric properties of surface layer (and/or the surface disclination) are obtained from the study of Pr-<E> hysteresis.

Polarization Reversal Kinetics in Ferroelectric Liquid Crystals

The Avrami model will be discussed in connection with the polarization reversal kinetics in ferroelectrics. The experimental results obtained with the surface stabilized ferroelectric liquid crystal DOBAMBC are presented, and analyzed on utilizing the Avrami model.

Switching Characteristics of Ferroelectric Liquid Crystal DOBAMBC (II)–Temperature Dependence and Scaling Rule–

The dependence of the switching characteristics of DOBAMBC on the temperature and the applied voltage to the SSFLC cell was observed. From the results obtained a scaling rule was found, i.e., the switching time, the number of nuclei and the domain wall velocities are only the functions of the applied voltage scaled by the temperature difference from the transition point. On the basis of it, we showed that the activation voltage, defined as the voltage over which the nucleation occurs, is proportional to the temperature difference from the transition point.

The Behaviour of Thin Samples of Ferroelectric Liquid Crystals

Abstract On a ferroelectric liquid crystal DOBAMBC transitions between helical and twisted structures are observed if changing the temperature. The twisted structure appears when the helical pitch becomes comparable to the sample thickness, From the switching process of the twisted DOBAMBC samples in an electric field the polarity of surface layers is determined. Low signal di- electric permittivity shows an increase below the transition to the ferroelectric phase, which is a consequence of the twisted structure deformation in the electric field.

Lifshitz-Like Behaviour in Ferroelectric Liquid Crystals

Abstract In order to explain the anomalous behaviour of the critical magnetic field and the helical pitch in the vicinity of the Λ-line and the Lifshitz point in the ferroelectric liquid crystal DOBAMBC a model involving not only chiral but also non-chiral biquadratic coupling between the polarization and the tilt has been proposed. The model predicts an “S - like” dependence of the modulation wave vector close to the smectic C* - smectic A transition. High temperature resolution measurements of the helical pitch and the intensity of the diffraction sattelites indeed support the proposed model and seem to show that the pitch is finite at Tc. In such a case the Lifshitz field HL is finite too.

Phase diagram of ferroelectric liquid crystals in an external magnetic field

Abstract The phase diagram of the ferroelectric smectic liquid crystal DOBAMBC in an external magnetic field H is analysed and the transition temperatures between the disordered Sm A, helicoidal Sm C* and homogeneously ordered Sm C phase are determined as functions of H by laser light diffraction measurements. Sm C* - Sm C transition line shows close to the transition temperature an anomalous behaviour of a similar character as the pitch of the helix. Both anomalies can be qualitatively explained by a non-linear coupling between the transverse polarization of molecules and their tilt from the normal to smectic layers.

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Abstract The structural phase transitions of DOBAMBC have been studied by energy-dispersive X-ray diffraction. In the smectic phase, a new phase SmA' is found between about 115°C and 117°C by observing the temperature dependence of the intensity of the lowest-energy peak, and of the interplanar distance between molecular layers. From the intensities in the vicinity of the SmC∗ to SmA transition, the helicoidal pitches observed by optical measurements are interpreted in terms of stacking disorder of the tilting of molecular directors along sequential layers in SmC∗

Two different types of hysteresis in dielectric property of ferroelectric liquid crystal

Abstract We have measured the field intensity dependence of D–E hysteresis in the ferroelectric liquid crystal DOBAMBC using Pepinsky bridge and also visually observed domain of smectic C* phase under ac field. Two different types of hysteresis are observed, one of which is ordinary D–E loop obtained under applying ac field and another is P r-⟨E⟩loop where P r is the residual polarization and ⟨E⟩the effective field amplitude. The stroboscopic observation synchronized with the applied field with a certain time delay has been done in order to understand the mechanism. The cause of the former hysteresis is unknown but at least does not seem to be directly associated with motion of helicoidal structure. The latter is caused by pinning of motion of the helicoidal structure and by the first order structural transition between helicoidal and nonhelicoidal smectic C* induced by the applied electric field.

Temperature dependence of the pitch of the helix in a chiral ferroelectric smectic liquid crystal

Abstract The temperature dependence of the helical pitch in the ferroelectric liquid crystal DOBAMBC near the C*-A transition (T c ) has been measured with a high temperature resolution. The pitch strongly decreases with temperature near the transition but reaches a finite value at T c. The consistency of the data with different theoretical models is studied.

Electric field effects of dielectric and ferroelectric properties in chiral smectic liquid crystal dobambc

Abstract Applied field dependence of static dielectric constant, and also dc bias field dependence of high frequency dielectric constant from 1 MHz to 100 MHz are measured in the helicoidal smectic C phase of ferroelectric liquid crystal (+)DOBAMBC. A very low characteristic threshold field, which may be thought as a coerceive field of Goldstone mode, E = 3.8 × 10−3 V/cm (at 82°C) was found in the static dielectric constant measurement by lock in amp and thermal noise method. The high frequency dielectric constant is temperature independent above from 10 MHz. A lucid ⊊ -E hysteresis loop by dc bias field E and its critical behaviour in 50 MHz region are observed.