We studied the change in polarization direction during field reversal in binary mixtures possessing antiferroelectric phase. Due to the coupling of the electric field with the spontaneous polarization of each smectic layer, the transition from antiferroelectric to the ferroelectric phase was induced. This transition was accompanied by changes in optical properties. Both three-state and V-shaped switching was observed. After removing the field, the sample transformed back to the antiferroelectric state. We have found that in some cases, the kinetics of this transition was very slow its time constant ranged from minutes to days, depending on temperature. In the same mixtures, we observed a large thermal hysteresis of the phase transition between the ferroelectric and antiferroelectric phases. We argue that all three phenomena: V-shaped antiferroelectric interactions acting between smectic layers are of comparable strength. We compare our results with the existing models of V-shaped switching and propose a modification of the models.
In some mixtures composed of ferroelectric components the induced antiferroelectric phase was observed. Two mixtures of this kind, both exhibiting antiferroelectric order in a broad concentration range, were investigated. The effect of electric field of various strengths on optical properties of these mixtures was studied. In small electric fields the modulation of light intensity occurred, which amplitude in ferroelectric phase was much higher than in the antiferroelectric one. In mixtures with induced antiferroelectric phase we detected a large hysteresis of the transition temperature between ferroelectric and antiferroelectric sates. In high electric fields thresholdless switching was observed in the hysteresis range. The presence of the thermal hysteresis of the CA* - C* phase transition can be considered as a mark of the V-shaped switching. In bulk samples the C*- phase is stable in, this temperature region, where the V- shaped switching was observed in thin samples.
The measurements described in this paper were carried out in smectic A and C phases of the chiral mixture named W98a. The results of measurements of the tilt angle and the electric permittivity were used to determine the components of the dielectric tensor as function of temperature. The values of the electric anisotropy (Delta) (epsilon) enable us to discuss dielectric torque (Gamma) D and its changes caused by temperature and frequency.
Investigations of a mixture possessing induced antiferroelectric phase are described. This mixture exhibited large thermal hysteresis of the transition between ferroelectric and antiferroelectric phases, which was demonstrated using dielectric and optical methods. Very slow kinetics (ranging from minutes to days) of the transition smectic C* $ARLR smectic CA* was revealed. The coexistence of ferroelectric and antiferroelectric states was noticed. In the investigated mixture the V-shaped switching was observed. The observed phenomena: hysteresis, slow kinetics and threshold-less switching are probably caused by competition of ferroelectric and antiferroelectric interactions acting between smectic layers.
Results of investigations of the switching phenomenon in antiferroelectric liquid crystals are presented. Mixtures possessing the smectic C*A phase in very broad temperature range, including the room temperature, were investigated. The influence of several experimental conditions on switching phenomenon was studied. The switching process was detected using two independent methods. In the electrooptic method, the intensity of light passing the sample placed between crossed polarizers was registered. In the second method the current flowing across the cell during the change of the polarization direction was measured. Both methods have given consistent results. The critical electric field strength needed for changing the polarization direction by 180 degrees has been determined as function of temperature, pulse shape and frequency. Two kinds of switching were observed. At low frequency of triangle-shaped of temperature, pulse shape and frequency. Two kinds of switching were observed. At low frequency of triangle-shaped voltage the switching between three stable states took place. At higher frequencies, or when the voltage has rectangular shape, we observed the switching between two ferroelectric states. A simple and accurate method of measurement of the threshold field for bistable switching is proposed.
Antiferroelectric liquid crystalline mixtures having smectic CA* phase in very broad temperature range have been investigated. Measurements of dielectric relaxation and spontaneous polarization were performed. Two absorption peaks in the existence range of the antiferroelectric CA* phase in the kHz-MHz range of frequency were observed. Both peaks are quite weak and of Debye type. Their characteristic dielectric strengths are almost temperature independent. The peaks are probably related to the motions of molecules on the side of a cone, defined by the molecule tilt angle, the slower process is related to the motions of molecules, tilted in opposite directions in subsequent layers and moving in the same direction. The process with shorter relaxation time is attributed to the movement of molecules in opposite directions. Both modes are active in dielectric measurements due to small residual polarization being a consequence of the helical superstructure.
Two kinds of twist grain boundary (TGB) phase have been observed until now: TGB-A and TGB-C, which are composed of grains, possessing the local structure of smectics A and C, respectively. According to Renn's theory, the third TGB phase may exist in the vicinity of the chiral NAC-point: the TGB-C* phase with the local smectic C structure. We investigated several mixtures possessing chiral NAC-point. In few systems of this kind we observed two TGB phases in place of the N*AC* point. One of these phases was the TGB-A phase. The other phase exhibited textures typical for both TGB and smectic C* phases. Texture observations and light diffraction experiments suggest that the low-temperature TGB phase might have the TGB-C* structure predicted by Renn.
A simple method for determination of the orientational order parameter in nematic liquid crystals is described. To determine the orientational order parameter S, we exploited the de Gennes observation, that any anisotropic physical quantity may be a measure of orientational ordering in the nematic phase. We prove that in the case of optical anisotropy, the discussion on microscopic level in determination of approximate S value can be omitted and the order parameter can be calculated directly from the macroscopic quantity measured in the experiment, i.e. the birefringence. A simple and precise method for the determination of the optical birefringence is described. Similarily as in the case of microscopic quantities, the value of anisotropic physical quantity for ideally ordered nematic can be extrapolated from its temperature dependence. Results obtained using the proposed method agree well with literature data. The method can be used also for the determination of orientational order parameter in cholesteric and some smectic liquid crystals.
Optical methods of detection of ferroelectric modes are described in detail and compared with the dielectric method. The advantages of both methods are discussed. It is concluded that the dielectric method is simple whereas the optical method is more sensitive. The optical response is free of the background steaming from nonferroelectric polarization mechanisms which are always present in the dielectric reponse. The realxation of the Goldstone- and soft modes in a room temperature ferroelectric liquid crystal has been investigated using dielectric and optical methods. Both methods have given concordant results.
A simple and experimentally convenient method for determination of the twist elastic constants of the c-director in tilted smectic phases is proposed. The method consists in measurement of changes in electrical permittivity at high frequencies (about 1 MHz) caused by a dc electric field applied in the direction parallel to the smectic layers. The computer controlled apparatus performing the whole experiment has been described. The results of measurement of an effective elastic constant in a ferroelectric liquid crystal are presented.
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