杉村 明彦

The surface anchoring energy of the Rapini-Papoular type for a nematic slab should be generally expressed as the interfacial energy per unit area for a two-dimensional deformation, which is a non-linear combination of the azimuthal and polar angles, that is, the unified surface anchoring energy. Surface anchoring energy dependencies of the Fréedericksz and saturation transitions have been calculated on the basis of general torque balance equations. The results show three types of optical switching in a super twisted nematic liquid crystal slab dependent on the unified surface anchoring energy. The director distribution in a nematic liquid crystal slab, confined between two glass plates, has been calculated numerically by a continuum theory approach involving the magnetic, electric, elastic, and unified surface anchoring energies. © 1998 Elsevier Science S.A. All rights reserved.

A saturation voltage method is described for determining the unified surface-anchoring energy, which has been proposed theoretically by Sugimura and co-workers [Phys. Rev. E, 52, 681 (1995)], at the interface of nematic (4-pentyl-4'-cyanobiphenyl) and its alignment layer (polyvinyl cinnamate). The method proposed can be employed as a standard measure of this energy without any additional numerical calculation. A conventional measurement method of an optical retardation from a liquid crystal slab is used to estimate the saturation voltage. (C) 1998 American Institute of Physics.

先に報告した一般化表面アンカリングエネルギ理論により、フレデリクス転移と飽和閾値が求められる。この飽和閾値を用いた表面アンカリング強度測定法を用い、ポリマー界面における一般化表面アンカリングエネルギの液晶分子構造依存性を実験的に調べた。本実験により、通常知られているシアノ系液晶と逆に、フッ素系液晶では、アンカリング強度の増大に伴いプレチルト角も増加することを初めて明らかにした。

We studied the behavior of the Maxwell displacement current for a cylindrically symmetric molecule with a point dipole at an angle to the symmetry axis, assuming that the biaxiality ordering of the molecular orientation of the monolayers at the liquid-air interface is induced by the image force of the dipole and is responsible for the generation of the current. The contribution of theta(D), the angle of the molecular dipole from the molecule long-axis, to the generation of the Maxwell displacement current by monolayer compression has been clarified by taking into account the parameter eta which represents the interaction working of the image forces. (C) 1997 Published by Elsevier Science B.V.

The desorption processes of adsorbed impurity ions on various alignment layers in nematic liquid crystal cells have been studied by measuring ac conductivity of the cells. The liquid crystal used here is 4-cyano-4'-pentyl biphenyl. The adsorption of the ions is induced by the application of de voltage to the cells. It is found in all alignment layers that the adsorbed ions decay exponentially after the cessation of the dc voltage application, and that the time constants of the decay exhibit thermally activated behavior. It is shown that the desorption process is governed by the surface energy of the alignment layers.

A steady state current flowing in nematic liquid crystals (NLCs) has been studied for NLC cells with and without polyimide alignment layers. For the cell without the polyimide layers, the steady state current is ascribed to the charge injection from the electrodes and the generation of mobile ions at lower and higher applied voltages, respectively. For the cell with the polyimide layers, on the other hand, the current is much smaller than that of the cell without the polyimide layers. This is due to the fact that the polyimide layers block the charge injection from the electrode to the NLC slab and that the external applied voltage falls entirely in the polyimide layers.

A theory for a twisted chiral nematic liquid crystal (TN) cell with weak anchoring boundaries has been developed. The anchoring energy at 4-pentyl-4'-cyano biphenyl/polyimide Langmuir-Blodgett alignment layer interfaces has been determined by measuring the saturation field of the TN cell. It is found that the anchoring energy is of the order of 10(-6) J/m(2) and decreases with increasing temperature toward the nematic-isotropic point.

The general torque balance equations have been derived for the liquid crystal cells with weak anchoring boundaries by minimizing the bulk and surface free energy simultaneously [A. Sugimura, G. R. Luckhurst, and Z. Ou-Yang, Phys. Rev. E, 52, 681 (1995)]. The torque balance equations have been numerically solved to obtain the director distribution in the nematic liquid crystal cells with weak anchoring boundaries and the influence of the anchoring energy on the director distribution is elucidated.

In a recent paper [Phys. Rev. E 52, 681 (1995)] we showed that in the absence of an external field, the twisted structure of substrates with weak anchoring can induce a deviation of the director at the surfaces from the easy axis and that the anchoring energy can be evaluated by measuring this deviation. To test the prediction, the anchoring energy for nematic 4-pentyl-4'-cyanobiphenyl and polyimide Langmuir-Blodgett (LB) substrates with thicknesses in a range less than 10 nm has been measured. The anchoring energy is found to increase and saturate with the number of LB layers. This result, unlike that expected for the isotropic van der Waals interaction, reveals the anomalous anisotropic interfacial tension between a liquid-crystal slab and polyimide LB films with nanometer thickness. The generalized nonretarded van der Waals energy for the chain-chain interaction is proposed to explain the experimental result.

The dielectric constant of a monolayer on a material surface was calculated with consideration of the local field acting on polar molecules with a permanent dipole moment, and the interaction working between the molecules and a material surface. In the calculation, rodlike molecules were assumed to be orientationally distributed on a material surface. It was shown that the dielectric constant of the monolayers depends on the dielectric constant of the materials which support the monolayers. It was also found that the interaction working between the rodlike molecules and the water surface makes a significant contribution when the molecular area A is close to the critical area A(C), at which the rodlike molecules can lie on a material surface, while in contrast, the interaction among molecules makes a major contribution when the molecular are A is much smaller than the critical area A(C).

A method for the determination of rotational viscosity gamma(1) and pretilt angle theta(o) in twisted nematic liquid crystal cells is proposed. The method is based on the analysis of transient current of a cell excited by a de voltage pulse. The accuracy of gamma(1) and theta(o) values determined by the method is demonstrated by means of numerical calculation of exact basic equations. Experimental results for a ZLI2293 (Merck) cell are presented, and the gamma(1) values obtained are found to be in excellent agreement with values in the literature.

A method for studying adsorption and desorption processes of impurity ions in nematic liquid crystal cells is proposed. The experimental results for 4-cyano-4'-pentyl biphenyl are interpreted in terms of a rate-equation approach and the time constants for the adsorption and desorption processes of the ions are determined.

The photocurrent transients induced by illuminating a nematic Liquid crystal sample with a light pulse under de voltage application are investigated. Three types of current peaks are normally observed in the photoinduced current transient. The anomalous photoinduced current transient, which does not originate from the transport of electronic carriers in the bulk, occurs in a time range of microseconds or less only in the nematic phase. In the similar time range, other two types of current peaks appear both in the nematic and isotropic phases. The occurrence of these current peaks, which is likely to be due to the generation and recombination process of electron-hole pairs, and the ionic carrier transport in the bulk induced by the light excitation, are studied experimentally and discussed briefly.

Transient photocurrent experiments in amorphous selenium and nematic liquid crystal double layers have been carried out to investigate the transient ion transport process in 4-cyano-4'-alkyl-biphenyl. It is found that the mobile charge carriers in the liquid crystal are positive ions with the drift mobility of 3.5 x 10(-6) cm(2)/Vs at 303 K and have the ionic radius of 0.32 nm. The numerical analysis is also carried out to gain insight into the injection process of photogenerated charge carriers from the amorphous selenium layer into the nematic liquid crystal layer.

The phase transition of an 8CB monolayer on a water surface was investigated experimentally and theoretically. A weak first-order phase transition of the nematic molecules from isotropic to polar orientation was clearly detected by compressing the film in the range of immeasurably low surface pressure by means of Maxwell-displacement-current-measuring techniques. On the basis of the interaction of dipolar molecules with a liquid surface, the thermodynamics of the molecular orientation and related mechanical and electric effects for Langmuir monolayers of molecules were also studied theoretically to elucidate the experimental results. It was shown that the compression of the molecular area should lead to a weak first-order phase transition of the orientation of molecules from the isotropic phase to the polar one. It was also predicted theoretically that a Maxwell displacement current with a sharp pulse shape is generated at, the onset of the transition.

Transient current technique is presented for measurement of rotational viscosity gamma(1) and pretilt angle theta(o) in nematic liquid crystal display devices. Theoretical expressions for the determination of gamma(1) and theta(o) from transient current are derived by taking account of the conduction of impurity ions and director rotation. Experimental results for gamma(1) and theta(o) are compared with those with other conventional methods.

Low-frequency dielectric properties of nematic liquid crystal (NLC) cells with and without polyimide alignment layers have been investigated with impedance spectroscopy. It is found that the dielectric behavior in the low frequency regime of NLC cells without polyimide alignment layers can be successfully explained in terms of the diffusive motion of impurity ions. The temperature dependences of the diffusion constant and the concentration of the impurity ions are determined from the dielectric behavior. NLC cells with polyimide alignment layers exhibit the interfacial polarization between the polyimide layers and NLC layer in the low frequency regime. The concentration of the impurity ions dissolved from polyimide layers to NLC layer is estimated.

A method is described for the measurement of the rotational viscosity gamma(1) in nematic liquid crystal cell assemblies. Theoretical formulas to determine gamma(1) from transient current are developed by considering both director rotation and conduction of impurity ions. The pretilt angle and the product of the drift mobility and density of the ions are also obtained with the method. Experimental results for 4-pentyl-4'-cyano biphenyl cells with or without polyimide alignment layers are reported.

Ultraviolet (UV)-light-soaked states in n-pentyl-p-n-cyanobiphenyl (5CB) have been investigated with transient current techniques. No change in the drift mobility of ions is detected, while the density of ions is increased after UV-light soaking because of the photochemical decomposition of 5CB. The origin of the impurity ions in 5CB is briefly discussed.

液晶分子の双極子相互作用に基づき、水面上液晶単分子膜の膜圧縮過程における分子配向の熱力学特性を、平均場近似法を用いて理論的に説明した。 単分子膜の膜圧縮過程での相転移現象は、マックスウェル変位電流法で、実験的に確認している。本理論により、等方性相から配向相への相転移現象に与える、剛直性極性分子の効果を明らかにした。

ホモジニアス配向ネマチック液晶セルヘのステップ電圧印加に伴う、過渡電流ピークの発生から、液晶固有の物理定数の回転粘性係数が測定できることを先に報告した。 本論文では、測定方法を、粘性・弾性・誘電トルクのバランス方程式を用いて理論的に説明するとともに、液晶バルク内での電界の空間的時間的変化に伴う変位電流を数値解析し、実験結果との対応から、本測定法の有効性を明確に示した。

回転粘性率は、液晶の重要を物理定数の－一つであり、従来、理論的にも実験的にも数多くの研究が行われてきている。 ここでは、電場印加に対するダイレクタ回転に着目し、理論的には、粘性トルクと誘電トルクのトルクバランス方程式から導かれる分子の動的挙動と、分子回転に伴う変位電流から回転粘性率が求められる事を示した。 また、本理論に基づき、実験的にも有効を回転粘性率の測定が可能である事を示した。

A new type of sensitive electro-optic effect in a nematic liquid-crystal (4-cyano-4'-5-alkyl-bipheneyl) cell with polymer thin films is reported. Optical switching in a transmitted intensity of light passing through a cell is experimentally observed by dual low-frequency modulation method using a triangular wave application. Dielectric relaxation effects in the thin polymer layer as part of the cell serve to reduce the effective voltage acting on the liquid-crystal layer for lower frequencies but not for higher frequencies. The mean effective applied voltage to a liquid-crystal layer is efficiently controlled by changing only the frequency.

Transient charging currents in nematic liquid crystals have been measured with a single- and a double-voltage pulse technique to elucidate the role of impurity ions incorporated in the materials in the transient charging processes. In the single pulse experiment, a peak in the current transient is observed in the nematic phase. It is shown that occurrence of the peak is due to the alignment of the director of the liquid crystal molecules in the direction of applied electric field. In the double pulse experiment, a voltage pulse (prepulse) is applied to the liquid crystal before the measurement of the transient current. It is observed that the application of the prepulse significantly alters the shape of the current. The changes in the current can be explained by considering the impurity ion distribution, modified by the prepulse, in the nematic liquid crystals. It is concluded that the double pulse experiment is a powerful tool for the examination of the influence of impurity ions in liquid crystals on the current transients.

A 4-component circuit is proposed to study the electric behavior of liquid crystal displays with polymer boundaries accompanying dc and ac voltage application. Two series components C1 and R1 are introduced to represent the capacitance and dielectric relaxation of a polymer boundary. Another two parallel components, R0 and C0, are used to take into account the bulk effect of liquid crystal layer. Transient currents induced by three types of switching of an applied voltage, single step, triangular and rectangular waveforms, are explained theoretically and experimentally. The theoretical results are in excellent agreement with the data from experiments. These provide the measurement method for obtaining the 4 components in a cell assembly. It is found that the effective voltage which is applied only to the bulk is different from an external applied waveform.

Dynamic behaviour of electrical properties accompanying an application of a DC step voltage to a liquid crystal cell, including polymer thin films, has been studied theoretically and experimentally. A 4 component equilibrium circuit of a liquid crystal cell is introduced to elucidate the electrical properties. In this circuit the electric response of a polymer film is represented by a series circuit of resistance R1 and a capacitance C1, considering dielectric loss of the layer. An effective voltage applied to a liquid crystal bulk (which is usually mistaken for an external applied voltage U0 in liquid crystal displays) is found to be apparently different from U0.

A new and simple electric charge interaction mechanism has been proposed to explain the behaviour of the tilt angle of liquid crystals by rubbing. The mechanism of the molecular alignment is elucidated based on the effect of a static electric charge on a substrate surface treated by rubbing. An electric field which is dependent on the topology of a substrate surface allows a planar orientation of molecules with positive dielectric anisotropy. With increasing rubbing strength, the tilt angle varies slowly. The director of molecules with negative dielectric anisotropy is uniform and has a tilt angle determined by the molecular permanent dipole direction. The air-liquid crystal interface case is also considered. This model enables us to give a unified picture of the molecular alignment mechanism.

The mechanism of the normally incident photoinduced current transients in homogeneous nematic samples in an applied dc voltage has been studied theoretically and experimentally. Explicit expressions for the photovoltaic emf are derived using the nonlinear optical Pockel's effect induced by the Freedericksz transition. The current transient is elucidated by the effect of dc field-induced ion drift to electrodes based on an equivalent circuit. The experimental results [briefly reported in Phys. Rev. Lett. 63, 555 (1989)] are completely explained by the present theory. According to the theoretical prediction, two methods to enhance the photoinduced current in liquid crystals, light exposure and dye doping, have been found and implemented.

The screening effect of dc voltage in a nematic liquid-crystal cell having polyimide alignment layers is studied experimentally and theoretically. The transmitted intensity of light passing through a liquid-crystal sample exhibits a dip arising from dc screening effects, which are never observed with ac voltage application. The dc screening effect is produced by two kinds of ionic charges in the boundary layer. One is adsorbed on the orientation film and the other compensates the adsorbed charges. The two kinds of charge model account quantitatively for the observed dip in transmission making its origin clear, and explain well the ionic conduction processes accompanying the application of dc voltage to a liquid-crystal sample.

The optical rectification of a nematic liquid crystal by the nonlinear optical Pockel's effect induced by a Freedericksz transition [Trans. Faraday Soc. 29, 919 (1933)] is presented as a mechanism for producing an anomalous photoinduced current transient. Explicit expressions for the photovoltaic emf and the current transient are derived and shown to be in good agreement with experiment. Motivated by the theory we have succeeded in finding and implementing a method, dye doping, to enhance the effect.

ネマチック液晶への印加直流電圧除去後の放電過程を実験的・理論的に説明した。放電過程におけるネマチック相、等方性相でのピーク電流の発生は通常の誘電帯では観測されない。 ピーク電流の発生を、液晶中の不純物イオンの拡張係数の違いを仮定し、電流連続の式をランダムウォーク法を用いた計算機シミュレーションによって解くことにより明らかにした。

Transient currents induced by the polarity reversal of a dc voltage applied to a liquid-crystal cell have been experimentally studied. For voltages less than a characteristic value, the time occurrence of the peak current decreases with decreasing applied voltage, which is contrary to behavior expected for a normal carrier-transport process. To explain the experiments, a simple model based on the time variation of an applied voltage and the internal electric field induced by the absorbed electric charges on the cell substrate has been proposed. The results from the model agree reasonably well with the experimental observations.

ネマチック液晶セルへの直流電圧印加状態で、光パルスを照射することにより、生体系での刺激伝搬電流と類似の異常過度光電流が光整流効果に基づき生じる。この過度電流の閥値特性を用いた液晶神経回路素子を提案した。すなわち、内部刺激としての直流印加電圧と、外部刺激としての光パルスの両者の閥値特性により、神経素子の出力の発火・制御を制御することができる。 さらに、この液晶神経素子を2次元配列させることにより、パターン認識が可能であることを示した。

A new type of high speed liquid crystal switching array is developed. The device has a simplified electrode configuration which allows a simple high speed operation and a small size cell design. Short response time of 0.5 msec and high ON/OFF light transmission ratio of 80 with 18% of total transmission are obtained for a switching array of 10 dots/mm. By applying this type of array to a character printer with loaded by amorphous silicon drum, a good hard copy is obtained at 1400 lines/min (14 pages/min for A4 size). © IOP Publishing Ltd.