Surface anchoring, polarization fields and memory states in polymer dispersed liquid crystals

We have investigated the formation and development of memory states in polymer dispersed liquid crystals induced by the application of a strong electric field. Both the optical transmittance and polarization field have been followed as functions of time. We have been able to distinguish between the contributions to the memory states arising from the surface anchoring of the liquid crystal at the droplet interface and from the electrical reorientation of the mesogenic molecules. The dependence of both residual transmittance and polarization field on temperature is reported and a simple model is proposed.     

Novel memory effect found in nematic liquid crystal/fine particle system

Novel liquid crystalline composites composed of a nematic two-frequency-addressing liquid crystal and organized clay mineral (about 1 wt %) have been prepared. The particles of clay mineral were dispersed homogeneously in the liquid crystal. The composite cells became transparent within 50ms when a 60 Hz electric field was applied. The transparent state was maintained after the field was switched off. It transformed into a turbid light scattering state by applying 1.5 kHz electric field which caused dynamic scattering in the cell. The light transmittance of both memory states did not change after 20 h without the electric field.     

Dielectric relaxation studies and electro‐optical measurements in poly(triethylene glycol dimethacrylate)/nematic E7 composites exhibiting an anchoring breaking transition

Electric field driven anchoring breakage in poly(triethylene glycol dimethacrylate)/nematic E7 composites was studied using dielectric spectroscopy and transmittance measurements. The transmittance hysteresis observed on increasing and decreasing an applied electric field, associated with different alignment states of the liquid crystal (LC), was monitored through dielectric loss. Essential changes are felt mainly in the δ‐peak, i.e. the dielectric response of the nematic when the director lies parallel to the applied electric field. An irreversible effect persists after the field had exceeded a critical value, which was manifest in a higher transmittance and a higher dielectric strength of the δ‐peak in the OFF state. The initial scattering/opaque state of the sample can only be recovered by heating to the clearing temperature of the nematic LC. The effect referred, commonly called memory effect, is rationalized in terms of anchoring breakage of the LC at the polymer–LC interfaces. The electro‐optical response was tested for different poly(triethylene glycol dimethacrylate)/nematic E7 composites in different composition ratios prepared by polymerisation‐induced phase separation. The lowest threshold field was observed for the 30:70 composite.     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.     

Topological ferroelectric bistability in a polarization-modulated orthogonal smectic liquid crystal

We report a bent-core liquid crystal (LC) compound exhibiting two fluid smectic phases in which two-dimensional, polar, orthorhombic layers order into three-dimensional ferroelectric states. The lower-temperature phase has a uniform polarization field which responds in an analog fashion to applied electric field. The higher-temperature phase is a new smectic state with periodic undulation of the polarization, structurally modulated layers, and a bistable response to applied electric field which originates in the periodically splay-modulated bulk of the LC rather than by surface stabilization at the cell boundaries.     

Stationary states of the surface stabilized ferroelectric liquid crystal layers in electric field

Stationary states of surface stabilized ferroelectric liquid crystal layers in an electric field are analysed by use of the Taylor expansion method based on catastrophe theory. Two kinds of director distribution within the flat smectic layers are taken into account: the uniform and the presplayed one. The butterfly catastrophe describes the properties of the cells correctly. The results have a qualitative character. Two categories of transitions can be predicted: switching between stable states characterized by opposite uniform orientations of the polarization vectors, and deformation of the director field which relaxes after removing the field. The threshold field strengths are found and the role of the system parameters is investigated.     

Reduced OFF-axis haze in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) are composite materials formed by micron-sized droplets of liquid crystals (LCs) dispersed in a polymer matrix, which can be turned from an opaque state to a transparent one by application of a suitable electric field. PDLCs have been proposed in applications related to the control of light transmittance on large surfaces (light shutters, displays, rear mirrors). Despite several advantages, PDLCs’ main drawback is haze, i.e. the fast decay of transmission at large viewing angles. In this paper, a method for achieving highly transparent PDLC devices over a wide range of viewing angles is proposed. The method is based on the use of PDLCs with tilted elongated LC droplets and driven by opportune electric fields, which are experimentally calculated and able to ensure an almost constant value for OFF-axis transmittance.     

Highly transparent memory states in polymer dispersed liquid crystal films

Electro-optical properties of polymer dispersed liquid crystal (PDLC) films which have reversed morphology are investigated. Highly transparent memory states, for which transmittances exceed more than 80 per cent, are observed in these PDLC films. The saturation voltage V₉₀ can be decreased by a PTF (a phase transition with a field) operation and becomes 10 times lower than that without PTF operation. A contrast ratio of more than 600 is obtained in the memory state of a thick PDLC film.     

Electrostatics and the electro-optic behaviour of chiral smectics C: 'block' polarization screening of applied voltage and 'V-shaped' switching

We present a new model for the physics of thresholdless switching in chiral smectics. In the limit of high polarization, the electro-optics of chiral smectic C liquid crystals are dominated by two distinct electrostatic effects. Complete (surface to surface) 'stiffening' of the polarization field by its charge self-interaction causes the polarization to orient as a uniform block. Complete screening of applied electric field by polarization charge leads to voltage-induced orientation where the electric field in the liquid crystal is exactly zero, These effects, both operative during the 'V-shaped' portion of thresholdless transmission vs. voltage curves, combine to produce 'V-shaped' switching.     

Calculation of the magnetophotoselective effect in reactions of triplet molecules

An expression for the initial chemically induced dynamic electron polarization produced according to a triplet mechanism under the conditions of pumping by polarized light has been obtained under the assumption that the fluctuations of the orientation of a molecule in a liquid have the character of an uncorrelated Markov process. The expression obtained is valid for arbitrary values of the magnetic field strength and the viscosity of the solvent. Numerical calculations of the chemically induced dynamic electron polarization have been carried out for slow molecular rotations and in weak magnetic fields.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 96–100, January–February, 1988.We thank V. E. Gluzberg for his assistance in carrying out the numerical calculations.     

Pyroelectric investigations of polarization and charge distributions in sandwich cells containing a ferroelectric liquid crystalline polymer

With the laser intensity modulation method (LIMM), polarization and charge profiles in sandwich cells containing the ferroelectric liquid crystal side group polymer LCP1 have been investigated. In a sample with a thickness larger than the helical pitch, contributions to the polarization profile due to surface anchoring were found. Additional contributions inside the cell occurred. The validity of the results, concerning numerical deviations in the deconvolution process of the pyrospectra, has been tested by simulations. The influence of space charges for measurements with a static external electrical field is discussed, giving contributions to the profile in addition to those arising from polarization.     

Copolymer composition-dependent light transmission of polymer/liquid crystals composite films

Electrooptic responses (voltage and angular-dependent transmittance) of polymer/liquid crystal composite films with H, V, and unpolarized lights have been studied based on a nematic liquid crystal (Ro-5921) and four types of homopolymers and copolymers from ethyl methacrylate and styrene with different compositions. In this way, the index ratio of the polymer (n_p) to the ordinary refractive index of liquid crystal (n_o)(n_p/n_o) has been varied systematically, and the effect of the index ratio on viewing angle, applied voltage, response times, and transient response have been investigated. With increasing styrene content in the copolymer, droplet size increased, threshold (V_th) and saturation (V_sat) voltage, and rise time decreased. With n_p ≲ n_o, maximum transmittance occurred at normal incidence, regardless of the type of polarization. On the contrary with n_p > n_o, V-polarization gave a peak in the transmittance-voltage curve, and transmittance overshot upon removal of the field, and these were interpreted in terms of effective refractive index and two-step relaxations. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 55–64, 1998     

Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR

The Haupt‐effect is a rather seldom used hyperpolarization method. It is based on the interdependence between nuclear spin states and rotational states of nearly free rotating methyl groups having C₃ symmetry. A sudden change in temperature from 4.2 K to room temperature by fast dissolution yields considerably enhanced ¹³C and ¹H resonance signals. This phenomenon is now termed quantum rotor induced polarization. More than 40 substances have been studied by this approach in order to identify them as polarizable by the ‘Haupt‐effect in the liquid state’. Influencing factors have been analyzed systematically. It could be concluded that substances having a high tunneling frequency, which is due to a small and narrow potential barrier, are most likely to feature quantum rotor induced polarization‐enhanced signals. Copyright © 2013 John Wiley & Sons, Ltd.     

Phase transitions in liquid-crystalline cyanoethyl cellulose solutions in magnetic field

The cloud-point method and polarization microscopy have been used to investigate the phase transitions and the phase state of cyanoethyl cellulose-dimethylacetamide and cyanoethyl cellulose-dimethylformamide systems in the presence and absence of magnetic field with the help of polarization photoelectric and magnetic setups. The temperature-concentration region of the existence of the liquid-crystalline phase in solutions widens in the magnetic field; the higher the field strength and polymer concentration, the more pronounced this widening. Cyanoethyl cellulose solutions are found to possess “memory”: after the magnetic field is switched-off, the orientation of macromolecules and the increased phase transition temperature are preserved for many hours.     

“Proximity effects” in radiative transitions: a model study of the role of vibronic coupling and static crystal field interactions

We report the results of a model study of the influence of vibronic coupling involving non-totally symmetric vibrations and static crystal field interactions on the spectral properties of molecules with close-lying excited electronic states. The presented results suggests that “proximity effects” brought about by solvent perturbation arise from two sources: (i) alterations in the energy separation between vibronically coupled electronic states and (ii) crystal field mixing of the isolated molecular electronic states. It is shown that crystal field mixing leads to the breakdown of the vibronic coupling scheme for non-totally symmetric vibrations in isolated molecules. This breakdown is shown to have a very pronounced effect on the spectral properties of molecules with close-lying excited electronic states. The effect of environmental perturbations on excited state frequencies, the breakdown of symmetry and polarization selection rules, and vibrational intensity distributions is discussed.     

Infrared and raman spectra,conformation and force field of 3-methoxy propyne

The infrared spectra of 3-methoxy propyne (CH₃OCH₂CsolutionCH) in and in the crystalline and vapour states have been studied. The Raman spectra of 3-methoxy propyne and 3-methoxy propyne- 1-d have also been measured. The spectra are relatively simple, and no significant spectral alterations are revealed on conversion from vapour, to liquid, to the crystalline state. Together with vapour phase band contours and Raman polarization measurements this indicates that there is predominantly one conformer in all three states. An assignment has been proposed, based on a gauche conformation and supported by normal coordinate analysis.     

Electrostatics and the electro-optic behaviour of chiral smectics C: 'block' polarization screening of applied voltage and 'V-shaped' switching

We present a new model for the physics of thresholdless switching in chiral smectics. In the limit of high polarization, the electro-optics of chiral smectic C liquid crystals are dominated by two distinct electrostatic effects. Complete (surface to surface) 'stiffening' of the polarization field by its charge self-interaction causes the polarization to orient as a uniform block. Complete screening of applied electric field by polarization charge leads to voltage-induced orientation where the electric field in the liquid crystal is exactly zero, These effects, both operative during the 'V-shaped' portion of thresholdless transmission vs. voltage curves, combine to produce 'V-shaped' switching.     

The two-photon spectrum of liquid pyridine by thermal lensing techniques

Thermal lensing is exploited to investigate the electronic properties of pyridine. The two-photon spectrum and polarization ratio of liquid pyridine has been measured from 2900 to 2000 Å. Absorptions corresponding to the two lowest ππ* states, of benzene parentage, and to the second excited nπ* state have been assigned. The latter, of A₂ symmetry, borrows two-photon intensity from A₁ states through vibronic coupling induced by a₂ vibrations. Calculations of allowed contributions to the two-photon cross sections for the lowest four excited levels support the assignment.     

Morphologies and electro-optic properties of phenoxy/liquid crystal composite films

Morphology and electro-optic properties of composite films composed of phenoxy and nematic liquid crystal (LC) have been studied at a film composition of 40/60 (polymer/LC by weight). Effects of temperature, frequency and voltage of applied a. c. electric field on the transmittance and response time of the films were measured.     

大分子引发剂分子量对接枝聚合物基体的PDLC电光性能的影响

以可逆加成-断裂链转移(RAFT)、引发转移终止(iniferter)活性自由基聚合相结合的方法,用一步法制备了不同分子量的大分子引发剂RAFT-PS-co-PCMSI(MI),并通过紫外光聚合诱导相分离法制备了以接枝聚合物为基体的聚合物分散液晶(PDLC)膜.研究了不同分子量的MI对PDLC的微观形貌,关闭状态透光率,阈值电压,饱和电压以及记忆效应等方面的影响.研究表明,降低PDLC中MI的分子量,会使得液晶微滴粒径增大,阈值电压(Vth)、饱和电压(Vsat)减小,记忆效应、关闭状态透光率升高.