液晶／高分子复合显示材料研究进展

综述了近年来液晶／高分子复合电光显示材料的研究进展。概括了液晶高分子复合膜显示材料的最新研究成果，就材料的制备方法、性能和应用前景等方面进行了探讨。     

黄原胶溶致液晶行为的研究

以小角激光光散射法、偏光显微镜法及旋转粘度计法研究了黄原胶水溶液在不同浓度和温度下的光学性质。结构区域及粘性行为。发现当溶液浓度达到3％(临界浓度值)时,呈现胆甾型液晶特征,液晶的形成对温度有显著的依赖性。     

基于胆甾相液晶的可调制光子晶体

将胆甾相液晶填充进胶体晶体内部空隙, 通过胆甾相液晶与胶体晶体的耦合, 构建了一种新型可调制液晶光子晶体. 填充于胶体晶体内部的胆甾相液晶织构呈现典型的手性近晶相(S)特征. 由于胆甾相液晶具有特定的选择性反射, 当胶体晶体的带隙处于胆甾相液晶的反射波长范围之内, 则随着温度的改变, 胶体晶体的带隙与胆甾相液晶的带隙同时发生蓝移. 在一定温度条件下, 胆甾相液晶的带隙将与胶体晶体的带隙发生耦合, 实现了光子晶体带隙在单峰与双峰之间的可逆切换.     

Polymer network structure and electro-optic performance of polymer stabilized cholesteric textures I. The influence of curing temperature

The effect of curing temperature on the electro-optic behaviour and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied. Scanning electron micrographs indicate that increasing the curing temperature generally results in a larger average void size within a polymer network, while the morphology of the individual strands (which varies significantly for different monomers) remains largely unchanged. PSCTs with sufficiently large network voids exhibit a two-stage switching behaviour consistent with a model in which the cholesteric liquid crystal is divided between two distinct environments one in which the liquid crystal is strongly dominated by the polymer network, the other in which a bulk-like behaviour, comparable to the unstabilized cholesteric material, is observed.     

Polymer network structure and electro-optic performance of polymer stabilized cholesteric textures II. The effect of UV curing conditions

The effect of UV curing intensity and curing time on the electro-optic behaviour and network morphology of reverse mode polymer stabilized cholesteric textures (PSCTs) has been studied. Scanning electron micrographs indicate that increasing the curing intensity generally results in a more open polymer network characterized by a larger average void size, while the morphology of the individual strands remains largely unchanged. In addition, as the polymerization process proceeds, voids within the network are observed to decrease in size. PSCTs with sufficiently large network voids exhibit a two-stage switching behaviour consistent with a model in which the cholesteric liquid crystal is divided between two distinct environments one in which the liquid crystal is strongly dominated by the polymer network, the other in which a bulk-like behaviour, comparable to the unstabilized cholesteric material, is observed.     

Control of the Induced Handedness of Helical Nanofilaments Employing Cholesteric Liquid Crystal Fields

In this paper, a simple and powerful method to control the induced handedness of helical nanofilaments (HNFs) is presented. The nanofilaments are formed by achiral bent-core liquid crystal molecules employing a cholesteric liquid crystal field obtained by doping a rod-like nematogen with a chiral dopant. Homochiral helical nanofilaments are formed in the nanophase-separated helical nanofilament/cholesteric phase from a mixture with a cholesteric phase. This cholesteric phase forms at a temperature higher than the temperature at which the helical nanofilament in a bent-core molecule appears. Under such conditions, the cholesteric liquid crystal field acts as a driving force in the nucleation of HNFs, realizing a perfectly homochiral domain consisting of identical helical nanofilament handedness.     

Utility of a liquid crystalline diacrylate for bistable switching cholesteric gel displays

We investigated cholesteric liquid crystals containing an anisotropic polymer network created by in situ photo-polymerization of a mesogenic diacrylate. In contrast to previous studies, we used a liquid crystalline diacrylate to achieve bistable switching between the reflecting Grandjean texture and the diffuse scattering focal conic texture. The switching behavior depends considerably on the orientation of the liquid crystal during the polymerization. For polymerization in the Grandjean texture, the response times for switching to the reflecting state were lowered compared to the pure liquid crystal. The chromaticity coordinates indicate that this new type of display is very promising for additive color mixing.     

Polarizerless electro-optic IR shutter based on a cholesteric scattering effect

The light scattering effect of the focal-conic to homeotropic texture transition of a cholesteric liquid crystal has been exploited to develop an electro-optic liquid crystal shutter for visible and infrared light without the utilization of polarizers. Using small helical pitches, the passive decay time from the homeotropic to the scattering state has been minimized to a size in the order of 2 milliseconds which is necessary for a 50 Hz shutter operation. To get active rise times of the same size without too large an increase in the necessary addressing voltage, while maintaining a good scattering contrast, the remaining cell and material parameters have been optimized. Using CaF₂ substrates, the shutter can be operated in the near and middle infrared region. Its contrast gradually decreases with increasing wavelength.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA–chitosan complexes. The accessibility of these ‘NA–liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA–chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Comparison of calculated and observed CD spectra of liquid crystalline dispersions formed from double-stranded DNA and from DNA complexes with coloured compounds

The CD spectra of dispersions of DNA, in the form of cholesteric liquid crystalline droplets, in an aqueous continuum have been studied. Calculated curves have been fitted to experimental spectra. The amplitude and the sign of the intense absorption band of the purine and pyrimidine bases vary with the droplet size, the pitch and the twist sense of the cholesteric phase. The CD spectra of dispersions of the complex formed by DNA and a coloured intercalating antibiotic have been similarly studied. A general satisfactory level of fitting between observed and calculated CD spectra was found.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA-chitosan complexes. The accessibility of these 'NA-liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA-chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Optical storage effects in liquid crystals

The electro-optic behavior of two chemically identical liquid crystal mixtures was compared. It was determined that ethoxybenzylidene-p′-n-butylaniline (Eobuta) doped with 10% 1-methanol exhibited pronounced optical storage, whereas Eobuta doped with 10% racemic menthol only exhibited dynamic scattering. Since Eobuta doped with 1-menthol is cholesteric and Eobuta doped with racemic menthol is nematic, it was concluded that optical storage capability is an intrinsic property of the cholesteric state, and in agreement with the observable textures, should be viewed as a reversible plane ? focal-canic texture change.     

Dynamics of chiral liquid crystals under applied shear

We simulate the alignment dynamics of cholesteric (chiral) rod-like liquid crystals by using a Landau-de Gennes (LdG) expression for microstructure evolution in response to flow. This study is motivated by recent advances in novel cholesteric nanorod dispersions. Prior work on the modelling of cholesterics has suffered from the restriction of helicity to only a single direction, often with a pre-imposed pitch, due to numerical difficulties. This has severely limited cholesteric modelling in regard to both accuracy and experimental relevance. Our simulations avoid this limitation. Relevant forces on rods include solvent-rod drag, nematic alignment, microstructure elasticity and chiral twist. Phase diagrams are developed to demonstrate the response of these systems to variations in chiral and flow forces. Our results indicate that for low shear rates, chiral and elastic forces prevent the rods from moving in response to flow. At high shear rates, the rods tumble and form unique transient structures (combinations of tumbling and cholesteric phases) as flow forces and chiral forces compete. Even if slight alignment is induced at the boundaries, the phase diagram substantially changes, chiefly by constraining the possible chiral phases. This work has immediate relevance to applications which exploit the optical properties of films solidified from cholesteric dispersions.     

Liquid crystals and biological morphogenesis: Ancient and new questions

Superposed series of arcs were observed in extracellular matrices of diverse tissues in plants and animals, and also within certain chromosomes. These bow-shaped patterns were shown to originate from oblique sections in a three-dimensional arrangement of fibrils, which was reminiscent of a well-known structure present in a specific type of liquid crystal, called “cholesteric”, since it was first described in several cholesterol esters. These liquid crystals were also obtained with numerous chiral polymers, such as double-stranded nucleic acids, diverse polypeptides and polysaccharides. However, the fibrous matrices with arced patterns are not at all fluid in general and are considered as stabilized analogues of cholesteric liquid crystals. These geometrical similarities between fibrous tissues and liquid crystals were at the origin of numerous works defining a new type of self-assembly involving two successive steps: a phase transition from an isotropic liquid phase to a liquid crystalline one, followed by a sol–gel stabilization of the liquid crystal. One is tempted to apply to these biological materials the main concepts introduced by physicists in the field of liquid crystals, but many aspects require a new discussion, especially because living cells are present in such systems, and this opens considerable perspectives for research.     

Restructuring space ordering of (DNA-protamine) complexes in liquid crystalline dispersions under proteolytic enzyme treatment

Complexes of DNA with the protamines stellin A and stellin B, in polymer-containing solutions, form both liquid crystalline phases and liquid crystalline dispersions. The 'non-specific' organization of the (DNA-protamine) phase is determined by the presence of protamine 'cross links' between the DNA molecules and not by the inherent anisotropy (cholesteric) double-stranded DNA molecules. Elimination of these 'cross links' by proteolytic enzyme action causes an increase in the distance between the DNA molecules which results in the appearance of an intense band in the CD spectrum and a 'fingerprint' (cholesteric) texture.     

Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.     

Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.     

Amplification of chirality in liquid crystals

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.     

Photo‐optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer–liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E–Z isomerization under UV irradiation was used. A merocianine‐type substance was selected as a dichroic dye. Introduction of a dye‐doped cholesteric mixture with a helical pitch higher than ～300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal–polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Amphiphilic cholesteric liquid crystals prepared from the quaternary ammonium surfactant S-(-)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide

We describe the first successful attempt to produce amphiphilic cholesteric liquid crystals using a chiral quaternary ammonium surfactant, namely S-(-)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide. Only amphiphilic cholesteric liquid crystal samples were made where the micelle structure is related to disc shaped micelles in the achiral N_D phase. The surfactant concentration dependence of the twist and the temperature dependence of the twist were made using laser diffraction. The twist in the amphiphilic cholesteric liquid crystal samples was too small to produce total iridescence.