How far has the molecular alignment of liquid crystals been elucidated?

Liquid crystal displays (LCDs) - born in late 1960s - have become a 45 billion-dollar industry in 2004. Today's LCD panels cover a wide range of sizes from 0.2 to 82 in diagonal. The electro-optical characteristics of any field effect LCD are determined by the orientation of its liquid crystal (LC) molecules at the display boundaries. Until recently most LCD alignment processes were based on rubbing polymer coated LCD substrates with a cloth. Despite its key importance, the complex mechanism governing the alignment of LC molecules on display substrates is still not well understood. Do the microgrooves generated by rubbing induce LC-alignment or does the stretching of polymer chains as a result of brushing cause it? To overcome the drawbacks of conventional aligning techniques, new alignment processes have recently been proposed and developed. Examples are micro embossing, ion bombardment, fringe field effects and photo-alignment. In this paper, some mechanisms of LC alignment are discussed and various alignment techniques and methods to investigate the interaction of LC molecules with substrates are reviewed.     

等离子体处理过的基板上液晶平行取向的两种模式

我们提出了一种用于液晶盒的新取向工艺,可以得到0或非0的预倾角。这项工艺是用定向的等离子体流倾斜辐照基板。我们用一个阳极层发射器(anodelagerthruster)作为等离子体辐射源,以产生层状的等离子体流。它适合于处理大面积基板,可以处理有机(聚合物)和无机(玻璃,ITO)层。等离子体流辐照可以得到两种类型的液晶取向:(1)最可及(择优)取向轴位于离子束方向和基板法线组成的入射平面上;(2)最可及取向轴垂直于入射面。随着照射总剂量的增加,取向方向可以从类型(1)向类型(2)转变。在第一种取向模式中,可以通过改变工艺参数,如入射角、离子流密度和离子能等来改变预倾角。第二种取向模式的特征是预倾角为0。第一种模式的方位锚泊能系数相对较弱(W=10-3 Erg/cm2),而第二种类型锚泊能很强(W>10-1 Erg/cm2),与摩擦聚合物基板相当。两种模式的取向特征可以用来产生满足所需参数的取向,和构图(pattern)液晶盒基板。这种工艺方法可以克服传统摩擦工艺的某些缺点。     

Addition of single-wall carbon nanotubes to a liquid crystal material: Impact on dielectric properties

In this study, the effect of carbon nanotube (CNT) on the dielectric parameters of (11-(4-cyanobiphenyl-4′-oxy)undecan-1-ol) (coded as) LC11 has been investigated by dielectric spectroscopy measurements. According to analysis of ac dielectric parameters׳ analysis, while relaxation frequency increases with CNT doping, the magnitude of energy loss decreases. In this respect, CNT doped LC11 material which can be utilized in the wide working frequency interval with low energy requirements, has a great potential for technological applications. On the other hand, CNT adding changes the relaxation type from nearly Debye to non-Debye type. In addition to these remarkable modifications on the dielectric parameters, it has been determined that CNT doping extends the frequency interval where the liquid crystal (LC) system shows negative dielectric anisotropy (NDA). In other words, observation of the NDA at the higher frequency region relative to LC11 means that the molecular alignment can be controlled by CNT. Hence, tuning the alignment of the LC11 molecules by CNT doping may lead its utility for various LC device applications.     

摩擦强度对取向膜表面液晶取向度的影响

利用红外二向色性吸收实验,定量分析了不同摩擦强度的取向处理后取向膜表面的液晶分子取向度。实验结果表明,取向膜表面的液晶分子取向度随摩擦强度的增加而增加,最后达到饱和。取向膜表面的液晶分子取向度饱和值仍低于体内部,大约为体内部的1／2。液晶界面层的厚度不随摩擦强度变化,均为7nm左右,此值具有分子相干长度的物理意义。     

一种采用内部线栅偏振片的半透半反射式液晶显示器

为了同时获得具有高透射率和反射率的液晶显示器,提出了一种采用内部线栅式偏振片(wiregridpolarizer,WGP)的单倍盒间距的半透半反式液晶显示器方法。在反射模式中,液晶盒内嵌的WGP作为依赖于偏振的外界光的反射器;在透射模式中,不需要消色差的四分之一波片。基于液晶盒的初始取向,该器件可用于正常显黑模式(垂直取向)和正常显白模式(扭曲向列相取向)。本文研究了这两种器件结构详细的电光特性,如电压依赖光效率特性和视角特性。     

An X-Ray Imaging-Based Layer Alignment and Tape Deformation Inspection System for Multilayer Ceramic Circuit Boards

Advances in conductor patterning and via preparation techniques have facilitated a continuous increase in the circuit packaging density of ceramic multilayer substrates. A decrease in feature size combined with the trend toward constantly increasing the processed panel size is placing extremely high demands for layer-to-layer alignment as well as ceramic green tape stability. This paper describes the main mechanisms contributing to layer-to-layer alignment errors in the ceramic tape alignment and stacking process and introduces a simple, nondestructive, X-ray imaging-based method for measuring alignment error in a ceramic multilayer substrate. A low-temperature cofired ceramic (LTCC) test panel was manufactured and analyzed using the introduced X-ray inspection method. In order to define the error caused by tape alignment and stacking as well as tape deformation, the alignment error between the layers was calculated over the whole panel area. The specific area of an LTCC panel that meets the required alignment tolerances can be identified from the calculated results.     

Plasma Modification of Fluoropolymers for Aligning Liquid Crystals

In this paper we describe methods to control liquid crystal (LC) alignment using plasma discharge on ferroelectric fluoropolymers. Two different plasma modification techniques were investigated: corona discharge and RF plasma in Ar gas. Corona discharge is a proven technique known to reorient the dipoles in poly (vinylidene fluoride) and its copolymers resulting in a strong remnant polarization. The polarization was patterned resulting in preferential LC alignment in selected regions. RF plasma in Ar gas defluorinates the polymer surface leading to planar alignment of positive dielectric anisotropy LCs. The defluorination of the alignment layer also causes low voltage switching of the LC.     

液晶共面取向的等离子处理方法

在受等离子辐照过的一些有机和无机的基板上，获得了高质量的液晶共面取向。与已知的用来改善顶部锚定及预倾角各向同性的等离子处理方法不同，新方法是将等离子束调整到倾斜射向待取向的基板。在所用的辐射参数范围内，所有基板上的LC取向的易取向轴(easy axis)都平行于等离子体传播方向。研究了LC的预倾角和锚定能与等离子束的入射角、辐照时间、能量以及辐照电流密度等的依赖关系。经等离子处理过的基板上，方位角、锚定能与用光取向方法得到的相近，而预倾角与摩擦产生的类似。透过率-电压曲线与等离子处理的和摩擦工艺处理的非常接近。与摩擦取向相同，等离子诱导的取向具有很高的温度和光照的稳定性。也考虑了采用等离子／偏振紫外光和等离子／摩擦处理等组合方法来制作LC图形。     

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid‐crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra‐thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid‐crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole‐conducting alignment layers in polymer light‐emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole‐transporting molecule (HTM) have been mixed into the PAP layer. These hole‐conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.‐%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m² and an efficiency of 0.3 cd/A could be realized.     

An optically controlled liquid crystal device using azopolymerfilms

We have developed a liquid crystal (LC) device that is totally controlled using light. The initial alignment is made by optically buffing the azopolymer film with two coherent argon laser beams. Surface relief gratings have been optically induced on an assembled cell filled with LC. A single linearly polarized argon laser beam is then employed to irradiate the sample and photoinduce a twisted alignment structure. This can then be erased by a circularly polarized beam. The alignment information can also be erased by heating to the glass transition temperature of the azopolymer film     

Alignment of liquid crystals on a topographically nano-patterned polymer surface prepared by a soft-imprint technique

In this study, we have fabricated polyacrylate substrates having a nano-patterned surface topography using a soft-imprint technique. The planar alignment of liquid crystals (LCs) along the direction of nanogrooves has been generated. Twisting behavior of nematic LCs has been also observed with a perpendicularly assembled LC cell and the cell parameters can be estimated by using the Soutar and Lu method. By comparing the anchoring energies obtained, accordingly, it has been demonstrated that the polymer nanogroove pattern has a comparable influence on LC alignment to the conventional rubbing process. It has been also shown that the artificial topography of the line grooves on the conventionally rubbed surface has a significant influence on the anchoring stability of the LC molecules.     

侧链含稠环的新型聚酰亚胺液晶垂直取向剂的制备及表征

以3,5-二硝基苯甲酰氯、雌二醇及溴代正辛烷为原料合成了含有稠环结构的新型功能性二胺(H8-c)。用1,2,3,4-环丁烷四甲酸二酐(CBDA)、对苯二胺(PDA)及H8-c共聚制备了一种聚酰亚胺(PI)液晶垂直取向剂。该PI膜作为取向层制得的液晶盒,在120℃下烘烤12h仍可垂直取向,拥有良好的垂直取向热稳定性。热重分析(TGA)测试表明其具有良好的热稳定性,在氮气中,这种PI的5%热失重温度(T5)和10%热失重温度(T10)分别高达400℃和480℃。紫外可见光谱(UV-vis)显示此PI膜在400nm以上具有高的透过率,可达95%以上。     

Super‐Fast Switching of Twisted Nematic Liquid Crystals on 2D Single Wall Carbon Nanotube Networks

We have developed a high performance liquid crystal (LC) alignment layer of ultra‐thin single wall carbon nanotubes (SWNTs) and a conjugated block copolymer nanocomposite that is solution‐processible for conventional twisted nematic (TN) LC cells. The alignment layer is based on the non‐destructive solution dispersion of nanotubes with a poly(styrene‐b‐ paraphenylene) (PS‐b‐PPP) copolymer and subsequent spin coating, followed by conventional rubbing without a post‐annealing process. Topographically grooved nanocomposite films with two dimensionally (2D) networked SWNTs embedded in a block copolymer matrix were created using a rubbing process in which bundles of SWNTs on the composite surface were effectively removed. The LCs were well aligned with a stable pre‐tilt angle of approximately 2° on our extremely transparent nanocomposite, which gave rise to superfast switching of the TN LC molecules that was approximately 3.8 ms, or four times faster than that on a commercial polyimide layer. Furthermore, the TN LCD cells containing our SWNT nanocomposite alignment layers exhibited low power operation at an effective switching voltage amplitude of approximately 1.3 V without capacitance hysteresis.     

A transflective liquid crystal display using an internal wire grid polarizer

To obtain high transmittance and reflectance simultaneously, a single cell gap transflective liquid crystal (LC) display using internal wire grid polarizer (WGP) is proposed. For the reflective mode, the imbedded WGP serves as a polarization-dependent reflector for the ambient light. For the transmissive mode, no achromatic quarter-wave film is needed. This device can be used as a normally black mode (using vertical alignment) and a normally white mode (using twist-nematic alignment), based on the initial LC cell alignment. Detailed electro-optic performance, such as voltage-dependent light efficiency and viewing angle of these two device configurations, is investigated.     

The formation of supramolecular liquid-crystal gels for enhancing the electro-optical properties of twisted nematic liquid crystals

We report that the supramolecular liquid-crystal (LC) physical gel can be formed through the fibrous self-assembly of the polyfluorene-based π-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in twisted nematic LC system for the first time. With the utility of alignment layers, the F8BT molecules can be aligned and formed the LC physical gels with the formation of self-assembled supramolecular structures in the twisted nematic LCs. In contrast to conventional LC physical gels, the presence of anisotropic π-conjugated structure makes the twisted nematic LC system exhibit excellent electro-optical properties of driving voltage reduction and contrast ratio enhancement owing to the conjugated polymer having a high π-electron delocalization degree which can efficiently drive LC molecules in much lower operating voltages. The self-assembled supramolecular network has revealed the potential for applying in various LC display devices with the ability of improving their electro-optical performance.     

Liquid-crystal polarization controller arrays on planar waveguidecircuits

In this letter, we have developed thin liquid crystal (LC) arrays for use as simple polarization-controller arrays on planar lightwave circuits (PLCs). The 30-μm thick LC arrays are vertically inserted into trenches cut across PLC substrates. Two such LC cells that are in line and parallel to each other, one of which has a direction of alignment that is rotated by 45° relative to that of the other, are able to convert an arbitrary input polarization to a desired one. Twisted-nematic LC cells can switch the polarization of input light between the TE and TM modes with an extinction ratio of 15 dB     

圆片级封装的新颖对准技术

对于3D互连、圆片级封装(WLP)和先进的MEMS器件的圆片键合,精密对准是一项关键技术,不同的MEMS,常常包含双面加工处理,而IC和CMOS制造业则只利用单面加工处理步骤,因此,圆片到圆片的对准必须使用设置在键合界面(也就是面对面)中的对准标记。论述了面对面对准方法的主要步骤,最新结果报导,用一种特殊开发的对准系统获得了≤1μm的对准精度。设备主要是为圆片对圆片的对准和键合而设计。     

The Directional Peeling Effect of Nanostructured Rigiflex Molds on Liquid‐Crystal Devices: Liquid‐Crystal Alignment and Optical Properties

We report a new strategy, the directional peeling of a rigiflex mold with a nanostructure, to overcome several problems with general patterning techniques for liquid‐crystal (LC) alignment. These include difficulty in generating the pretilt angle and in controlling the LC rising‐up direction, formation of local domains, and weak optical properties. The directional peeling of the rigiflex mold results in pretilt‐angle formation and controls the LC rising‐up direction. In addition, a nanostructure with small spacing aligns the LC with a high order parameter because of a strong confinement effect and suppresses diffraction due to its small spacing. Eventually, the nanostructure achieves improvements in the optical properties. In summary, while recent patterning techniques for LC alignment only solve one problem, the directional peeling of the rigiflex mold with a nanostructure simultaneously overcomes several problems with LC alignment and optical properties.     

Magnetic and Transparent Composites by Linking Liquid Crystals to Ferrite Nanoparticles through Covalent Networks

The fabrication of transparent, flexible, and optically homogeneous magnetic composites containing ferrite (Fe₃O₄) nanoparticles, liquid crystals (LCs), and siloxane backbones is reported. The transparent magnets are achieved by covalently bonding LCs to the siloxane backbones and then linking them to dopamine‐functionalized ferrite nanocrystals. They exhibit simultaneous high transparency and strong magnetic properties. A remarkable feature of these films is that the surface morphology of the LC‐attached ferrite films can be tuned by an external magnetic field, demonstrating a striped surface in the direction of the field. We show that the LC‐attached film can act as an alignment layer to orient LCs, enabling the development of LC alignment surfaces on the basis of these nanomagnet–LC polymer composites.     

Alignment of a Perylene‐Based Ionic Self‐Assembly Complex in Thermotropic and Lyotropic Liquid‐Crystalline Phases

The thermotropic and lyotropic liquid‐crystalline (LC) phases of the ionic self‐assembled complex N,N′,‐bis(2‐(trimethylammonium)ethylene)‐perylene‐3,4,9,10‐tetracarboxyldiimide‐bis(2‐ethylhexyl)sulfosuccinate have been studied using polarizing microscopy, differential scanning calorimetry (DSC), and X‐ray scattering techniques. A two‐dimensional (2D) columnar thermotropic LC phase with π–π stacking of the perylene tectonic units and a lyotropic LC phase in dimethyl sulfoxide (DMSO) have been found. Different techniques have been applied to align both systems and included: surface interactions, electric and magnetic fields, shear force, and controlled domain formation at the LC–isotropic phase‐transition front (PTF). Characterization of the alignment in films has been performed using polarized UV‐vis spectroscopy and transmission null‐ellipsometry. The best results have been obtained for alignment of the material in a lyotropic phase by controlled domain formation at the PTF of the LC–isotropic phase transition. In this case, a dichroic ratio of 18 is achieved with packing of columns of perylenediimide tectons perpendicular to the PTF.