Washing effects on the anchoring energy and surface order parameter on rubbed polymer surfaces containing the trifluoromethyl moiety

The washing effects on liquid crystal (LC) alignment capability in a NLC, 4-n-pentyl-4'-cyanobiphenyl (5CB), on a rubbed polyimide (PI) surface containing the trifluoromethyl moiety were studied. The extrapolation length d_e of 5CB on the rubbed PI surface decreases with the rubbing strength RS. Also, the large extrapolation length d_e of 5CB for washing with IPA was measured at RS=114 mm. The polar anchoring energy of 5CB on the rubbed PI surface was decreased by the washing process; it increased with the rubbing strength RS on the PI surface. The surface order parameter S_S of 5CB for all the washing processes is smaller than for the non-washing process. Consequently, the polar anchoring energy and surface order parameter S_s in 5CB are largely attributed to washing effects.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Preliminary Communication : Generation of high pretilt angles of nematic liquid crystal (5CB) on rubbed organic solvent soluble polyimide surfaces with helical backbone structure and trifluoromethyl moieties

High pretilt angles of nematic liquid crystals (NLC) have been successfully generated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. We have found that the pretilt angle of in 4-n-penty-4-cyanobiphenyl (5CB) on rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is larger compared to that on rubbed PI surfaces with trifluoromethyl moieties attached to the polymer backbone. The observed pretilt angle of 5CB on unidirectionally rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is about 15 degrees in the wide rubbing region. From these results, it is expected that the microscopic surface structure of the polymer strongly contributes to the pretilt angle generation of the LCs at the surface.     

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

The polar anchoring strength and pretilt angle generation in nematic liquid crystals (NLCs), on three kinds of rubbed polyimide (PI) surfaces, were investigated. The pretilt angle generated in 4-n-pentyl-4'-cyanobiphenyl (5CB) is large compared with ZLI-4792 (a fluorinated mixture type NLC) for all rubbed PI surfaces. The high pretilt angle in 5CB is attributed to a much larger birefringence and much larger perpendicular component of permittivity. The polar anchoring energy of ZLI-4792 is high compared with 5CB on all rubbed PI surfaces for a weak rubbing strength. It is suggested that the high anchoring energy of ZLI-4792 may be attributed to the low NLC pretilt angle.     

Investigation of surface anchoring strength and pretilt angle in NLC on rubbed polythiophene surfaces with alkyl chain lengths

A high pretilt angle for the nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), was observed on rubbed polythiophene (PTP) surfaces having alkyl chains with more than ten carbon atoms. We consider that this is due to a surface-excluded volume effect caused by the long alkyl chains between the LC molecules and the PTP surfaces. The polar anchoring strength in 5CB on rubbed PTP surfaces with long alkyl chains has also been successfully evaluated. The extrapolation length d_e of 5CB increases with increasing alkyl chain lengths above the seven carbons of alkyl chain R7; that is, it may be attributed to the high pretilt angle. An extrapolation length of 5 nm is observed in 5CB for the seven carbons alkyl chain R7 on the PTP surface; this indicates high anchoring strength.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

Insertion of the Liquid Crystal 5CB into Monovacancy Graphene

Interfacial interactions between liquid crystal (LC) and two-dimensional (2D) materials provide a platform to facilitate novel optical and electronic material properties. These interactions are uniquely sensitive to the local energy landscape of the atomically thick 2D surface, which can be strongly influenced by defects that are introduced, either by design or as a byproduct of fabrication processes. Herein, we present density functional theory (DFT) calculations of the LC mesogen 4-cyan-4′-pentylbiphenyl (5CB) on graphene in the presence of a monovacancy (MV-G). We find that the monovacancy strengthens the binding of 5CB in the planar alignment and that the structure is lower in energy than the corresponding homeotropic structure. However, if the molecule is able to approach the monovacancy homeotropically, 5CB undergoes a chemical reaction, releasing 4.5 eV in the process. This reaction follows a step-by-step process gradually adding bonds, inserting the 5CB cyano group into MV-G. We conclude that this irreversible insertion reaction is likely spontaneous, potentially providing a new avenue for controlling both LC behavior and graphene properties.     

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S_m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was A_ϕ ≈10^-4 J m^-2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.     

Surface morphology at nanometer level and optical phase retardation of rubbed alignment films

The surface morphology of rubbed polyimide LC aligning films has been studied by scanning force microscopy. We examined three types of alignment film: HT-210, AL-1051 (main chain type) and AL-8044 (side chain type) polyimide surfaces. The rubbed polyimide surfaces show anisotropic alignment of the polymer chain cluster, and a microgroove between the ripple pattern streaks. Also, they are periodically oriented along the rubbing direction. The optical phase retardation of the rubbed surface does not increase with increased cumulative time of rubbing, On the other hand, it increases monotonically with increasing rubbing depth.     

Surface morphology at nanometer level and optical phase retardation of rubbed alignment films

The surface morphology of rubbed polyimide LC aligning films has been studied by scanning force microscopy. We examined three types of alignment film: HT-210, AL-1051 (main chain type) and AL-8044 (side chain type) polyimide surfaces. The rubbed polyimide surfaces show anisotropic alignment of the polymer chain cluster, and a microgroove between the ripple pattern streaks. Also, they are periodically oriented along the rubbing direction. The optical phase retardation of the rubbed surface does not increase with increased cumulative time of rubbing, On the other hand, it increases monotonically with increasing rubbing depth.     

Tunable backflow in chiral nematic liquid crystals via twist-bend nematogens and surface-localised in-situ polymer protrusions

Dynamic electro-optic response of the liquid crystal (LC) director shows a backflow effect that is manifested as an optical bounce in chiral nematic LCs (N*LC) during field-induced homeotropic-twisted transition. The bend elastic constant (K₃₃) strongly influences the dynamics of backflow at the N*LC in homeotropic-twisted transition. The cyanobiphenyl LC dimers – CB7CB, CB9CB and CB11CB – possess a unique characteristic of inherent bend molecular configuration that lowers K₃₃. With the modulation of the effective K₃₃ in dimer-doped N*LCs, we report the tunability of the optical bounce that decreases with the increase in the length of flexible spacers in LC dimers. The doped LC dimers with short spacer lengths not only generate a strong backflow with an enhanced twist degeneracy of the LC director across the cell, but also prolong the time of disappearance of the optical bounce. Furthermore, we demonstrate the suppression of the optical bounce with surface localised polymer protrusions having 50–100 nm diameters, which allow faster dynamic relaxation process and reduced backflow. We envision a novel design of a tunable microfluidic device for precise flow control of organic or inorganic matter in LC medium that exploits the tunable backflow in LC dimer-doped N*LCs.     

Alignment of liquid crystals on ultrathin organized molecular films

Liquid crystal (LC) alignment techniques based on various kinds of ultrathin organized molecular films are reviewed. The mechanisms of LC alignment on the organized films are discussed. For the homeotropic alignment of LCs the main anchoring mechanism is due to the dipole–dipole interaction between polar groups of an aligning agent and LC molecules while the homogeneous alignment is mainly attributed to the orientation of polymer chains or polymer aggregates. An experimental system for an anchoring transition induced by a conformation change of aligning molecules is introduced. Finally the AFM experimental observations on the rubbed polymer films and its mechanisms are summarized.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Photoluminescence of a 5CB liquid crystal–organomontmorillonite nanoparticle heterocomposite

Photoluminescence (PL) of a heterocomposite, consisting of the nematic liquid crystal (LC) 4-pentyl-4´-cyanobiphenyl (5CB) and anisometric nanoparticles of montmorillonite (MMT) clay, modified by cetyltrimethylammonium bromide (CTAB) has been investigated at 4.2 and 300 K. The incorporation of this organoclay (B4) to 5CB decreases the emission intensity by 7–8 times due to efficient resonant quenching of the exciting energy by the organoclay. The spectrum shifts to a long-wave region, with this effect being considerably larger at low temperatures. Graphical separation of complex bands, corresponding to the bulk 5CB and 5СВ?+?В4 heterosystem at both temperatures revealed that the presence of the organoclay resulted in a significant growth of LC dimer quantity, shifting spectra towards longer wavelengths. Changes in the 5CB luminescence under organoclay influence can be explained by quite strong interphase interactions specified earlier by infrared spectroscopy between the MMT surface and LC, and by a realisation of more flat conformations of 5CB molecules. Confinement effects prevent full crystallisation of 5CB in the 5CB?+?B4 composite, and LC dimer structures located in the organoclay near-surface layers on the outer surface of the nanoparticles and inside its galleries remain in a larger amount, at low temperature, when compared to bulk 5CB. The remaining LC crystallises and photoluminescence from the 5CB monomers becomes intense.     

Influence of specific anions on the orientational ordering of thermotropic liquid crystals at aqueous interfaces

We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4'-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 ?(2)/molecule for NaClO(4)) and thus smaller molecular tilt angles (30° from the surface normal for NaClO(4)) than kosmotropic salts (5.0 mN/m at 38 ?(2)/molecule with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.     

利用液晶取向变化的光学免疫检测方法

利用共价固定方法将抗血清白蛋白固定到硅烷化玻璃基底上, 并通过摩擦其表面形成5CB液晶整齐均一取向排列的基底. 考察了不同浓度的人血清白蛋白与基底作用后液晶在基底上形成的偏光光学图像的差异, 并利用自行提出的“图像加权平均灰度值”定量分析了图像灰度与人血清白蛋白浓度的关系. 对比研究了基底上的特异性吸附与非特异性吸附引起的液晶偏光光学图像的差异以及调制偏振光能力, 结果表明, 该基底具有很高的特异性. 该方法可望发展成为一种灵敏、非标记的光学免疫检测方法.     

炭黑的“impurity-free”改性及其在聚酰亚胺薄膜中的分散特性研究

利用聚酰胺酸(PAA)在研磨过程形成炭黑(CB)的"impurity-free"分散剂制备纳米CB填充聚酰亚胺(PI)高性能复合薄膜.球磨CB和PAA/N-甲基-2-吡咯烷酮(NMP)溶液的混合液,PAA在研磨过程中降解形成活性分子,原位生成与CB表面具有反应活性和强烈物理吸附能力的"impurity-free"分散剂.拉曼、红外以及紫外-可见光吸收光谱证实了降解PAA分子对CB的改性作用.经改性的CB与PAA溶液共混,涂覆固化制备PI/CB复合薄膜.TEM照片表明该分散剂可以显著促进CB粒子在PI基体中的均匀分散,分散粒径约为200nm.力学性能测试和导电性能测试表明PI/CB复合薄膜的断裂伸长率大幅提高,电阻率(ρ)重复性浮动范围从2个数量级降到1个数量级.进一步研究发现,研磨过程中添加高分子量PAA更有利于CB在PI基体中的均匀分散.     

Influence of rubbing conditions of polyimide alignment layer on optical anisotropy of immobilized liquid crystal film

The relationship between the molecular orientation of a rubbed polyimide film (alignment layer) and that of mesogens in a photopolymerized liquid crystal (LC) coated on the film has been investigated using optical measurements. LC monomers were deposited on the alignment layer and were aligned in one direction. The LC monomers were subsequently photocured. Alignment layers under various rubbing conditions were prepared. It was found that the inclination angle of the refractive index ellipsoid and the optical retardation of photopolymerized LC films are strongly related to the optical anisotropy of the rubbed polyimide film. The photopolymerized LC film exhibited high optical anisotropy when alignment layers with an inclination angle of the refractive index ellipsoid smaller than 6° were used.