Electric force microscopy study of the surface electrostatic property of rubbed polyimide alignment layers

Two polyimides were synthesized for use as alignment layers. The pretilt angles of the liquid crystals, 4-cyano-4′-n-pentylbiphenyl, on the two polyimides were measured by the crystal rotation method. The relative surface atomic concentrations of F/C (%) were measured by X-ray photoelectron spectroscopy. Electric force microscopy was utilized to investigate the surface electrostatic property of the two thin polyimide alignment layers before and after rubbing. All results demonstrate that rubbing causes trifluoromethyl moieties to migrate towards the surface, absorb negative charges and orient along the rubbing direction. Thus, it is proposed that distributions of functional groups on the surface of the polyimide after rubbing are anisotropic and the van der Waals forces between the polar groups and liquid crystal molecules play an important role in the uniform orientation of the liquid crystal molecules.     

Anisotropy and Raman absorption of the polyimide surface irradiated by the ion beam for liquid crystal alignment

In this paper, polyimide surfaces irradiated by an ion-beam for liquid crystal alignment are investigated by using atomic force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. A liquid crystal cell aligned homogeneously through the ion-beam exposure exhibits electro-optic switching behavior similar to that of a rubbing-aligned liquid crystal cell. However, we found that the surface morphology and bonding molecules of ion-beam-treated polyimide surfaces show properties very different from mechanically-rubbed ones. Experimental results show that optical anisotropy of ion-beam-treated polyimide surfaces results in the formation of hydrogenated amorphous carbon-like structure with a short main-chain, while mechanical rubbing has little effect on structural and compositional variations of polyimide layers.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Chain flexibility versus molecular entanglement response to rubbing deformation in designing poly(oxadiazole-naphthylimide)s as liquid crystal orientation layers

Four poly(oxadiazole-imide)s containing naphthalene rings, with different flexibility and molecular weight, are investigated with respect to their rheological properties to establish the optimal processing conditions from solution phase to film state for liquid crystal orientation purposes. The film uniformity and strength are determined by monitoring the flow behavior and chain entanglements. The solution rheological data are in agreement with film tensile testing, revealing that higher molecular weight favors chain entanglements and implicitly the film mechanical resistance. In order to analyze the suitability of these films as alignment layers their surface is patterned by rubbing with two types of velvet. Liquid crystal alignment of 4′-pentyl-4-biphenylcarbonitrile nematic is tested by polarized light microscopy. The resulting behavior is correlated with the polyimide malleability and characteristics of the textile fibers, namely their polarity, size, and mechanical features. The competitive effects between chain flexibility and entanglements, together with the interactions occurring between the polymer and velvet are analyzed in order to explain the surface regularity, which influences the uniformity of the liquid crystal alignment. The contrast between dark and bright states recorded on the liquid crystal cell indicates that some of these polynaphthalimides are promising candidates for liquid crystal display devices.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

Orientational control of liquid crystal molecules by reformed poly(dimethylsiloxane) alignment layer via ion-beam irradiation

We introduce an alternative approach to making a reformed PDMS layer as a pretilt controllable alignment layer, which can be used in the vertical alignment (VA) and twisted nematic (TN) modes, by varying the ion beam (IB) energy. Depending on different the dielectric anisotropy of surfaces, PDMS layers have been demonstrated to align LC molecules homogeneously and homeotropically. The electro-optic characteristics of aligned VA and TN-LCD based on PDMS layer were comparable to those of VA and TN-LCD based on polyimide, showing good potential of PDMS film as an alignment layer.     

Preparation of nanoporous polyimide thin films via layer-by-layer self-assembly of cowpea mosaic virus and poly(amic acid)

Low dielectric (low-κ) materials are of key importance for the performance of microchips. In this study, we show that nanosized cowpea mosaic virus (CPMV) particles can be assembled with poly(amic acid) (PAA) in aqueous solutions via the layer-by-layer technique. Then, upon thermal treatment CPMV particles are removed and PAA is converted into polyimide in one step, resulting in a porous low-κ polyimide film. The multilayer self-assembly process was monitored by quartz crystal microbalance and UV-Vis spectroscopy. Imidization and the removal of the CPMV template was confirmed by Fourier transform infrared spectroscopy and atomic force microscopy respectively. The dielectric constant of the nanoporous polyimide film thus prepared was 2.32 compared to 3.40 for the corresponding neat polyimide. This work affords a facile approach to fabrication of low-κ polyimide ultrathin films with tunable thickness and dielectric constant.     

单层聚酰亚胺LB膜的STM成象

扫描隧道显微镜(STM)被用来表征聚酰亚胺LB膜的形貌及分子排列结构。本文介绍了单层聚酰亚胺LB膜样品制备过程,所用的STM系统及STM实验。给出了该LB膜亚胺化前后的STM图象。结果表明,所制备的LB膜的聚合链排列有序,测得链间距即横向周期～7(?),纵向上所谓的“之”字型结构的周期为11(?)。这些数值与根据分子面积在理论上所预估的结果相符。     

添加剂对聚酰亚胺液晶垂直取向剂性能的影响

研究了添加剂对PI取向性能、耐摩擦性、热稳定性以及透光率的影响。用均苯四甲酸二酐(PM-DA),对苯二胺(PDA),4-已氧基联苯酚-3’,5’-二胺基苯甲酸酯(C6-BBDA),制备了三元共聚型PI取向膜,并采用添加剂(封端剂和交联剂)对PI改性。通过调节添加剂的组成制备了4种PI。利用红外光谱仪(FT-IR)、热失重分析仪(TGA)、紫外分光光度计(UV-Vis)、偏光显微镜(POM)和预倾角测试仪对合成的PI进行了结构表征和性能测试。研究发现LC分子在4种PI膜上都获得了均匀的垂直取向;添加交联剂的PI膜具有良好的耐摩擦性,经摩擦强度为600mm摩擦后,仍对LC分子具有垂直取向的能力;加入添加剂的PI膜具有良好的热稳定性,此外,加入添加剂的PI膜仍具有较高的透光率。     

Spectral narrowing phenomena in the emission from a conjugated polymer

We report a study of the morphology dependent emission properties of a liquid crystalline polyfluorene in solid thin films. Spectral narrowing (SN) is observed in spin coated films, but after thermal treatment of the same films the SN is no longer observed. For films which are spin coated on a rubbed polyimide alignment layer, the situation is different. There is then no influence of thermal treatment on the observation of SN. However, specific combinations of the pump polarization direction, film alignment direction and the long axis of the rectangular excitation area now play an important role. The spectral location of SN peaks also varies with film morphology and for different regions of the same sample. The occurrence of SN is strongly dependent on film morphology and the excitation configuration.     

Fabrication and mechanism study of CuO layers on double surfaces of polyimide substrate using surface modification

Formation process and mechanism of continuous CuO layers on double surfaces of polyimide films were studied. The composite films were prepared using the facile surface modification and ion exchange technique. By alkaline-induced chemical modification and ion-exchange reaction, Cu²⁺ ions were incorporated into the surface of polyimide substrate. Thermal treatment in ambient atmosphere resulted in the formation of CuO particles that further agglomerated on the film surface and produced well-defined CuO thin layers on the double surfaces of polyimide films. The changes in the chemical structure, surface morphology, crystalline state and the surface roughness with the increase of ambient temperature were investigated. It was interesting to find that the conversion of metallic copper and low valence sub-oxide Cu₂O to high valence oxide CuO was observed in the thermal treatment process. The agglomeration mechanism for the CuO particles was proposed and proved by three steps, which illustrated that copper-catalyzed and oxygen-assisted decomposition of the polyimide overlayer resulted in the agglomeration of CuO particles. The final composite films retained the thermal stability of the pure polyimide.     

PMDA-ODA型聚酰亚胺制备工艺与聚集态结构的研究进展

综述了PMDA-ODA型聚酰亚胺的制备工艺,并阐述了酰亚胺化工艺对聚酰亚胺聚集态结构的影响。     

光刻法制备聚合物/液晶光栅

将明胶涂覆在表面经过取向处理的带有ITO电极的玻璃基板上，以紫外灯为光源，通过光掩模法，使明胶在光场的引发下发生光化学反应，样品显影刻蚀处理后呈栅状，将液晶注入光栅盒中，形成聚合物／液晶光栅。该光栅衍射效率可利用电场调控。采用He—Ne激光器对所制样品进行测试，结果表明，所制样品的栅结构较好。避免了液晶与聚合物相分离的不完全性和栅条边缘不整齐的现象。制得的光栅其第一衍射级次的开关比为87：1，显示了较好的开关能力。     

Nanoscale alignment and optical nanoimaging of a birefringent liquid

An anisotropic nanopatterning method, based on a technique of atomic force microscopy (AFM) scribing of a thin polyimide film, is used to generate an alignment layer whose topography depends on the writing direction. Detailed experimental measurements are presented for the topographical anisotropy that arises when the polyimide alignment layer is scribed parallel and antiparallel to the AFM cantilever orientation. By means of a novel nanotomographic approach, the optical retardation δ of an alignable birefringent liquid that covers the scribed substrate is measured with unprecedented resolution of only a few tens of nanometers. In this technique a thin optical fiber is raster-scanned at several fixed heights inside the birefringent liquid, and the transmitted polarized light is collected downstream. The optical retardation δ from the fiber's tip to the polyimide interface was measured as a function of position x,y,z, with the results reflecting the spatially varying depth of the medium due to the polymer film surface topography. Theoretical calculations for δ are in excellent agreement with both the topographical and the high resolution nanoimaging experimental results obtained.     

Anomalous light transmission by nematic liquid crystal cells

Light transmission through nematic liquid crystal (NLC) cells with a planar alignment and various boundary conditions has been studied as a function of the bias voltage in the regimes of maximum transmittance and complete extinction. The phenomenon of anomalous light leakage in the regime of complete extinction has been observed in the NLC sells with alignment layers of amorphous hydrogenated carbon (a-C:H) and poly(vinyl alcohol) (PVA), in which an anisotropy was induced by rubbing the electrode surface (for a-C:H) and the alignment layer (for PVA). The anomalous transmission is caused by the deformation of nematic molecules, which are situated near the surface, in the direction perpendicular to the direction of rubbing. The magnitude of anomalous transmission increases with the bias voltage. This behavior is explained by the anisotropy of the surface anchoring energy of nematic molecules at the interface.     

Effect of multi-dimensional ultraviolet light exposure on the growth of pentacene film and application to organic field-effect transistors

We report on the multi-dimensional alignment of pentacene molecules on a poly(methyl methacrylate)-based photosensitive polymer (PMMA-polymer) and its effect on the electrical performance of the pentacene-based field-effect transistor (FET). Pentacene molecules are shown to be preferentially aligned on the linearly polarized ultraviolet (LPUV)-exposed PMMA-polymer layer, which is contrast to an isotropic alignment on the bare PMMA-polymer layer. Multi-dimensional alignment of pentacene molecules in the film could be achieved by adjusting the direction of LPUV exposed to the PMMA-polymer. The control of pentacene molecular alignment is found to be promising for the field-effect mobility enhancement in the pentacene FET.     

Effect of UV/ozone treatment on interactions between ink-jet printed Cu patterns and polyimide substrates

In this study, we apply UV/ozone treatment to modify the surface properties of polyimide substrate and investigate the effects of UV/ozone treatment on surface properties of polyimide and the morphologies of ink-jet printed Cu patterns. Wettability of polyimide surface is enhanced by UV/ozone treatment as confirmed by contact angle analysis and XPS results. The change in wettability affected droplet sizes and morphologies of ink-jet-printed lines. The average droplet size increased from 26.8 μm on untreated polyimide to 42.5 μm after UV/ozone treatment. Furthermore, irregular patterns of coexisting beads and short lines were observed in untreated polyimide film, despite minimal overlap of dots. In contrast, patterns that were ink-jet printed on polyimide film that was modified by UV/ozone for 1 h showed continuous lines with straight edges.     

正交设计法研究聚酰胺酸分子量及其影响因素

采用正交试验设计方法,研究了聚酰胺酸固体含量、反应温度、加料间隔时间和反应时间对聚酰胺酸分子量的影响,采用凝胶渗透色谱仪测试了不同反应条件下合成的聚酰胺酸的分子量。正交设计试验结果表明,合成最高分子量聚酰胺酸的反应条件为:反应温度为15℃,固体含量为8%,加料间隔时间为9min,反应时间为3h。在此基础上,使用TGA测试了由不同分子量聚酰胺酸合成的聚酰亚胺薄膜的热稳定性。试验结果显示,聚酰胺酸分子量越大,薄膜的热稳定性越好。     

Formation of Cu or Cu2O nanoparticles embedded in a polyimide film for nanofloating gate memory

Cu and Cu2O nanoparticles were fabricated in polyimide by curing the stacked polyamic acid/Cu/polyamic acid on Si wafer and post heat treatment. Nanoparticle distribution in polyimide (a monolayer of vertically aligned nanoparticles or the randomly dispersed nanoparticles) can be controlled by changing the reactivity of Cu with PAA and curing atmosphere. About 6-7 nm sized Cu or Cu2O nanoparticles were observed in the polyimide film. The capacitance-voltage curves were measured with Al/particles in polyimide/p-Si(100) specimens at 300 K, and the capacitance hystereses were observed at different sweep voltage ranges, which indicates that Cu2O or Cu nanoparticles can be utilized in next generation flash memories.