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.     

Electro-optical characteristics of ZrO2 nanoparticle doped liquid crystal on ion-beam irradiated polyimide layer

It is well known that doping liquid crystals (LCs) with nanoparticles can readily change the physical and electro-optical properties of LC mixture. In this paper, we report on how the electro-optical properties and thermal stability of an LC system were enhanced by dispersing zirconia (ZrO2) nanoparticles in nematic LCs on ion-beam irradiated polyimide layers. Homogeneous LC alignment was achieved and ZrO2/LC mixture was applied in twisted-nematic (TN) mode. The addition of ZrO2 nanoparticles contributed to improvement of electro-optical properties in the TN LC cell by lowering voltage operation and decreasing response time. The TN LC cells with a ZrO2 nanoparticle concentration of 2.0 wt% showed the lowest threshold voltage of 2.0 V and the fastest response time of 15.3 ms. This enhanced electro-optical performance was likely due to van-der waals interactions and the screening effect of the ZrO2 nanoparticles in the LC medium. The thermal stability of the ZrO2/LC mixture was also improved compared to a pristine LC system.     

Preparation of porous ultra-fine poly(vinyl cinnamate) fibers

In this study, a new method of preparing porous ultra-fine fibers via photo-crosslinking was developed. Ultra-fine poly(vinyl cinnamate)/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PVCi/PHBV) blend fibers were electrospun and then the PVCi was photo-crosslinked by UV irradiation. PVCi and PHBV were immiscible and the phase separation proceeded during the electrospinning process. After the photo-crosslinking of PVCi, PHBV was extracted from the blend fibers with chloroform. The average pore sizes in the remaining ultra-fine PVCi fibers were increased with increasing the content of PHBV in the ultra-fine PVCi/PHBV fibers.     

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-Fluorinated Coatings

Fluoropolymers have many properties that are desirable in coatings. However, because of their poor solubility, formulating coatings containing fluoropolymers can be very difficult. Fluorine-based polymers that have been modified to increase their solubility have been used in a number of resin systems to bring the properties of fluoropolymers to the final coating. Typically these systems have fluorine throughout the entire resin and consequently have a high total fluorine level. We have found that by designing fluorinated molecules that stratify to the surface in the wet coating, we can deliver systems with the excellent surface properties of fluoropolymers but at very low total fluorine levels.     

Surface migration of fluorinated additive during the curing of epoxy resin

Driving forces, responsible for the migration of a fluorinated additive added to a thermoset resin, toward the air/solid interface, were investigated. On this subject, the surface chemistry as characterized by XPS analyses was compared to theoretical models based on the rheological properties of the blend. It appears that the migration of the fluorinated molecules toward the surface cannot be described by a conventional diffusion model. A second model derived from the latter suits better to the experimental data and should be presented as another proof that additional motion forces drive the fluorine molecules migration in curing epoxy resin.     

Poly(dimethylsiloxane)-polyimide blends in the formation of thick polyimide films

Thick polyimide layers can be formed by using some unique properties of poly(dimethylsiloxane)-polyimide (PDMS/PMDA–ODA) blends followed by surface modification and deposition of a second layer of polyimide precursor chemicals. The method is based on the micro-phase separation characteristics of these blends to yield surfaces that have PDMS-like character. Upon modification with UV/ozone treatment, a surface that is essentially SiO _x and hydrophilic in nature is produced. This surface is amenable to reaction and deposition of a second polyimide layer from polyimide precursors. The thicker polyimide layer has enhanced adhesion between the original layer of the blend and the new polyimide layer and this approach finds extensive applications for products that require thick polymer layers. Changes in surface energy for various blend compositions were monitored by measurement of advancing contact angle with de-ionized water. Contact angle for unmodified polyimide films was on the order of 70° and it increased to about 104° after blending with PDMS and curing. UV/ozone treatment reduced the contact angle of the doped polyimide to less than 5°. X-ray photoelectron spectroscopy (XPS) and angle resolved XPS (ARXPS) measurements were used to monitor the chemical compositions of the various surfaces. High-resolution XPS spectra in the Si2p region confirm the transformation of O–Si–C bonds in PDMS to SiO _x , where x is about 2. Scanning electron microscopy (SEM) of some selected samples shows that the blends contain phase separation of the polymers at the surfaces of the samples. Atomic force microscopy (AFM) of siloxane-free polyimide, and PDMS/PMDA–ODA blends both prior to and after UV/ozone exposure, show that the films are essentially flat at short treatment times (less than 60 min). AFM also reveals the separation of PDMS into micro-domains at the cured film surface and throughout the layer below the surface of the blended films. Adhesion of a subsequently deposited polyimide layer to the modified polyimide surface was found to be greatly improved when compared to the adhesion obtained for deposition onto a pristine polyimide surface.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

Abstract

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA?) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA? LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (～73%), response time and grayscale switching performance of the device.     

Surface modification of a flexible polyimide film using a reactive fluorinated polymer nanosheet

This paper describes an effective approach to surface modification of a flexible polyimide film using a reactive fluorinated polymer nanosheet. N-(1H,1H-pentadecafluorooctyl)methacrylamide copolymers containing carboxyl group as a reactive moiety form stable monolayer on the water surface and highly ordered reactive polymer nanosheets can be fabricated by the Langmuir-Blodgett technique. This reactive fluorinated polymer nanosheet was utilized to modify the surface properties of polyimide film through its immobilization using thermal treatment. The modification process was studied by X-ray photoelectron spectroscopy (XPS) and modified PI surface was characterized by contact angle measurements and atomic force microscopy (AFM).     

One-pot preparation of fluorinated mesoporous silica nanoparticles for liquid marble formation and superhydrophobic surfaces

One-pot synthesis of fluorinated mesoporous silica nanoparticles (FMSNs) is reported. Uniform mesoporous nanoparticles are prepared by condensation of tetraethyl orthosilicate (TEOS) and fluoroalkyl containing organotriethoxy silane monomers, respectively. The method enables selective deposition of fluorine atoms on the surface of the particles. FMSNs are used to prepare stable liquid marbles with water. An organo-modified silica sol is used with FMSNs to prepare mechanically stable superhydrophobic surfaces (water contact angle of 161°). The mechanical stability of the surface is investigated with water dripping and adhesive tape tests. The prepared FMSNs are promising building blocks for robust, large-area, and multifunctional self-cleaning surfaces.     

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.     

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.     

Homogeneously aligned liquid crystal display on silicon oxynitride thin film using ion beam bombardment

This paper introduces the homogeneously aligned liquid crystal (LC) display using ion beam (IB) bombardment of a new silicon oxynitride (SiON) alignment layer for the first time. The pretilt angle was shown to be a function of the IB incident energy, and possible mechanisms of LC alignment were investigated with physical and chemical methods. An X-ray photoelectron spectroscopy analysis showed that the LC alignment on the IB-bombarded SiON inorganic surface was due to the reformation of Si–O bonds as a major factor. The electro-optical characteristics were comparable to those of rubbed polyimide films.     

Isotope Engineered Fluorinated Single and Bilayer Graphene: Insights into Fluorination Selectivity,Stability, and Defect Passivation

Tailoring the physicochemical properties of graphene through functionalization remains a major interest for next-generation technological applications. However, defect formation due to functionalization greatly endangers the intrinsic properties of graphene, which remains a serious concern. Despite numerous attempts to address this issue, a comprehensive analysis has not been conducted. This work reports a two-step fluorination process to stabilize the fluorinated graphene and obtain control over the fluorination-induced defects in graphene layers. The structural, electronic and isotope-mass-sensitive spectroscopic characterization unveils several not-yet-resolved facts, such as fluorination sites and C F bond stability in partially-fluorinated graphene (F-SLG). The stability of fluorine has been correlated to fluorine co-shared between two graphene layers in fluorinated-bilayer-graphene (F-BLG). The desorption energy of co-shared fluorine is an order of magnitude higher than the C F bond energy in F-SLG due to the electrostatic interaction and the inhibition of defluorination in the F-BLG. Additionally, F-BLG exhibits enhanced light–matter interaction, which has been utilized to design a proof-of-concept field-effect phototransistor that produces high photocurrent response at a time <200 µs. Thus, the study paves a new avenue for the in-depth understanding and practical utilization of fluorinated graphenic carbon.     

静电纺技术在耐高温纤维材料领域研究进展

耐高温纤维在高温过滤膜、锂电池隔膜以及高温催化等方面有着良好的应用。结合纳米纤维高孔隙率、高比表面积等优点,将这些纤维原料经过静电纺丝技术制备成纳米纤维,应用于工业、国防、医疗、环境保护等领域已成为当今材料科学的研究热点。重点综述了静电纺丝技术制备耐高温性能纳米纤维材料的研究进展,如静电纺芳纶、聚酰亚胺、聚苯并咪唑、含氟高分子等聚合物纳米纤维及陶瓷无机纳米纤维等,为进一步开展静电纺丝制备耐高温纳米纤维的研究和应用提供参考。     

Effects of anisotropic diffractions on holographic polymer-dispersed liquid-crystal gratings

Volume gratings fabricated by interferometric exposure using composite materials composed of nematic liquid crystals (LC) and LC diacrylate monomers are discussed in the effects of diffraction properties on different grating formations, such as varying LC content ratios, film thicknesses, and the surface conditions composed of alignment layers and rubbing directions. Diffraction properties are experimentally investigated in the viewpoints of anisotropic diffraction and LC orientation. The polarization-azimuth dependence of diffraction efficiencies as functions of the incident polarization states shows the controllability of anisotropic diffractions based on the effects of different surface conditions.     

Effects of backbone conformation and surface texture of polyimide alignment film on the pretilt angle of liquid crystals

Polyimides (PIs) with different inclination angle of polymer backbones, together with polar hydroxyl group and/or nonpolar trifluoromethyl group at various sites of the backbone were synthesized and used as liquid crystal alignment layers. The molecular conformation, surface chemistry, surface energy, surface morphology, and pretilt angle of the PI film were investigated. The distributions of fluorinated group and hydroxyl group at different depths of the PI surfaces were analyzed by X-ray photoelectron spectroscopy. Effects of the conformation of the PI molecular backbone on the surface morphology of the rubbed PI layer, the pretilt angle and surface energy of the alignment film were studied. The PI which contains both nonpolar fluorinated groups sticking out of the surface and the polar hydroxyl groups on the surface exhibits high pretilt angle.     

Assemblies of fluorine containing bent-shaped liquid crystal molecules studied by using scanning tunneling microscopy

In this work we prepared a fluorine containing bent-shaped liquid crystal from biphenyl as the central core and rod-like azobenzene mesogens as the side arms, namely 4',3-biphenyl bis[4-(4'-hexadecanloxy-3-fluorophenylazo)benzoate] (L104). The self-assembly behaviors of L104 molecules on graphite surface were investigated by using scanning tunneling microscopy (STM) under ambient conditions. The high-resolution STM images of L104 assemblies revealed three kinds of structures showing the joint effects of dipole-dipole interactions originated from the fluorine and the bent-core alignments for maximizing pi-pi interactions. These observations may be beneficial for understanding the assembly mechanism and designs for novel banana-shaped liquid crystal molecules.