Rubbed Polyimide Surface Research Articles

Imaging of triboelectric charge distribution induced in polyimide film by using optical second-harmonic generation: Electronic charge distribution and dipole alignment

By using an optical second-harmonic generation (SHG) microscope, we visualized spatial distribution of triboelectric charge in polyimide films, which are negatively charged by rubbing with a cotton belt. The origins of this triboelectric charging were excessive electronic charges and aligned dipoles. The excessive electronic charges were imaged using the probe laser wavelength of 1140 nm (SHG wavelength 570 nm), whereas the aligned dipoles were visualized by choosing the laser wavelength of 570 nm (SHG wavelength 285 nm). Combining the two visualized images showed that the dipolar alignment region with a width of 10 μm was induced on the rubbed polyimide surface, and at the same time, excess electronic charges were generated in that region. We conclude that dipolar alignment and electronic charging are main contributors, and the interaction between the two contributors plays an essential role in triboelectrification.

Anchoring strength measurements of a lyotropic chromonic liquid crystal on rubbed polyimide surfaces

ABSTRACTTechniques to achieve planar anchoring of the director of a lyotropic chromonic liquid crystal (LCLC) have been developed only recently. These techniques range from the very weak anchoring achieved by simply scratching the glass with a very fine abrasive to the very strong anchoring achieved with lithographic techniques. A possible alignment technique is to use rubbed polyimide (PI) alignment layers as is routinely done to align thermotropic liquid crystals (TLCs). The anchoring strength of the LCLC disodium cromoglycate (DSCG) on different rubbed PI alignment layers is measured using a /2 twist cell and optical polarisation techniques. Although the anchoring strength is not as large as can be achieved with lithographic techniques, it is larger than scratched glass for one PI. Just as important, rubbed PI provides a much more uniform surface than is possible with scratched glass. Interestingly, addition of a small amount of surfactant to the LCLC, which results in improved alignment, does not increase the anchoring strength. Weak anchoring of LCLCs can also be achieved with a photo-patterned dye surface layer. Finally, atomic force microscopy measurements of the surface combined with prior theoretical work linking surface features and anchoring strength predict anchoring strengths consistent with experiment.

Comparison of photoinduced reorientation of ortho-, meta-, and para-methyl red-doped nematic liquid crystals on rubbed polyimide.

We compare the photoinduced reorientation of the easy axis on rubbed polyimide surfaces for the nematic E7 doped with three isomers of methyl red; ortho, meta, and para. Using optical techniques, the angle and the pitch of the director at the polymer surface were measured before, during, and after photoexcitation of the dye. Optical absorbances were also measured before and after photoexcitation. Extrapolation lengths, hence anchoring energies, were determined with the on/off application of a magnetic field for meta- and para-methyl red-doped nematics. Because of an elastic reorientation of the easy axis in the presence of the magnetic field, we could not determine the extrapolation length of the ortho-methyl red-doped nematic. Our results confirm that photoinduced reorientation is facilitated by desorption of all dyes from the polymer surface. While there is little evidence of weak photoinduced adsorption of meta- and para-methyl red to the surface during photoexcitation, there is strong evidence of photoinduced adsorption of ortho-methyl red, which is long lasting.

그라핀 양자점을 도핑한 TN 셀의 고속 스위칭 특성

In this study, we report the doping effect of graphene quantum dots (QDs) in nematic liquid crystal (NLC) system on rubbed polyimide (PI) surface. The good LC alignment and high thermal stability in QD-LC cell system on rubbed PI surfaces can be measured. Also, the low threshold voltage of QD-TN cell was observed about 2.77 V. The fast response time of 13.2 ms for QD-TN cell can be achieved. Finally, the good voltage holding ratio of QD-TN cell on rubbed PI surface was measured.

Bi-axially aligned crystallites of a fluorene–bithiophene co-polymer

Thin films of the soluble conjugated poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) on top of polyimide surfaces were investigated by X-ray diffraction measurements, atomic force and optical microscopy. In-situ temperature dependent specular X-ray diffraction experiments reveal the formation crystallites on heating. An orthorhombic unit cell of a=1.61nm, b=1.28nm and c=6.2nm was found which results in 16 monomer units being hosted in the unit cell at a density of 1.15gcm−3. Morphology investigations reveal random azimutal orientations of spherulite like structures are present for samples prepared on the isotropic surface. Using a defined preparation route allowed for the alignment of the crystallites with respect of a rubbed polyimide surface resulting in bi-axially aligned F8T2 crystals as determined by X-ray diffraction polefigure measurements and optical microscopy.

Asymmetrically Tilted Alignment of Rigid-Rod Helical Polysilanes on a Rubbed Polyimide Surface

The homogeneous alignments of helical rod-like polysilanes on a rubbed polyimide alignment layer were investigated by polarized optical microscopy (POM) and atomic force microscopy (AFM) analyses. The POM and AFM observations determined that polysilanes with a series of aliphatic side chains helically arranged around the main chains were tilted to the right and left by 33° from the rubbing direction when the handedness of the side-chain helical array is left and right, respectively. It is interesting to note that the side-chain arrays run perpendicular to the rubbing direction on the polyimide surface, which is different from intuitive "knob and hole" packing of the extended polyimide chain and the helical grooves between the side-chain arrays surrounding the polysilane backbone. More surprisingly, both right- and left-tilting smectic domains were simultaneously observed with an equal probability for an achiral polysilane, which apparently has the interconverting right- and left-handed helical segments separated by helical reversals. This might be the first observation of the chiral segregation of dynamic helical polymers.

Self-orienting liquid crystal doped with polymer-azo-dye complex

We investigate into a new type of liquid crystal (LC) heterogeneous system that contains hydrogen-bonded polymer-azo-dye complexes at a low concentration. The suspension shows a unique self-orienting property and a spontaneous anchoring transition from planar to homeotropic alignment on a rubbed polyimide surface. We suggest a simple polymer adsorption model to explain the universal homeotropic orienting property. The complex-doped LC suspension can be used as a new liquid crystalline material that does not require any additional alignment processing or treatment.

What are the differences of polymer surface relaxation from the bulk?

The polymer surface relaxation in thin films has been a long debating issue. We report a new method on studying surface relaxation behaviors of polymer thin films on a solid substrate. This method involved utilizing a rubbed polyimide surface with a pretilting angle in a liquid crystalline cell. Due to the surface alignment, the liquid crystals were aligned along the rubbing direction. During heating the liquid crystalline cell, we continuously monitored the change of orientation of the liquid crystals. It is understood that at a temperature where the orientation of liquid crystal is lost, the surface relaxation on the glass substrate takes place to lose the polyimide surface orientation. It was found that the relaxation temperature at which the liquid crystals lose their orientation depends on the film thickness of the polyimide. A quantitative linear relationship between the relaxation temperature and reciprocal of the film thickness can be observed. Furthermore, different topologies of the rubbed and relaxed thin films were amplified using the polyethylene decoration method and observed using atomic force microscopy.

Preferred Pre-tilt Direction of Liquid Crystal Molecules on Rubbed Polyimide Surfaces

We systematically investigated the relationship between the preferred pretilt direction (PD) of the liquid crystal (LC) directors and the physical parameters of the rubbing-treated polyimide (PI) surfaces via contact angle measurements and scanning probe microscopy (SPM) techniques. Rubbing treatment could induce anisotropy and asymmetry in the contact angle and the surface roughness of the PI surfaces, which played important roles in determining the preferred PD of the LC molecules. According to our results, the preferred PD of the LC directors aligns along a direction with low contact angle and low surface roughness.

A study of the transition of liquid-crystal alignment from homeotropic to planar on a polyimide layer

The alignment of nematic liquid crystals by rubbed polyimide surfaces has been well-studied and developed. A novel polyimide film which induced a homeotropic alignment of the nematic liquid crystal without rubbing or with weak rubbing strength was presented. However, there was a transition from homeotropic to planar alignment of the nematic liquid crystal after strong rubbing. In order to study the transition, the polyimide surface was investigated by atomic force microscopy, surface free energy measurement and angle-resolved analysis X-ray photo-electron spectroscopy before and after rubbing with a velvet fabric. It was found that both the change of surface polarity and surface morphology were not the reasons for the transition. The droop of the side chain on the polyimide surface after the rubbing treatment was detected by angle-resolved analysis X-ray photo-electron spectroscopy. Owing to the special structure of the novel polyimide, the X-ray photo-electron spectroscopy was successfully used for the first time to analyse the conformational change of the side chain of a polymer. In conclusion, the transition of nematic liquid crystal alignment from homeotropic to planar after rubbing was influenced by the side chain conformation of the polyimide.

Temperature induced structural evolution of a fluorene‐thiophene copolymer on rubbed surfaces

AbstractThe thermal alignment of the liquid crystalline fluorene‐thiophene copolymer (F8T2) on rubbed polyimide surfaces is investigated by ex‐situ and in‐situ X‐ray scattering experiments. The ex‐situ characterization allows an assignment of the observed diffraction peaks to distances between polymer backbones (1.6 nm), distances between the flexible side groups of the polymer chains (0.43 nm), and intramolecular distances of adjacent ring units (0.5 nm). The in‐situ characterization allows a temperature dependent observation of the polymer chain alignment. A gradual alignment process of the polymer backbones is observed for temperatures up to 563 K. Decreasing temperature after the polymer chain alignment is accompanied by a glass transition of the side chains at 380 K. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47:1599–1604, 2009

P‐195L: Late‐News Poster: Effect of the Surface Anisotropy of a Rubbed Polyimide Film on Liquid Crystal Alignment Properties

AbstractMorphological and molecular orientational anisotropies of a rubbed polyimide film, which is a general alignment layer in a mass production of the 42″ IPS‐mode LCD, were analyzed. The results of our AFM (Atomic Force Microscopy) and XRR (X‐ray Reflectivity) measurements of rubbed polyimide surface didn't show any morphological anisotropy between parallel and perpendicular to a rubbing direction. On the other hand, orientational anisotropy of polyimide chains on their surface was observed by a NEXAFS (Near Edge X‐ray Adsorption Fine Structure) analysis. The reason for this is that the hardness of a polyimide film and the rubbing process in LCD manufacturing were optimized to minimize a rubbing scratch, which deteriorates an image quality if any. From these results, we imply that only molecular orientation without any morphological anisotropy at the polyimide surface is enough to attain LC (Liquid Crystal) alignment ability for currently mass‐produced IPS‐mode LCD products. In order to reveal the effect of molecular anisotropy on LC alignment properties, we enhanced the orientational anisotropy of polyimide chains by changing their molecular structures. We then confirmed that the increase of the orientational anisotropy could improve contrast ratio of a 42″ IPS‐mode LCD TV.

Effects of Annealing on Rubbed Polyimide Surface Studiedby Grazing-Incidence X-Ray Diffraction

We investigated effect of annealing after rubbing by grazing incidence X-ray diffraction, since annealing process is performed in the actual process of liquid crystal display (LCD) fabrication. It was found that rubbed surface polymers were highly crystallized by annealing at 250°C after rubbing, and that the crystallization of surface polymers by annealing occurred in the aligned polymer region induced by rubbing. Crystallization of surface polymers by annealing increasingly occurred as increasing of rubbing strength. Thus, it is considered that annealing process after rubbing should also play an important role to control liquid crystal alignment in LCD.

Annealing Behavior of Pretilt Angles on Polyimide Surface with Rubbing and Ion Beam Irradiation

We have studied the liquid crystal (LC) alignment, the pretilt angle generation, and the annealing behavior for a nematic liquid crystal (NLC) on the homogeneous polyimide (PI) surfaces by using the rubbing method and the ion beam (IB) method. An excellent LC alignment of the NLC on the PI surface with rubbing and IB irradiation were observed. The pretilt angle of NLC on the homogeneous PI surface for the rubbing method is decreased from to as rubbing time is increased, that of the for the IB irradiation method is decreased from to as the time of IB irradiation is increased. After the annealing, the pretilt angles of the rubbed PI surfaces increased up to , these of the IB irradiated PI surfaces little increased. It is considered the side chain of the rubbed PI show the its abilities of the original capacities, while the side chain of the IB irradiated PI cannot show the its abilities of the original capacities due to the IB has already destroyed the side chain of the PI.

Phase Transition Behavior and Molecular Orientation of Oligo(9,9′‐dioctylfluorene‐alt‐bithiophene)

AbstractA novel conjugated oligomer, oligo(9,9′‐dioctylfluorene‐alt‐bithiophene) (OF8T2), was found to exhibit a unique phase transition between crystalline and liquid‐crystalline states, and a liquid‐crystalline glass was easily generated, offering better TFT device performance. In thin films, upon annealing the OF8T2 molecules oriented preferentially with their planes of conjugation being normal to the substrate, and both film thickness and annealing temperature were critical to the film morphology and the molecular orientation. When the OF8T2 film was deposited on a rubbed polyimide surface and annealed, the molecules aligned their long axes along the rubbing direction.magnified image

Cell processing on polyimide surface patterned by rubbing

AbstractIn this study, we prepared a novel rubbed fluorinated polyimide film using a rubbing machine with a rubbing cloth and determined the surface properties of the rubbed film using an atomic force microscope and contact angle measurements. In addition, we evaluated the cell adhesion behavior on the rubbed polyimide film using a phase contrast microscope. Interestingly, a rubbed polyimide surface having a micrometer‐scale grooved pattern was prepared by the rubbing method, and the morphologies of rat primary hepatocytes and human liver cell lines attached to the rubbed surface were three‐dimensional multicellular spheroids, while the cells on an unrubbed surface showed two‐dimensional monolayers. This initial study indicates that the rubbing method without any chemical modification is simple and can easily produce large surface areas, suggesting that the rubbing may become a novel cell culture method. Copyright © 2008 John Wiley & Sons, Ltd.

Recent Patents on Liquid Crystal Alignment

The fabrication of liquid crystal devices requires a reproducible and accurate method of aligning the LC on the surface of the electrodes. Rubbed polyimide surfaces have been successfully employed but suffer disadvantages. Recent developments in technology have seen new methods based on ion and plasma bombardment, UV exposure and electron irradiation. In addition, there has been much activity in achieving LC alignment through doping of the LC itself with suitable agents. Here, we review the recent developments in LC alignment technology and the patent literature in this area. Keywords: Liquid crystal, homeotropic, nematic, vertical alignment, pre-tilt angle

Alignment Capabilities and EO Performances of VA-LCD Using Transparent Conductive Al-doped ZnO Films

Transparent electrodes are indispensable for most of the flat panel display as liquid crystal display (LCD). We investigated liquid crystal (LC) aligning capabilities and electro-optical (EO) characteristics of transparent electrodes as Al-doped ZnO (ZnO:Al) substituting indium tin oxide (ITO). The experiment results show that a uniform vertical LC alignment on ZnO:Al electrodes based on a rubbed polyimide (PI) surface were achieved. A high pretilt angle of about 88° was obtained. EO performances of the VA-LCD on rubbed PI surfaces with ZnO:Al is almost the same as that of the VA-LCD with ITO electrodes. These results appeared that ZnO:Al electrodes as transparent electrodes of LCDs could substitute ITO electrodes.

Uniaxially aligned poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] thin films characterized by the X‐ray diffraction pole figure technique

AbstractAligned thin films of the liquid‐crystalline polymer poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐bithiophene] were prepared, and the correlation between the optical anisotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin‐casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a temperature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet–visible absorption spectroscopy and in‐plane X‐ray diffraction. Independently, each technique revealed Hermans orientation functions in the range of 0.75–0.8. Surprisingly, a layer‐thickness dependence was not observed. In addition, the X‐ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Aligning Capabilities for Vatical-Alignment (VA) Liquid Crystal Display using AZO Film

We have investigated liquid crystal (LC) aligning capabilities and electro-optical (EO) characteristics of transparent electrodes as Al-doped ZnO (AZO) substituting indium tin oxide (ITO). The experiment results show that a uniform vertical LC alignment on AZO electrodes based on a rubbed polyimide (PI) surface were achieved. A high pretilt angle of about <TEX>$88^{\circ}$</TEX> was obtained. EO performances of the VA cell on rubbed PI surfaces with AZO electrodes are almost the same as that of the VA cell with ITO electrodes. These results appeared that AZO electrodes as transparent electrodes of LCDs could substitute ITO electrodes.