Effect of photoreactivity of polyimide on the molecular orientation of liquid crystals on photoreactive polymer/polyimide blends

We prepared blend alignment layers from polymethacrylate with coumarin side chains (PMA-g-coumarin) and polyimides for the orientation of liquid crystals (LCs) using linearly polarized ultraviolet (UV) irradiation. We used two different polyimides, namely 4,4′-(hexafluoro-isopropylidene) diphthalic anhydride-3,5-diamino-benzoic acid (6FDA-DBA) and pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA). It was found that the molecular orientation of the LC depended on the type of polyimide in the blend alignment layer. The thermal stability of the LC orientation was enhanced regardless of the type of polyimide, while the direction of LC orientation was different for each type of polyimide. The photoreactivity of the polyimide was a very important factor in determining the molecular orientation of the LC on the blend alignment layer. This may be attributed to the different mechanisms of LC orientation on PMA-g-coumarin and polyimide induced by the polarized UV irradiation. The direction of the LC orientation could be changed by controlling the photoreaction of the polyimides using the appropriate UV filter for the polarized UV irradiation.     

Alignment mechanism of a nematic liquid crystal on a pre-rubbed polyimide film with laser-induced periodic surface structure

A periodic surface structure was prepared on a pre-rubbed polyimide (PI) film surface with a pulsed UV laser polarized perpendicular to the rubbing direction. The experimental results demonstrate that the rubbing-induced molecular anisotropic orientation was relaxed by the pulsed laser irradiation, and the laser induced molecular orientation was perpendicular to the line of the laser-induced periodic structure. The dichroism of the anisotropy of molecular orientation increased with the increase of laser energy. Since the direction of the laser-induced molecular anisotropy was perpendicular to the surface groove direction of the pre-rubbed PI surface, the effects of surface microgroove and anisotropic molecular orientation of the PI chain on liquid crystal (LC) alignment can be distinguished from each other. LC alignment was investigated by evaluating the anchoring energy of the PI surface, which was calculated according to Berreman's theory using the twist angle of the LC in the cells. The experimental results demonstrate that the exact alignment direction of the LC molecules is determined by the relative strength of both factors.     

Alignment mechanism of a nematic liquid crystal on a pre-rubbed polyimide film with laser-induced periodic surface structure

A periodic surface structure was prepared on a pre-rubbed polyimide (PI) film surface with a pulsed UV laser polarized perpendicular to the rubbing direction. The experimental results demonstrate that the rubbing-induced molecular anisotropic orientation was relaxed by the pulsed laser irradiation, and the laser induced molecular orientation was perpendicular to the line of the laser-induced periodic structure. The dichroism of the anisotropy of molecular orientation increased with the increase of laser energy. Since the direction of the laser-induced molecular anisotropy was perpendicular to the surface groove direction of the pre-rubbed PI surface, the effects of surface microgroove and anisotropic molecular orientation of the PI chain on liquid crystal (LC) alignment can be distinguished from each other. LC alignment was investigated by evaluating the anchoring energy of the PI surface, which was calculated according to Berreman's theory using the twist angle of the LC in the cells. The experimental results demonstrate that the exact alignment direction of the LC molecules is determined by the relative strength of both factors.     

Photo-induced liquid crystal alignment on polyimide containing fluorinated groups

Polyimides were prepared from pyromellitic dianhydride, 4,4'-bis[2-(4-aminophenyl)hexafluoroprop-2-yl]diphenyl ether and 4,4'-diamodiphenyl ether (PMDA-BDAF-ODA) and used for liquid crystal alignment using linearly polarized UV exposure. The alignment properties of a LC on the polyimide films were found to depend on the fluorine content in the PMDA-BDAF-ODA alignment layer and on the UV exposure time. Pretilt angles were obtained in the range 0° to 90° dependent upon the fluorine content in the polyimide film and the UV exposure time. These effects seem to be closely related to the surface energy of the photo-alignment layer.     

UV-driven switching of chain orientation and liquid crystal alignment in nanoscale thin films of a novel polyimide bearing stilbene moieties in the backbone

A novel photosensitive polyimide, poly(4,4'-stilbenylene 4,4'-oxidiphthalimide) (ODPA-Stilbene PSPI) was newly synthesized. The most surprising feature of this PSPI is that the PSPI films irradiated with linear polarized ultraviolet light (LPUVL) can favorably induce a unidirectional alignment of liquid crystals (LCs) in contact with the film surface and further switch the director of the unidirectionally aligned LCs from a perpendicular direction to a parallel direction with respect to the polarization direction of LPUVL by simply controlling the exposure dose in the irradiation process. These LPUVL-irradiated films were found to provide high anchoring energy to LCs, always giving very stable, homogeneous cells with unidirectionally aligned LCs regardless of the LC alignment directions. In the films, the PSPI polymer chains were found to undergo favorably unidirectional orientation via a specific orientation sequence of the polymer chain segments led by the directionally selective trans-cis photoisomerization of the stilbene chromophore units in the backbone induced by LPUVL exposure. Such unidirectionally oriented polymer chains of the films induce alignment of the LCs along the orientation direction of the polymer chains via favorable anisotropic molecular interactions between the oriented polymer chain segments and the LC molecules. In addition, the PSPI has an excellent film formation processibility; good quality PSPI thin films with a smooth surface are easily produced by simple spin-coating of the soluble poly(amic acid) precursor and subsequent thermal imidization process. In summary, this new PSPI is the promising LC alignment layer candidate with rubbing-free processing for the production of advanced LC display devices, including LC display televisions with large display areas.     

新型可溶性偶氮聚酰亚胺及其作为液晶光定向层应用的初步研究

以 4 ,4′ 二 (1,2 苯二酸酐 4 羧酸酯 )偶氮苯为二酐单体 ,4 十六烷氧基 4′ ,4″ 二氨基三苯甲烷为二胺单体 ,通过缩聚反应合成了可溶性偶氮聚酰亚胺 .采用红外光谱、氢核磁共振、紫外光谱和热分析等手段 ,对产物的结构、热性能及光学性能等进行了表征 .在紫外光辐照下 ,上述聚合物表现出明显的光色效应 .经线性偏振紫外光 (LPUV)辐射后的上述偶氮聚酰亚胺定向层能诱导液晶盒中液晶分子发生定向沿面排列 ,且取向均匀 .上述实验表明 ,合成的偶氮聚酰亚胺是一类潜在的液晶光定向材料 .     

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.     

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.     

Synthesis,photo-reaction and photo-induced liquid crystal alignment of soluble polyimide with pendant cinnamate group

In this paper, the synthesis, photo-reaction and photo-induced liquid crystal alignment of a polyimide with a pendant cinnamate group are reported. The polyimide was synthesized by the thermal imidization of the polyamic acid derived from 4,4'-(hexafluoro-isopropylidene)diphthalic anhydride and hydroxydiaminopropane, followed by the attachment of the cinnamate group to the main chain polyimide. The surface of thin layers of the polyimide was found to be preferentially occupied by the pendant cinnamate groups, and liquid crystal alignment on the polyimide thin film exposed to polarized UV was independent of the cinnamate content. The thermal stability of the photo-induced liquid crystal alignment was enhanced with decrease in the cinnamate content. This could be attributed to the strong interchain interaction of the polyimide chains which prevents thermal randomization of the photo-product of the pendant cinnamates. The dependences of the photo-reaction temperature and the annealing temperature of the alignment layer on the azimuthal anchoring energy of the photo-aligned liquid crystal suggest that the local stress developed during the UV irradiation profoundly influences the thermal stability of the liquid crystal alignment.     

Synthesis, photo-reaction and photo-induced liquid crystal alignment of soluble polyimide with pendant cinnamate group

In this paper, the synthesis, photo-reaction and photo-induced liquid crystal alignment of a polyimide with a pendant cinnamate group are reported. The polyimide was synthesized by the thermal imidization of the polyamic acid derived from 4,4'-(hexafluoro-isopropylidene)diphthalic anhydride and hydroxydiaminopropane, followed by the attachment of the cinnamate group to the main chain polyimide. The surface of thin layers of the polyimide was found to be preferentially occupied by the pendant cinnamate groups, and liquid crystal alignment on the polyimide thin film exposed to polarized UV was independent of the cinnamate content. The thermal stability of the photo-induced liquid crystal alignment was enhanced with decrease in the cinnamate content. This could be attributed to the strong interchain interaction of the polyimide chains which prevents thermal randomization of the photo-product of the pendant cinnamates. The dependences of the photo-reaction temperature and the annealing temperature of the alignment layer on the azimuthal anchoring energy of the photo-aligned liquid crystal suggest that the local stress developed during the UV irradiation profoundly influences the thermal stability of the liquid crystal alignment.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Preliminary communication Investigation of pretilt angle generation in nematic liquid crystal with oblique non-polarized UV light irradiation on polyimide films

We investigated new rubbing-free techniques for liquid crystal (LC) alignment with non- polarized ultraviolet (UV) light irradiation on plates coated with two kinds of the polyimide (PI) films. It was found that monodomain alignment of nematic (N) LC is obtained in the cell having a PI surface without a side chain. We successfully observed that the generated pretilt angle of the NLC is about 3 with an angle of incidence of 70 on the PI surface without side chains. This pretilt angle generation is attributed to interaction between the LC molecules and the polymer surfaces; the uniform alignment of NLC is attributed to anisotropic dispersion force effects due to photo-depolymerization of polymer on PI surfaces.     

Preliminary communication Generation of high pretilt angle in a nematic liquid crystal with single oblique polarized UV light irradiation on polyimide surfaces

We propose and investigate obtaining high pretilt nematic alignment using a new photoalignment method: single oblique polarized ultraviolet (UV) light irradiation on polyimide (PI) surfaces. It was found that the monodomain alignment in a nematic liquid crystal (NLC) is obtained with PI surfaces having side chains. We successfully observed that the generated high pretilt angles of the NLC are about 6 with an angle of incidence of 5 80 on the PI surface. Also, the generated pretilt angle of NLC is about 6 with an UV light irradiation of 30 min for angle of incidence of 5, and decreases with increasing UV light irradiation time. High pretilt orientation of the NLC is successfully observed in cells with a single oblique UV light irradiation on PI surfaces.     

Effect of chemical structures of polyimides on photosensitivity of liquid crystal alignment using a polarized UV exposure

Unidirectional liquid crystal (LC) alignment produced by polarized UV exposure was examined using polyimides (PIs) synthesized using different diamines. The dichroic ratio of the resulting LC cells suggests that the UV photosensitivity is primarily controlled by the chemical structures of the PIs used. The UV absorption and fluorescence spectra of the PI films, and molecular conformations of the diamines, indicate that the photosensitivity is controlled by the UV absorption efficiencies and molecular conformations of the PIs.     

Gas separation characteristics of isomeric polyimide membrane prepared under shear stress

We synthesized the isomeric polyimides, 6FDA-m-DDS and 6FDA-p-DDS, and investigated the gas selectivity of the asymmetric polyimide membranes with an oriented surface skin layer. Particularly, we focused on the effect of the chemical structure of the polyimide on the molecular orientation. The asymmetric membranes with the oriented skin layer were prepared by a dry–wet phase inversion process at different shear stresses. The gas permeances of the asymmetric polyimide membranes were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg. The molecular orientation in the asymmetric polyimide membranes was measured using polarized ATR–FTIR spectroscopy. The gas selectivity of the asymmetric 6FDA-m-DDS membrane increased with an increased in the shear stress and were greater than that of the dense membrane. In contrast, the gas selectivities of the asymmetric 6FDA-p-DDS membrane did not depend on the shear stress and were similar to those of the dense membrane. We clarified that a parallel oriented surface formed on the asymmetric 6FDA-m-DDS membrane caused the enhanced gas selectivity of the membrane.     

Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis[omega-[4-(4-cyanophenyl)phenoxy]hexyl] 4,4'-diamino-2,2'-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA--6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39 degrees to 43 degrees, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-theta(c)) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta(c)) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -theta(c) pretilt angle with respect to the rubbing direction.     

High-transmittance liquid crystal cell fabricated using nanoparticle-doped polyimide

We found that we can increase the transmittance of a liquid crystal (LC) cell by doping the alignment material with nanoparticles. We fabricated an LC cell using indium–tin–oxide glass substrates coated with SiO₂-doped polyimide. We confirmed that the transmittance of LC cells fabricated using SiO₂-doped polyimide was higher than that of LC cells fabricated using pure polyimide. To account for the increase of transmittance, we measured the order parameter of the fabricated LC cells and investigated the surface morphology of the alignment layer using atomic force microscopy.     

Synthesis of novel hyperbranched polyimide for liquid crystal alignment

A novel hyperbranched polyimide (HPPI) was synthesized from a new four‐functional amine and 1,2,3,4‐cyclobutanetetracarboxylic dianhydride. The polyimide with hyperbranched structure exhibited excellent alignment for nematic liquid crystal (LC) compared with traditional linear polyimides. The solution viscosities of the poly(amic acids) (PAAs) and the surface morphology of PI films were investigated using an Ubbelhode‐type viscometer and atomic force microscopy, respectively. The viscosities of the hyperbranched PAAs were much lower than that of the linear PAA solution at any monomer ratio (amine:dianhydride). Pretilt angles of LCs above 2.8° were achieved with HPPI.     

FT-IR study of liquid crystal molecules on alignment layers

Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.