Synthesis, characterization, and liquid crystal (LC) aligning capability of polyimide (PI) for the active matrix (AM) liquid crystal display (LCD) application are demonstrated and discussed. The synthesized PI materials in this research are featured by: 1) a low curing temperature below 200°C due to their organic-solvent-soluble property; 2) a good unidirectional LC alignment; 3) a useful capability for generating desired pretilt angles; and 4) an excellent voltage holding ratio. The methods and mechanisms of the unidirectional LC alignment, the generation of pretilt angle, and voltage holding ratio are discussed.

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Exposure of positive-type polyimide (PI) to linearly polarized deep UV (DUV) light (257nm) induced homogeneous alignment of nematic liquid crystals. The alignment state changed markedly at a particular energy of the UV light. Measurement of the UV absorption spectrum and retardation change generated in PI films by UV irradiation showed that anisotropic alignment was caused by a new mechanism, photodepolymerization of polyimide main chains parallel to the electric field of the DUV light.
We induced nematic monodomain homogeneous alignment by exposing positive-type PI to linearly polarized DUV light. The alignment direction was perpendicular to the electric field of the DUV, and its condition suddenly changed at 7J/cm2. We explained this phenomenon as resulting from a new mechanism, anisotropic photodepolymerization of PI.

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Effects of surface topology and polymer molecule orientation on the liquid crystal(LC) orientation have been examined using photolithography and Langmuir-Blodgett techniques. It has been clarified that the polymer orientation in the alignment layer affects more effectively than surface topology. UV/Visible spectroscopy has shown that the elevation angle of the polymer main chain in the alignment layer from the substrate plane does not correspond to the LC pretilt angle, which suggests that long alkyl side chains produce high pretilt angle for some polyimides.

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A photo-alignment of nematic liquid crystals (NLC) on a photopolymer film of polyvinyl cinnamate (PVCi) has been achieved by using linearly-polarized UV light. The mechanism of the LC alignment is clarified and good unidirectional LC alignment with pretilt angle is obtained by appropriate combination of normal and oblique exposure of linearly-polarized UV light to the PVCi films. This photo-alignment method is applied to fabricate super-multidomain (SMD) TN-LCDs making a feature of quartered pixels, and we demonstrate that reverse tilt disclination (RTD)-free SMD TN-LCDs is successfully fabricated by using the photo-alignment method.

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Molecular orientations and surface morphology of mechanically rubbed polyimide films have been extensively studied by FT-IR, ellipsometry and atomic force microscope. In-plane and out-of-plane molecular orientations of rubbed polyimide films have been examined by angular dependences of polarized infrared transmission spectra. It has been confirmed that the polyimide chain inclination against the surface plane is induced by the rubbing. It has been shown that mapping of molecular orientation can be obtained by a grazing angle FT-IR microspectroscopy.

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A new polyimide, PI(BTDA/ep-AHHFP) containing epoxy groups was synthesized by the reaction of epichlorohydrin with PI(BTDA/AHHFP) in the presence of benzyl trimethylammonium chloride at 110-120°C. This polyimide having epoxy groups is highly soluble in most common solvents. Photochemical reactivity of epoxy groups in PI(BTDA/ep-AHHFP) was examined by IR spectra in the presence of diphenyliodonium hexafluoroarsenate. The effects of the concentration of photo-acid generator were also studied. The PI(BTDA/ep-AHHFP) containing benzophenone groups reacts more rapidly than PI(6FDA/ep-AHHFP) without benzophenone group, and it reacts considerably in the presence of DPI-AsF₆ with photoirradiation and even without the post cure.

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Novel fluorine-containing aromatic polyimides having perfluorooctyl group were synthesized through the conventional two-step method; the ring-opening polyaddition of N, N-bis(4-aminophenyl)-4-perfluorooctylaniline to aromatic tetracarboxylic dianhydrides in a polar aprotic solvent at room temperature giving polyamic acids with inherent viscosities of 0.4-0.6dL/g, which were subsequently subjected to thermal cyclodehydration affording yellow to dark red, transparent, and flexible polyimide films. All the polyimides were insoluble in organic solvents. The polymer films had moderate tensile strength of 50-70MPa, high contact angles of water and methylene iodide of 98° and 69°, respectively, which indicating low surface free energy. The glass transition temperatures of polymers were in the range of 234-247°C and the 10% weight loss temperatures were around 500°C in air.

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Thermal and mechanical influence of high crosslinking on to rigid rod polyimide was examined through the cast film fabrication from crosslinkable groups-substituted biphenylene pyromelliticimide system. The crosslinking was fairly effective for the increase of heat resistivity, but not for the mechanical improvement due to thermal degradation. This means that the degradational factors propagated during crosslinking reaction must be exclussively removed before isotropically high-modulus diamond-like structure could be built up.

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New thermotropic liquid crystalline polyimides (ηinh : 0.58-1.20 dLg^-1) having aliphatic methylene chains and imide ring were prepared for the first time by the polycondensation under high pressure of the salt monomers derived from aliphatic diamines (number of methylene unit X=7-12) and diethyl 4, 4"-terphenyltetracarboxylate. Mesophases of the eight and eleven-membered polyimides were enantiotropic, whereas those of all the other polyimides were monotropic as observed by the DSC thermograms. According to the DSC and X-ray measurements, distinct odd-even effects were observed in which the mesophase properties and structures were significantly affected by the number of cabon atom in the flexible spacer. The smectic C_A phase could be confirmed for the odd-membered polyimides from their wide angle X-ray diffraction patterns which exhibited an inner sharp reflection, so-called layer reflection, and an outer broad split reflection. On the other hand, the nematic phase could be confirmed for the even-membered polyimides. The transition temperature from crystal to liquid crystal transition or isotropic temperature upon heating and all transition temperatures upon cooling exhibited an odd-even effect with the number of methylene unit.

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Polyimide-silica hybrid materials were prepared via a modified sol-gel and high pressure-thermal polymerization procedure. The precursor monomer salt was derived from 2, 5-diethoxycarboxyl terephthalic acid (p-PME) and 1, 9-diaminononane. Aqueous solutions of tetramethoxysilane (TMOS) and monomer salt were transformed into a gels with subsequent dehydration forming powders which were then subjected to high pressure thermal polymerization. By varying the ratio of TMOS to monomer salt, hybrids containing SiO₂ contents of 0 to 100wt% could be made. The thermal and physical behaviors of hybrids are discussed.

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Polyimide/silica composite membranes were prepared by the sol-gel reaction of tetraethoxysilane, methyltriethoxysilane or phenyltriethoxysilane in the solution of polyamic acid prepared by condensation of equimolar amount of pyromellitic dianhydride, 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride or 2, 2-bis(3, 4-dicarboxyphenyl)hexafluoropropane dianhydride with 4, 4′-oxydianiline in N-methyl pyrollidone. Silica-coated polyimide membranes were also prepared by the sol-gel method. Gas permeation for H₂, CO₂, CO, O₂, N₂ and CH₄ and water vapor permeation were reported. A slight reduction in gas permeability was observed for the composite membrane. The permeability of water vapor increased in polyimide/silica composite membranes, while silica coating resulted in a reduction of the permeability. Transient sorption and desorption experiments were also investigated at water vapor activity of 0.5. An increase in permeability of water vapor can be attributable to an increase in solubility of water vapor due to the presence of hydroxy species of silica.

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A positive working photosensitive polyimide precursor based on polyisoimide (PII) and 2, 3, 4-tris[1-oxo-2-diazonaphthoquinone-4-sulfonyloxy] benzophenone (D4SB) as a photoreactive compound has been developed. Polyisoimide (PII) was prepared by the ring-opening polyaddition of 3, 4, 3′, 4′-biphenyltetracarboxylic anhydride (BPDA) and 3, 3′-diaminodiphenyl sulfone (3, 3′-DDS), followed by treatment with trifluoroacetic anhydride-triethyl amine in N-methyl-2-pirrolidone. The PII film showed excellent transparency to UV light and solubility to common organic solvents. The dissolution behavior of PII film containing 20wt% of D4SB after exposure and post exposure bake (PEB) has been studied and found that the dissolution rate of exposed area was faster than that of unexposed area due to the photochemical reaction of D4SB in the PII film. The photosensitive polyimide-precursor containing 20wt% of D4SB showed a sensitivity 300mJ/cm2 and a contrast of 2.5 with 435nm-light when it was postbaked at 120°C for 10min followed by developing with 5% tetramethylammonium hydroxide solution at 25°C.

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Some photosensitive molecular systems are triggered by the photon absorption leading to the structural change of commander molecules which initiate the consequent processes consisting of structural as well as orientational modification of a large number of soldier molecules. The novel commander/soldier systems include photoinduced phase changes of liquid crystals, surface-mediated liquid crystal alignment photoregulation and photoregulation of the helical sense of a poly(isocyanate). Further discussions are made on the molecular amplification of photopolymer systems.

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