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.     

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.     

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

The polar anchoring strength and pretilt angle generation in nematic liquid crystals (NLCs), on three kinds of rubbed polyimide (PI) surfaces, were investigated. The pretilt angle generated in 4-n-pentyl-4'-cyanobiphenyl (5CB) is large compared with ZLI-4792 (a fluorinated mixture type NLC) for all rubbed PI surfaces. The high pretilt angle in 5CB is attributed to a much larger birefringence and much larger perpendicular component of permittivity. The polar anchoring energy of ZLI-4792 is high compared with 5CB on all rubbed PI surfaces for a weak rubbing strength. It is suggested that the high anchoring energy of ZLI-4792 may be attributed to the low NLC pretilt angle.     

Pretilt angle generation by UV exposure during imidization of polyimide

We investigated pretilt angle generation and liquid crystal (LC) alignment by ultraviolet exposure during the imidization of polyimide (PI). The generated pretilt angle of a nematic (N) LC using an in situ photo-alignment method is smaller than that using a conventional photo-alignment method on a surface of PI having side chains. The NLC pretilt angles using an in situ photo-alignment method injected at isotropic phase increased with annealing were observed.     

Pretilt angle generation by UV exposure during imidization of polyimide

We investigated pretilt angle generation and liquid crystal (LC) alignment by ultraviolet exposure during the imidization of polyimide (PI). The generated pretilt angle of a nematic (N) LC using an in situ photo-alignment method is smaller than that using a conventional photo-alignment method on a surface of PI having side chains. The NLC pretilt angles using an in situ photo-alignment method injected at isotropic phase increased with annealing were observed.     

Preliminary Communication : Generation of high pretilt angles of nematic liquid crystal (5CB) on rubbed organic solvent soluble polyimide surfaces with helical backbone structure and trifluoromethyl moieties

High pretilt angles of nematic liquid crystals (NLC) have been successfully generated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. We have found that the pretilt angle of in 4-n-penty-4-cyanobiphenyl (5CB) on rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is larger compared to that on rubbed PI surfaces with trifluoromethyl moieties attached to the polymer backbone. The observed pretilt angle of 5CB on unidirectionally rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is about 15 degrees in the wide rubbing region. From these results, it is expected that the microscopic surface structure of the polymer strongly contributes to the pretilt angle generation of the LCs at the surface.     

Generation of pretilt angle in NLCs and EO characteristics of a photo-aligned TN-LCD with oblique non-polarized UV light irradiation on a polyimide surface

We have investigated the generation of pretilt angle for a nematic liquid crystal (NLC) alignment in cells with oblique non-polarized ultraviolet (UV) light irradiation on polyimide (PI) surfaces. It was found that monodomain alignment of the NLC is obtained with an incident angle of 70 degrees and 75 degrees on the PI surface. It is considered that this alignment may be attributed to the anisotropic dispersion force due to photo-depolymerization of polymer on PI surfaces. Also, the generated NLC pretilt angles are all about 3 degrees at an incident angle of 70 degrees and 75 degrees for 1 h irradiation. Next, we observed that the voltage-transmittance characteristics for a photo-aligned twisted nematic (TN) LCD with an incident angle of 80 degrees on a PI surface were excellent. Also, we measured that the voltage-holding-ratio (VHR) of a photo-aligned TN-LCD is about 94%; it is almost same as obtained for rubbing-aligned TN-LCDs. Finally, the slow response time of photo-aligned TN-LCDs is attributable to their weak anchoring strength.     

Synthesis and Properties of a Novel Polyimide with Side Chain Containing Biphenyl Unit

A novel biphenyl side-chained diamine with alkyloxy spacer and alkyloxy tail,4'-butoxy-4-(3,5-diami-nobenzoyloxy)hexyloxybiphenyl(C6BBC4),was synthesized and characterized by FTIR and 1H NMR.A series o...     

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.     

Synthesis of new aromatic polyimides with various side chains containing a biphenyl mesogen unit and their abilities to control liquid-crystal alignments on the rubbed surface

New poly(m-phenylene 4,4′-oxydiphthalimide)s containing various side chains, such as 6-(4-biphenylmethoxy)hexyloxy group and 6-(phenylphenoxy)hexyloxy isomers, were synthesized, giving thin films of a high quality. All the polyimides apparently were almost amorphous, but exhibited short-range ordering in some extent, depending on the side chains. By incorporating side chains, the thermal properties, including stability, thermal expansivity, and glass transition temperature, were generally degraded, whereas the optical and dielectric properties were improved. All the polyimides exhibited a good rubbing processability and excellent performance in the controlling of both the alignment and the pretilt of LC molecules in the LC cell. The pretilt angle of LC molecules was easily achieved in a wide-angle range of 8–27°, depending upon the rubbing density as well as the incorporated side chains. The pretilting of LC molecules was very sensitive to all the molecular parameters (namely, the flexibility of polymer chain backbone as well as the isomeric structure of biphenyl mesogen end group, spacer length, and spacer conformation in the side chain) in the polyimide, in addition to the rubbing process. In particular, the side chains, which are much shorter in length than the long alkyl side chains in the polyimides being used widely as LC alignment layers, were evident to involve effectively in the alignment of and the pretilt of LC molecules, which are highly desired in the LC display industry. This might mainly be attributed to a strong interaction between the biphenyl mesogen end group of the side chain and the LC molecule. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2909–2921, 1999     

Pretilt angle for BTDA,PMDA and CPDA series polyimides with various side chain lengths

Polyamic acid precursors were prepared by mixing dianhydride of 3,3',4,4'-benzophenone-tetracarboxylic dianhydride (BTDA), 1,2,3,4-benzene-tetracarboxylic dianhydride (pyrromellitic dianhydride PMDA), cis-1,2,3,4-cyclopentane-tetracarboxylic dianhydride (CPDA), the diamine (alkyl 3,5-diaminobenzoate) with side chain, and 4,4'-oxydianiline (ODA) without side chain. Copolyimide films with various side chain lengths were prepared by thermal imidization of polyamic acid precursors. The roughness of rubbed polyimide surface increased with increase in the side chain length. The pretilt angle for the BTDA and PMDA series polyimide (PI) increased exponentially with increase in side chain length. Various pretilt angles were obtained on the synthesized polyimides. In the case of CPDA series PI, the pretilt angle was nearly constant at 0 until the alkyl side chain length reached 12 (C12) and then increased markedly at C18. Models of pretilt angle generation were tested.     

Orientational relationship among main and side chains of a polyimide alignment layer,liquid crystals monolayer and bulk pretilt angle

We have determined the orientational distribution of cyano-substituted side chains of a rubbed polyimide film, and a liquid crystal monolayer adsorbed on the film, by means of optical second harmonic generation. With the orientational distribution of a main chain that was measured in a previous study by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we have obtained the correlation of all the orientational distributions contributing to the alignment of LC molecules, i.e. pretilt angle. We find that the side chain plays a role in increasing the pretilt angle, but in the case of rubbing strength dependence, the main chain has stronger correlation with the pretilt angle than has the side chain.     

Orientational relationship among main and side chains of a polyimide alignment layer, liquid crystals monolayer and bulk pretilt angle

We have determined the orientational distribution of cyano-substituted side chains of a rubbed polyimide film, and a liquid crystal monolayer adsorbed on the film, by means of optical second harmonic generation. With the orientational distribution of a main chain that was measured in a previous study by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we have obtained the correlation of all the orientational distributions contributing to the alignment of LC molecules, i.e. pretilt angle. We find that the side chain plays a role in increasing the pretilt angle, but in the case of rubbing strength dependence, the main chain has stronger correlation with the pretilt angle than has the side chain.     

Anisotropic reactive mesogens transfer onto polyimides-mixture layer via imprinting method for continuous pretilt angle control

We present a new approach for the alignment process to orient liquid crystal (LC) molecules. The process consists of fabrication of a stamp composed of reactive mesogens (RMs), transfer of RM from the stamp onto polyimide (PI) layers, and LC device fabrication using the resulting RM/PI layers. The RM stamp was made anisotropic by a rubbing treatment, and this anisotropy was transferred to the PI layers via contact imprinting, leading to an achievement of uniform LC alignment. In addition, the pretilt angle of the LC molecules could be controlled by varying the content of homogeneous PI in the PI layers. We describe the effects of high pretilt angle upon the electro-optical (EO) characteristics of twisted-nematic (TN) cells based on the RM transfer film/blended PI layers.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Synthesis of novel polyimides with (tert-butyldimethylsiloxy)biphenyloxy groups and the effect of side-chains content on their properties

A novel aromatic diamine 4′-(tert-butyldimethylsiloxy)biphenyl-4-yl 3,5-diaminobenzoate (DPA) was synthesized through three steps including monoesterification, silylation and hydrogenation. DPA with different ratios was copolymerized with 4,4′-diaminodiphenyl ether (ODA) and 3,3′,4,4′-biphenyl dianhydride (BPDA) to give poly(amic acid)s which were converted into polyimides (PIs) by thermal imidization. The effect of side-chain content on the thermal stability, optical transparency and surface energies was investigated by DSC, TGA, UV-vis spectra and contact angle measurement. With the content of side-chains increasing, the PIs revealed lower thermal stability, higher optical transparency and lower surface energies. The pretilt angles of 4-pentyl-4′-cyanobiphenyl (5CB) on the PI films were measured by a crystal rotation method. For the same rubbing strength, the pretilt angles increased with the increasing of the content of side-chains. When the content of side-chains rose from 5% to 20%, the pretilt angles increased from 2.8° to 15.0° under 28.5 mm of rubbing strength. When the content of side-chains increased to 70%, 80%, 90%, and 100%, the pretilt angles was 75.0°, 82.9°, 87.0°, and 87.7°, respectively, under 28.5 mm rubbing strength.     

Relationship between alignment layer polymer surface structures and liquid crystal display parameters

The polar alignment layer (AL) surface provided relatively small liquid crystal (LC) pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles, in addition to an anchoring effect, are greatly affected by both electronic and steric interactions between LC molecules and a polyimide alignment layer surface. Rubbing with a cotton cloth induces functional groups, side chains, and repeat units at the surface of a liquid crystal polyimide AL to re-orient. It was discovered that rubbing induced polar functional groups and repeat units to re-orient out-of-the-plane of the surface, and it made non-polar aliphatic side chains partially re-orient inwards, toward the bulk of the film.     

Polar and azimuthal alignment of a nematic liquid crystal by alkylsilane self-assembled monolayers: effects of chain-length and mechanical rubbing

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.