Thermal stability of alignment of a nematic liquid crystal induced by polyimides exposed to linearly polarized light

The thermal stability of alignment of a nematic liquid crystal (LC) on three polyimide (PI) films exposed to linearly polarized light at 366 nm was investigated. Polarizing optical microscopy analysis indicates that the thermal stability of the LC alignment on the PI film without significant structural change was higher than that with obvious structural change.     

Twisted nematic LC mode with high electro-optical performance and high thermal endurance formed using IB-irradiated poly(methyl methacrylate) as an alignment layer

We obtained homogeneously aligned liquid crystals (LCs) on ion beam (IB) irradiated poly(methyl methacrylate) (PMMA) by controlling the IB energy. We then examined the LC alignment state using polarized optical microscopy and conducted thermal stability testing. We obtained homogeneous LC alignment at IB energies above 1,400 eV, indicating that strong IB energy facilitates the alignment of LCs on the PMMA surface. This surface was analyzed by atomic force microscopy, and the contact angles (CAs) were measured to elucidate the mechanism of LC alignment. The increased surface energy strengthened the van der Waals interaction between the surface and LCs, thereby inducing stable, homogeneous LC alignment. Electro-optical (EO) characteristics were measured using twisted nematic (TN) LC mode. Compared to LC cells with conventionally used rubbed polyimide (PI), the LC cells with IB-irradiated PMMA exhibited higher thermal budgets and good electro-optical characteristics. These new LC cells have promising potential for advanced LC displays.     

Synthesis of photosensitive polyimide for liquid crystal alignment under non-polarised UV ageing lamp irradiation and a study on the possible mechanism of alignment

ABSTRACT

A new photosensitive polyimide (PI) was successfully synthesised via esterification of PI containing hydroxyl groups with cinnamoyl chloride (CL). The cinnamate (CA) group contents of the PI was up to 90%. The PI films could induce uniform parallel alignment of liquid crystal (LC) molecules after the non-polarised ultraviolet ageing lamp exposure, which reduce the cost significantly compared with polarised ultraviolet light equipment. In addition, the PI exhibited good thermal stability. Meanwhile, the possible mechanism of PI alignment films induced by NPUVL was discussed in some detail.     

Stable photoalignment layer for nematic liquid crystals based on ladder-like polysilsesquioxanes

A new kind of aligning material for liquid crystal cells, ladder-like polysilsesquioxanes (LPS) grafted with cinnamoyl side groups, has been developed to improve the thermal stability of the photoalignment layer. The LC aligning ability of the LPS-based alignment layers, fabricated by linearly polarized UV-induced polymerization (LPP), was characterized by polarizing optical microscopy, conoscopic observations and electro-optic response measurements. In particular, a practical and severe annealing test was adopted to examine the thermal stability of the alignment layer; this showed that even when LC cells were annealed at 100 C (much higher than the clearing point of the LC) for several hours, good LC orientation could remain when the cell was cooled to a constant measurement temperature. The results confirmed that the photoalignment layers exhibited not only good LC aligning ability, but also excellent thermal stability, so heralding their potential application in LCDs.     

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.     

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.     

Hyperbranched polyimide application in liquid crystal alignment layers

Several hyperbranched polyimides (HBPIs) were applied in liquid crystal (LC) alignment layers and exhibited outstanding performance for LC alignment. The maximum pretilt angle was above 8°, and the minimum pretilt angle was 4.2°. The results of atomic force microscope measurement disclosed that a lot of grooves were aligned parallel to the rubbing direction and found that the grooves were not main factor for LC alignment. The LC alignment and pretilt angles are unambiguously associated with the intrinsic HBPI chemical structures. The results of thermal gravimetric analysis and ultraviolet–visible spectra showed that the HBPIs had good thermal stability and excellent transmittance. T₅ and T₁₀ were higher than 360°C and 400°C, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Preparation and characterization of a novel polyimide liquid crystal vertical alignment layer

A series of diamines with a side chain containing rigid biphenyl unit and nonpolar alkoxy side end group [4-alkoxy-biphenol-3′,5′-diaminobenzoate] (Cm-BBDA, m = 4, 6, 12) were synthesized and their chemical structures were confirmed by Fourier Transform Infrared Spectroscopy (FT-IR) and nuclear magetic resonance spectroscopy (¹H NMR). Then three polyimides (PIs) were prepared by copolymerization of pyromellitic dianhydride (PMDA), 4,4′-methylenedianiline (MDA) and Cm-BBDA in N-methyl-2-pyrrolodone (NMP), and chemical structures of all PIs were confirmed by FT-IR. Structural identification of all poly(amic acid)s (PAA) was performed by ¹H NMR. Liquid crystal (LC) cells were fabricated using these PIs as the alignment layer for characterization of the alignment properties of LCs. It was found that the planar alignment was obtained when PI with side chain containing alkoxy side end group of 4 carbon atoms was employed and the vertical alignment was observed when alkoxy side end groups of 6 or 12 carbon atoms were included. A uniform vertical alignment was validated by polarizing microscopy. It was testified that LC vertical alignment possessed high thermal stability.     

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.     

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.     

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.     

Novel alignment mechanism of liquid crystal on a hydrogenated amorphous silicon oxide

The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB.     

软光刻技术制备液晶显示用定向层

以偶氮聚合物光致表面起伏光栅为模板,制备聚二甲基硅氧烷(PDMS)弹性印章,再以可溶性聚酰亚胺(PI)为“墨水”,在石英玻璃上压印出具有规则起伏结构的PI薄膜.由此制备的PI薄膜显示出很好的使液晶分子定向排列的效果.此方法成本低、效率高,是一种实用的液晶定向层薄膜制备方法.     

Photo‐aligning of polyimide layers for liquid crystals

A series of soluble and highly transparent semi‐alicyclic polyimides (PIs) with designed flexible linkages have been synthesized derived from an alicyclic aromatic dianhydride (1,2,3,4‐cyclobutanetetracarboxylic dianhydride, CBDA) and various aromatic ether‐bridged diamines. The semi‐alicyclic PIs were evaluated as the photo‐alignment layers of liquid crystal (LC) molecules in liquid crystal display (LCD). Experimental results indicate that the photo‐alignment characteristics of LC molecules induced by the photo‐aligned PI layers and the electro‐optical (EO) properties of the LC cell devices are closely related with PI backbone structures. The retardation of the photo‐aligned PI layers is correlated with the ultraviolet (UV) absorption intensity of PI at 220 to approximately 330 nm. The higher UV absorption intensity PI has, the higher retardation and lower pre‐tilt angle the photo‐aligned PI layer exhibits. The defect‐free and photo‐aligned PI layer could result into the uniform LC texture, which is highly desired for in‐plane switching (IPS) mode LCD devices. In comparison, PI layer containing trifluoromethyl moiety shows poor photo‐aligning performance because of the strong electronic withdrawing effect of the fluorinated linkage.     

Response time mechanism for a photo-aligned vertical-alignment liquid crystal display on a homeotropic alignment layer

The electro-optical characteristics of the photo-aligned vertical-alignment liquid crystal display (VA-LCD) with a non-polarized UV exposure of 45° on homeotropic polyimide (PI) surface was investigated. The domain size of the photo-aligned VA-LCD increases proportionately with the UV exposure time. The LC alignment of the photo-aligned VA-LCD is attributed to photo-dissociation of the polymer by UV exposure on the homeotropic PI surface. Good voltage-transmittance characteristics of the photo-aligned VA-LCD without negative compensation film was measured. The response time of the photo-aligned VA-LCD was slow compared with a rubbing-aligned VA-LCD; this is considered to be due to the alignment of LC molecules.     

Response time mechanism for a photo-aligned vertical-alignment liquid crystal display on a homeotropic alignment layer

The electro-optical characteristics of the photo-aligned vertical-alignment liquid crystal display (VA-LCD) with a non-polarized UV exposure of 45° on homeotropic polyimide (PI) surface was investigated. The domain size of the photo-aligned VA-LCD increases proportionately with the UV exposure time. The LC alignment of the photo-aligned VA-LCD is attributed to photo-dissociation of the polymer by UV exposure on the homeotropic PI surface. Good voltage-transmittance characteristics of the photo-aligned VA-LCD without negative compensation film was measured. The response time of the photo-aligned VA-LCD was slow compared with a rubbing-aligned VA-LCD; this is considered to be due to the alignment of LC molecules.