Photoinduced alignment of polymerisable liquid crystals on photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone units in the main chain

Photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone (bisBC) units were synthesised and investigated as a photoalignment layer for polymerisable liquid crystals (PLCs) and liquid crystalline polymers (LCPs). The liquid crystalline materials were aligned homogeneously on the photoalignment layers in a wide range of irradiation dose of linearly polarised UV light (LPUVL). Specifically, for the photoalignment layer baked at 80°C, order parameters of the liquid crystalline materials were low due to the disturbance of oriented-photoreactive polymer caused by the contact with the solvent of liquid crystalline materials. However, the liquid crystalline materials were aligned homogeneously even at low irradiation doses on the thermally cured photoalignment layer baked at 180°C. In addition, the liquid crystalline materials were aligned perpendicular to the LPUVL electric field. The alignment mechanism is discussed by comparing the retardation of photoalignment layer with anisotropic polarisabilities of model molecules calculated by density functional theory (DFT). It is suggested that the liquid crystalline materials aligned along the unreacted chromophores in the photoreactive polymer.     

一种新型的氢键自组装液晶光控取向膜

报道了一种新型的以氢键为驱动力的液晶自组装光控取向膜, 研究了薄膜的制备方法与光敏特性. 通过聚(4-乙烯基吡啶)中的吡啶基团与光敏聚丙烯酰氧基肉桂酸间的氢键作用制备了LBL(layer-by-layer)型的自组装多层膜, 制备过程的紫外-可见光谱表明, 该组装过程为逐层、均匀沉积过程. 傅里叶变换红外光谱表明, 多层膜的成膜驱动力为氢键. 用线性偏振紫外光辐照该薄膜, 多层膜中与光矢量方向匹配的光敏基团发生[2+2]环加成反应, 形成表面张力各向异性的薄膜. 用该薄膜作为向列相液晶的取向膜制成平行液晶器件, 在偏光显微镜下观察, 发现获得了均一、稳定的取向效果.     

Photoalignment of liquid crystals by a covalently attached self-assembled ultrathin film

In this paper, the polyanion-containing cinnamoyl group (PACSS-CF3) was self-assembled with diazoresin (DR) to form a kind of stable covalent ultrathin film by irradiation with 365?nm UV light. The photoalignment properties of the DR/PACSS-CF₃ covalent film were investigated. The covalent film was found to have anisotropy after irradiation by 297?nm linearly polarised ultraviolet light (LPUVL), and could induce uniform alignment of liquid crystals (LCs). The pretilt angle of the LC was 2.5°. The stability of the film was enhanced by the covalent bonds. The films were thermally stable to 180°C. Polarised UV-Vis spectroscopy was utilised to investigate the photochemical process of the covalent film. It was found that cinnamoyl moieties parallel to the polarisation direction of the LPUVL were consumed by the photoreaction faster than those perpendicular to the polarisation direction. It can be concluded that the selective photoreaction induced the anisotropy of the films. The anisotropic films induced the homogeneous alignment of LC.     

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.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Photoalignmentability of silsesquioxane-containing citraconimide and its solvent-induced surface enrichment in PMMA matrix

Photo-reactive polymer of silsesquioxane-containing citraconimide (SQ-CI) was synthesised and evaluated as a photoalignment layer for polymerisable liquid crystals (PLC). A generation mechanism of the anisotropy by the linearly polarised ultraviolet light was discussed by comparing optical retardation of SQ-CI with theoretical calculations using density functional theory. In addition, the SQ-CI and poly(methyl methacrylate) were blended for a model of unification of photoalignment layer and protection layer in liquid crystal display. The orientation of PLC on the blend film was dominated by the solvent used for the sample preparation of PMMA/SQ-CI blend. We found that the SQ-CI was enriched at surface of the blend film prepared from the ?-butyrolactone solution, while not in the case of the film prepared from cyclopentanone solution. The surface enrichment in the film prepared from ?-butyrolactone solution does not follow the conventional mechanism which is that the lower surface free energy component segregates to the blend surface, because the surface free energy of SQ-CI is higher than that of PMMA. Based on the result of solvent-annealing effect and photoalignmentability, we concluded that the mechanism of surface enrichment was explained due to the difference of affinity between the polymer and the solvent.     

Surface-assisted photoalignment of discotic liquid crystals by nonpolarized light irradiation of photo-cross-linkable polymer thin films

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.     

A novel liquid crystals photoalignment layer-by-layer self-assembled film fabricated with long side-chain cinnamate polyelectrolyte

A novel photoalignment film for liquid crystals (LC) was prepared based on layer-by-layer self-assembly of photosensitive long side-chain cinnamate polyelectrolyte. A series of self-assembled films with different methylene spacer groups was prepared and used as alignment film. The film became anisotropic, and could induce uniform alignment of LC after irradiation by linearly polarised ultraviolet light (LPUVL). The effects of spacer chain lengths of the cinnamoyl polycations on the structure and photoalignment properties of the self-assembled film were studied. The polycation films with longer spacer chain obtained a larger dichroic ratio after LPUVL irradiation. The contrast ratio (T _max/T _min) of the LC cell increased with spacer chain length increase. However, it was found that the thermal stability of PSS/PACPYn films decreased with increasing chain length of polycation.     

Photocrosslinking reaction of vinyl‐functional polyphenylsilsesquioxane sensitized with aromatic bisazide compounds

Photochemical reactions of aromatic azide groups were applied for a novel photosensitive silicone ladder polymer, that is, partially vinyl‐substituted polyphenylsilsesquioxane sensitized with aromatic bisazide compounds as a photocrosslinker. The photocrosslinking reaction in this system was investigated from the viewpoint of the efficiency of the photocrosslinker, that is, the ratio of the photocrosslinker consumed for crosslinking. The numbers of photodecomposed azide groups and crosslinks in the polymer were determined by Fourier transform infrared measurements. At a higher bisazide concentration, the predominant reaction of nitrenes formed as the intermediary radical by the photolysis of azide was a coupling reaction that could not contribute to the gelation of the polymer. The ratio of the bisazide compound consumed for crosslinking showed the highest value at its concentration of 3 wt % and decreased with the addition of a larger amount. The semiempirical molecular orbital calculations were applied to the theoretical analysis of the photoreaction of nitrenes using phenylnitrene as a model structure. The calculation results indicated that the coupling reaction of nitrenes should proceed more easily than the photocrosslinking reaction in N₂ atmosphere, and the fact that the oxidation of nitrenes should proceed exclusively in the atmosphere including O₂ agreed with the experimental results. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4196–4205, 2001     

Synthesis and Characterization of a Soluble Polyimide Containing a Photoreactive 4‐Styrylpyridine Derivative as the Side Group

An aromatic polyimide bearing photoreactive 4‐(2‐(4‐oxyethylenyloxyphenyl)vinyl)pyridine side groups was synthesized and characterized. The polymer is stable up to 300°C and soluble in organic solvents, giving thin films in good quality. When exposed to UV light, it reorients favorably with an angle of 98° with respect to the electric vector of linearly polarized UV light. UV‐exposed films align liquid‐crystals (LCs) homogeneously along the preferential orientation of the polymer chains on the surface. The pretilt angle of the LCs is 0.32–0.92°, depending on the exposure dose and annealing. LC alignment is retained up to 210°C. Based on the optical retardation behavior and spectroscopic measurements, a photoalignment mechanism is proposed.     

Synthesis and properties of photoalignable aromatic polyesters containing phenylenediacrylate units in their backbones and n‐alkyl moieties in their side groups

A series of poly[oxy(4‐n‐alkyl‐3,5‐benzoate)oxy‐1,4‐phenylenediacryloyl]s (PPDA‐CnBZ polymers) with high molecular weights was synthesized. These polymers exhibit excellent solubility in some common organic solvents and produce good quality films using conventional spin‐casting and drying processes. The polymers are thermally stable up to 357–362 °C in a nitrogen atmosphere; their glass transition temperatures are greater than 121 °C. The photoreactions and photoalignments of the polymers were investigated using ultraviolet‐visible and infrared spectroscopy, and their liquid crystal (LC) alignment properties were examined. The phenylenediacrylate (PDA) chromophores in the polyesters were found to mainly undergo photocyclization upon ultraviolet light irradiation. Irradiation of the polyester films with linearly polarized ultraviolet light (LPUVL) induces preferential orientation of the polymer main chains, while the unreacted PDA chromophores are aligned along the direction perpendicular to the electric vector of the LPUVL. All the films irradiated with LPUVL were found to align LCs in a direction perpendicular to the electric vector of the LPUVL. Moreover, these LC alignments persisted even on irradiated films annealed at temperatures up to 210 °C, which is much higher than the glass transition temperatures of the polyesters. These LC alignment characteristics are due to the anisotropic interactions of the LC molecules with the oriented polymer chains and with the unreacted PDA chromophores. LC alignments on the polyester film surfaces have homeotropic to homogeneous characteristics, depending on the length of the n‐alkyl side group, providing strong evidence that the n‐alkyl side groups of the polyesters play a critical role in determining the pretilt angles of the LCs. The LC pretilt angles were also found to be influenced by the thermal annealing history of the irradiated films. In summary, the excellent properties of the PPDA‐CnBZ polymers make them promising candidate materials for use as LC alignment layers in advanced LC display devices. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1322–1334, 2004     

Sonochemical preparation of photochromic nanocomposite thin film based on polyoxometalates well dispersed in polyacrylamide

Novel photochromic nanocomposite thin film containing phosphotungstic acid entrapped in polyacrylamide was prepared using ultrasound technique. TEM image revealed that the polyoxometalates nanoparticles with narrow size distribution were finely dispersed in polymer matrix. IR spectra showed that the Keggin geometry of polyoxometalates was still preserved inside the composite film and strong Coulombic interaction was built between heteropolyoxometalates and polyacrylamide via hydrogen bond. It was found that the thermal stability of the hybrid film was lower than that of pure polymer but the film was stable enough for photochromic application from the TG-DTA curves. The transparent film changed from colorless to blue under UV irradiation and showed reversible photochromism. The bleaching process occurred when the film was in contact with air or O₂ in the dark. The photoreduced process was in accordance with radical mechanism.     

Radiation Emulsion Polymerization of n-Butyl Acrylate,Glycidyl Methacrylate,and Methacrylic Acid

The rate of emulsion polymerization of N-butyl acrylate and glycidyl methacrylate is influenced by the irradiation dose rate and the emulsifier type. With regard to the rate of polymerization and the stability of polymer emulsion, sodium dodecyl benzene sulfonate (SDBS) is found to be a better emulsifier than Tween 20. Irradiation dose, in order to obtain 90% conversion, is about 50 krd using SDBS emulsifier, compared to about 160 krd using Tween 20 emulsifier, both with an irradiation dose rate of about 80 krd/h. Irradiation dose, in order to obtain the same conversion, decreases with a decrease in irradiation dose rate. Thermal analysis showed that the glycidyl methacrylate content greatly influences the thermal stability of the polymer film.     

Photoinduced liquid crystal alignment by layer-by-layer ultrathin films with dual photoreaction moieties

A novel layer-by-layer (LBL) film containing dual photoreaction groups, cinnamoyl and azobenzene, was prepared from poly(diallyldimethylammonium chloride) (PDDA) and a photosensitive polyanion, PCAzo, in aqueous solution via electrostatic attraction. The film was able to induce uniform alignment of liquid crystals (LCs) with good stability and 2.3° pretilt angle by oblique irradiation with linearly polarised ultraviolet light (LPUVL). UV absorption and FTIR spectroscopic results indicate that the photoreactions of the two photoreactive groups jointly participate in generating the anisotropy of the film. The dichroic ratio of the film was found to depend on the number of adsorbed layers. The thicker film has the larger dichroic ratio after the LPUVL irradiation. The reorientation behaviour of the LC molecules was found to be associated with the LBL film thickness. Experiment results revealed that the photo-crosslinking of the cinnamoyl groups was responsible for the stability of the anisotropic orientation, and the isomerisation of the azobenzene chromophores led primarily to the appropriate pretilt angle.     

Surface patterns induced by laser irradiation on thin polymer bilayer films

We report on the preparation of wavelike surface patterns with characteristic wavelengths on thin bilayers of poly(methyl methacrylate) on azobenzene liquid crystalline polymer films (LCP/PMMA) by irradiation of a single polarized pulsed laser beam. The formation of such patterns was influenced by the thickness of the upper layer and the laser fluence. We were also able to guide the wavelike pattern to have a specific orientation by placing an elastic polydimethylsiloxane (PDMS) mold on the surface of bilayer film prior to laser irradiation. Moreover, the property of the laser irradiation, that is, the selectivity through mask-projection systems, allowed us fabricating complicated micropatterns for novel microdevices.     

新型光取向液晶聚合物的制备及其性能表征

目前,液晶分子常规的定向方法是对涂有定向膜的基片进行摩擦,这种方法简单、方便,然而在摩擦过程中却难以避免产生机械划痕、污染或静电,影响了液晶分子取向的均匀性,光控取向方法是近年来发展起来的一种液晶定向新技术,即通过激光或偏振紫外光照射,引发基片上的聚合物薄膜发生光致聚合、光致异构或光致分解反应,产生表面的各向异性,进而诱导液晶分子取向。     

Surface-mediated photoalignment of discotic liquid crystals on azobenzene polymer films

This paper describes a simple strategy for the formation of photoaligned and micropatterned discotic liquid crystal (DLC) film on the surface of photoirradiated azobenzene-containing polymer thin film. The key material for the surface-mediated photoalignment of the DLCs was poly[4-(4-cyanophenylazo)phenyl methacrylate] (pMAzCN). Optical anisotropy was generated in a pMAzCN film by oblique exposure to nonpolarized light which resulted in angle-selective photoisomerization and reorientation of the azobenzenes. Subsequent annealing of the film at 240 degrees C enhanced the photoaligned state of the p-cyanoazobenzenes due to strong intermolecular dipole-dipole interaction and semicrystalline nature of the pMAzCN. This combination of photoirradiation and subsequent annealing of the pMAzCN film made it possible to realize the surface-assisted orientation control of a DLC molecule, which displays both columnar (Col) and discotic nematic (N(D)) phases over 152 degrees C. When the pMAzCN film was exposed to linearly polarized light from the surface normal, the DLC molecules showed homeotropic orientation with the director perpendicular to the substrate surface. In the contrast, oblique irradiation of the pMAzCN film with nonpolarized light gave rise to tilted DLC orientation with well-ordered optical birefringence at the N(D) phase. Rapid cooling from the N(D) phase produced a well-aligned glassy N(D) state at room temperature, which was adequately stable for 10 months even though no covalent cross-linking among the DLCs was performed. The spatial orientation of photoaligned DLCs in both their bulk film and in their interface region was characterized by means of optical birefringence, X-ray diffraction, and fluorescence measurements. At the N(D) phase, the DLC molecules were aligned in a hybrid manner such that their tilt angles varied throughout the thickness of DLC film. The direction of tilted DLCs was opposite to the propagation of the actinic nonpolarized light. The photoaligned DLC films exhibited polarized fluorescence emission with an s-polarized/p-polarized intensity ratio of 4.1, despite the nonpolarized excitation of only DLC at outmost surface. These results indicate that the three-dimensionally aligned azobenzene moieties of the pMAzCN thin film were transferred to the tilted DLC molecules at air/DLC interface. Finally, we demonstrated micrometer-scale photopatterned orientation of DLC molecules on the pMAzCN surface by oblique nonpolarized irradiation of the film through a photomask.     

Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side group VII. On He-Ne laser beam irradiation

Photoinduced alignment in a polymer liquid crystal prepared from 6-{1-[4-(2-cyano-4-nitrophenylazo)phenyl]piperazino}hexyl acrylate and 4'-[6-(methacryloyloxy)hexyloxy]-4-cyanobiphenyl was investigated for the first time on irradiation with a polarized He-Ne laser beam at 633 nm. The azobenzene moieties as well as the inert cyanobiphenyl mesogenic units were aligned with the molecular long axis perpendicular to the polarization direction of the irradiation light. Alignment induced on short irradiation was reversible, while that induced under prolonged irradiation was irreversible due to the occurrence of crosslinking which might be caused by photoinduced decomposition of the azobenzene moieties during the photoirradiation process.     

TiO2/聚丙烯酸丁酯纳米复合薄膜的制备及结构表征

利用微乳液原位聚合法在普通玻璃表面上制备了TiO2/聚丙烯酸丁酯纳米复合膜.采用傅立叶变换红外光谱、高分辨透射电子显微镜和X射线光电子能谱对膜的结构进行了表征.结果表明， TiO2以纳米线的形式弥散在聚丙烯酸丁酯的高分子网络中，并且所制备的TiO2纳米线具有板钛矿相结构.