Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety

ABSTRACT

We designed and synthesised a reactive mesogen containing diphenylacetylene moiety in the mesogenic core and two polymerisable acrylate groups at both ends. By irradiating linearly polarised UV light on the conventional host LC mixture containing a small amount of the synthesised reactive mesogen in a sandwiched cell without an alignment layer, we demonstrated an in-situ photo-induced homogeneous alignment of liquid crystals without a pre-treated alignment layer, which was achieved by an irreversible polarisation-selective [2 + 2] photodimerization of diphenylacetylene moiety with linearly polarised UV irradiation at above the isotropic temperature of LC mixture. The resulting homogeneous alignment showed a superior initial dark state, negligible pretilt angle and excellent stabilities. Furthermore, the in-plane switching (IPS) LC cell prepared by this method exhibited a better dark state and electro-optic performance compared to that with conventional-rubbed polyimide alignment layer. The single photoirradiation process automatically resulted in a perfect alignment matching of optical axes between the top and bottom substrates in the LC cell, giving rise to an excellent dark state overcoming an intrinsic misalignment issue and complex fabrication process. The proposed in-situ alignment method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment technique in the manufacturing of high-resolution liquid crystal displays.     

Photoreactive main‐chain liquid‐crystalline polyesters: Synthesis,characterization, and photochemistry

Three series of semiflexible and rigid main‐chain polyesters containing photoreactive mesogenic units derived from p‐phenylenediacrylic acid (PDA) and cinnamic acid have been synthesized by high‐temperature polycondensation. The thermal and mesomorphic properties of the polymers have been determined. The photochemical behavior of polymer P‐[1]‐T, which contains a PDA unit, has been studied both in solution and in films. In solution, [2+2] photocycloaddition, E/Z photoisomerization, and photo‐Fries rearrangement can take place. In contrast, the dominant process in spin‐coated films is the [2+2] photocycloaddition reaction, which causes crosslinking of the polymer. In films, the photochemistry and induction of anisotropy are strongly influenced by the aggregation of the PDA phenylester unit. A dichroism of about 0.2 has been induced in films by irradiation with linearly polarized UV light, and thus the capability of these films to induce optical anisotropy and align liquid crystals has been demonstrated. Liquid‐crystalline cells have been made with polarized irradiated films of P‐[1]‐T as aligning layers. A commercial liquid‐crystalline mixture has been used for this study, and a similar liquid‐crystalline order determined by polarized Fourier transform infrared to a commercial cell with rubbed polyimide as an aligning layer has been detected. Because of crosslinking of the irradiated P‐[1]‐T photoaligning layer, the photoinduced anisotropy is stable at high temperatures, and the liquid‐crystalline molecules are insoluble in the irradiated polymer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4907–4921, 2005     

Increasing rewriting speed of optical rewritable e-paper by selecting proper liquid crystals

Effect of interaction between liquid crystal (LC) and photoalignment material on speed of optical rewriting process is investigated. The theoretical analysis shows that smaller frank elastic constant K₂₂ of liquid crystal corresponds to larger twist angle, which gives rise to larger rewriting speed. Six different LC cells with the same boundary conditions (one substrate is covered with rubbed polyimide (PI) and other with photo sensitive rewritable sulfuric dye 1(SD1)) are tested experimentally under the same illumination intensity (450 nm, 80 mW/cm²). The results demonstrate that with suitable liquid crystal, LC optical rewriting speed for e-paper application can be obviously improved. For two well known LC materials E7 (K₂₂ is larger) and 5CB (K₂₂ is smaller), they require 11 s and 6 s corresponding to change alignment direction for generating image information.     

Electro-optical properties of photo-aligned photonic ferroelectric liquid crystal fibres

Electro-optical and spectral studies of isotropic photonic crystal fibre (PCF) infiltrated with photo-aligned ferroelectric liquid crystal (FLC) under influence of external electric field are described. Desired alignment of FLC molecules is provided by generation of photo-aligning surface on the inner side of PCF structure. Propagation of light spectrum, as well as light attenuation have been investigated in details. Moreover, the electro-optical response times were measured and analysed.     

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.     

Analysis of the rewriting time of liquid crystal optical rewritable e-paper

Theoretical and experimental analysis of the rewriting time of liquid crystal (LC) optical rewritable (ORW) e-paper was conducted. The equations of rewriting time of alignment molecule SD1 film with and without interaction with LC based on diffusion model were derived, which shows that the rewriting time of LC ORW e-paper could be shortened by enlarging light intensity or decreasing azimuthal anchoring energy. The rewriting time of pure SD1 films and LC ORW cells was measured under different light intensities. And LC ORW cells with different azimuthal anchoring energy were prepared for rewriting time measurement. A good agreement between experimental and theoretical results was obtained, which indicates that using larger light intensity and making LC cell with smaller azimuthal anchoring energy, ORW rewriting time could be decreased to the amount suitable for practical use.