This review examines surface alignment of liquid crystals by exposure to light. Two distinct effects are discussed: illumination with polarized light induces a surface anisotropy to an alignment layer and hence a preferred in-plane orientation of the overlying liquid crystal director. Alternatively, a photochemical reaction of the alignment surface changes the liquid crystal anchoring conditions from homogeneous to homeotropic. We discuss how cis/trans isomerization, crosslinking and photodegradation are used to produce photoalignment layers and we show how the performance of the materials can be optimized by molecular design. Pretilted alignment is discussed and theoretical models are introduced to explain the photoalignment process. A range of display and non-display applications for photoalignment is presented.

https://iopscience.iop.org/article/10.1088/0022-3727/33/10/201/pdf

Photoinduced surface alignment for liquid crystal displays

Polyimides (PIs) exhibit excellent thermal stability, mechanical, dielectric, and chemical resistance properties due to their heterocyclic imide rings and aromatic rings on the backbone. Due to these advantageous properties, PIs have found diverse applications in industry. Most PIs are insoluble because of the nature of the high chemical resistance. Thus, they are generally used as a soluble precursor polymer, which forms complexes with solvent molecules, and then finally converts to the corresponding polyimides via imidization reaction. This complexation with solvent has caused severe difficulty in the characterization of the precursor polymers. However, significant progress has recently been made on the detailed characterization of PI precursors and their imidization reaction. On the other hand, much research effort has been exerted to reduce the dielectric constant of PIs, as demanded in the microelectronics industry, through chemical modifications, as well as to develop high performance, light-emitting PIs and liquid crystal (LC) alignment layer PIs with both rubbing and rubbing-free processibility, which are desired in the flat-panel display industry. This article reviews this recent research progresses in characterizing PIs and their precursors and in developing low dielectric constant, light-emitting, and LC alignment layer PIs.

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High performance polyimides for applications in microelectronics and flat panel displays

Photosensitive materials for photoalignment of liquid crystals (LCs) and photoinduced molecular reorientation are described, especially focusing on azobenzene-containing materials and photocrosslin...

Photoalignment and Photoinduced Molecular Reorientation of Photosensitive Materials

A soluble photoreactive polyimide (PSPI) with side-group cinnamate (CI) chromophores that forms good quality films through conventional solution spin-casting and drying processes was successfully synthesized with a reasonably high molecular weight:  poly(3,3‘-bis(cinnamoyloxy)-4,4‘-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-CI). This 6F-HAB-CI PSPI is thermally stable up to 340 °C, its glass transition temperature (Tg) is 181 °C, and it was determined by prism coupling to be positively birefringent. The photochemical reactions of the PSPI in solution and in films, as well as the molecular orientations of the PSPI that are induced by exposure of its films to linearly polarized ultraviolet light (LPUVL), were investigated in detail by nuclear magnetic resonance, ultraviolet−visible, infrared, and Raman spectroscopies and by dissolution testing and optical retardation measurements. It was confirmed that the CI chromophores of the PSPI undergo both photoisomerization and photodimerization. LPU...

Photoreactions and Photoinduced Molecular Orientations of Films of a Photoreactive Polyimide and Their Alignment of Liquid Crystals

Surface relief gratings have been optically inscribed on azopolymer films on the interior surface of an empty liquid crystal (LC) cell Uniform LC alignment is observed in the inscribed regions when the cell is filled Since the microgrooves are on both sides, this may provide a higher degree of order than other techniques The stability of the relief gratings and the transmittance of the azopolymer films can be further improved by photobleaching This alignment technique could have useful applications in the in-plane switching LC displays

Photoinduced liquid crystal alignment based on a surface relief grating in an assembled cell

The photopolymer material of poly(vinyl cinnamate) (PVCi) has been used to investigate the contribution of the anisotropic van der Waals force to the azimuthal anchoring energy in a pentyl cyanobiphenyl (5CB)/PVCi system. The optical anisotropy of a PVCi film is precisely controlled by varying the exposure time of linearly polarized UV light, and the azimuthal anchoring energy in the film is estimated by measuring the width of the Neel wall. The result shows that the azimuthal anchoring energy in the 5CB/PVCi system increases exponentially with increasing optical anisotropy, indicating the existence of additional contributions to the LC alignment.

https://iopscience.iop.org/article/10.1143/JJAP.36.L432/pdf

Measurement of azimuthal anchoring energy at liquid crystal/photopolymer interface

In-plane switching liquid crystal displays (IPS-LCDs) have been fabricated by a photo-alignment method using poly (vinyl cinnamate), and the effect of the azimuthal anchoring energy on the electrooptical performance has been studied theoretically and experimentally. The results show a large reduction in the driving voltage of IPS samples with decreasing azimuthal anchoring energy, which demonstrates the usefulness of the photo-alignment method for fabricating IPS-LCDs.

https://iopscience.iop.org/article/10.1143/JJAP.37.L743/pdf

Reduction in Driving Voltage of In-Plane Switching Liquid Crystal Displays Using Photo-Alignment Method

The photopolymer materials of poly(vinyl cinnamate) (PVCi) and polyimides showing homeotropic liquid crystal (LC) alignment have been used to control LC alignment on these films using linearly-polarized (LP)- UV exposure, and the LC alignment mechanism including pretilt angle generation on the films has been discussed. In the case of PVCi, an important contribution of non-dimerized side chains of PVCi molecules to the LC alignment is pointed out. On the other hand, LC alignment on homeotropically alignment polymer films exposed to LP-UV light is shown to be determined through the interaction of LC with alkyl branches attached to the polymer. It is also shown that the photo-alignment method using these photopolymer materials are very promising technique for the fabrication of some attractive LCD modes such as in-plane switching and vertically-aligned display modes, and an example of the application of the photo-alignment method to the fabrication of in-plane switching device is presented.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Photoalignment control of LC and its applications to LCD fabrication

The photopolymer material of poly(vinyl cinnamate) (PVCi) has been used to align liquid crystal molecules using linearly-polarized UV light, and the LC alignment mechanism on the films has been investigated. Comparing the LC alignment on rubbed PVCi films with that on UV-exposed PVCi film, it is shown that the side chains of PVCi molecules strongly interact with the LC molecules on the PVCi films and thus the alignment mechanism on linearly-polarized UV-exposed PVCi films is due to the anisotropic van der Waals interaction between LC molecules and optically-anisotropic PVCi films arising from the photo-induced anisotropic side chain distribution on the films. We also study the thermal stability of UV- exposed PVCi films, and clarify the improvement of the thermal stability with increasing UV exposure energy.

Mechanism of Photo-Induced Liquid Crystal Alignment on Poly(vinyl cinnamate) Surface.

A new type of waveguide-like nervous mode, in which the nematic liquid crystal (NLC) layer is used as the electrically controlled active material cladding and the channel waveguides are passive, is suggested Its synaptic weight effect is discussed theoretically with a specific numerical example, and a preliminary experiment shows the results supporting this idea

Xiang Tong Li

Papers

Measurement of azimuthal anchoring energy at liquid crystal/photopolymer interface

Reduction in Driving Voltage of In-Plane Switching Liquid Crystal Displays Using Photo-Alignment Method

Photoalignment control of LC and its applications to LCD fabrication

Mechanism of Photo-Induced Liquid Crystal Alignment on Poly(vinyl cinnamate) Surface.

Synapse Effect of Waveguide Modulator with a Nematic Liquid Crystal Cladding for Optical Neural Computing