Liquid-crystal photoalignment using low-molecular-weight photo-cross-linkable composites

Abstract

A liquid-crystal (LC) photoalignment method, based on the use of low-molecular-weight photo-cross-linkable (LMWPC) composites is proposed. The basic idea of this method is that cross linking or both cross linking and polymerization of LMWPC composites are realized immediately on the substrate simultaneously with the photoalignment or as a stage of the backing process. Providing the advantages of a conventional photoalignment method, the use of LMWPC composites simplifies the synthesis and alignment procedure, makes possible wide range control of pretilt angles, and enhances the thermal stability of the LC alignment. The abilities of this method are demonstrated using a series of cinnamate-containing monomers.

Photo-Alignment Technology

“…These polyimides can be small molecules [71][72][73], polymers [10,74,75], or polymer blends [76]. Linear photo-polymerization results in the preferred depletion of the cinnamic side-chain molecules by the [2 + 2] cycloaddition that is induced by irradiation with linearly polarized UV light.…”

“…One can fabricate materials containing several photosensitive fragments of different nature. The materials combining the fragments that undergo trans-cis isomerization and polymerization [82,83], trans-cis isomerization and [2 + 2] cycloaddition [84,85], and [2 + 2] cycloaddition and polymerization [72,86] have been demonstrated. Although mechanisms of photoalignment are not well established for these materials, the superior thermal stability, extraordinary optical and dielectrical properties make them very promising for some applications.…”

“…Obi et al [88] and Kawatsuki et al [89] identified other cinnamate polymers that exhibited "parallel" LC alignment. These polymers, as well as some of cinnamate monomers [72], gave both "parallel" and "perpendicular" alignment, depending on the degree of exposure [77].…”

“…The pretilt angle can be controlled by attaching hydrophobic groups such as fluorine, alkyl, alkoxy, fluoroalkyl, fluoroalkoxy and others to photosensitive molecules. These groups can be attached as individual side chains [90,91], as terminal groups of photosensitive side chains [72,92] or fragments of main polymer chains. They can also be blended with the photoaligning materials [76,93,94].…”

Bifunctional ITO layer with a high resolution, surface nano-pattern for alignment and switching of LCs in device applications

“…Although rubbing is a simple and inexpensive method, it has many intrinsic drawbacks such as generation of electrostatic charges, contamination from rubbing debris and high temperatures (up to ca 250 1C). As alternatives to the rubbing process, noncontact methods such as photo-alignment [2][3][4][5] and ion-beam irradiation 6 have been used but they also have critical drawbacks associated with controlling the orientation of photoswitchs on the surface and low anchoring energies. 7 Indium-tin oxide (ITO) has been widely used as an electrode for LC display because of its low electrical resistance and high transmittance in the visible range of the optical spectrum.…”