The Current Trends of Optics and Photonics by Cheng-Chung Lee

Author:Cheng-Chung Lee
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

11.3 Photo-Crosslinking in Cinnamic Side-Chain Polymers

LC molecules can be aligned by causing them interact with the side-chains of photo-crosslinkable polyimides that are coated on the surface of a substrate. These polyimides can be small molecules [71–73], 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. In the cycloaddition reaction, cyclobutane is formed from the double bonds following the delocalization of π electrons. The long axis of cyclobutane molecules preferably aligns perpendicular to the direction of the polarized UV light (). The van der Waals’ (or dispersion) interaction causes the LC molecules to align perpendicular to .

The [2 + 2] cycloaddition is a general reaction that is undergone by cinnamate [10, 74, 75], coumarin [12, 77–79], chalconyl [11], tetrahydrophthalimide [13] and maleimide [13, 14] photosensitive polyimides. The [2 + 2] cycloaddition of cinnamate and chalcone species, is accompanied by trans-cis isomerization [80, 81]. However, materials with a rigid molecular structure, such as coumarin, tetrahydrophthalimide and maleimide, cannot undergo trans-cis isomerization [77]. 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.

The photoaligning molecular structure is designed to enable the direction of the easy axis on its irradiated surface to be easily predicted. Reznikov et al. [87] found that the direction of the easy axis in the azimuthal plane depends upon the position of the cinnamate fragments in the side polymer chains. Owing to the [2 + 2] cycloaddition of the cinnamate moieties of the polymer, the direction of alignment is the direction of the axis of geometric anisotropy of the crosslinked fragments. 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 structure of photosensitive molecules can be manipulated to adjust the direction of the easy axis in the zenithal plane. 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]. The method by which photosensitive fragments are connected to the polymer chain is important. For example, the position of azobenzene fragments in the main chain can promote the photosensitivity and thermal stability of alignment. The attachment of the fragments to a side chain increases the reaction rate and the photoinduced ordering.

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