An Investigation of the Role of Cross-Linking and Photodegradation of Side-Chain Coumarin Polymers in the Photoalignment of Liquid Crystals

Abstract

A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.

Photoalignment and Photoinduced Molecular Reorientation of Photosensitive Materials

“…Additionally, the influences of the type of photoproduct of thin coumarin-containing polymethacrylate film on the LC alignment behavior have been investigated. 48,49 Whether parallel or perpendicular LC alignment is observed depends on the distribution of head-to-head crosslinked and head-to-tail crosslinked products. 48,49 Chen et al have systematically investigated alignment control of LCs using photoluminescent materials with several types of photoalignment coumarin-containing polymeric films.…”

“…48,49 Whether parallel or perpendicular LC alignment is observed depends on the distribution of head-to-head crosslinked and head-to-tail crosslinked products. 48,49 Chen et al have systematically investigated alignment control of LCs using photoluminescent materials with several types of photoalignment coumarin-containing polymeric films. 52,53 Calcone-containing polymers 1921 have also been employed as the LC alignment layer.…”

Macroscopic Photoinduced and Reversible Sol-Gel Conversions of Polymer Materials Containing Photoreactive Coumarin Moieties

“…Subsequent irradiation of the coumarin dimer at λ<300 nm induces cleavage of the cyclobutane ring and the monomer coumarins reform. Various coumarin-functionalized polymer materials, in particular, photo-cross-linking polymers have been reported thus far [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Chujo et al reported photocontrollable swelling behavior of hydrogel films obtained from polyoxazoline having a coumarin moiety [1].…”

Synthesis, characterization and properties of photoresponsive polymers comprising photocrosslinkable pendant chalcone moieties

“…These photopolymers have a wide variety of applications in the fields of macroand microlithography, 1,2 liquid crystals, 3,4 tissue engineering, 5 -7 biosensors, 8 UV-curing adhesives, 9 hole transporting materials in organic light emitting diodes 10 and optical anisotropy.…”