New Insight into Photoalignment of Liquid Crystals on Coumarin-Containing Polymer Films

Abstract

Polymers containing 6-and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the parts of the reacted and the unreacted coumarin moieties. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and the unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction.

Engineering of Complex Order and the Macroscopic Deformation of Liquid Crystal Polymer Networks

“…Herein, we demonstrate the straightforward engineering of complex order and macroscopic deformation of liquid crystal polymer networks that show well-defined deformation in three dimensions, through the use of patterned alignment layers in combination with polymerizable liquid crystals. During this procedure, photosensitive materials are treated with linearly polarized light, [9][10][11] allowing straightforward complex patterning using photomasks (Figure 1 a). [8] Photoalignment is an attractive method for the preparation of such alignment layers, and has previously been used to align liquid crystals, mainly for display applications.…”

Liquid Crystal Aligning Ability of Photosensitive Polymer Material

“…The introduction of some photosensitisers to the polyimide precursor mixture [12] or some photoreactive groups to the polyimide structure opened the path to easy chemical etching processes in manufacturing elements for the needs of the optoelectronic industry. These chemical agents, which have incorporated groups like cinnamates [13,14,15], coumarins [16,17] and chalcones [18], may also be used in the process of liquid crystal device preparation. Thus photoalignment techniques are those in which light is the factor that induces anisotropy of the aligning layer [19].…”

Room‐temperature processing of π‐conjugated oligomers into uniaxially oriented monodomain films on coumarin‐based photoalignment layers

“…Along the rubbing direction, grooves were generated and polyimide backbones were reoriented on the alignment layer surface, both conducive to uniaxial orientation of overlying nematic liquid crystal molecules thanks to the enhanced p-p interactions. 47,48 These two factors are absent on the Polymer 1 photoalignment layer without rubbing, where liquid crystal orientation is dictated by coumarin dimers through molecular interactions of dispersive and steric origins at the early stage of photodimerization before cross-over, 38 e.g., X ¼ 0.31 as implemented in this study.…”

“…Three distinct material approaches have been demonstrated: axisselective degradation of polyimides, [22][23][24][25][26] cis-trans isomerization of azobenzenes, [27][28][29] and cycloaddition of cinnamates, [30][31][32][33][34] or coumarins. [35][36][37][38][39][40][41][42] Orientational order parameter values emulating those on rubbed polymer alignment layers have been realized via thermal annealing of films comprising p-conjugated polymers 43,44 and oligomers 39,40 on photoalignment layers.…”