Thermo-, chiro- and photo-optical properties of cholesteric azobenzene-containing copolymer in thin films

Thin films (100-350 nm) of liquid crystalline copolymer containing the photoactive azobenzene and the chiral cholesterol groups were prepared by spin-coating technique and their optical and chiro-optical properties were investigated. In thick films (several microns), copolymer is characterized by the following phase transitions: g 27 SmA 83-85 N* 127-128 I, whereas spin-coated thin films are optically isotropic. Nevertheless, the films possess a noticeable circular dichroism with a maxima corresponding to the absorbance of the non-chiral azobenzene fragments. This phenomenon is evidence for the existence helical supramolecular structure elements in the polymer films. Annealing of the films at temperatures, higher glass transition leads to disappearance of circular dichroism and formation of the strong out-of-plane order with orientation of side groups of copolymer perpendicular to substrate. The annealed films are characterized by the high order degree comparable with that of SmA phase. The same effect was observed under visible light irradiation (436 nm). Action of UV light (365 nm) results in E-Z isomerization of azobenzene groups and significant drop in circular dichroism values (almost to zero) in the fresh films, and disruption of the out-of-plane order in the annealed films. Such complicated chiro-optical and ordering behaviour of the copolymer films opens new opportunities for creation active switchable optical devices.