Comparative study of photoorientation phenomena in photosensitive azobenzene-containing homopolymers and copolymers

Peculiarities of photoorientational behavior under the action of polarized and nonpolarized light of different wavelengths on films of a homopolymer and a related copolymer were studied. The homopolymer consists of polyacrylate main chain and photosensitive azobenzene side groups, whereas the copolymer also contains nonphoto sensitive bulky cholesterol-containing side groups. It was shown that the irradiation of thin spin-coated films of the polymers by polarized UV light leads to the appearance of small dichroism which is not stable under prolonged irradiation. This phenomenon associated with the photoselection and photoorientation processes takes place because only the azobenzenes with the molecular long axis perpendicular to the direction of the electric field vector of the incident light are inactive, but the others are activated for isomerization cycles accompanied by a motion of their molecular long axis until they are perpendicular to the direction of the electric field vector. The irradiation with linearly polarized visible light (436 nm) results in a strong increase of the in-plane orientation of the azobenzene groups. The maximal value of the photoinduced anisotropy is larger in the case of the copolymer. It was also found that the irradiation with nonpolarized visible light leads to an out-of-plane order in polymers films, i.e. in this case of the azobenzene moieties simultaneously adopt orientation along film normal. The comparative study of above-mentioned phenomenon allowed to find the influence of different factors, such as irradiation wavelength, polymer structure and film thickness on the kinetics, the photoinduced dichroism and the order parameter values. (C) 2004 Elsevier B.V. All rights reserved.