Photoinduced 3D Orientational Order in Liquid Crystalline Azopolymers Studied by the Method of Attenuated Total Reflection

Abstract

The attenuated total reflection (ATR) technique is applied to study 3D ordering in the films of liquid crystalline (LC) polymers. We used a series of LC sidechain polyesters differing by the tail substitutes (NO 2 , OCH 3 and OC 4 H 9 ) in azochromophore. The 3D order is studied in non-irradiated films as well as in the films irradiated by polarized light with k ex1 ¼ 365 nm or=and with k ex2 ¼ 457 nm strictly distinguished by absorption efficiency of azochromophores. The orientation under irradiation is compared to that after irradiation. Biaxial, uniaxial as well as isotropic 3D ordering of azochromophores is realized. The observed uniaxial spatial structures can be classified by analogy with crystal optics; 1) positive C films with preferential out-of-plane orientation of azochromophores; 2) negative C films with in-plane random orientation of azochromophores having symmetry axis in the film normal direction; 3) positive A films with in-plane uniaxial orientation, and 4) negative A films characterized by oblate orientational distribution having symmetry axis in-plane of the film. The initial states as well as the stationary states of the films are characterized by uniaxial order of azochromophores, except in the case of intensive pumping with 365 nm, which results in isotropic ordering. The change of the tail substitute in the succession NO 2 ! OCH 3 ! OC 4 H 9 leads to orientation transition from the structure of negative C plate to the structure of positive C plate (non-irradiated films), increase of the tendency of isotropic ordering (k ex1 ¼ 365 nm irradiation), and transition from the The studies were supported by personal NATO grant for Dr. O. Yaroshchuk. structure of positive A plate to the structure of negative A plate (k ex2 ¼ 457 nm irradiation).

Molecular Structure of Azopolymers and Photoinduced 3D Orientational Order. 1. Azobenzene Polyesters

“…The drawback of this method is that it requires an excellent optical quality of films and the optimization of films thickness. The first demand is difficult to satisfy for LC azopolymers, because of their domain structure, 38 whereas the second one complicates investigation of films having nonoptimized thickness. For these reasons, films for ATR studies should be thoroughly selected.…”