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

The combination of transmission null ellipsometry (TNE) and attenuated total reflection (ATR) methods supported by absorption measurements is shown to be an effective tool to study spontaneous and photoinduced 3D order in azopolymers. We investigated a series of azobenzene containing side-chain polyesters differing by the length of the main-chain spacer (CH2) m (m = 2, 8, 9, 10, 12, 13, 14, 16) and the tail substitutes (NO2, OCH3, and OC4H9) in azochromophore. The 3D order was induced by monochromatic polarized light of several wavelengths strictly distinguished by absorption efficiency of azochromophores. The orientational order under irradiation and after irradiation is studied. A big variety of 3D orientations (biaxial, uniaxial, and isotropic) is realized. The uniaxial and isotropic configurations correspond to the initial state and the saturation state of irradiation, while biaxiality is an attribute of the transient orientations. It is shown that both initial and photoinduced 3D orders are strongly determined by molecular structure of azopolymers. If we exclude the case of ultrathin polymer films (d < 200 nm), the observed regularities can be summarized as follow. The homologues with NO2 tail group and m > 8 demonstrate strong preference for in-plane alignment, which is random in the initial state and uniaxially ordered in the saturated state of irradiation. The change of the tail substitute in the succession NO2 → OCH3 → OC4H9 leads to transition from the in-plane to the out-of-plane alignment of azochromophores (nonirradiated films), increase of the tendency of isotropic ordering (excitation within ππ* absorption band), and transition from the positive (prolate) to the negative (oblate) in-plane uniaxial order (excitation within nπ* absorption band). The observed regularities are explained by competition of photoorientation determined by symmetry of light and intrinsic self-organization determined by the structure of polymer molecules.