We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced 3D orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, correspondingly. Using irradiation with the polarized light of two different wavelengths we find that the kinetics of photoinduced anisotropy can be dominated by either photo-reorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At early stages of irradiation the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photo-saturation. In the regime of photoselection the photo-saturated state of the film is optically isotropic. But, in the case of the photo-reorientation mechanism, anisotropy of this state is uniaxial with the optical axis dependent on the preferential alignment of azochromophores. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.

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.

Passivation of photoalignment films by continuous layers of reactive mesogen provides stable liquid crystal alignment and electro-optic performance equivalent to that of rubbed polyimide films.