ABSTRACT
The correlation between orientation ordering of polymer chains in the bulk of polymer film and at the polymer-liquid-crystal (LC) interface has been studied to determine it's role in LC alignment. The bulk and surface ordering of polymer were investigated by null ellipsometry and x-ray reflectivity, respectively. Two kinds of liquid-crystalline polymers were used; side-chain azopolymers with azochromophores containing hydrophobic OC4H9 alkyl chain (P1) and strongly polar NO2 group (P2) as the end substituents. The uniaxial tilt orientation of azochromophores in the films of both polymers was induced by the oblique irradiation with unpolarized UV light. The two polymers exhibit similar chain orientation but different ordering of azochromophores on the surface of the films of P1 and P2. Surface ordering of P1 films correlates very well with the order in the bulk of the film, which are essentially determined by the UV exposure. However, orientational order of polymer chains at the surface of P2 films is different from that in its bulk and is not determined by UV exposure. This is explained by strong aggregation of azochromophores during its self-assembling at the polymer-air interface. The LC alignment is determined by the surface ordering of azochromophores. The results imply that ordering tendency can be effectively transferred from polymer bulk to polymer surface and then to LC if it is not lost at the polymer-LC interface.
ABSTRACT
We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (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, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by either photoreorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At the early stages of irradiation, the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photosaturation. In the regime of photoselection, the photosaturated state of the film is optically isotropic. But, in the case of the photoreorientation 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.
