Optical investigations of photoinduced ordering in azo polymers.

摘要

There are four primary objectives of this thesis: to obtain an experimental handle on the anisotropic distribution of azobenzene chromophores in polymer films induced by polarized light; to model the material's anisotropy, thereby determining the anisotropic optical constants from the experimental data; to use this information to investigate the mechanism of the photo-induced ordering; and to study the dynamics of the processes involved. To achieve the first goal, a Stokes polarimeter was built and used to characterize the polarization of a probe beam transmitted through the ordered films. After visually inspecting the resulting polarization data and together with knowledge of the polarization of the incident probe beam, hypotheses were made as to the arrangement of the chromophores. From these hypotheses, models were constructed which describe the oriented structures and allow the polarization of the transmission to be calculated. Next, a fitting algorithm was devised based on an exhaustive search method, allowing the optical material constants to be determined from the experimental polarization data using the appropriate model.; This technique was applied to two studies. In the first investigation, an azo-containing liquid-crystalline polymer was irradiated with linearly-polarized light. In the region of the low-energy nπ* transition, the absorption coefficient is at times largest in the direction parallel to the pump beam polarization, contrary to what is typically observed for the photo-orientation of such materials. There is additional evidence to suggest that this likely results from a selective orientation of cis isomers from an optical Fréedericksz transition induced by the pump beam. In the second study, the same material was irradiated with circularly-polarized light and the transmitted polarization data shows evidence of photo-induced chirality. Structures are created with a handedness that depends on the handedness of the pump beam. Two models are described to interpret this result, one based on a uniaxial arrangement with the observed circular anisotropy included on a phenomenological level; the other based on a helical arrangement of chromophores.; A final project involves the study of the dynamics of the photo-orientation process when two amorphous azo polymers are irradiated with linearly-polarized light. A simple model of the pump-induced redistribution is proposed. (Abstract shortened by UMI.)