The Initial and Photoinduced 3D Orientational Order in Polymethacrylates with Azobenzene Side Groups

ABSTRACT

The combination of the transmission null ellipsometry and total absorption method, earlier approved for several polymer classes, is applied to study 3D orientational configurations of azochromophores in polymethacrylates with azobenzene side groups. The transformation of the initial orientation due to the photoexcitation of azochromophores is investigated. The dependence of the orientational configuration on the structure of azochromophore fragments and its concentration in the polymers are also studied. In the non-irradiated films, azochromofores strive to be aligned normally to the film plate. Under irradiation, if reorientation mechanism of the photoinduced ordering prevails, the azochromophores reorient perpendicularly to the polarization direction of the exciting light, E _ex, and in the saturation state they are randomly distributed in the plane perpendicular to E _ex. This implies that the induced structure is mainly influenced by the photoordering, while selfordering of polymer chains is not effective or overcome, presumably because of high T_g of polymethacrylates. In case when photoselection ordering mechanism dominates, 3D distribution of azochromophores in the saturation state of irradiation is isotropic due to a strong exhaustion of the number of anisotropic trans isomers. The transient photoinduced orientations are biaxial independently on the dominating ordering mechanism. The reduction of the azochromophore's concentration in polymers reduces the anisotropy rate but not influence the trends described above. The observed regularities were earlier described for several other classes of photosensitive polymers and so they may be common rules for photoordering in the photochromic media.

Keywords:

• azopolymer
• 3D orientational order
• photoinduced anisotropy
• polymethacrylate

Acknowledgments

These studies were carried out within the framework of the project “Ordering regularities and properties of nano-composite systems” of the NA Sci. of Ukraine. Partially, they were also supported by INTAS (project No 03-51-5448).