Comparative studies of polyimides with covalently bonded azo-dyes with their supramolecular analoges: Thermo-optical and photoinduced properties

The paper describes the synthesis and characterization of aromatic polyimides with azopyridine derivatives covalently attached as a side-chain and their supramolecular analoges based on the hydrogen bonds. Moreover, azopolymers differ in the chemical structure of the polymer backbone and the content of the chromophore. Azo-functionalized polymers were obtained by a two-step synthetic approach. This includes the preparation of a precursor with pendant hydroxyl groups followed by the covalent attachment of the chromophore, that is, 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine to the polyimide backbone. The degree of functionalization of the polymers was estimated by the UV-Vis spectroscopy. Supramolecular films were built on the basis of the hydrogen bonds between the hydroxyl groups of the polymer and azopyridine derivatives, that is, 4-(4-hydroxyphenylazo)pyridine and 4-[4-(6-hydroxyhexyloxy)phenylazolpyridine. The polymers were characterized and evaluated by FTIR, H-1 NMR, X-ray, UV-Vis, DSC and TGA methods. The synthesized precursor polymers revealed the glass transition temperature (T-g) in the range of 254-366 degrees C, whereas azopolymers and supramolecular systems exhibited lower T-g within the ranges 129-153 degrees degrees C and 53-171 degrees C, respectively. The polymers were easily soluble in common organic solvents enabling to obtain films of a good optical quality. The light-induced optical anisotropy was studied with the help of the holographic recording. The polymer films were investigated by an atomic force microscopy before and after the diffraction grating recording in order to confirm the formation of surface relief gratings. To the best of our knowledge, for the first time the photoinduced anisotropy in polyimides functionalized with azopyridine units has been compared with their supramolecular systems analoges. (C) 2013 Elsevier B.V. All rights reserved.