No abstract

No abstract

Two dicarboxylic acids were prepared by alkylation of 4-hydroxycinammic acid with 1,6-dibromohexane or 1,12-dibromododecane. The dichlorides of these dicarboxylic acids were polycondensed with mixtures of isosorbide and methylhydroquinone. All copolyesters then obtained showed nematic schlieren textures. Furthermore, three dicarboxylic acids were synthezised by alkylation of 4-hydroxycinammic acid with the ditosylates of di-, tri-, and tetraethylene glycol. Numerous copolyesters were prepared from the dicarboxylic acids and mixtures of isosorbide with methyl-, tert-butyl-, or phenylhydroquinone. Also 2,7-dihydroxynaphthalene was used as a comonomer. Almost all resulting copolyesters were cholesteric, and nearly 50% of them were capable of forming a Grandjean texture. The photochemical behavior of selected copolyesters was studied in solution in spin-coated films and in films with Grandjean texture. Depending on the reaction conditions, photo-cross-linking by 2 + 2 cycloaddition competed more or less successfully with the photo-Fries rearrangement and with the photoisomerization. UV irradiation (at 313 nm) of oriented films in the cholesteric melt caused disappearance of the Grandjean texture. However, irradiation below the glass-transition temperature caused stabilization of the Grandjean texture by photo-cross-linking.

Photochromic liquid crystalline main chain poly(ester imide)s with cinnamoyl moieties and chiral groups as part of the backbone were characterized concerning their liquid crystalline, molecular dynamical, and photochemical properties. Homo-and copolymers were synthesized by polycondensation of mixtures containing isosorbide, tert-butylhydroquinone, or catechol. The copolyesters were cholesteric, but exclusively the polymers containing the tert-butylhydroquinone moiety form Grandjean textures on annealing. These polymer films show blue iridescent color. UV irradiation causes photo-cross-linking of the polymers via the (2 + 2) photocycloaddition of the cinnamoyl moieties, resulting in insoluble films in which the iridescent supramolecular structure is frozen in without any changes compared to the initial films. Dichroism of about 0.07 is induced upon irradiation with linearly polarized UV light. In contact with liquid crystals, the photoinduced anisotropic interface of the films causes an efficient alignment of liquid crystals.

Light-induced ordering processes were investigated in spin-coated films of a smectic main-chain polyester with mesogenic 4,4‘-dioxyazobenzene groups in the backbone. Spin-coating results in aggregation due to π−π stacking and out-of-plane orientation. Both are destroyed by UV exposure. However, they are rebuilt by subsequent thermal Z/E isomerization of the azobenzene groups, forming a lamellar structure parallel to the substrate. The irradiation with linearly polarized visible light results in the orientation of the azobenzene moieties perpendicular to the electric field vector, preserving the lamellar structure with values of dichroism up to 0.43. However, the efficiencies of the E/Z photoisomerization and that of the photoorientation process strongly depend on the history of the sample. The scanning force microscopy (SFM) histogram indicates two types of domains in the top layer, one oriented in the normal and the second one formed by tilted azobenzene groups with an average tilt angle of about 50° to the normal. The last spacing is erased by annealing, but is rebuilt due to linearly polarized exposure. The photoinduced anisotropy of a “wet” dewetted ultrathin film of 5 nm is 1 order of magnitude smaller. It demonstrates that the photoreaction and the self-organization influence each other mutually.