Azobenzene polymers mechanism

Fukuda, X., Sumaru, K., Yamanaka, X, and Matsuda, H. Photo-induced formation of the surface relief grating on azobenzene polymers Analysis based on the fluid mechanics. Mol. Cryst. and Liq. Cryst. 2000, 345, pp. 263-268. 

Mechau N, Neher D, Borger V, Menzel H, Urayama K. 2002. Optically driven diffusion and mechanical softening in azobenzene polymer layers. Appl Phys Lett 81(25) 4715 4717. 

Mechau N, Saphiannikova M, Neher D. 2005. Dielectric and mechanical properties of azobenzene polymer layers under visible and ultraviolet irradiation. Macromolecules 38(9) 3894 3902. 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

Kumar, J., Li, L., Jiang, X. L., Kim, D.-Y., Lee, T. S., Tripathy, S. (1998). Gradient force the mechanism for surface relief grating formation in azobenzene functionalized polymers. Appl. Phys. Lett. 72, 2096-209S. 

The second mechanism (2) utilizes the change iMuced in the intramolecular interaction between pendant groups by photoirradiation. The system reported for the first time is polyfmethacrylic acid) with pendant azobenzene groups [4]. In an aqueous solution, the viscosity was found to increase by ultraviolet irradiation (Fig. 2(2a). The trans to cis photoisomerization was considered to decrease the hydro-phobic interaction between the azobenzene chromophor, allowing the polymer coil to expand. 

The third mechanism (3) is the simplest one. When trans-cis photoisomerizable chromophores are incorporated into the polymer backbone, the photoinduced configuration change of the chromophores is expected to induce a conformation change of the polymer chain. Azobenzene (4) is the most widely used as the trans-cis photoisomerizable photoreceptor molecule. It undergoes isomerization from the trans to the cis form under ultraviolet irradiation, while the cis form can return to the trans form either thermally or photochemically [11]. 

Neckers et al. [16, 17, 18] demonstrated that polyureas with backbone azobenzene groups (70) also underwent a photoviscosity effect when ultraviolet-irradiated. Stille et al. [19] reported that the intrinsic viscosity of polyquinoline (II) with backbone stilbene groups in di-m-cresyl phosphate/m-cresol decreased as much as 24% under ultraviolet light. The decrease was ascribed to the tram to cis isomerization of the stilbene groups. Because of its simplicity the mechanism (3) has been widely applied to other polycondensation or polyaddition polymers. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

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