Azobenzene polymers photomechanical effects

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

Another class of polymers equipped with azobenzene moieties comprises a-helical polypeptides, in particular pQly(L-glutamate)s and poIy(L-lysine)s. In solution, these azobenzene-modified polypeptides can undergo photoinduced helix-coil transitions. Polypeptides partially (30 to 50%) substituted with azobenzene moieties are surface active and form stable monolayers. Because of the partial substitution, there is sufficient free volume, and the azobenzene moieties can be isomerized in the monolayer. The photoisomerization changes the area per molecule, and the monolayer shows a photomechanical effect. LBK films of a photosensitive poly(L-lysine) with 31 mol... 

Seki, T., and Tamaki, T. Photomechanical effect in monolayers of azobenzene side ch.iin polymers. Chem. Lett., 1739 (1993). 

The photoinduced deformation phenomenon of materials is called a photomechanical effect, and it has been so far reported for photoresponsive polymer films and gels [35-43]. When azobenzene is isomerized from the trans form to the cis form, the length of the molecule is shortened from 0.90 to 0.55 nm. The size change of the molecule on photoirradiation is expected to alter the shape of the polymers which contain the azobenzene molecules. However, it is not the case in polymer systems. The transformation in polymer films does not change the polymer shape because of the large free volumes of the polymer bulk. Suitable organization or assembly of the molecules is required for the photoinduced deformation of materials. 

PHOTODEFORMABLE MATERIALS AND PHOTOMECHANICAL EFFECTS BASED ON AZOBENZENE-CONTAINING POLYMERS AND... 

Eisenbach (1980) investigated the photomechanical effect of poly(ethyl acrylate) networks (3) cross-linked with azobenzene moieties and observed that the polymer network contracted upon exposure to UV light (caused by the trans-cis isomerization of the azobenzene cross-links) and expanded upon irradiation of visible light (caused by cis-trans back-isomerization Fig. 3.5). This photomechanical effect is mainly due to the conformational change of the azobenzene crosslinks by the trans-cis isomerization of the azobenzene chromophore. However, the degree of deformation was small (0.2%). 

Matejka et al. synthesized several types of photochromic polymers based on a copolymer of maleic anhydride and styrene with azobenzene moieties both in the side chains and cross-links of the polymer network (Matejka et al., 1981 Matejka and Dusek, 1981 Matejka et al., 1979). The photomechanical effect was enhanced by an increase in the content of photochromic groups, and the photoinduced contraction of the sample amounted to 1% for a polymer with 5.4mol% azobenzene moieties. 

Fig. 5.13 Photomechanical effects. Schematic illustration of UV-light-induced bending of a cross-linked liquid-crystalline polymer film containing azobenzene groups. Light is absorbed at the upper surface layer of the film and causes anisotropic contraction. Adapted from Ikeda et al. [51] with permission from Wiley-VCH.

A photomechanical effect involves a reversible contraction and expansion of an oriented rubbery film after irradiation, based on the change in the dimension of chromophores embedded in the polymer networks. This effect was first described, by Agolini and Gay [5], for a partly crystalline polyimide [see structure (2)] with azo-aromatic chromophores in the backbone. They reported a contraction of the polymer film at 200°C of 0.5% by UV irradiation, which in principle could cause isomerization of azobenzene chromophores from the trans- to cw-form. A correlation between the isomerization and the film contraction was not obtained. At this high temperature, however, the content of cw-isomer in the photostationary state is believed to be very low because of rapid back-reaction from the cis- to trans-form. At the same time, the rapid thermal reaction made it difficult to measure any correlation between the contraction and the isomerization. It seems that the isomerization may not be the sole origin of the contraction of this system. 

The use of structural changes of photoisomerizable chromophores to change the size of polymers was first proposed by Merian (1966). He observed that a nylon filament fabric dyed with an azobenzene derivative shrank upon photoirradiation. This effect is ascribed to the photochemical structural change of the azobenzene moiety absorbed on the nylon fibers. However, the observed shrinkage was very small (only 0.1%), and subsequent to this work, much effort was made to find new photomechanical systems with an enhanced efficiency (Irie, 1990 Smets and De Blauwe, 1974). 

Kondo M, Sugimoto M, Yamada M, Naka Y, J-i M, Kinoshita M, Shishido A, Yu Y, Ikeda T (2010) Effect of concentration of photoactive chromophoies on photomechanical properties of crosslinked azobenzene liquid-crystalline polymers. J Mater Chem 20 117... 

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