Photodeformable Materials Based on Azobenzene-Containing Solid Films

PHOTODEFORMABLE MATERIALS BASED ON AZOBENZENE-CONTAINING SOLID FILMS 

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). 

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. 

The photoinduced expansion of thin films of polymers (4) containing azobenzene chromophores was explored in real time by single wavelength ellipsometry (Fig. 3.6 Tanchak and Barret, 2005). The initial expansion of the azobenzene polymer films of thickness ranging from 25 to 140 nm was irreversible and amounted to 1.5% %. Subsequently, reversible expansion was observed with repeated irradiation cycles the relative expansion was 0.6%-1.6%. 

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