Isomerization behavior of an azopolymer in terms of the Langmuir-Blodgett film thickness and the transference surface pressure

The photoinduced trans-cis and thermal cis-trans isomerizations of an azopolymer were investigated in terms of LB film thickness and the transference surface pressure by means of UV-vis spectroscopy and AFM microscopy. Three different photochemical behaviors have been observed depending on the number of layers: (i) for 1 layer, the percentage of cis isomers in the photostationary state is around 25%, (ii) in films of 5-20 layers, this percentage decreases to ca. 15%, and (iii) for 35 layers, the percentage of isomers cis formed after irradiation is more than 50%. Furthermore, the thermal cis-trans isomerization varies from a first-order kinetics to an expression with two exponential terms when the LB films change from 1 layer to multilayers. Morphologic changes in the surface of the films during the photoisomerization process were observed by means of AFM, and these changes also depend on the number of layers. In the cis state some hills were observed in the films of 1 and 35 layers, whereas practically no changes occurred in the films of 10 layers. These hills can be interpreted as the rupture of bidimensional structure of the film during the photoisomerization process. Three different schemes have been proposed to explain the observed behaviors.