Reversible wettability of photoresponsive fluorine-containing azobenzene polymer in Langmuir-Blodgett films

A polymer containing 3'-[trifluoromethyl-4,3'-dibenzoazo] dyes in the side chains has been synthesized, and a monolayer of this polymer has been trans ferred onto solid substrate by the Langmuir-Blodgett (LB) technique. The re versible processes of photoisomerization of the LB films induced by light a re observed by UV-vis absorption spectra, in situ atomic force microscopy, and friction force microscopy. In addition, the physisorbed monolayers of t he monomer of the polymer at liquid/solid interface are investigated by sca nning tunneling microscopy. The films of the polymer show the reversible ch ange of morphological and friction force responses under alternate UV(365 /- 10 nm) and visible (436 +/- 10 nm) irradiation. The films also undergo r eversible wettability changes with UV and visible light irradiation, which are directly confirmed by contact angle measurement. The largest photoinduc ed change of contact angle is 11 +/- 1 degrees. The larger value of the con tact angles measured corresponds to films mainly composed of the trans isom ers of the molecules. Evidence shows that the photoinduced surface energy c hanges have an important influence on the wettability changes.