Air-water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

Fabrication of Langmuir films at the air-water interface of four linear-dendritic block copolymers (LDBCs) is described. The LDBCs are composed of a linear hydrophilic chain of poly(ethylene glycol) (PEG) and the first four generations of hydrophobic aliphatic polyester dendrons functionalized at the periphery with cyanoazobenzene chromophores. Langmuir films of the LDBCs, coded as PEG-AZOn (n indicates the number of cyanoazobenzene units at the periphery of the dendritic block), have been characterized by a combination of surface pressure versus area per molecule isotherms, UV-vis reflection spectroscopy and Brewster angle microscopy. The observed PEG-AZOn Langmuir film behavior depends strongly on the hydrophilic/hydrophobic ratio. A typical transition, related to PEG chains desorption from the air-water interface into the water subphase is observed for all the LDBCs, except for PEG-AZO16. In addition, PEG-AZO2 and PEG-AZO4 show a second transition whose nature has been studied in detail. Azobenzene chromophore interactions have been shown to be relevant in the organization of PEG-AZOn (n=4, 8 and 16) Langmuir films. Moreover, for PEG-AZO16 the orientation of the azobenzene units has been determined, revealing the formation of a well organized structure of azobenzene moieties at the air-water interface.