title: Light-Induced Reversible Change of Roughness and Thickness of Photosensitive Polymer Brushes.
abstract: We investigate light-induced changes in thickness and roughness of photosensitive polymer brushes containing azobenzene cationic surfactants by atomic force microscopy (AFM) in real time during light irradiation. Because the cis-state of azobenzene unit requires more free volume than its trans counterpart, the UV light-induced expansion of polymer thin films associated with the trans-to-cis isomerism of azobenzene groups is expected to occur. This phenomenon is well documented in physisorbed polymer films containing azobenzene groups. In contrast, photosensitive polymer brushes show a decrease in thickness under UV irradiation. We have found that the azobenzene surfactants in their trans-state form aggregates within the brush. Under irradiation, the surfactants undergo photoisomerization to the cis-state, which is more hydrophilic. As a consequence, the aggregates within the brush are disrupted, and the polymer brush contracts. When subsequently irradiated with blue light the polymer brush thickness returns back to its initial value. This behavior is related to isomerization of the surfactant to the more hydrophobic trans-state and subsequent formation of surfactant aggregates within the polymer brush. The photomechanical function of the dry polymer brush, i.e., contraction and expansion, was found to be reversible with repeated irradiation cycles and requires only a few seconds for switching. In addition to the thickness change, the roughness of the brush also changes reversibly between a few Angstroms (blue light) and several nanometers (UV light). Photosensitive polymer brushes represent smart films with light responsive thickness and roughness that could be used for generating dynamic fluctuating surfaces, the function of which can be turned on and off in a controllable manner on a nanometer length scale.
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam , 14476 Potsdam, Germany.
authors.forename: Alexey
authors.initials: A
authors.lastname: Kopyshev
authors.affiliation: Department of Chemical & Biomolecular Engineering, North Carolina State University , Raleigh, North Carolina 27695-7905, United States.
authors.forename: Casey J
authors.initials: CJ
authors.lastname: Galvin
authors.affiliation: Department of Chemical & Biomolecular Engineering, North Carolina State University , Raleigh, North Carolina 27695-7905, United States.
authors.forename: Rohan R
authors.initials: RR
authors.lastname: Patil
authors.affiliation: Department of Chemical & Biomolecular Engineering, North Carolina State University , Raleigh, North Carolina 27695-7905, United States.
authors.forename: Jan
authors.initials: J
authors.lastname: Genzer
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam , 14476 Potsdam, Germany.
authors.forename: Nino
authors.initials: N
authors.lastname: Lomadze
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam , 14476 Potsdam, Germany.
authors.forename: David
authors.initials: D
authors.lastname: Feldmann
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam , 14476 Potsdam, Germany.
authors.forename: Juri
authors.initials: J
authors.lastname: Zakrevski
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam , 14476 Potsdam, Germany.
authors.forename: Svetlana
authors.initials: S
authors.lastname: Santer
keywords: azobenzene containing surfactants; domain memory in polymer brushes; light driven reversible change of surface topography and thickness; orientation of azobenzenes in polymer brushes; photosensitive brushes