Phototunable Response in Caged Polymer Brushes

A light-responsive brush was obtained by surface-initiated ATRP of a methacrylate monomer containing ionizable-COOH side groups caged with the photoremovable group 4,5-dimethoxy-2-nitrobenzyl (NVOC). In the caged form, the polymer brush (PNVOCMA) is neutral and hydrophobic due to the presence of the aromatic chromophore. Upon irradiation the NVOC group is removed and a polyanion (polymethacrylic acid, PMAA) chain is generated. The charged brush can swell and collapse depending on the pH and the exposure dose (i.e., uncaging degree). The behavior and properties of the brush layer for different photoconversion degrees were studied. On the basis of quartz crystal microbalance measurements, a threshold of 50% uncaging was identified in order to achieve significant swelling and pH response of the brush. Between 50 and 80% the photoconversion the response of the brush could be light-modulated. For photoconversions >80% only small changes in the response were detectable. X-ray reflectivity (XRR) and scanning force microscopy allowed us to measure thickness, roughness and swelling of the brushes at intermediate photoconversions. Combined XRR and grazing-incidence small-angle scattering experiments evidenced a change in the internal structure of the brush upon exposure and indicated the occurrence of domain segregation as a consequence of the coexistence of hydrophobic and charged groups in the brush structure.