Patterned Poly(N-isopropylacrylamide) Brushes on Silica Surfaces by Microcontact Printing Followed by Surface-initiated Polymerization

Patterned poly(N-isopropylacrylamide) (PNIPAAm) brushes were fabricated on oxidized silicon wafers by surface-initiated atom transfer radical polymerization of N-isopropylacrylamide from a micropatterned initiator. The patterned surface initiator was prepared by microcontact-printing octadecyltrichlorosilane and backfilling with 3-(aminopropyl)triethoxysilane followed by amidization with 2-bromo-2-methylpropionic acid. XPS and FTIR confirmed the chemical structure of the surface initiator and the PNIPAAm brushes. Surface analysis techniques, including ellipsometry, contact angle goniometry, and X-ray reflectometry (XRR), were used to characterize the thickness, roughness, hydrophilicity, and density of the polymer brushes. Tapping-mode AFM imaging confirmed the successful patterning of the PNIPAAm brushes on the oxidized silicon substrates. Variable temperature ellipsometry indicated that the lower critical solution temperature of the hydrated PNIPAAm brush was broad, occurring over the range of 20-35 degrees C. A solvatochromic fluorophore, 6-propionyl-2-dimethylaminonaphthalene (Prodan), in the PNIPAAm brush layers yielded a very similar emission to that in DMF, which can be attributed to the similarity of their chemical structures. Fluorescence microscopy further proved the successful patterning of the polymer brushes and suggested that the Prodan is localized in the patterned PNIPAAm brushes and excluded from the surrounding octadecyltrichlorosilane regions.