Polymerization, Nanopatterning and Characterization of Surface- Confined, Stimulus-Responsive Polymer Brushes
In this chapter we report the surface-initiated atom transfer radical polymerization (ATRP) of poly(N-isopropylacrylamide) (pNIPAAM), a stimulus-responsive polymer, from monolayers of x-mercapto-undecyl bromoisobutyrate on gold-coated surfaces. NIPAAM was polymerized in aqueous solution at low methanol concentrations ([MeOH] <3% by volume) at room temperature to maintain the pNIPAAM chains in a hydrophilic and extended conformational state. Under these conditions, thick polymer brush layers (up to 500 nm) are produced after 1 h of polymerization. Using dip-pen nanolithography (DPN), we fabricated for the first time patterned, stimulusresponsive pNIPAAM brushes with feature sizes on the order of 500 nm. The stimulus- responsive conformational change of bulk and patterned brushes was demonstrated by inverse transition cycling in water, and water-methanol mixtures (1:1, v:v). Diffusional processes are shown to be important for DPN fabrication of both dot and line patterns. Surface force measurements between pNIPAAM brush surfaces and a pNIPAAM decorated cantilever tip revealed large adhesion forces for the collapsed polymer brushes, suggesting strong hydrophobic interactions. The present studies are important because the triggered control of interfacial properties on the nanometer scale holds significant promise for actuation in bionanotechnology applications where polymeric actuators may manipulate the transport, separation, and detection of biomolecules.
