Stimulus-Responsive Poly(N-isopropylacrylamide) Brushes and Nanopatterns Prepared by Surface-Initiated Polymerization

In this paper we report the surface-initiated polymerization of poly(N-isopropylacrylamide) (pNIPAAM), a stimulus-responsive polymer, from monolayers of ω-mercaptoundecyl bromoisobutyrate on gold-coated surfaces. pNIPAAM was polymerized in aqueous solution at a low methanol concentration at room temperature to maintain the growing pNIPAAM chains in a hydrophilic and an extended conformational state. Under these conditions thick polymer brush layers (up to 500 nm in the swollen state) are produced after 1 h of polymerization. We present a new and simple strategy to fabricate stimulus-responsive, surface-confined pNIPAAM brush nanopatterns prepared in a “grafting-from” approach that combines “nanoshaving”, a scanning probe lithography method, with surface-initiated polymerization. The reversible, stimulus-responsive conformational height change of bulk and nanopatterned polymer brushes was demonstrated by repeated cycling in water and water/methanol mixtures (1:1, v/v). Our findings are consistent with the behavior of laterally confined and covalently attached polymer chains, where chain mobility is restricted largely to the out-of-plane direction. The present work is significant because the triggered control of interfacial properties on the nanometer scale holds significant promise for actuation in bio-nanotechnology applications where polymeric actuators may manipulate the transport, separation, and detection of biomolecules.