Salt-induced structural behavior for poly(N-isopropylacryamide) grafted onto solid surface observed directly by AFM and QCM-D

The salt-induced structural changes of an end-grafted poly(N-isopropylacrylamide) (PNIPAM) layer on a silica substrate were investigated in sodium sulfate solutions using an atomic force microscope (AFM) and a quartz crystal microbalance with dissipation (QCM-D). A PNIPAM layer was grafted onto the silicon wafer surface by free radical polymerization of NIPAM to obtain a high molecular weight polymer layer with low grafting density overall. AFM images of the coated surface were featureless at low salt concentrations commensurate with a brush-like layer. At salt concentrations >= 0.11 M, a large number of domain structures, with a characteristic size of similar to 100 nm, were seen on the surface commensurate with a collapse of the brush-like layer into mushroom-like aggregates. This critical concentration for brush collapse is in good agreement with the concentration range of between 0.1 and 0.2 M, at which the phase transition of the bulk PNIPAM is reported. Both the frequency and the dissipation data obtained using a QCM-D also showed a significant change at this concentration indicative of the layer collapse. Further analysis of these data confirmed that the observed PNIPAM structural transition was caused by a collapse of the brush-like structure as a result of dehydration of the polymer chains.