Microstructural Observation and Property Characterization of Stimuli-Responsive Polymer Grafted onto Solid Surface Using AFM and QCM-D

Abstract

It is well known that poly(N-isopropylacrylamide) (PNIPAM) exhibits thermally responsive proper ties in water. At a lower critical solution temperature (LCST) of around 32oC, PNIPAM chains show phase transition from coil to globule state. Therefore, solids on which the PNIPAM is grafted are expected to be applicable to a wide variety of industrial fields using expanding-shrinking proper ty depending on temperature. In this study, the structural changes of the PNIPAM chains grafted onto solid plates were investigated in aqueous solutions with an atomic force microscope (AFM) and a quartz crystal microbalance with dissipation (QCM-D). By AFM imaging, the transition of the grafted PNIPAM chains from brush to mushroom state was visualized. QCM-D signals showed significant change at around LCST, indicating that PNIPAM underwent a collapse by the dehydration of the chains. A complementary investigation indicated that a similar transition could be observed in solutions of high concentration electrolytes. It was also found that the grafting density of the PNIPAM chains had a critical role for the conformational behaviour of the PNIPAM.