Neutron characterization of stimuli-responsive polymer brushes for nanofluidic gating

 

Smart polymer brushes that are responsive to external stimuli are attractive candidates for nanofluidic applications requiring controllable gating properties, such as regulated fluid flow and particle sorting. The variation of polymer conformations in response to changes in the surrounding environment makes such materials well suited for designing functional surfaces. Here, we report controllable gating behavior of nanofluidic channels decorated with pH-responsive polymer brushes. The brushes are selectively grafted on the side walls of the channels. The conformational changes of the brushes under different solution conditions were non-invasively measured by high-resolution specular and off-specular neutron reflection. Simultaneous analysis of the specular and off-specular signals enabled a 3D reconstruction of the sample profile and yielded a detailed description of the brush thickness as a function of solution pH and ionic strength. Our analysis is based on a dynamical theory model employing an advanced computational optimization protocol. The fit results indicate collapsed and expanded polymer chain conformations under basic and acidic solution conditions, respectively. These findings show controlled channel switching between "open" and "close" states with pH regulation.