Softly supported polymer brushes confined in competition with ions

Abstract

Polymer brushes have a wide variety of applications both for technological systems and to enable fundamental interfacial physics research by controlling functionality density at a surface. Amine and catechol functionalities are often present in adhesives and interact with a range of different surfaces. Often these adhesives act in hydrated environments either from a water based glue or in technically challenging environments, such as tissue repairs or underwater applications. In these applications the adhesive also has to compete at the interface with ions from the often saline environment. We have made use of three techniques to study the interaction and hydration of polymer brushes under confinement, namely the surface forces apparatus (SFA), atomic force microscopy (AFM) and neutron reflectometry (NR). SFA provides force response characteristics by the approach, contact and retraction of two surfaces, while monitoring the distance via white light interferometry. For our amine terminated brushes these measurements led to our deeper understanding of the competition occurring with NaCl ions at a mica surface during binding. We observed a 90% decrease in the measured adhesion with a 0.01 to 1 M increase in electrolyte concentration. Although SFA provided excellent force response characteristics, NR is a powerful technique for determining thicknesses and hydration of layers independent from making contact between two surfaces allowing the native, unconfined, brush to be measured. Further, with the aid of a surface force-like confinement cell we have probed the hydration of the brush layers in confinement using NR. Thus, we have a much deeper insight into the fundamental interplay between hydration of the supported polymer brush and its adhesive interaction.