Layers of densely-tethered polymers (polymer brushes) are of interest due to their unique lubricating, antifouling, and/or biocompatible properties, which stem from their diffuse ‘brushy’ conformation. When these brushes are constructed from stimuli-responsive polymers their properties can be modulated via a stimulus-induced conformational change. These responsive systems enable advanced interface design, and as such it is important that their behaviour in complex environments be understood.

This work explores the interactions between a range of surfactants and a poly(N-isopropylacrylamide) (PNIPAM) brush layer, using a combination of ellipsometry, Neutron Reflectometry (NR) and quartz crystal microbalance techniques to quantify the structure of the layer and the location of the surfactant within the interface. We investigate the mechanical implications of the presence of surfactant by subjecting the brush to a confining force and measuring its conformation with NR. As part of this work, new modelling techniques for the analysis of NR data from diffuse interfaces and the quantification of uncertainty within NR experiments were developed.

Figure 1: Schematic of (a) different modes of surfactant swelling and (b) embedded surfactant increasing the resistance to confinement.

A rich surfactant-specific behaviour is observed and is found to be headgroup dependent, consistent with prior work on other PNIPAM architectures. Those surfactant species that do interact are found to swell the brush layer, even at concentrations well below the CMC. We show that the mechanism behind this response is the adsorption of surfactant micelles along the polymer chain, decorating the ordinarily neutral PNIPAM with charges. We report that these charges modify the mechanical properties of the brush, assisting it in resisting confinement. These findings elucidate the nature of the surfactant-NIPAM interaction, giving hints as to the mode of surfactant binding and revealing a method for modifying the mechanical properties of a brush layer.