The interfacial character of solids can be markedly altered by coating with a polymer brush to target a range of applications. Many techniques can reveal fundamental attributes of the brush such as thickness and solvation (ellipsometry), wettability (contact angle) or stiffness (quartz crystal microbalance with dissipation monitoring). However, only neutron reflectometry can reveal the internal brush nanostructure; the polymer volume fraction profile. We have used the Platypus reflectometer at ANSTO for the last decade to increase our understanding of brush structure in varied environments such as pH or ionic strength (for polyelectrolyte brushes [1,2]) or temperature (for thermoresponsive brushes [3,4]).
Recently we have focused considerable attention on improving the modelling of the diffuse polymer brush volume fraction (VF) profiles.[5] Subtle features in the brush depth profile have traditionally challenged the interpretation of the acquired relatively featureless reflectivity profiles. Confidence in fitting has been increased by using a Bayesian statistical approach [6] and co-refinement of directly comparable ellipsometric data.[7]
Using these approaches, subtle conformational changes in thermoresponsive polymer brush depth profiles have been revealed: these brushes undergo a well- to poorly-solvated phase transition over a given temperature range. These exemplar systems also readily reveal specific ion effects (character dependent on ion identity rather than solely ionic strength) when exposed to electrolytes from across the Hofmeister series.[4,8] Furthermore, nonmonotonic VF profiles have revealed (a) monomer enrichment near the substrate arising during copolymer brush synthesis,[9] or (b) ion binding to the brush.[10] Recent work has also deployed neutron reflectometry to probe brushes in nonaqueous solvents.[11] Finally, strong polyelectrolyte brushes are now helping us to probe the nature of the concentrated electrolyte phenomenon of underscreening which is not included in classical understanding of electrolytes yet is critical to interface optimisation in high salt environments such as batteries.