Salt-induced contraction of polyelectrolyte brushes

Abstract

We present an experimental study dedicated to understanding the behaviour ofpolyelectrolyte chains when salt goes into a polyelectrolyte brush. We use theability of asymmetric neutral-charged diblock copolymers to anchor to a selectiveinterface or to self-assemble in water, to examine polyelectrolyte brushes bothin planar and in spherical geometries. Using neutron reflectivity, the monomerprofile of planar brushes has been found to switch from a Gaussian profile to aparabolic profile, showing that at large spatial scale a salted planar brush behaveslike a neutral one in good solvent. Using small angle neutron scattering, it isfound that spherical brushes exhibit the same behaviour at a large spatial scalealthough polyelectrolyte chains remain rod-like at a small spatial scale whatever thesalinity is. The charged chains inside a polyelectrolyte brush could be viewedas ‘surveyor’s chains’, which fold their rod-like segments of persistence lengthlp. In the last part, spherical polyelectrolyte brushes in contact have been examined. Theirbehaviour is discussed in terms of interdigitation or contraction of the brushes.