Manipulating cylindrical polyelectrolyte brushes on the nanoscale by counterions: collapse transition to helical structures

Abstract

We present a study of the spatial structure of cationic cylindrical polymer brushes in the presence of counterions of different valencies. Dynamic light scattering (DLS), atomic force microscopy (AFM) and cryogenic transmission electron microscope (cryo-TEM) studies demonstrate that the brushes undergo a transition from an extended to a collapsed state when the ionic strength is raised. In the presence of a di- or trivalent salt this collapse passes through an intermediate state in which the cylindrical polymer brushes assume a helical conformation. The helical pitch (25 nm) is in the range expected from theoretical considerations. Moreover, we demonstrate that the collapse can be reversed by a photochemical cleavage of the complex trivalent counterion into one monovalent and one divalent ion (photoaquation). All data demonstrate that the spatial structure of cylindrical polymer brushes can be manipulated on the nanoscale in a defined fashion.