Swelling of a polyelectrolyte brush in humid air

We present an experimental study on the swelling of polyelectrolyte brushes covalently attached to planar solid surfaces in contact with humid air. Mo nolayers of poly-N-methyl-[4-vinylpyridinium]iodide (MePVP) with film thick nesses of several hundred nanometers were used in this study. The MePVP bru shes were attached to the surfaces of silicon wafers as well as to evaporat ed silicon oxide films on solid substrates by using self-assembled monolaye rs of an azo initiator and radical chain polymerization in situ. The film t hicknesses of the surface-bound monolayers were measured by optical wavegui de spectroscopy (OWS) as a function of the humidity of the environment. The MePVP brushes show strong increases in thickness as well as a strong decre ase of the refractive index of the surface-attached layer due to water inco rporation caused by the exposure to the humid environment.