Swelling and collapse of an adsorbed pH-responsive film-forming microgel measured by optical reflectometry and QCM

The swelling and deswelling of a pH-responsive electrosterically stabilized poly[2-(diethylamino)ethyl methacrylate] microgel adsorbed to silica surfaces have been quantified using the techniques of optical reflectometry (OR) and quartz crystal microbalance (QCM). It is shown that by utilizing and comparing OR measurements performed on wafers with differing oxide layer thicknesses the adsorbed amount and film thickness of the adsorbed microgel in both the swollen and deswollen forms can be determined. Also, the kinetics of the transition can be followed, revealing that collapse is a slower process than swelling, and direct support is provided for the formation of a dense outer layer or skin during collapse that slows the deswelling process. It is shown that the adsorption of this low glass transition temperature film-forming microgel latex is robust to changes in pH after an initial swelling event which is responsible for desorption of a large and variable fraction of the initially adsorbed polymer. Subsequent deswelling and swelling of the adsorbed film indicates that adsorption to a surface greatly hinders the volumetric swelling capacity of the microgel film. In its swollen state the film is only 3-4 times thicker than the collapsed film, whereas for particles in bulk the volume increases by a factor of 20 upon protonation of the tertiary amine residues. QCM results show that even in the collapsed form the film contains a considerable amount of water. Further, the viscoelasticity of the deswollen film is similar to that of the swollen film, suggesting that the degree of cross-linking is the primary determinant of viscoelasticity.