Forces between polymer-covered surfaces: a colloidal probe study

An atomic force microscope was used to measure the interaction forces betwe en a polymer-covered silica sphere and a polymer-covered silica plate at va rious pH values and electrolyte concentrations and for different polymer ch ain lengths. The polymer used was poly(ethylene oxide) (PEO). The force mea surements were performed in aqueous solution without dissolved polymer, at scan rates corresponding to the velocity of Brownian collisions between dis persed colloidal particles. In all cases the repulsion on approach was foun d to be electrostatic in nature: although the PEO adsorption layers are sat urated, there is no sign of steric repulsion before the distance of closest approach between the silica surfaces is reached. At pH 4 the approach curv es show, fbr separations smaller than 20 nm, an attractive component which partly compensates the electrostatic repulsion. On retraction a strong adhe sion is observed, which is attributed to bridging. At pH 8 and low electrol yte concentration (10(-3) M NaCl) the interaction is repulsive on approach and on retraction: no adhesion by bridging takes place. However, upon incre asing the NaCl concentration a weak adhesion is induced. At neutral pH (sim ilar to 6.5) the adhesion on separation depends on the force with which the surfaces have been pressed together (10(-3) M NaCl). The pH dependence of the interaction curves is discussed in terms of the segmental adsorption en ergy, which is known to decrease with increasing pH. Measurements at pH 4 s how a strong dependence of the adhesion force on the chain length. A linear relationship between the adhesion force and the surface coverage (in mass per unit area) is found. (C) 1998 Elsevier Science B.V. All rights reserved .