Adsorption kinetics of diblock copolymers from a micellar solution on silica and titania

The solution and adsorption behavior of a series of diblock copolymers of hydrophobic poly(dimethyl siloxane) and hydrophilic poly(2-ethyl-2-oxazoline) was studied. These block copolymers formed large polydisperse micelles in an aqueous solution. The critical micelle concentration was lower than 2 mg L-1. The adsorption kinetics of these polymers onto macroscopically flat oxide surfaces was studied with reflectometry in stagnation point flow. Both blocks of the copolymers had affinity for silica, and only the hydrophobic block had affinity for the titania surface. Nevertheless, the adsorption curves on silica and titania had similar features. The adsorption kinetics was affected by the exchange rate between micelles and free polymers. For short polymer chains the exchange rate was fast compared with the time necessary for diffusion across the diffusive layer. Before the micelles arrived at the surface, they had already broken up into unimers. Because the critical micelle concentration was very low, the experimental adsorption rate was determined by the diffusion of micelles toward the surface. This was not the case for the longest polymer chain; the exchange between micelles and unimers was relatively slow. The micelles did not adsorb directly, and the adsorption rate was determined by the exchange of polymers between micelles and solution. For all polymer samples the adsorption increases linearly as a function of time, up to very high adsorbed amounts where it reaches a plateau. The adsorbed amount on silica is considerably higher than found for titania. The poly(dimethyl siloxane) was anchored more strongly to the silica surface than to titania; the density of the adsorbed layer could therefore become higher.