ADSORPTION OF PEO PPO PEO BLOCK COPOLYMERS AT SILICA

The adsorption at silica of block copolymers of the type PEO-PPO-PEO [PEO and PPO being poly(ethylene oxide) and poly(propylene oxide), respectively], as well as that of PEO, has been studied. For a number of polymers of a total molecular weight of about 15 000, it was found that the adsorbed amount is rather low (0.35-0.40 mg/m2), independent of the PO content in the range 0-30% PO. For a copolymer of a total molecular weight of 4000 and a PO content of 50% the adsorbed amount was about 0.20 Mg/M2. All polymers investigated formed thin adsorbed layers, with hydrodynamic thicknesses of about 2-5 nm. The pH dependence of the adsorbed amount and the hydrodynamic thickness is similar to that displayed by PEO homopolymers. Ellipsometry experiments on silica surfaces showed good agreement with the experiment on silica dispersions. Furthermore, ellipsometry experiments provided information on both adsorption and desorption kinetics. Both adsorption and desorption are fast processes in these systems, occurring over minutes. Finally,ellipsometry experiments showed that, just prior to solution micellization, an abrupt increase in the adsorbed amount occurred. At temperatures above the critical micellization temperature, the adsorbed amount remained independent of temperature in the temperature range studied. At all temperatures the hydrodynamic thickness was much smaller than the hydrodynamic diameter of the solution micelles. On hydrophobic surfaces, no abrupt increase in the adsorbed amount was observed. The experimental findings were interpreted with a modified mean-field theory.