Adsorption of graft copolymers onto silica and titania

The adsorption of graft copolymers of poly(acrylamide) (PAAm, backbone) and poly(ethylene oxide) (PEO, side chains) from aqueous solution onto silica and titania was studied with reflectometry. Two high-molar-mass copolymers were used with different PEO graft densities (10 and 18% w/w PEO in copolymers G10 and G18, respectively). On titania only the PAAm backbone adsorbs and the PEO does not. This results in adsorbed amounts of 0.83 and 0.85 mg m(-2), respectively, which is about the same as that for a PAAm homopolymer. On silica the situation is reversed: now the PEO side chains adsorb and the PAAm backbone does not. The adsorption as a function of time shows a maximum, before the stable plateau is reached. The adsorbed amount on silica is much higher than that on titania: in the final plateau it is 1.35 and 1.2 mg m(-2) for 018 and G10, respectively. On silica the polymers form longer loops and tails so that more molecules can be accommodated at the surface. The overshoot on silica depends on the polymer concentration, suggesting that it is not caused by a conformational change of the adsorbed layer but by exchange with polymer molecules from solution. Differences in graft distribution and graft density in the polymer sample are probably responsible for the displacement. The average number of grafts per polymer is rather low. On statistical grounds there should be an appreciable polydispersity in graft distribution and in graft density. Molecules in which the grafts are clustered in a few groups can displace molecules with more regularly separated grafts, and molecules with a high graft density can displace those with a lower number of side chains. The newly arriving molecules can then adsorb in a flatter conformation with a lower adsorbed amount as the extra loss in conformational entropy is compensated by the gain in adsorption energy.