Monolayers of symmetric triblock copolymers at the air-water interface. 1.Equilibrium properties

Surface pressure isotherms and ellipsometric measurements of monolayers of two triblock symmetric copolymers, poly(ethylene oxide)-poly(propylene oxid e)-poly(ethylene oxide) (PEO-PPO-PEO), at the air-water interface have been carried out. These copolymers are water-soluble, and the difference in hyd rophobicity between the blocks is small. This represents a different scenar io for brush formation than for most of the hydrophobic--hydrophilic block copolymers reported so far. The surface pressure curves show two different phase transitions. The ellipsometric measurements indicate a thickness tran sition when the monolayer saturates, which supports the hypothesis for brus h formation. The experimental data have been analyzed in terms of the scali ng theory of adsorption of polymer brushes. Despite the possibility of diff usion from the interface, the PPO block acts as an efficient anchoring elem ent in the formation of an adsorbed brush, once the adsorption sites at the interface are fully occupied. This is analogous to what has been reported for diblock copolymers with a much larger difference in the hydrophobicity of the blocks.