Polymer brushes formed by end-capped poly(ethylene oxide) (PEO) at the air-water interface

We study monolayers formed at an air-water interface on a Langmuir trough by telechelic poly(ethylene oxide) polymers end capped with hydrophobic alkane groups (C-12 and C-16) The pressure-area isotherms show two plateaus: the first plateau at low coverage already exists for nontelechelic PEO and is attributed to the formation of loops in the solution; after this plateau, the pressure rises stongly as the hydrophobic groups anchor the polymers on the surface, which leads to the formation of a grafted polymer layer; the second plateau at high density is due to a dissolution of the polymer chains in the bulk water. Starting from a monolayer at high density, we also study experimentally the relaxation of the surface pressure with time due to the dissolution of the polymer. A theoretical approach of this problem based on polymer brush theory is proposed. Both the static properties of the layer and the desorption kinetics are calculated. The model accounts well for the shape of the isotherms and for the relaxation kinetics. It gives a good interpretation of the role of various parameters such as the compression velocity, the molecular weight, and the hydrophobicity of the chain ends. The anchoring energy of the hydrophobic groups is determined by comparison with experiments.