Wetting Transition on Hydrophobic Surfaces Covered by Polyelectrolyte Brushes
Résumé
We study the wetting by water of complex “hydrophobic-hydrophilic” surfaces made of a hydrophobic substrate covered by a hydrophilic polymer brush. Polystyrene (PS) substrates covered with polystyrene-block-poly(acrylic acid) PS-b-PAA diblock copolymer layers were fabricated by Langmuir-Schaefer depositions and analyzed by atomic force microscopy (AFM) and ellipsometry. On bare PS substrate, we measured advancing angles θA ) 93 ( 1° and receding angles θR ) 81 ( 1°. On PS covered with poorly anchored PS-b-PAA layers, we observed large contact angle hysteresis, θA ≈ 90° and θR ≈ 0°, that we attributed to nanometric scale dewetting of the PS-b-PAA layers. On well-anchored PS-b-PAA layers that form homogeneous PAA brushes, a wetting transition from partial to total wetting occurs versus the amount deposited: both θA and θR decrease close to zero. A model is proposed, based on the Young-Dupre´ equation, that takes into account the interfacial pressure of the brush Π, which was determined experimentally, and the free energy of hydration of the polyelectrolyte monomers ΔGPAA hyd , which is the only fitting parameter. With ΔGPAA hyd ≈ -1300 J/mol, the model renders the wetting transition for all samples and explains why the wetting transition depends mainly on the average thickness of the brush and weakly on the length of PAA chains.
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