Annealed polyelectrolyte brushes under normal and lateral compressions

We report here compression force measurements of diblock copolymers contain ing a short neutral hydrophobic sequence (polystyrene) and a long water-sol uble weak polyelectrolyte sequence (poly(2-vinylpyridine)). This study focu ses on the polyelectrolyte sequence and has been restricted to the case whe re the subphase is at pH 2. The lateral compressions were done using a Lang muir trough whereas the normal compressions were carried out with an atomic force microscope with a spherical particle attached to the tip. The thickn ess of the brush is considered as a function of the grafting density and is determined from the AFM force curves without reference to any theoretical model. At high grafting densities an "anomalous" behavior is observed, wher e the thickness of the brush decreases with increasing the grafting density . This is attributed to the annealed character of the polyelectrolyte.