VISCOELASTICITY OF ADSORBED POLYMER LAYERS

We discuss theoretically dynamic measurements with a surface force app aratus composed of a plane and a sphere coated with adsorbed polymer l ayers in a good solvent. The hydrodynamics are studied within a simple two fluids model where the friction between polymer and solvent is de scribed by the so-called Brinkman approximation. In a steady compressi on experiment the distance between the sphere and the plane varies at a constant velocity and the polymer layers have a hydrodynamic thickne ss e(H) of the order of their radius of gyration. In a periodic compre ssion experiment, the distance has a periodic modulation of small ampl itude at a finite frequency; the results are recast in terms of a comp lex modulus G. At a low frequency, the modulus has the standard Maxwel l behavior (G' congruent-to omega2, G'' congruent-to omega). The contr ibution of the polymer to the loss modulus G'' is small when the polym er layers do not overlap; it is of the same order of magnitude as the pure solvent contribution when they do overlap. The elastic modulus in creases with the thickness of the adsorbed layers. At a high frequency , the complex modulus increases as G congruent-to omega2/3 and is inde pendent of the thickness of the polymer layers. When the adsorbed poly mer layers overlap, there is an intermediate regime where the elastic part of the modulus increases as G' congruent-to omega4/3.