RESPONSE OF THIN POLYMER LAYERS CONFINED BETWEEN SOLID-SURFACES AND SUBMITTED TO NORMAL OSCILLATORY DEFORMATIONS

The viscoelastic properties of liquids close to solid surfaces differ from the bulk. Nanorheology has been performed by using a surface forc e apparatus adapted to operate as a rheometer in a sphere-plane geomet ry. Axial oscillatory measurements have been carried out with high pol ymer solutions filling the gap. The deformations were kept sufficientl y small not to perturb the film structure and were applied in a large range of frequency (10(-3) to 10(2) s(-1)). It is shown that the compl ex modulus characterizing the confined medium can be split into two co mponents: a shear modulus (it accounts for the viscous dissipation due to the flow of solvent molecules through the mesh created by the long polymer chains which connect the two solid surfaces) and a compressio n modulus which is related to the normal stress response of the chains confined between the solid surfaces. The hydrodynamic screening lengt h xi(h) and the correlation length xi deduced from the two moduli are compared and are found to scale in the same way as a function of the d istance between the two surfaces.