The effect of mobile polymers on the normal and shear forces between polymer brushes

We used the surface force balance technique to investigate the influence of free polymer chains on the equilibrium and dynamic properties of dense polymer brushes, focusing on their extension and swelling under oscillatory shear flow. In particular, we extend earlier results on shear of brushes (Nature 1991, 352, 413) both to additional molecular weights of the brush and especially, for the first time, to the effect of the added free polymer. We present experimental studies on how the equilibrium properties of these polymer brushes are influenced by the presence of free, chemically identical polymer chains well above the overlap concentration, c approximate to 40-50 c*. We observe a strong influence of the free chains on the behavior of the brushes when high shear rates are applied, even when the brushes are well away from contact, indicating a clear coupling between the brush and the free polymer. The onset of shear-induced swelling in our experiments correlates reasonably well with the relaxation properties of the outer regions of the brushes.