SHEAR-INDUCED DEFORMATION AND DESORPTION OF GRAFTED POLYMER LAYERS

We describe the behaviour of grafted polymer layers in strong solvent shear flows within a model where only a subset of chains are exposed t o the flow (hence to the tension arising from hydrodynamic drag forces ), leaving the remainder protected. We show that for quite small value s of the shear rate, gamma, the system reaches a self-regulating state where the lowest possible fraction of grafted chains is exposed to th e how. This brings quantitative corrections to previous models (all ba sed on the assumption that the chains behave alike) which correspond t o a higher susceptibility of the layer to shear fields: the onset of s ignificant swelling occurs at a lower shear rate and at high shear rat es the asymptotic value of the relative swelling is somewhat larger. F urthermore we find that the behaviour of the layer strongly depends on both the index of polymerisation of the chains and the grafting densi ty. In particular, for thick brushes, our model predicts a discontinuo us (first order) swelling transition at a critical shear rate. The mod el is used to study the rate of desorption of individual chains grafte d via compact end-stickers and insoluble polymer blocks. In both cases , there is a strong increase in desorption at the swelling transition. For the case of end-sticker grafting, we find the desorption rate R o beys R similar to gamma(3) for large shear rates; while in the case of diblock grafting, we find that the barrier height to desorption is a strong function of shear rate, leading to an exponentially enhanced de sorption rate for large gamma: R similar to e(gamma tau 0).