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 to the flow (hence to the tension arising from hydrodynamic drag forces), leaving the remainder protected. We show that for quite small values of the shear rate, I, the system reaches a self-regulating state where the lowest possible fraction of grafted chains is exposed to the flow. This brings quantitative corrections to previous models (all based on the assumption that the chains behave alike) which correspond to a higher susceptibility of the layer to shear fields: the onset of significant swelling occurs at a lower shear rate and at high shear rates the asynlptotic value of the relative swelling is sonlewhat larger. Furthermore we find that the behaviour of the layer strongly depends on both the index of polymerisation of the chains and the grafting density. In particular, for thick brushes, our model predicts a discontinuous (first order) swelling transition at a critical shear rate. The nlodel is used to study the rate of desorption of individual chains grafted ma 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 7Z obeys 7Z -J i~ 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 desorption rate for large I: 7Z ~W ei"