The slip transition at the polymer-solid interface

Direct measurements of the velocity of a high-molecular-weight polydimethylsiloxane melt in the immediate vicinity of a solid wall (100 nm from the interface) have been conducted, using a novel optical technique based on evanescent wave induced fluorescence and pattern photobleaching. On silica, quite distinct behaviours are observed: (i) for weak polymer-surface interactions (silica modified by grafting of C18 hydrocarbon chains), a sharp transition between almost no and strong slippage at the wall appears for high enough shear rate, in good agreement with a recent model that ascribes the onset of slippage to a stretching transition of a few chains attached to the surface; (ii) for strong polymer-surface interactions (bare silica) chains irreversibly adsorbed build a pseudo-brush; only weak slippage develops with b (the length under the surface where the velocity extrapolates to zero) independent of the shear rate. No sharp transition appears within the explored shear rate range.