SHEAR DEFORMATION OF 2 INTERPENETRATING POLYMER BRUSHES - STOCHASTIC DYNAMICS SIMULATION

The interaction between two polymer brushes (layers of polymer chains grafted at one end onto an impermeable surface) brought into a close c ontact and moving with respect to each other in the lateral direction is studied by the method of stochastic dynamics simulation. The struct ural properties of the brushes such as the monomer density profile, th e distribution of the free ends of the chains, as well as the disjoini ng pressure have been studied as functions of the separation between g rafting planes (the degree of static overlapping of brushes) and of th e lateral displacement rate. We have found that at zero shear rate the extension of grafted chains perpendicular to the grafting planes mono tonically decreases with decreasing distance between the planes, but n ot as rapidly so that significant overlapping and interpenetration of brushes occurs. At strong compression the free ends of the chains are distributed all over the gap between grafting planes. An increase in t he shear rate is accompanied by inclination and extension of chains in the direction of shear and by a simultaneous decrease in the chain di mensions in the direction perpendicular to the grafting planes: the fr ee ends an localized near the middle plane and progressive decoupling of brushes occurs. At large shear rate the width of the interpenetrati on zone almost vanishes. The inclination of chains at given shear rate increases with decreasing grafting density and decreasing width of th e gap between the grafting planes. The disjoining pressure exhibits a weak maximum as a function of the shear rate that is in qualitative ag reement with the predictions of scaling theories. (C) 1998 American In stitute of Physics.