CONFORMATION OF POLYMER BRUSHES UNDER SHEAR - CHAIN TILTING AND STRETCHING

We have studied the conformation of a polymer brush in equilibrium wit h a solvent that is subject to a shear flow. The interplay between the polymer brush and the hydrodynamic flow of the solvent has been model ed, with simple but largely justifiable approximations. The main techn ique used in our study is a Monte-Carlo simulation algorithm that is d istinct from many standard numerical methods used in studies of polyme r brushes in that it combines an off-lattice description of polymer br ushes-the Edwards Hamiltonian-with a modification of the standard Metr opolis Monte-Carlo transition probability to take into account the eff ective force acting upon the polymer molecules by the moving solvent. The conformation of the polymer brush, the configurations of each indi vidual chain in particular, is investigated in detail. It is found tha t the significant response of the brush to the solvent shear flow mani fests principally in the form of the chain tilting toward and stretchi ng along the direction of the flow, whereas the overall conformational properties, such as the averaged local monomer density, and the linea r span of the brush in the direction normal to that of the flow remain essentially unaffected by the flow. Such response can be understood b oth qualitatively and semiquantitatively in terms of a notion of the m echanical balance of the different physical forces involved, which was used in the theory of Rabin and Alexander (Rabin, Y.; Alexander, S. E urophys. Lett. 1990, 13, 49). The relevance of our study to some recen t experiments is briefly discussed.