We have recently studied the surface segregations of amphiphilic block copolymers blended in a matrix of cross-linked poly(dimethyl siloxane). In this system, while the hydrophilic block with high surface energy avoids air surface, it starts to segretage to form a brush layer when the surface is placed in water. This is an anti-fouling polymer brush surface having a self-repairing ability and hopeful for medical applications. We have already conducted preliminary studies which suggest the formation of high density polymer brush. We propose a neutron reflectivity investigation on (1)the structures of low density brushes of low copolymer concentration samples with high and low molecular weight copolymers and (2) the formation process of the brush layers by time-depend analysis from the contact with water to the equilibrium.