Hydrodynamic-flow-driven phase evolution in a polymer blend film modified by diblock copolymers

We have studied surface-directed phase separation in thin films of deuterat ed polystyrene and poly(bromostyrene) (with 22.7% of monomers brominated) u sing He-3 nuclear reaction analysis, dynamic secondary ion mass spectroscop y and atomic force microscopy combined with preferential dissolution. The c rossover from competing to neutral surfaces of the critical blend film (cas t onto Au) was commenced: polyisoprene-polystyrene diblock copolymers were added and segregated to both surfaces reducing in a tuneable manner the eff ective interactions. Two main stages of phase evolution are characterised b y i) the growth of two surface layers and by ii) the transition from the fo ur-layer to the final bilayer morphology. For increasing copolymer content the kinetics of the first stage is hardly affected but the amplitude of com position oscillations is reduced indicating more fragmented inner layers. A s a result, a faster mass flow to the surfaces and an earlier completion of the second stage were observed. The hydrodynamic flow mechanism, driving b oth stages, is evidenced by nearly linear growth of the surface layer and b y mass flow channels extending from the surface layer into the bulk. The fi nal bilayer structure, formed even for the surfaces covered by strongly ove rlapped copolymers, is indicative of long-range (antisymmetric) surface for ces.