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We have studied the surface-induced spinodal decomposition in thin films of deuterated and protonated polystyrene, using 3 He nuclear-reaction analysis and dynamic secondary-ion mass spectroscopy. We found that the amplitude of this process may be modified by a polyisoprene-polystyrene diblock copolymer, which segregates predominantly to the surface when admixed to the isotopic polystyrene blend. This is due to the reduced surface attraction of deuterated polystyrene for the increased surface coverage by diblocks. Finally, the surface directed mode of the spinodal decomposition is observed to be extinct for the surface completely covered by copolymers.

We have studied surface-directed phase separation in thin films of deuterated polystyrene and poly(bromostyrene) (with 22.7% of monomers brominated) using 3 He nuclear reaction analysis, dynamic secondary ion mass spectroscopy and atomic force microscopy combined with preferential dissolution. The crossover from competing to neutral surfaces of the critical blend film (cast onto Au) was commenced: polyisoprene-polystyrene diblock copolymers were added and segregated to both surfaces reducing in a tuneable manner the effective interactions. Two main stages of phase evolution are characterised by i) the growth of two surface layers and by ii) the transition from the four-layer to the final bilayer morphology. For increasing copolymer content the kinetics of the first stage is hardly affected but the amplitude of composition oscillations is reduced indicating more fragmented inner layers. As a result, a faster mass flow to the surfaces and an earlier completion of the second stage were observed. The hydrodynamic flow mechanism, driving both stages, is evidenced by nearly linear growth of the surface layer and by mass flow channels extending from the surface layer into the bulk. The final bilayer structure, formed even for the surfaces covered by strongly overlapped copolymers, is indicative of long-range (antisymmetric) surface forces.PACS. 64.75.+g Solubility, segregation, and mixing; phase separation -68.55.-a Thin film structure and morphology