We have used 3He nuclear reaction analysis to measure the growth of the wetting layer as a function of immiscibility (quench depth) in blends of deuterated polystyrene and poly(alpha-methylstyrene) undergoing surface-directed spinodal decomposition. We are able to identify three different laws for the surface layer growth with time t. For the deepest quenches, the forces driving phase separation dominate (high thermal noise) and the surface layer grows with a t(1/3) coarsening behavior. For shallower quenches, a logarithmic behavior is observed, indicative of a low noise system. The crossover from logarithmic growth to t(1/3) behavior is close to where a wetting transition should occur. We also discuss the possibility of a "plating transition" extending complete wetting to deeper quenches by comparing the surface field with thermal noise. For the shallowest quench, a critical blend exhibits a t(1/2) behavior. We believe this surface layer growth is driven by the curvature of domains at the surface and shows how the wetting layer forms in the absence of thermal noise. This suggestion is reinforced by a slower growth at later times, indicating that the surface domains have coalesced. Atomic force microscopy measurements in each of the different regimes further support the above. The surface in the region of t(1/3) growth is initially somewhat rougher than that in the regime of logarithmic growth, indicating the existence of droplets at the surface.

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

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.

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Blends of atactic and syndiotactic polystyrene have a single composition dependent glass transition temperature. This does not confirm the miscibility of both polymers, however, unambiguously because there is only a small difference of 10°C in the glass transition temperatures of both neat polymers. Diffusion measurements of syndiotactic polystyrene and deuterated atactic polystyrene using the nuclear reaction D(3He,a)p confirm at least partial miscibility of both components. The average diffusion coefficient at 190°C is 2.3 t 0.2 x cm2/s. 0 1997, Huthig 8c