Wetting in a phase separating polymer blend film: Quench depth dependence

We have used He-3 nuclear reaction analysis to measure the growth of the we tting layer as a function of immiscibility (quench depth) in blends of deut erated polystyrene and poly(alpha-methylstyrene) undergoing surface-directe d 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 thigh thermal noise and the surface layer grows with a t(1/3) coarsening behavior. For shallower quenches, a logarit hmic 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 tran sition should occur. We also discuss the possibility of a "plating transiti on" extending complete wetting to deeper quenches by comparing the surface field with thermal noise. For the shallowest quench, a critical blend exhib its a t(1/2) behavior. We believe this surface layer growth is driven by th e 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 farce microscopy measurements in each of the different regimes furt her support the above. The surface in the region of t(1/3) growth is initia lly somewhat rougher than that in the regime of logarithmic growth, indicat ing the existence of droplets at the surface.