STRUCTURE FORMATION VIA POLYMER DEMIXING IN SPIN-CAST FILMS

The domain structure in thin films of an immiscible polystyrene/poly(m ethyl methacrylate) (PS/PMMA) blend was studied after spin-casting fro m a common solvent. Atomic force microscopy (AFM) combined with select ive dissolution was used to obtain three-dimensional information on th e domain morphology in thin films. Three different common solvents and three different substrate surfaces were studied. Distinct differences in the thin film domain structure and surface topography are observed depending on the substrate surface energy and the solubility of the t wo polymers in the three solvents. The topographic modulation can be e xplained by a different rate of solvent evaporation during spin-coatin g for the two phases. The normal and lateral organization of the phase -separated domains is governed by a complex interplay between preferen tial aggregation of one phase at the substrate and phase segregation i n the film. Additionally, some of the results suggest that a dewetting process may be involved in the domain formation. The structures obtai ned after spin-casting are far from thermodynamic equilibrium. The equ ilibration of the films during annealing depends strongly on the phase morphology, and long-lived metastable configurations are found.