Surface relief terraces and self-assembled nanostructures in thin block copolymer films with solvent annealing

We have investigated the temporal evolution of relief terraces and microphase-separated nanodomains in poly(styrene-block-ethylene oxide) P(S-b-EO) on silicon substrates (SiOx/Si) annealed under toluene/butanol vapors. For thin films, solvent annealing causes the formation of relief terraces. With solvent annealing for 12 h, hexagonal arrays of nanospheres of three sizes are dominant over the surface morphology of relief-terraced microstructures with epitaxial packing. The coexistence of nanospheres of three sizes is ascribed to a morphology triggered by the constraint of the film thickness. Upon exposure to toluene/butanol vapors for 16 and 20 h, these nanospheres become metastable and grow further into nanocylinders lying on the surface. During dewetting, the undulations of the local thickness and the nanodomain transitions are inseparable, consequently forming relief terraces comprising multiple layers of lying nanocylinders. The mechanisms and causes of nanodomain transitions of varied routes in solvent-annealed P(S-b-EO) films are analyzed and discussed.