Film Instability of Amphiphilic Block Copolymer Thin Films Driven by Solvent Annealing and Drying

This paper investigates the film instability and structural evolution of solvent-annealed polystyrene-block-poly(4-vinylpyridine), P(S-b-4VP), as thin films of initially monolayer thickness on bare substrates (SiO_x/Si) and substrates coated with end-grafting PS (PS-SiO_x/Si) under tetrahydrofuran vapor. At an early stage of solvent annealing, merging of nanospheres to form nanocylinders produces film rupture, creating small holes within a continuous layer. Because of the connectivity, the coalescence of incipient holes is driven by 2D pressure relaxation. Upon further iterative solvent annealing and drying, flat islands progressively grow outward at the expense of expanding holes until they contact with the edges of holes to form a network-like cellular structure, and then decay into hemispherical droplets. The capillary driving forces associated with the negative excess interfacial entropy are responsible for the film instability of micelles on PS-SiO_x/Si. In contrast, thin P(S-b-4VP) films on SiO_x/Si reveal only small holes during iterative swelling and drying. The presence of these small holes is ascribed to an incomplete coverage during the merging of nanospheres and the anchoring of P(S-b-4VP) chains on SiO_x/Si. Adsorption in micellar films has three important effects - retarding the motion of micelles, increasing the film stability, and favoring complete wetting.