Selective swelling induced pore generation of amphiphilic block copolymers: The role of swelling agents

Swelling of block copolymers by selective solvents has emerged as an extremely simple and efficient process to produce nanoporous materials with well-controlled porosities. However, the role of the swelling agents in this pore-making process remains to be elucidated. Here we investigate the evolution of morphology, thickness, and surface chemistry of thin films of polystyrene-block-poly (2-vinyl pyridine) (PS-b-P2VP) soaked in a series of alcohols with changing carbon atoms and hydroxyl groups in their molecules. It is found that, in addition to a strong affinity to the dispersed P2VP microdomains, the swelling agents should also have a moderate swelling effect to PS to allow appropriate plastic deformation of the PS matrix. Monohydric alcohols with longer aliphatic chains exhibit stronger ability to induce the pore formation and a remarkable increase in film thickness is associated with the pore formation. High-carbon alcohols including n-propanol, n-butanol, and n-hexanol produce cylindrical micelles upon prolonged exposure for their strong affinity toward the PS matrix. In contrast, methanol and polyhydric alcohols including glycol and glycerol show very limited effect to swell the copolymer films as their affinity to the PS matrix is low; however, they also evidently induce the surface segregation of P2VP blocks.