Orientation Control of the Microphase-separated Nanostructures of Block Copolymers on Polyimide Substrates

This study aimed to form a perpendicularly orientated lamellar structure with polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) by its microphase separation on a polyimide substrate. Eight types of polyimides were prepared from different combinations of monomers and tested as the bottom layer for the microphase separation of PS-b-PMMA. The surface free energies of those polyimide substrates were evaluated by the Owens-Wendt method. Thin films of PS-b-PMMA were prepared on the polyimide substrates and the self-assembly was induced by thermal annealing, and the surface architecture was observed by atomic force microscopy. In the tested polyimide substrates, only one from 2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl]propane dianhydride (tetracarboxylic dianhydride) and 1,12-bis (4-aminophenoxy) dodecane (diamine) showed a perpendicularly oriented lamellar structure. This polyimide substrate shows one of the smallest polar components in its surface free energy. Relatively large domain size and long correlation length in the PS-b-PMMA layer were obtained by optimizing the conditions for fabrication of the polyimide substrate, which were prepared by casting the polyimide onto a silicon wafer, followed by thermal annealing. These results suggest that the combination of one monomer with relatively large molecular weight, which will result in a low density of imide groups, and the other monomer with long alkyl chains, which will reduce the polarity of the resulting polyimide, contributes to providing a perpendicular orientation.