Hierarchically-responded assembly of block copolymer thin films with stimuli of varied solvent selectivity

Abstract

Through a combination of micro-contact imprinting and block copolymer self assembly, a highly dewetting process of a symmetric diblock copolymer occurs, leading to hierarchical development of microstructures (microdroplets or microterraces) and nanodomains. The process is driven by the macroscopically hexagonal-ordered circular geometry of the PS-grafted pattern on silicon substrates and by solvent annealing in solvent vapors of varied selectivity. We investigate how relief P(S-b-4VP) microstructures adjust their internal nanostructures and microscopic shape on a patterned substrate in response to solvent vapors of varied selectivity. Whereas solvent annealing in THF vapor causes formation of nanosphere-comprised droplets with a hemispherical cap, solvent annealing in chloroform and acetone vapors leads to the development of terraced stacks of parallel-oriented lamellae. Each lamellar layer within the terraced stacks has the thickness of an entire PS-P4VP/P4VP-PS layer. In contrast, three types of relief microstructures, nanosphere-comprised terraces, nanocylinder-comprised droplets and nanolamella-comprised terraces, are found in a dynamic process under DMF vapor. For DMF-annealed terraces, the stacks comprise PS-P4VP lamellar layers and thus the thickness of each PS-P4VP lamellar layer is effectively quantized by a half thickness of one PS-P4VP/P4VP-PS layer. The variances in nanoscale and microscale structures are due to the adaption of stimuli-responsive polymer materials to surrounding environments under the confinement of the patterned substrate.