Ya-Sen Sun

Micellar transitions in solvent-annealed thin films of an amphiphilic block copolymer controlled with tunable surface fields.

We investigated the response of symmetric poly(styrene-block-4vinylpyridine) P(S-b-4VP) diblock copolymer micelles to surface fields of variable strength at free surfaces and substrate interfaces when the micelles as spun were subjected to solvent annealing. Free surface interactions were controlled with solvent annealing in solvents of varied selectivity. On exposure to vapors of a solvent strongly selective for PS, the micelles retained their spherical shape but grew into cylindrical micelles or lamellar nanostructures via fusion on exposure to slightly selective or neutral solvent vapors. Giant 2D disks that completely wetted PS-grafted substrates resulted when spherical micelles were exposed to vapors of a highly selective solvent for P4VP. The interfacial interactions were controlled through subjecting them to UV/ozone (UVO) substrates initially coated with an end-grafted layer of short PS chains, with which the grafted PS chains became oxidized, degraded, or totally removed through UVO treatment for a controlled duration. When thin films were annealed in vapors of THF, the structural transition from spherical to cylindrical micelles depended on the interfacial field. On applying selective UVO exposure of optimal duration, we fabricated a substrate with two interfacial chemistries that promoted varied micellar species (spherical and cylindrical micelles) with a sharp boundary developed within thin films through solvent annealing for a controlled duration.

Kinetically controlled self-assembly of monolayered micelle films of P(S-b-4VP) on bare and PS-grafted substrates†

We investigated the temporal evolution of micellar nanostructures in solvent-annealed poly(styrene-block-4-vinylpyridine) P(S-b-4VP) monolayer films on bare and PS-grafted substrates under tetrahydrofuran (THF) vapor. The size of end-grafted PS homopolymers on Si substrates has a large effect on the kinetics of transitions between spherical and cylindrical micelles at the early and intermediate stages of solvent annealing. At these stages, cylindrical micelles developing via micellar merger were dominant over spherical micelles growing via micellar splitting on substrates grafted with a layer of short PS brushes (PS4k–SiOx/Si). On substrates having an attractive surface (SiOx/Si) or a grafted layer of long PS chains (PS19k–SiOx/Si), the dominant surface morphology was inverted through the dominance of micellar splitting. Upon solvent annealing for a given interval in THF, micellar structures existed as a single morphology of cylindrical micelles on PS4k–SiOx/Si, but appeared as spherical micelles on SiOx/Si and PS19k–SiOx/Si. On immersion of these micelles in ethanol to give rise to opening of the micelles, dimple-like nanopores in hexagonal arrays were induced on SiOx/Si but stripe-like nanogrooves were induced on PS4k–SiOx/Si. We demonstrate a novel method of kinetically controlled hierarchical self-assembly of P(S-b-4VP) micellar species. On combining the chemical micropatterning of the grafted layer of PS brushes with UV/ozone etching, we prepared P(S-b-4VP) films that have dimple-like nanopores and stripe-like nanogrooves coexisting with a micro-scale order within the same monolayer.