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

Abstract

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 (PS_4k–SiO_x/Si). On substrates having an attractive surface (SiO_x/Si) or a grafted layer of long PS chains (PS_19k–SiO_x/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 PS_4k–SiO_x/Si, but appeared as spherical micelles on SiO_x/Si and PS_19k–SiO_x/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 SiO_x/Si but stripe-like nanogrooves were induced on PS_4k–SiO_x/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.