Template Cross-Linking Effects on Morphologies of Swellable Block Copolymer and Mesostructured Silica Thin Films

Abstract

Mesostructured silica films were formed from cross-linked poly(d8-styrene-block-2-vinylpyridine) (dPS-b-P2VP) diblock copolymer template films by swelling with silica precursor solutions. The mesoscopic and macroscopic morphologies of the resulting materials depended strongly on the extent of P2VP cross-linking. Without cross-linking, extensive rearrangement of the initial 2D hexagonal arrangement of dPS cylinders took place during swelling, leading to wormlike mesostructures. At low extents of template cross-linking, osmotic stresses led to a surface-wrinkling instability with a morphology directed by the anisotropic mechanical properties of the swelled block copolymer mesostructure. At higher degrees of P2VP cross-linking, wrinkling was suppressed, and the composite silica/polymer mesostructures corresponded to essentially affine expansions of the initial template structures normal to the film plane. At low extents of P2VP cross-linking, the mesostructures of the hybrid block copolymer/silica films were preserved during oxidation and removal of the block copolymer template species, leading to stable mesoporous silica films. At high extents of P2VP cross-linking, however, insufficient amounts of silica were incorporated to form stable porous inorganic materials, leading to mesostructural collapse. An intermediate level of cross-linking allowed the preparation of stable mesoporous silica films with structures that closely resembled those of the initial template films.