Epitaxial growth of a surfactant-silica mesostructure on oriented polyimide films

In this paper, I review our past works on macroscopic control of in-plane arrangement of mesopores in mesoporous silica films. Mesostructured silicas are grown epitaxially on polyimide coatings with surface anisotropy to provide silica films with fully controlled mesoporous structures over the whole of the substrate surface. The anisotropic polyimide coatings are formed by a mechanical rubbing post-treatment of isotropic polyimide coatings as well as by thermal imidation of a Langmuir–Blodgett (LB) film of the precursor polyamic acid. The chemical anisotropy of the polyimide underlayer is shown to be more important for the epitaxial growth than the morphological anisotropy. Not only the uniaxially aligned two-dimensional (2D) hexagonal mesostructure but also the single-crystalline three-dimensional (3D) hexagonal mesostructure, in which the in-plane arrangement of the spherical mesopores is entirely controlled, are formed on the anisotropic polyimide. The epitaxial growth takes place hydrothermally through heterogeneous nucleation and growth of mesostructured silica seeds on the anisotropic polymer surfaces. It is considered that the hydrophobic interactions between the anisotropic polymer chains and the alkyl chains of the surfactants cause the observed epitaxial growth. It is shown that, in contrast to the conventional epitaxial growth of crystals, the observed epitaxial growth of silica mesostructures does not require molecular-scaled size matching with the chemical structure of the underlying polyimide.