Crosslinked Poly(styrene)-block-Poly(2-vinylpyridine) Thin Films as Swellable Templates for Mesostructured Silica and Titania

Traditional approaches to the formation of mesostructured inorganic films from block-copolymer and surfactant templates rely on simultaneous assembly of amphiphilic structure-directing agents and inorganic sol–gel precursor species into ordered hybrid structures. Variations in sample composition and processing conditions have afforded considerable control over the local (nanometer-scale) morphology of the composite mesostructures formed via such routes. However, the ability to manipulate mesophase orientation and longrange ordering in these systems has been relatively limited so far, largely due to restrictions on the timescale and conditions for processing that are imposed by network formation of the inorganic component. Here, we demonstrate an alternative approach to forming mesostructured inorganic thin films that can potentially circumvent these limitations. Pre-organized block-copolymer films are first crosslinked, allowing their morphology to be preserved as they are subsequently infiltrated with sol–gel species to form hybrid inorganic/organic materials. Specifically, thin films of poly(d8-styrene)-blockpoly(2-vinylpyridine) (dPS-b-P2VP) diblock copolymer were thermally annealed to produce a well-ordered mesostructure consisting of dPS cylinders surrounded by a P2VP matrix. The cylinders were oriented parallel to the film plane due to preferential wetting of dPS at the free interface and P2VP at the substrate. As shown schematically in Figure 1, this morphology was fixed by chemical crosslinking of the majority P2VP component, allowing films to be swelled by aqueous solutions with essentially affine deformation of the polymer mesostructure. Sol–gel precursors for silica or titania present in the swelling solution were sequestered into the P2VP component of the polymer, producing composite inorganic/organic replicas of the initial block-copolymer morphology. Subsequent oxidation to remove the block-copolymer template species led to mesoporous inorganic oxide films that closely resembled the original block-copolymer mesostructure. This decoupling of self-assembly from inorganic infiltration provides the possibility of first processing the block-copolymer film to obtain the desired mesostructural ordering, domain alignment, and surface patterning (using a variety of recently reviewed methods) and subsequently forming a mesostructured inorganic replica of the starting template film. Recently, other approaches have been described for the preparation of mesoporous silica from pre-formed films of organic templates. Sol–gel precursors were infused via supercritical CO2 [14] or via the vapor phase into pre-formed films of non-crosslinked structure-directing agents, where they were subsequently condensed to produce ordered silica/organic hybrid materials. These approaches, and the current technique, can be thought of as an extension to thin films of the “true liquid-crystal templating” approach for the formation of bulk mesoporous silica, wherein a pre-formed lyotopic liquid-crystal phase can be used as a template. However, as Nishiyama and co-workers demonstrated, the use of a non-crosslinked organic structure-directing agent allows mesostructural rearrangement to occur until halted by the formation of a percolating silica network, potentially resulting in re-organization or even disorganization of the initial structure. In this key respect, these preparation techniques are similar to previous methods for the production of mesoporous silica films: The ultimate structure depends on a cooperative assembly of the structure-directing agent and the inorganic component, and is therefore not a direct replica of the pre-existing template film. C O M M U N IC A IO N S