A simple method for creating nanoporous block-copolymer thin films

Abstract

We introduce a simple method to create block copolymer films with controlled porosity. We show that the pore structure can be varied over a broad range of length scales not obtainable in homopolymer blend films. The morphology is a random two phase kinetically trapped structure that is not limited by the equilibrium block copolymer structure. The morphology is obtained through blending homopolymer poly(4-vinylpyridine) (P4VP) with block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and then removing the homopolymer P4VP by washing with ethanol. The structure obtained prior to washing (which templates the nanoporous structure) is stabilized in the kinetically trapped morphology during spincoating and is not obtainable from either homopolymer blends or the pure block copolymer. When PS/P4VP blend solutions in tetrahydrofuran were spincoated at 25% relative humidity, continuous films with raised P4VP nanodomains were formed due to a preferential affinity of the spinning solvent for polystyrene. In a similar manner, when PS-b-P4VP/P4VP block copolymer/homopolymer solutions were spincoated, the P4VP homopolymer was solubilized in the P4VP block domains during spincoating, suppressing macro-phase separation. The film morphology is generated at the air surface and then propagates through the film, resulting in P4VP nanodomains oriented vertically to the substrate. In the resulting films, the size of P4VP nanodomains were varied by increasing the amount of P4VP homopolymer. The subsequent extraction of P4VP homopolymer from the PS-b-P4VP/P4VP blend films in ethanol resulted in nanopores with a distribution of length scales. The morphology of these materials makes the films potentially suitable for a range of applications such as anti-reflective coatings, nanoporous membranes and low-k materials. An illustrative example of an anti-reflective coating will be presented.