Nanoporous block copolymer films using highly selective solvents and non-solvent extraction

Abstract

Nanoporous block copolymer thin films are fabricated by selective solvent swelling of the majority phase and subsequent rapid extraction with a miscible non-solvent (water). Selection of the initial solvent provides a facile route to tune the porosity of the films. Poly(butylnorbornene)-block-poly(hydroxyhexafluoroisopropyl norbornene) (BuHFA) is used to generate these porous thin films due to its high T_g (>300 °C) and the selectivity of primary alcohols towards poly(hydroxyhexafluoroisopropyl norbornene) (pHFANB) to enable a relatively environmentally benign process. As the solvent quality for the HFA increases from ethanol to isopropanol to n-butanol, the porosity of the film developed by water extraction increases up to 69%. Aqueous mixtures of ethanol provide an addition handle to tune the porosity between 10 and 54%. These nanoporous films are robust with the porosity nearly unchanged after extended heating at 160 °C. Their elastic moduli are investigated using surface wrinkling and the modulus, E, scales with the film density, ρ, as E ∼ ρ^2.2, which is similar to cellular solids. The nanopores are templated by the self-assembled structure of the block copolymer, so these coatings are transparent despite the high porosity. These thin films act as anti-reflective coatings for glass slides. Spin coating provides a coating on both sides and processing to generate 55% porosity leads to an increase in transmittance from approximately 92% to 99.1% (average for the full range of visible light). A maximum transmittance of 99.8% is found at 523 nm. This methodology is simple and highly tunable; extension to other block copolymer systems is likely possible if sufficient solubility contrast between segments exists.