Broad range thickness dependence of gas separation properties for carbon molecular sieve membranes based on a hydroxyl-functionalized microporous polyimide

Abstract

Carbon molecular sieves (CMS) are a promising class of membrane materials that exhibit excellent gas separation properties originating from their well-developed structures containing micropores (<20 Å), ultramicropores (<7 Å) and submicropores (<4 Å). In the first stage of a membrane development process CMS materials are usually characterized in form of thick (50–100 μm), self-standing isotropic membrane films. For practical applications, however, CMS materials need to be converted into thin, selective layers with thicknesses on the order of several micrometers or less. Reduction of the film thickness can often lead to significant differences in the gas separation performance of the CMS materials similar to the well-known deviations from bulk behavior found in glassy polymers (e.g. glass transition temperatures, density, chain dynamics, physical aging rate, etc.). However, despite its practical importance the thickness-dependence of CMS membranes has been rarely studied systematically. Here, we present a detailed study of the gas separation properties of CMS films derived from a promising intrinsically microporous polyimide precursor (6FDA-HTB) over a broad thickness range of 0.55–100 μm, including free standing (10–100 μm) and supported <1 μm samples. This allows us to directly compare the properties of thick, self-standing films with those of thin, supported films representative of practical CMS membranes. Our results indicate a strong but relatively regular reduction of permeability with decreasing film thickness that suggests a similar mechanism of microporosity evolution for thick and thin films. Despite the reduction of permeability, the thin films still possess quite favorable combinations of permeances and selectivities even after 28 days of physical aging, e.g. O₂/N₂ > 8, O₂ permeance ∼20–30 GPU, CO₂/CH₄ > 100, CO₂ permeance ∼150–300 GPU. The presented results are of significant importance for the design of efficient molecularly sieving membranes based on amorphous microporous CMS materials.