Influence of Aliphatic and Aromatic Fluorine Groups on the Gas Permeability and Morphology of Fluorinated Polyimides

Partially fluorinated polymers show exceptional separation performance for a large variety of gas pairs, in part due to inefficient packing caused by the comparatively large size of fluorine versus hydrogen atoms. While the incorporation of aliphatic fluorine groups in the polymer backbone is relatively commonplace (e.g., CF3 groups as a bulky side group), there are very few studies on the impact of aromatic fluorine groups on gas transport. Here, we present an analysis of two pairs of structurally-analogous, partially fluorinated polyimides – one pair altered via aliphatic fluorination, and the other via aromatic fluorination. Challenges related to the synthesis of highly fluorinated polyimides are discussed and the effects of differing fluorine group connectivity on solid-state morphology are related to observed changes in permeability and permselectivity, diffusion and diffusion selectivity, and their energetics. Developing an understanding of structure-property relationships related to fluorination can allow for improved rational design of high-performance polymers in the future.