(663e) Free Volume Manipulation of a 6FDA-Hab Polyimide Using a Solid-State Protection/Deprotection Strategy

Polymer membranes have shown great promise for applications in gas separations as they are energy-efficient and easily processable. In order to be suitable for the gas separation industry, membrane materials should be solution-processable, as well as highly permeable and selective. Since transport in polymer membranes is largely governed by free volume and free volume distribution, it has been shown in recent years that polymers with rigid backbones and contorted structures possess both high permeability and selectivity due to their inefficient chain packing that leads to the formation of larger free volume elements. However, free volume elements are formed via a “bottom-up” method in which careful polymer chemistry design is required, which can lead to limited opportunities to rationally and selectively manipulate free volume without changing polymer chemistry.

In this study, we report a free volume modification (FVM) method through the use of solid-state deprotection chemistries. Tert-butoxycarbonyl (t-BOC), a common chemical protecting group, was appended onto a polyimide consisting of 2,2’-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3’-dihydroxy-4,4’-diamino-biphenyl (HAB). This polymer (6FDA-HAB-t-BOC) was formed into a self-standing film, after which, different thermal treatments were performed to selectively remove t-BOC. However, despite performing thermal treatments well below the glass transition temperature of 6FDA-HAB (~300 °C), this particular FVM approach only produced slight changes relative to the unprotected polyimide in terms of polymer density, fractional free volume, average free volume element size, and gas transport properties. While these findings suggest that thermal deprotection of functional groups in glassy polymer films can be used to selectively manipulate free volume, as well as control gas transport performance, more robust systems than linear polyimides are required to preserve free volume elements that are generated via this FVM method. Additional techniques for FVM are also explored for future studies, including UV irradiation and cross-linking of polymer backbones.