Gas diffusion in polymers occurs through free volume elements, which are the nanoscopic openings within polymer media devoid of electron density. The average size and shape of these free volume elements helps determine gas diffusion rates, and their size distribution helps determine gas selectivity. Ideally, a polymer matrix should be formed with precisely controlled free volume elements that promote fast diffusion of certain molecules and a barrier to diffusion for larger molecules. Unfortunately, the disordered nature of amorphous polymers discourages facile design and control of isoporous free volume elements, and well-ordered, highly crystalline polymers are generally impermeable to gases. A major challenge is developing strategies to create ordered free volume elements in entropically disordered, yet highly permeable, amorphous polymers. To address this pressing need, this project will develop chemistries to form free volume elements in polymers by post- polymerization modification of solution-cast polymer films.
This proposal is inspired from the seminal work on chemically amplified photoresists developed by Ito, Willson, and Frechet,1-2 who discovered that poly(hydroxystyrene) derivatives with irradiatively photolabile chemical functionalities could easily be treated with UV light to induce acid-catalyzed chemical reactions. These so-called “t-BOC resists”, named for UV active tert-butyl carbonates, were developed by IBM as a key technological advancement for dynamic random access memory (RAM) devices used in computers. The t-BOC system is compatible with an extraordinarily diverse range of hydroxyl-functional polymers, and, surprisingly, acidolysis reactions with these systems can occur deep within the glassy state for a variety of amorphous polymers. This reactivity has profound implications for the formation of free volume elements: chemical bonds can be broken and labile chemical moieties, referred to hereafter as “porogens”, can diffuse as gaseous products from a polymer matrix.
Essential to this project is the identification of high glass transition temperature polymers with phenolic moieties, and polyimides and polymers of intrinsic microporosity (PIMs) are uniquely suited to meet these requirements. Initial studies will focus on appending porogens (e.g., t-BOC) onto the backbones of these polymers, removing them using UV light, and characterizing them for pure-gas diffusion rates for a variety of molecular diluents including CO2, N2, and CH4. Ultimately, if pore architectures can be templated into inexpensive and easily processable amorphous polymers, this strategy could be used for designing membranes and adsorbents with predictive property sets for a variety of applications related to CO2 capture and separations in the petrochemical and chemical industries.