(82a) High Aspect Ratio Mixed Matrix Membranes: Advantages and Challenges

Membranes offer attractive alternatives to thermally-driven separations. Asymmetric hollow fiber membranes enable transforming intrinsically useful materials into structures to maximize the productivity achievable in a modular device. Despite the attractiveness of the membrane platform, intrinsic transport properties of pure polymeric membranes are limited by a tradeoff between productivity and selectivity. This tradeoff can be partially overcome by broadening the spectrum of materials to include hybrid composites (commonly referred to as mixed matrix membranes) of polymer and a dispersed phase of either inorganic or carbon materials. These materials offer processing options that combine the attractive, low cost processing approaches that are already available for the manufacture of pure polymer-based membranes. Two primary challenges must be addressed in the development of a mixed matrix membrane: material selection and process optimization. Ideally, polymer and sieve materials can be chosen based on an optimum pairing between permselectivities, availability and processibility. The first step in process optimization involves determination of the proper compositions of organic solvents, inorganic fillers and procedures to compatibilize and form stable ?dopes.? Subsequent processing steps must transform these dopes into high performance asymmetric hollow fibers. Traditionally, mixed matrix membranes used for gas separations have been comprised of zeolite sieves within a polymer matrix. These hybrid materials have been shown to give increased separation efficacy. This fact notwithstanding, such hybrids require achieving high sieve solid volume percents in order to provide properties that approach those of the pure component high performance dispersed sieve phase itself. Recent developments have considered high aspect ratio sieving materials (i.e. flakes or platelets). These materials have a significantly higher aspect ratio (l/d) than traditional zeolites. With l/d values greater than 10 (versus standard zeolites at l/d ~ 1), a higher tortuosity results at any given volume fraction of filler. Therefore, less sieving material is required to achieve high performance. As noted above, the production of mixed matrix membranes utilizing zeolites involves the optimization of processing steps. The key challenges faced in this optimization include not only stable dispersion of the inorganic sieves in the polymer matrix, but also engineering of the polymer-zeolite interface to achieve adequate bonding. Post-spinning adhesion is crucial to achieve the full benefit from the hybrid structure. High aspect ratio, platelet type mixed matrix membranes face these same problems, but with added complications associated with rheological challenges due to the complex shape of the dispersed sieve phases. To fully benefit from the high aspect ratio it is essential that the particles be aligned orthogonal to the radius of the hollow fiber during processing. A discussion of other added challenges (such as exfoliation of the individual platelets) will also be included. Some preliminary results based on model platelets in Ultem® (polyetherimide) matrices will be presented and discussed, as well. Projected benefits from the use of these high aspect ratio fillers will also be discussed in terms of the observed results.