Significance and Impact: This novel strategy opens up an alternative avenue for preparing microporous polymer membranes for gas separations requiring both high permeability and selectivity
Details: High-performance polymeric membranes for gas separation are attractive for molecular-level separations in industrial-scale chemical, energy and environmental processes. Molecular sieving materials are widely regarded as the next-generation membranes to simultaneously achieve high permeability and selectivity. However, most polymeric molecular sieve membranes are based on a few solution-processable polymers such as polymers of intrinsic microporosity. Here we report an in situ cross-linking strategy for the preparation of polymeric molecular sieve membranes with hierarchical and tailorable porosity. The process uses nonporous polystyrene (PS) membranes as a precursor membrane template. Then, based on the Friedel-Crafts reaction, PS membranes are readily cross-linked in situ to produce hypercross-linked porous polymeric membranes. In the cross-linking process, formaldehyde dimethylacetal is used as a cross-linker and FeCl3 as a catalyst. These membranes demonstrate exceptional performance as molecular sieves with high gas permeabilities and selectivities for smaller gas molecules, such as carbon dioxide and oxygen, over larger molecules such as nitrogen. Hence, these membranes have excellent potential for large-scale gas separations of commercial and environmental relevance. Moreover, this strategy could provide a possible alternative to “classical” methods for the preparation of porous membranes and, in some cases, the only viable synthetic route toward certain membranes.
The Kirkendall effect describes the diffusion of atoms at an interface between two different materials where the flux of atoms proceeds through vacancies rather than direct interchange between atoms. In this mechanism, atoms switch with vacancies creating a flow of atoms in one direction and a flow of vacancies in the opposite direction. During the synthesis of the porous polymer membrane, the outer surface is cross-linked first creating a thin, porous layer that acts as an interface. In order to cross-link the interior of the membrane, there must be a flow of the cross-linking agent through the interface to the interior. At the same time, there is a flow of polystyrene molecules from the interior to the outer surface. Because these diffusion rates are different, there is a net flow of vacancies toward the interface as well as coalescence of vacancies in the interior. The diffusion of the vacancies only arises because of the cross-linking process. Therefore this is called a polymerization-induced Kirkendall mechanism.
Reference: Z.-A. Qiao, S.-H. Chai, K. Nelson, Z. Bi, J. Chen, S. M. Mahurin, X. Zhu, and S. Dai, “Polymeric Molecular Sieve Membranes via In-Situ Cross-Linking of Nonporous Polymer Membrane Templates,” Nature Commun. 2014, 5:3705 doi:10.1038/ncomms4705