Synthesis of Two-Dimensional Polymers as Ultrathin Membranes for Molecular Sieving

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Micro- and nanoporous materials act as selective barriers and play important roles as energy efficient gas and liquid separation membranes in industrial processes as well for environmental technologies, such as CO2 capture or water treatment. One of the goals in developing new membrane materials is to achieve a high flux as well as a high selectivity, while maintaining a high mechanical stability. Since the flux through a membrane is inversely proportional to the thickness of the selective layer, robust two-dimensional (2D) sheets with thicknesses of one to a few atoms or molecules would be excellent materials for ultrathin high-performance membranes. However, most 2D sheets do not have pores with well-defined dimensions at the molecular scale and it is thus a great challenge to develop homoporous membranes (films with homogenous pores open at both ends) with pore diameters below 3 nm and a thickness of less than 50 nm. In the proposed project, a class of such membranes with pore sizes between 0.25 nm and 2.6 nm will be fabricated from freestanding 2D polymers. It will be explored how stable and flexible they are and if molecules and atoms can be translocated through them. The project aims to establish versatile and reliable interfacial synthesis methodologies (gas-water, surfactant-water and liquid-liquid interfaces) towards highly crystalline 2D polymers with well-defined pores (size, shape, pore-density and functional group at the periphery of the pores), which can be rationally designed on demand at the molecular level. The project will produce macroscopic-sized 2D polymer crystals with a thicknesses from single layer to multilayers. Both in-situ (grazing-incidence wide-angle X-ray scattering) and ex-situ (STM, TEM) techniques will elucidate the structure of the precursors, model monomers, reaction intermediates at the interface, model compounds and 2D polymer crystals. Based on the experimental results together with simulation, correlations between structures of raw materials and final products will be analyzed to deduce the reaction mechanism for the formation of 2D polymers. Furthermore, critical effects of monomer structures, polymerization degrees, domain sizes, grain boundaries, edge structures, pore structures and thickness on the permeance and selectivity of the 2D polymer crystals will be addressed. Key functions for the operation of this intriguing class of 2D polymer crystals as molecular sieves for separation will be explored. As the key achievements, the applicants expect reliable interfacial synthesis methodologies, delineation of structure-property relationships and superior separation performance of the 2D polymer membranes. In the proposed project, the groups of the PIs join forces and combine their proven expertise in 2D materials, polymer chemistry, nanostructure fabrication and membrane science to explore how freestanding homoporous 2D polymers can be created and what their structure-property correlations are.