The synthesis of a polymer that combines processability of plastics with extreme rigidity of cross-linked organic networks is highly attractive for molecular sieving applications. However, cross-linked networks are typically insoluble or infusible, preventing them from being processed as plastics. Therefore, there is a need to identify routes to make cross-linked microporous materials that can be processed like plastics without suffering from the collapse of inner pore structures. One possibility is to identify a crystal constructed of monomers held by van der Waals interactions, which can be melted and covalently fused by thermosetting polymerization. The monomers have to be designed such that the cross-linking occurs in an orderly manner to generate porosity defined by the topology of the linkers.
In our recent paper "A solution-processable and ultra-permeable conjugated microporous thermoset for selective hydrogen separation", we designed a precursor molecule, which sublimes, melts and polymerizes in sequence during heating, therefore the precursor in gas or/and liquid phases was processed into various shapes and morphologies before polymerization. In this way, conjugated microporous thermoset (CMT) based on strong aryl-aryl bonds were prepared into patterned 3D monoliths, ultrathin 2D sheets and 1D nanotubes via a simple heating process (Scheme 1). CMT has an extremely narrow pore size distribution centered at ~0.4 nm. This is uncommon, especially when compared to conjugated microporous polymers synthesized in solvents with noble metal catalysts under stirred conditions. In our strategy, the endogenous liquid-state polymerization (that is, without the presence of solvents, initiators, or catalysts) was able to achieve a homogeneous porous structure, because precursor molecules were polymerized in the isotropic liquid state, without randomization effects of solvents and catalysts.
Scheme 1. Schematic illustration of the thermosetting process to make 3D, 2D, and 1D CMT, depending upon the substrates.
Monolayer CMT sheets were synthesized at gram-scale with controllable thickness. More importantly, the obtained ultrathin sheets are highly dispersible in common organic solvents, thus allowing them to be processed into large-area molecular sieve membranes with controllable thickness by simple filtration. With contributions from both intrinsic microporosity and interlayer spacing, CMT membranes exhibit ultrahigh H2 gas permeability with good selectivity and excellent long-term stability at high temperatures. The combined processability, structural rigidity, semiconductivity, microporousity and easy feasibility make this polymeric membrane promising for not only large-scale gas separation but also ionic nanofiltration. We are now focusing on designing new thermoset structures and their applications in energy storage devices.