Low cost, scaleable method for producing 2-D polymer layers

20 Feb 2017. NUS chemists have developed a method to make two-dimensional (2D) polymers with high energy storage performance for batteries.

Researchers have developed a low cost, highly scaleable method to produce thin films of polymeric material. This method uses heat to start the chemical reactions and does not need solvents or catalysts. Chemical reactions using solvents or catalysts tend to be more randomised. The polymers produced have uniform pore sizes. They showed excellent energy storage performance when used in sodium ion batteries.

The discovery of graphene, the well-known 2D sheet of sp2-hybridised carbon, has stimulated interest in the organic synthesis of π-conjugated 2D polymers. However, constructing stable 2D conjugated polymers with strong linkages, such as the carbon-carbon (C-C) bond, presents complex challenges in synthetic chemistry. This is due to the characteristic of these coupling reactions that once the bonds are formed, “repair” is not allowed (i.e. the reaction is not reversible).

Prof Kian Ping LOH and his Ph.D. student LIU Wei from the Department of Chemistry, NUS, applied an ingenious strategy based on solid-state polymerisation to produce 2D polymers using C-C coupling reactions. Solid-state polymerisation allows the monomers (basic molecular unit of polymer) to couple and extend in space from an existing solid. This research breakthrough is achieved in collaboration with Prof Su Ying QUEK from the Department of Physics, NUS. She used molecular dynamics simulation to provide the team with a better understanding of the molecular constraints favouring the polymerisation of 2D monomers.

The researchers discovered that applying heat to a set of carefully designed, flat monomers which are pre-packed in a certain way forms a 2D crystalline polymer. Ultrathin polymeric sheets (micrometre-sized sheets of nanometre thickness) can be easily peeled from it. This method employing solid-state polymerisation offers many advantages over wet chemistry methods that use noble metal catalysts. The process involved only thermal annealing without the use of solvents, initiators or catalysts. It is also low cost and highly scaleable. This opens up a new way to produce crystalline 2D conjugated materials for various applications.

The polymeric sheets produced have very narrow pore size distribution in the range of 4 to 6 angstroms, depending on the packing of the monomers. This is very useful for ion storage and molecular separation applications. When used as the anode material in sodium ion batteries, the polymeric sheets exhibit unprecedented cycle stability and rate capability in an ambient temperature setting.

This solid-state polymerisation method can potentially be applied to other classes of halogenated monomers to produce polymer sheets for possible applications in rechargeable batteries and supercapacitors.

Figure shows a 2D polymer created via solid-state polymerisation of pre-arranged monomers in their crystalline state.

References

1. Liu W; Luo X; Bao Y; Liu YP; Ning G; Ibrahim A; Li Lj; Nai CT; Hu ZG; Zhao D; Liu B; Quek SY*; Loh KP*, “A two-dimensional conjugated aromatic polymer via C–C coupling reaction” NATURE CHEMISTRY DOI: 10.1038/nchem.2696 Published online: 2017.

2. M Ebrahimi*; F Rosei*, “Organic analogues of graphene” NATURE, doi:10.1038/nature21503 Published online: 2017.