Enhancing the Gas Transport Properties of Network Polyimide Membranes by Reducing Cross-Linking Density and Thermal Treatment

Herein, we developed a new diamine monomer termed TA-BOC that features a tert-butoxycarbonyl (t-Boc) protected group. This t-Boc group could shield one of the reactive amine sites present in the commercial tris(4-aminophenyl)amine (TAPA) monomer. Then, this difunctional TA-BOC analogue is employed as a comonomer alongside TAPA and 6FDA to regulate the cross-linking density of polyimide material by varying the molar ratio of TAPA to TA-BOC. Moreover, due to the bulky steric hindrance of the t-Boc group, we attempted to remove the t-Boc group via thermal treatment to create more microporosity within membranes. Therefore, because of the synergistic effect of reduced cross-linking density and heat treatment, the gas separation performance of the prepared membranes is greatly improved. Furthermore, the presence of trifunctional TAPA monomer in the network structures significantly enhances the membrane's resistance to plasticization. For instance, 6F-TA:TA-BOC(1:3)-450 has a high CO2 permeability of 865.3 Barrer and an excellent CO2/CH4 selectivity of 41.8, meanwhile its CO2 plasticization pressures exceeding 35 bar. These values surpass the 2008 Robeson upper bound,, indicating its great potential for efficient gas separation processes. We expect that this study will provide new perspectives for the reasonable design of network polyimide membranes possessing an enhanced gas separation performance and strong plasticization resistance.