Enhancing the\nGas Transport Properties of Network\nPolyimide Membranes by Reducing Cross-Linking Density and Thermal\nTreatment

Herein, we developed a new diamine monomer termed TA-BOC\nthat features\na <i>tert</i>-butoxycarbonyl (<i>t</i>-Boc) protected\ngroup. This <i>t</i>-Boc group could shield one of the reactive\namine sites present in the commercial tris(4-aminophenyl)amine (TAPA)\nmonomer. Then, this difunctional TA-BOC analogue is employed as a\ncomonomer alongside TAPA and 6FDA to regulate the cross-linking density\nof polyimide material by varying the molar ratio of TAPA to TA-BOC.\nMoreover, due to the bulky steric hindrance of the <i>t</i>-Boc group, we attempted to remove the <i>t</i>-Boc group\nvia thermal treatment to create more microporosity within membranes.\nTherefore, because of the synergistic effect of reduced cross-linking\ndensity and heat treatment, the gas separation performance of the\nprepared membranes is greatly improved. Furthermore, the presence\nof trifunctional TAPA monomer in the network structures significantly\nenhances the membrane’s resistance to plasticization. For instance,\n6F-TA:TA-BOC(1:3)-450 has a high CO<sub>2</sub> permeability of 865.3\nBarrer and an excellent CO<sub>2</sub>/CH<sub>4</sub> selectivity\nof 41.8, meanwhile its CO<sub>2</sub> plasticization pressures exceeding\n35 bar. These values surpass the 2008 Robeson upper bound,, indicating\nits great potential for efficient gas separation processes. We expect\nthat this study will provide new perspectives for the reasonable design\nof network polyimide membranes possessing an enhanced gas separation\nperformance and strong plasticization resistance.