Design and synthesis of novel polymers of intrinsic microporosity for gas separations

Abstract

Polymers of Intrinsic Microporosity (PIMs) are a new class of microporous material combining excellent solution processability with great structural diversity. The microporosity of PIMs derives from the inefficient packing of rigid and contorted polymeric chains. PIM membranes show great potential in gas separations (e.g., CO2/CH4, CO2/N2, O2/N2, H2/N2). However, they suffer from a well-defined tradeoff relationship between gas permeability and selectivity. Novel PIMs were designed and synthesised to investigate structure-property relationships and to provide a guideline for designing a new membrane material, which can provide a good balance of gas permeability and selectivity.

In the first part of this thesis, two fluorinated benzotriptycene polybenzodioxin polymers were prepared. These benzotriptycene-based PIMs exhibit ultra-high permeability with remarkable selectivity for carbon capture (CO2/N2) and natural gas purification (CO2/CH4), and defined the new upper bounds for these gas pairs together with four other benzotriptycene PIMs. In particular, the fluorinated polymers showed a better performance for CO2/CH4 and He/CH4 than alkyl substituted benzotriptycene based PIMs due to their higher solubility selectivity resulting from the low CH4 sorption in the fluorinated polymers.

In the second part, a fluorinated and non-fluorinated benzomethanoanthracene PIMs (PIM OCF and PIM-OCP) were designed and synthesized in which fluorine atoms illustrated a minor effect on polymer microstructure confirmed by molecular simulation. The improved CO2/CH4 solubility selectivity exclusively derives from the inclusion of the fluorocarbon unit. The perfluoroalkyl chain substituent was also observed to reduce the CH4 sorption and improve the solubility selectivities of CO2/CH4, He/CH4 and H2/CH4, although the reduced size selectivity of perfluoroalkyl chain substituted polybenzodioxin polymer and low surface area of the perfluoroalkyl chain substituted Tröger’s base polymer limit their potential applications.

In the last chapter, dibenzeno-6,13-methanopentacene (DBMP), a bridged bicyclic structure with a bulkier structure than that of triptycene, was utilized for PIM synthesis. The membrane of the homopolymer (PIM-DBMP) proved too fragile for membrane fabrication. Therefore, the potential of DBMP for gas separations was investigated using a series of copolymers composed of DBMP and 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI). The gas permeability of the copolymers demonstrated that the introduction of DBMP improves the gas permeability and selectivity simultaneously, overcoming the challenging tradeoff relationship. PIM-DBMP was further modified via amidoxime functionalisation, which was anticipated to improve the membrane’s mechanical properties. As expected, a robust membrane of amidoxime-PIM-DBMP was achieved which showed promising gas separation performance.