Synthesis and Characterization of High Performance Polymers for Gas Separation Membranes
This dissertation focuses on the synthesis and characterization of high performance polymers, especially polyimides, polybenzoxazoles and polybenzimidazoles for gas separation applications. An abundance of monomers and novel polymers were synthesized and fabricated into membranes.
Thermally rearranged polybenzoxazoles and their precursor polyimides were systematically studied with regard to size of pendant functional groups, thermal rearrangement conversion, and relationship of backbone structure/gas transport properties. 3,3'-Diamino-4,4'-dihydroxybiphenyl was synthesized using an economical route. Meta and para oriented polyimides with different ortho-functionality were synthesized and these polymers were thermally rearranged into polybenzoxazoles. The polar hydroxyl functional groups on the polyimide backbone diminished the meta/para isomer effect of the permeability coefficients of the polymers and only a small difference between meta- and para-oriented polyhydroxyimides in permeability coefficients was observed. The TR polybenzoxazoles derived from meta/para-oriented isomeric polyimides with ortho functionality had similar gas separation properties, especially for CO2/CH4 separation, and it is hypothesized that this is due to a lack of intersegmental mobility distinction between the two isomeric TR polymers. The TR polymers derived from the polyimides with acetate ortho-functional groups had significantly better gas separation properties than ones derived from the precursor with hydroxyl ortho-functional groups.
Polybenzimidazoles were also investigated for use as gas separation membranes. Polybenzimidazoles are some of the most thermally stable polymers. However, commercial polybenzimidazoles do not have good solubility in common solvents. The solubility issue was solved by incorporating sulfonyl linkages into the polybenzimidazole backbone using a 3,3',4,4'-tetraaminodiphenylsulfone (TADPS) monomer. 3,3',4,4'-Tetraaminodiphenylsulfone was synthesized by a novel route with higher overall yield and less steps than the traditional synthetic method. The TADPS based polybenzimidazoles also demonstrated better thermal stability than commercial polybenzimidazole. The meta/para oriented isomer effect on gas transport properties is discussed. TADPS-based polybenzimidazoles exhibited H2/CO2 gas separation properties near or surpassing the upper bound with H2 permeabilities from 3.6 to 5.7 Barrer and selectivities from 10.1 to 32.2 at 35 °C.