This dissertation is concerned with characterization of thermally rearranged (TR) polymer membranes, which have been investigated for membrane gas separation applications. Gas transport properties of gas separation membrane have been widely studied to investigate and evaluate membrane performances according to the solution-diffusion model. The gas solubility and permeability were dealt with thermally rearranged polymer membranes and the gas diffusivity was determined by the relationship of gas permeability, diffusivity, and solubility.
This dissertation is organized into seven chapters including the introduction chapter of recent progress on microporous polymers as gas separation membranes. Microporous polymers have received much attention for various applications in gas separation, gas storage, and for clean energy resources due to their easy processability for mass production, as well as microporosity for high performance. The new classes of microporous polymers, so-called TR polymers and polymers of intrinsic microporosity (PIMs), have been reviewed. They have been developed by enhancing polymer rigidity to improve microporosity with sufficient free volume sizes. Their rigidity improves separation performance and efficiency with extraordinary gas permeability. Moreover, their solubility in organic solvents allows them to have potential use in large-scale industrial applications.
In Chapter 2, the solution-diffusion model of gas transport through polymer membrane was dealt with in theoretical aspect. The solution-diffusion model has been developed over the past 40 years as the most widely accepted transport model through polymer membranes in dialysis, reverse osmosis, gas transport, and pervaporation. The theoretical explanation from Fick’s first law was discussed into the solution-diffusion model where the permeability coefficient is defined as a product of the diffusivity coefficient and the solubility coefficient.
In Chapter 3, the gas solubility of thermally rearranged polybenzoxazole (TR-PBO) membranes derived from hydroxyl polyimide (HPI) precursors, which was named as TR-α-PBO, was determined for small gas molecules. Sorption isotherms of TR-PBO followed the dual-mode sorption model, which is regarded as a typical sorption model for glassy polymers. The Henry’s law coefficient (kD), Langmuir affinity parameter (b), and Langmuir capacity parameter (C’H) were determined from non-linear fitting for the dual-mode sorption equation. During the thermal rearrangement process, excess free volume in the polymer membrane matrix increased and gas transport property was improved with the increase in gas permeability, diffusivity, and solubility. The relationship between gas permeability and diffusivity of TR-PBO membranes were also studied using the solution-diffusion model.
In Chapter 4, a copolymer of thermally rearranged polybenzoxazole with rigid polyimide (TR-PBO-PI) was introduced and the gas solubility and permeability has been investigated. Copolymer membranes of polyimides and TR-PBO might be desirable to generate efficient gas transport properties as well as to process polymers into fiber or film forms. Gas permeability, diffusivity, and solubility of the precursor polyimide and TR-PBO-PI membranes were investigated to characterize gas transport properties for small gas molecules including H2, O2, N2, CH4, and CO2. Thermal rearrangement process improves the diffusion and sorption coefficients, resulting in an increase in free volume elements.
In Chapter 5, the gas solubility of TR-PBO membranes derived from poly(o-hydroxylamide) (PHA) precursors, which was named as TR-β-PBO, was investigated. TR-β-PBO membranes have been developed to improve H2/CO2 separation properties with high H2 permeability even at high temperature operation conditions. The cavity sizes of TR-β-PBO have been tuned from the other thermal rearrange route of dehydration reaction of poly(o-hydroxylamide)s by thermal reaction. The gas solubility of TR-β-PBO and their precursor membranes was investigated to study on the gas transport properties of TR-β-PBO membranes.
In Chapter 6, the temperature dependence in gas transport property of TR-β-PBO membranes was investigated. The gas permeation, diffusion, and sorption were thermodynamic phenomena and they would increase or decrease by operation temperature. Moreover, TR-β-PBO membranes have been developed for H2/CO2 separation of pre-combustion carbon capture process at the high temperature. Therefore, the actual gas transport property at the operation temperature should be investigated. The gas permeation, diffusion, and sorption properties at the elevated temperature were studied. The gas transport properties followed Arrhenius relationship and the activation energy of permeation, the activation energy of diffusion, and the enthalpy of sorption were determined from temperature dependent gas permeation and sorption properties.
In Chapter 7, the conclusions, evaluation, and directions for further studies were presented regarding the study on gas transport properties of highly permeable membrane materials for gas separation applications. Finally, mixed gas sorption and transport of highly permeable microporous polymers were presented as recommendations for future researches.