기체분리용 열전환 고분자 분리막

Abstract

This dissertation is concerned with development of novel polymer membrane materials for gas separation applications, investigation of the transport mechanism and comparison with recently developed other superior materials with respect to their chemical, physical properties as well as gas permeation properties. This dissertation is organized into seven chapters, including introductory chapter where gas separation membranes, gas transport mechanism and recent trend of high performance membrane materials are briefly reviewed. In Chapter 2, the structure, preparation methods and characterizations of thermally rearranged (TR) polymer membranes are introduced. TR-polymer is a kind of microporous material prepared by chain rearrangement of polyimide with ortho-functional groups at solid state. Conversion of aromatic polyimides into highly rigid polybenzazoles at elevated temperature results in the increase of fractional free volume in the polymer matrix up to 30 %. The evolution of microcivities in the polymer membranes helps accelerate mass transport phenomena on sub-nano scale, providing significant technological applications for adsorption, separation and storage. In Chapter 3, a series of copolymer membranes was prepared using polyimide (PI) and hydroxyl containing polyimide (HPI) precursors. Free volume cavities produced during thermal conversion were easily controlled by varying HPI composition in the copolymer. Evidence of thermal conversion was confirmed using spectroscopic and thermogravimetric analysis. O2 permeability of copolymer membranes varied from 0.17 Barrer (1 Barrer = 1?I10-10 cm3 (STP) cm/cm2∙ s∙ cmHg) to 220 Barrer depending on membrane composition without a significant loss in selectivity. The copolymer membranes presented here easily overcome the conventional polymeric upper bound limit, and was also expected to improve the shape properties of the polymer membrane. In Chapter 4, it is investigated on the effect of several imidization methods on the properties of TR-polymer membranes because synthetic methods to prepare precursor polyimide are important for the resultant thermally rearranged (TR) polymer membranes. We synthesized ortho-functional polyimides from 4,4?f-hexafluoroisopropylidene diphthalic anhydrides and 2,2?f-bis(3-amino-4-hydroxyphenyl) hexafluoropropane by three different imidization methods. Acetate-containing polyimide by chemical imidization and further silylation treatment as well as hydroxyl-containing polyimides by thermal and azeotropic imidization are compared with respect to various properties of both precursor polyimides and thermally rearranged polybenzoxazole (TR-PBO) membranes. By using thermogravimetric analysis, free volume measurement, positron annihilation lifetime spectroscopy and gas permeation properties, we confirmed that TR-PBO membranes exhibited high free volume and superior gas transport behaviors. In Chapter 5, a new synthesis route, for the first time, is introduced to prepare thermally rearranged microporous polybenzimidazole (TR-PBI) membranes, exhibiting exceptionally highly permeable characteristics to small gas molecules as well as excellent molecular sieving properties. Generally, common PBIs have very rigid, well-packed structures due to their strong intermolecular interactions, resulting in very low gas permeation properties which prevent PBIs from applying in gas separation. Here, we demonstrate new synthesis route to obtain highly permeable TR-PBI membranes having microporous characters (i.e., high fractional free volume), prepared by the alkaline hydrolysis of polypyrrolone followed by simple thermal treatment. In Chapter 6, as a next step to realize advanced gas separation membranes from the high performance membrane materials, membrane modules were fabricated by means of hollow fiber membranes and composite membranes for application to gas separation, especially air and flue gas separation. Hollow fiber membranes derived from thermally rearranged polymers and composite membranes from carbon molecular sieve materials were successfully prepared as obtaining 1-3 ??m of effective thickness. Carbon composite membranes were obtained by dip-coating of polymer solution followed by pyrolysis of the composite membranes. Membrane modules composed of the composite membranes were tested for the compositions of air and flue gas as feed pressure, pressure ratio, stage-cut and feed compositions were controlled to optimize the membrane process with high recovery ratio. In Chapter 7, the conclusions, evaluation, and directions for further studies regarding highly permeable membrane materials and modules for gas separation are presented.