Thermal Rearrangement Behavior of Polymer Membranes Controlled by Molecular Architecture

Abstract

The purposes of this dissertation are to understand the thermal rearrangement behaviors of thermally rearranged (TR) polymers and to design molecular architecture to produce gas separation membrane with excellent gas transport properties. This dissertation starts with an introduction that presents advantages of membrane gas separation, operational conditions for obtaining the effective separation performance, and basic characteristics of thermally rearranged polymers, known as a representative microporous polymer membrane material. The dissertation consists of 6 chapters including the main text and conclusions. This dissertation is intended to understand fundamental characteristics of thermal rearrangement processes by studying thermal rearrangement behaviors of amide based TR polymer structures and bismaleimide based TR polymer structures. These structures represent significant structural characteristics such as low thermal conversion temperatures with high glass transition temperatures, and high crosslinking density respectively. Therefore, the dissertation enables comprehensive understanding on how glass-rubber transition temperature and crosslinking configurations affect the thermal rearrangement process of TR polymers. In Chapter 2, the mixed gas permeation properties of TR polymers were investigated to provide invaluable insights into practical applications as well as physical properties of the TR membrane materials. The plasticization tests on binary mixture of CO2 and CH4 demonstrated that both cross-linked and liner type of TR polymers exhibited strong resistance against plasticization. In addition, the H2/CO2 permeation tests at the elevated temperatures revealed that both the H2 permeability and H2/CO2 selectivities was gradually improved at the higher temperatures. However, the mixed gas experiments revealed that permeabilities of H2 were compromised by competitive sorption with CO2. Finally, mixed gas experiments on the hollow fiber membrane demonstrated that separation outcomes in the practical separation process are largely varied depending on employed operating conditions. In Chapter 3, rigidly structured poly(o-hydroxy amide)s were prepared and their properties were controlled by introducing flexible ether linkage. Since the TR-β polymer with rigid biphenyl structure demonstrated a high glass transition temperature and low thermal cyclization temperature, the glass-rubber transition was occurred during the thermal conversion reaction. As a result, two distinct thermal rearrangement behaviors were observed before and after the glass-rubber transition. The thermal cyclization before glass transition leads to chemical transformation to chemically rigid structure without segmental rearrangement of the polymer chains, resulting in improvements in the membrane gas selectivities. The thermal cyclization after the glass transition, on the other hand, improved the gas permeabilities through segmental rearrangement of the polymer chains. The flexible biphenyl ether groups, introduced into the polyamide polymer through copolymerization, could change both thermal cyclization temperature and glass transition temperature while improving the processability. The introduction of small portion of the flexible group helped to improve selectivities by accelerating the thermal conversion before reaching the glass transition temperature. As the proportion of the flexible group increased, the amount of the thermal cyclization reaction after Tg became dominant, and the gas permeabilities were improved. In Chapter 4, the thermodynamic properties and gas transport properties of biphenyl based TR-β polymers were examined by changing the structures of acid chloride moieties. As a result, a significant portion of the thermal conversion occurred before the glass transition temperature in a rigidly structured membrane, while the membranes with flexible structures exhibited the substantial thermal cyclization reactions after the glass transition temperature. Based on these results, non-TR-able diamine structures were introduced through copolymerization to observe the thermal behavior and gas transport properties. As a result, the membranes incorporating the rigid diamine structures demonstrated significant enhancements in selectivities. However, the membrane incorporating relatively flexible diamine exhibited increase in permeabilities. In Chapter 5, bismaleimide based polymers having crosslinked structure at a high density were designed and demonstrated excellent gas permeabilities. The crosslinked bismaleimide precursor membranes were simply prepared by thermally induced crosslinking of monomer building blocks during membrane formation. The thermal rearrangement of bismaleimide precursor generated the microporous structure with an extremely increased surface area. In addition, bismaleimide based membranes could be easily applied to other polymer matrix to form semi-interpenetrating networks through in-situ thermal crosslinking. Therefore, in this thesis, extensive studies on the effect of various structural characteristics on thermal conversion and gas permeation performance are presented. Based on these results, the materials with high gas selectivities were developed by devising a new control method on thermal rearrangement process. In addition, a new class of TR polymers with highly enhanced permeabilities were designed through the thermal rearrangement process under the highly crosslinked structures. Therefore, it can be concluded that the desired performance could be obtained by controlling the thermal cyclization reaction according to the rigidity of each structure.