This dissertation discusses the effect of synthesis route and ortho-position group on the thermal and transport properties of thermally rearranged polymers. Thermally rearranged polymers are polybenzoxazoles formed via the solid state rearrangement of ortho-functional polyimides. In this study, polymers were derived from 3,3'-dihydroxy-4,4'-diamino-biphenyl and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB-6FDA). These HAB-6FDA polymers were synthesized using chemical and thermal imidization, and hydroxyl, acetate, propanoate, or pivalate ortho-position groups were considered. In these polymers, gas permeability increases as a function of conversion for all samples. The polyimide synthesis route does not affect the thermal or transport properties. However, the precursor ortho-position group strongly influences the thermal and transport properties of TR polymers. Additionally, it was determined that an increase in gas diffusivity was the primary cause of increased permeability as a function of thermal rearrangement. / text

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. / Ph. D.

Thermally rearranged (TR) polymers have outstanding gas separation properties, but are limited in their industrial application due to being mechanically brittle. A series of low volume fraction of a poly(arylene ether sulfone) (PAES) block was introduced into the TR precursor polyhydroxyimide (PI) chain to improve mechanical properties without compromising gas transport properties. The multiblock copolyhydroxyimide incorporated the PAES in systematically varied amounts and copolymerized it with 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 3,3’-dihydroxy-4,4’-diaminobiphenyl. Before thermal rearrangement, the PI-co-PAES precursors exhibited much more improved mechanical properties (tensile stress and strain at break) than those of homo polyimide precursor. After thermal rearrangement, tensile stress and strain at break of all TR copolymers decreased comparing to their corresponding precursors, but improved comparing to the homo TR polymer. Poly(phenylene oxide) (PPO) based copolymers (Chapter 4) and polymer blends (Chapter 5) were also studied for use as gas separation membranes. The polymer materials were cast into films, then crosslinked in the solid state with UV light. The ketone and benzylic methyl groups crosslinked upon exposure to UV light. For the study of PPO copolymers, copolymers were prepared by polycondensation of a difunctional PPO oligomer with 4,4’-difluorobenzophenone or 1,3-bis(4-fluorobenzoyl)benzene respectively. This study offers a means for fabrication of membrane films, fibers or composites, as well as tuning of gas transport properties through crosslinking in the solid state. While for the study of PPO polymer blends, PPO polymers with Mn’s from 2000-22,000 g/mole were synthesized and blended with a poly(arylene ether ketone) derived from bisphenol A and difluorobenzophenone (BPA-PAEK). The crosslinked blends had improved gas selectivities over their linear counterparts. The 90/10 wt/wt 22k PPO/BPA PAEK crosslinked blends gained the most O2/N2 selectivity and maintained a high permeability. / Ph. D. / Membrane gas separation has become a fast-growing industrial technology because of its many advantages over traditional separation technologies including low capital cost and energy consumption relative to thermal distillation methods, and higher operational flexibility. Currently, membrane gas separation is widely used in processing raw natural gas to meet certain specifications before delivery to pipelines. Researches in this dissertation mainly focus on synthesis and characterization of membrane gas separation materials. In chapter 3, one type of thermally rearranged copolymer membrane was obtained, which could be potentially used in industrial field due to its improved mechanical property. In chapter 4 and 5, a series of poly(phenylene oxide) based copolymers and blends were studied. After UV-crosslinking reaction, poly(phenylene oxide) membranes showed improved gas selectivities over their linear counterparts.

This dissertation focuses on the synthesis and characterization of high performance polymers, specifically polybenzimidazoles (PBIs) for gas separation applications and polyimides (PI) for water purification and as interfacial agents for thermoplastic carbon fiber composites. Two methods for improving the gas transport properties (for H2/CO2 separation) of a tetraaminodiphenylsulfone (TADPS)-based polybenzimidazole were investigated. Low molecular weight poly(propylene carbonate) (PPC) and poly(ethylene oxide) (PEO) were incorporated as sacrificial additives that could be removed via a controlled heat treatment protocol. PBI films containing 7 and 11 wt% PPC (blend) and 13 wt% PEO (graft) were fabricated and the gas transport properties and mechanical properties after heat treatment were measured and compared to the PBI homopolymer. After heat treatment, the 7 wt% PPC blend exhibited the highest performance while retaining the toughness exhibited by the PBI homopolymer. Novel sulfonated polyimides and their monomers were synthesized for use as interfacial agents and water purification membranes. Polyimides are high performance polymers that have high thermal, mechanical, and chemical stability. The objective was to assess structure-property relationships of novel sulfonated polyimides prepared by direct polymerization of the diamine monomers. A series of sulfonated polyimides was synthesized using an ester-acid polymerization method with varying degrees of sulfonation (20%, 30%, and 50% disulfonated and 50% and 100% monosulfonated polyimides). The results showed that the toughness of the polyimides in the fully hydrated state was much better than current commercial cation exchange membranes. A 100% disulfonated polyimide (sPI) and poly(amic acid) salt (PAAS) using the same monomers used for the synthesis of Ultem® were utilized as suspending agents for the fabrication of coated sub-micron polyetherimide (PEI) particles. Sub-micron particles were obtained using 1 wt% PAAS and 4 wt% sPI to coat the PEI. The PEI particles were coupled onto ozone treated carbon fibers using a silane coupling agent. SEM images showed a significant amount of particle coating on the treated carbon fibers compared to the non-silane treated carbon fibers. / PHD / This dissertation describes synthetic and processing techniques used to fabricate materials for applications such as, water purification and gas separation. The polymers included in this dissertation include polybenzimidazoles and polyimides, which are materials that have exceptional mechanical and thermal properties. The polybenzimidazoles were specifically used for gas separation, while the polyimides were synthesized for use as water purification membranes and surfactants for coating polyimides and carbon fibers. Gas separation membranes are effective tools for purifying gas mixtures (e.g. H₂/N₂, O₂/N₂, CO₂/CH₄). Additionally, they offer the advantage of being economical and environmentally-friendly compared to other methods of separation (e.g. cryogenic distillation). Many synthetic membranes made from polymers are used commercially, however, very few polymers can be used for elevated temperature separations. Because polybenzimidazoles exhibit high thermal stability, they are excellent candidates for high temperature gas separations (specifically H₂/CO₂ gas mixture). However, polybenzimidazoles have inherently low gas permeabilities. Thus, the focus of this research was to develop a simple method to introduce “pores” into the polymer membrane to improve gas permeability. Water purification is a very important process as the demand for clean water increases with the growing global population. Currently, one-third of the global population experience water scarcity, and by 2025, two-thirds of the world’s population may face water shortages. Multi-stage flash distillation is the most widely used method for water desalination from sea water but it is also the most energy intensive process. Water desalination using polymer membranes (e.g. reverse osmosis, nanofiltration, electrodialysis) has been developed as low energy and environmentally-friendly alternatives for producing clean water. The current state-of-the-art membranes used for water purification lack mechanical integrity and chemical resistance, which complicate and reduce the overall efficiency of the separation process. Thus, the focus of the research was to synthesize polyimide membranes with improved toughness and chemical stability.