Series of homo- and copolyimides containing controlled degrees of sulfonic acid ion conducting pendant groups have been synthesized from both phthalic (five-) and naphthalic (six-membered) dianhydrides and appropriate wholly aromatic diamines and heterocyclic analogues. The goal is to identify thermally and hydrolytically stable ion conducting polymers (ICP) suitable as proton exchange membranes, PEM, for fuel cells. The candidate ICP's have been synthesized and characterized for molecular weight, chemical composition, film forming properties, thermal transition behavior, boiling water stability, solvent solubility and water absorption and conductivity. Commercially available five-membered ring dianhydrides such as 6FDA, BPDA, and six-membered ring dianhydrides such as naphthalene tetracarboxylic dianhydride (NDA) have been used. High molecular weight five-membered ring polyimides were obtained from an equimolar ratio of diamines and dianhydride using a one-pot ester-acid procedure by initially converting the dianhydride to a diester-diacid derivative, followed by the reaction with sulfonated and unsulfonated aryl diamines. The sulfonated diamine monomer was allowed to oligomerize with the diester-diacid of the dianhydride for 2-3 hours, before unsulfonated diamine was charged into the reaction flask. The levels of sulfonation in the polymer backbones were controlled by varying the mole ratio of sulfonated diamine to unsulfonated diamine. For the six-membered ring polyimides, phenolic solvents, e.g. m-cresol, have been used. In general, 4,4′-diamino-biphenyl-2,2′-disulfonic acid (DPS) has been employed as the source of the sulfonated unit. The chemical compositions of both sulfonated and unsulfonated polyimides were obtained using ¹H-NMR and FT-IR. The sulfonic acid contents in both diamine monomers, as well as the sulfonated polyimides were also analyzed by acid-base potentiometric titration. In all cases, high inherent viscosity values and good film forming ability of the polymers were the key indications of high molecular weight. The viscosity values increased with an increase of sulfonation degree in the polymers. This increase of viscosity in these ionomers can be attributed to the increase of polymer chain aggregation with their increasing ionic character. Polymers were fabricated into membranes via solution casting or spin casting from DMAc or m-cresol in order to study film-forming properties. The solution cast dry films of the sulfonated polyimide membranes gave tough, ductile membranes and demonstrated moderate to high water absorption, which is necessary for PEM fuel cells. However, swollen films, in general, showed poor hydrolytic stability which resulted in brittle membranes. The solution-cast membranes were thermally analyzed to study the effect of the degree of sulfonation on the thermal properties of sulfonated polymers. All the thermograms of the sulfonated polyimide films exhibited a two-step degradation behavior. The first weight loss, observed between 300-400 °C, corresponds to desulfonation in the sulfonated block, and the second weight loss, observed for a temperature around 500 °C or above, corresponds to the polymer backbone degradation. The TGA thermograms indicated that the initial weight losses were steeper for polymers with higher sulfonation degrees. Furthermore, the weight loss temperature of sulfonated polyimides decreased and broadened with increasing sulfonation levels. However, the onset temperature of the first weight loss was independent of the degree of sulfonation. Weight loss data in TGA curves of the sulfonated polymers were used to calculate the degree of sulfonation. Experimental and theoretical values were in good agreement with each other. The sulfonated five-membered polyimide membranes were aged in an air-oven at increasing temperatures (30-220 °C) for 30 min and then titrated with TMAH using non-aqueous potentiometric titration. All the films that were aged up to 220 °C were still completely soluble in DMAc. Moreover, the sulfonic acid groups were unchanged. In addition, several new flexible sulfonated and unsulfonated diamines and bis(naphthalic anhydride) monomers containing phosphineoxide [-P(O)-] or sulfone [-S(O)₂ -] moieties in their structure have been synthesized and characterized with various analytical techniques. The structural design of naphthalic polyimides by incorporating bis(naphthalic anhydrides) was one approach to give a better solubility and processability of their related products. Development of an iterative approach for defining the optimum degree of sulfonation that will produce the highest ionic conductivity while still retaining other important properties such as flexibility, strength, hydrolytic stability has been a goal of this research and will be discussed in the thesis. / Ph. D.

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.