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This study examined several features of flexible polyurethane foams from a structure-property perspective. A major part of this dissertation addresses the issue of connectivity of the urea phase and its influence on mechanical and viscoelastic properties of flexible polyurethane foams and their plaque counterparts. Lithium salts (LiCl and LiBr) were used as additives to systematically alter the phase separation behavior, and hence the connectivity of the urea phase at different scale lengths. Macro connectivity, or the association of the large scale urea rich aggregates typically observed in flexible polyurethane foams was assessed using SAXS, TEM, and AFM. These techniques showed that including a lithium salt in the foam formulation suppressed the formation of the urea aggregates and thus led to a loss in the macro level connectivity of the urea phase. WAXS and FTIR were used to demonstrate that addition of LiCl or LiBr systematically disrupted the local ordering of the hard segments within the microdomains, i.e., it led to a reduction of micro level connectivity or the regularity in segmental packing of the urea phase. Based on these observations, the interaction of the lithium salt was thought to predominantly occur with the urea hard segments, and this hypothesis was confirmed using quantum mechanical calculations. Another feature of this research investigated model trisegmented polyurethanes based on monofunctional polyols, or "monols", with water-extended toluene diisocyanate (TDI) based hard segments. The formulations of the monol materials were maintained similar to those of flexible polyurethane foams with the exceptions that the conventional polyol was substituted by an oligomeric monofunctional polyether of ca. 1000 g/mol molecular weight. Plaques formed from these model systems were shown to be solid materials even at their relatively low molecular weights of 3000 g/mol and less. AFM phase images, for the first time, revealed the ability of the hard segments to self-assemble and form lath-like percolated structures, resulting in solid plaques, even though the overall volume of the system was known to be dominated by the two terminal liquid-like polyether segments. In another aspect of this research, foams were investigated in which the ratios of the 2,4 and 2,6 TDI isomers were varied. The three commercially available TDI mixtures, i.e., 65:35 2,4/2,6 TDI, 80:20 2,4/2,6 TDI, and 100:0 2,4/2,6 TDI were used. These foams were shown to display marked differences in their cellular structure (SEM), urea aggregation behavior (TEM), and in the hydrogen bonding characteristics of the hard segments (FTIR). Finally, the nanoscale morphology of a series of 'model' segmented polyurethane elastomers, based on 1,4-butanediol extended piperazine based hard segments and poly(tetramethylene oxide) soft segments, was also investigated using AFM. The monodisperse hard segments of these 'model' polyurethanes contained precisely either one, two, three, or four repeating units. Not only did AFM image the microphase separated morphology of these polyurethanes, but it also revealed that the hard domains preferentially oriented with their long axis along the radial direction of the spherulites which they formed. / Ph. D.

Cellulose is the most abundant polysaccharide on earth and it is relatively a simple homopolymer with three hydroxyl groups, differing only subtly in reactivity. The position of substitution has a powerful influence on physical properties of cellulose derivatives. To better understand the structure and property relationships of cellulose derivatives, it is critical to have all homopolymers related to important cellulose ethers and esters available. However, regiocontrol in cellulose chemistry is still a difficult, mostly unconquered frontier. In this dissertation, the main objective is to develop novel synthetic methods to synthesize regioselectively substituted cellulose derivatives including cellulose ethers and esters, and apply advanced characterization tools to understand structure and its influence on properties, which will give us deep insights into the composition of more random commercial derivatives, maximizing the content of advantageous monosaccharides. Several strategies to regioselectively synthesize cellulose derivatives are discussed in detail. The obtained regioselective cellulose derivatives were fully characterized analytically. Structure-property relationships of these regioselectively substituted cellulose derivatives were also studied. / Ph. D.

Poly(arylene ether)s are a well-established class of thermoplastics that are known for their mechanical toughness, thermal stability, and fabrication into membranes. These materials can undergo a myriad of modifications including backbone structure variability, sulfonation, and crosslinking. In this dissertation, structure-property relationships are considered for poly(arylene ether)s with regard to membrane applications for proton exchange and gas separation membranes. All of the proton exchange membranes in this dissertation focus on a disulfonated poly(arylene ether sulfone) based hydrophilic structure to produce hydrophilic-hydrophobic multiblock copolymers. The hydrophobic segments were based upon poly(arylene ether benzonitrile) polymers and copolymers. The oligomers were synthesized and isolated separately, then reacted under mild conditions to form the alternating multiblock copolymers. Structure-property relationships were considered for two different proton exchange membrane applications. One multiblock copolymer system was for H2/air fuel cells, and the other for direct methanol fuel cells (DMFCs). The H2/air fuel cells operate under harsh conditions and varying levels of relative humidity, while the DMFCs operate in an aqueous environment with a methanol-water mixture (typically 0.5-1 M MeOH). Thus two different approaches were taken for the multiblock copolymers. All of the multiblock copolymers were cast into membranes and after annealing resulted in drastically reduced water uptake as compared to random and non-annealed systems. The membranes were characterized with regard to composition, mechanical properties, morphology, water uptake, proton conductivity, and molecular weight. Membranes were also sent to collaborators to elicit the fuel cell performance of the proton exchange membranes. In H2/air fuel cells the approach was to increase charge density by bisphenol choice in the hydrophilic phase. This was performed by switching to a lower molecular weight monomer, hydroquinone, and a monosulfonated hydroquinone. This produced higher charge density in the hydrophilic phase, and the corresponding multiblock copolymer. With increased hydrophilicity the multiblock copolymers showed increased phase separation, proton conductivity, and better performance under relative humidity testing. In the second system for DMFCs, the primary goal was to reduce methanol permeability by bisphenol selection in the hydrophobic phase. This was done with by replacing fifty mole percent of the fluorinated monomer with a series of increasing hydrophobicity bisphenols. Addition of benzylic methyl groups on the bisphenols, was the method undertaken to increase the hydrophobicity. The combination of reduced fluorine content along with the addition of methyl groups resulted in multiblock copolymers with extremely low water uptake and methanol permeability. This allowed for a PEM with better performance than Nafion® in 1M MeOH in DMFC testing. The gas separation membranes presented in this dissertation are based upon poly(arylene ether ketone)s. Two systems were presented: one with a polymer directly synthesized with a bisphenol containing benzylic methyl groups and 4,4'-difluorobenzophenone, and the other a difunctional poly(phenylene oxide) oligomer polymerized with 4,4'-difluorobenzophenone. These systems were crosslinked via UV light through excitation of the ketone group to the triplet state and then hydrogen abstraction from the benzylic methyl. Confirmation of crosslinking was performed via differential scanning calorimetry and infrared spectroscopy. Changes in the glass transitions between crosslinked and non-crosslinked materials were characterized with respect to the concentration of ketones to elicit the effects of crosslink density on the polymers and copolymers. Gas transport properties showed a strong dependence on the ketone percentage as the selectivity was much higher for the homopolymer, while the permeability was higher for the PPO copolymer in the CO2/CH4 and O2/N2 gas pairs. / Ph. D.

Polymers containing less than 10 mol % of ions (ionomers) have been studied in depth for their potential in producing polymers with tailored properties for specific applications. A small molar percentage of ions can be incorporated into a polymer to drastically enhance the properties of the polymer. An ionomer that has been studied is that of isoprene copolymerized with sodium styrene sulfonate (poly(I-co-NaSS)). Research has been performed relating to the synthesis and chemical characterization of the copolymers. However, an in depth study of the way the physical properties are affected by a change in ion concentration has not been presented. Thus, it is the goal of this thesis to synthesize a series of poly(I-co-NaSS) copolymers with varying levels of sulfonated styrene and characterize their physical properties. The poly(I-co-NaSS) polymers, containing a range of 1.15 to 4.74 mol % NaSS, were polymerized using free radical emulsion polymerization. The copolymer compositions were confirmed using combustion sulfur analysis. Dynamic light scattering indicated that large aggregates were present in solution. These aggregates were large enough that capillary intrinsic viscosities could not be measured. Small angle x-ray scattering (SAXS) and thermal analysis showed little change as the ion concentration was increased, while tensile, stress relaxation and adhesion properties were improved. The absence of changes in the SAXS patterns indicated that there was an absence of a well-defined ionic aggregate, while the mechanical properties showed evidence of electrostatic interactions. This can be at least partially attributed to ionic interactions on a smaller scale (doublets, triplets). / Master of Science