Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Colton, Jonathan; Committee Member: Schultz, John; Committee Member: Zhou, Min. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Gall, Ken; Committee Co-Chair: McDowell, David; Committee Member: Guldberg, Robert; Committee Member: Sanderson, Terry; Committee Member: Shofner, Meisha; Committee Member: Tannenbaum, Rina.

Thesis (M.S.M.E)--University of Delaware, 2006. / Principal faculty advisor: Jian-Qiao Sun, Dept. of Mechanical Engineering. Includes bibliographical references.

This dissertation is composed of 5 chapters detailing advances in the synthesis and post-modification of polymers using tetrazines. The research described herein conveys three new discoveries each of which should forge new fields of research concerning tetrazines in polymer chemistry, that is the use of tetrazines as chemical blowing agents in generating polymer foams, as agents for incorporating large concentrations of antioxidants into polymers, and their attachment to polymers, adding a photodegradable functional unit which alters the refractive index of the materials after degradation. The first chapter is a review which highlights the use of tetrazines in polymer chemistry as repeat units incorporated into polymers, tools for post-modification, and cross-linkers for the formation of gels. Tetrazines are unique molecules which been utilized in many different types of materials. Their tetra-azo core has provided the basis for preparing high energetic explosive materials, and the high nitrogen content has also been exploited in metal chelating polymers. Being highly electron deficient, the tetrazine ring has found use in donor – acceptor (D – A) copolymers for use in polymer solar cells (PSCs). Its ability to undergo an irreversible cycloaddition reaction without the requirement of metal catalysis, has created an entire field of research in bioorthogonal ligation and has also found use in polymer chain extension, post-modification, and gel formation. The versatility of the tetrazine ring is demonstrated in this review with the shear variety of its applications in polymer chemistry. The second chapter reports on the discovery of 3,6-dichloro-1,2,4,5-tetrazine's (DCT) use as a chemical blowing agent in preparing polymer foams. This discovery demonstrates the first reported use of tetrazines as chemical blowing agents, and the production of a new class of polymers foams through exploitation of this chemistry. Nitrogen is released from a cycloaddition reaction between DCT and polybutadiene (PBD). As the reaction proceeds, the foam grows and increases in viscosity eventually setting up into a solid material. The product of the cycloaddition reaction, a dihydropyridazine ring, is demonstrated to provide the foams with a built-in antioxidant, and a change in fluorescence of the foams provides indication for extent of oxidation. Also in this chapter, a new method is proposed for comparing antioxidant properties of small molecules, and two dihydropyridazines are shown to outperform commercial antioxidant BHT with this method. Chapter 3 follows up on the results from the previous chapter, with the synthesis of four new polymers through solution-based cycloaddition reactions between dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (DMDT) and PBD. Near quantitative conversions results in the formation of two new classes of dihydropyridazine polymers, providing an entirely new class of polymer which contains its own built-in antioxidant on nearly every repeat unit along the polymer backbone. Oxidation of these materials results in an additional 2 new classes of heteroaromatic pyridazine polymers reported for the first time in this chapter. The properties of these new materials are reported and the antioxidant properties of the dihydropyridazine ring are further explored through cyclic voltammetry (CV). Quenching of fluorescence is observed upon oxidation of the materials, providing a visual indicator for extent of oxidation, and an explanation for the quenching is also provide through CV analysis. Chapter 4 diverges from tetrazine IEDDA modification of polybutadienes and instead focuses on the post-modification of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) with DCT to generate photobleachable materials. This chapter represents the first reported use of tetrazine photobleaching in modifying the optical properties of polymers, and the generation of a new copolymer containing pendant tetrazine repeat units. The modified copolymer is demonstrated to be solvent processable, but crosslinks upon heat treatment. Extended UV or ambient light excitation of the copolymer results in degradation of the tetrazine ring and bleaching of the polymer. A refractive index change for the polymer is observed after degradation, leading to the hypothesis that these polymers may find use in optical waveguide materials. Finally, chapter 5 provides a summation of the dissertation and discusses unfinished projects that can be pursued further by future Loy group graduate students.

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Syntactic foams (SFs) are characterized by a unique combination of low density, high mechanical properties, and low thermal conductivity. Moreover, these properties can be tuned to specific applications. In contrast to traditional foams, where porosity is formed during the foaming process, porosity in syntactic foams can be obtained simply by incorporating pre-formed bubbles. Because of their high chemical stability and buoyancy, these type of foams found their first application in the marine industry. Nowadays, they are utilized in many other fields, such as in the aerospace and automotive industry. In this work, the multifunctionality of epoxy-based and polypropylene (PP)-based SFs was increased by including a microencapsulated Phase Change Material (PCM), able to impart Thermal Energy Storage (TES) capability at phase transition temperatures of 43 °C and 57 °C. The rheological, morphological, thermal, and mechanical properties of the prepared materials were systematically investigated. A final comparison of the two systems was performed to obtain a better comprehension of their potential in emerging industrial applications.

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In this study, the viscoelastic and morphological characterization of molded foams was the main focus. A series of molded foams based on toluene diisocyanate (TDI) and glycerol initiated polyethylene-oxide-capped propylene-oxide was studied in terms of the structure property features. The results were in many instances compared to those obtained on conventional slabstock foams based on TDI and glycerol initiated propylene-oxide. These comparisons were made to delineate and clarify distinct differences between these two different and very important systems. It was found that high temperatures and humidities "plasticized" the viscoelastic behavior of molded foams to a greater extent than that of slabstock foams; the molded foams displayed higher load decay values in the viscoelastic measurements than slabstock foams. In an attempt to understand these dramatic differences, the two types of “cross-links" (covalent cross-links and urea based phase separated hard segment domains) were evaluated. It was discovered that the structure of the hard segment domains dictated the foam's behavior, especially at elevated temperatures and humidity. Furthermore, it was found that the hard segment domains in slabstock foams had a much higher level of short range ordering. This was confirmed by wide-angle x-ray scattering (WAXS) and fourier transform infrared (FTIR) which revealed that the conventional slabstock foams had much more organized hard segment domains. It is thus concluded that the dramatic differences between the mechanical properties of molded and slabstock foams are due to the lower and weaker ordering of the hard segments in molded systems making these physical "cross-links" more labile at higher temperatures and humidities. These morphological differences were shown to be due primarily to differences in the formulation components between the two studied systems. First, the ethylene-oxide capping used in the polyol of molded foams to increase the reactivity is known to also increase the compatibility between the hard and soft segments thus promoting some phase-mixing. Second, the addition of diethanolamine (DEOA) added in the molded foam formulation to decrease demold times by enhancing cross-linking clearly resulted in the prevention of the full development of the hard-segment domains. It was also found that the copolymer polyol particles (CPP), added to molded foams to increase load bearing capabilities, had a negative effect on the viscoelastic properties. The viscoelastic properties of the CPP containing foams were more time-dependent than those of the foams lacking these particles. As expected, the incorporation of these particles increased the initial load and decreased the initial strain over the foams lacking the particles suggesting that the initial stiffness of these materials was increased. However, over a period of time, the amount of this initial load that decayed was greater for the CPP containing foams and furthermore, at elevated conditions, the load decreased to levels below those of the CPP lacking foams. A series of slabstock foams was also studied to evaluate the effect of toluene diisocyanate (TDI) index on the physical properties, and morphology of the foams. Extraction experiments using dimethyl formamide (DMF) showed that increasing the index increased the level of covalent cross-linking with perhaps a maximum being reached at an index of 100. Viscoelastic measurements also supported the claim of increased crosslinking with TDI index. The initial load in load relaxation experiments systematically increased with increasing TDI while the percent decay in a three hour period decreased. Temperature and/or humidity "plasticized" the load relaxation behavior in all the foams studied indicating that the hard segment domain physical "cross-links" play a significant role in the properties of these materials. The morphology of the foams was also found to be influenced by the TDI index. Small angle x-ray scattering (SAXS), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) all provided evidence that an increase in the TDI index promoted phase mixing. FTIR and WAXS showed that the short range ordering within the hard segment domains displayed a maximum at an index of 100 and decreased as the index was increased. Finally, the influence of transient moisture conditions on the viscoelastic behavior was also investigated. In creep extractions, as the moisture conditions were cycled from low to high humidity while maintaining constant temperature, the compressive strain increased in subsequent steps where the strain levels under cyclic moisture conditions surpassed those observed at the highest constant relative humidity. This overall phenomenon of enhanced creep under cyclic moisture levels was attributed to water interacting with the hydrogen bonded structure within the foam. These hydrophillic interactions, principally promoted within the hard segment regions due to high hydrogen bonding, are disrupted causing slippage and increases in strain. As the foam is rapidly dried, regions of free volume are induced by the loss of water thus causing further increases in strain prior to the re-establishment of well ordered hydrogen bonding. / Ph. D.

A series of four water-blown flexible polyurethane foams was produced in which the water content was varied from 2 to 5 pph at a constant isocyanate index of 110. A portion of each foam was thermally compression molded into a plaque. The morphology of the foams and plaques was investigated using DMS, DSC, FTIR, TEM, SEM, swelling, WAXS, and SAXS. A high degree of phase separation occurs in these foams and the degree of phase separation is independent of water (hard segment) content. In the foam with the lowest water content the morphology is similar to that of typical segmented urethane elastomers. Small hard segment domains are present with a correlation distance of roughly 7.0 nanometers. When the water content is increased a binodal distribution of hard segments appears. There are the small hard segment domains typical of segmented urethane elastomers as well as large hard segment aggregates greater than 100 nanometers in diameter. The large domains are thought to be aggregates of polyurea that precipitated during the manufacture of the foam. The foam making process successfully incorporated the trifunctional polyols into a network indicating a high degree of polymerization for the hydroxyl-isocyanate reaction. Unreacted isocyanate is present in the foams a month after curing. It is believed to be trapped in the large urea aggregates. WAXS patterns of the foams suggest hard segment ordering that may be of a paracrystalline nature but certainly lacking in true crystallinity. / Master of Science