Thesis (Ph. D.)--School of Textile and Fiber Engineering, Georgia Institute of Technology, 2004. Directed by C.P. Wong. / Vita. Includes bibliographical references (leaves 197-210).

Herein is contained details of a comprehensive finite element survey and experimental investigation into the buckling and postbuckling characteristics of thin laminated square Carbon-Epoxy panels with various cutout geometries, subjected to uniaxial compression. The plate edges are considered to be fully fixed with constant edge displacement loading. The panels were quasi isotropic in nature with a stacking sequence of (0/90/±45)2,. Square, circular and elliptical centrally located cutouts were considered with cutout dimension/panel widths ranging from 0.1 to 0.7 in increments of 0.1. Eccentrically located circular and square cutouts were considered for cutout dimension/panel width ratios ranging from 0.1 to 0.4 with vertical and horizontal eccentricity varying from 0 to 20% of the panels width. Multiple circular cutouts with cutout dimension/panel width ranging from 0 to 0.3 with separation distance/panel width ratios ranging from 0.2 to 0.65. A finite element eigenvalue analysis was adopted to determine the critical buckling loads and buckle mode shapes for the panels. The postbuckling response of the panels were investigated by adopting a non-linear finite element analysis approach using an Incremental Newton-Raphson Iterative solution scheme. A limited experimental test programme was undertaken to act as verification to the finite element solutions. A purpose built buckling rig was designed and manufactured for the purposes of the tests. It has been confirmed that the critical buckling loads for centrally located circular and square cutouts initially reduces as the cutout size increases. After reaching a minimum value it thereafter increases with large cutout sizes, the exact changeover point being dependant upon the shape of the cutout. The orientation of ellipse major axis significantly affects the critical buckling load of a panel. A horizontally aligned ellipse exhibits similar behaviour as that to a circular or square cutout. However when the major axis is rotated relative to the horizontal axis its buckling capacity reduces monotonically till it has a buckling load less than that for an unperforated panel when vertical aligned. It has been shown when a circular cutout is eccentrically placed in a panel, for small cutout sizes the buckling load reduces with horizontal eccentricity while a small increase is experienced for vertical eccentricity. Multiple circular cutouts significantly reduce the buckling capacity of the panel for all cutout sizes and separation distances. Initial geometric imperfection in the panel does not affect the critical buckling load significantly. The postbuckled response of such panels are also insensitive to the magnitude of imperfection. Panels with circular, square and elliptical cutouts exhibit substantial postbuckled strength. The post buckling response of such panels are insensitive to cutout geometry shape.

Oat is known to synthesize several epoxy fatty acids in seeds using peroxygenase (PXG), a type of hydroperoxide-dependent epoxygenase. This thesis aims to molecularly clone and functionally characterize the PXG genes from oat developing seeds. The research started with identifying additional PXG genes from oat expressed sequence tag (EST) databases using a previously identified oat peroxygenase AsPXG1 as a query sequence. This resulted in the identification of six homologous contig sequences from the EST data bases. Of them, two contigs with high sequence similarity and alignment with plant PXG/caleosin proteins were selected for cloning and functional analysis. Rapid amplification of cDNA ends (RACE) and reverse transcriptase-polymerase chain reaction (RT-PCR) were used to retrieve the full length cDNAs of the contigs, which resulted in identification of three putative PXG genes, AsPXG2, AsPXG3 and AsPXG4. Open reading frame (ORF) of AsPXG2 is 702 bp long encoding a polypeptide of 233 amino acids, while ORFs of both AsPXG3 and AsPXG4 are 627 bp in length coding for 208 amino acids. All these putative peroxygenases comprise a single transmembrane domain, presumably for lipid droplet anchoring, conserved hisditine residues for heme-binding and a conserved EF-hand motif for calcium-binding. To functionally characterize the three genes, their ORFs were individually expressed in Escherichia coli/Pichia pastoris. The enzymatic assays showed that all transformants produced 9,10-epoxystearic acid methyl ester in the presence of oleic acid methyl ester and cumene hydroperoxide, indicating all three genes encode functional peroxygenase. AsPXG3 has the highest specific activity at 42 mol/mg/min with about 25% substrate conversion efficiency. Substrate specificity assays on free fatty acids showed that AsPXG3 could epoxidize all mono- and poly-unsaturated fatty acids tested, with linolenic acid (C18:3-9c,12c,15c) being the most preferred substrate. Site-directed mutagenesis of three conserved histidines and nine conserved residues surrounding the histidines in AsPXG3 showed that substitution of the first conserved histidine at position 32 (H1) and the third conserved histidine at position 102 (H3) with alanine respectively resulted in complete loss of the enzymatic activity, while substitution of the second conserved histidine at position 98 (H2) resulted in only slight reduction of the activity, indicating that only H1 and H3 are absolutely essential and probably involved in heme-binding for the peroxygenase. Substitution of leucine at position 29 (M1), isoleucine at position 97 (M5), and lysine at position 101 (M8) with alanine reduced the enzymatic activity on oleic acid methyl ester by more than 80% relative to the wild type enzyme, indicating these three residues are also very important for catalytic activity. The activity of M1, M5 and M8 mutants was also drastically reduced on all other free mono-unsaturated fatty acids tested (>60%). However, to linolenic acid, M5 showed only slight reduction of the activity (~15%) and M8 even increased the activity by 12% relative to the wild type enzyme. These results suggest that these conserved residues might play roles in defining the shape and size of the catalytic site for interaction of the heme with fatty acid substrates.

RESUM Les reïnes epoxi constitueixen un dels polímers més emprats en el món de la industria, si bé presenten una sèrie d’inconvenients, els més importants dels quals són: la seva inherent fragilitat, la seva excessiva resistència tèrmica que en dificulta l’eliminació d’un substrat un cop finalitzada la seva vida útil i l’encongiment que experimenten durant el procés de curat. Per tal de reduir o eliminar aquests problemes aquesta tesi proposa l’ús de polímers hiperramificats així com polímers estrella i copolímers lineal-hiperramificat de bloc com a modificants químics de reïnes comercials. Amb aquesta estratègia s’han aconseguit millorar la tenacitat degut a efectes flexibilitzants o a separacions de fase del modificant en la matriu epoxídica, així com reduir l’encongiment en el curat o la degradabilitat de les reïnes, sense afectar altres propietats de la reïna com la seva Tg o la seva duresa. / ABSTRACT Epoxy resins are one of the most used polymers in the field of technological applications. However, they present some drawbacks being the most important the following: they are inherently brittle materials; they present excessive thermal resistance that limits their reworkability; and the shrinkage they experiment during curing. To overcome these problems this thesis proposes the use of hyperbranched polymers, as well as star polymers and lineal-hyperbranched block copolymers as chemical modifiers of commercially available epoxy resins. With this strategy tougher materials have been obtained due to either a flexibilizing effect or a phase separation of the modifier within the epoxy matrix. Moreover, the shrinkage on curing and the degradability of the thermosets have been improved without compromising other properties of the resin such as its Tg or its hardness.

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Some Gulf of Mexico (GOM) wells destroyed by hurricanes have become environment and safety hazard and cannot be abandoned by conventional methods since pumping and circulating cement into the casing is impossible when the platforms have been completely destroyed and toppled. This project tested the curing properties of several epoxy resin systems in different environments. A bisphenol-F/epichlorohydrin (BPF) resin cured by curing agent MBOEA system was successfully tested in the laboratory as a potential plugging material to abandon wells destroyed in the GOM. The BPF/MBOEA resin system had the most suitable curing time in a synthetic seawater environment. The system could be successfully weighted by barite up to 16.8 ppg and cured properly. Weighting allows the resin system fall more efficiently through the casing annulus. This laboratory verification of properties will lead to field test in the test wells.

Using both artificial and natural templates, biomimetic micro-structures are fabricated on conventional coating materials (epoxy and silicone elastomers) to mimic both artificial and natural templates through effective pattern transfer processes. The pattern transfer processes use a soft-polymer negative stamp, where the flexibility of the stamp allows for easy conformation to both flat and curved surfaces. Patterns have been successfully transferred as a rigid epoxy to complex surfaces or as a soft elastomer replica of a hydrophobic Trembling Aspen leaf. The hydrophobicity and friction behaviour of the resulting micro-patterned surfaces are systematically investigated, showing that surface patterning can be used as an effective way to improve hydrophobicity while reducing the surface adhesion and friction without a loss of the structural integrity or rigidity typical of epoxy coatings. The relative strength of the micro-pattern was determined through indentation testing in order to support the claim of a robust pattern on the micro-scale that is able to withstand the harsh environment of industrial application or weather exposure. With the well characterized patterned epoxy material fabricated and able to be transferred to many different surfaces, the potential for the patterned surface to act as an icephobic coating was pursued. The robustness of the epoxy material with the unique ability to coat surfaces that are typically unable to possess a micro-structure makes this coating an ideal candidate for large-scale icephobic application. The potential use of a micro-patterned epoxy coating is investigated against comparable surface coatings within an innovative experimental set-up to measure the relative ice-adhesion strength of different substrates. In characterizing the relative shear-force required to remove frozen water droplets from the coating surface at the interface, several variables and factors were explored. The addition of a surface pattern was found to impact the icephobic ability of several materials, where different materials with the same pattern were compared to identify that the surface energy of the substrate influences the icephobic nature of a surface. Moreover, previous studies that relate the water contact angle or hysteresis to ice-adhesion strength are questioned through a preliminary qualitative analysis of ice adhesion strength data. This work demonstrates a potential process for the utilization of biomimetic epoxy micro-patterns as an enhanced hydrophobic and icephobic option for large scale protective coatings.

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