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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Synthesis and characterization of poly(3,3,3-trifluorolactic acid)

McKie, Derrick Bernard 01 January 1997 (has links)
A long-standing goal in polymer research is to predictably relate the primary molecular structural features of polymeric materials with ultimate macroscopic physical characteristics. This continues to be a challenging undertaking in polymer science, considering the statistical nature of synthetic macromolecules. The science and technology of naturally occurring polymers in the form of proteins offer an opportunity from which potentially substantial advances can be made in synthetic polymer science. Application of what is understood about the molecular structures of natural macromolecules and processes by which they are formed can be applied in synthetic polymer science The investigation of polymers of $\alpha$-hydroxyacids represents a reasonable starting point for application of what is known about proteins and applying it to wholly synthetic materials. Lactic acid is the simplest asymmetric $\alpha$-hydroxyacid. A model monomer, trifluorolactic acid, is formed when the methyl group is replaced by a trifluoromethyl group. Acidity of the methine hydrogen of an $\alpha$-hydroxyacid is expected to increase when the strongly electron withdrawing trifluoromethyl group is introduced to the stereogenic carbon. In this way, the hydrogen bonding capability of the polymer molecule would be enhanced. Polymerization of trifluorolactic acid represents a part of the effort toward the ultimate goal of the production of bio-inspired synthetic polymers with predictable and enhanced physical characteristics. Racemic 3,3,3-trifluorlactic acid ((RS)-3,3,3-trifluorolactic acid) was characterized and contrasted with lactic acid. Unlike lactic acid, it does not form intermolecular esters in the presence of trace water. Poly((RS)-3,3,3-trifluorolactic acid) was synthesized by esterification with 1,3-diisopropylcarbodimide condensing agent and catalytic salts of 4-dimethylaminopyridine and protic acids. Infrared and NMR spectroscopy as well as molecular weight characterization by inherent viscosity, GPC, and light scattering showed that the target polymer was produced. The DSC thermal trace indicated a T$\sp{\rm g}$ of 36$\sp\circ$C. TGA indicated that the fluoropolyester compared favorably with poly(sc D,L-lactide) in thermal stability. Surface properties of poly(sc D,L-lactide) blends with poly((RS)-3,3,3-trifluorolactic acid) showed increased contact angle with the presence of the fluorinated polymer. Wide angle x-ray diffraction patterns indicated the emergence of a band indicative of weak interchain correlation despite the lack of chain stereoregularity. The analogous poly(sc D,L-lactide) showed no such correlation.
42

Capillary electrophoresis of polyelectrolytes in dilute neutral polymer solutions: Simulations and model experiments

Starkweather, Margaret Ellen 01 January 1998 (has links)
The electrophoretic separation of polyelectrolytes in dilute neutral polymer solutions are investigated through computer simulations and capillary electrophoresis experiments. The isolated entanglement dynamics of a mobile "probe" polyelectrolyte and "host" neutral polymer are simulated, using a Monte Carlo algorithm, providing insight into the effect of various probe and host parameters on molecular weight discrimination. Experiments employing narrow dispersity molecular weight fractions of pullulan as model most and poly(styrene sulfonate) as model probe, are used for investigation of bulk separation trends, in particular, to assess the role of host molecular weight. Both simulation and experiments indicate that probe separation in dilute matrices is dominated by the duration of probe/host entanglements. Single probe/host pairs alone are shown to impart a strong molecular weight discrimination, and this discrimination is highly affected by the size of the matrix polymer. A plot of probe mobility versus log of molecular weight is sigmoidal with the "best" separation occurring at comparable probe and host molecular weights. Probe separations are noted at concentrations as low as c*/8. A new understanding of the underlying molecular mechanisms of separation in dilute polymer solution matrices is achieved.
43

Smectic ordering of rod-like polymers owing to monodispersity of chain length: Synthesis and characterization of benzyl and (4-hexadecyloxy)benzyl esters of monodisperse derivatives of poly(alpha,L-glutamate)

Yu, Michael Seungju 01 January 1998 (has links)
Two types of monodisperse rod-like polymers, poly($\gamma$-benzyl $\alpha$,L-glutamate) (PBLG) and poly($\gamma$-4-(hexadecyloxy)benzyl $\alpha$,L-glutamate) with backbone sequences of have been prepared by chemical modification of monodisperse poly($\alpha$,L-glutamic acid) (PLGA) that was synthesized by recombinant DNA biosynthesis techniques. The monodisperse PLGA () was produced in Escherichia coli as a fusion to mouse dihydrofolate reductase and purified to homogeneity by metal affinity chromatography, CNBr digestion and ion-exchange chromatography.$$\rm GluAsp(Glu\sb{17}Asp)\sb{x}GluGlu\quad x = 3, 4, 5, 6\eqno {\bf1}$$ The monodisperse PBLG derived from showed smectic ordering both in solution and in films as characterized by polarized light optical microscopy and X-ray diffraction. The layer spacings of smectic order were nearly identical to the expected length of the rods, given the axial rise per residue of 1.5 A for the $\alpha$-helix. X-ray diffraction patterns of magnetically oriented films were consistent with the supramolecular structure in which helical rods are arranged in layers with their helical axes approximately perpendicular to the smectic layers. Transmission electron microscopy and electron diffraction on the PBLG films revealed a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in cholesteric order. Detailed examination of the relative orientation of the banding in the morphology image and the reflections in the electron diffraction pattern leads to the conclusion that the structure of the unoriented smectic PBLG is that of a smectic A*. The monodisperse poly($\gamma$-4-(hexadecyloxy)benzyl $\alpha$,L-glutamate)s exhibit strong melting transitions at around $40\sp\circ$C; however no ordered melt was observed due in part to the reduced aspect ratio brought about by the long alkyl side groups.
44

The interfacial behavior of Bombyx mori silk fibroin

Valluzzi, Regina 01 January 1998 (has links)
A new crystal structure has been observed for Bombyx mori silk fibroin at air-water interfaces. This structure, silk III, incorporates a left-handed three-fold polyglycine II conformation and an approximately hexagonal lattice. Detailed crystallographic studies using electron diffraction data have been used to characterize the silk III crystal structure. There is a hexapeptide repetitive sequence found in the crystallizable portions of silk fibroin. When this sequence, Gly-Ala-Gly-Ala-Gly-Ser, is in a threefold helical conformation, a row of alternating glycine and serine residues parallel to the helical axis results. One third of the helix thus becomes slightly hydrophilic, whereas the other two thirds consist of glycine and hydrophobic alanine residues. The data indicate that the helices are arranged so that the serine residues pack preferentially in serine-rich sheets in the (110) planes of the crystal. The result is a monoclinic crystal structure, where the basal plane angle $\gamma$ is 116$\sp\circ$ rather than the 120$\sp\circ$ expected for perfect hexagonal packing, due to the distortion in nearest neighbor interhelical packing distances that results when the serine residues have a preferred packing. The separation of hydrophobic and hydrophilic residues in a threefold helical conformation of fibroin suggests that the air-water interface may stabilize the threefold conformation because this conformation allows the fibroin to behave as a surfactant at the interface. The sheet-like arrangement of serine residues deduced for the monoclinic silk III crystallites also supports a role for surfactancy in stabilizing the silk III structure at the air-water interface. If the crystallizable portions of fibroin are behaving as a surfactant, then the three-fold helical silk III structure should be oriented with the axes of the three-fold fibroin helices in the plane of the interface. This orientation is observed in uncompressed fibroin films which were picked up onto TEM grids. In LB films compressed to 16.7 mN/meter on the trough prior to being picked up onto TEM grids a uniaxial orientation is observed for silk III, with the helical axes perpendicular to the plane of the sample film. A surface compression of 34 mN/meter results in films containing silk II crystallites with the same uniaxial orientation, placing the helical axes perpendicular to the plane of the film. In addition to air-water interface experiments several experiments were carried out using aqueous-organic interfaces. A hydrated crystal structure incorporating a left-handed 6/2 helix which is still roughly three-fold, is observed at the water-hexane and water-chloroform interfaces. Large lamellar crystallites possessing a hexagonal habit were observed at both of these interfaces. A banded cholesteric mesophase which in some regions crystallizes into the same silk III hydrate structure as the lamellae was also observed and characterized.
45

An analysis of molecular parameters governing phase separation in a reacting polyurethane system

Yontz, Dorie J 01 January 1999 (has links)
The evolution of microstructure in polyurethane foams is a complex process involving competing kinetics of polymerization, crosslinking, and phase separation. Hard segments form in-situ and the increase in molecular weight induces phase separation. Coincident with these processes is a significant rise in viscosity through crosslinking of the trifunctional polyether soft segment. The inherent temperature rise that accompanies foaming complicates matters further and makes morphology control difficult. The resulting morphologies are complex and cannot be explained by traditional views of polyurethane morphology. Based on morphological and mechanical data, a new model of the phase-separated structure is proposed. This model incorporates the notion of interconnectivity of ordered hard domains through bridges of either extremely long hard segments or constrained soft material. To identify factors responsible for the final morphology in these polyurethanes and to correlate mechanical properties to the polymer microstructure without the complication of a cellular structure, films were synthesized isothermally. By exploiting the influence of temperature on the relative rates of phase separation and crosslinking, it was possible to create very different morphologies. Infrared spectroscopy, atomic force microscopy, and transmission electron microscopy show that films prepared at low reaction temperatures have organized, continuous hard domains. These materials have very high moduli and mechanically behave as continuous structures. In contrast, films prepared at high temperatures exhibit isolated, poorly ordered hard domains. These films are much weaker mechanically, although the volume fraction of hard segment is the same. The differences in morphology are accredited to varying rates of crosslinking relative to phase separation. At low reaction temperatures, phase separation proceeds faster than chemical crosslinking, but at higher temperatures, crosslinking dominates. Factors potentially responsible for the change in relative rates are addressed. Analysis of hard segment mass distribution by Matrix-Assisted Laser Desorption Ionization mass spectrometry shows that hard segment length is not a critical factor governing the phase separation behavior. Instead, an increase in viscosity associated with an increase in crosslinking rate with temperature is responsible for the observed homogeneous morphology in films prepared at high reaction temperatures. However, hard segment length does contribute to the mechanical properties by providing connectivity between domains.
46

Synthesis and characterization of chlorinated bisphenol-based polymers and polycarbodiimides as inherently fire -safe polymers

Stewart, Jennifer Rebecca 01 January 2000 (has links)
Two different types of polymers were synthesized and their degradation and combustion behavior were investigated. The first class, 1,1-dichloro-2,2-(4-hydroxyphenyl)ethylidene (bisphenol C) based polymers, were found to be among the most fire-resistant polymers with peak heat release capacities as low as 20 J/g-K. Polymers containing bisphenol C all exhibited exothermic decomposition behavior. When compared to corresponding bisphenol-A-based polymers, these bisphenol-C-containing polymers had higher char yields and lower decomposition temperatures. The presence of bisphenol C in materials, whether as a co-monomer or blends, showed a char enhancement effect; yielding higher char than what is expected by a purely additive affect. Bisphenol C polyarylates and polycarbonates yielded large amounts of HCl and carbon dioxide upon decomposition. Compared with other bisphenol-based polymers, polycarbonates and polyarylates containing bisphenol C yielded significantly less amounts of monomer. Decreasing the concentration of bisphenol C in the copolymers or blends yielded relatively more monomer in the degradation products. The second class of polymers studied were polycarbodiimides, which generally decompose in an endothermic manner to yield quantitative amounts of monomer. The incorporation of TEMPO-containing side chain substituents altered the degradation. TEMPO-based polycarbodiimides decomposed in an exothermic fashion and yielded several other degradation products in addition to the original monomer. These free radical containing polymers showed a 25% reduction in the peak heat release capacity when compared with the control polymer. Neither polymer was found to be fire resistant which is due to their high organic content and essentially 100% weight loss during decomposition.
47

Model polypeptides and molecular recognition at a monolayer interface

Dawson, Susan Lee 01 January 2000 (has links)
The enormous interest in organic thin films during the last fifteen years has been driven by the prospect of utilizing these films in applications, particularly in the areas of nonlinear optics, pyroelectric materials, sensors and protective layers. The ease of formation and monolayer stability of self-assembled films, has been responsible, in part, for this renaissance. In fact, these self-assembled monolayers are now widely used to study more complex systems. Presented herein, is an example of a self-assembling process, wherein a relatively weak hydrogen bonding interaction (molecular recognition) leads to the formation of bilayers. The molecular recognition at a 2,4-diaminopyrimidine terminated monolayer (host) by a succinimide derivative (guest) has resulted in the formation of bilayers. The bilayer structures and the hydrogen bonding interactions were analyzed by external-reflection Fourier transform infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. The lateral stability of self-assembled monolayers is limited by the strength of the van der Waal interaction, although, the stability is improved over Langmuir-Blodgett systems. We propose to improve these systems further. We have been exploring the self-assembly of artificial proteins on metals. The hydrogen bonding capacity of protein monolayers would be expected to provide enhanced stability for these films due to the multiplicity of hydrogen bonds. Polymers of sequence [-(AlaGly)3CysGly(AlaGly)3GluGly-] n have been designed to adopt b-sheet structures on metallic and oxide surfaces. To better understand and control polypeptide adsorption, we have focused on the self-assembly of model compounds which capture the substrate and lateral interactions. Results are reported for the synthesis of layered arrays of C17H35COCysOCH3 and CH3CO(AlaGly) 3CysOCH3 on gold and the characterization of these arrays via ellipsometry, vibrational spectroscopy and X-ray photoelectron spectroscopy.
48

Solid-state NMR studies of PEO in various environments: Conformation, chemical shift, and dynamics

Harris, Douglas Jeffrey 01 January 2000 (has links)
Double-quantum NMR spectroscopy was employed to determine the torsion angle distributions of the OC-CO bonds in poly(ethylene oxide), PEO, in various environments: crystalline PEO, PEO-p-nitrophenol (PEO/PNP) and PEO-resorcinol molecular complexes, PEO nanocomposites with a smectite clay and MoS2, and PEO-poly(α,L-glutamate) blends. The technique not only allowed quantitative determination of both the relative ratios and average torsion angles for the trans and gauche bonds, but also set an upper limit for the width of the torsion angle distribution. The studies confirmed that the OC-CO bonds in crystalline PEO all adopt a gauche torsion angle and determined an average angle of Ψ = 74 ± 4° with a distribution width of σΨ < 9°. The spectra of the PEO-resorcinol and PEO-poly(α,L-glutamate) blends showed little difference from that of crystalline PEO. PEO/PNP was shown to be a suitable model system for studying conformation because 1/3 of OC-CO bonds are trans and the complex has extremely slow dynamics at room temperature. Simulations of the double-quantum spectra set an upper limit for the trans torsion angle distribution width of σ Ψ < 7°. Finally, the OC-CO bonds of the PEO chains in the nanocomposites were found to be 90 ± 5% gauche, which provides valuable constraints on the possible chain conformation in the intercalation gaps. The effects of packing, hydrogen bonding, and conformation on 13C chemical shift were studied in crystalline PEO and the PEO/PNP complexes. CP-MAS spectra of crystalline PEO at 200 K, acquired under strong 1H decoupling and after filters to suppress the mobile components, resolved four distinct peaks. Deconvolution of the 1D spectrum showed chemical shifts ranging from 74.7 to 71.6 ppm for the 14 carbons in the 72 helical repeat unit. Dynamics of the conductive phase was studied in polymer electrolytes and block copolymers from poly(oligo(oxyethylene) methacrylate) macromonomer (POEM), lauryl methacrylate, n-butyl methacrylate, and methyl methacrylate. Enhanced conductivity of diblock copolymer electrolytes with a low Tg non-conductive phase is at least partially due to faster chain dynamics in the conductive phase. A higher Tg non-conductive block shifts the observed dynamics curves towards a higher temperature by approximately 5°C. The dynamic inhomogeneity in the POEM side chain was determined by wideline separation experiments to be on a length-scale of less than 5 nm. (Abstract shortened by UMI.)
49

The effect of branch distribution on morphology, chain dynamics and rheological behavior of metallocene and Ziegler -Natta linear low density polyethylenes

Gelfer, Mikhail Y 01 January 2000 (has links)
The effects of the branching distribution in ethylene-α-olefin copolymers (LLDPEs) on morphology and molecular mobility in the solid state, and rheological properties in the melt and during crystallization were studied. Two types of ethylene-α-olefin-copolymers were examined: (I) Ziegler-Natta LLDPE containing significant amounts of highly and low-branched chains, and (II) metallocene LLDPE where branched α-olefin units are statistically distributed among the molecules. Solid-state NMR techniques, WAXS, DSC, TEM and Raman spectroscopy were utilized to characterize LLDPEs in the solid state. A novel solid-state NMR technique for the determination of the lamellar thickness distribution was developed and tested during this investigation. The crystallization kinetics was measured by DSC. Rheological properties in the melt and during crystallization were characterized using oscillatory and steady shear techniques. In Ziegler-Natta copolymers the crystalline domains are predominantly formed by almost linear chains while highly branched molecules are excluded into the amorphous region. In the metallocene system, crystalline and amorphous domains are formed by segments belonging to the same chains. So numerous covalent links exist between crystalline and amorphous regions. As a result, the metallocene system has thinner crystalline and amorphous domains. It shows lower molecular mobility in the amorphous phase than its more branched Ziegler-Natta analog. The existence of a crystalline-amorphous interface formed by all-trans yet partially mobile chains was proven for LLDPEs by NMR. The morphological partitioning of branched units and local chain conformation near the crystalline defects in LLDPEs was characterized by solid-state NMR techniques. It was shown that for both metallocene and Ziegler-Natta LLDPEs the critical crystallinity &phis;cr, at the gel point, i.e. the melt-solid transition, does not exceed 5% [w/w]. A higher crystallization rate and a narrower solidification interval is observed for a Ziegler-Natta copolymer and can be related to the significant content of almost linear molecules. Step-crystallization and partial melting temperature programs for the preparation of stable near-critical physical gels, whose crystallinity remains in the vicinity of &phis;cr on the timescale of hours, were developed. The overall crystallinity and local chain conformation in stable near-critical gels prepared from metallocene LLDPE were studied using Raman spectroscopy.
50

Atom transfer radical polymerization: Fundamentals, challenges, and application

Kwark, Young-Je 01 January 2001 (has links)
Various aspects of atom transfer radical polymerization (ATRP) were investigated. In an attempt to find a novel catalyst system for ATRP, we screened the activities of various metal complexes using a combinatorial approach. Several new catalyst systems including FeCl2/bam(TMS) were found to be active ATRP catalysts in the polymerization of styrene and MMA. In order to make this combinatorial screening a viable method of quickly discovering usable systems, we tried to find a fast and reliable method to evaluate the catalysts. A parameter estimation method based on nonlinear regression was developed to evaluate various catalyst systems by determining kinetic parameters of polymerization. From our model system considering small molecular atom transfer addition reaction, we found that equilibrium constant of atom transfer reaction could be successfully estimated. A new model dealing polymerization itself was also developed, and we could demonstrate that each values of activation and deactivation reaction rate constant can be estimated unambiguously. On screening the catalyst systems for ATRP, we found some titanium complexes gave a control in the polymerization of styrene without the aid of Group I–III cocatalysts. A series of experiments to elucidate the mechanism of polymerization all support that radical mechanism is involved in the polymerization using bis-(cyclopentadienyl)titanium dichloride. A possibility of ATRP mechanism was checked by isolating intermediate species. It is found that the polymerization is not followed the pure ATRP pathway, but is comprised of various competing reactions. Several strategies has been developed to prepare polymers having higher order structure including branched, hyperbranched, star, and dendrigrafts. The combination of nitroxide mediated SFRP and ATRP techniques successfully provided relatively simple routes to from branched and hyperbranched polymers in controlled structures. To overcome this limitation of backbone polymer prepared by SFRP, a new strategy using protection-deprotection chemistry was employed. Among the various protected monomers tested, we could prepare branched polystyrene having controlled structure using VBt-BOC and 4-methyl styrene. As an example of diversity of this strategy, we also could prepare the branched acrylate polymer having controlled structure.

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