Department: Polymer Science ![[clear]](close-x.png "Remove this limiter")
79 matches in the database.
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1.
Akgun, Bulent.
SURFACE AND INTERFACE STRUCTURE OF DIBLOCK COPOLYMER BRUSHES.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► The main objective of the work presented in this thesis is to…
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▼ The main objective of the work presented in this thesis is to understand the surface and interface structure and dynamics of diblock copolymer brushes (DCBs). DCBs are stimuli-responsive materials and the surface properties of a DCB can be changed from those characteristic of one polymer block to those characteristic of the other one by treating the DCB with a solvent selective for one of its blocks. For this purpose, polystyrene-block-polyacrylate or polyacrylate-block-polystyrene brushes were synthesized using the “grafting from” technique in combination with atom transfer radical polymerization (ATRP). In the first part of this project the internal structure of DCBs after the synthesis and surface rearrangement were investigated using neutron reflectivity (NR) and grazing incidence small angle X-ray scattering (GISAXS). It was found that the internal brush structure depends strongly on the synthesis sequence of polymer blocks and the value of xN. For small values of xN (xN is smaller than 11), a model of two layers with an interfacial region of finite width provides a good description of the data. The interface width is found to be larger for DCBs which have the polymer block with the lower surface energy synthesized next to the substrate. A three layer model must be used to describe the structure of DCBs of larger xN values (xN is greater than 23) and of sufficiently asymmetric composition. The necessity of including a third layer is consistent with the presence of a lateral ordering of some type in the center of the brush, as evidenced by correlation peaks in the GISAXS data. The spacing of the in-plane ordering varies with the thickness of the poly(methyl acrylate) (PMA) block. After a DCB is treated with a selective solvent, Bragg rods appear in the GISAXS pattern. The appearance of Bragg rods indicates the formation of a new 2D structure which has a lateral spacing on the order of the total thickness of the brush. The Bragg rods disappear upon heating to 80 degree celcius. Roughness correlation between the interfaces of a DCB has been detected using longitudinal diffuse X-ray scattering and X-ray reflectivity. The amplitude of the fringes in the longitudinal diffuse scattering decreases with increasing thickness of the DCB, indicating that the interactions between substrate and brush surface decrease as the thickness of the DCB increases. Roughness correlation of the brush is observed after annealing the brush but, it is lost after the brush is swollen in the vapor of a nonselective solvent, dichloromethane. A remarkable suppression of polymer film surface fluctuations by the tethering of one end of each chain in a densely grafted brush has been demonstrated experimentally using X-ray Photon Correlation Spectroscopy (XPCS), a direct measure of dynamics. Within the range of time and length scale investigated, 0.2 s to 1100 s, and 200 nm to 5 um, no relaxation of the brush surface could be observed, even 170 degree celcius above the bulk glass transition temperature of corresponding untethered chains, suggesting that the surface relaxation time is increased by at least three orders of magnitude as compared to a film of untethered chains of comparable thickness. The observation of the suppression of long wavelength fluctuations on the surface of a dry homopolymer brush is consistent with theoretical predictions.
Advisors/Committee Members: Foster, Mark D.
Keywords: Polymer brushes, diblock copolymer brushes, surface fluctuations,stimuli-responsive thin polymer films, interface structure of thin films.
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2.
Barrios, Carlos A.
Modified Scanning Probes for the Analysis of Polymer Surfaces.
Degree: Doctor of Philosophy, Polymer Science, 2009, University of Akron
► Scanning probe microscopies (SPMs) allow the observation and measurement of surface properties…
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▼ Scanning probe microscopies (SPMs) allow the observation and measurement of surface properties on a highly local level based on the interaction between a very sharp probe and the surface. Interaction stability and probe integrity, at least during the scanning time, are necessary, but still a challenge. Conventional scanning probes have been used for revealing morphological features connected with macroscopic properties of complex latex films. These films, when dry, are sufficiently hard to be studied with minimal distortion of the surface by the probe. By comparing formulations with various fluorosurfactant concentrations, it was demonstrated how the fluorosurfactant, which is known heuristically to improve leveling, gives the system more mobility before drying. Specifically, in styrene-butadiene copolymer formulations, sub-micrometer differences in surface morphology, observed as a function of fluorosurfactant concentration, correlated with differences in gloss. In some cases, surfaces are too soft or too adhesive to be studied by conventional probes. Modification of scanning probes with hydrophobic layers having methyl or fluorinated end groups has been used to weaken the probe-surface interaction to study model adhesive surfaces. Acrylic block copolymers with well-defined molecular weight and composition were synthesized by controlled radical polymerization to create models of pressure sensitive adhesives to evaluate performance after aging or humidity exposure.Macroscopically, tack measurements demonstrated that blending a triblock copolymer with a homopolymer of the midblock polymer can be used to tailor the tack value. Microscopically, force-penetration curves of the acrylic blends showed a superficial phase separation into two mixed phases as homopolymer content was increased. Hydrophobic modification made it possible to measure these surfaces that would otherwise be too adhesive to study with conventional probes. Modification of scanning probes has also been used to bring robustness to high resolution chemical imaging. Addition of a 2-3 nm Al2O3 protective layer on silver-coated scanning probes dramatically reduced degradation of the probes with time, without sacrificing initial probe efficiency. In addition, the protective layer improved markedly the wear resistance of the probes. The surface morphology of the silver structure was not altered and the signal enhancement decay was completely stopped. These results agree with the conjecture that an ideal dielectric coating shifts the epicenter of the electromagnetic field enhancement in plasmonic structures, a behavior previously simulated, but not demonstrated. New types of plasmonic structure fabrication were also explored as means to surpass currently available signal enhancements by controlled size and shape, increased roughness or higher crystallinity. A single metal nano-pillar deposited by electron beam induced deposition (EBID), crystalline dendritic structures deposited by metal replacement reactions, and layers with sharper asperities deposited by ion sputtering were all fabricated on conventional scanning probes. The last produced a significant enhancement. In addition, strong signal fluctuations in Raman band intensity (“blinking”), similar to those seen in single molecule surface enhanced Raman spectroscopy, were also observed. This approach opens exciting new possibilities.
Advisors/Committee Members: Foster, Mark.
Subjects: Analytical chemistry; Materials science; Optics; Physics; Polymers
Keywords: scanning probe microscopy; SPM; AFM; modified probe; tip enhanced Raman spectrocopy; TERS; nanoRaman; plasmonics; ultrathin; dielectric; SERS; acrylic block copolymer; surface segregation; latex films; fluorosurfactant; SAM; scanning probes
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3.
Begen, Burak.
INFLUENCE OF PRESSURE ON FAST DYNAMICS IN POLYMERS.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► One of the biggest challenges in solid state physics today is understanding…
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▼ One of the biggest challenges in solid state physics today is understanding the nature of the glass transition. Dynamic studies are critical in solving some of the problems in the field. Until recently, investigations of dynamics in glass formers were mostly carried out as a function of temperature. However, with the advancements in experimental techniques and methods, the interest towards using pressure as an additional experimental variable increased. The advantages of pressure over temperature are two-fold: First, it only alters the density of the system, whereas temperature changes both the thermal energy and the density, and secondly, one can achieve significant density changes (~20%) with pressure, whereas temperature creates smaller density changes (~5%). These advantages let researchers make direct comparisons of the results with glass transition models (i.e. free volume ideas). The dynamics in the frequency range between 1 GHz and 5 THz (fast dynamics), are thought to have a crucial role. Crystals in this frequency range have a Debye-like density of vibrational states. Glasses, however, have two extra contributions when compared to crystalline structures: (i) an anharmonic relaxation-like contribution that appears as a quasielastic scattering (QES) and (ii) a harmonic vibrational contribution, which shows up as the boson peak (BP) in light and neutron scattering spectra. It has also been shown experimentally that fast dynamics in glasses are strongly correlated with the temperature dependence of structural relaxation.In this dissertation the influence of pressure on fast dynamics in polyisobutylene, polyisoprene and low molecular weight polystyrene is investigated using inelastic light, neutron and X-ray scattering techniques. The results are compared to the predictions of the existing models.The results for all polymers studied showed that the boson peak shifts more strongly than sound modes, suggesting that the variations cannot be fully described by the transformation of elastic continuum as has been claimed in a few recent publications. It is also observed that the boson peak intensity decreases significantly under pressure. However, when scaled to the Debye level, the boson peak intensity increases under pressure for all polymers. The analysis of the QES showed a strong decrease of intensity under pressure. These variations strongly correlate with the change in the boson peak intensity under pressure, suggesting a relationship between QES and BP.
Advisors/Committee Members: Sokolov, Alexei P.
Keywords: pressure, high pressure, high pressure cell, diamond anvil cell, DAC, polymers, fast dynamics, boson peak, quasielastic scattering
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4.
Bhargava, Prachur.
SELF-ASSEMBLED POLYSTYRENE-BLOCK-POLY (ETHYLENE OXIDE) (PS-b-PEO) MICELLE MORPHOLOGIES IN SOLUTION.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► In this research, we have investigated the self-assembly behavior of the amphiphilic…
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▼ In this research, we have investigated the self-assembly behavior of the amphiphilic diblock copolymer polystyrene-b-poly(ethylene oxide) PS962-b-PEO227 in DMF/water and DMF/acetonitrile mixtures. We have used solution conditions, namely copolymer concentration, solvent composition and temperature to control the morphology of aggregates. With increasing the water concentration in the DMF/water or the acetonitrile concentration in the DMF/acetonitrile system, the micelle morphology observed in transmission electron microscopy changed from spheres to worm-like cylinders and then, to vesicles. Critical micelle concentrations were determined by static light scattering. Morphological diagrams were constructed from the study of the micelle morphology changes in different copolymer concentrations. Based on the observations of morphological reversibility and annealing experiments, these two morphological diagrams were proven to be in thermodynamic equilibrium. Although the trend in morphological changes was identical in these two systems, there were remarkable differences in the morphological diagrams of PS962-b-PEO227. At higher copolymer concentrations a ‘worm-network’, which led to an order of magnitude increase in the inherent viscosity of the colloidal system was formed only in the DMF/water system.4 The percentage of selective solvent required to induce morphological changes was much higher in case of DMF/acetonitrile system. The driving force for these morphological changes was understood to approach micelle free energy minimization. Approximate micelle free energy calculations confirmed that the free energy decreases as the morphology changes from spheres to worm-like cylinders and then to vesicles with an increase in the selective solvent concentrations, and is dominated by the free energy of the interface. Further insight into the thermodynamic nature of morphological changes was achieved by inducing temperature driven morphological changes in DMF/water mixtures. With increasing the temperature, changes in the morphology from vesicles to worm-like cylinders and then to spheres were observed. Cooling the system back to room temperature regenerated the vesicle morphology indicating that the morphological changes are reversible. No hysteresis was observed in the morphological transitions during heating and cooling, indicating that the system is in thermodynamic equilibrium. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer concentration and water content in the DMF/water mixture. The driving force for these morphological changes was understood to be reduction in the free energy of the corona and interfacial energy, which resulted in minimization of the micelle free energy.
Advisors/Committee Members: Cheng, Stephen Z.D.
Subjects: Chemistry, Polymer
Keywords: block copolymer; micelle; self assembly; POLYSTYRENE-BLOCK-POLY (ETHYLENE OXIDE)
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5.
Bhargava, Sphurti.
Submicron Structures, Electrospinning and Filters.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► Electrospun fibers have been used in high efficiency filters for over two…
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▼ Electrospun fibers have been used in high efficiency filters for over two decades. The electrospun fibers are responsible for better performance of the filters. In this work, experiments were done in which electrospun nanofibers were exposed to a stream of oil fog. The growth rate of drops, by coalescence of oil droplets on a single fiber, was followed using optical microscopy. Different mechanisms by which the aerosol droplets can be captured include: interception, Brownian motion of droplets, inertial impaction and vapor deposition by diffusion. A quantitative model for predicting the growth rate by coalescence was developed and tested. The drop growth mechanism on a nanofiber can be attributed to both interception and Brownian diffusion mechanism in the creeping flow conditions. The relative contribution by the individual mechanisms was estimated. Merging of growing drops on the fiber was modeled and experimentally observed. The experimental merging time for drops is close to that predicted. We have discovered a novel technique of forming polymer nanostructures on various substrates, which involves vaporizing cyanoacrylate monomer. Cyanoacrylate monomer vapor was collected on a solid surface and polymerized to form nanofibers. Tiny spots of initiator on the surface of a substrate and small monomer droplets in the monomer vapor appear to be required for the growth of the polycyanoacrylate nanofibers. Morphology and size of the structures can be controlled by varying temperature, amount of monomer, amount of initiator and setup parameters. The polycyanoacrylate nanofibers create a network and increase the specific surface area significantly. Such structures can help in controlling the porosity and improving the structural stability of a filter. We have investigated the manufacturing process of yarns from electrospun nanofibers. The process involves electrospinning between two surfaces both grounded and placed at a certain distance apart. One of the collector surfaces is rotated at a particular speed to give twist to the fibers collected, thus obtaining yarn. Pulling the other collector away from the rotating collector, leads to continuous yarn manufacture. This method is useful for manufacture of composite and hollow yarn structures.
Advisors/Committee Members: Reneker, Darrell H.
Subjects: Engineering, Materials Science
Keywords: Electrospinning; cyanoacrylate; structures; nanofibers; yarn; filters; coalescence
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6.
Boonkerd, Kanoktip.
EFFECT OF EFFICIENCY OF SULFUR VULCANIZATION ON PROPERTIES OF GUM AND BLACK-FILLED NATURAL RUBBER VULCANIZATES.
Degree: Doctor of Philosophy, Polymer Science, 2006, University of Akron
► The effect of the efficiency of sulfur vulcanization on cure behavior and…
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▼ The effect of the efficiency of sulfur vulcanization on cure behavior and tensile strength of gum and black-filled NR vulcanizates was studied. NR vulcanizates with different sulfur to accelerator (DCBS) ratios were prepared. In addition, one vulcanizate was prepared using TMTD, without free sulfur. All vulcanizates had about the same crosslink density. The vulcanizate cured with the highest S/DCBS ratio (lowest cure efficiency) is expected to contain mainly polysulfidic linkages, while the vulcanizate cured with TMTD (highest cure efficiency) should contain monosulfidic linkages. For both gum and black-filled compounds, reversion was highest at intermediate S/DCBS ratios, and compounds with the highest ratio had the lowest tensile strength. Crystallizability is proposed to control the strength of the NR vulcanizates, rather than just the type of sulfidic linkages. Adding carbon black caused more reversion. Cure efficiency strongly influenced the tensile strength of pre-cut gum specimens. The gum vulcanizate with the highest cure efficiency had the highest cut growth resistance. Gums also showed a large drop in strength at a critical cut size, ccr, which increased with increasing cure efficiency. Cure efficiency had less effect on the cut growth resistance of filled compounds. No ccr was found in these. However, the black-filled compound with the highest cure efficiency still showed the highest strength. Gums showed simple lateral crack propagation, while black-filled compounds developed longitudinal cracks. Carbon black accelerates strain-induced crystallization, resulting in sufficient anisotropy to cause longitudinal cracking. Five different crack patterns were identified. These depended on cut size and efficiency of cure.
Advisors/Committee Members: Hamed, Gary R.
Subjects: Chemistry, Polymer
Keywords: sulfur vulcanization; cure efficiency; crosslink type; natural rubber; tensile strength; cut tensile strength
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7.
Boukany, Pouyan E.
Nonlinear Flow Behavior of Entangled DNA Fluids.
Degree: Doctor of Philosophy, Polymer Science, 2008, University of Akron
► Understanding the nonlinear flow behavior of entangled fluids including shear thinning (related…
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▼ Understanding the nonlinear flow behavior of entangled fluids including shear thinning (related to stress plateau) or transient stress overshoot upon startup shear remains a fundamental challenge in polymer rheology. For more than five decades, a conventional rheometric measurement was carried out to investigate the nature of these nonlinear viscoelastic properties in entangled fluids by treating them as liquids that could deform homogenously. Recent particle-tracking-velocimetric (PTV) measurements based on 1,4 polybutadiene (PBD) solutions has found that shear inhomogeneity occurs after stress overshoot when sheared on time scale faster than terminal relaxation time (γ̇appτ > 1.0). Because of experimental difficulties such as edge fracture during shearing, it is challenging to achieve reliable steady state in synthetic entangled polymeric liquids. In this work, we choose DNA solutions as model entangled systems. Since the required DNA concentration is rather low, between 5 to 22 mg/mL, these samples can be sheared for an infinitely long time without experiencing significant edge instability. A wide variety of DNA solutions with various number Z of entanglements per chain and solvents are studied by combination of PTV and rheometric measurements. In the Newtonian regime (γ̇appτ < 1.0), the uniform flow was displayed at all time of shearing for entangled DNA solutions and there is no stress overshoot during measurement. For least entangled DNA solution with Z = 24, transient shear inhomogeneity was presented after stress overshoot, however at long time the velocity field returned to linearity across the gap. For the strongly entangled DNA solutions (Z ≥ 60), the permanent shear banding was observed in both startup shear and creep in the stress plateau (shear thinning) regime. The thickness of high-shear band eventually grows to span the entire gap at the upper end of the plateau region. It was shown that using of glycerol as a solvent minimized interfacial wall slip, permitting shear banding to develop in the bulk at rates that can be studied with the current PTV method. Finally, we found that strong shear banding during startup shear can be avoided by either slow ramp-up from low rate (γ̇ ~ 10-4 < τ-1, s-1) or quenching down from homogenous shear at sufficiently high rate to the specific rate. The final steady shear stress in presence or absence of shear banding is the same exhibiting the limitation of conventional rheometric measurements to depict nonlinear flow behavior of entangled fluids.
Advisors/Committee Members: Wang, Shi-Qing.
Subjects: Polymers
Keywords: Non-linear flow; DNA; Entangled fluids; Rheology; Particle-tracking-velocimetric
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8.
Buehler, Betul.
Molecular Adhesion and Friction at Elastomer/Polymer Interfaces.
Degree: Doctor of Philosophy, Polymer Science, 2006, University of Akron
► We have studied the contact interface between elastomeric poly(dimethyl siloxane) (PDMS) lenses…
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▼ We have studied the contact interface between elastomeric poly(dimethyl siloxane) (PDMS) lenses with various solid surfaces during adhesion and friction using IR-visible sum frequency generation spectroscopy (SFG). SFG in total internal reflection (TIR) geometry can be used to determine molecular structure at the polymer/solid and polymer/polymer contact interfaces. It is a nonlinear optical technique, which detects the orientation and density of molecules at interfaces. In this study, we have designed a novel approach to couple SFG with adhesion and friction experiments. The solid surfaces were chosen to be octadecytrichlorosilane monolayer (OTS), poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC), polystyrene (PS), poly(n-butyl methacrylate) (PnBMA), and poly(n-propyl methacrylate) (PnPMA). In the first part of the research, we have concentrated on the importance of characterizing the static contact interface in relation to adhesion. Our results for the OTS in contact with oxygen plasma treated PDMS show surprising surface restructuring, which results in adhesion hysteresis. The short PDMS chains generated during plasma treatment are locally confined and are as strongly ordered as OTS. SFG spectra from other surfaces (sapphire substrates and fluorinated monolayers (FC)) indicates that short PDMS chains require not only confinement but also an ordered template provided by the methyl groups of OTS. In the second part, we have studied the sliding contact interfaces of various polymers with PDMS. The friction forces between PDMS lenses and glassy PS are about four times higher than PDMS sliding on crystalline well-packed PVNODC surfaces. This cannot be explained by the difference in adhesion energy or hysteresis. The in-situ SFG measurements indicate local interdigitation during contact, which is evident from the change in orientation of PS phenyl groups upon mechanical contact and during sliding compared to that at the PS surface. Such a local penetration is unexpected at room temperature (T R) that is much below T g. For comparison, we have also studied PnBMA and PnPMA having T gbelow and above T R, respectively. Preliminary studies at the contact and sliding interfaces with PDMS exhibited similar interpenetration based on adhesion, friction and SFG results regardless of the bulk T g. In addition, our results indicate that the adhesion energy and hysteresis of surfaces are not sufficient to predict their friction properties, which makes the characterization of the molecular structure at the static and dynamic contact essential. Finally, we have reported a fabrication process of constructing polymer surfaces with multiwalled carbon nanotube (MWNT) hairs. The force measurements with scanning force microscope (SPM) indicated strong nanometer level adhesion forces, 200 times higher than those observed for Gecko foot-hairs. These forces are a combination of van der Waals forces and energy dissipation during the elongation of the carbon nanotubes which comes from their material properties. In the velocity region of 0.50Hz up to 14Hz there is no detectable change in the energy dissipation and contact area. In conclusion, we have demonstrated that structures found in Gecko’s feet can be fabricated on polymer surfaces by using multiwalled carbon nanotubes.
Advisors/Committee Members: Dhinojwala, Ali.
Subjects: Chemistry, Polymer
Keywords: adhesion; friction; adhesion and friction of elastomer; elastomer polymer interfaces; sum frequency generation spectroscopy; SFG; molecular adhesion and friction; Gecko; Gecko foot-hair; mimicking Gecko foot-hair; synthetic adhesives
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9.
Chai, Xiaoli.
Reinforcement and Cut Growth in Swollen and Unswollen Filled Rubber Compounds.
Degree: Doctor of Philosophy, Polymer Science, 2008, University of Akron
► In reinforced rubber, filler and rubber molecules interact, forming additional “effective crosslinks”.…
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▼ In reinforced rubber, filler and rubber molecules interact, forming additional “effective crosslinks”. During deformation, rubber chains slide (disbond /rebond) over the filler surface. This causes energy dissipation and stress softening, which make it difficult to study filler-rubber interactions. In this thesis, stress-softening was depressed using controlled swelling techniques. Natural rubber compounds filled with different levels of carbon black and silica were made, and cure characteristics determined. Cured samples were swollen to various degrees under controlled conditions. Dynamic mechanical thermal analysis (DMTA), tensile tests and torsion tests were performed using unswollen and swollen samples. Dynamic modulus, Young’s modulus, shear modulus and stiffness were compared. At a critical degree of swelling, stiffness and strength dropped greatly, indicating loss of reinforcement. With further swelling, dynamic and tensile moduli were independent of strain, and stress-strain curves started to superimpose. This is hypothesized to result from the disruption of filler-filler and filler-rubber bonds, an important source of filler reinforcement. Cut growth tests were performed on swollen black-filled natural rubber vulcanizates at different strain rates. The effects of swelling and strain rate on cut growth were studied, and crack patterns were observed. Critical cut size was independent of the degree of swelling and strain rate. When equilibrium swelling was reached, crack deviation, due to marked anisotropy of strength in stretched rubber, was suppressed. These observations support the hypothesis that sufficient swelling disrupts the filler network.
Advisors/Committee Members: Hamed, Gary.
Subjects: Polymers
Keywords: reinforcement, cut-growth, filled rubber, swollen
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10.
Chang, Tae-Eun.
Microscopic mechanism of reinforcement and conductivity in polymer nanocomposite materials.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► Modification of polymers by adding various nano-particles is an important method to…
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▼ Modification of polymers by adding various nano-particles is an important method to obtain effective enhancement of materials properties. Within this class of materials, carbon nanotubes (CNT) are among the most studied materials for polymer reinforcement due to their extraordinary mechanical properties, superior thermal and electronic properties, and high aspect ratio. However, to unlock the potential of CNTs for applications, CNTs must be well dispersed in a polymer matrix and the microscopic mechanism of polymer reinforcement by CNTs must be understood. In this study, single-wall carbon nanotube (SWNT) composites with polypropylene (PP)-SWNT and polystyrene (PS)-SWNT were prepared and analyzed. Microscopic study of the mechanism of reinforcement and conductivity by SWNT included Raman spectroscopy, wide-angle X-ray diffraction (WAXD) and dielectric measurement. For PP-SWNT composites, tensile tests show a three times increase in the Young’s modulus with addition of only 1 wt% SWNT, and much diminished increase of modulus with further increase in SWNT concentration. For PS-SWNT composites, well-dispersed SWNT/PS composite has been produced, using initial annealing of SWNT and optimum sonication conditions. The studies on the tangential mode in the Raman spectra and TEM indicated well-dispersed SWNTs in a PS matrix. We show that conductivity appears in composites already at very low concentrations, hinting at the formation of a ‘percolative’ network even below 0.5% of SWNT. The Raman studies for both composites show good transfer of the applied stress from the polymer matrices to SWNTs. However, no significant improvement of mechanical property is observed for PS-SWNT composites. The reason for only a slight increase of mechanical property remains unknown.
Advisors/Committee Members: Sokolov, Alexei P.
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11.
Chen, Chong.
Investigating Nonnative Contacts in Protein Folding.
Degree: Master of Science, Polymer Science, 2009, University of Akron
► Proteins are organic compounds, consisting of amino acids (residues) bound by peptide…
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▼ Proteins are organic compounds, consisting of amino acids (residues) bound by peptide bonds into polypeptide chains, and can fold into their unique functional structures (native state) without any chaperones. There is more than one simple model of proteins, among which Go model, HP model and “perturbed homopolymer model” are widely used. Amino acids in these models are considered as beads lying on the sites of a lattice. By extracting information from the system with different kind of computer algorithms, one hopes to predict the folding process. How and why do protein fold in this particular way is still not clear although it has been extensively investigated for more than half a century. Experimental techniques (NMR, X-ray crystallography, etc) as well as computer aided theoretical works (energy landscape, homology, etc) have been used to understand how a protein starts from its amino acids sequence into a functional structure. Nonnative contacts are defined as contacts (nearest-neighbor) not found in the native state, and can be used to study the process of folding. According to the recent literature, nonnative contacts affect protein folding rates, and make contributions to the unfolding pathways. Folding intermediates are also found rich in nonnative contacts. We have investigated nonnative contacts for proteins in a modified HP model by generating all possible conformations (enumeration) which enable us to carry out exact calculation for the nonnative contact density nc(e) as a function of the energy e as well as thermodynamically average nonnative contacts as a function of temperature of T. We have investigated different sequences to understand the sequence dependence of nc(e) and nc(T) . These results provide us with new understandings of the role for nonnative contacts play in the protein folding process, as are listed below: 1) The density nc(e)is always monotonically increasing in the standard model. In weakly interacting and strongly interacting model, the density nc(e)is spread out, but generally increasing. 2) The nc(T) for all models is generally monotonically increasing. A few violations can be seen. 3) The protein property is sequence dependent.
Advisors/Committee Members: Gujrati, Purushottam.
Subjects: Polymers
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12.
Chen, Yan.
SINGLE CRYSTAL ENGINEERING OF AMORPHOUS-CRYSTALLINE BLOCK COPOLYMERS CRYSTALLIZATION, MORPHOLOGY AND APPLICATIONS.
Degree: Doctor of Philosophy, Polymer Science, 2005, University of Akron
► Single crystals of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers with different molecular weights…
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▼ Single crystals of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers with different molecular weights and compositions were grown in dilute solution, and were utilized as ideal models for studying: (1) crystallization in nano-confined environments; (2) fabrication of uniform polymer brushes of varying tethering density; (3) modification of chemical and physical conditions for crystal growth. These studies have been referred to as “Single Crystal Engineering”. The glass transition temperature of the PS layer in the single crystal of a PEO-b-PS copolymer (Mn(PS)=17k g/mol, Mn(PEO)=11k g/mol) is higher than the melting temperature of PEO layer. Therefore, the melting and crystallization behavior of this single PEO nano-layer confined between two PS glassy walls can be studied. Results show that the crystal orientation of the confined PEO nano-layer depends on the recrystallization temperatures. After crystallization of PEO blocks, PS blocks are tethered to the basal surfaces of the crystal. The PS brushes generated from single crystals are uniform, because of the precision of the molecular weight and tethering density. The tethering density of PS brushes is manipulated by changing crystallization temperatures. It has been found that the PS brushes start overcrowding when the reduced tethering density of PS brushes is 3.7 ± 0.1, independent of solvent. During crystallization of PEO blocks in solution, the PS chains can chemically and physically affect the crystal growth front of PEO lamellae. The single crystals of a series of PEO-b-PS diblock copolymers were used as nucleation sites for the crystal growth of a homo-PEO fraction in solution. In the copolymers, Mn(PEO) is similar, and the Mn(PS) ranges from 4.6k to 17k g/mol. The results indicate that increasing the Mn(PS) leads to a higher reduced tethering density of the PS blocks on both the basal surfaces of the single crystals. The repulsion generated among the tethered PS blocks caused the PS blocks located near and at the edges to advance along the [120] direction. Novel channel-wire arrays on a submicrometer length scale having chemical and geometric recognitions have been fabricated via alternating crystal growth of PEO-b-PS and homo-PEO. The spacing between arrays can be controlled down to 50 nm.
Advisors/Committee Members: Cheng, Stephen.
Subjects: Chemistry, Polymer
Keywords: block copolymer; single crystal engineering; confinement; crystallization, nano film; nano channel; polymer brush
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13.
Constable, Andrew N.
Functionalization of Silica Micro-capillaries and Silica Nanoparticles via Polymer Brushes.
Degree: Doctor of Philosophy, Polymer Science, 2008, University of Akron
► Polymer brushes were synthesized on the interior of micro-capillaries to study the…
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▼ Polymer brushes were synthesized on the interior of micro-capillaries to study the influence of brushes on solvent flow through a confined space. The synthesis of several polymer brush coatings was performed in fused silica micro-capillary tubing by atom transfer radical polymerization using the grafting from approach. Characterization of the polymer coatings inside of the capillaries was challenging due to inaccessibility of the sample geometry. Fluorescence spectroscopy was used as a technique to verify that covalently-attached polymer brushes were present. Capillary rise measurements demonstrated that the Zisman critical surface energy changes as the polymer coatings were modified from either a hydrophilic or a hydrophobic surface. The hydrophilic and hydrophobic nature of polymers leads to flow in micro-capillaries that can be manipulated and controlled in a passive fashion without external stimulus. Backpressure measurements were performed to show how selective solvents can be used to alter the backpressure required for flow. The measurements were used to correlate the flow of good/bad solvents with different polymer coatings; we speculate how these solvents alter brush conformation as an explanation for differentiated backpressure measurements. An analogous study involved capillaries functionalized with small molecule silanes which supported the hypothesis that the polymer brush is in an extended (solvated) state in good solvent and in a collapsed stated in bad solvent. Preferential flow experiments were designed to measure the preferred path an aqueous solvent would take when comparing two capillaries containing polymer coatings of different surface energies. Water flow preferred the hydrophilic, higher surface energy coated capillary when comparing capillaries of the same internal diameter. When the difference in internal size became too large, the surface energy effect was overwhelmed and the aqueous solvent flowed down the larger, hydrophobic capillary. The second portion of this dissertation describes the in situ formation of functionalized silica nanoparticles. The reactive stabilizers used in the study were (3-(2-bromoisobutyryl)propyl)triethoxysilane and (3-(2-bromoisobutyryl)propyl)ethoxy-dimethylsilane. Both stabilizers have an ATRP initiator at the non-condensable end yielding an initiator-immobilized silica nanoparticle. With the initiator-functionalized silica nanoparticles, ATRP synthesis was performed with styrene, tert-butyl acrylate and methyl acrylate. The size of these functionalized silica nanoparticles was controlled by varying the reactive stabilizer concentration and the time of addition. This work was extended by using preformed polymer chains as the reactive stabilizer. The polymer chain contained a monoethoxysilane-functional group which condensed to form hybrid polymer/silica nanoparticles. The relationships among the molecular weight, time of addition and concentration of the polymeric stabilizer to the nature of the resulting nanoparticle were studied. The large polymer reactive stabilizers did not afford the control in particle size observed with the small molecule reactive stabilizers.
Advisors/Committee Members: Quirk, Roderic P.
Subjects: Polymers
Keywords: ATRP; capillary rise; polymer brushes; silica nanoparticles
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14.
Contractor, Asfiya Q.
ANIONIC SYNTHESIS OF FUNCTIONAL POLYMERS USING MULTIFUNCTIONAL EPOXIDES AS LINKING AGENTS.
Degree: Master of Science, Polymer Science, 2005, University of Akron
► The linking reaction of poly(styryl)lithium with the difunctional epoxide linking agent, 1,3-butadiene…
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▼ The linking reaction of poly(styryl)lithium with the difunctional epoxide linking agent, 1,3-butadiene diepoxide, was investigated. Poly(styryl)lithium was prepared by anionic polymerization in benzene at room temperature. The linked polymer was found to have in-chain hydroxyl groups. It was characterized by NMR spectroscopy, size exclusion chromatography, MALDI-TOF MS, as well as with thin layer and column chromatography. The coupled product containing in-chain alkoxylithium groups was used to initiate the polymerization of ethylene oxide in the presence of a phosphazene base with the objective of synthesizing a hetero, four-armed, (PS)2-star-(PEO)2 polymer. Ethylene oxide was polymerized at 45°C for two weeks. The product obtained was characterized by NMR spectroscopy, size exclusion chromatography and MALDI-TOF MS.
Advisors/Committee Members: Quirk, Roderic Paul.
Subjects: Chemistry, Polymer
Keywords: anionic polymerization, linking agents, diepoxide, in-chain, hydroxyl-functionalized polymers.
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15.
Cyrus, Crystal Dawn.
Synthesis and Formulation of Novel Polymers for the Design of Extended Wear Contact Lens Materials and Surfaces.
Degree: Master of Science, Polymer Science, 2009, University of Akron
► Controlled free radical polymerization techniques have been employed for the synthesis of…
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▼ Controlled free radical polymerization techniques have been employed for the synthesis of polymers for contact lens application. Diblock polyampholyte copolymer brushes of Si/SiO2//poly(acrylic acid-block-2(4)-vinyl pyridine) were synthesized by atom transfer radical polymerization (ATRP) using a “grafting from” strategy. These polyampholytes showed stimuli responsive behavior with pH. Polymer brushes can be attached to a lens surface. The advantages to using a controlled radical polymerization technique for polymer brush synthesis are control over the brush thickness, via control of molecular weight, synthesis of polymers possessing narrow molecular weight distributions, and the ability to prepare block copolymers by re-initiation of dormant chain ends and subsequent extension of the polymer chains. Polyelectrolytes and hydrophilic polymers were synthesized using reversible-addition fragmentation chain transfer (RAFT) polymerization. Copolymerization with monomers containing an epoxy group allows polymers to be attached to the lens surface.
Advisors/Committee Members: Pugh, Coleen.
Subjects: Polymers
Keywords: Contact Lens Materials Surfaces
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16.
Daum, Jeremy L.
NOVEL NETWORKS BY THE POLYMERIZATION OF CYCLIC SILOXANES.
Degree: Doctor of Philosophy, Polymer Science, 2005, University of Akron
► The objective of this research was the syntheses of three types of…
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▼ The objective of this research was the syntheses of three types of novel networks by the polymerization of cyclic siloxanes and to study the networks’ properties. Earlier work by Kurian and Kennedy in the area of designed membranes led to the synthesis of a novel series of amphiphilic networks for potential biological/medical applications. These networks were prepared by the use of di-vinyl telechelic polydimethylsiloxane, di-allyl telechelic poly(ethylene glycol), and a cyclic crosslinker/reinforcing agent, pentamethylcyclopentasiloxane (D5H). The networks’ ability to simultaneously combine a series of demanding requirements for implantation, including biocompatibility, and the exclusion of large defensive proteins while allowing rapid diffusion of smaller proteins/molecules, were investigated in this work. Recently we designed, synthesized and characterized a novel family of siloxane-based polymers, termed cyclolinear polysiloxanes (CLPSs). CLPSs are polymers that contain cyclic siloxane units covalently connected to form linear chains. The cyclic units we designed and used to prepare polymers are diacetoxy-diethyltetramethylcyclotetrasiloxane and diacetoxy-triethylpentamethylcyclopentasiloxane. These novel monomers were catalytically condensed with water to form CLPSs with vinyl termi, and were co-condensed with di-hydroxyl telechelic PDMS to form extended CLPSs (eCLPSs) with PDMS spacers between the cyclic units. Divinyl CLPSs were crosslinked by hydrosilation, and the oxygen permeabilities and mechanical/ thermal properties of the networks were investigated. Based upon insight gained in this work, we developed a new polymerization/curing strategy for PDMS. This strategy involved synthesizing PDMS by ring opening polymerization of a cyclic siloxane (D4) while simultaneously copolymerizing it with our novel eCLPS. Thus, the cyclic units in the eCLPS function as crosslinking agents for the siloxane network. The properties of these siloxane networks were controlled by varying the ratio of eCLPS to D4.
Advisors/Committee Members: Kennedy, Joseph P.
Subjects: Chemistry, Polymer
Keywords: Tricomponent networks, artificial pancreas, cyclolinear polysiloxanes, siloxane networks, oxygen permeability, network formation by ring opening polymerization and ring opening polymerization.
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17.
Ding, Yifu.
Influence of Molecular Weight and Architecture on Polymer Dynamics.
Degree: Doctor of Philosophy, Polymer Science, 2005, University of Akron
► Molecular weight (MW) and architecture are two important parameters of a synthetic…
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▼ Molecular weight (MW) and architecture are two important parameters of a synthetic polymer. Their roles on polymer properties including dynamics have not been well understood yet. In this thesis, we have used various techniques including light, neutron scattering and dielectric spectroscopy to elucidate their influences on polymer dynamics within a broad time (frequency) range, covering chain, segmental relaxation and fast dynamics. Comparisons between different polymers were made to understand the role of chemical structure in determining MW dependence of the dynamic behavior. Experimental results showed that different physical properties studied appear to have similar molecular weight dependence in the sense that they all saturate when chains approach Gaussian coil behavior. We demonstrate that the difference in the molecular weight dependence for various polymers does not correlate with either the difference in the Kuhn segment length or molecular weight between entanglements. Instead, we propose to introduce an additional parameter, mR (molecular weight associated with each step of the Random walk in the approximation of Gaussian chain) that might be important for characterizing the molecular weight dependence of chain statistics and many physical properties. The most intriguing result is that the molecular weight dependence of the fast dynamics, sound velocity and fragility observed in polystyrene is opposite to the one observed in polyisobutylene, although Tg increases with molecular weight in both cases. We speculate that difference in symmetry of the monomer structures is responsible for the opposite behavior. Studies of the influence of architectures on fast and segmental relaxation were also carried out. We found that in the case of polybutadiene both of them scale better with total molecular weight instead of the molecular weight of each arm, as suggested by the chain end free volume model. Analysis of the branching effect on the segmental relaxation illustrates similarity to the blending of the same polymer with different molecular weights.
Advisors/Committee Members: Sokolov, Alexei P.
Keywords: Polymer Dynamics, Glass Transition, Fast dynamics, Fragility, Molecular Weight Effect, Depolarized Light Scattering
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18.
Dirama, Taner E.
DYNAMICS OF PROTEINS IN GLASSY SOLVENTS.
Degree: Doctor of Philosophy, Polymer Science, 2005, University of Akron
► The main objective of the work presented in this thesis is to…
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▼ The main objective of the work presented in this thesis is to understand the dynamic behavior of proteins immersed in bio-preserving liquids and glasses. For this purpose, the model protein lysozyme was chosen. The two solvents selected were glycerol, a triol, and trehalose, a carbohydrate, which are known to be very effective bio-preserving agents. In the first part of this research project the dynamics of glycerol, trehalose and their mixtures were investigated using atomistic molecular dynamics simulations. It was found that a mixture of 5% glycerol and 95% trehalose had the most suppressed dynamics in a one-nanosecond time window. This result agreed with the experimental analysis of the mean-square displacement of the hydrogen atoms, as measured by neutron scattering. Moreover, this result correlates with the experimentally observed enhancement of the stability of some enzymes immersed in a trehalose-glycerol mixture with this particular concentration. The microscopic analysis suggested that the formation of a robust intermolecular hydrogen bonding network was most effective at this concentration and was the main mechanism for the suppression of the fast dynamics. Afterward, the study of the dynamics of the protein embedded in the same unary solvents and binary mixtures was done. The results showed that the protein had the most suppressed dynamics in a 10% glycerol and 90% trehalose mixture, a result that correlated with the internal dynamics of the glass surrounding the protein. It was also found that the hydrogen bonding network formed between the protein and the solvent was more robust for this particular mixture. Possible molecular mechanisms behind the coupling of protein and solvent dynamics were also explored for lysozyme in pure glycerol and trehalose systems. The dynamics of the hydrogen bonding network between the solvent molecules in the first shell and the surface residues of the protein were found to control the structural relaxation of the whole protein. Additionally, a study of the dynamics of the solvent as a function of the distance from the surface of the protein indicated that the viscosity seen by the protein was not the one of the bulk solvent. The presence of the protein changed the dynamics of the surrounding solvent. This implies that the protein sees an effective viscosity that can be higher or lower than the one of the bulk solvent. Moreover, the dynamics of the surface and core residues of the protein were found to differ significantly. The former followed the dynamics of the solvent more closely than the latter. Based on these results a molecular mechanism for the coupling of the solvent-protein dynamics was proposed.
Advisors/Committee Members: Carri, Gustavo A.
Keywords: protein dynamics; molecular dynamics simulation; glassy solvents; bio-preservation
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19.
Dos Santos Freire, Lucas M.
Synthesis of Arborescent Model Polymer Structures by Living Carbocationic Polymerization for Structure-Property Studies.
Degree: Doctor of Philosophy, Polymer Science, 2009, University of Akron
► Polyisobutylene is fully saturated, therefore exhibits outstanding chemical, oxidative and thermal stability,…
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▼ Polyisobutylene is fully saturated, therefore exhibits outstanding chemical, oxidative and thermal stability, which makes it ideally suitable as a model to study mechanical and viscoelastic properties of elastomers, and to correlate properties with structure. The main objective of this dissertation was to develop a fundamental understanding of the mechanism of the synthesis of arborescent (hyperbranched) polyisobutylene (arbPIB) by inimer-type (initiator-monomer) living carbocationic polymerization. The strategy for the effective synthesis of arbPIBs consists of copolymerizing the 4-(2-methoxyisopropyl)styrene inimer (MeOIM) and isobutylene (IB) via controlled/living carbocationic polymerization using TiCl4 coinitiator. In situ FTIR monitoring showed that the self-condensing vinyl polymerization (SCVP) of MeOIM is possible, and that when copolymerizing MeOIM and IB, a nearly alternating structure and multiple end groups are obtained. arbPIB was synthesized and the repeatability of the polymerization was demonstrated. It was found that higher branching was obtained with increasing [MeOIM] and that branching did not further increase if additional IB was added after the MeOIM had reacted completely. No evident changes were observed when switching solvents from Hx/MeCl to a MeCHx/MeCl mixture. Branching parameters showed that arbPIBs have a behavior between polydisperse stars and polycondensates with the number of branches increasing linearly with molecular weight. Novel arbPIB-based block copolymers (TPEs) were synthesized and it was found that copolymers with low Tg short end blocks and less than 5 mol% of a second monomer exhibit thermoplastic elastomeric properties. The materials were strongly reinforced when compounded with carbon black. arbPIB-b-PS are prospective biomaterials and the establishment of reliable methods for evaluating their short and long term properties is a subject of great importance. A dynamic fatigue testing methodology was developed for small, soft rubbery specimens that can be implanted into small animals and re-tested after explantation. Higher ultimate tensile strength and lower elongation at break were measured on microdumbbells than on standard dumbbells. Fatigue testing microdumbbells induced higher stresses at the same strain rate, thus presenting the worst case scenario and being appropriate for fatigue tests. The fatigue limit of linear polystyrene-b-polyisobutylene-b-polystyrene (SIBS) and analogous arbPIB-b-PS samples were determined by using stepwise increasing strain tests developed for fatigue analysis. The presence of branching and a broader molecular weight distribution arbPIB-b-PS led to better mechanical stability and higher fatigue limit than in analogous linear SIBS samples.
Advisors/Committee Members: Puskas, Judit.
Subjects: Chemistry; Materials science; Plastics; Polymers
Keywords: arborescent; hyperbranched; polyisobutylene; polyisobutylene-b-polystyrene; branching; fatigue; block copolymers; pilot plant
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20.
Erdem, Haci Bayram.
Synthesis and Characterization of Thermoplastic Polyphenoxyquinoxalines.
Degree: Doctor of Philosophy, Polymer Science, 2008, University of Akron
► This research was divided into two main parts. In the first part,…
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▼ This research was divided into two main parts. In the first part, a new facile route to relatively inexpensive thermoplastic polyphenoxyquinoxalines was developed. The synthetic route involves the aromatic nucleophilic substitution reaction of bisphenols with 2,3-dichloroquinoxaline. The dichloro monomer was prepared in two steps. In the first step, oxalic acid was condensed with o-phenylenediamine to give 2,3-dihydroxyquinoxaline. In the second step, 2,3-dihydroxyquinoxaline was treated with thionyl chloride to give 2,3-dichloroquinoxaline. This monomer was successfully polymerized with bisphenol-A, bisphenol-S, hexafluorobisphenol-A and 9,9-bis(4-hydroxyphenyl)fluorenone. Hydroquinone and biphenol, however, can not be polymerized to high molecular weight polymers because of the premature precipitation of crystalline oligomers. The glass transition temperatures of the high molecular weight polymers prepared from a series of bisphenols range from 191 °C to 279 °C, and their thermal decomposition temperatures are around 500 °C. The polymers are soluble in a wide range of solvents and can be solution-cast into thin films that are colorless and transparent. The polymers have tensile strengths ranging from 61 to 107 MPa, and tensile moduli ranging from 3.5 to 2.3 GPa. The synthesis of polymer obtained from 2,3-dichloroquinoxaline and bisphenol-A was scaled up to afford 500 g of material. This polymer is a thermoplastic with a melt-viscosity less than 1000 Pa.s. at 300 °C. The notched Izod impact strength of injection-molded samples of this polymer is 40.7 J/m. In the second part of this research, the synthetic method has been modified to allow the preparation of quinoxaline containing polyimides. Thus, 2,3-dichloroquinoxaline was treated either with p-nitrophenol followed by reduction of nitro groups, or with p-aminophenols to directly obtain the desired 2,3-(4-aminophenoxy)quinoxaline. This diamine was polymerized with 3,3',4,4'-biphenyldianhydride, 4,4'-oxydiphthalic anhydride and 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. The polymerizations were carried out by the two step method. The poly(amic acid) intermediates were thermally imidized. Although they have high molecular weights judged by their inherent viscosities ranging from 0.51 to 1.01, thin films of all these polyimides were brittle. The glass transition temperatures of the polyimides range from 259 °C to 282 °C with thermal decomposition temperatures around 550 °C. The polyimide obtained from 2,3-(4-aminophenoxy)quinoxaline and 3,3',4,4'-biphenyldianhydride was found to be semi-crystalline.
Advisors/Committee Members: Harris, Frank.
Subjects: Polymers
Keywords: Condensation Polymerization; High Temperature Polymers; Quinoxaline; Polyquinoxaline; Polyphenoxyquinoxaline
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21.
Erguney, Fatih M.
COARSE-GRAINED MC SIMULATIONS OF POLYMER NANOCOMPOSITES.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► A new generation of MC Simulations for dense melts of coarse-grained chains…
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▼ A new generation of MC Simulations for dense melts of coarse-grained chains on a high coordination lattice has been developed so that the simulations incorporate nanoscale filler particles. In this unique approach, filler particles and matrix chains are built from the same chemical structural building unit. With this special construction method, pair wise enthalpic interactions between the two species are the same. A desired number of linear chains are collapsed to become nanoparticles. The collapse process is intramolecular in its origin and achieved by the enhancement of the attractive core of the discretized Lennard-Jones potential. Even when the nanoparticles are completely collapsed to dense, impermeable objects, they still have some degrees of freedom left in their internal structure. Hence, the translational motion of their centers of masses remains comparable in magnitude to that of the matrix chains. Degree of collapse of the filler particles can be manipulated by changing the magnitude of the enhancement in their attractive LJ potential. Therefore, it is also possible to create partially collapsed nanoparticles with moderate permeability against the matrix chains. Clustering of nanoparticles is normally not the case provided that they are strongly collapsed. Not similarly, weakly collapsed filler particles may show some spontaneous clustering behavior. However, this aggregation upon clustering is not due to energetic interactions, but is rather because of the reduction in the conformation entropy of the matrix chains when they fill the extra space provided by permeable filler particles. In this report, most efforts focus on the investigation the response of the mean square radius of gyration, „´s2„Äm, of the matrix chains to random arrays of the nanoscale filler particles. The changes in „´s2„Äm are targeted as a function of compactness, mobility, or composition of the nanoparticles. The relative size effects of matrix chains and filler particles are also of concern. In this simple model, a detailed description of surface - matrix interactions is missing. Nevertheless, the simulated response of „´s2„Äm to the insertion of the filler particles seems quite complicated and very much indeed system dependent. There exists a non-monotonic dependence on the extent of the collapse of the filler particles. The mobility and concentration of the filler particles, as well as the relative molecular masses of the two species, are determined to affect the resulting behavior. Distortion of the matrix chains is seen when the particles and the matrix chains are both represented by a relatively small number of beads. On the contrary, contraction of matrix chains may also be expected if the two species have sufficiently large molecular masses. In the former case, mobility of the filler particles is speculated to account for the unexpected behavior of the matrix chains, whereas the latter behavior is most probably favored by an entropic driving force appearing as a result of a huge gain in the conformation entropy when the matrix chains travel into to a confined geometry defined by the nanoparticles through nano channels in which the number of available configurations for the matrix chains is very limited.
Advisors/Committee Members: Mattice, Wayne L.
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22.
Eryazici, Ibrahim.
SUPRAMOLECULAR CHEMISTRY OF FUNCTIONALIZED TERPYRIDINES.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► Highly ordered, regularly repeating molecular architectures, constructed via self-assembly techniques, have gained…
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▼ Highly ordered, regularly repeating molecular architectures, constructed via self-assembly techniques, have gained importance over the last three decades due to their potential utilitarian applications. A key construction strategy has relied on the synthesis of specific building blocks capable of forming “higher-ordered” stable structures that have useful properties that can be used as molecular and supramolecular devices. To this end, mono and bis(terpyridine) ligands have been widely used because of their well known photochemical and electronic properties, as well as their ability to facilitate directed, metal-mediated self-assembly. However, limited accessibility to unsymmetrically functionalized terpyridines has restricted their potential use in the construction of more complex infrastructures. For this purpose, methyl-, methoxy-carbonyl- and cyano-substitution patterns on the 4,4''-positions of 4'-arylterpyridine were chosen since these functionalities afforded simple routes to a variety of useful substituted building blocks for higher-ordered supramacromolecular architectures. Single crystal X-ray studies of these terpyridines revealed that molecules of the diester terpyridine (approximately coplanar) are stacked by the overlap of the central pyridine rings in consecutive layers with mean interplanar distances of 3.4 Å (π – π interactions) in the solid state. Moreover, functionalized bis(terpyridine) ligands were achieved via the Kröhnke method and Pd(0) coupling strategy using either 1,3-toluenylbisboronic acid or 1,3-diethynyltoluene with meta- or para-I or Br-phenylterpyridines. A dinuclear tetracationic Fe(II) complex was prepared via metal-directed self-assembly. The chair-like molecular architecture was primarily characterized by X-ray crystallography, mass spectroscopy (ESI-MS), as well as 1H NMR, UV-vis, and CV experiments. Crystal packing of this metallomacrocycle revealed that it formed channels that encapsulated water and MeCN. The low temperature 1H NMR studies suggested that tpy-Fe-tpy moieties in the dimer were interlocked and resembled a spur gear relationship. Surprisingly, dinuclear and trinuclear metallomacrocycles were formed when 1,3-bis(2,2';6',2''-terpyridine-4'-phen-3-ylethynyl)toluene was treated with equimolar amount of Ru(II) that was confirmed by MALDI-TOF mass and NMR spectroscopy. The construction of a heteronuclear (Ru4Fe2) hexameric metallomacrocycle with methyl- and carbonyl-functionalized bis(terpyridyl) moieties was achieved by Pd(0) coupling strategy for potential solar cell applications and supramolecular aggregation of the resulting hexamers through H-bonding. Carboxylic acid functionalized mono- and dinuclear homo- and heteroleptic Ru(II) precursors were also prepared for the same purposes. The single crystal X-ray structure of a homoleptic Ru(II) complex with tetra-ethoxycarbonyl and di(iodo) groups revealed short iodo-carbonyl interactions.
Advisors/Committee Members: Newkome, George R.
Subjects: Chemistry, Polymer
Keywords: Supramolecular chemistry; Terpyridine; Self-assembly; Metallomacrocycles; Molecular machines; Nanotechnology
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23.
Gautriaud, Emilie.
Synthesis of polyisobutylene-polyisoprene diblock copolymer based on natural rubber biosynthesis.
Degree: Master of Science, Polymer Science, 2006, University of Akron
► A detailed review of the biochemical literature concerning the biosynthesis of polyisoprenes,…
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▼ A detailed review of the biochemical literature concerning the biosynthesis of polyisoprenes, including natural rubber (NR, cis-1,4-polyisoprene cPIP) by rubber producing plants, and the polymer chemical literature on biomimetic and related syntheses, was made. It led us to postulate that the biosynthesis of polyisoprenoids in general and that of NR in particular, may proceed by a living carbocationic polymerization process. Our analysis led to the formulation of a “Natural Living Carbocationic Polymerization” (NLCP) mechanism in terms of accepted polymer chemical formalism, i.e., initiation, propagation, and equilibria between active and dormant species. A thorough analysis of the intermediates known to be involved in the biosynthesis of NR is consistent with our postulate. This review of the biochemical literature also inspired the concept of producing new block copolymers utilizing this process and a synthetic macroinitiator. A feasible synthetic pathway was developed to yield the target macroinitiator by combining conventional syntheses and natural rubber biosynthesis. This macroinitiator will consist of a polyisobutylene (PIB) chain carrying a neryl pyrophosphate functional group (PIB-NPP) that will be synthesized from PIB-OH made by “traditional” living carbocationic polymerization. The purpose is to explore if the cis-prenyltransferase enzyme involved in NR biosynthesis would recognize this new synthetic molecule as an initiator of cis-1,4- polyisoprene, so if this synthetic PIB-NPP macroinitiator would induce NR biosynthesis in vitro, and if successful, to produce PIB-b-NR block copolymer. My Thesis involved the development of a six-step synthetic strategy for the preparation of the PIB-NPP macroinitiator. Three of them were successfully completed. The others were studied by carrying out experiments using model compounds, and the results obtained were extremely promising.
Advisors/Committee Members: Puskas, Judit E.
Subjects: Chemistry, Polymer
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24.
Ghatty Venkata Krishna, Pavan K.
Structure and Dynamics of Proteins in Bio-protective Solvents.
Degree: Doctor of Philosophy, Polymer Science, 2009, University of Akron
► The aim of this dissertation is to study the structure and dynamics…
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▼ The aim of this dissertation is to study the structure and dynamics of proteins immersed in trehalose – water and glycerol – water binary mixtures which are known to have bio – protective properties. Fully atomistic molecular dynamics simulations were employed for the study. Hen Egg-White Lysozyme was used as the model protein. In the first part of the study the behavior of the protein in 0, 10, 20, 30 and 100% by weight trehalose – water binary mixtures was studied. The global structure of the protein in the mixtures was found to be insensitive to the concentration of trehalose while the local structure showed a strong dependence on it. The dynamic behavior of the protein, as quantified by the Incoherent Intermediate Scattering Function, showed a non-monotonic dependence on trehalose content. Indeed, in the case of 30% trehalose - 70% water mixture the protein relaxes faster than in the case of 20% trehalose - 80% water mixture. This counterintuitive behavior is rationalized in terms of the behavior of the solvent - protein hydrogen bonds and indicates the onset of preferential hydration. The dynamic behavior of the hydration layer was also explored. The short-length-scale dynamics of this layer were found to be insenstive to trehalose concentration. However, the long-length-scale behavior showed a significant dependence on trehalose content. Insights into the dynamics of bound and mobile hydration water are also presented. In the second part of the study the behavior of the protein in 0, 10, 20, 30 and 100% by weight glycerol – water binary mixtures was studied. Similar to the case of trehalose – water binary mixtres, the global structure of the protein was found to be insensitive to the concentration of glycerol while the local structure showed a strong dependence on it. The dynamic behavior of the protein showed a monotonic dependence on glycerol content. The fluctuations of the protein residues with respect to each other were found to be similar in all water – containing solvents but different from those in pure glycerol. The increase in the number of protein – glycerol hydrogen bonds with increasing glycerol content explains the slowing down of protein dynamics as the glycerol content increases. The dynamics of the hydration water were found to be similar to those in trehalose – water mixtures.
Advisors/Committee Members: Carri, Gustavo.
Subjects: Biophysics; Polymers
Keywords: glycerol; trehalose; lysozyme; protein dynamics; protein structure
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25.
Graham, Matthew.
DEVELOPMENT OF HIGH REFRACTIVE INDEX POLY(THIOPHENE) FOR THE FABRICATION OF ALL ORGANIC 3-D PHOTONIC MATERIALS WITH A COMPLETE PHOTONIC BAND GAP.
Degree: Doctor of Philosophy, Polymer Science, 2006, University of Akron
► The field of photonics hopes to harness light to supercede in performance…
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▼ The field of photonics hopes to harness light to supercede in performance many of the functions carried out by electronics. To accomplish this, the flow of light can be controlled by means of a photonic band gap (PBG) the same way electronic band gaps can control the flow of electrons. PBGs, through the coherent backscattering of radiation, create frequency ranges in which light propagation is forbidden. A PBG is created when a wave propagates through a periodic array of materials with sufficient refractive index (n) contrast (n1/n2) where the dimensionality of the periodicity defines the dimensionality of the PBG. The n contrast required to open a PBG increases as the dimensionality increases. Currently, only inorganic materials have a sufficiently high n to open a complete 3-D PBG. The goal of this project is to fabricate a polymeric material with a complete 3-D PBG, to bring the tailorable physical, electrical, and optical properties of polymeric materials to 3-D PBG materials. The first step was to develop a polymer with a sufficiently high n. Because of its conjugated nature and the presence of a heavy sulfur atom in its repeat unit, poly(thiophene) (PT) is predicted to have one of the highest polymeric refractive indices with n = 3.9 at 700 nm, but the reported n value for PT is 1.4 at 633 nm. This discrepancy is because the potential needed to electrosynthesize PT, the only method available to synthesize thick and high quality PT films, is higher than its degradation potential. It was found that by polymerizing thiophene with an optimized monomer concentration, proton trap concentration, and reaction temperature in a strong aprotic Lewis acid solvent, the polymerization potential could be reduced below the degradation potential of PT. The resultant PT film had a maximum n of 3.36, which is sufficiently high to open a 3-D PBG. Photonic templates were then constructed using a combination of Colvin’s method3 with monodisperse spheres and mechanical annealing. High n PT was used to infiltrate the templates, and the templates were removed, leaving a polymeric inverse opal with the possibility of a complete 3-D PBG.
Advisors/Committee Members: Cheng, Stephen Z. D.
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26.
Han, Tao.
Electrospinning and Nanofibers.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► Electrospinning offers a unique way to produce submicron and nanofibers. It utilizes…
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▼ Electrospinning offers a unique way to produce submicron and nanofibers. It utilizes an electrical force to generate a fine, charged jet from the surface of a viscous liquid. This jet moves straight towards a grounded collector for a certain distance, then bends into spiral coils; finally, the jet solidifies and collects as nonwoven cloth. Determination of the jet path near the onset of the electrical bending instability is important for the orderly collection of the electrospun fibers. The onset and development of the electrical bending instability were investigated. Under certain conditions, high applied voltage prohibited the onset of bending instability and a straight jet reached the collector. Micron and submicron fibers were produced by collecting a straight jet on a moving collector. The diameter, velocity and the longitudinal stress along the jet axis of an electrospinning jet were measured using custom-built equipment. A new apparatus was designed to measure the longitudinal stress along the electrospinning jet. The relaxation of longitudinal stress along the electrospinning jet was experimentally verified. The elongational relaxation time and elongational viscosity of the polymer solution were characterized. Buckling instabilities of electrospinning jets were observed and compared with the behavior of uncharged jets. Various two-dimensional and three-dimensional micron size patterns were produced by the buckled electrospun fibers. A novel pendulum-like motion of a straightelectrified jet was observed and the resulting hierarchical structures made from buckled fibers were presented. A theoretical model was proposed and simulated results showed reasonable agreement with the experimental observations. This work extends the understanding of the complicated electrospinning process.
Advisors/Committee Members: Reneker, Darrell H.
Keywords: electrospinning; nanofibers; buckling instability; bending instability; pendulum-like motion
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27.
Hartschuh, Ryan D.
OPTICAL SPECTROSCOPY OF NANOSTRUCTURED MATERIALS.
Degree: Doctor of Philosophy, Polymer Science, 2007, University of Akron
► Significant interest in nanotechnology is stimulated by the fact that materials exhibit…
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▼ Significant interest in nanotechnology is stimulated by the fact that materials exhibit qualitative changes of properties when their dimensions approach nanometer scales. Quantization of electronic, optical, and acoustic energies with nanoscale dimensions provides exciting, novel functions and opportunities, with interests spanning from electronics and photonics to biology. Characterizing the behavior of nanoscale materials is critical for the full utilization of such novel properties, but metrology for nanostructures is not yet well developed. In particular, mechanical properties of nanoscale particles or features are critical to the manipulation and stability of individual elements, yet changes in mechanical and thermodynamic properties in nanostructured materials create complications in fabrication. This thesis involves the application of Brillouin light scattering to quantify and utilize confinement induced vibrational spectra to understand phononics and elastic properties of nanostructured materials. Measurement and proper interpretation of acoustic waves in polymeric, inorganic, and biological nanostructures provides information about elastic properties and self-assembly. Brillouin light scattering was used to study the vibrational spectra of two-dimensionally confined photoresist and silicon oxide nanolines and three-dimensionally confined poly(methyl methacrylate) spheres and spherical-like viruses. These applications extend the capabilities of Brillouin from characterization of thin films and well-defined spheres to more complex structures. Acoustic waves propagating along the polymeric and silicon oxide lines allowed determination of modulus and its anisotropy. An unexpected acoustic mode was identified in the spectra from nanolines that provided a means to measure mechanical anisotropy. In polymeric lines as narrow as 88nm, neither a change in elastic properties relative to bulk elastic values nor anisotropy in elastic constants was observed. The acoustic waves propagating within polymeric and silicon oxide nanolines were mostly similar, but differed somewhat due to the ratio of elasticities between the lines and the substrate. More localized vibrations were observed in the photoresist lines than in the silicon oxide lines, both of which rested on a silicon oxide substrate. Analysis of the Brillouin spectra from Wiseana Iridoviruses (WIV) revealed strong mechanical coupling between close-packed viruses, unlike the behavior of polymeric colloidal particles. In contrast to traditional model assumptions, Brillouin spectra indicated that their DNA core, rather than their protein shell, dominated the elastic properties of WIV.
Advisors/Committee Members: Sokolov, Alexei P.
Keywords: Brillouin light scattering; acoustic surface waves; mechanical properties of nanostructures; mechanical properties of viruses; mechanical properties of photoresist features; mechanical properties of thin films; optical characterization; spectroscopy of na
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28.
Hayat Soytas, Serap.
Living Carbocationic Polymerization of Isobutylene by Epoxide/Lewis Acid Systems: The Mechanism of Initiation.
Degree: Doctor of Philosophy, Polymer Science, 2009, University of Akron
► The objective of the work presented in this dissertation was to generate…
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▼ The objective of the work presented in this dissertation was to generate a fundamental understanding of the synthesis of star-branched polyisobutylenes (PIBs) arising from hexaepoxysqualene (HES)/Lewis acid (LA) initiating systems, using BCl3 and TiCl4 as LAs. The understanding of initiation and propagation mechanisms by HES is crucial to control the number of arms and arm lengths of star PIBs expected from this initiator. The initiation by monofunctional epoxides, such as α-methylstyrene epoxide (MSE), 1,2-epoxy-2,4,4-trimethylpentane (TMPO-1), and 2,3-epoxy-2,4,4-trimethylpentane (TMPO-2), was investigated. In situ FTIR spectroscopy, which was highly utilized in this research, provided valuable information. Most importantly, the ability to identify the head group by in situ FTIR of growing PIB chains initiated by an epoxide, i.e. the –C–O–LA complex, contributed significantly to the understanding of initiation of IB polymerization. This technique allowed the monitoring of the rate of initiation with the multifunctional epoxy initiator. Previous research showed that TiCl4 gave 40% initiating efficiency in conjunction with the aromatic epoxy initiator MSE, whereas the aliphatic initiators TMPO-1 and TMPO-2 gave only 3 and 10% efficiency, respectively. In this research it was found that BCl3 is more efficient with the aliphatic initiator, TMPO-1, yielding an asymmetric telechelic PIB carrying an α-primary OH and an ω-tertiary Cl functional group with 70% initiator efficiency, while MSE gave only 1-4% efficiency. The TMPO-2/BCl3 system gave 20% initiator efficiency. The various initiation mechanisms were discussed. IB polymerization was successfully initiated by HES in the presence of excess BCl3 and monitored by in situ FTIR spectroscopy. The gradual increase of the IR band assigned to the –C–O–BCl2 group demonstrated that slow initiation was occurring. Chain extension with the HES/BCl3 initiated PIB was achieved leading to high molecular weight PIBs in the presence of TiCl4. It was also demonstrated for the first time that BCl3 invariably leads to β-proton expulsion, leading to chain transfer in IB polymerization. In addition, 1,2-epoxycyclohexane and epoxycyclohexyl-functional siloxanes, i.e. epoxycyclohexylisobutyl polyhedral oligomeric silsesquioxane (POSS) and bis[3,4-(epoxycyclohexyl)ethyl]tetramethyldisiloxane, were found to be initiators for the polymerization of IB. 1,2-epoxycyclohexane/TiCl4 was an efficient initiating system for the IB polymerization yielding up to 45% initiator efficiency. It was proposed that initiation of IB polymerization involves an SN2 reaction between IB and TiCl4-coordinated epoxide.
Advisors/Committee Members: Puskas, Judit E.
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29.
Higgins, Bernadette Ann.
Carbon Nanofiber-Polymer Composites for Electronic Applications.
Degree: Doctor of Philosophy, Polymer Science, 2006, University of Akron
► Carbon nanofiber (CNF) and carbon nanotube (CNT) composites have interesting mechanical and…
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▼ Carbon nanofiber (CNF) and carbon nanotube (CNT) composites have interesting mechanical and electrical properties that make these composites interesting for reinforcing applications. These applications require good dispersion of CNF within a polymeric matrix. Presently high shear methods, such as twin screw extrusion, are used to make well dispersed CNF composites but these methods reduce the physical properties due to a reduction in the aspect ratio of the CNF. Low shear methods to functionalize CNT and CNF have been used to obtain good dispersion while maintaining the high aspect ratio. In this research three ways of making CNF/polymer composites by low shear methods were explored. The first reaction used bisphenol A cyclic carbonate oligomer as a low molecular weight precursor. The oligomers were polymerized to disperse the CNF within the matrix. These composites were characterized by electrical resistivity, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravametric analysis (TGA) and gel permeation chromatography (GPC). The composites had a percolation threshold at 6 wt % CNF decreasing the resistivity to 10 4ohm•cm. The second way used heterocoagulation where a cationic polystyrene latex was combined with anionically charged oxidized CNF. The composites were melt pressed and characterized using electrical resistivity, SEM, and TGA. The percolation threshold was 2 wt % and the resitivity dropped to 10 6ohm•cm. Finally, it was found that synthesizing a hyperbranched polyol was possible by chemically modifying oxidized CNF with glycidol and BF 3OEt 2. The resulting polyol CNF were characterized by TGA, Fourier transform infrared spectroscopy (FTIR), TEM, and X-ray photoelectron spectroscopy (XPS). The OH groups were reacted with heptafluorobutyryl chloride to determine the amount of OH in the sample. The resulting fluorinated composite was characterized by FTIR and elemental analysis. The amount of OH for the polyol CNF increased by 550% compared with oxidized CNF. It was also found that purification of CNF was possible using methylene chloride and an increase in the thermal stability of CNF was observed. Purified CNFs were also reacted with the glycidol and characterized by FTIR, TGA, TEM, and XPS. The heptafluorobutyryl chloride reaction was also done with the purified CNF and purified polyol CNF and characterized by the same methods mentioned for the previous sample. The amount of OH increased by 440% for the purifed polyol CNF compared with the purified CNF. The polyol functionalized CNF should aid dispersion into hydrophilic polymers and the oxidized CNF, oxidized polyol CNF, purified CNF and purified polyol CNF were combined with an epoxy matrix. The resulting composites were characterized using electrical resistivity, TGA, SEM, and TEM. There was no percolation threshold observed for any of the composites except or the purified polyol CNF which had a percolation threshold at 5 wt % to 10 6ohm•cm which is significantly higher than most CNT/epoxy composites reported in the literature.
Advisors/Committee Members: Brittain, William J.
Subjects: Chemistry, Polymer
Keywords: carbon nanofiber; polymer; styrene; carbonate cyclic oligomers; epoxy; glycidol; hyperbranched; cationic polymerization; electrical properties
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30.
Hiza, Sarah B.
THE EFFECT OF AGING AND AGING UNDER STRESS ON THE TEAR STRENGTH OF FILLED NATURAL RUBBER VULCANIZATES.
Degree: Doctor of Philosophy, Polymer Science, 2005, University of Akron
► The effects of aging and aging under stress on the tear strength…
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▼ The effects of aging and aging under stress on the tear strength of filled natural rubber vulcanizates were investigated. Tear strengths were evaluated using a trouser tear geometry with a constrained tear path. Fracture energies and crack growth rates were determined. Four distinct cracking patterns were observed and characterized. Tear strength significantly decreased and crack deviation was absent at long cure times. Anaerobic heat aging at 120ºC resulted in little change in the tear strength, while aging at 80ºC, 100ºC or 140ºC resulted in a lowering of strength. Heat aging under stress (mechano-chemical) resulted in anisotropy. Tear results were assessed using double network concepts. After mechano-chemical aging at low strain, tear strength decreased slightly, but with enough deformation during aging, it decreased dramatically. In addition, the effect of various bonding agents on the fracture behavior was studied. A new commercial agent resulted in higher tear strengths than seen for traditional bonding agents.
Advisors/Committee Members: Hamed, Gary R.
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