Department: Polymer Science ![[clear]](close-x.png "Remove this limiter")
132 matches in the database.
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1.
Adepetun, Adeyemi Adedayo.
Effect of Carbon Black Loading and Temperature on Cut Growth in N990-Filled Natural Rubber Vulcanizates.
Degree: MS, Polymer Science, 2011, University of Akron
► The tensile and tear properties of edge-cut specimens of natural rubber vulcanizates…
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▼ The tensile and tear properties of edge-cut specimens of natural rubber vulcanizates containing varying amounts of a coarse (particle size: 250 nm, surface area: 13 m2/cc) carbon black (N990) were determined at different temperatures. The properties of these vulcanizates at 25°C were compared with those determined by other researchers that contained N115 (particle size: 27 nm, surface area: 200 m2/cc) and N660 black (particle size: 55 nm, surface area: 63 m2/cc). At low carbon black loadings (6-12 phr), N990 black gave a higher tear strength than that with the finer N115 and N660, but, the converse was true at higher carbon black loadings (25-75 phr)! Multiple cracks at the tip of an edge-cut were first observed at 55 phr of N990 while 18 phr and 40 phr have been reported previously for N115 and N660, respectively. Furthermore, unlike in the case of the N115- and N660- vulcanizates, multiple crack formation in edge-cut N990 samples did not give much reinforcement; even a vulcanizate with100 phr loading of N990 was only marginally stronger than the gum vulcanizate at all the cut depths. For a given carbon black loading, crack complexity generally increased with increase in temperature, while tear resistance decreased.
Advisors/Committee Members: Hamed, Gary.
Subjects: Materials Science; Polymers
Keywords: Natural Rubber; Fracture Mechanics; Tensile Strength; Tear Strength
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2.
Agapov, Alexander.
Decoupling Phenomena in Dynamics of Soft Matter.
Degree: PhD, Polymer Science, 2011, University of Akron
► Despite significant work done in the field of the dynamics of soft…
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▼ Despite significant work done in the field of the dynamics of soft matter, the microscopic mechanisms controlling various relaxation processes (structural relaxation, diffusion, chain dynamics in polymers) in vitreous materials still remain unclear. The situation is even worse when one tries to understand the interrelation between different relaxation phenomena. Resolving the mechanisms controlling various aspects of glassy structure and dynamics is of obvious fundamental and industrial importance. In this work, mechanisms controlling different relaxation phenomena occurring over a broad range of length and time-scales in vitreous materials were characterized with several experimental techniques: dielectric spectroscopy, light and neutron scattering, and rheology. The effect of polar interactions on the temperature dependences of structural (segmental) and chain dynamics in polymeric liquids was studied. The glass transition temperature, Tg, and fragility index, m, are found to depend on the monomer’s polarity and the relative position of the polar group. The effect of polar interactions on Tg and m is discussed in terms of a balance between changes in the cohesive energy and conformational rigidity. Intriguing and unusual results were obtained on the effect of polar interactions on chain dynamics. It was demonstrated that the presence of polar interactions leads to a significant coupling of segmental and chain dynamics in polar materials in comparison to non-polar polymers with similar fragilities. Apparently, polar interactions have long range effects that are yet to be understood. The temperature dependences of the self-diffusion rate and structural dynamics in a room temperature ionic liquid (RTIL) were analyzed in an extremely broad temperature range. The results clearly demonstrate that the self-diffusion coefficient in this molecular liquid has a temperature dependence that is weaker than the temperature dependence of the structural relaxation time. However, the observed decoupling of the structural relaxation and self-diffusion in the polar RTIL is much weaker than expected for a van der Waals molecular glass-forming liquid with comparable fragility. Combination of these results with the data obtained on dynamics in polar polymers allows one to speculate that the large length scale dynamic processes can be significantly affected by the presence of delocalized polar interactions in a broad range of organic glass-forming liquids, polymers and molecular liquids. The relationship between the temperature dependences of ionic conductivity and segmental dynamics in polymers uncovered here also points to intriguing aspects of dynamics in soft matter. Ionic conductivity was found to decouple from segmental dynamics with the degree of decoupling correlating with the fragility of the polymer. It is speculated that frustration in packing is the main driving mechanism of the decoupling between ionic and segmental dynamics. These results suggest that ionic conductivity can be decoupled from segmental dynamics in a material specific way. With the right approach it should be possible to decouple the ionic motion from segmental dynamics in a manner that would result in high conductivity even in the glassy state. This opens new routes for material design of truly solid polymer electrolytes.
Advisors/Committee Members: Sokolov, Alexei P.
Subjects: Polymers
Keywords: soft matter; liquids; glass; polymers; dynamics; segmental; chain; viscosity; Rouse; polar; conductivity; electrolyte; ion; diffusion; decoupling; dielectric; ultra slow process; bead size; Kuhn segment
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3.
Agapov, Rebecca L.
Advanced Scanning Probe Techniques for the Study of Polymer Surfaces.
Degree: PhD, Polymer Science, 2012, University of Akron
► Important contributions to our understanding of polymer surfaces rely very heavily on…
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▼ Important contributions to our understanding of polymer surfaces rely very heavily on the development of new techniques for the study of those surfaces. The unifying aspect of the research described in this dissertation is the exploitation of advances in polymer surface characterization for purposes of elucidating surface behavior and properties. The results have implications for an interesting diversity of polymer science applications ranging from the design of superior latex films, to detection of trace components on surfaces, to the engineering of blend surface properties by varying chain molecular architecture. To design superior latex films it is important to connect mesoscale morphological features as well as adhesion properties of dried latex films to their macroscopic properties observable by eye. Using conventional scanning probe microscopy (SPM) probes, but exploiting in particular highly resolved adhesion mapping, dried latex films containing various fluorosurfactants, polymers, and cross-linking agents were studied. It was found that a complex that forms between the fluorosurfactant and a zinc cross-linking agent leads to mesoscale lateral phase separation. The presence of these lateral inhomogeneities correlates well with the poor performance on the macroscopic level. While conventional SPM techniques can analyze surface properties of samples, there is still a need for a non-invasive, robust imaging technology capable of simultaneously collecting topographic and chemical information with nanoscale resolution. This has been realized with tip enhanced Raman spectroscopy (TERS). Metallized probes are a key enabling component for this technique, which has the potential for high sensitivity detection and surface chemical imaging with very high (nm) resolution. However, the robustness of the metallized probes has been a hindrance to the application and commercialization of the technique. An ultrathin protective coating of aluminum oxide for the metallized probe was developed to extend the storage life of these probes to three months in a dessicator and to double the scanning life under harsh scanning conditions. These robust protected probes were then used to investigate the mechanism of “blinking” of the Raman signal. Results are consistent with the contention that thermal diffusion of molecules is the major mechanism behind blinking. Using the extreme enhancement from protected probes, individual species in an isotopically labeled polymer blend were detected for the first time using molecules that were not Raman resonant. TERS has limited potential for use with samples engineered to contain two species that have the same chemistry but differ in chain architecture, such as a blend of linear and cyclic polystyrenes. To quantify the surface composition of such a blend, a new mass spectrometry technique was developed. This technique, which we term surface layer matrix assisted laser desorption/ ionization time-of-flight mass spectrometry (SL-MALDI-TOF MS), has the ability to unambiguously identify species in the top molecular layer of a film with the same repeat chemistry as long as each species has a unique mass to charge ratio (m/z).
Advisors/Committee Members: Foster, Mark D.
Subjects: Polymers
Keywords: tip enhanced Raman spectroscopy; surface chemical imaging; robust plasmonics; polymer blends; surface MALDI; adhesion mapping
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4.
Akgun, Bulent.
SURFACE AND INTERFACE STRUCTURE OF DIBLOCK COPOLYMER BRUSHES.
Degree: PhD, 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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5.
Baker, James Stewart Jr.
Synthesis of Functional Vinylbenzocyclobutenes for Use as Crosslinkers in the Preparation of Amphiphilic Nanoparticles.
Degree: PhD, Polymer Science, 2011, University of Akron
► Synthetic routes to 1-substituted 4-vinylbenzocyclobutenes (VBCBs) were developed. The VBCBs were generated…
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▼ Synthetic routes to 1-substituted 4-vinylbenzocyclobutenes (VBCBs) were developed. The VBCBs were generated by a [2+2]-cycloaddition between benzyne and a vinyl-substituted compound. Routes to aromatic halogenation of 1-substituted-BCBs were explored. The use of a substrate bearing an aromatic halogen for the cycloaddition reaction was a more efficient route to 1-substituted-4-halo-BCBs than post-cycloaddition halogenation. Conversion of the aromatic halogen to a vinyl group using both a Wittig reaction and a Kumada coupling with vinyl bromide was examined with the latter being the preferred route. These VBCB monomers were copolymerized with styrene derivatives via atom transfer radical polymerization (ATRP). The VBCB monomer was present in 5-20 mol% to act as a thermally induced crosslinker. Random copolymerizations of 1-ethoxy-4-vinylbenzocyclobutene (VEBCBE) with both styrene and 2,3,4,5,6-pentafluorostyrene (PFS) were conducted. Polymerizations using VEBCBE were conducted at polymerization temperatures of 60 °C or less to prevent premature reaction of the BCB unit. Differential scanning calorimetry (DSC) analysis was used to characterize the ring-opening behavior of the BCB compounds. Dropwise addition of polymers containing these VBCB units to a heated solvent allowed preparation of spherical, polymeric nanoparticles at lower temperatures than those previously described in the literature. The polymerization behavior of VEBCBE was examined. The chain transfer constant for this compound in the free radical polymerization of both styrene and PFS in bulk at 60 °C was measured as Cx= 0.006 and 0.015, respectively. The reactivity ratios for PFS and VEBCBE at 70 °C in bulk free radical polymerization were also measured. The polymerization exhibited an alternating tendency with r1=rVEBCBE=0.05-0.12, r2=rPFS=0.504-0.735, and r1r2=0.025-088.
Advisors/Committee Members: Pugh, Coleen.
Subjects: Chemistry; Polymer Chemistry; Polymers
Keywords: benzocyclobutene; vinylbenzocyclobutene; orthoquinodimethane
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6.
Banerjee, Abhishek.
Functionalizable Biodegradable Polyesters for Drug Delivery Applications.
Degree: PhD, Polymer Science, 2012, University of Akron
► Current biodegradable polymers, like poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their…
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▼ Current biodegradable polymers, like poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their copolymers (PLGA) do not have functionalities on their backbones. Such biodegradable polymer systems are therefore not able to covalently attach drugs or other therapeutic molecules, which could be useful for making drug delivery devices. Instead, the therapeutic molecules must be physically entrapped into these polymers, either by forming micelles or by nano-encapsulation, thereby limiting their loading capacity. Our research deals with the polyesterification of 2-bromo-3-hydroxypropanoic acid which is a halogenated isomer of lactic acid, yet has a primary alcohol group like glycolic acid. It is therefore an ideal co-monomer for incorporation into PLGA. Such co-polyesters are potentially biodegradable with halogen functionalities on the main chain. We have synthesized brominated copolymers with LA and GA of number average molecular weights around 20,000 Da (PS standards), under bulk co-polymerization conditions. The number of functionalizable sites on the main chain of this polyester is controlled by varying the feed ratio of the halogen co-monomer. The biodegradability of the polymer can also be tailored by varying the lactic acid and glycolic acid feed ratios. Solution polymerization using carbodiimide chemistry at room temperature has also been explored to prepare these co-polymers, with lesser success. The biodegradation behavior of these brominated polyesters were studied in the form of compression molded tablets under physiological conditions, Phosphate buffer saline (pH 7.4), at 37 °C and these polymers were found to degrade to around 80% of their initial molecular weights.
Advisors/Committee Members: Pugh, Coleen.
Subjects: Polymer Chemistry
Keywords: Biodegradable, PLA, PLGA
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7.
Barrios, Carlos A.
Modified Scanning Probes for the Analysis of Polymer Surfaces.
Degree: PhD, 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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8.
Begen, Burak.
INFLUENCE OF PRESSURE ON FAST DYNAMICS IN POLYMERS.
Degree: PhD, 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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9.
Bhargava, Prachur.
SELF-ASSEMBLED POLYSTYRENE-BLOCK-POLY (ETHYLENE OXIDE) (PS-b-PEO) MICELLE MORPHOLOGIES IN SOLUTION.
Degree: PhD, 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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10.
Bhargava, Sphurti.
Submicron Structures, Electrospinning and Filters.
Degree: PhD, 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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11.
Bhawalkar, Sarang.
Colloidal Lithography for Patterning Non-Planar Surfaces.
Degree: PhD, Polymer Science, 2012, University of Akron
► A colloidal lithography method has been developed for patterning nonplanar surfaces. Hexagonal…
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▼ A colloidal lithography method has been developed for patterning nonplanar surfaces. Hexagonal noncontiguously packed (HNCP) colloidal particles 127 nm - 2.7 µm in diameter were first formed at the air-water interface and then adsorbed onto the substrate coated with a layer of polymer adhesive. The adhesive layer plays the critical role of securing the order of the particles against the destructive lateral capillary force generated by a thin film of water after the initial transfer of the particles from the air-water interface. The adhesive could either be attached to the substrate or simply be coated on it. Poly(n-butyl acrylate) and poly(n-butyl acrylate-random-N,N-diethylaminoethyl acrylate) brushes were grafted on the substrates via the “graft-from” method using atom transfer radical polymerization (ATRP) or these polymers were simply synthesized separately and coated on the substrates via dip coating. Silicon-containing polymers were also tested as successful adhesives. Several parameters that influence preservation of the order of the particle arrays were examined. The collidal lithography method is robust and very simple to carry out. It is applicable for a variety of surface curvatures and for both inorganic and organic colloidal particles. Hierarchical structures of colloidal particles were successfully created on substrates with the use of the colloidal lithography method. Coating these substrates with a low energy coating afforded superhydrophobic surfaces. It was also possible to tune the hydrophobicity of the surface by varying system parameters such as interparticle distance between the particles and the nature of the surface coating. The discrete nature of the particles made it possible to predict the wetting properties of the surface.
Advisors/Committee Members: Jia, Li.
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12.
Boonkerd, Kanoktip.
EFFECT OF EFFICIENCY OF SULFUR VULCANIZATION ON PROPERTIES OF GUM AND BLACK-FILLED NATURAL RUBBER VULCANIZATES.
Degree: PhD, 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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13.
Boukany, Pouyan E.
Nonlinear Flow Behavior of Entangled DNA Fluids.
Degree: PhD, 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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14.
Buehler, Betul.
Molecular Adhesion and Friction at Elastomer/Polymer Interfaces.
Degree: PhD, 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 (TR) that is much below Tg. For comparison, we have also studied PnBMA and PnPMA having Tg below and above TR, 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 Tg. 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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15.
Cao, Yan.
Understanding Molecular Epitaxial Mechanism and Chain Folding in Determining Chain Packing and Crystalline Morphology of Isotactic Polypropylene.
Degree: PhD, Polymer Science, 2012, University of Akron
► Although polymer crystal polymorphisms and their crystallographic relationships of isotactic polypropylene (i-PP)…
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▼ Although polymer crystal polymorphisms and their crystallographic relationships of isotactic polypropylene (i-PP) have been studied and understood for the past fifty years, we are still curious on some aspects of epitaxial mechanism between the alpha phase and the gamma phase. This research topic is designed to investigate how the epitaxial domination of the crystal morphologies takes place in the gamma-phase of the chain-folded crystals using high molecular weight isotactic polypropylene (i-PP) samples with a controlled number of stereodefects. Due to the specific epitaxial growth of the gamma-phase on the elongated alpha-phase single crystal, two different morphologies have been identified via transmission electron and atomic force microscopies (TEM and AFM). The epitaxial growth of the gamma-phase crystal is generated from the stem direction in the initial alpha-phase single crystal. The select area electron diffraction (SAED) results indicate that the chain orientations in the “flat” lamellae are tilted at ±40° from the thin film (lamellar) normal within the ab-plane of the gamma-phase. Based on the tilted SAED results from TEM, the microscopic formation mechanism of this morphology reveals that the initial alpha-phase single crystal has to have a stem orientation tilted 25 degree or 17 degree away from the thin film normal within the ac-plane around the b-axis. Elongated alpha2-form lath-like single crystals can be found by using both commercial sample and one with stereodefects. Based on our SAED experimental results, the stems in these lath-like single crystals are tilted at an unusual 17 degree angle around the b-axis. This 17 degree stem tilt in the alpha2-form single crystals with the (10-2) fold surface appears to depend upon both conformational and chain folding constraints.
Advisors/Committee Members: Cheng, Stephen.
Subjects: Polymers
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16.
Chai, Xiaoli.
Reinforcement and Cut Growth in Swollen and Unswollen Filled Rubber Compounds.
Degree: PhD, 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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17.
Chang, Tae-Eun.
Microscopic mechanism of reinforcement and conductivity in polymer nanocomposite materials.
Degree: PhD, 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.
Keywords: SWNT; nanotubes; Raman; composites; annealed SWNT; CNT; PS
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18.
Chan, Yi-Tsu.
Design and Construction of Metallo-Supramolecular Terpyridine Architectures.
Degree: PhD, Polymer Science, 2010, University of Akron
► Beyond molecular chemistry, supramolecular chemistry focuses on constructing highly complicated and functional…
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▼ Beyond molecular chemistry, supramolecular chemistry focuses on constructing highly complicated and functional systems from chemical species held together by intermolecular non-covalent interactions. Metal-ligand coordinative interaction plays an important role in construction of supramolecules; 2,2':6',2"-terpyridine (tpy) is one ligand that has been widely used in this arena. Bis(terpyridine) ligands with suitable geometry are able to form cyclic structures upon complexation with metal ions. Either by a stepwise procedure or by a self-assembled process, a series of metallomacrocyles containing tpy-M-tpy units (M = metal) has been constructed and isolated. Self-assembly using the labile tpy-CdII-tpy connectivity provides access in quantitative yields to hexacadmium macrocycles that were characterized by traveling wave ion mobility mass spectrometry (TWIM-MS), which enhances the resolving power of electrospray ionization mass spectrometry by adding shape-dependent dispersion and completely deconvolutes the isotope patterns of different charge states, as well as avoids isomer superposition. The self-assembly of a series of CdII metallomacrocycles with different sizes and architectures has been achieved by controlling the stoichiometry and shapes of the building blocks. Novel approaches to the development of artificial multivalent carbohydrate conjugate objects remain an important area of research due to their strong and specific interactions with receptors. Integrating carbohydrates with metallomacrocycles, linear or branched sugar-functionalized 3,5-di(terpyridinyl)arenes have been synthesized and shown to self-assemble using FeII into hexameric and pentameric macrocycles. The self-assembled nanofibers of both macrocycles were generated by slowly diffusing the non-polar solvent hexane into a homogeneous solution of the complexes in CHCl3/MeOH/MeCN. In order to fabricate nanometer-sized materials from predesigned building blocks, a bottom-up approach has been used to incorporate Au nanorods (NRs) into multi-component structures containing tpy-FeII-tpy connectivity. This was the first example of Au NR assembly to afford predominately end-to-end linear and branched nanoarchitectures using metal complexes, which can be disassembled under mild conditions.
Advisors/Committee Members: Newkome, George R.
Subjects: Chemistry; Polymers
Keywords: Calcd; TERPYRIDINE; Hz; 2H; NMR; 4H
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19.
Chavan, Vijay S.
Controlled Synthesis and Characterization of Branched, Functionalized, and Cyclic Polymers.
Degree: PhD, Polymer Science, 2011, University of Akron
► A variety of methods were used to make polymers with different architecture…
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▼ A variety of methods were used to make polymers with different architecture and functionalities. The linking chemistry of vinyldimethylchlorosilane (VDMCS) with poly(styryl)lithium (Mn = 1,700-3,000 g/mol) was studied. The average degree of branching varied from 7.5 to 9.4 with an increase in concentration of VDMCS (1.2 to 5.2 eq). The intrinsic viscosities and melt viscosities (at 160 °C) of the star polymers were found to be less than half of that of the corresponding linear polystyrenes. α-Pyrrolidine-functionalized polystyrene (Mn = 2,700 g/mol, Mw/Mn = 1.03, 92.5%) was successfully synthesized from α-chloromethyldimethylsilane-functionalized polystyrene(Mn = 2,600 g/mol, Mw/Mn = 1.02) based on NMR spectroscopy, MALDI-TOF and ESI mass spectrometry. The stability of silyl hydride groups under atom transfer radical polymerization conditions was proven by copolymerizing methyl methacrylate and (4-vinylphenyl)dimethylsilane (VPDS). Tapered block copolymers of isoprene, VPDS, and styrene with narrow molecular weight distributions (1.04 and 1.05) were synthesized via anionic polymerization. Evidence regarding the topology of cyclic polybutadienes was obtained by Atomic Force Microscopy of grafted polymers obtained by grafting an excess of silyl hydride functionalized polystyrene (Mn = 8,300 g/mol, Mw/Mn =1.01) onto cyclic polybutadiene(Mn=88,000 g/mol, Mw/Mn = 2.0). The reactivity of polyisobutylene carbocations was compared with respect to competitive electrophilic addition to a vinyl group versus silyl hydride transfer by investigating the reaction with VPDS. Based on GPC results, and 1H and 13C NMR spectroscopy, no evidence for any vinyl group addition was observed. A successful attempt was made to prepare electrospun fibers from fluorofunctionalized styrene-butadiene elastomers. The water contact angle of these surfaces was found to be 162.8o ± 3.8o for the fibrous mat of the fluorinated polymers as compared to 151.2o ± 2.4o for the analogous fibrous mat of the non-fluorinated polymers. In-chain functionalization of tapered styrene butadiene rubber using chloromethydimethylsilane was quantitatively done via a hydrosilation reaction. Pyrrolidine-functionalized styrene butadiene rubber was obtained in 71% yield after reacting pyrrolidine with chloromethyldimethylsilane-functionalized styrene butadiene rubber. In-chain, silyl hydride-functionalized, deuterated polystyrene (Mn = 2,100 g/mol,Mw/Mn = 1.01) was functionalized with allyl cyanide in the presence of Karstedt's catalyst to obtain in-chain cyano-functionalized, deuterated polystyrene (45% based on the mass of in-chain, cyano-functionalized deuterated polystyrene obtained).
Advisors/Committee Members: Quirk, Roderic.
Subjects: Polymers
Keywords: Anionic Polymerization; Cationic Polymerization; Ring Opening Metathesis Polymerization; ATRP; Hydrosilation; Functionalization; Electrospinning; Star Polymers; Branched Polymers; Deuterated Polymers; Cyclic Polymers; AFM; GPC; DSC; TGA; MALDI-TOF
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20.
Chen, Chong.
Investigating Nonnative Contacts in Protein Folding.
Degree: MS, 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
Keywords: PROTEIN; NONNATIVE CONTACTS; residues; conformations; interacting model; FOLDING
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21.
Chen, Yan.
SINGLE CRYSTAL ENGINEERING OF AMORPHOUS-CRYSTALLINE BLOCK COPOLYMERS CRYSTALLIZATION, MORPHOLOGY AND APPLICATIONS.
Degree: PhD, 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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22.
Constable, Andrew N.
Functionalization of Silica Micro-capillaries and Silica Nanoparticles via Polymer Brushes.
Degree: PhD, 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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23.
Contractor, Asfiya Q.
ANIONIC SYNTHESIS OF FUNCTIONAL POLYMERS USING MULTIFUNCTIONAL EPOXIDES AS LINKING AGENTS.
Degree: MS, 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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24.
Cyrus, Crystal Dawn.
Synthesis and Formulation of Novel Polymers for the Design of Extended Wear Contact Lens Materials and Surfaces.
Degree: MS, 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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25.
Daum, Jeremy L.
NOVEL NETWORKS BY THE POLYMERIZATION OF CYCLIC SILOXANES.
Degree: PhD, 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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26.
Ding, Yifu.
Influence of Molecular Weight and Architecture on Polymer Dynamics.
Degree: PhD, 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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27.
Dirama, Taner E.
DYNAMICS OF PROTEINS IN GLASSY SOLVENTS.
Degree: PhD, 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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28.
Dos Santos Freire, Lucas M.
Synthesis of Arborescent Model Polymer Structures by Living Carbocationic Polymerization for Structure-Property Studies.
Degree: PhD, 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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29.
Erdem, Haci Bayram.
Synthesis and Characterization of Thermoplastic Polyphenoxyquinoxalines.
Degree: PhD, 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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30.
Erguney, Fatih M.
COARSE-GRAINED MC SIMULATIONS OF POLYMER NANOCOMPOSITES.
Degree: PhD, 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.
Keywords: filler particles; matrix chains; filler; chains; POE; POE chains; particles
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