Department: Polymer Engineering ![[clear]](close-x.png "Remove this limiter")
59 matches in the database. These are records: 1 - 30.
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1. Acharya, Sunil. Micromechanics of Asperity Interaction in Wear – A Numerical Approach.
Degree: Doctor of Philosophy, Polymer Engineering, 2005, University of Akron
▼ In last 50 years, research on elastomer wear has produced qualitative and statistical data regarding wear debris and associated morphologies. However, the exact wear mechanism and the evolution of wear morphologies is not understood to the level where a quantitative prediction or description of wear is possible In this study a numerical analysis (FEA) has been used to understand mechanical interactions related to pattern wear. A blunt surface crack and it’s interaction with a single penetrating asperity has been modeled for varying frictional, material and kinematic conditions. An interacting asperity creates a deformation field in an elastomeric body. This stress field is altered in the presence of a crack. The resulting stress relief is quantitatively estimated for varying geometric, material and friction parameters. Consequently, the energy available for a new crack to propagate in the vicinity of the existing crack decreases leading to a characteristic spacing between successive cracks. Energy release rate data from fracture experiments on thin rubber sheets is used to calculate the spacing between the cracks. The approach sheds some light on crack propagation characteristics in pattern wear. Other numerical experiments in this study analyze: (1) Elastomer response to dynamic asperity loading (2) Asperity loading at micro-scale where filled rubber has high degree of non-homogeneity (3) Effect of asperity loading at an angle on a rubber flap. As a result, we now better understand the evolution of wear related morphologies.
Advisors/Committee Members: Leonov, Arkady I.
Keywords: Wear, Rubber,Elastomer,Contact Mechanics,Fracture Mechanics,Energy Release Rate,Finite Element Analysis
2. Adames, Juan M. Characterization of Polymeric Binders for Metal Injection Molding (MIM) Process.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ The Metal Injection Molding (MIM) process is an economically attractive method of producing large amounts of small and complex metallic parts. This is achieved by combining the productivity of injection molding with the versatility of sintering of metal particulates. In MIM, the powdered metal is blended with a plastic binder to obtain the feedstock. The binder imparts flowability to the blend at injection molding conditions and strength at ambient conditions. After molding, the binder is removed in a sequence of steps that usually involves solvent-extraction and polymer burn-out. Once the binder is removed, the metal particles are sintered. In this research several topics of the MIM process were studied to understand how the polymeric binder, similar to the one used in the sponsoring company, works. This was done by examining the compounding and water debinding processes, the rheological and thermal properties, and the microstructure of the binder/metal composite at different processing stages. The factors studied included the metal contents, the composition of the binder and the processing conditions. The three binders prepared during the course of this research were blends of a polyolefin, polyoxymethylene copolymer (POM) and a water-soluble polymer (WSP). The polyolefin resins included polypropylene (PP), high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). The powdered metal in the feedstocks was 316 L stainless steel. The compounding studies were completed in an internal mixer under different conditions of temperature, rotational speed and feedstock composition. It was found that the metal concentration was the most important factor in determining the torque evolution curves. The observation of microstructure with Scanning Electron Microscope (SEM) at different stages during compounding revealed that the metal particles neither agglomerate nor touch each other. The liquid extraction of the water-soluble polymer (WSP) from the molded parts (or water debinding) was investigated using two configurations of flow of water relative to the samples. Both permitted the reduction of the mass transfer resistance outside the parts, revealing information on the diffusion of the WSP inside the part exclusively. The debinding studies showed that a single effective diffusivity could be used to model the extraction process of the binder from molded parts. This approach is more accurate when the debinding time is above 2 hours. Steady shear and dynamic experiments were conducted on the binder and feedstocks samples containing LLDPE. The results of both experiments revealed that the feedstocks did not show yield stress even though the highest metal content was 64 % by volume. Therefore, it was concluded that there were only hydrodynamic interactions between the metal particles. The thermal characterization of binders, polymers and feedstocks included differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC tests were performed after preheating and quenching of the samples. The heating rate was 20 °C/min. The TGA scans were conducted from room temperature to 700 °C at 20 °C/min. The DSC tests revealed that the melting point of the polymers depressed when blended in the binders and feedstocks. The depression was more intense for POM and the water-soluble polymer than for the polyolefins. Therefore, it was concluded that the melting point depression of POM and the water-soluble polymer was caused by their entrapment in the polyolefin matrix and in between the metal particles. The TGA scans showed that the feedstocks with higher metal concentration had higher final decomposition temperature, but similar onset temperature. The reason was that the higher the metal concentration the more difficult the diffusion of the products of the decomposition of the binder out of the samples. The morphological studies revealed that the binders were heterogeneous showing domains of the polar resins, embedded in a continuous phase composed of polyolefin. This distribution of phases was the result of the immiscibility between the polymeric components, and of the higher concentration (>70 vol%) of the polyolefin with respect to the polar components (polyoxymethylene and water-soluble polymer). The deformation during steady shear testing and compounding of the binder with the metal modified the size of the dispersed domains. The steady shearing increased the size of the dispersed domains by coalescence of the particles. On the other hand, the presence of powdered metal during compounding forced a redistribution of the dispersed phases. Apparently, a thin heterogeneous layer of binder surrounded the metal particles while most of the polyolefin occupied the space between the coated metal particles. The SEM study on samples obtained after water debinding revealed that the water-soluble polymer did not distribute uniformly on the surface of the molded disk of feedstock used for water debinding tests.
Advisors/Committee Members: Leonov, Arkady I.
Keywords: Metal Injection Molding; Binders; Polymer
3. Ahn, Dae Up. Well-Aligned 3-Dimensional Self-Assembly in Block Copolymers and Their Nanotechnological Applications.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ We have prepared well-aligned 3-dimensional block copolymer nano-cylinders over the entire sample area and thickness without any additional external field applications such as mechanical and electric fields. Self-assembled 3-dimensional perpendicular cylinder orientation was achieved by thermodynamic controls of incompatibility between the block components, and further elaborate modification of size and hexagonal alignment of perpendicular cylinders was also accomplished by kinetic controls of diffusive molecular mobility of block copolymer microdomains. Since those two controls have been mainly achieved by simple blending of minority homopolymer, the intrinsic advantages of block copolymer nanopatterning, such as fast and spontaneous 3-dimensional nanopatterning with a high thermodynamic stability and reproducibility, have been completely preserved in this fabrication strategy. After preparing block copolymer masks containing perpendicularly and hexagonally well-aligned nano-cylinders, a top-down method using excimer laser was applied to the block copolymer masks for a fast nanopattern transfer to organic and inorganic substrates in the form of nano-dots. Mask-image-like high-density polystyrene and silicon nano-dots were readily obtained after the one-step excimer laser irradiation on the block copolymer masks without any additional selective staining and/or etching steps before a non-selective etching process. The numerical analysis on the photothermal excimer laser ablation of periodically nanostructured block copolymer masks revealed that sufficiently low laser intensity was suitable for the one-step fabrication of mask image-like topographic nanopatterns on the surface of silicon substrates, as long as the intensity was high enough to induce a matrix-assisted photothermal excimer laser ablation in less UV-sensitive block component. Therefore, we illustrate a novel nanofabrication technique using a top-down after bottom-up method to create new opportunities for the fabrication of low-cost and high-throughput nanostructured materials with highly ordered 3-dimensional nanopatterns.
Advisors/Committee Members: Sancaktar, Erol.
Keywords: Block Copolymer Self-Assembly; Excimer Laser; Top-Down after Bottom-Up Methods; Two-Dimensional Thermal Diffusions; Nanotechnology; Thermodynamics and Kinetics of Block Copolymer Self-Assembly
4. Al-Yamani, Faisal M. A route to enhanced intercalation in rubber silicate nanocomposites.
Degree: Master of Science in Polymer Engineering, Polymer Engineering, 2005, University of Akron
▼ Addition of organophilic layered silicates (montmorillonite) to the polymer matrix produces effective polymer nanocomposites by intercalation of macromolecules into the interlayer spaces. Rubber nanocomposites represent an untapped application that is still in an early stage of development. This research work focuses broadly on the property enhancement of rubber matrices through the incorporation of different types of layered silicates and a dispersion agent, Hexamethoxymethylmelamine (HMMM), to the composite formulation. Styrene butadiene rubber (SBR) and acrylonitrile butadiene rubber (NBR) rubber nanocomposites were prepared using the commercially viable rubber compounding process and investigated through wide angle x-ray diffraction to determine the resulting clay morphology. The mechanical evaluation of the rubber nanocomposites was performed by tensile and tear testing. The effect of the dispersing agent was manifested in both the morphology and mechanical properties, which showed significant increase when added to the right combination of the modified organoclay and rubber type due to an increase d-spacing distance in the gallery height shown through WAXD. This work thus proves the positive benefit of such dispersion agents.
Advisors/Committee Members: Goettler, Lloyd.
Subjects: Chemistry, Polymer
Keywords: Hexamethoxymethylmelamine,layered silicates ,Rubber nanocomposites ,SBR,NBR
5. Aussawasathien, Darunee. ELECTROSPUN CONDUCTING NANOFIBER-BASED MATERIALS AND THEIR CHARACTERIZATIONS: EFFECTS OF FIBER CHARACTERISTICS ON PROPERTIES AND APPLICATIONS.
Degree: Doctor of Philosophy, Polymer Engineering, 2006, University of Akron
▼ The advantages of conducting materials in the non-woven nanofiber-mat form prepared by the electrospinning technique are proposed for sensing nanocomposite applications. Due to the sub-micron size of electrospun conducting fibers, a large specific surface area is generated, while the fiber web contains high fiber aspect ratio and high interconnecting network compared to the same materials in film and short fiber forms. Consequently electrospun lithium perchlorate doped polyethylene oxide (LiClO4-doped PEO) fibers and electrospun carbon black filled LiClO4-doped PEO composite fibers were prepared to be used as humidity sensors. The measurements of humidity dependent resistance of these conducting fibers were carried out for different humidity changes. Electrospun camphosulfonic acid doped polyaniline-polystyrene (HCSA doped PANI-PS) fibers were also produced for glucose sensing measurements. Glucose oxidase (GOX), an enzyme, was immobilized on as-spun fiber surfaces prior to glucose sensing detection. Pristine H2O2 solution was used for testing the glucose sensing electrode composed of HCSA doped PANI-PS fibers. The cyclic voltammetry method was used to detect the redox currents for varied glucose concentrations at the oxidative potential of the glucose oxidation. The surface morphology before and after sensing measurements of as-prepared fibers for both types of sensors were investigated. Sensitivity comparison was performed between the fiber and film-type sensors as indicated by the slopes of humidity versus logarithm of resistance lines and glucose concentration versus redox current lines. Electrospun polyacrylonitrile (PAN) fiber precursor based carbon fiber (CNF) as well as electospun nickel (Ni) nanofiber-based mats were also produced and impregnated with epoxy resin. The electrical and mechanical properties of as-prepared nanofibers in the mat and short fiber filled epoxy nanocomposite forms were determined to demonstrate the effect of fiber aspect ratio and interconnecting network on those properties. Furthermore, the improvement of CNF conductivity was achieved by using in-situ silver (Ag)-PAN fibers as precursors, as well as by chemically coating Ag on CNF mat surfaces. For electrospun Ni nanofibers, Ag was chemically coated on fiber surfaces to prevent metal oxidation, and to increase conductivity. The characterization of samples was performed by using scanning electron microscope (SEM), Raman spectrometer, wide angle x-ray diffractometer (WAXD), Thermal gravimetric analyzer (TGA), Fourier transform infrared spectrometer (FT-IR), dynamic mechanical analyzer (DMA), voltmeter, and tensile tester. The effect of high thermal conductivity of CNF mat on curing reaction of epoxy nanocomposites was also studied by isothermal and dynamic experiments using DSC together with the gel content determination. Kamal’s model was employed for curve fitting the conversion rate versus the reaction rate at the initial stages of curing, at low temperatures.
Advisors/Committee Members: Sancaktar, Erol.
Subjects: Engineering, Materials Science
6. Baena, Johanna. PROCESSING AND KINETIC STUDIES OF THE REACTIVE BLENDS OF POLY(VINYL CHLORIDE) AND THERMOPLASTIC POLYURETHANES.
Degree: Doctor of Philosophy, Polymer Engineering, 2006, University of Akron
▼ A novel reactive blending process was investigated to produce poly(vinyl chloride)/thermoplastic polyurethane (PVC/TPU) blends of significant commercial interest. The reactive blending process took place in two stages. The thermally stabilized PVC was plasticized with the polyol monomers of TPU, and then the polyols reacted with diphenylmethane diisocyanate (MDI) to polymerize TPU in-situ with the PVC. In the first stage of the reactive blending, either a polyester or a polyether polyol and 1,4-butanediol (BDO), which forms the TPU’s hard segment, were blended with PVC. The polyester polyols included poly(butylene adipate) (PBA), poly(hexamethylene adipate) (PHA) and poly(propylene adipate) (PPA), while the polyether polyol was poly (propylene glycol) (PPG). In the second stage of the reactive blending, several commercial MDIs, including 4,4’-diphenylmethane diisocyanate (SMDI), a mixture of 4,4’-diphenylmethane diisocyanate and 2,4’- diphenylmethane diisocyanate (LMDI), and carbodiimide Modified 4,4 Diphenylmetane Diisocyanate (CDMDI) were used. Initially, blends of PVC with the various polyols were prepared in a batch internal mixer. The PBA and PHA were partially miscible with the PVC in a wide range of compositions. The PPA turned out to be partially miscible with PVC only at high concentrations of PVC, and the polyether (PPG) was immiscible with PVC. The miscibility and resultant morphology of the blends were determined by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. The reactive blending studies were performed in both a batch internal mixer and a continuous counter-rotating intermeshing twin-screw extruder. The miscibility, morphology, and mechanical properties of the blends were determined by differential scanning calorimetry (DSC), Fourier transformed infrared (FTIR), scanning and transmission electron microscopy (SEM and TEM), and Instron tensile testing respectively. The study in the batch internal mixer revealed that PVC was miscible with the TPUs produced when the TPU was formed from the polyester polyols, i.e. PBA, PHA and PPA. TEM revealed that the PVC/TPU blends had a multiphase morphology at room temperature, with varying morphologies that depended on the monomers used. The mechanical properties of the reactive blends depended on the MDI used and they showed superior tensile properties than the analogous melt blends. The reactive extrusion (REX) in a continuous counter-rotating intermeshing twin-screw extruder was shown to improve the PVC/TPU (LMDI) properties compared to those obtained by a batch internal mixer. In REX a liquid MDI was required to assure accurate MDI feeding, given the difficulties in feeding SMDI, which dimerizes at room temperature. Additionally, the use of a liquid MDI provides the fulfillment of the stoichimetric ratio that is critical to obtaining high molecular weight polyurethane. The reactive blending studies with the different MDIs resulted in blends with different properties and phase morphology, but similar thermal properties. The kinetics of TPU polymerization was studied in a rheometer and in a home-made vessel under various conditions of shear and pressure, respectively. This study revealed that pressure had negligible effect on the polymerization under the conditions studied, but that the shear rate influenced the rate of TPU polymerization appreciably. The effect of different polyols (PBA and PHA) and a variety of MDIs (SMDI, LMDI and CDMDI) on the TPU polymerization was studied. It was observed that the kinetic parameters depended on the chemical structure, functionality and reactivity of the monomers. Lastly, the effect of the addition of the PVC on the kinetics of TPU polymerization was studied. It was observed that the PVC stabilizer works as a TPU polymerization catalyst. The kinetic equation obtained that includes the effects of temperature, pressure, and shear would be useful for applications in twin screw extruders.
Advisors/Committee Members: Min, Kyonsuku.
Subjects: Plastics Technology
Keywords: REACTIVE EXTRUSION; POLY(VINYL CHLORIDE); THERMOPLASTIC POLYURETHANE; KINETICS OF POLYMERIZATION
7. Bajpai, Vardhan. SYNTHESES, CHARACTERIZATION AND APPLICATIONS OF MICRO-/NANO-STRUCTURED CONDUCTING POLYMERS AND CARBON NANOTUBES.
Degree: Doctor of Philosophy, Polymer Engineering, 2005, University of Akron
▼ The fabrication, characterization and manipulation of micro-and nano-systems have caused science and engineering to become interwoven closer together in a way that is unprecedented. Since the discovery of carbon nanotubes by Iijima in 1991 the quest for developing new micro- and nano-structures has been ever growing. On the other hand, discovery of inherently conducting polymer opened up many new possibilities for devices combining their unique optical, electrical and mechanical properties. The first part of my work deals in synthesis, characterization and application demonstrations for two microstructures of inherently conducting polymer (ICP), polypyrrole, namely, microcontainers and microhorns. These electrochemically-synthesized microstructures were characterized using various tools such as cyclic voltammetric technique, FT-IR, SEM and TEM. Apart from the region-specific growth of polypyrrole microcontainers it has been shown that they possess immense potential to be applied in the field of chemical/bio-sensing and controlled release. Polypyrrole microhorns have been demonstrated to form better chemical and biosensors. Nanostructures from conducting polymer will be better for such applications due to their higher surface energies and interactive area. Aligned helical carbon nanotubes were synthesized in large scale to function as a template for further synthesis of coaxial aligned helical carbon nanotube/polyaniline nanowires. Apart from that, polyaniline nanowires were also synthesized separately to compare the sensing properties of both nanostructures of polyaniline. These nanostructures were characterized using various tools such as cyclic voltammetric technique, FT-IR, Raman, XPS, SEM and TEM. Their application as electrochemical capacitors and chemical sensors were also demonstrated.
Advisors/Committee Members: Goettler, Lloyd.
Subjects: Engineering, Materials Science
Keywords: carbon nanotube, polypyrrole, polyaniline, nanowires, microcontainers,nanotechnology
8. Ban, Kyunha. Mechanism and Significance of Slip and New Mixing Elements During Flow in Modular Intermeshing Co-Rotating Twin Screw Extruders.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ We have investigated slippage effect on melt flow of various polyolefins and their compounds in modular intermeshing co-rotating twin screw extruder, which include high density polyethylene (HDPE), isotactic polypropylene (iPP), isotactic polybutene-1 (PB1), isotactic poly(4-methyl pentene-1) (P4MP1) and two different kinds of particle filled polypropylenes (PP/carbon black and PP/Silica). To induce slippage during the process, octadecanoic acid was introduced on the second port of the extruder. Length of fill, die pressure and screw characteristics in twin screw extruder were studied under varying processing parameters: volumetric flow rate, screw rotational speed, and die geometry. The effort to account for these variations on slippage effect was combined with considerations of the structures of polyolefins and polarities of fillers. One of five different polyolefins, CPO, has different backbone structure and the others have different pendant group. The order of pendant group size from small to big was found out to be HDPE > PP > PB1 > P4MP1. Two different kinds of inorganic particle fillers (carbon black and silica) were compounded to study the effect of polarity of inorganic particles on the slippage behavior. Carbon black represented non-polar filler and silica represented polar filler. In order to make objective and quantitative predictions in twin screw extrusion process, it was necessary to figure out slip velocity - shear stress relation since the boundary conditions on the barrel, screw and die surfaces are determined by slip velocities which are only can be predicted from applied shear stress fields. From the Mooney's method, we could find out slip velocity - shear stress relations using three different diameters of capillary dies having same L/D ratio. A numerical method (Flow Analysis Network method) was applied to simulate the effect of slippage on the flow in twin screw channel based on the slip velocity and shear stress relations obtained from capillary experiments. To confirm the simulation, length of fills for various process conditions were predicted by simulation and they were compared with experimental results. In addition, the screw characteristics and flow patterns for two different special mixing elements (SME, ZME) were obtained to investigate the mechanism and functions of these elements using the FAN method. The simulation of these special mixing elements were compared with conventional screw elements which having same helix angle, diameter and length.
Advisors/Committee Members: White, James.
Subjects: Polymers
Keywords: "wall slip; twin screw extruder, polyolefins"
9. Bas, Serkan. Synthesis of Hybrid Latexes and Polymerization Kinetics of Functional Latexes.
Degree: Doctor of Philosophy, Polymer Engineering, 2009, University of Akron
▼ Hydrophilic or hydrophobic functional monomers impart unusual properties to latexes. The type, amount, and addition sequence of functional monomers affect the colloidal stability, film formation and mechanical properties of latexes. Carboxylic acid and hydroxyl functional monomers provide reactive sites for crosslinking. The colloidal stability of latex particles can be enhanced by functional groups such as carboxylic acids. The latexes with functional groups can also be used to graft inorganic materials to form hybrid materials. Functional groups on the latexes not only determine the morphology of the latexes, but also the polymerization kinetics. The present work focused on assessing the effects of the type and the amount of functional monomers on the physical properties of hybrid latexes (particle size, solid content, and glass transition temperature, etc.), polymerization kinetics of core-shell latexes and mechanical properties of thermoset latex films. The first aim was to investigate the effect of hydrophobic groups such as polysiloxane on the physical properties of latexes. Polysiloxane-functionalized acrylic latexes were prepared by three different grafting techniques. In the first method, an acrylic core was prepared with the addition of a coupling agent, 3-(trimethoxysilyl) propyl methacrylate, after which a cyclic siloxane monomer (octamethylcyclotetra-siloxane, D4) was grafted onto the coupling agent. In the second method, a methacrylate- terminated polysiloxane was copolymerized with ethyl acrylate (EA) and 2-ethylhexyl acrylate (EHA) in batch emulsion polymerization. In the third method, D4 was added during emulsion polymerization of EA, EHA and 2-hydroxyethyl methacrylate. A core-shell morphology was observed in transmission electron microscopy (TEM) for the first preparation method. Microphase separation was observed by atomic force microscopy (AFM) after polysiloxane-functionalization for all latex films. Energy dispersive X-ray data indicated that only the hybrid latex by copolymerization of methacrylate-terminated polysiloxane (second grafting method) resulted in higher silicon content at the film-air interface than the film-substrate interface. In all methods, storage modulus and surface energy of latex films decreased after polysiloxane-functionalization of latexes. Secondly, the effect of polymerization of hydrophilic functional monomers with different types of surfactant on the polymerization kinetics was investigated. A semi-batch emulsion copolymerization of butyl acrylate (BA), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and methacrylic acid (MAA) was performed in which the concentration of HEMA (in core), MAA (in shell) and the type of surfactant (two anionic and two nonionic) were varied. New particle formation occurred throughout the polymerization even under almost starved-monomer conditions. The instantaneous rate of polymerization was inversely proportional to the concentration of HEMA and MAA. Secondary nucleation and limited coagulation were more significant when the anionic surfactant (Triton X-200) was used. In general, the smallest particle size was obtained when Triton X-200 was used. Generally, the anionic surfactant (Aerosol MA-80) yielded slower polymerization reactions which were attributed to high critical micelle concentration (CMC) compared to the other surfactants. Finally, the latexes with hydrophilic functional monomers were crosslinked to study the effects of crosslinker type on mechanical properties. The latexes with varying concentrations of HEMA, MAA and two types of surfactants (Tergitol XJ, Triton X-200) were crosslinked with five different types of crosslinkers. Melamine-formaldehyde (MF) resin was employed to crosslink hydroxyl functionalities in the core. Carboxylic acid groups in the shell were crosslinked with zinc ammonium carbonate and N, N′- dicyclohexylcarbodiimide. Cycloaliphatic diepoxide and hexamethylene diisocyanate (HDI) isocyanurate were used to crosslink with hydroxyl or carboxyl functional groups in the core and the shell. The toughest films were obtained when MF resin was used as crosslinker in the tensile test. However, zinc crosslinker yielded brittle films with very low toughness and pencil hardness. The highest Young’s modulus was obtained for the latex films when HDI isocyanurate or carbodiimide were used as crosslinker. In general, anionic surfactant (Triton X-200) showed higher crosslink density compared to nonionic surfactant (Tergitol XJ). This was attributed to the broader particle size distribution of the latexes with Triton X-200.
Advisors/Committee Members: Soucek, Mark.
Subjects: Polymers
Keywords: emulsion polymerization; latex; hydrid materials; acrylic; silicone; design of experiment; dual-cure
10. Cano, Camilo I. Polyimide Microstructures From Powdered Precursors: Phenomenological and Parametric Studies on Particle Inflation.
Degree: Doctor of Philosophy, Polymer Engineering, 2005, University of Akron
▼ Polyimide foams have emerged as high performance cellular materials with great potential for applications in aerospace due to their excellent mechanical, chemical and electrical properties. A novel technology in the development of polyimide foams consists of solid precursors in the form of powders with embedded blowing agent that can be converted by simple thermal treatment into single polymeric spherical microstructures or inflated together to produce foams with varying ranges of density and properties. This technology is hereafter referred to as solid-state powder foaming. Solid-state foaming from poly(amic acid) precursor particles was studied by examining concurrent and competitive phenomena that determine the morphology and physical properties of resulting polyimide microstructures. Phenomenological analysis of morphological relations, as well as physicochemical processes provided a comprehensive understanding of the governing principles by which potential particle inflation is achieved. Resulting polyimide microstructures present morphologies that are the result of the combined effect of morphological features on the precursor particles, blowing agent concentration and processing conditions exerted on the powders during the inflation process. A strong interrelation was found between different controlling factors such as precursor morphology and processing conditions. The balance between local temperature and concentration inside the particles can be manipulated by these controlling factors in such a way that a particle under a certain set of conditions might experience multiple bubble growth, while under slightly different conditions might present single bubble inflation or no inflation at all. Results from the phenomenological analysis served as the basis of a numerical model and corresponding parametric study where the different parameters of the governing phenomena were evaluated and studied. This parametric study provided a comprehensive understanding of the interrelation of different parameters and conditions which govern the inflation of powdered precursors. By having separated each of the different phenomena, as well as their sources and effects, future research in the field of powder foaming can center on specific issues where optimization or fundamental understanding is required. The comprehensive understanding achieved from the present work lays the groundwork for further optimization of the powder foaming process towards better control of foam properties and novel application development.
Advisors/Committee Members: Kyu, Thein.
Subjects: Chemistry, Polymer
Keywords: Polyimide; Foams; Powder; Microspheres
11. Cao, Feina. Shape Memory Polyurethane Nanocomposites.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ Shape memory polymers are smart materials which can remember their original shapes. However, the low recovery stress and low mechanical strength limit the commercial applications of shape memory polymers. In this study, nanoclays were introduced to shape memory polyurethanes (SMPU) to augment these properties by enhance the network of SMPU. Several factors which influence the shape recovery stress were evaluated, including the nature of polymer chain by using different monomers, type of clay particles, extent of filler dispersion, clay content and deformation conditions. It was found that only reactive clay particles were well dispersed into polyurethane matrix by the tethering between –CH 2CH 2OH functional groups in clay surfactants and polyurethane chains. Two different shape memory polyurethanes ( Systems I and II) prepared by bulk polymerization were compared. The shape memory effect of System Iwas triggered by melting of the soft segment crystals, while that of System IIwas by glass transition of the soft segments. It was seen that the reactive clay particles dispersed well in both polyurethane matrices and augmented the recovery stress, e.g., 20% increase with 1 wt % nanoclay in System Iand 40% increase with 5 wt % nanoclay in System IIwere observed. In System I, clay particles interfered with soft segment crystallization, and promoted phase mixing between the hard and soft segments, thus affecting the fixity and recovery ratio. Nevertheless, the soft segment crystallinity was still enough and in some cases increased due to stretching to exhibit excellent shape fixity and shape recovery ratio. The higher loading of clay particles accelerated the stress relaxation, resulting in reduction of recovery stress. In System II, no significant effect of clay particles in phase separation was observed, so there was no influence of clay on shape fixity and recovery ratio. The recovery stress increased with reactive nanoclay content. It was also found that the recovery stress could be tailored by the processing conditions. The recovery stress increased with decrease of stretching rate, and increase of stretching temperature and stretch ratio. The recovery stress of polyurethane/clay nanocomposites largely depended on the degree of clay exfoliation. Higher recovery stress was found in nanocomposites with better clay dispersion. The dependence of stress relaxation on stretching conditions, clay type, and clay content was also investigated and related to shape recovery stress. It was found that stress relaxation occurred more easily in the presence of nanoclay.
Advisors/Committee Members: Jana, Sadhan C.
Subjects: Engineering, Materials Science
Keywords: Shape memory; Polyurethane; Nanoclay; Nanocomposite
12. Carrillo, Antonio J. Residual Stresses and Birefringence in Gas-assisted Injection Molding of Amorphous Polymers: Simulation and Experiment.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ This study presents measurements and simulations of the distribution of the birefringence components, [Delta]n and nrr-nθθ, and the average birefringence, nzz-nθθ, and polymer/gas interface in polystyrene (PS) and polycarbonate (PC) spiral-shaped tubular moldings obtained by Gas-Assisted Injection Molding (GAIM) under various processing conditions. Furthermore, free quenching experiments in cylindrical samples were carried out at different initial temperatures, and various components of birefringence and stresses were measured and simulated. The flow- and thermally-induced stresses were simulated using linear and nonlinear viscoelastic theories, respectively. The flow- and thermally-induced birefringence components were calculated using the stress-optical rule and photoviscoelastic model, respectively. The governing equations of the GAIM process were derived using a nonlinear viscoelastic model and solved using a hybrid finite element/finite difference/control volume method. The governing equations to calculate the thermally-induced stresses were discretized using finite differences. The numerical results predicted qualitatively the effect of the processing variables on the polymer/gas interface distribution of the GAIM moldings. It was found that the processing variables that strongly affected the interface distribution were the injection speed, gas-delay time, and the shot size. The processing variables that exerted less influence on the polymer/gas distribution were the gas pressure and the melt and mold temperatures. The processing variables that strongly affected the birefringence [Delta]n were the melt temperature, injection speed, and gas-delay time. The mold temperature, gas pressure and shot size exerted less influence on [Delta]n. For PS GAIM moldings the measured birefringence near to the outer wall did not relax when a section was annealed at a temperature close to the glass transition temperature while the birefringence near to the inner wall did relax significantly. This observation was consistent with the idea that the birefringence at the outer wall was mainly induced by shear and normal stresses, while that at the inner wall was induced by thermal stresses. For PC GAIM moldings the measured [Delta]n and nrr-nθθ relaxed significantly near the inner and outer walls, suggesting that the thermally-induced stresses exerted a strong influence on the birefringence throughout the wall thickness. The simulated birefringence qualitatively described the measurements. Summation of simulated thermally- and flow-induced birefringence provided a better description of measured birefringence in PS GAIM moldings. However, for PC GAIM moldings, simulations showed large discrepancies. This was attributed to the strong influence of thermally-induced birefringence throughout the wall thickness of the molding. Melt temperature, gas-delay time, and injection speed strongly affected the average birefringence in PS GAIM moldings while the shot size and gas pressure exerted less influence. For PS GAIM moldings the contribution of the thermally-induced average birefringence was minimal while for PC GAIM moldings it was significant.
Advisors/Committee Members: Isayev, Avraam.
Subjects: Polymers
Keywords: Gas-assisted Injection Molding, Birefringence, Residual Stresses
13. Chakraborty, Ruby. Development of Novel Cycloaliphatic Siloxanes for Thermal and UV-curable Applications.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ Siloxanes have been extensively used as additives to modulate surface properties such as surface tension, hydrophobicity/hydrophobicity, and adhesion, etc. Although, polydimethyl -siloxane and polydiphenylsiloxane are the most commonly used siloxanes, the properties are at extremes in terms of glass transition temperature and flexibility. It is proposed that the ability to control the properties in between the these extremes can be provided by cycloaliphatic substitutions at the siloxane backbone. It is expected that this substitution might work due to the intermediate backbone rigidity. In order to achieve the above objectives, a synthetic route was developed to prepare cycloaliphatic (cyclopentane and cyclohexane) silane monomers followed by subsequent polymerization and functionalizations to obtain glycidyl epoxy, aliphatic amine and methacrylate telechelic siloxanes. The siloxanes were either thermally or UV- cured depending on end functionalizations. Chemical characterization of monomers, oligomers and polymers were performed using 1H, 13C, 29Si-NMR, FT-IR and GPC. The curing kinetics of photo-induced reactions were investigated through photo-differential scanning calorimetry (PDSC). The oxygen permeability, mechanical, coatings, and release properties of siloxanes were studied as a function of the backbone substitutions. The mechanical, coatings and released properties of cycloaliphatic siloxanes improved with respect to polydimethylsiloxanes. The thermal analysis of the cured films were carried out using differential scanning calorimetry (DSC). Viscoelastic properties of the cured siloxanes due to the variation of substitution at the siloxane backbone were measured using dynamic mechanical thermal analysis (DMTA). The cycloaliphatic substituted siloxanes showed an increased glass transition temperature and permeability but reduced crosslink density, conversion, and rate of curing with respect to polydimethylsiloxanes. Hybrids of siloxanes were prepared with linseed oil based alkyds to study the effect of variation of alkyd oil lengths and cycloaliphatic substitutions on siloxane backbone. The oil length of an alkyd resin is defined as the number of grams of oil used to produce 100 grams of resin. Three linseed oil based alkyds representing long, medium, and short oil lengths were grafted with siloxanes substituted with methyl, cyclopentyl, and cyclohexyl groups. The reaction was monitored through FTIR and 1H-NMR. The hybrids were formulated with standard drier package and thermally cured for detailed film characterization. Improvement in crosslink density, flexibility, and reverse impact resistance were found as function of oil length. However, tensile modulus, elongation, glass transition temperature, drying time and fracture toughness decreased with increase in oil length. For hybrids, the cycloaliphatic substituents at the siloxane backbone showed enhanced mechanical and coating properties as compared to hybrids with polydimethylsiloxanes. Random and block copolymer of polydimethylsiloxanes with polydicycloaliphatic- siloxanes were synthesized and compared with homopolymers of polydicycloaliphatic siloxanes. The chemical characterization of the copolymers and homopolymers were carried out through 1H, 13C, 29Si-NMR, and FT-IR. The glass transition temperatures (Tg) of the synthesized polymers were obtained through DSC and advanced rheometric expansion system (ARES). The Tg of random copolymers were found to be higher than the corresponding block copolymers. There was very small difference in Tg between cycloaliphaticsiloxanes homopolymers and corresponding random copolymers. From the above results, it can be inferred that the cycloaliphatic substitutions at the siloxane backbone can be used as a means to obtain properties intermediate to polydimethyl- and polydiphenyl siloxanes.
Advisors/Committee Members: Soucek, Mark.
Subjects: Chemistry; Experiments; Materials science; Polymers
Keywords: cycloaliphatic; siloxanes; hybrids; backbone rigidity; hydrophobicity; thermal; UV; curable
14. Chen, Hongyan. Simulations of Shearing Rheology of Thermotropic Liquid Crystalline Polymers.
Degree: Master of Science in Polymer Engineering, Polymer Engineering, 2008, University of Akron
▼ The simulations present a first attempt to describe the rheological properties of the liquid crystalline polymers (LCPs), using recently developed thermodynamic theory of weakly viscoelastic nematodynamics. In this theory the complicated rheological properties of nematic LCPs are modeled by a set of quasi-linear anisotropic viscoelastic constitutive equations (CE)with anisotropy described by director, whose viscoelastic evolution is coupled with the evolution of director. Although this theory has been developed for relatively small viscoelastic effects, it is still possible to compare the simulations with experimental data. A new mathematical tool, algebra of nematic operators, is helpful in operations with this multi-parametric theory. There are eight parameters in the theory which include 3 viscosities, 3 elastic moduli, and 2 tumbling (elastic and viscous) parameters. These parameters established for steady shearing are used for the calculations of evolution of shear stress and first normal stress difference with corresponding evolution of director, during relaxation and start up flow. The problem with initial conditions for director in start up flow is resolved in the following way. We preliminarily fitted the experimental data for stresses in steady shearing with following adjustment of parameters to describe also the relaxation of stresses. In this case the parameters of evolution equation for director, along with its orientation in steady shearing were also established. The orientation of director during stress relaxation was then easily calculated, and its final orientation at the rest state was taken as initial one for the start up flow. We demonstrate that our simulations are at least in a semi-quantitative agreement with experimental data obtained for two industrial and two academic LCPs. Dependencies of fitted theoretical parameters on temperature and structure of LCPs have also been discussed.
Advisors/Committee Members: Leonov, Arkady I.
Subjects: Chemical engineering; Engineering; Fluid dynamics; Materials science; Plastics; Polymers
Keywords: simulations; liquid crystalline polymers; rheology; nematodynamics
15. Dayal, Pratyush. DYNAMICS AND MORPHOLOGY DEVELOPMENT IN ELECTROSPINNING OF POLYMER SOLUTIONS.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ In the process of solution (dry, wet or electro) spinning of polymer fibers removal of the solvent is one of the most critical steps to produce dry fibers. In dry spinning as well as electrospinning the solvent is removed through evaporation from the surface of the fiber, whereas in wet spinning it is removed by exchange with a non-solvent. Therefore, morphology of the fiber is directly influenced by the interactions between the polymer and solvent or non-solvent. In other words, the structure development in polymer fibers is a function of spinning conditions and phase equilibria of the system. The primary objective of the present work is directed to the elucidation of solvent evaporation process and its effect on the structure evolution in fibers spun from polymer solutions. As the solvent concentration decreases, due to evaporation from the surface of the fiber, the system moves from low concentration to high concentration region in the phase diagram. In doing so, it traverses through different coexistence regions of the phase diagram in a matter dependent on the rate of solvent loss. To mimic the morphology development in binary systems comprised of polymer and solvent and ternary systems containing polymer, solvent and non-solvent,temporal evolution of the concentration order parameter was modeled in the framework of Cahn-Hilliard equation in conjunction with Flory-Huggins (FH) free energy of mixing. It turns out that, the competition between the phase separation dynamics and solvent evaporation rate determines the structure of the spun fiber. The aforementioned model was subsequently extended to main chain liquid crystalline polymer (MCLCP) solutions. The structure evolution in these systems is influenced by the mesophase transitions of MCLCP as well as the trajectory of the concentration sweeps across the phase diagrams of these systems. A generalized theoretical scheme was developed for the binary phase diagrams of crystal-liquid crystal mixtures by a combination of a phase field model of solidification, FH theory for liquid-liquid mixing and Maier-Saupe theory for nematic and Maier-Saupe-McMillan theory smectic ordering in liquid crystals. Subsequently, a non-equilibrium thermodynamic approach was developed to describe the emergence of fiber morphologies from MCLCP solution undergoing solvent evaporation. Matsuyama-Kato free energy was utilized which incorporated chain-stiffening, combined with Flory-Huggins free energy of mixing. The temporal evolution of concentration and nematic order parameters pertaining to the above free energy density of liquid crystalline polymer solution was simulated in the context of time dependent Ginzburg Landau (TDGL-Model C) theory coupled with the solvent evaporation rate equation under the quasi-steady state assumption. The morphology development of polymer systems investigated thus far were driven by concentration sweeps under quiescent conditions. In the actual electrospinning process the fiber morphology is immensely influenced by the fiber spinning conditions such as applied voltage, flow rate, viscosity of the solutions, solubility of polymer solutions among others. The dynamics of electrospinning process was modeled in terms of an array of beads connected by Maxwell's elements in a cylindrical coordinate system through the force balance between Coulombic and viscoelastic forces. The phase separation dynamics was calculated in the framework of Cahn-Hilliard equation in conjunction with solvent evaporation through the fiber surface. The simulations based on the coupling of these two processes revealed in-situ morphology development registering all structural forming process including polymer droplet, interconnected bi-continuous structure, porous structure and solid fiber. The fiber formation dynamics and associated morphology development were demonstrated as a function of applied voltage, electrical charge density and flowrate.
Advisors/Committee Members: Kyu, Thein.
Keywords: Electrospinning, Solvent evaporation, Concetration sweeps, Morphology development, Phase separation
16. Dharaiya, Dhawal. EFFECTS OF NANOCLAY AND CONDUCTIVE CARBON BLACK ON MORPHOLOGY DEVELOPMENT IN CHAOTIC MIXING OF IMMISCIBLE POLYMERS.
Degree: Doctor of Philosophy, Polymer Engineering, 2006, University of Akron
▼ Chaotic mixing of immiscible polymer blends has been known to produce morphological features such as lamellas, fibrils and droplets. In this research work, we studied the effect of fillers, such as carbon black (CB) and organically treated nanoclay, on morphology development in an immiscible polymer system, consisting of polyamide 6 (PA6) and polypropylene (PP) in a chaotic mixer. Operating conditions were chosen such that chaotic mixing was widespread inside the mixer. The filler particles were mixed with minor component PP before blending with PA6. It was found that continuous lamellar and fibrillar morphology of PP formed early in mixing produced double percolating conductive networks with only 1 wt% CB particles. The conductive networks sustained their existence even after fibrils broke into droplets. This was attributed to migration of CB particles from the bulk of PP droplets and selective localization at the interfaces of closely spaced PP droplets. It was also found that much smaller PP droplets resulted in the presence of CB particles. Prior reports in literature indicated that organically treated nanoclay particles can act as compatibilizer of immiscible polymer blends, although no study showed that how nanoclay would influence morphology development. In this study, we showed that clay particles helped produce PP droplets of much smaller size and with narrower size distribution due to their direct influence on breakup of PP domains. The clay particles reduced interfacial tension between PP and PA6 phases. Consequently, the PP domains sustained lamellar and fibrillar forms and significantly thin fibrils were formed. These thin fibrils in turn broke rapidly into smaller droplets. It was also found that a large fraction of clay particles migrated into PA6 phase and contained intercalated PA6 chains in their galleries. This indicated that clay particles did not participate in compatibilization in this system. The effect of degradation of surface treatment of nanoclay was also characterized using contact angle measurements. It was observed that the surface polarity of treated clay reduced significantly owing to degradation of the organic modifier of clay. This degradation had significant consequences on morphology development in the blending of PP and PA6.
Advisors/Committee Members: Jana, Sadhan C.
17. Du, Ling. HIGHLY CONDUCTIVE EPOXY/GRAPHITE POLYMER COMPOSITE BIPOLAR PLATES IN PROTON EXCHANGE MEMBRANE (PEM) FUEL CELLS.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ In this work, highly conductive carbon-filled epoxy composites were developed for manufacturing bipolar plates in proton exchange membrane (PEM) fuel cells. These composites were prepared by solution intercalation mixing, followed by compression molding and curing. The in-plane and through-plane electrical conductivity, thermal and mechanical properties, gas barrier properties, and hygrothermal characteristics were determined as a function of carbon-filler type and content. For this purpose, expanded graphite and carbon black were used as a synergistic combination. Mixtures of aromatic and aliphatic epoxy resin were used as the polymer matrix to capitalize on the ductility of the aliphatic epoxy and chemical stability of the aromatic epoxy. The composites showed high glass transition temperatures (Tg ~ 180°C), high thermal degradation temperatures (T2 ~ 415°C), and in-plane conductivity of 200-500 S/cm with carbon fillers as low as 50 wt%. These composites also showed strong mechanical properties, such as flexural modulus, flexural strength, and impact strength, which either met or exceeded the targets. In addition, these composites showed excellent thermal conductivity greater than 50 W/m/K, small values of linear coefficient of thermal expansion, and dramatically reduced oxygen permeation rate. The values of mechanical and thermal properties and electrical conductivity of the composites did not change upon exposure to boiling water, aqueous sulfuric acid solution and hydrogen peroxide solution, indicating that the composites provided long-term reliability and durability under PEM fuel cell operating conditions. Experimental data show that the composites developed in this study are suitable for application as bipolar plates in PEM fuel cells.
Advisors/Committee Members: Jana, Sadhan C.
Subjects: Engineering, Chemical
Keywords: bipolar plates, proton exchange membrane fuel cells, conductive polymer composites, electrical conductivity, hygrothermal effects, thermal conductivity
18. Dziczkowski, Jamie S. Advances in Acrylic-Alkyd Hybrid Synthesis and Characterization.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ In situ graft acrylic-alkyd hybrid resins were formed by polymerizing acrylic and acrylic-mixed monomers in the presence of alkyds by introduction of a free radical initiator to promote graft formation. Two-dimensional NMR, specifically gradient heteronuclear multiple quantum coherence (gHMQC), was used to clarify specific graft sites of the hybrid materials. Both individual and mixed-monomer systems were produced to determine any individual monomer preferences and to model current acrylic-alkyd systems. Different classes of initiators were used to determine any initiator effects on graft location. The 2D-NMR results confirm grafting at doubly allylic hydrogens located on the fatty acid chains and the polyol segment of the alkyd backbone. The gHMQC spectra show no evidence of grafting across double bonds on either pendant fatty acid groups or THPA unsaturation sites for any of the monomer or mixed monomer systems. It was also determined that choice of initiator has no effect on graft location. In addition, a design of experiments using response surface methodology was utilized to obtain a better understanding of this commercially available class of materials and relate both the chemical and physical properties to one another. A Box-Behnkin design was used, varying the oil length of the alkyd phase, the degree of unsaturation in the polyester backbone, and acrylic to alkyd ratio. Acrylic-alkyd hybrid resins were reduced with an amine/water mixture. Hydrolytic stability was tested and viscoelastic properties were obtained to determine crosslink density. Cured films were prepared and basic coatings properties were evaluated. It was found that the oil length of the alkyd is the most dominant factor for final coatings properties of the resins. Acrylic to alkyd ratio mainly influences the resin properties such as acid number, average molecular weight, and hydrolytic stability. The degree of unsaturation in the alkyd backbone has minimal effects on resin and film performance. Reversible-addition fragmentation polymerization techniques were employed to create a new class of acrylic-alkyd hybrid materials. Medium and long oil alkyds made from the monoglyceride process using soybean oil, glycerol, and phthalic anhydride were modified with a RAFT chain transfer agent. The alkyd macro-RAFT agents were reached by end-capping a medium oil soya-based alkyd with a carboxy-functional trithiocarbonate. The alkyd macro-RAFT agents were then used to create acrylic-alkyd block structures by polymerizing different acrylic monomers, including both acrylates and methacrylates in the presence of the macro-RAFT agent and 2, 2'-azobisisobutyronitrile (AIBN). Co-acrylic segments were reached by complete polymerization of one monomer followed by the addition of a second monomer and additional free radical initiator. The alkyds, macro-RAFT agents and, acrylic-alkyd blocks were characterized by size-exclusion chromatography (SEC), FTIR, and 1H-NMR. Pseudo-first-order kinetics behavior and conversion vs. molecular weight plots show that the RAFT-mediated reaction afforded a more controlled process for the synthesis of acrylated-alkyd materials. Preliminary coatings tests showed that material properties of acrylated-alkyds achieved by RAFT polymerization exhibit good overall coatings properties including adhesion, gloss, hardness, and impact resistance.
Advisors/Committee Members: Soucek, Mark.
Subjects: Polymers
Keywords: alkyd; acrylic-alkyd; RAFT; gHMQC; 2D-NMR
19. Ertekin, Ayca. Analysis of Wetting, Flow and End-use Properties of Resin Transfer Molded Nanoreinforced Epoxy-glass Fiber Hybrid Composites.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ In this research, the usage of single wall carbon nanotubes (SWNTs) and nanoclays in the resin transfer molding (RTM) of biaxially stitched micro-fiber reinforced epoxy matrix composites was investigated to evaluate the role of nanoscale reinforcements on the wetting, flow and end-properties of composites through multi-scale effects. The study primarily focused on characterization of the state of dispersion and curing of nanoscale reinforced epoxy polymers, assessment of the relative importance of viscous forces over interfacial forces and the wettability of glass fabrics by the nanoscale reinforced epoxy polymers, analysis of complex flow of nanoreinforced epoxy systems through glass fiber porous media by several flow properties and evaluation of the properties of hybrid epoxyglass fiber composites enriched with nanoscale particles. The presence of nanoreinforcements retarded the cure kinetics to some degree such that the activation energies increased with the nanoreinforcement content. Both the unsteady-state and steady-state relative tow permeabilities were observed to decrease as the nanoclay amount was increased. The presence of nanoclay was observed to reduce the "tow wet-out" with almost 50 % reduction in the steady-state tow permeability with the addition of 4 wt % nanoclay to the reactive epoxy. Contact angle measurements indicated, approximately 21 % increase in the contact angle with the addition of 4 wt % nanoclay to epoxy. It was found that beyond 0.3 wt % SWNT, RTM of epoxy-60 % biaxially stitched glass fiber systems was not feasible. It was also observed that an addition of SWNT at only 10 % the level of nanoclay caused almost a 25 % increase in steady-state pressure level along with almost an 18 % decrease in permeability. It is believed that nanoreinforcements affected flow rate somewhat differently along the various fabric capillary paths and thereby leading to preferential flow paths in the mold cavity. It is proposed that nanoparticles, particularly at high weight percentages, agglomerate during flow inside the mold cavity and block some intra-tow regions and lead to instabilities in the flow resulting in anomalous pressure differentials at different regions of the flow and unusual permeability results. While the use of nanoreinforcements reduced oxygen and moisture transport along with thermal expansion coefficient, the mechanical properties were found to decrease due to the several defects, such as voids and agglomerates introduced during RTM as a result of the differential micro- and macro-flows.
Advisors/Committee Members: Jana, Sadhan C.
Subjects: Engineering; Polymers
Keywords: resin transfer molding, wetting, nanoreinforcements, epoxy, glass fiber, hybrid composites
20. Fujiyama-Novak, Jane Hitomi. The Role of Nanoclay on the Deformation Behavior of Polypropylene/Maleic Anhydride Modified Polypropylene Films and Fibers in Full and Partially Molten State Processing.
Degree: Doctor of Philosophy, Polymer Engineering, 2009, University of Akron
▼ The behavior of polypropylene nanocomposite fibers and films under uniaxial deformation in a partial and fully molten state was investigated. A fiber that exited the die was found to contain orientation gradient in the radial direction that was preserved even after solidification without application of a take up. The shearing effect in the die resulted in a band of oriented outer layers in which broad surfaces of the clay particles became parallel to the surface of the fibers. The polymer phase trapped between these particles exhibited moderate to high preferential orientation levels. Conversely, in the core low levels of preferred orientation were found in both the clay and the polymer phases. Upon application of take up, the presence of clay particles substantially enhanced the orientation of amorphous and crystalline phases in PP/PPgMA (maleic anhydride modified polypropylene) fibers. This was due to the substantial decrease in chain relaxation in the proximity of the clay platelets and enhancement of orientation in the polymer phase in the vicinity of particles that created amplified deformation field by their relative motions. Measurements of the clay orientation in the melt-spun fibers as they undergo “confined melting” in constrained state revealed that these naturally anisotropic nanoplatelets contributed positively to the birefringence of the fibers. A hybrid real-time spectral birefringence technique depolarized light intensity method was used to capture the mechanistic changes that take place during heating, stretching, holding and cooling cycles of PP/PPGMA and nanocomposite films in a partially molten state. During the heating stage, the birefringence and the degree of melting of the as-cast films were determined by real-time depolarizing light intensity technique. The results indicated the initial fraction of crystallites, which govern the deformation behavior of the PP films, remained in dynamic equilibrium with the molten phase prior to the deformation after the initial transient stage. Throughout the uniaxial deformation of PP/PPgMA at this “mushy” (partially molten) state, the true stress-birefringence results showed three-regime stress optical behavior. Regime I was characterized by the slight decrease of birefringence with stress. At this small strain level, the original spherulites deformed and some of these superstructures were destroyed, resulting in broken and deformed lamellar remnants. Regime II showed a rapid rise of birefringence accompanied by orientation and stress induced crystallization. During this stretching microfibrillar morphology developed. Polymer chains approached their limit of extensibility in Regime III, where birefringence approached a plateau. After the deformation, the film was held in a stretched state at the processing temperature to simulate the holding stage. During this period, birefringence and strain increased, while stress decreased. This stage was found to involve the growth of lamellae, primarily in the transverse direction on the previously developed fibrillar morphology. During cooling, the stress initially decreased, and birefringence and strain showed significant development. Following this stage, the thermal and stress-induced crystallization under the influence of substantial contraction forces developed. The results of the experimental investigation on as-cast nanocomposite films indicated the nanoplatelet poles are highly oriented perpendicular to the plane of the film. This leaded to a uniplanar texture in PP crystalline as well as clay phase. During the heating cycle, real-time depolarized light intensity and independent IR spectroscopy measurements revealed the presence of clay particles reduced significantly the rate of PP/PPgMA melting. The uniaxial stretching of nanocomposite films at partially molten states (T
Advisors/Committee Members: Cakmak, Mukerrem.
Subjects: Materials science; Packaging; Polymers
Keywords: polypropylene; nanocomposites; nanoclays; montmorillonite; nanoplatelets; nanoparticles; films; fibers; structural evaluation; real-time measurements; birefringence; optical properties; film processing; fiber processing; maleic anhydride modified polyprop
21. Gagov, Atanas. INSTABILITIES IN ELONGATION FLOWS OF POLYMERS AT HIGH DEBORAH NUMBERS.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ The objective of this work is to study the instabilities of the contraction flows at high Deborah numbers of various polymers. The following topics will be discussed: 1. Linear and nonlinear stability analysis of isothermal fiber spinning instabilities. 2. Linear stability analysis of nonisothermal fiber spinning instabilities 3. Linear and nonlinear stability analysis of contraction flow instabilities 4. Propagation of reservoir instabilities in capillary The analysis of these flow problems requires solution of the closed set of PDE’s (or ODE’s), consisting of equations for conservation of mass and momentum, along with an adequate viscoelastic constitutive equation, with appropriate initial/boundary conditions. The goal of this work is to demonstrate a procedure for determining the critical regime beyond which the process becomes unstable and also to determine weather the process is stable when the disturbances grow to a finite size. Linear and non-linear stability theories have been used to describe the fluctuations of fiber spinning and contraction flow. Linear stability analysis determines the onset of the instabilities of the process while nonlinear analysis establishes the complete range of the stable and unstable conditions. The melt fiber spinning is the most common of polymer fiber processing. Finding critical process conditions and the stabilizing effect of the cooling is described in this work. The critical draw ratio is established using linear stability analysis and the effect of the finite size imposed disturbances is studied through nonlinear stability analysis. Contraction flow is one of the benchmark problems in computational polymer fluid mechanics and polymer processing. In this modeling, the whole flow region is divided in naturally introduced sub-regions with well-known and highly simplified types of flow. Thus, the model analyzes the entire flow region in a simplified geometric manner with properly matched conditions between adjacent sub-regions. The propagation of the disturbances formed in the reservoir region has been analyzed. Employing the isothermal “Jet approach” followed by linearized perturbation approximation of the governing equations for finding the onset of the instabilities supplies information about the stability of the contraction flow which has been used to describe the mechanism of propagation of the disturbances into capillary up to the die exit, and its numerical implementation.
Advisors/Committee Members: Leonov, Arkadii I.
22. Gintert, Michael Jason. A NOVEL APPROACH TO OBTAIN HIGH PERFORMANCE LAYERED SILICATE THERMOSET POLYIMIDE MATRIX NANOCOMPOSITES.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ The use of exfoliated layered silicates as reinforcing filler in polymer composites shows great potential for drastically improving the material properties. Much work has been done with thermoplastic systems showing improved composite properties from exfoliated clay reinforcement. More recently, thermoset systems have been increasingly studied, however little progress has been made in achieving full clay exfoliation in high performance thermoset polyimides, which otherwise possess desirable processability, thermal, and mechanical properties. Literature has shown that attaining necessary levels of clay exfoliation and dispersion in these materials is a challenge using current technology. This study explores a novel method of clay exfoliation, which involved prior intercalation of organoclay with lower molecular weight PMR-5 oligomer and incorporation of clay into higher molecular weight PMR-15 resin. In the process, the elastic force within clay galleries grows at a higher rate and pushes the clay platelets apart to yield an exfoliated clay structure. Process of clay intercalation by lower molecular weight PMR monomer is studied under conditions of low and high shear mixing and sonication, in order to determine effect of processing on properties. In addition, this work sought to determine an optimum organic treatment for nanoclay to be compatible with the method stated above. Specifically, an organic treatment should promote PMR-5 intercalation of clay and be stable at very high curing temperature, ~315„aC. In this work four surfactants were used to treat clay, including hydrochloride salts of aliphatic and aromatic diamines and, reactive and non-reactive amines. The resultant mechanical, thermal, and rheological properties were evaluated as functions of clay type, degree of clay exfoliation, and clay intercalation strategies and compared with those available in literature. It was found that method of mixing plays a key role in achieving high degree of exfoliation, and a 1:3 ratio by weight of APND and C12 combination of surfactants are optimum to achieve maximum clay exfoliation and improvements in thermal and mechanical properties of PMR-15 composites. The resultant composites showed increased barrier properties, improved thermal stability, and increased flexural strength and strain in carbon fiber laminate composites compared to composites of neat PMR-15 resin. The new and novel method of clay exfoliation in PMR-15 resin explored in this work was met with success.
Advisors/Committee Members: Jana, Sadhan C.
Subjects: Engineering, Materials Science
Keywords: Polymer Nanocomposites; Layered Silicate Clay; Polyimide; Thermoset
23. Guha Thakurta, Soma. Anhydrous State Proton and Lithium Ion Conducting Solid Polymer Electrolytes Based on Sulfonated Bisphenol-A-Poly(Arylene Ethers).
Degree: Doctor of Philosophy, Polymer Engineering, 2009, University of Akron
▼ Sulfonated polymer based solid polymer electrolytes (SPEs) have received considerable interest in recent years because of their wide variety of applications particularly in fuel cells, batteries, supercapacitors, and electrochromic devices. The present research was focused on three interrelated subtopics. First, two different bisphenol-A-poly(arylene ethers), polyetherimide (PEI) and polysulfone (PSU) were sulfonated by a post sulfonation method to various degrees of sulfonation, and their thermal and mechanical properties were examined. The effects of poly(arylene ether) chemical structure, reaction time, concentration, and types of sulfonating agents on sulfonation reaction were investigated. It was found that deactivation of bisphenol A unit caused by the electron withdrawing imide, retarded the sulfonation of PEI compared to PSU. Sulfonation conducted with a high concentration of sulfonating agent and/or prolonged reaction time exhibited evidence of degradation at the isopropylidene unit. The degradation occurred through the same mechanistic pathway with the two different sulfonating agents, chlorosulfonic acid (CSA) and trimethylsilyl chlorosulfonate (TMSCS). The degradation was faster with CSA than its silyl ester, TMSCS, and was evident even at low acid concentration. Second, novel anhydrous proton conducting solid polymer electrolytes (SPEs) were prepared by the incorporation of 1H-1,2,4-triazole (Taz) as a proton solvent in sulfonated polyetherimide (SPEI) matrix. The size, shape, and state of dispersion (crystal morphology) of triazole crystals in SPEI were examined as a function of degree of sulfonation and triazole concentration. Increasing sulfonic acid content caused reduction of triazole crystallite size, hence the depression of melting temperature and their uniform distribution throughout the sulfonated polymer matrix. The increased rate of structure diffusion within the smaller size crystals due to the improved molecular mobility contributed significantly to the anhydrous state proton conductivity. Third, a new category of single lithium ion conducting SPEs was developed by crosslinking a polyether epoxy, poly(ethylene glycol)diglicidyl ether (PEGDGE) (lithium ion solvent), in sulfonated polysulfone (SPSU) matrix. The effects of degree of sulfonation and electrolyte composition on ionic conductivity, thermal, and tensile properties of SPEs were investigated. It was found that ion-dipole interactions between lithium sulfonate (SO3Li) and PEGDGE were responsible for the reduction in size of the dispersed epoxy phase and increased thermal stability. Lithium sulfonate promoted compatibilization and also caused improvement in elongation at break. A low molecular weight electrolyte salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was further dissolved in PEGDGE phase prior to its crosslinking in SPSU matrix, and the ionic conductivity and thermal properties were evaluated as a function of doping level. The ionic conductivity showed remarkable improvement compared to the undoped system.
Advisors/Committee Members: Min, Kyonsuku.
Subjects: Polymers
Keywords: solid polymer electrolytes; sulfonated bisphenol-A-polyetherimide; sulfonated bisphenol-A-polysulfone; sulfonation; trimethylsilyl chlorosulfonate; degree of sulfonation; triazole; proton solvent; anhydrous state proton conductivity; crystal morphology; s
24. Gunes, Ibrahim Sedat. Analysis of Shape Memory Properties of Polyurethane Nanocomposites.
Degree: Doctor of Philosophy, Polymer Engineering, 2009, University of Akron
▼ The effects of nano-size fillers on shape memory (SM) properties of polyurethane (PU) nanocomposites were evaluated. Organoclay, carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), silicon carbide (SiC), and carbon black (CB) were selected as the fillers in an attempt to reinforce the PU and to obtain significantly increased shape recovery stress. The shape memory PU was synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol, the latter with a molecular weight of 4000 g/mol. The composites were prepared by melt mixing of extended chain PU with the fillers. The shape memory behavior was triggered by heating the specimen above the melting point of the crystalline soft segment. Our results indicated that exfoliated organoclay significantly augmented SM performance, while CNF and SiC diminished it by interfering with crystallization of the soft segment. The shape memory properties reduced significantly beyond a certain loading of CB. Better SM performance with organoclay can be attributed to significant mechanical reinforcement and very little detrimental effect on the soft segment crystallinity. The reduction of soft segment crystallinity in the presence of CNF and SiC was analyzed. It was found that the extent of crystallinity, as well as the crystallization temperature, was significantly reduced in the presence of these fillers. The effects of thermal expansion on shape memory performance of SMPU composites were also evaluated. It was observed that strain due to thermal expansion can significantly reduce the recoverable strain. Such reduction depends on the magnitude of temperature gradient in shape recovery (heating) step, thermal expansion coefficient of system, and the level of recoverable strain. The effectiveness of carbonaceous, electrically conductive fillers, CNF, ox-CNF, and CB, in shape memory actuation of polyurethane composites by resistive heating was also evaluated. Specifically, the dependence of electrical resistivity on specimen temperature and imposed tensile strains encountered in shape memory test cycles was determined for CNF/SMPU, ox-CNF/SMPU, and CB/SMPU composites. A reduction in soft segment crystallinity was observed in the presence of CNF and ox-CNF; the reduction was smaller in the case of ox-CNF. Only the composites of CB showed pronounced positive temperature coefficient (PTC) effects. The observed PTC effects bore a close relationship with non-linear thermal expansion during heating. The composites of CNF and ox-CNF did not show PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CB increased by several orders of magnitude with imposed strain while composites of CNF and ox-CNF showed weak dependence. The filler-matrix interactions were anticipated to be stronger in ox-CNF/SMPU composites compared to those in CNF/SMPU composites due to the presence of polar functional groups on ox-CNF particles. However experimental analysis of hard segment interactions with CNF and ox-CNF particles as revealed from fluorescence emission spectroscopy data could not distinguish between the relative strengths of these interactions. The study also investigated if styrene-isoprene-styrene (SIPS) and styrene-isobutylene-styrene (SIBS) copolymers would exhibit shape memory properties, especially in light of the possibility of strain induced crystallization of these polymers. It was found that the pristine and silica filled SIBS and SIPS copolymers did not exhibit any shape memory properties due to absence of crystals at room temperature.
Advisors/Committee Members: Jana, Sadhan.
Subjects: Polymers
Keywords: Shape Memory Polymer; Nanocomposites; Polyurethane
25. Hill, Meagan E. Adding Value to Recycled Polyethylene Through the Addition of Multi-Scale Reinforcements.
Degree: Master of Science in Polymer Engineering, Polymer Engineering, 2005, University of Akron
▼ As a result of the degradation experienced by polymers during their use and the impurities acquired during the recycling process, recycled polymers tend to have weaker mechanical properties than their virgin counterparts. Efforts have been made to upgrade recycled high-density polyethylene so that it may compete with virgin material, both economically and performance wise. This study focuses on the improvement of the mechanical properties of recycled high-density polyethylene (RHDPE) through the addition of multi-scale reinforcements. RHDPE composites reinforced with nanoclay, cellulose fibers, and a combination of the two were made. The effects of the reinforcements on the mechanical properties of the composites as well as the effect of compatibilization on the composite properties are highlighted. To address the economic objective of this study, bentonite is investigated as a possible alternative to montmorillonite as the nanofiller in the hybrid system. Bentonite is the ore from which montmorillonite is refined. So, it is less expensive than montmorillonite, but possesses the same layered silicate structure. In this study, a hybrid composite was made and shown to have increased tensile strength and elastic modulus compared to that of virgin HDPE. The nanoscale clay reinforcement was found to provide effective stiffening to the composite, while the microscale cellulose reinforcement was found to have effective strengthening.
Advisors/Committee Members: Goettle, Lloyd A.
26. Huang, Wenyi. Synthesis, Characterization, and Rheology of Functional and Heterocyclic Liquid Crystalline Polymers.
Degree: Doctor of Philosophy, Polymer Engineering, 2006, University of Akron
▼ Three segmented main-chain thermotropic liquid crystalline polymers (TLCPs) functionalized with nitrogen-containing heterocyclic groups were synthesized; namely PyHQ12 having pendent pyridyl group, PABP having side-chain azopyridyl group with flexible spacer, and PTBP containing side-chain terpyridine group with flexible spacer. The principles of specific interactions were applied to prepare supramolecular structures and organoclay or clay nanocomposites based on these functional TLCPs. Three combined main-chain/side-chain liquid-crystalline polymers (MCSCLCPs) were prepared via hydrogen bonding or ionic interactions based on PyHQ12 and PABP. The presence of hydrogen bonds in self-assembled PyHQ12-7CNCOOH and PABP-AA, and the presence of ionic interactions in self-assembled PABP-TSA, which exist above their respective clearing temperatures, were confirmed using Fourier transform infrared (FTIR) spectroscopy, the thermal transitions in each MCSCLCP were determined using differential scanning calorimetry (DSC), and the mesophase structures of each self-assembled MCSCLCP were characterized using polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). PyHQ12 was observed to be very effective in exfoliating organoclay (Cloisite 30B) aggregates, because of the formation of hydrogen bonds, as determined by in situ FTIR spectroscopy, between the pendent pyridyl group in PyHQ12 and the hydroxyl groups in the surfactant MT2EtOH residing at the surface of Cloisite 30B. Thus, functionality in TLCP is necessary to obtain highly dispersed organoclay nanocomposites, but at the same time there is a possibility to lose some degree of liquid crystallinity in the TLCP, due to the proximity of the pendent pyridyl group to the mesogenic main-chain backbone. Thus, another functional TLCP, PABP having side-chain azopyridyl group with flexible spacer, was synthesized. It has been found that the liquid crystallinity of PABP in the organoclay nanocomposites was more or less intact and yet organoclay (Cloisite 30B) aggregates were very well dispersed, which was attributed to the fact that the pyridyl group in the side-chain azopyridine of PABP is located sufficiently far away from the mesogenic main chain. The rheological properties of functional TLCP/organoclay nanocomposites were also investigated. A unique polymer/ruthenium complex [RuII(PTBP)(6TPy)](PF6)2based on PTBP was used to prepare natural clay (Montmorillonite, MMT) nanocomposite. X-ray diffraction and transmission electron microscopy indicate that MMT has a high degree of exfoliation in the matrix of PTBP/ruthenium complex. UV-vis spectroscopy and FTIR spectroscopy show that the driving force for exfoliating MMT in PTBP/ruthenium complex is attributable to the Coulombic interactions between the positively charged ruthenium center in the side chain of the polymer and the negatively charged clay surfaces.
Advisors/Committee Members: Han, Chang Dae.
Keywords: Liquid crystalline polymers; heterocylic polymers; clay (organoclay) nanocomposites; rheology; pyridine, azopyridine, terpyridine; ruthenium complex; exfoliation; combined main-chain/side-chain liquid crystalline polymers; self-assembled supramolecular st
27. Jenkins, John A. Jr. ULTRASONIC DECROSSLINKING OF CROSSLINKED POLY (ETHYLENE).
Degree: Master of Science in Polymer Engineering, Polymer Engineering, 2007, University of Akron
▼ Decrosslinking of crosslinked polyethylene with the aid of ultrasonics is considered in this study. Polyethylene is crosslinked with peroxide by heating and compression molding and then subjected to various levels of ultrasonic energies to break the bonds in the crosslinked polyethylene network. Properties of the resulting materials are then compared to original properties to determine the usefulness of the process. Apparatus used to perform continuous decrosslinking includes a one-inch single screw Killion extruder for pumping of the crosslinked polyethylene into a die fitted with the ultrasonic device. The die is designed to allow the material to pass through an area where it is uniformly subjected to ultrasonic energies of various levels. The ultrasonic energy is introduced to the material through a stepped horn using a Branson power source. During processing, conditions were set that included pressures and time for the static experiments, barrel temperatures, flow rates, and ultrasonic power consumption for the continuous experiment. Static experiments on crosslinked polyethylene were performed where specific amounts of crosslinked material were exposed to controlled ultrasound energy. This apparatus used the same power source but was discontinuous. Properties of treated and untreated material such as gel fraction, crosslink density, viscosity, and mechanical properties were measured. Comparisons of collected samples showed that the process is highly effective for decreasing linking of crosslinked polyethylene at specific processing conditions. Further study of this process will surely be beneficial.
Advisors/Committee Members: Isayev, Avraam I.
Subjects: Engineering, Materials Science
Keywords: crosslinking, decrosslinking, ultrasonic, polyethylene
28. Jiang, Qibo. Modeling Flow, Melting, Solid Conveying and Global Behavior in Intermeshing Counter-Rotating Twin Screw Extruders.
Degree: Doctor of Philosophy, Polymer Engineering, 2008, University of Akron
▼ Intermeshing counter-rotating twin screw extruders are widely applied in polymer processing industry, especially in compounding and PVC profile processing. However, the design of this type of machines is generally based on experiences and error-and-try. In addition, most of the investigations on intermeshing counter-rotating twin screw extruders were made on the melt conveying region. There is a lack of adequate study on a complete extrusion process to this type of machines. In this study, models were developed to simulate the extrusion processes, including solid conveying, melting and metering, evaluate the performance of intermeshing counter-rotating twin screw extruders, and optimize the design of machines and operating conditions. Experiments were carried out on a laboratory modular intermeshing counter-rotating twin screw extruder to observe solid conveying, the melting process and the global behavior of this type of machine. The solid bed is formed in the solid conveying region. The inter-screw region plays a dominant role in the melting process. Based on our observations, models were developed to describe both the solid conveying and the melting process. Based on hydrodynamic lubrication theory, a melt conveying model was developed to characterize the pumping capacity of screw elements in intermeshing counter-rotating twin screw extruders. The effect of screw channel aspect ratio (screw channel depth / width) was incorporated into the melt conveying model to improve the prediction of screw pumping capacity. Calculations were made to investigate the effect of geometrical parameter on screw pumping capacity. Models of solid conveying, the melting process and melt conveying were integrated together and a global composite model was developed to characterize the whole intermeshing counter-rotating twin screw extrusion process. The global model is intended for both flood fed and metered starved fed conditions. This is the first composite model designed for this type of machines. Simulations and experiment results were compared and it was found that they match very well. This global model was further successfully developed into user-friendly software, which is used to design, test and optimize intermeshing counter-rotating twin screw extruders.
Advisors/Committee Members: White, James.
Subjects: Plastics
Keywords: model flow; melting; solid conveying; composite model; software; intermeshing counter-rotating twin screw extruders
29. Jimenez, Guillermo Alfonso. Characterization of Poly(Methyl Methacrylate) and Thermoplastic Polyurethane-Carbon Nanofiber Composites Produced by Chaotic Mixing.
Degree: Doctor of Philosophy, Polymer Engineering, 2007, University of Akron
▼ Chaotic mixing is a novel mixing technique offering high mixing efficiency even under mild shearing conditions. In this work, chaotic mixing was used to prepare composites of carbon nanofibers and two thermoplastic polymers – poly (methyl methacrylate) (PMMA) and thermoplastic polyurethanes (TPU) – and their electrical, mechanical, and thermal properties were evaluated. The TPU systems were based on the reaction products of 4,4’-diphenylmethane diisocyanate, (MDI), soft segment polyol, and 1,4-butanediol as chain extender. Soft segment polyols in the form of poly(propylene glycol) (PPG), and poly(ε-caprolactone)diol (PCL) were used to obtain respectively amorphous and crystalline soft segments. Of these, the TPU system based on crystalline soft segment exhibited shape memory effects. Both, as-received untreated carbon nanofibers (CNF) with a very low amount of atomic oxygen on the surface, and oxidized carbon nanofibers (CNFOX) were used. CNFOX was also modified by esterifying with PPG to produce a third type of carbon nanofiber named CNFOL. These carbon nanofibers were examined by X-ray photoelectron spectroscopy to determine the elemental composition of the surface, and by scanning electron microscopy and transmission electron microscopy to determine the surface morphology.
Advisors/Committee Members: Jana, Sadhan C.
Keywords: Polymer composites; Chaotic mixing; Carbon nanofibers; Carbon nanotubes
30. Jung, Changdo. SYNTHESIS OF THERMOPLASTIC POLYURETHANES AND POLYURETHANE NANOCOMPOSITES UNDER CHAOTIC MIXING CONDITIONS.
Degree: Doctor of Philosophy, Polymer Engineering, 2005, University of Akron
▼ The self-similar mixing microstructures generated in chaotic mixing of prepolymer and chain extender were utilized to augment conversion and molecular weight in the synthesis of thermoplastic polyurethanes (TPU) and polyurethane nanocomposites. The values of time scales of mixing and chemical reaction were varied so as to obtain the best possible product. The chaotic mixing parameter (è), catalyst concentration, and chemical structures of polyols and diisocyanates were used as factors to exert influence on the speed of conversion, molecular weight, and phase separation during synthesis. The time to reach maximum torque during mixing and tensile properties of the products were evaluated. In addition, the barrier to diffusion of water vapor through composites of reactive and non-reactive clay particles and pristine polyurethanes was determined. The time scales of reaction and mixing were obtained respectively from the values of kinetic rate constant of the reaction system and Liapunov exponents, the latter characterizes fluid element separation. As a general observation, it was found that best polyurethane products were obtained when the time scale of mixing and the time scale of reaction between –NCO and –OH groups were close to each other. It was found that a value of è=1440o with sinusoidal rotor speed variation provided the best mixing condition and the time scale of reaction was gradually shortened using higher concentration of catalysts. In addition, the chaotic mixing protocol with cylindrical rotors produced better products than a mixer formed by a set of commercial neutral kneading discs. The effective shear rate was kept the same in both cases for fair comparison. It was also found that phase separation of hard segments in systems with 38 wt% hard segments limited the maximum value of conversion even though the time scales of mixing and chemical reactions were matched. This indicates that the time scale of phase separation must also be considered in the analysis. Composites of polyurethanes with reactive nanoclay contained micrometer size clay particles. However, they provided significant reduction in water permeation due to alignment of clay particles by chaotic mixing flow. It was found that very little clay-polymer tethering reaction occurred due to strong imbalance in reactivity–the reactivity between prepolymer and butanediol were found to be 25 times faster than those between prepolymer and reactive clay. The knowledge gleaned from the study can be extended to other reactive systems and to designing of continuous chaotic mixing devices.
Advisors/Committee Members: Jana, Sadhan C.
Keywords: chaotic mixing, synthesis of thermoplastic polyurethanes, polyurethane nanocomposites
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