Browsing by Author "Saad A. Khan, Committee Member"

Now showing 1 - 20 of 24

Asphaltene Self-Assembly
(2008-11-07) Verruto, Vincent J.; Peter K. Kipatrick, Committee Chair; Jan Genzer, Committee Member; Orlin D. Velev, Committee Member; Saad A. Khan, Committee Member
Despite a strong push for alternative energy, fossil fuels remain an important energy source given an ever-increasing global energy demand. As crude oil prices continue to soar, petroleum producers and refiners are looking to â€œunconventionalâ€ crudes, such as bitumen and heavy crude oils, to meet their needs. Unlike light-sweet â€œconventionalâ€ feedstocks, heavy crudes are often rich in a fraction that is characteristically polydisperse, of high-MW, polyaromatic, polar, and surface-active. Consequently, asphaltenes present expensive challenges associated with aggregation, flocculation, precipitation, deposition, and emulsion stabilization. The scope of the work here focuses on two important aspects of asphaltene self-assembly: bulk phase aggregation and interfacial film formation. Using small-angle neutron scattering (SANS) we expand the description of these aggregates beyond their size (~50-100 Ã…), shape (discoidal), and degree of solvent entrainment (30-50% by volume), to also include the entrained solvent composition when dissolved in binary solvent mixtures. We then use SANS to evaluate the physical and chemical properties of the stabilizing interfacial films in water-in-model oil emulsions. In Part I of this SANS of emulsions investigation, we unravel the thickness and asphaltenic composition of the interfacial films from emulsions made in three solvents of varying aromaticity. We will show that for these three systems, emulsion stability depended on the asphaltenic composition in the films as opposed to the film thickness, which was nearly constant among the three solvents. In the Part II we seek a more thorough definition of the interfacial film composition by using neutron contrast variation to illuminate not just the asphaltenic makeup, but the solvent, water, and, when applicable, additive composition within the films. Finally, through the use of interfacial shear and dilatational rheology, we explore the various interactions at model oil/water interfaces that influence interfacial film assembly. We find that electrostatic interactions between charged adsorbed species largely dictate the transient evolution of the interfacial elasticity at acidic, neutral, and basic pH. Furthermore, through our comparisons of the interfacial rheological behavior of asphaltenes and model polycyclic compounds, we are able to better understand the physicochemical phenomena that contribute to asphaltene interfacial dynamics.
Behavior of Adhesive Materials in Screening Devices for Paper Recycling
(2004-04-28) Lucas, Bradley Earle; Hasan Jameel, Committee Co-Chair; Martin A. Hubbe, Committee Member; Saad A. Khan, Committee Member; Richard A. Venditti, Committee Co-Chair
The objective of this research is to understand the behavior of pressure sensitive adhesive (PSA) materials in industrial and laboratory screening devices. The research was initiated by screening pulp containing PSA using an industrial pressure screen with fine slots. The industrial pressure screen removed less than 80% of the feed PSA contaminant. In contrast, an atmospheric laboratory screen of the same slot width, 0.006 inches, removed 99% of the PSA. One of the reasons for the lower removal efficiency was found to be a significant breakage of PSA particles into smaller particles within the industrial pressure screen. The breakage of PSA particles in a shear field was studied using a high shear mixer and the consistency was found to be the most significant variable affecting the breakage of the PSA. It was also of interest to investigate how the PSA material could deform and pass through the fine slot of a pressurized industrial screen. A laboratory screening device with a single slot was developed to study the passage of PSA particles through the slot as a function of pressure drop across the slot. It was observed that at moderate pressure drops, particles with their smallest dimension up to five times the slot width passed through the slot. Increased temperature promoted the passage of the particles through the slot, indicating that the physical properties of the PSA influenced its passage. To understand the relationship between PSA formulation, physical properties, and passage of particles through a fine slot, several PSA formulations of known composition were prepared for analysis. Differences in the PSA formulations were found to affect the yield strain, yield stress, and modulus of the PSA films, which correlated with particle breakage in the pulper. Yield stress and particle area correlated with particle passage through the slot. A mechanical model for passage is in agreement with the experimental results, indicating that yield stress and particle size are significant for PSA particle passage through fine slots. These studies have resulted in important information for screen manufacturers, paper recyclers, and PSA producers to improve the removal of PSA particles from recovered paper.
Concurrent and Sequential Surface Modification of Electrospun Polymer Micro/Nano-Fibers
(2009-04-23) Sun, Xiaoyu; Orlin D. Velev, Committee Member; Tushar K. Ghosh, Committee Member; Saad A. Khan, Committee Member; Richard J. Spontak, Committee Chair
Surface modification of nano-fibers with bioactive functional groups has become an arresting research area in recent decades, which provides possibility for the invention of bioactive materials for textiles and biomedical applications e.g. tissue engineering. The major objective of this research is to develop a novel single-step processing route for the production of synthetic fibers possessing specific bioactive surface functionalities at nano/submicron scale. Unlike traditional sequential surface modification of nanofibers, sequence-defined oligo-peptide that carries biofunctionality was synthesized separately before incorporated onto the electrospun fibers as surface functionalities by a single-step spinning process, so as to avoid the effect from chemical synthesis on fiber processing. As one of the most widely-used technologies for the production of polymeric nanofibers, electrospinning was chosen to achieve the single-step surface modification. Conventional homopolymer in conjunction with the biofunctional oligopeptide-incorporated block copolymer were co-electrospun. Nanofibers at submicron scale with surface enrichment of block copolymer were achieved due to phase separation caused by polarizability difference under static electric field. The surface segregation of peptide block was proved by the nitrogen enrichment measured from X-ray Photoelectric Spectroscopy (XPS). The proposed mechanism is discussed based on mainly the model homopolymer system of polyethylene oxide (PEO), and extended to the ternary polymer blends composed of thermoplastic polymethyl methacrylate (PMMA), PEO and block copolymer. The surface modification technique introduced deep insight into the electrospinning process with its effect to the polymer blends microphase separation, and leads to a promising perspective for biomaterial engineers to produce nanofibers with certain surface bio-functional groups.
Continuous Precipitation Polymerization of Acrylic Acid in Supercritical Carbon Dioxide
(2006-04-07) Liu, Tao; Joseph M. DeSimone, Committee Co-Chair; Alan E. Tonelli, Committee Member; Saad A. Khan, Committee Member; George W. Roberts, Committee Co-Chair
The precipitation polymerization of acrylic acid in supercritical carbon dioxide (scCO2) was carried out in a continuous stirred tank reactor. The product polymer was a white, dry, fine powder that dissolved in water. A wide range of polymer molecular weights (5 to 200 kg/mol) was obtained. The effect of the operating variables on the polymerization rate and on the polymer molecular weight was evaluated. The polymerization showed distinct deviations from the normal solution polymerization. By assuming that chain initiation occurs in the solution phase, but chain propagation and chain termination occur in the polymer phase, a 'surface polymerization model' and a 'particle polymerization model' both described the polymerization well. Scanning electron micrographs showed that three types of polymer particles were obtained: agglomerates of primary particles of about 100 nanometers in size, irregular particles of 5—20 micrometers, and spheres of 10—100 micrometers. It is speculated that the agglomerates were produced when the polymerization temperature (TP) was below the polymer glass transition temperature (Tg), the irregular particles were obtained when TP was close to Tg, and the spheres were prepared when TP was above Tg. The CO2 absorption into poly(acrylic acid) (PAA) was measured with a quartz crystal microbalance. The Tg depression by scCO2 was calculated with the Chow's equation. The calculated results lent strong support to the proposed particle formation mechanism. Cross-linking polymerization of acrylic acid in scCO2 was studied in a batch reactor at 50°C and 207 bar. All products were white, dry, fine powders. By adjusting the cross-linker concentration, water-soluble and water-insoluble PAAs were synthesized. The water-insoluble PAA was neutralized by ammonia gas and sodium hydroxide alcohol solution to make superabsorbent polymers.
Controlling the Stereoregularity of Polyacrylonitrile and Its Determination Using Small-Molecule Host Inclusion Compounds
(2006-11-02) Yang, Hyungchol; Wendy E. Krause, Committee Member; Alan E. Tonelli, Committee Chair; Charles M. Balik, Committee Member; Saad A. Khan, Committee Member
This research focuses on synthesizing highly stereoregular polyacrylonitrile (PAN) and determining its tacticity (predominantly isotactic or syndiotactic), utilizing guest monomer (acrylonitrile = AN) host inclusion polymerization. Highly stereoregular PAN, with a meso or racemic diad content ~ 80%, was prepared by γ-ray irradiation of an AN urea canal complex at a low temperature (–78° C). Several essential experimental factors for ensuring the highly stereoregular PAN production were considered. After γ-ray irradiation polymerization, the tacticity of PAN was determined from triad peak intensities of the methine (CH) and nitrile (–C≡N) carbons in the 13C-NMR spectra, assuming Bernoullian statistics. When the AN guest forms an inclusion compound (IC) with urea host, it was expected that there is a structural transformation of urea into the hexagonal crystal lattice structure with a narrow channel diameter (5.25-5.5Å). However, in our FTIR observations run at room temperature, a different type of transformation was detected. AN urea IC before and the PAN urea IC after low temperature γ-ray irradiation polymerization are both large tetragonal structures, which have a larger channel diameter (> 5.5Å) at room temperature. Because these infrared observations were not carried out below -20.8° C, known as the decomposition temperature of the hexagonal IC structure, the fact that the structures of both AN urea IC and PAN urea IC are the large tetragonal does not necessarily prove that during polymerization below –20.8°C the AN urea IC was also the large tetragonal structure. If PAN was polymerized in the hexagonal (or pseudo-hexagonal) urea canal lattice (5.5Å), which provides a more confined environment for its conformation and configuration, it would likely be syndiotactic and adopt the all trans conformer. Because of the flexible nature of urea when it forms inclusion compounds with guest molecules, if AN was polymerized in the large tetragonal (> 5.5.Å) lattice structure of urea, which gives more freedom to PAN during its inclusion polymerization or inclusion compound formation, it could have either an isotactic or a syndiotactic configuration. Because no definitive evidence has been previously reported in the determination of γ-ray irradiated PAN by NMR spectroscopy, an effort to prepare PAN in another molecular host crystalline lattice, α-cyclodextrin (CD), was made. Synthesis of PAN in the columnar structure of AN α-CD–IC is a very promising method to reveal the original tacticity of highly stereoregular (~80%) PAN due to the fact that α-CD has a rigid small diameter (4.9Å) channel cavity, and only syndiotactic PAN in the all trans conformation is likely to be produced. However, γ-ray irradiation of a channel structure AN α-CD IC did not produce any PAN, implying that the AN urea IC that produced stereoregular PAN upon γ-irradiation was likely in a large tetragonal structure. Alternatively, because of the disparity in AN:α-CD and PAN:α-CD stoichiometries [1:1 (experimental) versus 3:1 (expected)], after γ-irradiation initiation of AN α-CD IC, a shortage of AN would result in the α-CD IC channels, possibly interrupting polymerization. By analogy to polypropylene (PP) polymerized in host perhydrotriphenylene (PHTP) IC (d ~ 5Å) and polyvinylchloride (PVC) polymerized in urea canals, which are both found to be syndiotactic, we suggest that stereoregular PAN polymerized in urea canals is also predominantly syndiotactic.
Cooling Effects on the Funcitonality and Microstructure of Processed Cheese
(2003-09-25) Zhong, Qixin; Saad A. Khan, Committee Member; Christopher R. Daubert, Committee Chair; Orlin D. Velev, Committee Co-Chair; Brian E. Farkas, Committee Member
Cooling is the final stage of process cheese manufacturing, and a slower cooling process generates a stronger structure requiring a larger force to shear cheese loaves while yielding a poor melt quality. To interpret cooling mechanisms, a model rennet casein system was formulated to include emulsifying salts and rennet casein powder, as used in processed cheese analogs. At pH 5.8 and 6.5, protein networks were discontinuous, and no cooling trends on rheological data were observed when cooled from 80 to 5 °C at 0.025, 0.05, 0.1, and 0.5 °C/min. At pH 7.2 and 12, networks were formed, and slower cooling delivered a firmer gel. The rennet casein network was described as cross-linked flocs, and the aggregation of two particles into a doublet was the first stage of floc formation. At lower pHs, the overall force between protein particles was more attractive, the doublet formation time was short, and therefore the number of doublets formed was not affected by cooling rates. Accordingly, floc numbers and rheology did not show a trend with cooling effects at the lower pHs. At pH 7.2 and above, the doublet formation time was comparable to the time for temperature changes during cooling because of stronger repulsive forces. As a result, doublet formation was influenced by cooling rate, generating more doublets and more sites for floc growth at slower rates. A larger number of smaller flocs were created, delivering a stronger gel at slower cooling rates. More flocs at higher protein concentrations and slower cooling rates simplified the network formation. Finally, cooling effects on processed cheese resulted from more uniform structures at slower cooling rates. A more uniform protein network formed prior to fat crystallization and confined the volume for fat crystallization, creating a more uniform cheese matrix and a firmer product. Cheese manufacturers can benefit from this research by adopting an appropriate cooling schedule to deliver specific functional properties.
Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide
(2008-02-28) Ahmed, Tamer Samir; Joseph M. DeSimone, Committee Co-Chair; Saad A. Khan, Committee Member; Alan E. Tonelli, Committee Member; George W. Roberts, Committee Co-Chair
This thesis details research to study the copolymerization of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) in supercritical carbon dioxide (scCO2). Another objective of this thesis is to understand the origin of the bimodal molecular weight distribution (MWD) that results under certain conditions during the precipitation polymerization of poly(vinylidene fluoride) (PVDF) in scCO2. The copolymerization of VF2 with HFP was carried out in scCO2 using a continuous stirred tank reactor (CSTR). The experiments were done at 40 oC with pressure in the range of 207-400 bar using perfluorobutyryl peroxide as the free radical initiator. Four different copolymer compositions were studied: ca. 10, 23, 26, and 30 mole % HFP. The 10%-copolymer was collected as a dry free-flowing semicrystalline powder while the other compositions were amorphous elastomeric materials collected continuously using acetone. Most of the polymerizations were heterogeneous, i.e., polymer particles precipitated during the reaction. However, some were homogenous, especially in the higher range of HFP content. The effects of feed monomer concentration and reaction pressure were both explored at otherwise constant conditions. The rate of polymerization (Rp) and the number-average molecular weight (Mn) increased linearly with the total monomer concentration up to about 6 M, the highest concentration investigated. In addition, the Rp and the Mn increased with reaction pressure. The MWDs of the synthesized copolymer showed a long tail that increased to become a broad shoulder with increasing total monomer concentration. This tail decreased with HFP content in the copolymer and increased with reaction pressure. The experimental results of VF2 homopolymerization and copolymerization with HFP in scCO2 were tested against three kinetic models to determine the main locus of polymerization. The first model, the "solution polymerization" model, is based on the assumption that all the polymerization reactions place in the continuous, CO2-rich phase, with no reaction in the polymer phase. In the second model, the ?surface polymerization" model, chain initiation occurs exclusively in the continuous phase, while chain propagation and termination occur in a thin zone on the surface of the polymer particles. The third model, the "interior polymerization" model, is similar to the "surface polymerization" model, except that propagation and termination take place uniformly throughout the polymer particles. Both the surface and the interior polymerization models failed to fit the experimental results. On the other hand, the solution polymerization model was able to describe the experimental results of the polymerizations fairly well over the whole range of polymer compositions. This suggests that the CO2-rich continuous phase is the main locus of polymerization in the precipitation polymerization of VF2 homopolymer and VF2⁄HFP copolymers scCO2. Finally, a homogenous model is presented to account for the bimodal MWDs of PVDF. The model takes into account both the change of termination scheme of the polymeric radicals with chain length from chemically-controlled termination to diffusion-controlled termination and chain transfer to polymer reaction. The model was successful in accounting for the change of modality with reaction conditions such as monomer concentration, average residence time at low and high monomer concentrations, and the reaction temperature. In addition, the model could capture the occurrence of gelation, which was responsible for an inoperability region that was observed in the polymerization experiments.
Directed Assembly and Manipulation of Anisotropic Colloidal Particles by External Fields
(2010-01-12) Gangwal, Sumit; Gregory N. Parsons, Committee Member; John F. Muth, Committee Member; Saad A. Khan, Committee Member; Orlin D. Velev, Committee Chair
The application of external fields to anisotropic particles can be an efficient means of programmed assembly of novel materials and is a rapidly expanding research field. We report a series of studies on the assembly and manipulation of surface patterned anisotropic colloidal particles (whose surfaces are physically or chemically different) by external alternating current (AC) electric and magnetic fields. The fundamental results include the first experimental observation of induced-charge electrophoretic (ICEP) motion of asymmetric metallodielectric microspheres and the formation of novel assembled structures of these particles by dielectrophoresis (particle interaction with external AC electric Ã¯Â¬Â eld gradients) and by magnetophoresis (migration and interaction of particles in an inhomogeneous magnetic field). The experimental and modeling techniques developed and fundamental principles uncovered could be used to engineer the processes of directed and/or programmed assembly of other types of anisotropic particles. Ã¢â‚¬Å“JanusÃ¢â‚¬Â particles were prepared by coating dielectric, polystyrene latex microspheres with a conductive metal layer on one hemisphere. The phase space for AC electric field intensity and frequency was explored for these particles on a glass surface between two electrodes. A rich variety of metallodielectric structures and dynamics were uncovered, which are very different from those obtained from directed dielectrophoretic assembly of plain dielectric or plain conductive particles. The application of low frequency AC Ã¯Â¬Â elds to aqueous suspensions of the Janus particles leads to unbalanced liquid Ã¯Â¬â€šows around each half of the particle causing nonlinear, ICEP particle motion (perpendicular to the Ã¯Â¬Â eld direction). Above 10 kHz field frequency, the metallodielectric particles assemble into new types of chain structures, where the metallized halves of neighboring particles align into lanes along the Ã¯Â¬Â eld direction. These staggered chains were confined together to form two-dimensional metallodielectric crystals. The experimental results of the orientation of Janus particles in the electric field and the formation of staggered chains were interpreted by means of numerical simulations of the electric energy of the system. The assembly of Janus metallodielectric particles may Ã¯Â¬Â nd applications in liquid-borne microcircuits and materials with directional electric and heat transfer. The electrokinetic motion of the particles may Ã¯Â¬Â nd applications in microactuators and microÃ¯Â¬â€šuidic devices. The assembly of magnetic Janus colloids (having 50% surface coating of iron on polystyrene microspheres) under the combined (and sometimes competing) dielectrophoretic and magnetophoretic forces was investigated. The structures formed by magnetic fields have the advantage that the particle interactions are bistable. They can result in permanent structures, which could be disassembled on demand by remote demagnetization and then reassembled into new stable structures, thus recycling the building blocks. The assembly of magnetic anisotropic particles may find numerous potential applications, among which are bifunctional drug delivery agents and novel flexible displays. We found that even more unusual types of new structures are formed when high frequency (> 50 kHz) AC electric fields are applied to suspensions of Ã¢â‚¬Å“patchyÃ¢â‚¬Â particles. The microspheres, produced by glancing angle metal deposition, have either a single patch that is less than 50% of the total latex particle surface or two metallic patches on each pole of the particle. These patchy particles assemble in electric fields by interacting with each other in two or more directions, pre-programmed by the patch size and orientation. The multi-directional chains were confined together to form a percolated network of particles and lattices of unusual symmetry. Simulation results indicate that the assembly pattern of these particles into multi-directional chains is guided by quadrupolar and multipolar interactions, which allow for the future development of new strategies for highly controlled Ã¢â‚¬Å“programmedÃ¢â‚¬Â assembly by external fields.
Electroactive Behavior of Nanostructured Polymers
(2008-08-03) Shankar, Ravi; Richard J. Spontak, Committee Co-Chair; Saad A. Khan, Committee Member; Russell E. Gorga, Committee Member; Tushar K. Ghosh, Committee Co-Chair; Stephen Michielsen, Committee Member
Formation and properties of surface-anchored polymer assemblies with tunable physico-chemical characteristics
(2003-06-16) Wu, Tao; Christine S. Grant, Committee Member; Jan Genzer, Committee Chair; Christopher B. Gorman, Committee Member; Richard J. Spontak, Committee Member; Saad A. Khan, Committee Member
We describe two new methodologies leading to the formation of novel surface-anchored polymer assemblies on solid substrates. While the main goal is to understand the fundamentals pertaining to the preparation and properties of the surface-bound polymer assemblies (including neutral and chargeable polymers), several examples also are mentioned throughout the Thesis that point out to practical applications of such structures. The first method is based on generating assemblies comprising anchored polymers with a gradual variation of grafting densities on solid substrates. These structures are prepared by first covering the substrate with a molecular gradient of the polymerization initiator, followed by polymerization from these substrate-bound initiator centers ("grafting from"). We apply this technique to prepare grafting density gradients of poly(acryl amide) (PAAm) and poly(acrylic acid) (PAA) on silica-covered substrates. We show that using the grafting density gradient geometry, the characteristics of surface-anchored polymers in both the low grafting density ("mushroom") regime as well as the high grafting density ("brush") regime can be accessed conveniently on a single sample. We use a battery of experimental methods, including Fourier transform infrared spectroscopy (FTIR), Near-edge absorption fine structure spectroscopy (NEXAFS), contact angle, ellipsometry, to study the characteristics of the surface-bound polymer layers. We also probe the scaling laws of neutral polymer as a function of grafting density, and for weak polyelectrolyte, in addition to the grafting density, we study the affect of solution ionic strength and pH values. In the second novel method, which we coined as "mechanically assisted polymer assembly" (MAPA), we form surface anchored polymers by "grafting from" polymerization initiators deposited on elastic surfaces that have been previously extended uniaxially by a certain length increment, Λx. Upon releasing the strain in the substrate after completion of polymerization, we show the grafting density of the polymers grafted to flexible substrates can be tuned as a function of Λx.
Fumed oxide-based nanocomposite polymer electrolytes for rechargeable lithium batteries
(2003-03-18) Zhou, Jian; Peter S. Fedkiw, Committee Chair; Saad A. Khan, Committee Member; Daniel L. Feldheim, Committee Member; John H. van Zanten, Committee Member
Rechargeable lithium batteries are promising power sources for portable electronic devices, implantable medical devices, and electric vehicles due to their high-energy density, low self-discharge rate, and environmentally benign materials of construction. However, the high reactivity of lithium metal limits the choice of electrolytes and impedes the commercialization of rechargeable lithium batteries. One way to tackle this problem is to develop electrolytes that are kinetically stable with lithium. Composite polymer electrolytes (CPEs) based on fumed oxides presented in this work are promising candidates for rechargeable lithium batteries. The effects of fumed oxides (SiO2, Al2O3, TiO2) and binary mixtures of oxides (SiO2/Al2O3) on ionic conductivity of CPEs based on poly(ethylene oxide) (PEO) oligomers (Mw =250, 200, 1000, and 2000) + lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li:O=1:20) are studied using electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy. Fillers show similar effect on conductivity in all systems: no distinguishable effect is found with filler type, and addition of filler decreases conductivity at temperatures above the melting point (Tm) but increases conductivity at temperatures below. The insulating nature of fillers and stiffening of the polymer solvent (as evidenced by FTIR and DSC data) in the presence of fillers cause a decrease in conductivity at temperatures above Tm, which remains constant upon addition of fillers. The increase in conductivity at temperatures below Tm can be attributed to faster ion transport along the filler surface. Addition of fumed oxides increases electrolyte viscosity (and elasticity) and the extent of enhancement varies with filler type: fumed silica shows the strongest and titania the least. Elastic modulus, yield stress, and normalized viscosity of gel-type composite electrolytes decrease with increasing oligomer Mw when electrolytes are amorphous. The reduction in structure strength may be ascribed to the enhanced interactions between surface hydroxyl groups on fumed oxides and polyether oxygens. Thus, the number of accessible ?OH groups is reduced for interactions among fumed oxide particles, which dictates the strength of solid-like structure. The interfacial stability between electrolyte and lithium is enhanced in the presence of fumed silica. The enhancement in interfacial stability is seen as a decrease in interfacial resistance and cell polarization, and an increase in lithium cycleability and cell capacity. The improved interfacial stability between CPE and lithium is attributed to less lithium corrosion (fillers scavenge water impurities that corrode lithium) and dendrite formation (electrolyte elasiticty inhibits dendrite formation). The extent of the enhancing effect of fumed silica depends on its surface chemistry, with the largest effect seen with hydrophilic fumed silica, which has the largest scavenging capacity and highest elasticity. The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Cells with composite electrolytes show higher capacity and less cell polarization than those with filler-free electrolyte. Among the three active materials studied (LiCoO2, V6O13, and LixMnO2), V6O13 cathodes deliver the highest capacity and LixMnO2 cathodes render the best capacity retention. Discharge capacity of Li/LiCoO2 cells is affected greatly by cathode carbon type and discharge capacity increases with decreasing carbon particle size. Current collector materials also play a significant role in cell cycling performance: Li/V6O13 cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil. In summary, fumed oxide-based nanocomposite electrolytes are promising candidates for lithium battery applications with high room-temperature conductivity, good mechanical strength, stable interface between lithium metal and electrolytes, and reasonable capacity and capacity retention with optimized cathode compositions.
A Fundamental Study of the Molecular Structure, Interactions and Self-Organization of 1,3:2,4-Dibenzylidene-D-Sorbitol
(2003-04-16) Wilder, Elizabeth A; Keith E. Gubbins, Committee Member; Saad A. Khan, Committee Member; Richard J. Spontak, Committee Chair; Carol K. Hall, Committee Co-Chair
1,3:2,4-Dibenzylidene-D-sorbitol (DBS) is a relatively low-molecular-weight amphiphile that is capable of self-organizing into nanoscopic fibrils. At sufficiently high DBS concentrations, these fibrils assemble into a nanofibrillar network in a wide variety of organic solvents and polymer melts to produce "organogels." DBS has been shown to induce physical gelation at surprisingly low concentrations (< 1 wt%), making it ideal for applications requiring uncompromised physical or chemical properties of the matrix medium. Contemporary applications of DBS include personal cosmetics, biomedical materials, and (opto)electronic devices. Despite the many and diverse uses of DBS in existing, as well as emerging, technologies, a comprehensive study addressing the molecular structure, intermolecular interactions, nanofibrillar morphology and macroscopic properties of DBS-containing systems remains lacking. In this work, we seek to elucidate the molecular interactions governing DBS self-assembly, the impact of molecular structure on resultant nanofibrillar morphology, and the effect of this nanostructure on macroscopic mechanical properties. Molecular mechanics calculations performed with Cerius2 and InsightII software reveal two important features of the DBS molecule: (i) the pendant hydroxyl group tends to form intramolecular hydrogen bonds, and (ii) the phenyl rings prefer to lie in an equatorial position. The terminal hydroxyl group, however, possesses tremendous flexibility, indicating that it may be able to participate in intermolecular interactions. Molecular self-organization of DBS molecules, as discerned from both molecular mechanics calculations and molecular dynamics simulations of dimers, is sensitive to hydrogen bonding of the hydroxyl groups and pi interactions between phenyl rings, suggesting that the mechanism of network formation is complex, involving more than one type of local interaction. Transmission electron microscopy of organogels composed of poly(ethylene glycol) (PEG) and DBS reveals that DBS nanofibrils measure from about 10 to 70 nm in diameter, with a primary nanofibrillar diameter closer to 10 nm. Dynamic rheological measurements of DBS-containing PEG and PEG derivatives differing in endgroup substitution and, hence, polarity exhibit several interesting features. The rate of gelation, the gel dissolution/formation temperatures, and the magnitude of the dynamic elastic modulus are all sensitive to both DBS concentration and matrix polarity. Hydroxy-endcapped PEG/DBS systems require more time to gel and dissolve faster than their methoxy-endcapped analogs at constant DBS concentration. The elastic modulus, however, is less dependent on matrix polarity. Time-temperature superposition analyses provide evidence that the activation energy of gelation increases linearly with: (i) decreasing DBS concentration at constant matrix polarity and (ii) increasing matrix polarity at constant DBS concentration. Addition of DBS to a series of amphiphilic polypropylene glycol-b-polyethylene glycol-b-polypropylene glycol (PPG-b-PEG-b-PPG) triblock copolymers yields organogels with properties intermediate between those observed in PEG/DBS and PPG/DBS systems. Dynamic rheology reveals a maximum in the elastic modulus at temperatures near the gel dissolution and formation temperatures, both of which increase with increasing DBS concentration and PPG content. The magnitude of the elastic modulus is sensitive to copolymer composition and block length at low DBS concentration, but becomes matrix-independent as the DBS network saturates at a DBS concentration in excess of about 1 wt%. Transmission electron microscopy and microtomography of DBS networks in a nonpolar thermoplastic such as poly(ethyl methacrylate) reveal the existence of DBS nanofibrils measuring ca. 10 nm in diameter and ranging up to several hundred nanometers in length. At sufficiently high DBS concentration, these nanofibrils form a highly interconnected 3D network that can be altered through the further addition of a siliceous nanoparticle, such as colloidal silica. Dynamic mechanical property analysis reveals that, while DBS has little effect on glassy PEMA, it serves to increase, in systematic fashion, the elastic modulus of molten PEMA above the glass transition temperature.
How Asphaltenes Aggregate: Role of Chemistry and Solvent
(2005-05-26) Gawrys, Keith Louis; Saad A. Khan, Committee Member; Jan Genzer, Committee Member; Stefan Franzen, Committee Member; Richard J. Spontak, Committee Member; Peter K. Kilpatrick, Committee Chair
Asphaltenes were separated into several subfractions by sequential precipitation from n-heptane and toluene. Three distinct solubility regimes were indicated where the subfraction chemical and colloidal behaviors varied significantly with total precipitated asphaltenes. The earliest fractions precipitated had lower than average aromaticity and atomic N/C ratios and contained significant inorganic solids contents. Subfractions isolated in the second regime varied significantly in aromaticity and had systematically decreasing N/C ratios and increasing O/C ratios with increasing asphaltene yield. The most aromatic subfractions formed the largest aggregates in solution. Subfractions in the most soluble regime were more 'resin-like' in chemical composition and aggregation behavior. Application of various geometric form factors to the SANS scattering spectra of asphaltenes suggested the aggregates are polydisperse radius oblate cylinders. A polydisperse cylinder model provided ranges of average particle thicknesses (5-32Å), radii (25-125Å), and polydispersity (~30%). Calculation of aggregate molar masses suggested solvent entrainment within the aggregates from 30-50% (v/v) that was consistent with previous viscosity measurements. Changes in the apparent aggregate mass with concentration indicated deviations from ideal solutions that were quantified through the calculation of second virial coefficients (A2). A2 values varied significantly with solvent conditions, concentration, and chemical composition of the solute. Results suggested that interactions of asphaltenes, resins, and solvent are dominated by dispersion and p-bonding interactions. Experimentally measured A2 values under-predicted those calculated on an excluded volume basis, suggesting energetic interactions of the solute and entrained solvent are significant. UV-vis spectroscopy was used to determine the solubility of poly-nuclear aromatics in binary solvent mixtures with the intention of extending the methods to asphaltenic systems. Binary solvent mixtures were selected to probe specific intermolecular solvent-solute interactions (i.e., dispersion, polar, and hydrogen bonding). Solubility data were fit to a three-dimensional solubility parameter model that provided accurate solubility predictions in some solvents with less than 30% error. The predictive capability of the model decreased as one or more contributions to the solvent solubility parameter deviated from the solute. Experiments measuring solubility in a multi-component solute mixture suggested a need to incorporate aggregation in the model.
Investigating Aqueous PEO-PPO-PEO Triblock Copolymer Dispersion Dynamics with Colloidal Sphere Thermal Motion
(2007-04-27) Kloxin, Christopher James; Carol K. Hall, Committee Member; Michael Rubinstein, Committee Member; John H. van Zanten, Committee Chair; Saad A. Khan, Committee Member; Christopher R. Daubert, Committee Member
The objective of the thesis is to investigate the structural and dynamical behavior of aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock surfactants, commercially known as PluronicsTM, by studying the Brownian motion of embedded tracer probe particles. Recent advances in the understanding of diffusive light transport in highly scattering media have seeded a novel dynamic light scattering technique in the multiple scattering limit known as diffusing wave spectroscopy (DWS). This technique allows for the determination of probe motion at frequencies greater than 1 MHz and at a spatial resolution of several angstroms. Thus, DWS provides unique access to early thermal relaxation modes in Pluronic samples, which is the proposed origin of viscoelastic behavior observed on a macroscopic level. In general, Pluronic-type macromolecular surfactants associate into spherical micelles, leading to complex structures and rich dynamic behavior when dispersed in aqueous solution. In the first part of the thesis, we demonstrate the utility of DWS microrheology to studying the short-time dynamics of aqueous Pluronic L64 [(EO)13(PO)30(EO)13] solutions, revealing a temperature independent high frequency viscosity above the micellization temperature. The dynamics at high temperatures are dominated by an apparent attractive inter-micellar potential, consistent with our inverse osmotic pressure measurements. We confirm the presence of a short-lived elastic gel at high temperatures, which we attribute to a spanning cluster indicating the crossing of a dynamic percolation threshold. In the second part of this thesis, we employ a high-pressure scattering cell to examine the phase space of aqueous Pluronic P85 [(EO)25(PO)40(EO)25] solutions. We demonstrate that subtle changes in water by densification via increased hydrostatic pressure, by increasing thermal energy, or even by isotopic substitution, lead to discernable large-scale effects in aqueous P85 Pluronic samples. More generally, we show the utility of DWS tracer studies to explore complex fluids in high pressure and temperature environments, allowing for the construction of phase diagrams based on dynamical pathways.
Lithium Alloy-Carbon Composite Nanofibers for Energy Storage by Electrospinning and Carbonization
(2009-10-30) Ji, Liwen; Peter S. Fedkiw, Committee Co-Chair; Saad A. Khan, Committee Member; Russell E. Gorga, Committee Member; Xiangwu Zhang, Committee Chair
Nanoscale Engineering Materials with Supercritical Fluid and Atomic Layer Deposition
(2009-08-04) Peng, Qing; Gregory N. Parsons, Committee Chair; Jan Genzer, Committee Member; Ruben G. Carbonell, Committee Member; Saad A. Khan, Committee Member
With the development of material science and technology, modification of substrates, which have random geometry and high aspect ratio three dimensional (3D) complex structures, with desired functional, reactive and stable coatings becomes important and challenging. The ability to fabricate mono- or multi-layers of functional materials with precisely controlled dimensions, finely tuned composition and molecular structures, attracts significant interests in materials science and is the key to construct such devices and structures at nano- and micro- scale with desired properties. In this study, supercritical carbon dioxide (scCO2) has been studied as an alternative route for modifying substrates due to the unique gas-like (low viscosity, high diffusivity and zero surface tension) and liquid-like properties (high density). 1) The reaction kinetics of metal oxides thin film deposition from pyrolysis of metal organics in scCO2 was studied in detail. This method was demonstrated as a powerful technique to coat oxides, including Al2O3, Ga2O3 and others, into 3D high aspect ratio complex structure of carbon nanotubes (CNTs) forest. 2) The low temperature scCO2 based hydrogenolysis process was developed as a useful way to functionalize aligned CNTs forest with dense Nickel nanoparticles. On the second part of this work, atomic layer deposition (ALD) /molecular layer deposition (MLD), as a vapor phase, stepwise and self-limiting vacuum based deposition process, was demonstrated as a powerful way to form highly conformal and uniform film onto substrates, even into highly complex 3D complex structures. In this study, 4) Metal oxide ALD is applied onto 3D electrospun polymer microfiber mats template to illustrate an effective and robust strategy to fabricate long and uniform metal oxide microtubes with precisely controllable wall thickness. Designer tubes of various sizes and different materials were demonstrated by using this method. 5) By further extending this technique, complex coaxial Al2O3/ZnO/Al2O3 multilayed microtubular structure is fabricated, which provides an unique platform to study the solid state reaction and diffusion process (Kirkendall Effect) between Al2O3 shells and the confined middle ZnO layers by annealing the samples at 700 Ã‹Å¡C. 6) The extension of ALD-MLD process of polyamides, zinc hybrid, aminosilane self assembly monolayers were studied by various techniques to illustrate the surface reaction mechanism.
Nanostructred Polymeric Membranes for Selective CO2 Removal from Light Gas Mixtures
(2004-06-27) Patel, Nikunj Pragjibhai; Steve D. Smith, Committee Member; Saad A. Khan, Committee Member; Richard J. Spontak, Committee Chair; John H. van Zanten, Committee Member
Two primary materials strategies have been developed to produce nanostructured polymer membranes for selective CO2 removal from mixed light-gas streams. In one approach, a microphase-ordered poly(styrene-b-ethylene oxide-b-styrene) (SEOS) triblock copolymer and its miscible blends with poly(ethylene glycol) (PEG) differing in molecular weight have been investigated to establish structure-transport property relationships. These membranes exhibit high CO2/H2 selectivity due to the affinity of CO2 for the ether moiety in the copolymer/homopolymer backbone. Crystalline regions in the EO microphase or introduced by relatively high-molecular-weight PEG serve as impermeable barriers to penetrating gas molecules and therefore compromise membrane performance. This drawback can be overcome through the physical addition of low-molecular-weight PEG, which behaves as a diluent. Upon PEO crystal melting at elevated temperatures, the CO2/H2 selectivity undergoes an abrupt increase consistent with the hypothesis that only amorphous regions can participate in penetrant transport. An alternative approach to near-equilibrium block copolymer/homopolymer blends is the introduction of a B-compatible homopolymer into a swollen ABA triblock or higher-order multiblock copolymer. The resultant "mesoblends" are reproducible, nonequilibrium blends that do not undergo the same morphological transitions induced in the near-equilibrium blend analogues. This procedure has been adopted here to generate novel morphologies in the SEOS triblock copolymer and a poly(amide-b-ethylene glycol) (AEG) multiblock copolymer with PEG homopolymers. Solvent quality, solution concentration and temperature have a profound impact on PEG solubility within the copolymer. Incorporation of amorphous PEG into the AEG copolymer is found to enhance CO2 permeability, as well as CO2/H2 selectivity. The second approach examined here relies on chemically crosslinked PEG diacrylate (PEGda) oligomers differing in molecular weight, as well as their nanocomposites prepared with up to 10 wt% methacrylate-functionalized fumed silica (FS) or an organically-modified nanoclay. The mechanical, thermal and morphological characteristics of these membranes have been probed by dynamic rheology, thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM), respectively. These PEGda membranes exhibit exceptionally high acid-gas selectivity coupled with high gas permeabilities that tend to increase with increasing oligomer molecular weight. Addition of FS results in improved mechanical properties without deteriorating transport properties. Temperature-dependent permeation studies demonstrate Arrhenius behavior with considerably lower activation energy of permeation for CO2. The polarity of the matrix, represented by PEGda oligomer molecular weight, and the transmembrane pressure allow systematic tuning of CO2/H2 selectivity and CO2 permeability. Crosslinked poly(propylene glycol) diacrylate (PPGda) membranes with various additives have also been synthesized due to their reportedly higher CO2 solubility. Gas transport and rheological properties are extremely sensitive to the molecular weight of oligomer, as in the case of the corresponding PEGda membranes. The major difference between these two membranes is the higher CO2 permeability, but lower CO2/H2 selectivity, in the PPGda membranes. Gas transport properties vary according to the rule of mixtures in PPGda/PEGda membranes blended prior to chemical crosslinking.
Physiological, Biochemical and Biotechnological Characterization of Glycoside Hydrolases from the Hyperthermophilic Bacterium
(2003-11-19) Chhabra, Swapnil R; Robert M. Kelly, Committee Chair; David F. Ollis, Committee Member; Denns T. Brown, Committee Member; Saad A. Khan, Committee Member
The genome sequence of Thermotoga maritima MSB8, encodes for the highest number of glycoside hydrolase genes amongst hyperthermophilic Bacterial and Archaeal genome sequences reported to date. The ability of T. maritima to utilize the polysaccharides galactomannan and CM cellulose as carbon sources can be attributed at least in part due to the presence of the genes cel5A (TM1751), man5 (TM1227) and cel74 (TM0305). The encoded proteins Tm Man5 and Tm Cel74 are extracellular marked by the presence of N-terminal signal peptides whereas Tm Cel5A is intracellular. Biochemical properties of recombinant versions of Tm Man5 and Tm Cel74, expressed in Escherichia coli, correlated well with predictions made by sequence comparisons. Thus, Tm Man5 was found to be a strict -mannanase while Tm Cel74 was found to be a strict endoglucanase. In contrast, although Tm Cel5A shows sequence similarity to an endoglucanase, its biochemical characteristics point to dual substrate specificity such that Tm Cel5A was found to hydrolyze both -mannan and -glucan polysaccharides. Glu-137 (proton donor) and Glu-253 (nucleophile) were found to be the catalytic residues in Tm Cel5A while Glu-329 was the catalytic nucleophile in Tm Man5. A mutation of these residues in each protein resulted in a complete loss of hydrolytic activity. Currently, Tm Cel74 is the only endoglucanase in Family 74 of glycoside hydrolases that lacks the presence of a cellulose-binding module at its C- terminus. Fusion of a binding module to the C-terminus of Tm Cel74 allowed the chimeric protein to bind and hydrolyze ii microcrystalline cellulose. Gene expression profiles of cel5A and man5 using Northern hybridizations and cDNA microarrays suggested co-regulation during growth on mannose and -1,4 mannan polysaccharides. Overall expression levels of cel74 were several fold lower than the other extracellular endoglucanase gene cel12A (TM1524) during growth on -1,4 glucan polysaccharides. Global gene expression analysis using a targeted cDNA microarray indicated the presence of tight regulatory mechanisms for glycoside hydrolase expression in T. maritima during growth on different carbon sources. Mixed model data analysis revealed co-regulation of genes within potential operons as well as sets of spatially distant gene strings with similar expression profiles, suggesting the presence of regulons in the T. maritima genome. This information in conjunction with biochemical characteristics of encoded proteins, was used to predict pathways for polysaccharide uptake and utilization in T. maritima. The research presented in this work provides a framework for future studies using full genome microarrays of T. maritima and other hyperthermophiles for the identification of glycoside hydrolases with novel sequences and substrate specificities.
Platinum and Platinum Alloy-Carbon Nanofiber Composites for Use as Electrodes in Direct Methanol Fuel Cells
(2010-04-20) Lin, Zhan; Xiangwu Zhang, Committee Chair; Wendy E. Krause, Committee Co-Chair; Saad A. Khan, Committee Member; Samuel M. Hudson, Committee Member
In response to the energy needs of modern society and emerging ecological concerns, the pursuit of novel, low-cost, and environmentally friendly energy conversion and storage systems has raised significant interest. Among various energy conversion and storage systems, fuel cells have become a primary research focus since they convert chemical energy directly into electrical energy with high efficiency and low pollutant emissions. For example, direct methanol fuel cells (DMFCs), which supply the electrical energy by converting methanol to energy, are an ideal fuel cell system for applications in electric vehicles and electronic portable devices due to their relatively quick start-up, rapid response to catalyst loading, and low operating temperature. However, the wide commercial use of DMFCs in advanced hybrid electric vehicles and electronic portable devices is hampered by their high cost, poor durability, and relatively low energy and power densities. In order to address these problems, their research focuses on the development of highly active electrode catalysts coupled with a suitable electrode structure for the oxidation of methanol at the anode and the reduction of oxygen at the cathode to attain high efficiency of DMFCs, and subsequently lowering the cost. In this dissertation, the fabrication of novel platinum and platinum alloy nanoparticle-loaded carbon nanofibers (CNFs) for use as electrodes in DMFCs is demonstrated through electrospinning, carbonization, and deposition. The resulting CNF-based electrodes possess the properties of high electroactive surface area, good catalytic abilities towards the oxidation of methanol and the reduction of oxygen, and great long-time stability. As a result, DMFCs using these CNFs-supported platinum and platinum alloy nanoparticles as electrodes offer many advantages, such as improved electrocatalytic abilities, long-term stability, easy fabrication, low cost, and environmental benignity. Therefore, this new technology opens up new opportunities to develop high-performance electrode materials in the future for high-performance DMFCs, which are one of the promising power sources for consumer devices and electric vehicles, and play a critical role in solving the worldwide critical energy issue.
Rheological Characterization of Alginate Microbead Gels and Suspensions
(2009-06-29) Yaniv, Yifat Rahel; Brian E. Farkas , Committee Member; Allen E. Foegeding, Committee Member; Saad A. Khan, Committee Member; Christopher E. Daubert , Committee Chair
As the use of microgel beads in food, pharmaceutical, and other industries becomes more prevalent, additional data is needed regarding the rheology of suspensions and gels composed of microgel beads. The goal of this work was to evaluate the rheological properties of alginate microbead gels and suspensions. The microgel beads used in this research were produced by an air-assisted atomization process that was designed for this work. The process was based on disintegration of alginate solution jet by exposure to high-velocity air, and yielded alginate microgel beads ranging in size from several microns to several hundred microns. The bead diameter was a function of solution viscosity and air velocity. Decreasing solution viscosity and increasing air velocity contributed to the disintegration of the atomized liquid jet and produced smaller beads. A correlation between the bead mean diameter and the Weber and Reynolds numbers was obtained by dimensional analysis: DÃ¢Ë†Â (WeÃƒâ€”Re)^(-0.25). The beads produced were used for construction of the Ã¢â‚¬Å“microbead gelsÃ¢â‚¬Â and suspensions. The mechanical properties of the microbead gels were investigated as a function of bead mechanical properties and particle size distribution. Bulk gels were used to evaluate the mechanical properties of the beads, which depended on alginate type and concentration. The microbead gels were constructed from different bead types and the viscoelastic properties of the microbead gels were related to the viscoelastic properties of the bulk gels. While higher modulus beads contributed to the modulus of the microbead gel, they were also less deformable and therefore decreased interparticle contact. As smaller particles were introduced into the system, the importance of particle deformability and interparticle interactions increased. The rheology of suspensions composed of viscoelastic particles was investigated as a function of shear rate, volume fraction, particle size distributions, and mechanical properties of the beads. The viscoelastic properties of the beads proved to be very important in the analysis of the effects shear rate, particle volume fraction, and particle size distribution had on suspension viscosity. The diverse flow behavior could not be fully explained by the models presented in literature; therefore, several mechanisms were proposed to explain suspension rheology. The proposed mechanisms were based on the hydrodynamic forces, interactions between the particles, and the beads viscoelastic properties.