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The synthesis and study of poly(N-isopropylacrylamide)/poly(acrylic acid) interpenetrating polymer network nanoparticle hydrogels.

Description: Homogeneous hydrogels made of an interpenetrating network of poly(N-isopropylacrylamide) (PNIPAm) and poly(acrylic acid) (PAAc) are synthesized by a two-step process; first making PNIPAm hydrogels and then interpenetrating acrylic acid throughout the hydrogel through polymerization. The kinetic growth of the IPN is plotted and an equation is fitted to the data. When diluted to certain concentrations in water, the hydrogels show reversible, inverse thermal gelation at about 34°C. This shows unique application to the medical field, as the transition is just below body temperature. A drug release experiment is performed using high molecular weight dyes, and a phase diagram is created through observation of the purified, concentrated gel at varying concentrations and temperatures.
Date: August 2006
Creator: Crouch, Stephen Wallace
Partner: UNT Libraries

Growing carbon nanotubes by chemical vapor deposition technique.

Description: Carbon nanotubes were synthesized in the laboratory using chemical vapor deposition at different methane concentration. I found that a methane concentration of 4 sccm was ideal for well recognizable carbon nanotubes. A higher concentration led to fewer nanotube growth and silicon carbide structure. Coating the sample first with Fe(NO3)3 created a catalyst base on the substrate for the nanotube to adhere and grow on.
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Date: May 2000
Creator: Rajan, Harihar V.
Partner: UNT Libraries

Photoelectric Emission Measurements for CVD Grown Polycrystalline Diamond Films

Description: We examined CVD grown polycrystalline diamond films having different methane concentrations to detect defects and study the possible correlation between the methane concentration used during the growth process and the defect density. SEM and Raman results show that the amorphous and sp2 carbon content of the films increases with methane concentration. Furthermore, photoelectric emission from diamond is confirmed to be a two-photon process, hence the electrons are emitted from normally unoccupied states. We found that the photoelectric yield, for our samples, decreases with the increase in methane concentration. This trend can be accounted for in two different ways: either the types of defects observed in this experiment decrease in density as the methane concentration increases; or, the defect density stays the same or increases, but the increase in methane concentration leads to an increase in the electron affinity, which reduces the overall photoelectric yield.
Date: August 1999
Creator: Hassan, Tarek
Partner: UNT Libraries

Temperature dependent rheology of surfactant-hydroxypropyl cellulose solutions.

Description: The rheology of 1-8% hydroxypropyl cellulose (HPC) solutions has been studied in the temperature range of 20-45 degrees Celsius. The results showed that the relative viscosity at each HPC concentration decreases with increasing temperature. The relative viscosity decreases drastically at about 43 degrees Celsius due to a phase transition. The influence of anionic surfactant, sodium dodecylsulfate (SDS), induced gelation of a 2% HPC solution. The HPC solutions gelled at surfactant SDS concentrations ranging from 0.4 to 1.0 critical micelle concentration (CMC). The gelation of the HPC/SDS hydrogel is explained in the surfactant SDD - bridged HPC linear polymer chains. The complex viscosity - concentration profile was determined below the CMC of the SDS - water pair. The peak itself was a function of frequency indicating the presence of two relaxation times within the gelled network.
Date: December 2002
Creator: Snively, C. Todd
Partner: UNT Libraries

Work Function Study of Iridium Oxide and Molybdenum Using UPS and Simultaneous Fowler-Nordheim I-V Plots with Field Emission Energy Distributions

Description: The characterization of work functions and field emission stability for molybdenum and iridium oxide coatings was examined. Single emission tips and flat samples of molybdenum and iridium oxide were prepared for characterization. The flat samples were characterized using X-ray Photoelectron Spectroscopy and X-ray diffraction to determine elemental composition, chemical shift, and crystal structure. Flat coatings of iridium oxide were also scanned by Atomic Force Microscopy to examine topography. Work functions were characterized by Ultraviolet Photoelectron Spectroscopy from the flat samples and by Field Emission Electron Distributions from the field emission tips. Field emission characterization was conducted in a custom build analytical chamber capable of measuring Field Emission Electron Distribution and Fowler-Nordheim I-V plots simultaneously to independently evaluate geometric and work function changes. Scanning Electron Microscope pictures were taken of the emission tips before and after field emission characterization to confirm geometric changes. Measurement of emission stability and work functions were the emphasis of this research. In addition, use of iridium oxide coatings to enhance emission stability was evaluated. Molybdenum and iridium oxide, IrO2, were characterized and found to have a work function of 4.6 eV and 4.2 eV by both characterization techniques, with the molybdenum value in agreement with previous research. The analytic chamber used in the field emission analysis demonstrated the ability to independently determine the value and changes in work function and emitter geometry by simultaneous measurement of the Field Emission Energy Distribution and Fowler-Nordheim I-V plots from single emitters. Iridium oxide coating was found to enhance the stability of molybdenum emission tips with a relatively low work function of 4.2 eV and inhibited the formation of high work function molybdenum oxides. However, the method of deposition of iridium and annealing in oxygen to form iridium oxide on molybdenum emitters left rather severe cracking in the protective oxide ...
Date: August 1999
Creator: Bernhard, John Michael
Partner: UNT Libraries

Polymer Gels: Kinetics, Dynamics Studies and Their Applications as Biomaterials

Description: The polymer gels especially hydrogels have a very special structure and useful features such as unusual volume phase transition, compatibility with biological systems, and sensitivity to environmental stimuli (temperature, pH value, electric field, light and more), which lead to many potential applications in physical and biochemical fields. This research includes: (1) the theoretical and experimental studies of polymer gels on swelling kinetics, spinodal decomposition, and solution convection in gel matrix; (2) applications of polymer gels in wound dressing, tissue-simulating optical phantom and gel display. The kinetics of gel swelling has been theoretically analyzed by considering coupled motions of both solvent and polymer network. Analytical solutions of the solvent and the network movement are derived from collective diffusion equations for a long cylindrical and a large disk gel. Kinetics of spinodal decomposition of N-isopropylacrylamide (NIPA) polymer gel is investigated using turbidity and ultrasonic techniques. By probing movement of domains, a possible time-dependent gel structure in the spinodal decomposition region is presented. Theoretical studies of solution convection in gel matrix have been done and more analysis on dimensionless parameters is provided. To enhance the drug uptake and release capacity of silicone rubber (SR), NIPA hydrogel particles have been incorporated into a SR membrane. This SR/NIPA composite gel has promising attributes for wound dressing and other uses. Tissue-simulating optical phantom has been synthesized and studied using NIPA solution trapped inside a hydrogel. Polymer gels with engineered surface patterns were implemented. NIPA gel deposited on the surface of an acrylamide gel can be used as responsive gel display. A dynamically measurement technique of local shear modulus and swelling ratio of gel is presented based on an engineered periodic surface pattern as square array.
Date: December 2003
Creator: Wang, Changjie
Partner: UNT Libraries

Oligonucleotide guanosine conjugated to gallium nitride nano-structures for photonics.

Description: In this work, I studied the hybrid system based on self-assembled guanosine crystal (SAGC) conjugated to wide-bandgap semiconductor gallium nitride (GaN). Guanosine is one of the four bases of DNA and has the lowest oxidation energy, which favors carrier transport. It also has large dipole moment. Guanosine molecules self-assemble to ribbon-like structure in confined space. GaN surface can have positive or negative polarity depending on whether the surface is Ga- or N-terminated. I studied SAGC in confined space between two electrodes. The current-voltage characteristics can be explained very well with the theory of metal-semiconductor-metal (MSM) structure. I-V curves also show strong rectification effect, which can be explained by the intrinsic polarization along the axis of ribbon-like structure of SAGC. GaN substrate property influences the properties of SAGC. So SAGC has semiconductor properties within the confined space up to 458nm. When the gap distance gets up to 484nm, the structure with guanosine shows resistance characteristics. The photocurrent measurements show that the bandgap of SAGC is about 3.3-3.4eV and affected by substrate properties. The MSM structure based on SAGC can be used as photodetector in UV region. Then I show that the periodic structure based on GaN and SAGC can have photonic bandgaps. The bandgap size and the band edges can be tuned by tuning lattice parameters. Light propagation and emission can be tuned by photonic crystals. So the hybrid photonic crystal can be potentially used to detect guanosine molecules. If guanosine molecules are used as functional linker to other biomolecules which usually absorb or emit light in blue to UV region, the hybrid photonic crystal can also be used to tune the coupling of light source to guanosine molecules, then to other biomolecules.
Date: August 2008
Creator: Li, Jianyou
Partner: UNT Libraries

The Effects of Cesium Deposition and Gas Exposure on the Field Emission Properties of Single Wall and Multiwall Carbon Nanotubes

Description: The effects of Cs deposition on the field emission (FE) properties of single-walled carbon nanotube (SWNT) bundles were studied. In addition, a comparative study was made on the effects of O2, Ar and H2 gases on the field emission properties of SWNT bundles and multiwall carbon nanotubes (MWNTs). We observed that Cs deposition decreases the turn-on field for FE by a factor of 2.1 - 2.9 and increases the FE current by 6 orders of magnitude. After Cs deposition, the FE current versus voltage (I-V) curves showed non-Fowler-Nordheim behavior at large currents consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I-V curves before and after Cs deposition was approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current. Our results show that cesiated SWNT bundles have great potential as economical and reliable vacuum electron sources. We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurred after operation in UHV. The higher operating voltages in an O2 environment caused a permanent decrease of FE current and increase in turn-on field of MWNTs. The ratios of the slopes before and after O2 exposure were approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively. SWNTs compared to MWNTs would appear to make more economical and reliable vacuum electron sources.
Date: May 2002
Creator: Wadhawan, Atul
Partner: UNT Libraries

The Effect of Average Grain Size on Polycrystalline Diamond Films

Description: The work function of hydrogen-terminated, polycrystalline diamond was studied using ultraviolet photoelectron spectroscopy. Polycrystalline diamond films were deposited onto molybdenum substrates by electrophoresis for grain sizes ranging from 0.3 to 108 microns. The work function and electron affinity were measured using 21.2 eV photons from a helium plasma source. The films were characterized by x-ray photoelectron spectroscopy to determine elemental composition and the sp2/sp3 carbon fraction. The percentage of (111) diamond was determined by x-ray diffraction, and scanning electron microscopy was performed to determine average grain size. The measured work function has a maximum of 5.1 eV at 0.3 microns, and decreases to 3.2 eV at approximately 4 microns. Then the work function increases with increasing grain size to 4.0 eV at 15 microns and then asymptotically approaches the 4.8 eV work function of single crystal diamond at 108 microns. These results are consistent with a 3-component model in which the work function is controlled by single-crystal (111) diamond at larger grain sizes, graphitic carbon at smaller grain sizes, and by the electron affinity for the intervening grain sizes.
Date: May 2002
Creator: Abbott, Patrick Roland
Partner: UNT Libraries

Multifunctional Organic-Inorganic Hybrid Nanophotonic Devices

Description: The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as it undergoes its phase change creates a controllable mechanism for adjusting the transmittance of light frequencies through a linear defect in a photonic crystal. The NIPA infiltrated photonic crystal shows greater shifts in the bandwidth per ºC than any liquid crystal methods. This dissertation demonstrates the versatile uses of hydrogel, as a means of control in nanophotonic devices, and will likely lead to development of other hybrid applications. The development of smaller light based applications will facilitate the need to augment the devices with control mechanism and will play an increasing important role in the future.
Date: May 2008
Creator: Garner, Brett William
Partner: UNT Libraries

The Concept of Collision Strength and Its Applications

Description: Collision strength, the measure of strength for a binary collision, hasn't been defined clearly. In practice, many physical arguments have been employed for the purpose and taken for granted. A scattering angle has been widely and intensively used as a measure of collision strength in plasma physics for years. The result of this is complication and unnecessary approximation in deriving some of the basic kinetic equations and in calculating some of the basic physical terms. The Boltzmann equation has a five-fold integral collision term that is complicated. Chandrasekhar and Spitzer's approaches to the linear Fokker-Planck coefficients have several approximations. An effective variable-change technique has been developed in this dissertation as an alternative to scattering angle as the measure of collision strength. By introducing the square of the reduced impulse or its equivalencies as a collision strength variable, many plasma calculations have been simplified. The five-fold linear Boltzmann collision integral and linearized Boltzmann collision integral are simplified to three-fold integrals. The arbitrary order linear Fokker-Planck coefficients are calculated and expressed in a uniform expression. The new theory provides a simple and exact method for describing the equilibrium plasma collision rate, and a precise calculation of the equilibrium relaxation time. It generalizes bimolecular collision reaction rate theory to a reaction rate theory for plasmas. A simple formula of high precision with wide temperature range has been developed for electron impact ionization rates for carbon atoms and ions. The universality of the concept of collision strength is emphasized. This dissertation will show how Arrhenius' chemical reaction rate theory and Thomson's ionization theory can be unified as one single theory under the concept of collision strength, and how many important physical terms in different disciplines, such as activation energy in chemical reaction theory, ionization energy in Thomson's ionization theory, and the Coulomb logarithm in ...
Date: May 2004
Creator: Chang, Yongbin
Partner: UNT Libraries

The preparation and characterization of thermo-sensitive colored hydrogel film and surfactant-free porous polystyrene three-dimensional network.

Description: Polymer hydrogel films change their properties in response to environmental change. This remarkable phenomenon results in many potential applications of polymer hydrogel films. In this thesis colored thermo-sensitive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel film was prepared by firstly synthesizing polymer latex and secondarily crosslinking the nanoparticles and casting the polymers onto glass. The shape-memory effect has been observed when changing the environmental temperature. The temperature-dependent of turbidity of polymer hydrogel film was measured by HP UVVisible spectrophotometer. This intelligent hydrogel might be used in chemomechanical systems and separation devices as well as sensors. Polymer adsorption plays an important role in many products and processes. In this thesis, surfactant-free three-dimensional polystyrene (PS) nanoparticle network has been prepared. The infrared spectroscopy and solubility experiment are performed to prove the crosslinking mechanism, also the BET method was used to measure the adsorption and desorption of polystyrene network. The BET constant (C) is calculated (C=6.32). The chemically bonded polymer nanoparticle network might have potential applications as catalyst or used for chromatographic columns.
Date: December 2001
Creator: Zhou, Bo
Partner: UNT Libraries

Characterization, Properties and Applications of Novel Nanostructured Hydrogels.

Description: The characterization, properties and applications of the novel nanostructured microgel (nanoparticle network and microgel crystal) composed of poly-N-isopropylacrylanmide-co-allylamine (PNIPAM-co-allylamine) and PNIPAM-co-acrylic acid(AA) have been investigated. For the novel nanostructured hydrogels with the two levels of structure: the primary network inside each individual particle and the secondary network of the crosslinked nanoparticles, the new shear modulus, drug release law from hydrogel with heterogeneous structure have been studied. The successful method for calculating the volume fraction related the phase transition of colloid have been obtained. The kinetics of crystallization in an aqueous dispersion of PNIPAM particles has been explored using UV-visible transmission spectroscopy. This dissertation also includes the initial research on the melting behavior of colloidal crystals composed of PNIPAM microgels. Many new findings in this study area have never been reported before. The theoretical model for the columnar crystal growth from the top to bottom of PNIPAM microgel has been built, which explains the growth mechanism of the novel columnar hydrogel colloidal crystals. Since the unique structure of the novel nanostructured hydrogels, their properties are different with the conventional hydrogels and the hard-sphere-like system. The studies and results in this dissertation have the important significant for theoretical study and valuable application of these novel nanostructured hydrogels.
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Date: December 2006
Creator: Tang, Shijun
Partner: UNT Libraries

Theoretical and Experimental Investigations Concerning Microgels of Varied Spherical Geometries

Description: Polymer gels have been studied extensively due to their ability to simulate biological tissues and to swell or collapse reversibly in response to external stimuli. This work presents a variety of studies using poly-N-isopropylacrylamide (PNIPA) hydrogels. The projects have been carried out both in the lab of Dr. Zhibing Hu and in collaboration with others outside of UNT: (1) an analysis of the swelling kinetics of microgel spherical shells prepared using a novel design of microfluidic devices; (2) a comparison of the drug-release rates between nanoparticle structures having either core or core-with-shell (core-shell) designs; (3) an investigation into the thermodynamics of swelling for microgels of exceedingly small size.
Date: August 2012
Creator: Wahrmund, Joshua Joseph
Partner: UNT Libraries

Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy

Description: Dental amalgams have been used for centuries as major restorative materials for decaying teeth. Amalgams are prepared by mixing alloy particles which contain Ag, Sn, and Cu as the major constituent elements with liquid Hg. The study of microstructure is essential in understanding the setting reactions and improving the properties of amalgams. Until the work reported in this dissertation, optical microscopy (OM), scanning electron microscopy (SEM), and x-ray diffractometry (XRD) were used commonly to analyze amalgam microstructures. No previous systematic transmission electron microscopy (TEM) study has been performed due to sample preparation difficulties and composite structure of dental amalgams. The goal of this research was to carry out detailed microstructural and compositional studies of dental amalgams. This was accomplished using the enhanced spatial resolution of the TEM and its associated microanalytical techniques, namely, scanning transmission electron microscopy (STEM), x-ray energy dispersive spectroscopy (XEDS) and micro-microdiffraction (μμD). A new method was developed for thinning amalgam samples to electron transparency using the "wedge technique." Velvalloy, a low-Cu amalgam, and Tytin, a high-Cu amalgam, were the two amalgams characterized. Velvalloy is composed of a Ag₂Hg₃ (γ₁)/HgSn₇₋₉ (γ₂) matrix surrounding unreacted Ag₃Sn (γ) particles. In addition, hitherto uncharacterized reaction layers between Ag₃Sn(γ)/Ag₂Hg₃ (γ₂) and Ag₂Hg₃ (γ₁)/HgSn₇₋₉ (γ₂) were observed and analyzed. An Ag-Hg-Sn (β₁) phase was clearly identified for the first time. In Tytin, the matrix consists of Ag₂Hg₃ (γ₁) grains. Fine precipitates of Cu₆Sn₅ (η') are embedded inside the γ₁ and at the grain boundaries. These precipitates are responsible for the improved creep resistance of Tytin compared to Velvalloy. The additional Cu has completely eliminated the γ₂ phase which is the weakest component of amalgams. Ag-Hg-Sn (β₁) and large grains of Cu₆Sn₅ (η') are found adjacent to the unreacted alloy particles. Tytin alloy particles contain Cu₃Sn (ε) precipitates in a matrix of Ag₃Sn ...
Date: May 1997
Creator: Hooghan, Tejpal Kaur
Partner: UNT Libraries

Temperature Dependency of Some of the Thermodynamical Properties of Aqueous Binary-Mixture Systems

Description: The temperature dependence of the refractive index and the density of aqueous binary mixtures of water and ethyl alcohol (C₂H₅OH) were measured by using a modified Michelson interferometer and a narrow glass capillary tube over the temperature range of 278≤T≤353 K for solutions of 100, 75,65, 50, 25, 10 and 0 volume percent ethyl alcohol. The temperature was cycled over both increasing and decreasing directions to explore hysteresis in the cycling. The data are discussed and compared with the Lorentz-Lorenz (LL) formula. A more accurate formula which fits the experimental data better than the LL relation was derived. An attempt was made to determine the nature of the solvent-solute interaction through any changes that were found in the refractive index for He-Ne laser light and IR diode signals and to analyze the refractive index and density results to test the accuracy of the available mixing rules in predicting the refractive index values and the density of binary systems. Conductivity measurements (d. c.) over the temperature range 278≤T≤353 K of aqueous solutions of NaCl at various concentrations were made and used to establish transport properties of ions in solution. The dynamical properties of the electrolytes were used to establish the nature of hydrogen bonding in aqueous binary mixture systems. Rate equations for ion formation and recombination were used to establish the temperature ranges in which hydrogen bonding dominated in forming polymeric species. From experimental data on the binary mixtures with water, a better understanding of water in its different functions and aggregation is possible. The water molecule itself and its response to the environment are understood when suitable studies are made of the forces in the system. In this work, some qualitative aspects of the interactions and dynamics of the water molecule have been investigated. Classical molecular dynamics simulations were tried ...
Date: August 1996
Creator: Zheng, Yueming
Partner: UNT Libraries

Dielectric Relaxation of Aqueous Solutions at Microwave Frequencies for 335 GHz. Using a Loaded Microwave Cavity Operating in the TM010 Mode

Description: The frequency dependence and temperature dependence of the complex dielectric constant of water is of great interest. The temperature dependence of the physical properties of water given in the literature, specific heat, thermal conductivity, electric conductivity, pH, etc. are compared to the a. c. (microwave) and d. c. conductivity of water with a variety of concentration of different substances such as HC1, NaCl, HaS04, etc. When each of these properties is plotted versus inverse absolute temperature, it can be seen that each sample shows "transition temperatures". In this work, Slater's perturbation equations for a resonant microwave cavity were used to analyze the experimental results for the microwave data.
Date: August 1994
Creator: Wang, Henry F. S. (Henry Fu-Sen)
Partner: UNT Libraries

Nested Well Plasma Traps

Description: Criteria for the confinement of plasmas consisting of a positive and negative component in Penning type traps with nested electric potential wells are presented. Computational techniques for the self-consistent calculation of potential and plasma density distributions are developed. Analyses are presented of the use of nested well Penning traps for several applications. The analyses include: calculations of timescales relevant to the applications, e.g. reaction, confinement and relaxation timescales, self-consistent computations, and consideration of other physical phenomenon important to the applications. Possible applications of a nested well penning trap include production of high charge state ions, studies of high charge state ions, and production of antihydrogen. In addition the properties of a modified Penning trap consisting of an electric potential well applied along a radial magnetic field are explored.
Date: August 2000
Creator: Dolliver, Darrell
Partner: UNT Libraries

Synthesis and host-guest interaction of cage-annulated podands, crown ethers, cryptands, cavitands and non-cage-annulated cryptands.

Description: Symmetrical cage-annulated podands were synthesized via highly efficient synthetic strategies. Mechanisms to account for the key reaction steps in the syntheses are proposed; the proposed mechanisms receive support from the intermediates that have been isolated and characterized. An unusual complexation-promoted elimination reaction was studied, and a mechanism is proposed to account for the course of this reaction. This unusual elimination may generalized to other rigid systems and thus may extend our understanding of the role played by the host molecules in "cation-capture, anion-activation" via complexation with guest molecules. Thus, host-guest interaction serves not only to activate the anion but also may activate the leaving groups that participate in the complexation. Complexation-promoted elimination provides a convenient method to desymmetrize the cage while avoiding protection/deprotection steps. In addition, it offers a convenient method to prepare a chiral cage spacer by introducing 10 chiral centers into the host system in a single synthetic step. Cage-annulated monocyclic hosts that contain a cage-butylenoxy spacer were synthesized. Comparison of their metal ion complexation behavior as revealed by the results of electrospray ionization mass spectrometry (ESI-MS), alkali metal picrate extraction, and pseudohydroxide extraction with those displayed by the corresponding hosts that contain cage-ethylenoxy or cage-propylenoxy spacers reveals the effect of the length of the cage spacer upon the host-guest behavior. A series of cage-annulated cryptands, cavitands and the corresponding non-cage-annulated model compounds have been synthesized. These host molecules display unusual behavior when examined by using ESI-MS techniques, i.e., they bind selectively to smaller alkali metal ions (i.e., Li+ and Na+), a result that deviates significantly from expectations based solely upon consideration of the size-fit principle. It seems likely that this behavior results from the effect of the host topology on host-guest behavior. A series of non-cage-annulated cryptands also have been synthesized. These compounds can serve as starting ...
Date: May 2003
Creator: Chen, Zhibing
Partner: UNT Libraries

Synthesis and study of crystalline hydrogels, guided by a phase diagram.

Description: Monodispersed nanoparticles of poly-N-isopropylacrylamide-co-allylamine (PNIPAM-co-allylamine) and PNIPAM-co-acrylic acid (AA) have been synthesized and used as building blocks for creating three-dimensional networks. The close-packed PNIPAM-co-allylamine and PNIPAM-co-AA nanoparticles were stabilized by covalently bonding neighboring particles at room temperature and at neutral pH; factors which make these networks amicable for drug loading and release. Controlled release studies have been performed on the networks using dextran markers of various molecular weights as model macromolecular drugs. Drug release was quantified under various physical conditions including a range of temperature and molecular weight. These nanoparticle networks have several advantages over the conventional bulk gels for controlling the release of biomolecules with large molecular weights. Monodispersed nanoparticles of poly-N-isopropylacrylamide-co-allylamine (PNIPAM-co-allylamine) can self-assemble into crystals with a lattice spacing on the order of the wavelength of visible light. By initiating the crystallization process near the colloidal crystal melting temperature, while subsequently bonding the PNIPAM-co-allylamine particles below the glass transition temperature, a nanostructured hydrogel has been created. The crystalline hydrogels exhibit iridescent patterns that are tunable by the change of temperature, pH value or even protein concentration. This kind of soft and wet hydrogel with periodic structures may lead to new sensors, devices, and displays operating in aqueous solutions, where most biological and biomedical systems reside. The volume-transition equilibrium and the interaction potential between neutral PINPAM particles dispersed in pure water were investigated by using static and dynamic light-scattering experiments. From the temperature-dependent size and energy parameters, the Sutherland-like potential provides a reasonable representation of the inter-particle potential for PNIPAM particles in swollen and in collapsed phases. An aqueous dispersion of PNIPAM particles can freeze at both high and low temperatures. At low temperatures, the freezing occurs at a large particle volume fraction, similar to that in a hard-sphere system; while at high temperature, the freezing occurs at ...
Date: December 2004
Creator: Huang, Gang
Partner: UNT Libraries

Fabrication and light scattering study of multi-responsive nanostructured hydrogels and water-soluble polymers.

Description: Monodispersed microgels composed of poly-acrylic acid (PAAc) and poly(N-isopropylacrylamide) (PNIPAM) interpenetrating networks were synthesized by 2-step method with first preparing PNIPAM microgel and then polymerizing acrylic acid that interpenetrates into the PNIPAM network. The semi-dilute aqueous solutions of the PNIPAM-PAAc IPN microgels exhibit an inverse thermo-reversible gelation. Furthermore, IPN microgels undergo the reversible volume phase transitions in response to both pH and temperature changes associated to PAAc and PNIPAM, respectively. Three applications based on this novel hydrogel system are presented: a rich phase diagram that opens a door for fundamental study of phase behavior of colloidal systems, a thermally induced viscosity change, and in situ hydrogel formation for controlled drug release. Clay-polymer hydrogel composites have been synthesized based on PNIPAM gels containing 0.25 to 4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. Surfactant-free hydroxypropyl cellulose (HPC) microgels have been synthesized in salt solution. In a narrow sodium chloride concentration range from 1.3 to 1.4 M, HPC chains can self-associate into colloidal particles at room temperature. The microgel particles were then obtained in situ by bonding self-associated HPC chains at 23 0C using divinyl sulfone as a cross-linker. The volume phase transition of the resultant HPC microgels has been studied as a function of temperature at various salt concentrations. A theoretical model based on Flory-Huggins free energy consideration has been used to explain the experimental results. Self-association behavior and conformation variation of long chain branched (LCB) poly (2-ethyloxazoline) (PEOx) with a CH3-(CH2)17 (C18) modified surface are investigated using light scattering techniques in various ...
Date: December 2003
Creator: Xia, Xiaohu
Partner: UNT Libraries

Electrochemical synthesis of CeO2 and CeO2/montmorillonite nanocomposites.

Description: Nanocrystalline cerium oxide thin films on metal and semiconductor substrates have been fabricated with a novel electrodeposition approach - anodic oxidation. X-ray diffraction analysis indicated that as-produced cerium oxide films are characteristic face-centered cubic fluorite structure with 5 ~ 20 nm crystal sizes. X-ray photoelectron spectroscopy study probes the non-stoichiometry property of as-produced films. Raman spectroscopy and Scanning Electron Microscopy have been applied to analyze the films as well. Deposition mode, current density, reaction temperature and pH have also been investigated and the deposition condition has been optimized for preferred oriented film formation: galvanostatic deposition with current density of -0.06 mA/cm2, T > 50oC and 7 < pH < 10. Generally, potentiostatic deposition results in random structured cerium oxide films. Sintering of potentiostatic deposited cerium oxide films leads to crystal growth and reach nearly full density at 1100oC. It is demonstrated that in-air heating favors the 1:2 stoichiometry of CeO2. Nanocrystalline cerium oxide powders (4 ~ 10 nm) have been produced with anodic electrochemical synthesis. X-ray diffraction and Raman spectroscopy were employed to investigate lattice expansion phenomenon related to the nanoscale cerium oxide particles. The pH of reaction solution plays an important role in electrochemical synthesis of cerium oxide films and powder. Cyclic voltammetry and rotation disk electrode voltammetry have been used to study the reaction mechanisms. The results indicate that the film deposition and powder formation follow different reaction schemes. Ce(III)-L complexation is a reversible process, Ce3+ at medium basic pH region (7~10) is electrochemically oxidized to and then CeO2 film is deposited on the substrate. CE mechanism is suggested to be involved in the formation of films, free Ce3+ species is coordinated with OH- at high basic pH region (>10) to Ce2O3 immediately prior to electrochemically oxidation Ce2O3 to CeO2. CeO2 / montmorillonite nanocomposites were electrochemically produced. X-ray ...
Date: December 2003
Creator: Wang, Qi
Partner: UNT Libraries

Synthesis and Characterization of Crystalline Assemblies of Functionalized Hydrogel Nanoparticles

Description: Two series monodispersed nanoparticles of hydroxylpropyl cellulose (HPC) and functionalized poly-N-isopropylamide (PNIPAM) particles have been synthesized and used as building blocks for creating three-dimensional networks, with two levels of structural hierarchy. The first level is HPC nanoparticles were made from methacrylated or degradable cross-linker attached HPC. These nanoparticles could be stabilized at room temperature by residual methacrylate or degradable groups are present both within and on the exterior of HPC nanoparticles. Controlled release studies have been performed on the particle and networks .The nearly monodispersed nanoparticles have been synthesized on the basis of a natural polymer of hydropropylcellulose (HPC) with a high molecular weight using the precipitation polymerization method and self-assembly of these particles in water results in bright colors. The HPC nanoparticles can be potential using as crosslinkers to increase the hydrogels mechanical properties, such as high transparency and rapid swelling/de-swelling kinetics. The central idea is to prepare colloidal particles containing C=C bonds and to use them as monomers - vinylparticles, to form stable particle assemblies with various architectures. This is accomplished by mixing an aqueous suspension of hydrogel nanoparticles (PNIPAM-co-allylamine) with the organic solvent (dichloromethane) to grow columnar crystals. The hydrogels with such a unique crystal structure behavior not only like the hydrogel opals, but also have a unique property: anisotropy.
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Date: December 2005
Creator: Cai, Tong
Partner: UNT Libraries