Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6990. Adviser: Richard C. Alkire. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

<p> Mechanics of emulsion droplets is crucial in applications where the encapsulated payload needs to be released under mechanical stimulus. This dissertation explores dumbbell nanoparticles as emulsifiers with focus on the emergent mechanical stability of the particle assembly at interfaces. Using a combination of freeze fracture shadow casting cryo-scanning electron microscopy and analytical modelling, I first investigate the complex adsorption behavior of individual dumbbells and discuss the corresponding implications for particle assembly at the interface. I then investigate the onset of mechanical instabilities in droplets stabilized by dumbbells using micropipette aspiration. I compare my findings to the control experiments of bare droplets and droplets stabilized with molecular surfactant under aspiration. In all three cases, the magnitude of the critical pressure for the onset of instabilities is set by the fluid surface tension. While particles have a dramatic impact on the mechanism of failure, the mechanical strength of the droplets is only modestly increased. This work provides experimental handles that can be tuned to aid the mechanical stability of emulsion droplets. The findings also inform advances in the mechanics of highly bendable sheets.</p><p>

<p> The catalytic hydrogenations of CO and CO2 to more useful chemicals is not only beneficial in producing more valuable products and reducing dependence on fossil fuels, but present a scientific challenge in how to control the selectivity of these reactions. Using colloidal chemistry techniques, a high level of control over the synthesis of nanomaterials can be achieved, and by exploiting this fact a simple model system can be realized to understand the reaction of CO and CO2 on a molecular level. Specifically, this dissertation focuses on understanding cobalt materials for the conversion of CO and CO2 into more useful, valuable chemicals. </p><p> Colloidally prepared cobalt nanoparticles with a narrow size distribution were supported in mesoporous SiO2 and TiO2 to study the effect of the support on the Co catalyzed hydrogenation of CO and CO2. The 10nm Co/SiO2 and Co/TiO2 catalysts were tested for CO and CO2 hydrogenation at 5 bar with a ratio to hydrogen of 1:2 and 1:3, respectively. In addition, the effect of Co oxidation state was studied by using different reduction pretreatment temperatures (250&deg;C and 450&deg;C). The results showed that for both hydrogenation reactions, Co/TiO2 had a high activity at both reduction temperatures compared to Co/SiO2. However, unlike Co/SiO2 which showed higher activity after 450&deg;C reduction, Co/TiO2 had a higher activity after reduction at 250&deg;C. Through synchrotron x-ray spectroscopy, it was concluded that the TiO2 was wetting the Co particle at higher reduction temperatures and dewetting at lower reduction temperatures. In addition to the wetting, CoO was observed to be the surface species on Co/TiO2 catalyst after reduction at low temperatures, which catalyzed both CO and CO2 hydrogenation reactions with higher activity than the Co metal obtained after reduction at 450&deg;C. </p><p> Classical steady-state measurements are limited in so much as they are often unable to provide information on individual reaction steps in complex reaction pathways. To attempt to circumvent this, a chemical transient kinetics (CTK) reactor was designed and built. Verification of the reactor was performed by evaluating a catalyst from the literature and confirming the results. A CoMgO catalyst was used to accomplish this, and our original findings show that at short time scales steric hindrances at the surface may push the product distribution towards olefinic rather than branched compounds. </p><p> Continuing work on the CTK, two distinct particle sizes of Co nanoparticles were synthesized and tested under atmospheric conditions (H2:CO = 2:1) on the transient reactor. 4.3 nm Co and 9.5 nm Co were supported on MCF-17 to study the previously observed size effect, where Co nanoparticles lose activity at smaller sizes. It was found that indeed, the 4.3 nm Co are less active because they contain less CO dissociation sites, which are necessary for populating the surface with carbon monomers and spurring subsequent chain growth. The specific CO dissociation site was identified as the Co (221) step, of which larger Co particles have more and smaller Co particles have less. </p><p> To investigate the nature of the MnO / Co3O4 interface, an in situ study using synchrotron radiation was undertaken. A sample of 6nm MnO nanoparticles loaded on mesoporous Co3O4 was studied with ambient pressure x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy at the Mn and Co L edges, and scanning transmission x-ray microscopy. X-ray measurements show that under reducing conditions of CO + H2, the MnO nanoparticles wet the Co surface until it is completely covered by a layer of MnO. Through the combination of techniques, it is shown that the system is catalytic active at the low pressures studied, and that the nature of the interface between MnO and Co3O4 is highly dependent on the temperature and gaseous environment it is prepared in. (Abstract shortened by ProQuest.)</p><p>

Similar to many self-assembling materials such as surfactants or liquid crystals, block copolymers undergo mesophase transitions due to the changes of external fields. This thesis focuses on the effects of copolymer architecture, concentration fields, and external shear flow fields on the self-assembly of block copolymers. Theories of the morphology transition of diblock copolymers at equilibrium have been studied for more than two decades. However, relatively little work has been done on more complicated architectures. The first part of this thesis uses a combination of the density functional theory (DFT) and the theory of composition fluctuations to examine the morphology transition in block copolymers of topologically distinct architectures. The morphology diagrams and corresponding properties are calculated for A-B-A triblock copolymer with given architectural parameters. The second part of this thesis focuses on the phase behavior of polymer blends consisting of amphiphilic block copolymers and selective homopolymer solvents. Phase diagrams are calculated for binary and ternary systems and the formation of induced morphologies due to the change of concentration fields are studied. The effects of architecture and sizes of constituent blocks of copolymers and the sizes of homopolymers are investigated by comparing various systems. Also, the critical Lifshitz point is examined and discussed. The third part of the thesis focuses on the order-disorder transition (ODT) and order-order transition (OOT) of block copolymers induced by an external shear flow field. The presence of shear alters the microscopic evolution of fluctuations and leads to a suppression of the fluctuation field. Consequently, the effective potential caused by excitations of monomer densities is reduced, resulting in an apparent increase of the ODT temperature. The self-consistent equation of the fluctuation spectrum is solved numerically, and the transition curve with respect to the applied shear rate is computed. This result resolves the recent controversy between experimental data and the previous perturbation analysis and shows good agreement with experimental results.

In aqueous solutions diblock polyelectrolytes with amphiphilic character form aggregate structures, which affect physical properties such as viscosity, elasticity, surface tension, and film hydrophilicity. Potential applications for diblock polyelectrolyte solutions include coatings, inks, oil recovery agents, personal care products, and biomaterials. By varying the diblock polyelectrolyte and solution properties, the solutions can be tuned to meet the needs of particular applications. The research objective was to identify the influences of block length, pH, and ionic strength on the rheological and interfacial properties of poly(styrene- b-acrylic acid) (PS-PAA) solutions. Six polymers with varied PS and PAA block lengths were examined, all at 1.0 wt% in aqueous solutions. The hydrophobicity of the PS block causes the formation of spherical micelles in aqueous solutions. Increasing the solution pH ionizes the PAA block, which leads to an increase in micelle corona thickness due to repulsions between chains. Major trends observed in the rheological and interfacial properties can be understood in terms of expected changes in the micelle size and interfacial self-assembly with pH, ionic strength, and block length. Addition of NaOH was found to increase the solution pH and initially led to increases in solution viscosity, elasticity, surface tension, and film hydrophilicity. This effect was attributed to creation of larger micelles and greater inter-micellar repulsions as the PAA chain became more fully charged. However, when the concentration of NaOH exceeded a critical value, the solution viscosity, elasticity, and film hydrophilicity decreased. It is believed this was due to charge shielding by excess sodium ions, leading to shrinkage of the micelle corona and smaller micelles. Increasing the PS-PAA solution ionic strength by adding NaCl also provided charge shielding, as observed by decreases in solution viscosity and elasticity. Increasing the length of either block led to an increase in the solution viscosity, presumably due to formation of larger micelles and hence an increased effective micellar volume fraction. The dependence of the interfacial properties on block length was less straightforward. The solution surface tension and film hydrophilicity were both found to correlate well with the ratio of styrene units to acrylic acid units.

Ordered nanostructured materials are of enormous interest in photonics, sensing and detection arrays, catalysis, separations, microfluidics and low dielectric constant (low k) thin films. The traditional approach to preparation of these nanostructured films involves cooperative self-assembly of organic templates (including surfactants) and precursor molecules in solution. The approach has been successful in controlling the pore order at the local length scale, but has limited promise for controlling long-range pore order and orientation over the length scales of the device. We have developed a novel technique for the preparation of nanostructured metal oxide films by a rapid replication procedure of structured organic templates in supercritical carbon dioxide. The technique enables us to separate the self-assembly of the ordered template film from the formation of the metal oxide network. The structure of the pre-formed template on both the local and device levels can be achieved in three dimensions using established techniques prior to infusion of the inorganic phase. This approach also offers flexibility with regard to framework chemistry and the nature of the copolymer template, which can now be chosen independently without regard to compatibility in solution or concerns about disrupting the coordinated self-assembly process. The application of the technique to preparation of ordered mesostructured silicate/organosilicate films with dielectric constants below 2.2 and desirable mechanical properties is discussed. Post-synthesis treatments of mesostructured silicate and organosilicate films were also investigated with the aim of reducing the shrinkage associated with the template removal by calcination. In addition, blends of semicrystalline amphiphilic block copolymers and amorphous homopolymers was also studied to improve the wetting properties and reduce the crystallinity. Mesostructured silicate and organosilicate films that were templated with the blends exhibited a higher degree of mesostructural order as compared to the films templated by the amphiphilic block copolymers only.

Polyimides have been the material of choice for use as an insulating layer in electronic circuit boards. The solvent resistance and temperature stability as well as the insulating properties that polyimides possess make them ideally suited for this application. Polyimides are applied to circuit boards by spin-coating or sputter-coating a poly(amic-acid) precursor solution. Curing, solvent removal and imidization of the poly(amic-acid) to the polyimide, causes shrinkage stress. This shrinkage stress can cause failure of the film by cracking or delamination. An understanding of the stress in these systems would allow for better design of the circuit boards thereby avoiding these modes of failure. The swelling stresses which occur in fully cured polyimide films exposed to solvents/permeants is the focus of this dissertation. A small strain linear elastic analysis has been performed and has shown the applicability of the theory to transient swelling. Techniques have been developed to measure the swelling stresses and swelling strains of these films. The swelling behavior of poly(N,N$\sp\prime$-bis(phenoxyphenyl)-pyromellitimide) (PMDA-ODA) has been fully characterized in water and in methylene chloride. It was found that the swelling stress of uniaxially constrained PMDA-ODA in water is approximately seven MPa and in methylene chloride is greater than twenty MPa. The actual swelling stress on PMDA-ODA in methylene chloride is difficult to measure since the swelling stress is so large that the uniaxially constrained film buckles when exposed. The corresponding strains are approximately 0.3% for water exposure and 3.8% for methylene chloride exposure. The swelling coefficient of a material is the amount of length change experienced upon exposure to a permeant normalized by the mass uptake of the permeant. These experiments showed that the swelling coefficient of PMDA-ODA in water is 0.17 (m/m)/(g/g) and in methylene chloride/nitrogen vapor is 0.025 (m/m)/(g/g). The swelling coefficient of PMDA-ODA in methylene chloride is much lower than the swelling coefficient in water despite PMDA-ODA's greater strain in methylene chloride. This would indicate a much larger mass uptake of methylene chloride by the PMDA-ODA. Coupling between stress, temperature and solvent content was also observed. These observations indicate that a PMDA-ODA film under stress will not completely dry after solvent exposure unless the stress is lowered or the temperature is raised. This effect was most noticeable in the films exposed to methylene chloride, chloroform and acetone. PMDA-ODA films pretensioned to 50 MPa, held at constant strain and allowed to stress relax to an apparent equilibrium of 35 MPa, experienced a stress of zero when exposed to methylene chloride, chloroform or acetone. The large change in stress is the result of swelling and not the result of viscoelastic time effects.

<p> Colloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. </p><p> Organic ligand-capped CuInSe₂ (CIS) and Cu(In_XGa_1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSe_XS_2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence.</p><p> The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique.</p>

<p> Retinal prosthesis relies on the stimulation of living nerve tissue behind the rods and cones of the eye. The current state of the art relies on electrodes controlled by cameras which directly stimulate the nerve tissue to elicit a response to an image. These types of retinal implants have allowed for short-term crude vision in patients but have had limited long term success due to external battery packs and electroplating of the implanted electrodes.</p><p> Ionic stimulation is one of the principle mechanisms that sensory neurons utilize in the generation of an action potential. In a complex transduction pathway, ionic gradients are constantly altered inside the neuron by voltage sensors or mechanically controlled gates embedded in the neuronal cell membrane; responsible for the open and close state of these ion channels.</p><p> It has been demonstrated that local concentration increases of K⁺ by direct injection proximal to the nerve can elicit nerve firing at a concentration of 15-20 mM (3-4X normal concentration) increase in K⁺ concentration. As part of a larger concept of integrating biotechnology with nanofabrication, the materials for the development of potassium selective sequestration/storage and delivery were developed in the form of a redox-gated K⁺ selective crown ether.</p><p> The structure of the anthraquinone-based crown was deduced by computational simulation and stoichiometry of the complex confirmed by mass spec. along with 2D diffusion NMR techniques. In this instance, the stoichiometry could be controlled by the addition of different salts to give a 1:1 complex with large, aromatic anions and a 2:1 complex with smaller anions such as triflate. The synthesis of the molecule was optimized by computational modeling and simulations of transport through an artificial membrane. The selectivity of the architecture developed was specific for K⁺ over Na+, the other major ionic species present in the blood. The mechanism influencing the self-assembly of this class of compounds has much to do with the breakage of intramolecular &pi;-stacking interactions and the formation of stronger intermolecular &pi;-stacking interactions.</p><p> Finally, the transport of K⁺ through nanoporous membranes and single nanopores with novel PEG-type polymeric dispersions is demonstrated. This thesis concludes with future work toward developing more advanced transporters and proposes novel uses for anthraquinone-appended polymers as proton exchange membranes and DNA-base pair interchelators.</p>

<p> The realization of nanostructured functional materials by self-assembly in polymers and polymer nanocomposites is adversely affected by persisting structural defects which greatly diminish the performance of the material. The use of magnetic fields to impose long-range order is investigated in three distinct systems - ion-conducting block copolymers, semiconducting nanowire-polymer composites and lyotropic surfactant mesophases. The alignment process is quantitatively studied with X-ray scattering and microscopic methods. Time and temperature resolved data collected <i>in situ</i> during the magnetic experiments provide an insight into the thermodynamic and kinetic aspects of the process. These data together with simultaneous electrical conductivity measurements allow relating fundamental structural properties (e.g., morphology and long-range order) to transport properties (i.e., conductivity). In particular, it is demonstrated that magnetic fields offer a viable route for improvement of electric conductivity in these systems. More than an order of magnitude increase in conductivity is recorded in magnetically-annealed materials. The resulting aligned nanostructured systems are attractive for ordered solid polymer electrolyte membranes, heterojunction photovoltaic devices and generally help to understand charge transport mechanisms in anisotropic heterogeneous systems.</p>