Dissertation (Ph.D.)--Auburn University, 2009. / Abstract. Includes bibliographic references.

In this dissertation, I have investigated applying the laser ablation of microparticle aerosol (LAMA) process to the production of nanocomposite polymer membranes for olefin/paraffin separation. Experimental results for three major thrusts are presented: 1) an investigation into the scalability of the LAMA process, 2) a new laser ablation technique for nanoparticle production from aqueous feedstocks, and 3) characterization of olefin-selective polymer nanocomposite membranes produced using LAMA. The propensity for Ag nanoparticles to form agglomerates in LAMA is investigated. Nanoparticle samples were collected on TEM grids at several feedstock aerosol densities. As the density increased, the particle morphology shifted from single nanoparticles 5 nm in diameter to chained agglomerates of 20 nm diameter primary particles. The results are in agreement with a numerical model of Brownian agglomeration and diffusion. Factors influencing nanoparticle morphology, such as temperature, initial nanoparticle charge, and feedstock aerosol density are discussed. It is shown that agglomeration occurs on a much longer timescale than the other processes, and can be treated independently. A new nanoparticle synthesis technique is presented: laser ablation of aqueous aerosols. A Collison nebulizer is used to generate a mist of ~10 [mu]m diameter water droplets containing dissolved transition metal salts. Water from the droplets quickly evaporates, leaving solid particles which are ablated by an excimer laser. Ablation results in plasma breakdown and photothermal decomposition of the feedstock material. For AgNO₃ ablated in He gas, metallic Ag nanoparticles were produced. For Cu(NO₃)₂ ablated in He gas, crystalline Cu₂O nanoparticles were produced. For Ni(NO₃)₂ ablated in He gas, crystalline NiO nanoparticles were produced. A combination of AgNO₃ and Cu(NO₃)₂ ablated in a reducing atmosphere of 10%H₂/He yielded nonequilibrium Ag-Cu alloy nanoparticles. Membranes composed of poly(ethylene glycol diacrylate) (PEGDA) and Ag nanoparticles were produced by the LAMA process. Permeation and sorption measurements for the light olefins and paraffins were conducted for these membranes. The membranes showed very little improvement in olefin/paraffin selectivity compared with neat PEGDA membranes. Using the LAMA implementation described here, it was impossible to produce membranes with high Ag loading. Whether membranes containing more Ag would exhibit improved selectivity remains an open question. / text

Flame retardant (FR) intumescent additives and montmorillonite (MMT) organoclay incorporated nylon-6 nanocomposite (FR-NC-PA6) fibers with a diameter of about 200 nm were fabricated by electrospinning. Before electrospinning, dispersion and exfoliation of the FR additive and MMT in nylon-6 were achieved by twin-screw extrusion. Tensile, TGA and UL-94 flammability tests were first performed using injection-molded bulk samples. The tensile modulus of FR-NC-PA6 was 45% higher than that of neat PA6, but tensile strength and elongation at break decreased by 23% and 98.7%, respectively. It is worth noting that although the TGA results show that FR-NC-PA6 has a slightly earlier decomposition temperature than neat PA6, it did not drip under fire and had the best rating (V-0) in UL 94 test, while neat PA6 is only rated as V-2. SEM and EDX of char residues after the UL 94 test clearly show the oxygen-rich protective char layer on the surface. These results indicate the advantage of using clay and FR additive in bulk-form PA6. Flammability of electrospun nanocomposite fibers was characterized by Micro-combustion calorimeter (MCC), a small-scale test to screen flammability of polymer materials. The MCC results show that the nano-fillers in both bulk and fiber form could effectively improve flame retardant properties of the material. Electrospun fibers had similar combustion properties as bulk materials. In addition to FR applications, superhydrophobic surface was another area that was explored using the electrospun nanocomposite fibers. Static water contact angle (WCA) test showed that samples with 5wt% clay even without plasma treatment greatly improved the WCA to 140°, probably due to the barrier effect of nanoclay platelets. Plasma treatment was used to modify the surface energy, further improving WCA to as high as 160°. However, fiber structure was partially etched away when overexposed to the plasma. This etching effect increased the surface roughness. Clay incorporated samples had higher level of surface roughness and better resistance to plasma etching compared to neat nylon 6. / text

Nanoparticles have garnered much attention in pharmaceutical and biomedical fields because their small size and high surface area facilitate drug absorption, improve access to cells and organs, and enhance optical imaging. However, delivery of nanoparticles to the body is not always feasible or effective. Here, nanoparticle assemblies (flocs or clusters) for pulmonary drug delivery and biomedical imaging in cells are shown to facilitate delivery, interactions with cells, and manipulation of optical properties of inorganic/organic nanocomposites. The formation of aggregates by physical techniques and their mechanisms are described in detail. For pulmonary delivery, particles with aerodynamic diameters between 1-5 [mu]m deposit efficiently in the deep lungs. However, crystalline, non-porous, poorly water soluble drugs of this size require long dissolution times, limiting absorption by the body. Therefore, drug dissolution must be “decoupled” from deposition to improve absorption. To address this challenge, drug nanoparticles were dispersed within 4-[mu]m water droplets when administered via nebulization or as micron-sized flocs using a pressurized metered dose inhaler (pMDI). Upon deposition in aqueous media, the aerosolized nanoparticle assemblies dissociated into constituent nanoparticles, raising the available surface area for dissolution and increasing dissolution rates, relative to solid particles. Poorly water soluble drug nanoparticles were prepared using a controlled precipitation (CP) or thin film freezing (TFF) process, in which stable nanoparticles (30-300 nm in diameter) with high potencies (>90 wt% drug) were produced by rapidly nucleating drug solutions in the presence of strongly adsorbing polymers or by freezing, respectively. Amorphous, nanoparticles prepared by CP produced stable aqueous dispersions with high fine particle fractions (FPF) of 77% and total emitted doses (TED) of 1.5 mg/min upon nebulization. CP and TFF also produced anisotropic particles (aspect ratios >5), which formed stable suspensions in a hydrofluoroalkane propellant. Inefficient packing of anisotropic particles formed loose, open flocs that stacked upon each other to prevent settling. Upon pMDI actuation, atomized propellant droplets shear apart and template portions of the floc to yield porous particles with high FPFs (49-64%) and TEDs (2.4 mg/actuation). The controlled assembly of gold nanoparticles into clusters is also of great interest for biomedical imaging and therapy because clusters exhibit improved near infrared absorbance (where blood and tissue are most transparent), relative to single spherical particles, and can biodegrade into clearable particles. Gold nanoparticles (5 nm) were assembled into clusters between 30 to 100 nm in diameter with high gold loadings, resulting in strong NIR absorbance. The assembly was kinetically controlled with weakly adsorbing polymers by manipulating electrostatic, van der Waals, steric, and depletion forces. Furthermore, clusters assembled with a biodegradable polymer deaggregated back into primary particles in physiological media and within cells. This kinetic assembly platform is applicable to a wide variety of fields that require high metal loadings and small particle sizes. / text

Conductive and flexible nanocomposites can have wide applications in textiles, including wearable sensors, antenna, electrodes, etc. The objective of this research is to develop electrically conductive fibers and films that are flexible and deformable for use in textile structures able to accommodate the drape and movement of the human body. To achieve this objective, we evaluate the electrical properties of PEDOT:PSS/nickel nanostrand as well as nylon 6/nickel nanostrand nanocomposites. Nickel nanostrands (NiNS) were first used to reinforce an intrinsically conductive polymer, Poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), in order to fabricate nanocomposite films with high electrical conductivity. The electrical properties of the films were evaluated by the Van der Pauw method. The addition of 10 wt% nanostrands in PDOT:PSS provided a two order of magnitude improvement in electrical conductivity. In addition to PDOT:PSS, nylon 6/NiNS nanocomposite fibers were produced using electrospinning and exhibited diameters in the sub-micron range. The NiNS-reinforced fibers had electrical conductivity that exceeded the ESD range, which offers the potential for use in protective textile applications. / text

D. Tech. Chemical Engineering. / This research aims to improve the current state of wastewater treatment technologies by exploiting the characteristics and capabilities of nanomaterials. Also, it aims at protecting the environment and human health by minimizing exposure of toxic contaminants found in waters sources by treatment with cheaply engineered materials. The nanocomposites that will be employed in this study have shown to be effective for removing a number of heavy metals from aqueous solutions during trial experiment. The study is therefore carried out to reduce the water scarcity in South Africa by minimizing the contamination of remaining water resources. With industrial effluents the main targets, the aim is to design systems that will enable industries to recycle their wastewater instead of discharging into the environment. This study will therefore benefit the communities who solely depend on surface and ground water and again it will safe industrial bodies high costs of treating their wastewater with ineffective conventional methods. The research focuses on the application of polypyrrole-clay nanocomposites for removing heavy metals from wastewater streams. The research conducted hereby highlights the application of polymer based nanocomposites as suitable adsorbents for the remediation of the toxic chromium(VI) [Cr(VI)] from water. The work describes the preparation and characterization of the nanocomposites, their application to wastewater laden with Cr(VI) in both batch and continuous adsorption and finally understanding the adsorbent-adsorbate interactions and sorption mechanisms under various physico-chemical conditions.

M. Tech. Department of Chemical, Metallurgical and Materials Engineering. / Discusses how to synthesize a titanium-manganese oxide composite electrode with improved supercapacitive properties. The research aim was achieved through the following objectives: 1. the mechanisms of the synergistic incorporation of manganese oxide for improving the supercapacitive properties of titanium oxide electrodes. 2. Investigate possible metallurgical interactions and phenomenon during the sintering of the composite. 3. Investigate the electrochemical characteristics of titanium-manganese composite electrodes.

D.Tech. Chemical Engineering / This research emphasizes the potential application of conducting polymer based nanocomposites for the remediation of contaminants from water. This study facilitates the preparation of conducting polymer based nanomaterials for the efficient removal of fluoride and toxic chromium(VI) from water. This work also identifies the importance of understanding the physico-chemical properties of the synthesized nanomaterials which greatly influence the materials performance in removing contaminants from water.

This research focuses on the modeling of surfaces decorated by grafted polymers in order to understand their structure, energetics, and phase behavior. The systems studied include flat and curved surfaces, grafted homopolymers and random copolymers, and in the presence of solvent conditions, homopolymer melt conditions, and diblock copolymer melt conditions. We use self-consistent field theory to study these systems, thereby furthering the development of new tools especially applicable in describing curved particle systems and systems with chemical polydispersity. We study a polymer-grafted spherical particle interacting with a bare particle in a good solvent as a model system for a polymer-grafted drug interacting with a blood protein in vivo. We calculate the energy of interaction between the two particles as a function of grafting density, particle sizes, and particle curvature by solving the self-consistent field equations in bispherical coordinates. Also, we compare our results to those predicted by the Derjaguin approximation. We extend the previous study to describe the case of two grafted particles interacting in a polymer melt composed of chains that are chemically the same as the grafts, especially in the regime where the particle curvature is significant. This is expected to have ramifications for the dispersion of particles in a polymer nanocomposite. We quantify the interfacial width between the grafted and free polymers and explore its correlation to the interactions between the particles, and use simple scaling theories to justify our results. In collaboration with experimentalists, we study the behavior of the glass transition of polystyrene (PS) films on grafted PS substrates. Using the self consistent field theory methods described above as well as a percolation model, we rationalize the behavior of the glass transition as a function of film thickness, chain lengths, and grafting density. Grafting chemically heterogeneous polymers to surfaces in melt and thin film conditions is also relevant for both particle dispersion and semiconductor applications. To study such systems, we model a random copolymer brush in a melt of homopolymer that is chemically identical to one of the blocks. We modify the self-consistent field theory to take into account the chemical polydispersity of random copolymer systems and use it to calculate interfacial widths and energies as well as to make predictions about the window in which perpendicular morphologies of diblock copolymer are likely to form. We also explore the effect of the rearrangement of the chain ends on the surface energy and use this concept to create a simple modified strong stretching theory that qualitatively agrees with our numerical self-consistent field theory results. We explicitly study the system that is most relevant to semiconductor applications - that of a diblock copolymer melt on top of a substrate modified by a random copolymer brush. We explore the morphologies formed as a function of film thickness, grafting density, chain length, and chain blockiness, and make predictions about the effect of these on the neutral window, that is, the range of brush volume fractions over which perpendicular lamellae are expected to occur. / text

<p>Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems. </p><p>This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs. </p><p>Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg. </p><p>Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention). </p><p>Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.</p> / Dissertation