Browsing by Subject "Nanocomposites"
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Item Combustion characteristics of A1 nanoparticles and nanocomposite A1+MoO3 thermites(2005-05) Granier, John J.; Pantoya, Michelle; Seshaiyer, Padmanabhan; Oler, James W.; Levitas, Valery; Berg, Jordan M.Scientific advances in material synthesis such as exploding wire technology, plasma nucleation and wet precipitation have enabled industrial manufacturers to produce metal and metal oxide powders with nanometer-sized particles. These processes have enabled better overall quality control (i.e. more definitive particle size, smaller particle size distributions, oxide coating control and decreased contaminate concentration) and faster production rates. Much interest has been formed in the science and application of nano-sized aluminum (nm-Al) combustion. A thermite (or aluminothermic) reaction is an oxidation reaction between aluminum and a metal oxide with highly exothermic energy release. Thermite reactions of traditional Al powder (typically micron-sized particles) and Iron-oxide have been used for decades in welding and other intense heat applications. Nano-thermite reactions, have shown unique properties in ignition sensitivity and deflagration (flame propagation) speeds which have propelled thermites to new realms of applications. The decrease in required ignition stimuli of nano-thermites is an improvement for many payload critical applications, but the ignition sensitivity also creates various hazards during material handling and seems to be a factor in decreased reactivity of aged nano-thermites. Nano-thermites have displayed reaction rates near detonation speeds presenting applications as more efficient incendiary devices. The precise particle size control of nano-thermites is leading researchers to develop highly-tunable energy release mechanisms that can be applied as heat signature flare decoys. Studies have shown that the thermite reaction of nm-Al+MoO3 has a large theoretical energy density [19], increased ignition sensitivity [23][8], and near detonation flame propagation speeds [5][6] in comparison to traditional micron-particle thermites. This work will present macroscopic combustion behaviors (such as flame speed) along with experimental results focusing on the molecular reactions and thermal properties of nanocomposite Al+MoO3 thermite materials This work will outline the successes and precautions of several nm-Al+MoO3 powder mixing methods and several cold-pressing techniques used to form compressed solid samples. A general relationship of sample density as a function of pressing force and with a systematic methodology is presented to allow other researchers to produce similar samples for future comparison. Second, results from laser experiments performed to determine flame speeds of nano and micron-sized Al+MoO3 composites through a range of sample densities. Flame propagation speeds were measured using high-speed digital video. Samples were also tested to determine thermal conductivity, specific heat and thermal diffusivity as a function of compressed sample density. Theories are presented for the unique trends of the nano and micron-composite results. Third, experimental work is presented analyzing the effects of pre-heated compressed nm-Al+MoO3 samples. Sample pre-heating is achieved by volumetric heating using an isothermal oven and by varying the applied laser power to allow conductive heating. Both methods of preheating show unique behaviors and elevated flame propagation speeds compared to previous results. Results and discussion of this work also discuss the difficulties and critical time response of using bare-wire thermocouples to accurately measure nano-thermite reaction temperatures. Fourth, a series of DSC/TGA experiments were performed on the reaction of Al and gaseous oxygen to analyze the purest and ¡¥simplest¡¦ form of the Al oxidation (void of any reaction mechanisms dependent on the metal-oxide decomposition). Results are presented showing unique reaction onset temperatures, oxidation rates and activation energies for nano and micron-Al reacting in a gaseous oxygen environment. Fifth, a series of DSC/TGA experiments were performed on the reaction of Al and nano-MoO3. Results are presented for reaction onset temperatures, peak temperatures, heat of reaction values, and activation energies for Al+MoO3 composites with Al particles ranging from 50 nm to 20 ƒÝm. A final set of experiments was designed using the DSC/TGA to determine reaction duration and reaction self-propagation criteria for Al particle sizes ranging from 50 nm to 20 ƒÝm. Heating programs were manipulated for micron and nano-Al+MoO3 samples to determine the relationship between sample heating rate and reaction mechanisms. DSC tests were done using isothermal time intervals displaying that the nm-Al+MoO3 reactions are temperature dependent and not self-sustaining. Isothermal time intervals applied to ƒÝm-Al+MoO3 reactions displayed a delayed peak temperature. Finally, all of the results and experiments are combined as evidence in support of a single theory of the oxidation reaction of spherical Al particles. The presented results portray unique evidence in support of the nano and micron-sized Al reaction characteristics.Item Combustion characteristics of aluminum nanoparticles and nanocomposite aluminum+moly-trioxide thermites(Texas Tech University, 2005-05) Granier, John J.; Pantoya, Michelle; Seshaiyer, Padmanabhan; Oler, James W.; Levitas, Valery; Berg, Jordan M.Scientific advances in material synthesis such as exploding wire technology, plasma nucleation and wet precipitation have enabled industrial manufacturers to produce metal and metal oxide powders with nanometer-sized particles. These processes have enabled better overall quality control (i.e. more definitive particle size, smaller particle size distributions, oxide coating control and decreased contaminate concentration) and faster production rates. Much interest has been formed in the science and application of nano-sized aluminum (nm-Al) combustion. A thermite (or aluminothermic) reaction is an oxidation reaction between aluminum and a metal oxide with highly exothermic energy release. Thermite reactions of traditional Al powder (typically micron-sized particles) and Iron-oxide have been used for decades in welding and other intense heat applications. Nano-thermite reactions, have shown unique properties in ignition sensitivity and deflagration (flame propagation) speeds which have propelled thermites to new realms of applications. The decrease in required ignition stimuli of nano-thermites is an improvement for many payload critical applications, but the ignition sensitivity also creates various hazards during material handling and seems to be a factor in decreased reactivity of aged nano-thermites. Nano-thermites have displayed reaction rates near detonation speeds presenting applications as more efficient incendiary devices. The precise particle size control of nano-thermites is leading researchers to develop highly-tunable energy release mechanisms that can be applied as heat signature flare decoys. Studies have shown that the thermite reaction of nm-Al+MoO3 has a large theoretical energy density [19], increased ignition sensitivity [23][8], and near detonation flame propagation speeds [5][6] in comparison to traditional micron-particle thermites. This work will present macroscopic combustion behaviors (such as flame speed) along with experimental results focusing on the molecular reactions and thermal properties of nanocomposite Al+MoO3 thermite materials This work will outline the successes and precautions of several nm-Al+MoO3 powder mixing methods and several cold-pressing techniques used to form compressed solid samples. A general relationship of sample density as a function of pressing force and with a systematic methodology is presented to allow other researchers to produce similar samples for future comparison. Second, results from laser experiments performed to determine flame speeds of nano and micron-sized Al+MoO3 composites through a range of sample densities. Flame propagation speeds were measured using high-speed digital video. Samples were also tested to determine thermal conductivity, specific heat and thermal diffusivity as a function of compressed sample density. Theories are presented for the unique trends of the nano and micron-composite results. Third, experimental work is presented analyzing the effects of pre-heated compressed nm-Al+MoO3 samples. Sample pre-heating is achieved by volumetric heating using an isothermal oven and by varying the applied laser power to allow conductive heating. Both methods of preheating show unique behaviors and elevated flame propagation speeds compared to previous results. Results and discussion of this work also discuss the difficulties and critical time response of using bare-wire thermocouples to accurately measure nano-thermite reaction temperatures. Fourth, a series of DSC/TGA experiments were performed on the reaction of Al and gaseous oxygen to analyze the purest and ¡¥simplest¡¦ form of the Al oxidation (void of any reaction mechanisms dependent on the metal-oxide decomposition). Results are presented showing unique reaction onset temperatures, oxidation rates and activation energies for nano and micron-Al reacting in a gaseous oxygen environment. Fifth, a series of DSC/TGA experiments were performed on the reaction of Al and nano-MoO3. Results are presented for reaction onset temperatures, peak temperatures, heat of reaction values, and activation energies for Al+MoO3 composites with Al particles ranging from 50 nm to 20 ƒÝm. A final set of experiments was designed using the DSC/TGA to determine reaction duration and reaction self-propagation criteria for Al particle sizes ranging from 50 nm to 20 ƒÝm. Heating programs were manipulated for micron and nano-Al+MoO3 samples to determine the relationship between sample heating rate and reaction mechanisms. DSC tests were done using isothermal time intervals displaying that the nm-Al+MoO3 reactions are temperature dependent and not self-sustaining. Isothermal time intervals applied to ƒÝm-Al+MoO3 reactions displayed a delayed peak temperature. Finally, all of the results and experiments are combined as evidence in support of a single theory of the oxidation reaction of spherical Al particles. The presented results portray unique evidence in support of the nano and micron-sized Al reaction characteristics.Item Electrical and Thermal Experimental Characterization and Modeling of Carbon Nanotube/Epoxy Composites(2012-10-19) Gardea, FrankThe present work investigates the effect of carbon nanotube (CNT) inclusions on the electrical and thermal conductivity of a thermoset epoxy resin. The characterization of electrical and thermal conductivity of CNT/epoxy composites is presented. Pristine, oxidized, and fluorine-functionalized unpurified CNT mixtures ("XD grade") were dispersed in an epoxy matrix, and the effect of stirring rate and pre-curing of the epoxy on the dispersion of the CNTs was evaluated. In order to characterize the dispersion of the CNTs at different length scales, Optical Microscopy (OM), Raman Spectroscopy, and Scanning Electron Microscopy (SEM) was performed. Samples of varying CNT weight fractions were fabricated in order to find the effect of CNT weight fraction on thermal and electrical conductivity. Electrical conductivity was measured using a dielectric spectrometer, and thermal conductivity was determined by a transient plane source thermal analyzer. It was found that electrical conductivity increases by orders of magnitude for the pristine and oxidized XD CNT composites relative to the neat epoxy matrix, while fluorinated XD CNT composites remain electrically non-conductive. A small, but significant, increase in thermal conductivity was observed for pristine, oxidized, and fluorinated XD CNT composites, showing a linear increase in thermal conductivity with increasing CNT weight fraction. Pristine XD CNTs were ball-milled for different times in order to reduce the degree of agglomeration and entanglement of CNTs, and composites were fabricated using the same technique as with non-milled XD CNTs. Using ball-milled CNTs shows improved dispersion but results in an electrically non-conductive composite at the CNT weight fractions tested. The thermal conductivity of the ball-milled CNT samples shows an initial increase higher than that of non-milled pristine, oxidized, and fluorinated XD CNTs, but remains constant with increasing CNT weight fraction. A micromechanics model based on the composite cylinders method was implemented to model the electrical and thermal conductivity of the CNT/epoxy composites. Nanoscale effects in electrical and thermal conduction, such as electron hopping and interface thermal resistance, respectively, were incorporated into the model in order to accurately predict the acquired results. Modeling results show good agreement with acquired experimental results.Item Fire retardant polyamide 11 nanocomposites/elastomer blends for selective laser sintering(2016-05) Ortiz, Rogelio; Bourell, David Lee; Koo, Joseph H.Additive manufacturing (AM) had previously been used solely for prototyping and visualization purposes, but in recent years, this technique has shifted to the idea of producing end-use parts. This has already been successfully done in some areas via selective laser sintering (SLS). Unfortunately, current polymeric materials for processing via SLS do not meet the requirements of the majority of commercial applications. Hence, this thesis presents efforts to develop a multifunctional polyamide 11 (PA11) polymer with enhanced thermal, mechanical, and flammability properties for SLS through the use of nanotechnology.Item Flame retardant nylon 6 nanocomposite fibers : processing and characterization(2016-08) Wu, Hao, Doctorate in materials science and engineering; Krifa, Mourad; Koo, Joseph H.; Li, Wei; Ellison, Christopher J; Chen, Jonathan YOne type of engineering thermoplastic polymers that has significant commercial application is nylon. However, flammability and melt dripping is a major problem for polymers like nylon 6 because it can cause fire to spread to other flammable objects and escalate the fire in a short amount of time. Although high performance inherently flame-retardant (FR) fibers have been discovered and various durable FR finishes for nylon have been developed, cost-effective flame retardant nylon and nylon blend fabrics remain a challenge. The goal of this research is to develop non-drip inherently FR nylon 6 fibers as a cost-effective alternative for use in high volume FR fabrics. In this dissertation, a cost effective alternative of producing non-drip inherently flame retardant nylon 6 fibers with balanced performances was developed based on polymer nanocomposite systems incorporating intumescent FR and nanoclay additives. Nanoclay was added to the system to reduce FR particle loading and capitalize on the synergistic effect between nanoclay and intumescent additives. Adequate dispersion of the additives with exfoliation of the nanoclay platelets was observed using TEM and XRD. Injection molding was used as a tool for screening the performance of the nanocomposite formulations in bulk form before the fiber spinning process. Results of injection molded FR PA6 nanocomposites suggest that although a good FR performance could be achieved, mechanical properties, especially ductility, were significantly compromised. To solve this problem, rubber toughening was achieved using a thermoplastic elastomer with significant success in recovering material ductility without compromising FR performance. Ultra-sonication of the FR additives prior the fiber spinning could effectively reduce the FR particle size distribution. Single fiber tensile tests show that PA6/FR/elastomer/nanoclay formulation is able to improve both the tenacity and elongation at break from the original PA6/FR system. Moreover, flammability tests suggest that the nanocomposite FR fibers have significantly lower heat release properties and are able to retain a fibrous shape after combustion indicating the non-dripping property. Therefore, our experiments have yielded improved non-drip FR properties in PA6 through the infusion of nanoclay and non-halogenated intumescent particles (FR) via co-rotating twin-screw extrusion. One major implication of these results is that with the new non-drip FR nylon 6 fiber, it would be possible to achieve blends with higher nylon content than customary and not compromise the FR performance of the fabric, thus providing a cost effective solution for high-volume applications.Item Functional nanocomposite fibers through electrospinning : flame retardant and superhydrophobic(2012-05) Wu, Hao; Krifa, Mourad; Koo, Joseph H.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.Item Melt processed polymer-organoclay nanocomposites(2011-12) Spencer, Matthew Walter; Paul, Donald R.; Freeman, Benny D.; Ellison, Christopher J.; Kovar, Desiderio; Sanchez, Isaac C.Polymer nanocomposites with organoclay fillers offer improved properties and performance, providing opportunities for commercial applications. The key to significant property enhancement is to exfoliate the individual organoclay platelets into the polymer matrix to utilize their high aspect ratio and modulus. The affinity between the polymer matrix and the organoclay is one of the most important factors for determining the exfoliation level. Although polar polymers, such as nylon 6, exfoliate the organoclay well, hydrophobic matrices, such as polyolefins, generally do not effectively exfoliate the organoclay. Thus, a significant part of this work investigates various routes to improve polyolefin-organoclay interactions and organoclay exfoliation in these systems. Nanocomposites formed from organoclay and blends of high density polyethylene and maleic anhydride-grafted high density polyethylene over the entire range of compositions were melt processed to obtain further insights into the 'compatiblizing' role of maleated polyolefins. The organoclay particle aspect ratio was found to initially increase drastically, reach a maximum, and slightly decrease with increased maleation. As the maleation level increases, the relative modulus increases initially and then levels off at higher loadings To a certain extent, the affinity between the polymer and the organoclay can be enhanced by optimizing the organoclay structure for a given polymer matrix. A silanized organoclay was investigated to determine if reduced agglomeration, improved exfoliation, and matrix reinforcement could be achieved in a polypropylene matrix without using a more costly compatibilizer. The silanized organoclay was found to be superior to the non-silanized precursor, but did not achieve the benefits obtained with a compatibilized matrix. Ionomer matrices have also been used as a means of improving organoclay exfoliation. This study examined the effects of ion type (K⁺, Na⁺), neutralization level, and melt index on the nanocomposite morphology and properties. The Na⁺ ionomers appear to have more favorable interactions with the organoclay. Exfoliation and matrix reinforcement tend to increase with decreased melt index and with increased neutralization, except at high levels. In these cases, it is possible that the additional exfoliation results in particles with lower aspect ratios. Composite properties are highly dependent on the particle aspect ratio. Several theories were used to predict the modulus and the thermal expansion coefficient of composites based on the filler aspect ratio. Novel two-population approaches were applied to enable the modeling of nanocomposites containing organoclay tactoids and single platelets, organoclay particles and glass fibers, or organoclay and elastomer particles. The quantitative agreement between the values predicted using experimentally determined particle aspect ratios and experimental modulus and thermal expansion was vastly improved using these methods.Item Modification of surfaces using grafted polymers : a self consistent field theory study(2011-08) Trombly, David Matthew; Ganesan, Venkat; Peppas, Nicholas; Ren, Pengyu; Sanchez, Isaac; Truskett, ThomasThis 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.Item Multiphase polymer nanocomposites(2010-08) Yoo, Youngjae; Paul, Donald R.; Freeman, Benny D.; Sanchez, Isaac C.; Kovar, Desiderio; Ellison, Christopher J.Polymer nanocomposites with organoclay fillers offer improved performance and opportunities for commercial applications. The key to significant property enhancement is to exfoliate the individual organoclay platelets into the polymer matrix to utilize their high aspect ratio and modulus. The affinity between the polymer matrix and the organoclay is one of the most important factors for determining the exfoliation level. To a certain extent, the affinity can be enhanced by optimizing the organoclay structure for a given polymer matrix. Numerous studies have demonstrated that nanocomposites provide significant enhancements in stiffness and strength, flame retardancy, gas barrier properties, thermal stability and ionic conductivity. However, most polymer nanocomposites have decreased toughness relative to that of the matrix polymer. One exception to this general rule was found for nanocomposites based on poly(ethylene-co-methacrylic acid) ionomer prepared by melt compounding. My initial work investigated this system using an instrumented impact test. The data were analyzed using the essential work of fracture (EWF) methodology. Transmission electron microscopy (TEM) revealed that the clay platelets were well exfoliated in this matrix. It has also been observed that addition of organoclays to polymer blends can greatly reduce the size of the dispersed phase in some cases. It was thought that this feature might be useful for controlling rubber particle size, and, therefore, the toughness of polyamide/elastomer blends. Initially, I investigated the effect of the organoclay structure on the extent of exfoliation and properties of the nanocomposites. Nanocomposites based on the organoclays with one alkyl tail and hydroxyl ethyl groups gave well-exfoliated structures and high matrix reinforcement while nanocomposites from two-tailed organoclay contain a considerable concentration of intercalated stacks. Nanocomposites from the organoclays with one alkyl tail showed slightly better exfoliation and matrix reinforcement than those from the organoclays with hydroxyl ethyl groups. Based on this research result, the toughening response of amorphous polyamide nanocomposites using two types of elastomers, EOR and EOR-g-MA, four types of organoclays, M3(HT)1, M2(HT)2, M1H1(HT)2 and (HE)2M1T1, and two mixing protocols, has been investigated. Glass fibers (diameter ~ 12 m) are frequently used to reinforce polyamides. However, there is a practical limit to the amount of fiber that can be added while maintaining processability. Another possible use of organoclays is as an additional filler that acts on the nanoscale to complement the micro-scale reinforcement of the glass fibers. The possible synergies of simultaneous reinforcement at these very different length scales were explored and the composite moduli were compared to theoretical predictions using aspect ratios determined from TEM images.Item Multiscale analysis of nanocomposite and nanofibrous structures(2009-05-15) Unnikrishnan, Vinu UnnithanThe overall goal of the present research is to provide a computationally based methodology to realize the projected extraordinary properties of Carbon Nanotube (CNT)- reinforced composites and polymeric nanofibers for engineering applications. The discovery of carbon nanotubes (CNT) and its derivatives has led to considerable study both experimentally and computationally as carbon based materials are ideally suited for molecular level building blocks for nanoscale systems. Research in nanomechanics is currently focused on the utilization of CNTs as reinforcements in polymer matrices as CNTs have a very high modulus and are extremely light weight. The nanometer dimension of a CNT and its interaction with a polymer chain requires a study involving the coupling of the length scales. This length scale coupling requires analysis in the molecular and higher order levels. The atomistic interactions of the nanotube are studied using molecular dynamic simulations. The elastic properties of neat nanotube as well as doped nanotube are estimated first. The stability of the nanotube under various conditions is also dealt with in this dissertation. The changes in the elastic stiffness of a nanotube when it is embedded in a composite system are also considered. This type of a study is very unique as it gives information on the effect of surrounding materials on the core nanotube. Various configurations of nanotubes and nanocomposites are analyzed in this dissertation. Polymeric nanofibers are an important component in tissue engineering; however, these nanofibers are found to have a complex internal structure. A computational strategy is developed for the first time in this work, where a combined multiscale approach for the estimation of the elastic properties of nanofibers was carried out. This was achieved by using information from the molecular simulations, micromechanical analysis, and subsequently the continuum chain model, which was developed for rope systems. The continuum chain model is modified using properties of the constituent materials in the mesoscale. The results are found to show excellent correlation with experimental measurements. Finally, the entire atomistic to mesoscale analysis was coupled into the macroscale by mathematical homogenization techniques. Two-scale mathematical homogenization, called asymptotic expansion homogenization (AEH), was used for the estimation of the overall effective properties of the systems being analyzed. This work is unique for the formulation of spectral/hp based higher-order finite element methods with AEH. Various nanocomposite and nanofibrous structures are analyzed using this formulation. In summary, in this dissertation the mechanical characteristics of nanotube based composite systems and polymeric nanofibrous systems are analyzed by a seamless integration of processes at different scales.Item Roles of nanofiller structure on mechanical behavior of thermoplastic nanocomposites(Texas A&M University, 2006-10-30) Weon, Jong IlTraitedness has been described as the ??????the degree to which a particular trait structure is approximated in a given person?????? (Tellegen, p. 28, 1991) and has been hypothesized as one explanation for findings of weak trait-behavior relationships. That is, if traits are differentially applicable to different individuals, then trait-behavior relationships may be moderated based on the strength with which an individual fits with a given trait model. This study used moderated multiple regression to test the moderating effects of four different traitedness indicators to increase the prediction of diagnostic consistency in four personality disorders, and also tested the main effects of traitedness estimates to predict cross-situational consistency of functional impairment. Traitedness estimates performed better in the prediction of increased diagnostic consistency, though there were some isolated findings of traitedness increasing crosssituational consistency of functional impairment. orientation of the clay in the nanocomposite and the simple shear process. It is found that the modulus, strength, and heat distortion temperature of the nanocomposites decrease as the clay aspect ratio and degree of orientation are reduced. The micromechanics-based models accurately describe the relationship between clay structural parameters and the corresponding moduli for exfoliated nanocomposites. The impact fracture mechanisms of polypropylene (PP)-calcium carbonate (CaCO3) nanoparticles have been investigated. A detailed investigation reveals that the CaCO3 nanoparticles act as stress concentrators to initiate massive crazes, followed by shear banding in the PP matrix.Item Synthesis, stabilization, and controlled assembly of organic and inorganic nanoparticles for therapeutic and imaging applications(2009-12) Tam, Jasmine Man-Chi; Johnston, Keith P., 1955-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.