Materials Science and Engineering Commons^™

Structure And Properties Of Cnt Yarns And Cnt/Cnf Reinforced Pan-Based Carbon Fibers, Nitilaksha Phalaxayya Hiremath

Doctoral Dissertations

There is continuing effort to enhance the strength and modulus of carbon fibers by various combinations of materials and processing. Carbon fibers are produced from various precursors, and the strength of the CFs are directly related to the type of precursor used to make them. Carbon Nanotubes (CNTs) have received a great deal of attention due to their unique structure and properties. Major focus of this research is on the evaluation of processing, structure and properties of CNT based yarns and composite fibers.

High strength and low cost carbon fibers (CFs) are needed for today’s applicatio ns. A low cost …

Strategies For Controlling Bulk Heterojunction Morphology, Zach Daniel Seibers

Doctoral Dissertations

Organic photovoltaic devices have been extensively studied as a means to produce sustainable energy. However, the performance of organic-photovoltaic (OPV) devices is dependent upon a number of factors including the morphology of the active layer, device architecture, and processing conditions. Recent research has indicated that fullerenes in the bulk heterojunction are entropically driven to the silicon and air interfaces upon crystallization of P3HT, which occurs during thermal annealing. The first chapter of this research focuses on investigating the structure and function of end-tethered poly(3-hexylthiophene) chains to a transparent electrode as an anode buffer layer. Neutron reactivity reveals that these P3HT …

Nanoscience At Interfaces And Surfaces: From Jamming To Electrode Texturing, Mengmeng Cui

Doctoral Dissertations

This dissertation focuses on the nanoparticle self-assembly on the liquid/liquid interface and the nanomaterial modification on surface. The self-assembly of nanoparticles at the liquid/liquid interface was utilized to trap non-equilibrium morphology when the nanoparticles reach jamming state. The dynamics of jammed systems were further studied by X-ray photon correlation spectroscopy. For the surface part, the nanomaterials were modified on the electrodes to improve the performance of microbial electrosynthesis. Also, a novel and simple method was developed to prepare nanomaterials including nanoparticle surfactants and carbon nanotubes (CNTs).

Evaporation Induced Self-Assembly And Characterization Of Nanoparticulate Films: A New Route To Bulk Heterojunctions, Yipeng Yang

Doctoral Dissertations

Polymer-based semiconducting materials are promising candidates for large-scale, low-cost photovoltaic devices. To date, the efficiency of these devices has been low in part because of the challenge of optimizing molecular packing while also obtaining a bicontinuous structure with a characteristic length comparable to the exciton diffusion length of 10 to 20 nm. In this dissertation we developed an innovative evaporation-induced nanoparticle self-assembly technique, which could be an effective approach to fabricate uniform, densely packed, smooth thin films with cm-scale area from home-made P3HT nanoparticles. Unlike the previous reports of nanoparticle-based film formation, we use a mixture of two solvents so …

Biopolymer Electrospun Nanofiber Mats To Inactivate And Remove Bacteria, Katrina Ann Rieger

Doctoral Dissertations

The persistence of antibiotic resistance in bacterial pathogens remains a primary concern for immunocompromised and critically-ill hospital patients. Hospital associated infections can be deadly and reduce the successes of medical advancements, such as, cancer therapies and medical implants. Thus, it is imperative to develop materials that can (i) deliver new antibiotics with accuracy, as well as (ii) uptake pathogenic microbes. In this work, we will demonstrate that electrospun nanofiber mats offer a promising platform for both of these objectives because of their high surface-to-volume ratio, interconnected high porosity, gas permeability, and ability to contour to virtually any surface. To provide …

Engineering Polymers Through Impact Modification And Superheated Liquid Processing, Gregory Connor Evans

Doctoral Dissertations

A new approach to toughen anionically polymerized polyamide 6 (aPA6) was applied using reaction induced phase separation (RIPS). This method solved issues with particle dispersion, mixture viscosity, and additive concentration common with conventional rubber toughening thereby making it an ideal candidate for fiber reinforced aPA6 reaction injection molding (RIM). Octamethylcyclotetrasiloxane (D₄) was used as a functional additive that undergoes RIPS during aPA6 polymerization and polymerizes to polydimethylsiloxane (PDMS). Controlled phase separation, modulus retention, and increased crystallinity were achieved at low additive concentrations. Optimal properties were achieved with 2 wt% D₄. Fracture energy was measured at high …

Polymer And Small Molecule Designs For Anion Conducting Membranes: Connected Ion-Channel Morphologies And Highly Alkaline Stable Ammonium Cations, Sedef P. Ertem

Doctoral Dissertations

Fuel cells are one of the oldest sustainable energy generation devices, converting chemical energy into electrical energy via reverse-electrolysis reactions. With the rapid development of polymer science, solid polymer electrolyte (SPE) membranes replaced the conventional liquid ion transport media, rendering low-temperature fuel cells more accessible for applications in portable electronics and transportation. However, SPE fuel cells are still far from commercialization due to high operation cost, and insufficient lifetime and performance limitations. Anion exchange membrane fuel cells (AEMFCs) are inexpensive alternatives to current proton exchange membrane fuel cell (PEMFC) technology, which relies on utilizing expensive noble-metal catalysts and perfluorinated SPE …

Interaction Between Charge-Transfer States Studied By Magnetic Field Effects, Mingxing Li

Doctoral Dissertations

Organic semiconducting materials, consisting mostly of carbon and hydrogen atoms, provide remarkable promise for electronic applications due to their easy processing, chemical tenability, low costs and environmental-friendly characteristics. For realizing electronic applications such as light emitting diodes and photovoltaic cells, charge-transfer states act as an important intermediate state for recombination and dissociation. Interestingly, magnetic field effects on semiconducting materials have been realized based on the suppression of spin mixing in the charge-transfer states. Although lots of studies have been carried out on investigating the properties of charge-transfer states, little has been done to consider the interaction between them. This thesis …

Exploring Thermoelectric Effect Based On Multi-Layer Conductor/Organic/Conductor Devices, Qing Liu

Doctoral Dissertations

Thermoelectric phenomena involve the simultaneous presence of both electrical and thermal currents. The entropy has been heavily used as the driving force to diffuse charge carriers between high and low temperature surfaces towards the development of Seebeck effects in thermoelectric devices. However, this driving force from entropy difference can cause an inverse relationship between Seebeck coefficient and electrical conductivity in the thermoelectric developments. Increasing the charge density can decrease the entropy difference to diffuse the charge carriers at a given temperature difference and lead to a decrease on the Seebeck coefficient developed by the entropy difference. Therefore, it is necessary …

Morphological And Material Effects In Van Der Waals Interactions, Jaime C. Hopkins

Doctoral Dissertations

Van der Waals (vdW) interactions influence a variety of mesoscale phenomena, such as surface adhesion, friction, and colloid stability, and play increasingly important roles as science seeks to design systems on increasingly smaller length scales. Using the full Lifshitz continuum formulation, this thesis investigates the effects of system materials, shapes, and configurations and presents open-source software to accurately calculate vdW interactions. In the Lifshitz formulation, the microscopic composition of a material is represented by its bulk dielectric response. Small changes in a dielectric response can result in substantial variations in the strength of vdW interactions. However, the relationship between these …

Post-Wrinkling Behaviors In Layered Elastic Polymers, Anesia D. Auguste

Doctoral Dissertations

Surface instabilities include a variety of different modes such as wrinkles, folds, and creases. Such surface instabilities have been used in numerous contexts including changing the wetting, optical, and mechanical properties of different material surfaces for applications in flexible electronics, tissue engineering and biosensors. Interesting surface morphologies can arise due to differences in pre-strain, stiffness, and thickness between the film and substrate; however, the effects of these differences are not well understood. We have developed an experimental system, complemented by finite element simulations, to systematically vary the pre-stretch of the substrate, modulus contrast and thickness contrast to study the transitions …

Magnetic, Optical And Electrical Properties Of Electron-Hole Pairs In Polymer And Organo-Metal Halide Perovskite Photovoltaic Cells, Yu-Che Hsiao

Doctoral Dissertations

Organic polymer and organo-metal halide perovskite (OMHP) materials have attracted extensive attention during the past decade due to their various applications, like solar cells, light emitting diode, even lasing action (OMHP). Especially, the organo-metal halide perovskite solar cell shows a remarkable power conversion efficiency of about 20%, which is comparable to the amorphous silicon solar cell. Therefore, OMHP solar cell had been considered as a promising substitution for the next generation of renewable energy source. The OMHP materials contain both advantages of organic and inorganic semiconductors, like solution processable thin film fabrication, long-range ambipolar transport characteristics, high dielectric constants, low …

Inter-Droplet Membranes For Mechanical Sensing Applications, Nima Tamaddoni Jahromi

Doctoral Dissertations

This dissertation combines self-assembly phenomena of amphiphilic molecules with soft materials to create and characterize mechanoelectrical transducers and sensors whose sensing elements are thin-film bioinspired membranes comprised of phospholipids or amphiphilic polymers. We show that the structures of these amphiphilic molecules tune the mechanical and electrical properties of these membranes. We show that these properties affect the mechanoelectrical sensing characteristic and range of operation of these membrane transducers. In the experiments, we construct and characterize a membrane-based hair cell embodiment that enables the membrane to be responsive to mechanical perturbations of the hair. The resulting oscillations of membranes formed between …

Solution, Interfacial, And Interlayer Studies Of Electronically Active Polymers, Hsin-Wei Wang

Doctoral Dissertations

This thesis describes the solution behavior and interfacial properties of electronically active polymers. The performance of such polymers in devices is often determined by their chain conformation and morphology in solution and in thin films. For example, the intricate balance between polymer domain size and crystalline packing of electron donor and acceptor components, as well as the properties at the polymer-metal interface, are crucial for achieving optimal performance in devices, such as solar cells. Chapter 1 presents the current progress in polymer-based solar cells, their fundamental principles, and key factors to improve their efficiency. Literature precedents on the development of …

Diffusion Of Polymers In Polyelectrolyte Gels, Anand Rahalkar

Doctoral Dissertations

Polyelectrolyte gels have the ability to undergo volume transitions in response to external stimuli. As a result, they are used in a wide variety of applications such as sensors, actuators, artificial implants, and controlled drug delivery. In addition, diffusion of trapped macromolecules inside gels is of tremendous significance in drug delivery systems. In this thesis, the dynamics of trapped polyelectrolytes in a polyelectrolyte gel is studied using dynamic light scattering technique. The effect of the crosslink density and the charge density of the gel, as well as of solvent quality and salt concentration are investigated. In general, two distinct diffusive …

Refractive Index Engineering And Optical Properties Enhancement By Polymer Nanocomposites, Cheng Li

Doctoral Dissertations

The major part of this dissertation discusses the engineering of the refractive index of materials using solution-processable polymer nanocomposites and their applications in building optical components and devices. Three particular polymer nanocomposites have been introduced to achieve materials with tunable refractive indices and enhanced optical properties, which can be used to manipulate the behavior of light or electromagnetic radiations. In the first system, polyhedral oligomeric silsesquioxane (POSS)/polymer nanocomposites are developed. Thin films with tunable, low refractive indicies were fabricated from the composites. The mechanical strength of these films was characterized, and their application in antireflective coatings is discussed. In the …

(I) Polymer Nanocomposites: Rheology And Processing For Mesoporous Materials And (Ii) Nanopatterning Of Metal Oxides Using Soft Lithography, Rohit Kothari

Doctoral Dissertations

The research in this dissertation is categorized into two parts. The first part is focused on investigation of order-to-disorder transitions (ODT) in nanocomposites of an amphiphilic block copolymer containing various hydrogen-bonded additives, and fabrication of novel mesoporous silica based materials by utilizing such nanocomposites as templates. Disordered Pluronic®, poly(ethylene oxide) (PEO)−poly(propylene oxide) (PPO)−PEO triblock copolymer upon blending with small molecule additives containing hydrogen-bond-donating functional groups (carboxyl or hydroxyl) result into ordered nanoscale morphologies by preferentially interacting with the hydrophilic PEO chains in the Pluronic®. The dependence of ODT-temperature in these novel Pluronic®/small-molecule-additive complexes on composition, number and type of functional …

Model Systems For Characterizing The Intrinsic Properties Of Polymer Semiconductors: Oligomers And Single Crystals, Nicholas S. Colella

Doctoral Dissertations

Polymer semiconductors hold great promise for the realization of inexpensive, flexible electronic devices. One of the greatest strengths of these materials, their low-cost processability from solution, results in a wide range of solid-state structures. The crystallinity and morphology of these materials strongly impacts their performance as charge carriers. Furthermore, the nature of polymerization inherently leads to the production of materials which are disperse in their molecular character, with regard to both coupling and molecular weight. Thus, it is difficult to deconvolute the intrinsic properties of polymer semiconductors from their synthetic and processing conditions. Polythiophenes are particularly susceptible to performance variations …

Controlling The Assembly Of Nanoparticles In Polymer Blends, Kyle C. Bryson

Doctoral Dissertations

While many novel methods have been devised for directing the assembly of nanoparticles in block copolymers, the topic has not reached the same level of sophistication for polymer blends. The assembly of particles at the interface between phase-separated domains can serve as a means to compatibilize polymer blends, reducing domain sizes and enhancing interdomain adhesion by impeding coalescence and decreasing interfacial tension. Compatibilization optimizes the performance of blended materials in applications where the properties of both components must be expressed synergistically, such as in plastics requiring both high strength and high toughness and in photovoltaic films. Thus, approaches to robustly …