Cooperativity in the enhanced piezoelectric response of polymer nanowires

We provide a detailed insight into piezoelectric energy generation from arrays of polymer nanofibers. For sake of comparison, we firstly measure individual poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFe)) fibers at well-defined levels of compressive stress. Under an applied load of 2 mN, single nanostructures generate a voltage of 0.45 mV. We show that under the same load conditions, fibers in dense arrays exhibit a voltage output higher by about two orders of magnitude. Numerical modelling studies demonstrate that the enhancement of the piezoelectric response is a general phenomenon associated to the electromechanical interaction among adjacent fibers, namely a cooperative effect depending on specific geometrical parameters. This establishes new design rules for next piezoelectric nano-generators and sensors.Comment: 31 pages, 11 figures, 1 tabl

Models of polymer solutions in electrified jets and solution blowing

Fluid flows hosting electrical phenomena make the subject of a fascinating and highly interdisciplinary scientific field. In recent years, the extraordinary success of electrospinning and solution blowing technologies for the generation of polymer nanofibers has motivated vibrant research aiming at rationalizing the behavior of viscoelastic jets under applied electric fields or other stretching fields including gas streams. Theoretical models unveiled many original aspects in the underpinning physics of polymer solutions in jets, and provided useful information to improve experimental platforms. This article reviews advances in the theoretical description and numerical simulation of polymer solution jets in electrospinning and solution blowing. Instability phenomena of electrical and hydrodynamic origin are highlighted, which play a crucial role in the relevant flow physics. Specifications leading to accurate and computationally viable models are formulated. Electrohydrodynamic modeling, theories for the jet bending instability, recent advances in Lagrangian approaches to describe the jet flow, including strategies for dynamic refinement of simulations, and effects of strong elongational flow on polymer networks are reviewed. Finally, the current challenges and future perspectives of the field are outlined and discussed, including the task of correlating the physics of the jet flows with the properties of realized materials, as well as the development of multiscale techniques for modelling viscoelastic jets.Comment: 135 pages, 42 figure

Production and Characterization of Electrospun Metal Nanofibers for Applications in Nanoaerosol Filtration

Airborne particles have a large impact on human health and the environment. They can be as small as a few nanometers. Electrospun nanofibrous materials have shown strong potential in filtering airborne nanoparticles for their high efficiency and low energy consumption. Electrospinning has been investigated for decades. Unlike polymeric fibers, the production of fibers made from non-polymeric materials is limited. Additionally, the effects of single parameters on the fiber size are not well understood. To better understand the effects of electrospinning parameters on nanofiber size, the following tasks have been conducted: 1. Understand electrospinning using polymer-based samples for air filtration 2. Determine a reliable way of producing metal-based fibers, focusing on composition of the electrospinning solution and calcining atmosphere 3. Conduct a parametric study using metal-based fibrous filter samples 4. Conduct dimensionless parametric studies aiming at predicting the size of the fibers produced by electrospinning Before producing metal-based fibers, polymer fibers were produced using an existing apparatus in the lab. CA solutions were prepared by diluting various concentrations of CA in a 2:1 (w:w) ratio of N,N-dimethylacetamide (concentration 10 wt.% to 20 wt.%). The electrospinning voltages ranged from 8 to 12 kV with distances from 10 to 15 cm and deposition times of up to 30 minutes. The produced nanofibrous filter samples were then analyzed in terms of fiber size distribution and filter quality factor using nanosized NaCl particles ranging from 4 to 240 nm in diameter. The maximum filtration efficiency measured was 99.8 % for filter samples obtained with an overall deposition time of 30 minutes. The maximum filter quality factor was 0.14 Pa-1 for a CA concentration of 20 wt.% and a tip-to-collector distance of 15 cm. The average fiber diameters of the fibers were between 175 and 890 nm, and CA concentrations below 15 % led to the formation of beads. Then ceria and alumina-based filters were fabricated using the same setup with different operating parameters. Results showed that a solution mix with a ratio of 2:1 ethanol:water with a solid concentration of 15% in a weight ratio of 1:2 w:w metal nitrate:polymer yields the best fibers in terms of size distribution. The average fiber diameter was reduced by calcination due to the loss of polymer. The average diameter of the fibers was as small as 200 nm after calcination. Additionally, the produced metal-based fibers were tested for filtration and the filtration quality was 0.07 Pa-1, which is comparable to those of polymeric fibers. The importance of different solution and operating parameters were evaluated. The trial series was planned according to orthogonal two factorial experimental design. Four parameters, each with two levels were chosen for this study. The solution parameter chosen was concentrations of polymer and salt; process parameters included voltage, nozzle size and feed rate of the solution. It was found that the concentration of the precursor solution had a dominant effect on the fiber size, while the effects of electric field strength, flow rate and needle diameter were comparable in their effect on the fiber size. Dimensionless numbers have been developed using the Pi-theorem aiming at the prediction of electrospinnablitiy. The development of the dimensional tables and the identification of suited parameters for the dimensional table show that the processing parameters electric field strength, needle diameter and solution feed rate; the solution parameters, including viscosity, surface tension and solution conductivity, are the most appropriate for characteristic numbers describing the electrospinning process

3D computational simulation and experimental characterization of polymeric stochastic network materials : case studies in reinforced eucalyptus office paper and nanofibrous materials

The properties of stochastic fibrous materials like paper and nanowebs are highly dependent on those fibers from which the network structure is made. This work contributes to a better understanding of the effect of fiber properties on the network structural properties, using an original 3D fibrous material model with experimental validation, and its application to different fibrous materials used in reinforced Eucalyptus office paper and nanofibrous networks. To establish the relationships between the fiber and the final structural material properties, an experimental laboratorial plan has been executed for a reinforced fibrous structure, and a physical based 3D model has been developed and implemented. The experimental plan was dedicated to an important Portuguese material: the reinforced Eucalyptus based office paper. Office paper is the principal Portuguese paper industry product. This paper is mainly produced from Eucalyptus globulus bleached kraft pulp with a small incorporation of a softwood pulp to increase paper strength. It is important to access the contribution of different reinforcement pulp fibers with different biometry and coarseness to the final paper properties. The two extremes of reinforcement pulps are represented by a Picea abies kraft softwood pulp, usually considered the best reinforcement fiber, and the Portuguese pine Pinus pinaster kraft pulp. Fiber flexibility was determined experimentally using the Steadman and Luner method with a computerized acquisition device. When comparing two reinforcement fibers, the information about fiber flexibility and biometry is determinant to predict paper properties. The values presented correspond to the two extremes of fibers available as reinforcement fibers, regarding wall thickness, beating ability and flexibility values. Pinus pinaster has the thickest fiber wall, and consequently it is less flexible than the thinner wall fibers: Pinus sylvestris and Picea abies. Experimental results for the evolutions of paper properties, like paper apparent density, air permeability, tensile and tear strength, together with fiber flexibility for the two reinforcement fibers, constitute valuable information, also applicable for other reinforcement fibers, with fiber walls dimensions in this range. After having quantified the influence of fiber flexibility, we identified that this is as a key physical property to be included in our structural model. Therefore, we chose to develop a 3D network model that includes fiber bending in the z direction as an important parameter. The inclusion of fiber flexibility was done for the first time by Niskanen, in a model known as the KCL-Pakka model. We propose an extension of this model, with improvements on the fiber model, as well as an original computational implementation. A simulator has been developed from scratch and the results have been validated experimentally using handmade laboratory structures made from Eucalyptus fibers (hardwood fibers), and also Pinus pinaster, Pinus Sylvestris and Picea abies fibers, which are representative reinforcement fibers. Finally, the model was modified and extended to obtain an original simulator to nanofibrous materials, which is also an important innovation. In the network model developed in this work, the structure is formed by the sequential deposition of fibers, which are modeled individually. The model includes key papermaking fiber properties like morphology, flexibility, and collapsibility and process operations such as fiber deposition, network forming or densification. For the first time, the model considers the fiber microstructure level, including lumen and fiber wall thickness, with a resolution up to 0.05μm for the paper material case and 0.05nm for the nanofibrous materials. The computational simulation model was used to perform simulation studies. In the case of paper materials, it was used to investigate the relative influence of fiber properties such as fiber flexibility, dimensions and collapsibility. The developed multiscale model gave realistic predictions and enabled us to link fiber microstructure and paper properties. In the case of nanofibrous materials, the 3D network model was modified and implemented for Polyamide-6 electrospun and cellulose nanowebs. The influence of computational fiber flexibility and dimensions was investigated. For the Polyamide-6 electrospun network experimental results were compared visually with simulation results and similar evolutions were observed. For cellulose nanowebs the simulation study used literature data to obtain the input information for the nanocellulose fibers. The design of computer experiments was done using a space filling design, namely the Latin hypercube sampling design, and the simulations results were organized and interpreted using regression trees. Both the experimental characterization, and computational modeling, contributed to study the relationships between the polymeric fibers and the network structure formed.As propriedades de materiais estocásticos constituídos por fibras, tais como o papel ou nanoredes poliméricas, dependem fortemente das fibras a partir das quais a estrutura em rede se forma. Este trabalho contribui para uma melhor compreensão da influência das propriedades das fibras nas propriedades estruturais das redes, utilizando um modelo original 3D para materiais constituídos por fibras, com validação experimental, bem como a sua aplicação aos materiais utilizados no papel de escritório de Eucalyptus, com fibras de reforço, e a redes de nanofibras. Para estabelecer as relações entre a fibra e as propriedades estruturais do material, executou-se um planeamento experimental para uma estrutura fibrosa reforçada, e desenvolveu-se e implementou-se um modelo 3D de base física. O plano experimental teve como objecto um material relevante em Portugal: o papel de escritório de Eucalyptus com fibras de reforço. O papel de escritório é o produto principal da indústria de papel Portuguesa. Este tipo de papel é produzido a partir da pasta kraft branqueada de Eucalyptus globulus, com incorporação de uma pequena quantidade de pasta de reforço, “softwood”, para melhorar a resistência do papel. É importante avaliar a contribuição de diferentes fibras de reforço, com biometria e massas linear distinta, nas diferentes propriedades finais do papel. Os dois extremos das fibras de reforço estão representados pela pasta kraft de Picea abies, usualmente considerada a melhor fibra de reforço, e a pasta kraft Portuguesa de Pinus pinaster. A flexibilidade da fibra determinou-se experimentalmente utilizando o método de Steadman e Luner, com um dispositivo de aquisição automatizado. A informação relativa à flexibilidade e biometria da fibra é fundamental para inferir sobre as propriedades do papel. Os valores determinados correspondem a valores dos extremos, paras as fibras de reforço disponíveis no mercado, no que diz respeito a espessura de parede, refinabilidade e valores de flexibilidade. Pode considerar-se a fibra de Pinus pinaster num extremo, sendo a fibra de paredes mais espessas, e consequentemente menos flexível que as fibras de paredes mais finas: Pinus sylvestris e Picea abies. Desta forma, os resultados experimentais obtidos para estas fibras, relativos à evolução de propriedades do papel, nomeadamente densidade, permeabilidade ao ar, resistência à tracção e ao rasgamento, entre outros, constituem informação importante que pode ser aplicada a outras fibras de reforço, que se situem nesta gama. Como consequência lógica da identificação da flexibilidade da fibra como uma propriedade física determinante, e após a quantificação experimental, a escolha do modelo de papel recaiu sobre um modelo que inclui a flexibilidade como propriedade chave. Assim, desenvolvemos um modelo 3D que inclui a flexão das fibras na direcção transversal, isto é, a direcção da espessura do papel, também reconhecida como direcção da coordenada z. A inclusão da flexibilidade da fibra baseia-se no modelo de Niskanen, conhecido como o modelo KCL-Pakka. Apresenta-se uma extensão deste modelo, com modificações no modelo da fibra, bem como uma implementação computacional original. Desenvolveu-se um simulador para matérias em rede, que se validou com resultados experimentais. Efectuaram-se, também, as modificações necessárias para obter um simulador para nanomateriais, o que constitui uma inovação relevante. No modelo deste trabalho, desenvolvido para materiais fibrosos em rede, as fibras modelam-se individualmente e a estrutura forma-se sequencialmente pela sua deposição e conformação à estrutura existente. O modelo inclui propriedades das fibras determinantes, tais como morfologia, flexibilidade e colapsabilidade. Bem como etapas do processo, nomeadamente a deposição das fibras e a formação da rede, isto é, a densificação da estrutura. De uma forma original, o modelo da fibra inclui a espessura do lúmen e da parede da fibra, com uma resolução de 0.05μm para as fibras do papel e 0.05nm no caso das nanofibras. O modelo computacional desenvolvido utilizou-se na realização de estudos de simulação. No caso dos materiais papeleiros, utilizou-se para investigar a influência das propriedades das fibras, tendo-se obtido previsões realistas. No caso dos nanomateriais, o modelo foi modificado e implementado para as fibras electrofiadas de Poliamida-6 e redes de nanocelulose. O plano de experiencias computacionais utilizou uma distribuição no espaço “Latin hypercube” e os resultados das simulações organizaram-se recorrendo a árvores de regressão. Tanto a caracterização experimental, como a modelação computacional, contribuíram com valiosa informação para o estudo das relações entre as fibras poliméricas e as estruturas em rede por elas formadas

FABRICATION AND CHARACTERIZATION OF MANGANESE OXIDE SURFACES FROM POLYMER-BASED FIBER PRECURSORS

Supercapacitors are an energy storage technologycombining properties of both capacitors and batteries to deliver high power and energydensities. Supercapacitors store charge through electrostatic and faradaic interactions between the electrode and ionic electrolyte. By improving the physical structure of electrodes, the interfacial area where energy storage occurs can be increased and electrochemical performance improved. New, fibrous, manganese oxide web-based structures tested for use as a supercapacitor electrode were fabricated by electrospinning and direct calcinationof metal salt-containing polymer fibers, and the effects of fabrication parameters on electrochemical performance were investigated. Data show that high polymer concentrations and low oxide precursor concentrations during electrospinning form porous fibers with increased surface area, resulting in capacitance values up to 76 % greater than electrodes prepared with low polymer and high precursor concentrations. Post-electrospinning vapor melting treatments improved mechanical stability of the fiber mats to prevent delamination during calcination, increasing active mass of the prepared electrodes and improving performance by over 50 %. Calcining the structures for at least 4 h improves structural and electrical properties, increasing capacitance by up to 140 % compared to 2 h calcination, but extended calcination times past 4 h have no further beneficial effects.Electrochemical impedance spectroscopy and linear sweep voltammetry on electrospun web electrodes areused to extract system parameters including double layer capacitance and charge transfer resistance. The measured parameters are combined with mathematical models to develop a theoretical description of discharge behavior in electrospun electrodes with varying fiber sizes, porosities, and materials. Modeled discharge curves are used to calculate power and energy densities over current densities ranging from 5 to 5000 A/g and predict that the electrospun electrodes should exhibit remarkably stable power density over a large range of energy densities. The geometry of a fabricated electrode is used to predict relationships between fiber diameter, ivporosity, and surface area. The predictions are used to examine the effect of fiber diameter on the performance of an electrospun system. At low porosity, electrode energy density is maximized by minimizing void space in the electrode. Parametric manipulation of the model shows thatimprovements to electrode conductivity and the material’s specific capacitance are promising, high-impact areas for optimization, while electrolyte conductivity and exchange current density have minimal effects. The model is also expanded to MnO2, Fe2O3, Co3O4, V2O5, and WO3in order to predict suitability for use in electrospun web electrodes. The unexpectedly good performance of low-capacitance materials with high conductivities reveals the complex relationship between material parameters and electrospun electrode performance. The model is useful for predicting the effects of changing electrospun electrode parameters while decreasing the amount of necessary experimentation.The work presented in this dissertation has demonstrated the suitability of electrospun structures for use assupercapacitor electrodesand provides insight into the fabrication conditions that improve capacitance. The model produced is a powerful tool for predicting materials and fiber sizes that are well-suited to the application, providing the potential for electrospun electrode fabrication methods to be expanded into higher-performing materials for improved low cost energy storage

EXPERIMENTAL ANALYSIS OF FLOW REGIMES PERTAINING TO ELECTROSPINNING FROM A POLYMER DROP

Ph.DDOCTOR OF PHILOSOPH