O presente trabalho relata a investigação de uma rota reprodutível de obtenção de materiais de carbono a partir do tratamento térmico de pré-polímero fenol formaldeído (resina fenólica) levando em consideração características morfológicas e estruturais.O controle da composição de resina bem como o estabelecimento cuidadoso da rampa de aquecimento foram fatores decisisvos para a obtenção de materiais análogos ao carbono vítreo de maneira reprodutível. Os materiais obtidos apresentaram reduzida presença de poros e condutividade térmica apreciável uma vez que foram testados como eletrodos de trabalho em ensaios eletroquímicos . A análise estrutural e morfológica dos compostos sintetizados demonstrou a presença de estrutura semelhante ao carbono vítreo relatado na literatura com presença de poucos poros superficiais da ordem de micrômetros até nanômetros. Já em relação aos materias de carbono porosos, buscou-se obter compostos com arranjo ordenado de mesoporos. Resultados preliminares demonstraram que os materiais apresentaram poros micrométricos superficiais organizados. / The present work reports on the investigation of reproducible route to synthesize carbon materials from phenolic resin take into account the structural and morphological features. The control or the resin composotion and the careful establinshing of the ramp heating were decisive for achievement the materials analogous to the glassy carbon (vitreous carbon) in a reproducicle way. The materials obtained materials revealed the glassy carbon structure with the presence of the superficial pores in the range of micrometers to nanometrers. In relation to the carbon materilas, they organized arrangement of the porous in the micrometric scale.

Pertencendo à família dos eletrodos de carbono, o carbono polimérico vítreo (CPV) apresenta condutividade elétrica, estabilidade térmica, resistência mecânica e grande intervalo de potencial. Dessa forma, se torna possível sua aplicação como eletrodo sólido para o monitoramento de processos de transferência de carga, suporte para polímeros eletroativos e modificadores de superfície. O processo de obtenção do CPV se baseia na carbonização em ambiente inerte de materiais precursores, dentre os quais, podem-se destacar as resinas fenólicas, como por exemplo, resol (C7H8O2). Neste contexto, este trabalho investiga a obtenção do CPV a partir da resina fenólica resol, bem como o efeito da incorporação de íons metálicos (crômio, ferro e cério) na estrutura cristalina e nas propriedades eletroquímicas. Os materiais foram caracterizados por análise térmica, microscopia eletrônica de varredura, difratometria de raio-X, espectrofotometria no infra-vermelho e por voltametria cíclica. O difratograma de raio-X confirmou a obtenção de um material com estrutura similar a do carbono vítreo e, através da microscopia eletrônica de varredura, ficou comprovada a baixa porosidade do CPV. Por espectrofotometria no infra-vermelho, ficou demonstrada a presença de grupos funcionais, como carbonilas e hidroxilas, na estrutura do material. Os resultados mostram que tanto o CPV quanto o CPV modificado por íons metálicos possuem boa estabilidade térmica, pois não houve nenhuma decomposição desses materiais abaixo de 400ºC. E, tanto o CPV quanto o CPV modificado por íons metálicos apresentam comportamento eletroquímico similar no sistema ferricianeto/ferrocianeto. Concluindo, o método desenvolvido para obtenção do CPV modificado por íons metálicos é satisfatório, reprodutivo além de proporcionar uma dispersão homogênea dos íons na fase do CPV. Desta forma, possibilita uma nova frente de estudos destes materiais eletródicos em eletrocatálise e eletroanalítica. / Belonging to the family of the carbon electrodes, the glassy polymeric carbon (GPC) presents electric conductivity, thermal stability, mechanical resistance mechanics and great potential interval. In this way, it is possible its application as solid electrode in order to follow electron transfer process, act as support of electroactive molecules and surface modifier. The glassy polymeric carbon production is based on carbonization of organic precursors in inert atmosphere such as phenolic resins (for example, resol - C7H8O2) In this context, this work investigates the production of GPC from phenolic resin, as well as the effect of incorporation of metallic ions (chromium, iron, and cerium) on crystalline structure and electrochemical properties. The materials were characterized by thermal analysis, scanning electron microscopy, X-ray diffraction, infrared spectrophotometry, and cyclic voltammetry. X-ray diffraction patterns had confirmed the presence of a material with similar structure of that found in conventional glassy carbon, and through SEM images it has shown a low porosity carbon material. From infrared spectrum, it can be observed carbonyl and hydroxide groups. The found results showed that both GPC and metallic ion modified GPC exhibit thermal stability, since no decomposition has occurred up to 450 oC. And, both carbon materials present similar electrochemical behavior in hexacyanoferrate system. In conclusion, the method developed for attainment of the GPC modified for ions metallic is satisfactory, and reproductive. Besides, it provides a homogeneous dispersion of ions in the bulk phase of the GPC. As a consequence, it makes possible a new front of studies of these materials in electrocatalysis and electroanalysis.

Il est bien établi que les propriétés mécaniques et rhéologiques d'une large classe de matériaux vitreux amorphes met en jeu - contrairement aux dislocations dans les cristaux - des rearrangements structuraux localisés formant par un processus de cascade des bandes de cisaillements. Cette localisation de la déformation est observée dans divers systèmes vitreux ainsi que dans des simulations numériques. Cette réponse mécanique complexe reste mal comprise à une échelle microscopique et il n'est pas clair si l'écoulement plastique peut être associé à une origine structurale locale ou à des processus purement dynamiques.Dans cette thèse nous envisageons ces problématiques à l'aide de simulations atomiques athermales sur un système Lennard-Jones modèle. Nous calculons le tenseur élastique moyenné localement sur une échelle nanométrique. A cette échelle, le verre est assimilable à un matériau composite comprenant un échafaudage rigide et des zones fragiles. L'étude détaillée de la déformation plastique à différents taux de cisaillement met en évidence divers régimes d'écoulement. En dessous d'un taux de cisaillement critique dépendant de la taille du système, la réponse mécanique atteind une limite quasistatique (effets de taille fini, cascades d'événements plastiques, contrainte seuil) alors que pour des taux de cisaillement plus importants les propriétés rhéologiques sont fixées par le taux de cisaillement imposé. Dans ce régime nous mettons en évidence la croissance d'une longueur de coopérativité dynamique et discutons de sa dépendance avec le taux de cisaillements.

<p>In this thesis, I present the electrochemical studies of three dendrimeric polypropylene imine (PPI) nanomaterials and their applications as a platform in the development of a novel label free DNA nanobiosensor based on electrochemical impedance spectroscopy. Cyclic voltammetry (CV), differentia pulse voltammetry (DPV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) techniques were used to study and model the electrochemical reactivities of the nanomaterials on glassy carbon electrode (GCE) as the working electrode.</p>

The performance of some polymeric materials is profoundly affected by long-term exposure to moisture during service. This poses problems for high precision and/or load bearing components in engineering applications where moisture-induced changes in mechanical properties and dimensional stability could compromise the reliability of the device or structure. In addition to external factors such as moisture, the material properties are also evolving due to inherent structural relaxation within the polymeric material through a process known as physical aging. Based on the current knowledge of both mechanisms, they have opposite effects on material properties. The common approach to studying the effects of moisture is to expose the polymeric material to combined moisture and heat, also referred to as hygrothermal conditions. The application of heat not only increases the rate of moisture diffusion but also accelerates physical aging processes which would otherwise be very slow. In spite of this coupled response, nearly all hygrothermal studies ignore physical aging in their investigations due to the complexity of the coupled problem. The goal of this work is to develop a numerical model for simulating the interactive effects of moisture diffusion and physical aging in a glassy polymer. The intent is to develop a capability that would also allow one to model these effects under various mechanical loading and heat transfer conditions. The study has chosen to model the response of polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS), which is a glassy polymer blend that has very similar behaviour to polycarbonate. In this study, a comprehensive approach which considers four physical mechanisms – structural mechanics, moisture diffusion, heat conduction, and physical aging – has been applied. The most current analytical models in the literature usually attempt to model two or three coupled physical phenomena. To develop the four coupled phenomena model, the current work has undertaken an extensive scope of work involving experimental characterization and finite element modeling. In the experimental part of this work, seven sets of different tests were conducted to characterize the behaviour of PC/ABS exposed to moisture diffusion and accelerated physical aging. These experiments provided a comparative study between the effects of physical aging and moisture diffusion on the material’s behaviour; and at the same time, provided data for the numerical modeling. The dual glass transition temperatures (Tg) of the material were determined using two techniques: dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA). The DMA tests provided data for studying the effects of hygrothermal aging on the Tg’s of the material and were also useful for mechanical tests such as creep and stress relaxation performed using the DMA. The Tg’s obtained using the TMA were also required for physical aging experiments using the dilatometry mode of TMA. Structural relaxation of the blend was studied by aging the material at 80 °C for 7 aging times in the TMA. These experiments gave an insight into the volume relaxation behaviour of the blend at a constant temperature. Specific heat capacity of the PC/ABS blend was also measured using another thermal analysis technique; i.e., differential scanning calorimeter (DSC), before and after test specimens were exposed to hygrothermal aging for 168 hours. The interactive effects of physical aging and moisture diffusion on the moisture uptake of the material were studied using gravimetric experiments performed at 5 different hygrothermal conditions. The experimental results were used to determine the coefficient of diffusion as well as the equilibrium moisture uptake of the samples. Furthermore, the effects of both moisture diffusion and physical aging on the mechanical behaviour of the polymer blend were investigated using stress relaxation tests. The comparison of the results of the tests performed on un-aged specimens with those of thermally and hygrothermally aged samples showed how physical aging effects competed with moisture diffusion. Also, the coefficient of hygroscopic expansion of the PC/ABS blend was determined using a so-called TMA/TGA technique. The numerical modeling of the four-coupled physics was achieved using the governing equations in the form of partial differential equations. Modeling was performed using the commercial finite element software package, COMSOL Multiphysics®. First, the uncoupled physical mechanisms of structural mechanics, moisture diffusion, and heat conduction were modeled separately to investigate the validity of the PDEs for each individual phenomenon. The modeling of the coupled physics was undertaken in two parts. The three coupled physics of structural mechanics, moisture diffusion, and heat conduction was first simulated for a gas pipe having a linear elastic behaviour. The comparison of the results with similar analysis available in the literature showed the capability of the developed model for the analysis of the triple coupled mechanisms. The second part modeled the four coupled phenomena by incorporating the experimentally determined coupling coefficients. In the developed numerical model, the material behaviour was considered to be linear viscoelastic, which complicated the model further but provided more realistic results for the behaviour of the polymer blend. Moreover, an approximation method was proposed for estimating the coupling coefficients that exist between different coupled physics in this study. It was also suggested that the anomalous moisture diffusion in the material can be modeled using a time varying boundary condition. Finally, the model was successfully verified and demonstrated using test case studies with thin thermoplastic plates. The proposed four-coupled physics model was able to predict with good accuracy the deflection of thin thermoplastic plates under bending for a set of hygorthermal test condition.

The dibenzo[a,c]phenazine dimers with the branched alkoxy chain of disc were synthesized simplely. We now show on the synthesis of this series of compounds which were found to exhibit mesomorphism as determined by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). These materials showed polymeso- morphism and glass formation. In our investigations, we also observed that the effects of branched chains on dibenzo[a,c]phenazine enhanced columnar liquid crystal phase stability indeed.

- / An amorphous system can be viewed as a point in con guration space spending most of its time vibrating about local minima of the energy hypersurface. Transitions to adjacent minima, which correspond to elementary events of structural rearrangement, are rare, since the vibrational energy of the system is not high enough to overcome the surrounding energy barriers. Therefore, molecular dynamics simulations, which can track a system's behavior over at most a few nanoseconds, fail to give us information about the atomistic nature and characteristics of such minimum-to-minimum transitions. In this work we try a di erent approach: we construct molecular con gurations of an amorphous Lennard-Jones solid, which, for given values of the temperature and stress/pressure, constitute local minima of the free energy under the quasi-harmonic approximation (QHA). From the volumetric behavior of these con gurations for various values of the temperature and/or pressure we conclude that the QHA is very reasonable for our system and we calculate the values of the isothermal compressibility and of the elastic constants. We then identify representative paths in con guration space leading from one free energy minimum to an adjacent one at xed temperature and pressure. For each of these transitions or elementary structural relaxation events we determine the corresponding rate constant using the principles of multidimensional transitionstate theory. The distribution of free energy barriers is found to be strongly asymmetric and extremely broad, whereas the corresponding distribution of activation entropies is narrow. There is a strong positive correlation between the volume change and the free energy change accompanying each of these elementary transitions, as would be expected from observed volume relaxation phenomena in glasses. The physical phenomenon of physical ageing can be described as a sequence of elementary relaxation events. Therefore, our next step is the analysis of sequential minimum-to-minimum transitions. This analysis is performed via a novel kinetic Monte-Carlo simulation method, which we call quasi-MD. The quasi-MD method tracks di erent stable initial con gurations "escape" through one of the transition states surrounding their current minimum and get trapped inside a neighboring one, which becomes the new current state for the system. Each of the observed transition events is associated with a characteristic time related to the randomly chosen escape route, integrating-out the vibration inside the minimum. This way, it is possible to follow the time-dependence of the system's properties during the relaxation process. One of the most characteristic such properties for the phenomenon of physical ageing is the self-part of the intermediate scattering function Ss(q; t). Calculation of the timedependence of Ss(q; t) for the glassy Lennard-Jones using quasi-MD clearly shows the three known characteristic regions: a) initial rapid decay which corresponds to local motion of the particles within their "cage", b) the slowly decaying plateau region corresponding to relaxation of the "cage", called the -relaxation, and c) the nal decay which corresponds to the breakup of the "cage" and escape of the particles, designated as the -relaxation. The glassy Lennard-Jones system used so far is an ideal system for the development of novel simulation methods and the analysis of the validity of di erent approaches. However, more complex systems are interesting from a technological point of view. Therefore, our nal step is the analysis of a glassy polymer. We focus on the validity of the QHA in glassy atactic polypropylene. Following the same procedure as in the case of Lennard-Jones spheres, we construct di erent stable amorphous con gurations for a series of temperatures and pressures. Using these con gurations we calculate the values of the thermal expansion coefficient and isothermal compressibility, which agree reasonably well with available experimental data. Finally, the amorphous polypropylene con gurations get uniaxially strained leading to the estimation of the value of Young modulus.

<p>In this thesis, I present the electrochemical studies of three dendrimeric polypropylene imine (PPI) nanomaterials and their applications as a platform in the development of a novel label free DNA nanobiosensor based on electrochemical impedance spectroscopy. Cyclic voltammetry (CV), differentia pulse voltammetry (DPV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) techniques were used to study and model the electrochemical reactivities of the nanomaterials on glassy carbon electrode (GCE) as the working electrode.</p>

The performance of some polymeric materials is profoundly affected by long-term exposure to moisture during service. This poses problems for high precision and/or load bearing components in engineering applications where moisture-induced changes in mechanical properties and dimensional stability could compromise the reliability of the device or structure. In addition to external factors such as moisture, the material properties are also evolving due to inherent structural relaxation within the polymeric material through a process known as physical aging. Based on the current knowledge of both mechanisms, they have opposite effects on material properties. The common approach to studying the effects of moisture is to expose the polymeric material to combined moisture and heat, also referred to as hygrothermal conditions. The application of heat not only increases the rate of moisture diffusion but also accelerates physical aging processes which would otherwise be very slow. In spite of this coupled response, nearly all hygrothermal studies ignore physical aging in their investigations due to the complexity of the coupled problem. The goal of this work is to develop a numerical model for simulating the interactive effects of moisture diffusion and physical aging in a glassy polymer. The intent is to develop a capability that would also allow one to model these effects under various mechanical loading and heat transfer conditions. The study has chosen to model the response of polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS), which is a glassy polymer blend that has very similar behaviour to polycarbonate. In this study, a comprehensive approach which considers four physical mechanisms – structural mechanics, moisture diffusion, heat conduction, and physical aging – has been applied. The most current analytical models in the literature usually attempt to model two or three coupled physical phenomena. To develop the four coupled phenomena model, the current work has undertaken an extensive scope of work involving experimental characterization and finite element modeling. In the experimental part of this work, seven sets of different tests were conducted to characterize the behaviour of PC/ABS exposed to moisture diffusion and accelerated physical aging. These experiments provided a comparative study between the effects of physical aging and moisture diffusion on the material’s behaviour; and at the same time, provided data for the numerical modeling. The dual glass transition temperatures (Tg) of the material were determined using two techniques: dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA). The DMA tests provided data for studying the effects of hygrothermal aging on the Tg’s of the material and were also useful for mechanical tests such as creep and stress relaxation performed using the DMA. The Tg’s obtained using the TMA were also required for physical aging experiments using the dilatometry mode of TMA. Structural relaxation of the blend was studied by aging the material at 80 °C for 7 aging times in the TMA. These experiments gave an insight into the volume relaxation behaviour of the blend at a constant temperature. Specific heat capacity of the PC/ABS blend was also measured using another thermal analysis technique; i.e., differential scanning calorimeter (DSC), before and after test specimens were exposed to hygrothermal aging for 168 hours. The interactive effects of physical aging and moisture diffusion on the moisture uptake of the material were studied using gravimetric experiments performed at 5 different hygrothermal conditions. The experimental results were used to determine the coefficient of diffusion as well as the equilibrium moisture uptake of the samples. Furthermore, the effects of both moisture diffusion and physical aging on the mechanical behaviour of the polymer blend were investigated using stress relaxation tests. The comparison of the results of the tests performed on un-aged specimens with those of thermally and hygrothermally aged samples showed how physical aging effects competed with moisture diffusion. Also, the coefficient of hygroscopic expansion of the PC/ABS blend was determined using a so-called TMA/TGA technique. The numerical modeling of the four-coupled physics was achieved using the governing equations in the form of partial differential equations. Modeling was performed using the commercial finite element software package, COMSOL Multiphysics®. First, the uncoupled physical mechanisms of structural mechanics, moisture diffusion, and heat conduction were modeled separately to investigate the validity of the PDEs for each individual phenomenon. The modeling of the coupled physics was undertaken in two parts. The three coupled physics of structural mechanics, moisture diffusion, and heat conduction was first simulated for a gas pipe having a linear elastic behaviour. The comparison of the results with similar analysis available in the literature showed the capability of the developed model for the analysis of the triple coupled mechanisms. The second part modeled the four coupled phenomena by incorporating the experimentally determined coupling coefficients. In the developed numerical model, the material behaviour was considered to be linear viscoelastic, which complicated the model further but provided more realistic results for the behaviour of the polymer blend. Moreover, an approximation method was proposed for estimating the coupling coefficients that exist between different coupled physics in this study. It was also suggested that the anomalous moisture diffusion in the material can be modeled using a time varying boundary condition. Finally, the model was successfully verified and demonstrated using test case studies with thin thermoplastic plates. The proposed four-coupled physics model was able to predict with good accuracy the deflection of thin thermoplastic plates under bending for a set of hygorthermal test condition.

Philosophiae Doctor - PhD / In this thesis, I present the electrochemical studies of three dendrimeric polypropylene imine (PPI) nanomaterials and their applications as a platform in the development of a novel label free DNA nanobiosensor based on electrochemical impedance spectroscopy. Cyclic voltammetry (CV), differentia pulse voltammetry (DPV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) techniques were used to study and model the electrochemical reactivities of the nanomaterials on glassy carbon electrode (GCE) as the working electrode. / South Africa