Cette thèse se décompose en deux parties : i) l'étude du creusement d'un canal dans un plasma sous-dense (0.1nc

Anisotropy in the spatial arrangement of species and also geometric anisotropy in the components of liquid and soft matter systems gives rise to complex phase and non-trivial dynamic behaviors. Many systems are encompassed by this description including polymers, guest molecules in porous gels or nano-structured materials and also orientable fluids. In order to understand these systems, computational studies provide valuable insight. The study of such systems computationally is difficult if not prohibitive thus it is necessary to reduce these systems to simple models that capture the essential physical processes that govern the dynamics and phase behavior. Two simple models fit into this category: systems where the surrounding solvent is held stationary, a Lorentz model and also systems where a liquid crystal is formed from hard spherocylinders and is driven by an electric field. The Lorentz models studied provide a description of the dynamical regimes accessible when the probe and/or scatterers are given geometric anisotropy. The resulting dynamics are studied when order is present in the stationary solvent, i.e. structured versus isotropic solvent structure. Effective channels depending upon the competition of length scales as well as the structure of the stationary solvent leads to transitions in the dynamics of the traversing probe. Geometric anisotropy in nematogens in liquid crystals leads to the formation of mesophases indicative of a liquid crystal. When coupled to a rotating electric field, the dynamics and phase of the nematogens can be controlled.

A distribuição de massa molar (DMM) e seus parâmetros são de fundamental importância na caracterização dos polímeros. Por este motivo, o desenvolvimento de técnicas que permitam a determinação da DMM de forma mais rápida e a menor custo é de grande importância prática. Os principais objetivos deste trabalho foram a implementação de alguns dos modelos baseados da teoria da reptação dupla propostos na literatura para descrever o mecanismo de relaxação das cadeias poliméricas, a avaliação dessas implementações e a análise de dois passos fundamentais na obtenção da DMM a partir de dados reológicos que são a metodologia de cálculo do espectro de relaxação baseado no modelo de Maxwell e a estratégia para a avaliação numérica das integrais que aparecem nos modelos de relaxação. Foi resolvido o problema denominado problema inverso, ou seja, a determinação da DMM a partir de dados reológicos usando um modelo de relaxação especificado e uma função de distribuição imposta. Foi usada a função Exponencial Generalizada (GEX) para representar a probabilidade de distribuição, sendo consideradas duas abordagens: i) cálculo explícito do espectro de relaxação e ii) aproximações paramétricas de Schwarzl, que evitam a necessidade do cálculo explícito do espectro de relaxação. A metodologia de determinação da DMM foi aplicada para amostras de polietileno e foram estimadas distribuições com boa representação dos dados experimentais do GPC, ao considerarem-se amostras com polidispersões inferiores a 10. Com relação a metodologia de cálculo do espectro de relaxação, foi realizado um estudo comparativo da aplicação de espectros de relaxação discreto e contínuo, com o objetivo de estabelecer critérios para especificação do número ótimo de modos de Maxwell a serem considerados. Ao efetuar-se a comparação entre as técnicas, verificou-se o espectro discreto apresenta como um sistema melhor condicionado, permitindo assim obter maior confiabilidade dos parâmetros estimados. Também é proposta uma modificação da metodologia de determinação da DMM, em que é aplicada a quadratura de Gauss-Hermite para a resolução numérica da integral dos modelos de relaxação. / The molecular weight distribution (MWD) and its parameters are of the fundamental importance in the characterization of polymers. Therefore, the development of techniques for faster and less time consuming determination of the MWD is of great practical relevance. The goals of this work were the implementation of some of the relaxation models from double reptation theory proposed in the literature, the evaluation of these implementations and the analysis of two key points in the recovery of the MWD from rheological data which are the methodology for calculation of the relaxation spectrum based on the Maxwell model and the numeric strategy for the evaluation of the integrals appearing in the relaxation models. The inverse problem, i.e., the determination of the MWD from rheological data using a specified relaxation model and an imposed distribution function, was solved. In the analysis of the inverse problem, the Generalized Exponential (GEX) was used as distribution function and two approaches were considered: i) explicit calculation of the relaxation spectrum and ii) use of the parametric method proposed by Schwarzl to avoid the explicit calculation of the relaxation spectrum. In the test of commercial samples of polyethylene with polidispersity less than 10, the application of this methodology led to MWD curves which provided good fit of the experimental SEC data. Regarding the methodology for calculation of the relaxation spectrum, a comparison between the performance of discrete and continuous relaxation spectrum was performed and some possible a criteria to determine the appropriate number of relaxation modes of Maxwell to be used were evaluated. It was found that the technique of discrete spectrum leads to better conditioned systems and, consequently, greater reliability of the estimated parameters. With relation to the numeric strategy for the evaluation of the integrals appearing in the relaxation models, the use of Gauss-Hermite quadrature using a new change of variables was proposed.

A distribuição de massa molar (DMM) e seus parâmetros são de fundamental importância na caracterização dos polímeros. Por este motivo, o desenvolvimento de técnicas que permitam a determinação da DMM de forma mais rápida e a menor custo é de grande importância prática. Os principais objetivos deste trabalho foram a implementação de alguns dos modelos baseados da teoria da reptação dupla propostos na literatura para descrever o mecanismo de relaxação das cadeias poliméricas, a avaliação dessas implementações e a análise de dois passos fundamentais na obtenção da DMM a partir de dados reológicos que são a metodologia de cálculo do espectro de relaxação baseado no modelo de Maxwell e a estratégia para a avaliação numérica das integrais que aparecem nos modelos de relaxação. Foi resolvido o problema denominado problema inverso, ou seja, a determinação da DMM a partir de dados reológicos usando um modelo de relaxação especificado e uma função de distribuição imposta. Foi usada a função Exponencial Generalizada (GEX) para representar a probabilidade de distribuição, sendo consideradas duas abordagens: i) cálculo explícito do espectro de relaxação e ii) aproximações paramétricas de Schwarzl, que evitam a necessidade do cálculo explícito do espectro de relaxação. A metodologia de determinação da DMM foi aplicada para amostras de polietileno e foram estimadas distribuições com boa representação dos dados experimentais do GPC, ao considerarem-se amostras com polidispersões inferiores a 10. Com relação a metodologia de cálculo do espectro de relaxação, foi realizado um estudo comparativo da aplicação de espectros de relaxação discreto e contínuo, com o objetivo de estabelecer critérios para especificação do número ótimo de modos de Maxwell a serem considerados. Ao efetuar-se a comparação entre as técnicas, verificou-se o espectro discreto apresenta como um sistema melhor condicionado, permitindo assim obter maior confiabilidade dos parâmetros estimados. Também é proposta uma modificação da metodologia de determinação da DMM, em que é aplicada a quadratura de Gauss-Hermite para a resolução numérica da integral dos modelos de relaxação. / The molecular weight distribution (MWD) and its parameters are of the fundamental importance in the characterization of polymers. Therefore, the development of techniques for faster and less time consuming determination of the MWD is of great practical relevance. The goals of this work were the implementation of some of the relaxation models from double reptation theory proposed in the literature, the evaluation of these implementations and the analysis of two key points in the recovery of the MWD from rheological data which are the methodology for calculation of the relaxation spectrum based on the Maxwell model and the numeric strategy for the evaluation of the integrals appearing in the relaxation models. The inverse problem, i.e., the determination of the MWD from rheological data using a specified relaxation model and an imposed distribution function, was solved. In the analysis of the inverse problem, the Generalized Exponential (GEX) was used as distribution function and two approaches were considered: i) explicit calculation of the relaxation spectrum and ii) use of the parametric method proposed by Schwarzl to avoid the explicit calculation of the relaxation spectrum. In the test of commercial samples of polyethylene with polidispersity less than 10, the application of this methodology led to MWD curves which provided good fit of the experimental SEC data. Regarding the methodology for calculation of the relaxation spectrum, a comparison between the performance of discrete and continuous relaxation spectrum was performed and some possible a criteria to determine the appropriate number of relaxation modes of Maxwell to be used were evaluated. It was found that the technique of discrete spectrum leads to better conditioned systems and, consequently, greater reliability of the estimated parameters. With relation to the numeric strategy for the evaluation of the integrals appearing in the relaxation models, the use of Gauss-Hermite quadrature using a new change of variables was proposed.

A distribuição de massa molar (DMM) e seus parâmetros são de fundamental importância na caracterização dos polímeros. Por este motivo, o desenvolvimento de técnicas que permitam a determinação da DMM de forma mais rápida e a menor custo é de grande importância prática. Os principais objetivos deste trabalho foram a implementação de alguns dos modelos baseados da teoria da reptação dupla propostos na literatura para descrever o mecanismo de relaxação das cadeias poliméricas, a avaliação dessas implementações e a análise de dois passos fundamentais na obtenção da DMM a partir de dados reológicos que são a metodologia de cálculo do espectro de relaxação baseado no modelo de Maxwell e a estratégia para a avaliação numérica das integrais que aparecem nos modelos de relaxação. Foi resolvido o problema denominado problema inverso, ou seja, a determinação da DMM a partir de dados reológicos usando um modelo de relaxação especificado e uma função de distribuição imposta. Foi usada a função Exponencial Generalizada (GEX) para representar a probabilidade de distribuição, sendo consideradas duas abordagens: i) cálculo explícito do espectro de relaxação e ii) aproximações paramétricas de Schwarzl, que evitam a necessidade do cálculo explícito do espectro de relaxação. A metodologia de determinação da DMM foi aplicada para amostras de polietileno e foram estimadas distribuições com boa representação dos dados experimentais do GPC, ao considerarem-se amostras com polidispersões inferiores a 10. Com relação a metodologia de cálculo do espectro de relaxação, foi realizado um estudo comparativo da aplicação de espectros de relaxação discreto e contínuo, com o objetivo de estabelecer critérios para especificação do número ótimo de modos de Maxwell a serem considerados. Ao efetuar-se a comparação entre as técnicas, verificou-se o espectro discreto apresenta como um sistema melhor condicionado, permitindo assim obter maior confiabilidade dos parâmetros estimados. Também é proposta uma modificação da metodologia de determinação da DMM, em que é aplicada a quadratura de Gauss-Hermite para a resolução numérica da integral dos modelos de relaxação. / The molecular weight distribution (MWD) and its parameters are of the fundamental importance in the characterization of polymers. Therefore, the development of techniques for faster and less time consuming determination of the MWD is of great practical relevance. The goals of this work were the implementation of some of the relaxation models from double reptation theory proposed in the literature, the evaluation of these implementations and the analysis of two key points in the recovery of the MWD from rheological data which are the methodology for calculation of the relaxation spectrum based on the Maxwell model and the numeric strategy for the evaluation of the integrals appearing in the relaxation models. The inverse problem, i.e., the determination of the MWD from rheological data using a specified relaxation model and an imposed distribution function, was solved. In the analysis of the inverse problem, the Generalized Exponential (GEX) was used as distribution function and two approaches were considered: i) explicit calculation of the relaxation spectrum and ii) use of the parametric method proposed by Schwarzl to avoid the explicit calculation of the relaxation spectrum. In the test of commercial samples of polyethylene with polidispersity less than 10, the application of this methodology led to MWD curves which provided good fit of the experimental SEC data. Regarding the methodology for calculation of the relaxation spectrum, a comparison between the performance of discrete and continuous relaxation spectrum was performed and some possible a criteria to determine the appropriate number of relaxation modes of Maxwell to be used were evaluated. It was found that the technique of discrete spectrum leads to better conditioned systems and, consequently, greater reliability of the estimated parameters. With relation to the numeric strategy for the evaluation of the integrals appearing in the relaxation models, the use of Gauss-Hermite quadrature using a new change of variables was proposed.

Růst krystalů zásadně ovlivňuje morfologii a tím také mechanické vlastnosti semikrystalických polymerů. Tato PhD práce přináší alternativní pohled na popis kinetiky krystalizace v polyolefinech plněných slabě interagujícími částicemi. V nanokompozitních materiálech vysoký specifický povrch plniva i při nízkých plněních zásadně ovlivňuje dynamiku řetězců. V blízkosti povrchu plniva začíná hrát významnou roli zpomalená reptace způsobená jak vzájemnými interakcemi plnivo-polymer tak prostorovým omezením mezi nanočásticemi. Růst krystalů byl zkoumán pomocí polarizovaného optického mikroskopu vybaveného horkým stolkem. Výsledky byly korelovány s teoretickými modely a rozsáhlými počítačovými simulacemi na molekulární úrovni. Pozorovaný pokles rychlosti růstu sférolitů v závislosti na obsahu plniva a molekulové hmotnosti matrice je interpretován na základě imobilizační teorie, tedy, zpomalení reptačního pohybu.

Entangled semiflexible polymer networks are usually described by the tube model, although this concept has not been able to explain all experimental observations. One of its major shortcomings is neglecting the thermal fluctuations of the polymers surrounding the examined test filament, such that disentanglement effects are not captured. In this study, we present experimental evidence that correlated constraint release which has been predicted theoretically occurs in entangled, but not in crosslinked semiflexible polymer networks. By tracking single semiflexible DNA nanotubes embedded both in entangled and crosslinked F-actin networks, we observed different reptation dynamics in both systems, emphasizing the need for a revision of the classical tube theory for entangled polymer solutions.

Cette thèse se décompose en deux parties : i) l’étude du creusement d'un canal dans un plasma sous-dense (0.1nc<n<nc, nc étant la densité critique) de plusieurs centaines de microns par une impulsion laser de durée 1-10 ps et d'intensité 10^18 à 10^20 W/cm² ; ii) les mécanismes de saturation de la diffusion Raman arrière stimulée d'une impulsion laser de durée ps et d'intensité 10^14 à 10^16 W/cm². Le creusement d’un canal plasma par un laser très intense a fait l’objet d’une étude paramétrique à l’aide d’un code PIC (Particle In Cell) 2D. On obtient différents types de canaux en fonction des paramètres du laser et du plasma, reproduisant ainsi et élargissant des résultats précédent. De plus, la vitesse de creusement du canal a été mesurée, et des lois d’échelle ont été établies pour les plasmas homogènes. Elles sont ensuite appliquées à des plasmas inhomogènes, du type de ceux rencontrés lors de la fusion par confinement inertiel (FCI). Cela permet de prévoir l’énergie nécessaire pour creuser un canal jusqu’à la densité critique, étape importante de la FCI par allumage rapide. La saturation du Raman a été étudiée d'un point de vue numérique, pour déterminer si la cause de la saturation était due au déphasage ou à la croissance d'une onde satellite (« sideband »), en utilisant diverses approches. La première est de regarder les résultats de simulations Raman (donc électromagnétiques) à partir de codes cinétiques PIC et Vlasov. La deuxième, consiste à regarder ce qui se passe lorsque l'on initialise un plasma avec une fonction de distribution issue de la théorie adiabatique à l'aide d'un code Vlasov (donc dans une version purement électrostatique). Dans ce cas, on observe bien la croissance d'une onde satellite, dont le nombre d'onde dominant, ainsi que le taux de croissance sont en bon accord avec ce que l'on observe dans les simulations cinétiques. Au final, la saturation de l’onde plasma peut être causée par les deux mécanismes de saturation. / This thesis is divided in two parts : i) the laser channeling in hundreds of microns long under-dense plasmas (0.1nc<n<nc, nc being the critical density) of a laser pulse of intensity 10^18 to 10^20 W/cm² and duration 1-10 ps; ii) the saturation mechanisms of stimulated Raman back-scattering of a laser pulse of intensity 10^14 to 10^16 W/cm² and duration of about 1 ps. A parametric study was performed to study the channeling of a very intense laser pulse, using a 2D PIC (Particle In Cell) code. Various kinds of channels were obtained depending on the laser and plasma parameters, thereby reproducing and enlarging previous studies. Moreover, the channeling velocity was measured and scaling laws were established for homogeneous plasmas. They are then applied to inhomogeneous plasmas, similar to those encountered in inertial confinement fusion (ICF). It is then possible to estimate the energy necessary to channel to the critical density, an important step for the fast ignition scheme of ICF. Raman saturation was studied using numerical simulations, in order to determine if it is due to dephasing or to the growth of sidebands, using different approaches. The first is to study Raman simulations (electromagnetic) performed with kinetic PIC and Vlasov codes. The second, is to study the evolution of a plasma initialized with a distribution function after the adiabatic theory, using a Vlasov code (electrostatic). In this case, we observe the growth of a sideband, with dominant wavenumber and growth rate in good agreement with kinetic simulations. The saturation of the plasma wave can be caused by both saturation mechanisms.

Η παρούσα εργασία αφορά την ανάπτυξη μαθηματικών μοντέλων και την εκτέλεση υπολογιστικών προσομοιώσεων Μοριακής δυναμικής για την περιγραφή και μελέτη της δυναμικής συμπεριφοράς συστημάτων αποτελούμενων από μακρομόρια πολυαιθυλενοξειδίου. Δύο κατηγορίες συστημάτων εξετάστηκαν, πολυμερικά τήγμα κυκλικών καθώς και γραμμικών αλυσίδων του πολυαιθυλενοξειδίου. Ένα εύρος δομικών θερμοδυναμικών αλλά και δυναμικών ιδιοτήτων περιγράφεται από το προτεινόμενο μοριακό μοντέλο και τα αποτελέσματα συγκρίνονται με τις προβλέψεις των δύο επιτυχημένων θεωριών για τα πολυμερή, τις θεωρίες Rouse και ερπυσμού (Reptation Theory). Δίνεται έμφαση στη σύγκριση των αποτελεσμάτων που προκύπτουν από τις προσομοιώσεις των γραμμικών και κυκλικών συστημάτων έτσι ώστε να κατανοηθεί η φύση της κυκλικής αλυσίδας. Η μελέτη αυτή διαπραγματεύεται ταυτόχρονα ένα αυστηρά καθορισμένο πρόβλημα από φυσική άποψη αλλά και ένα ιδιαίτερο θέμα σύγχρονου τεχνολογικού ενδιαφέροντος. Η εργασία αρχικά εισάγει το γενικότερο πλαίσιο κάτω από το οποίο θα γίνει η μελέτη των συστημάτων πολυαιθυλενοξειδίου. Γίνεται αναφορά σε θεωρητικά θέματα όπως η ανάπτυξη της θεωρίας Rouse και ερπυσμού τόσο για τα γραμμικά όσο και για τα κυκλικά πολυμερή και έπειτα παρουσιάζονται αποτελέσματα προσομοιώσεων των προαναφερθέντων συστημάτων σε ένα εύρος θερμοκρασιών. / This work involves developing mathematical models and molecular dynamics computer simulations which are performed to describe and study the dynamic behavior of systems composed of polyethylene macromolecules. Two types of systems examined, polymer melt cyclic and linear chains of polyethylene. A range of structural, thermodynamic and dynamic properties are described by the proposed molecular model and the results are compared with the predictions of two successful theories for polymers, Rouse and Reptation Theory. Emphasis is placed on comparing the results of simulations of linear and cyclic systems so as to understand the nature of the circular chain. This study deals with a strictly defined physical problem but also an issue of distinct technological interest. This work initially introduces the general framework under which the study will be made of polyethylene systems. Refer to theoretical issues such as the development of Rouse and Reptation theory for both linear and cyclic polymers and then presented simulation results of the above systems in a range of temperatures.

Made available in DSpace on 2016-12-08T17:19:29Z (GMT). No. of bitstreams: 1 Elementos pre-textuais.pdf: 356258 bytes, checksum: 9fa8daf60f02856f1b21e1882468c798 (MD5) Previous issue date: 2006-12-13 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The weld line is a common phenomenon in molded plastic articles and its consequences can be evaluated by several manners. This study treat weld line as a sharp crack, which length can be estimated based on the self-diffusion of molecular chains across the polymer-polymer interface. The temperature gradient from the contact surface mold/polymer up to the centre of the thickness was determined by simple heat transfer analyses equations. By combining the temperature gradient behavior through the part thickness with the polymer properties it is possible to determine the diffusion value across the interface and the interpenetration as a result of that. Above a critical interpenetration value, the weld line crack length will fall below a critical crack length, whereby the weld line will no longer initiate brittle fracture. The critical interpenetration is a function of the melt temperature and it is determined by strength tests for samples molded in several temperatures in the polymer manufacture recommended range. The results from strength tests, by which were possible analyze the weak area generated by weld line, were compared with theoretical results. It was evident, by comparing experimental and theoretical results, that weld lines are determined to be near-surface flaws, which embrittle notch sensitive materials such as most amorphous polymers. A software was developed to become possible to determine very quickly the crack length calculations as a function of the melt temperature and other properties related to diffusion. The software can be used to the calculations of any amorphous polymer, process condition and part thickness, obviously by the use of proper adjust equations. / A linha de solda é um fenômeno muito comum em produtos plásticos injetados e as conseqüências da mesma podem ser analisadas de várias maneiras. Este trabalho considerou a linha de solda como sendo uma trinca, cujo comprimento foi estimado a partir da difusão molecular na interface entre as duas frentes convergentes de fluxo. Um gradiente de temperatura desde a superfície de contato molde/polímero até o centro da espessura da peça foi determinado com equações para análise de troca térmica. E o comportamento deste gradiente de temperatura ao longo da espessura do produto somado as propriedades do polímero determinaram valores para a difusão molecular na interface e a conseqüente interpenetração. Acima de um determinado valor crítico de interpenetração, o comprimento da trinca da linha de solda cai para valores abaixo de um determinado valor crítico, pelo qual a linha de solda não mais será responsável por uma fratura frágil. A interpenetração crítica é função da temperatura de injeção e foi determinada via ensaios de tração para amostras injetadas em várias temperaturas dentro da faixa recomendada pelo fabricante da matéria-prima. Os resultados dos ensaios de tração, que possibilitaram a análise da perda de resistência à tração devido à trinca gerada pela linha de solda, foram comparados com os resultados obtidos teoricamente. Ficou evidente, devido a convergência dos resultados teóricos e experimentais, que a linha de solda é um fenômeno que produz um defeito superficial que fragiliza de maneira mais acentuada os materiais poliméricos amorfos, devido a sensibilidade dos mesmos ao entalhe. Um aplicativo foi desenvolvido para proporcionar o cálculo do comprimento da trinca em função da temperatura de injeção e demais propriedades. O aplicativo pode ser utilizado para o cálculo de qualquer polímero de estrutura amorfa, em qualquer condição de processo e espessura, mediante a utilização das adequadas equações de ajuste.