The outstanding properties of carbon nanotubes (CNTs) keep attracting the attention of researchers from different fields. CNTs are promising candidates for applications e.g. in lightweight construction but also in electronics, medicine and many more. The basis for the realization of the manifold applications is a detailed knowledge of the material properties of the carbon nanotubes. In particular for applications in lightweight constructions or in composites, the knowledge of the mechanical behavior of the CNTs is of vital interest. Hence, a lot of effort is put into the experimental and theoretical determination of the mechanical material properties of CNTs. Due to their small size, special techniques have to be applied. In this research, a modified molecular structural mechanics model for the numerical determination of the mechanical behavior of carbon nanotubes is presented. It uses an advanced approach for the geometrical representation of the CNT structure while the covalent bonds in the CNTs are represented by beam elements. Furthermore, the model is specifically designed to overcome major drawbacks in existing molecular structural mechanics models. This includes energetic consistency with the underlying chemical force field. The model is developed further to enable the application of a more advanced chemical force field representation. The developed model is able to predict, inter alia, the lateral and radial stiffness properties of the CNTs. The results for the lateral stiffness are given and discussed in order to emphasize the progress made with the presented approach.

Kohlenstoffnanoröhren (Carbon Nanotubes, CNTs) gelten seit einigen Jahren als vielversprechendes neuartiges Material für verschiedenste Anwendungen in der Technik unterschiedlicher Fachgebiete. Von besonderem Interesse, z.B. in Leichtbaustrukturen, sind die postulierten exzellenten mechanischen Eigenschaften der einzelnen CNTs hinsichtlich Steifigkeit und Festigkeit. Diese auf der Nanoskala identifizierten Eigenschaften sollen auch in makroskopischen Bauteilen zu besonders guten mechanischen Eigenschaften führen. Demonstriert werden kann dies zum Beispiel an einer neuartigen Faser, die aus einer Vielzahl individueller Kohlenstoffnanoröhren gesponnen wurde. An dieser Faser durchgeführte Tests zeigen jedoch, dass die Eigenschaften nicht in der gewünschten Höhe von der Nanoskale auf die Makroskale übertragen werden. Um diesen Effekt erklären und evtl. beheben zu können, sowie für das Design von Strukturen aus Nanoröhren ('Superstrukturen') und einige weitere Anwendungen, sind Simulationsmodelle nötig, die die grundlegenden mechanischen (elastischen) Eigenschaften beschreiben können und zudem mit einer sehr großen Anzahl beteiligter CNTs und damit Atome umgehen können. Betrachtet man dies zusätzlich unter dem Aspekt, dass, beispielsweise zu Designzwecken, jeweils Rechnungen zu mehreren Varianten notwendig sind, ist verständlich, dass für jeden Durchlauf nur eine begrenzte Menge an Rechenzeit aufgebracht werden soll. Daher wird in der vorliegenden Arbeit ein mechanisches Modell der Kohlenstoffnanoröhren entwickelt, das die geforderte Aufgabe um ein Vielfaches schneller als quantenmechanische Methoden oder auch klassische Molekulardynamik behandeln kann. Basis hierfür ist ein molekularmechanischer Ansatz, der ein Ersatzmodell der betrachteten Kohlenstoffnanoröhre aus Balkenelementen erzeugt. Die zur Definition des Balkenfachwerks nötigen Balkeneigenschaften werden hierbei aus einem zugrundeliegenden chemischen Kraftfeld abgeleitet, das die kovalenten Bindungen zwischen den Atomen der Nanoröhre beschreibt. Der Ansatz ist damit in die Klasse der 'molecular structural mechanics' (MSM) Ansätze einzuordnen. Ausgangspunkt der vorliegenden Arbeit ist zunächst ein etabliertes MSM-Modell, dessen Schwächen in der vorliegenden Arbeit analysiert werden. Dabei wird festgestellt, dass der bisher verwendete MSM-Ansatz nicht energetisch konsistent zum zugrundeliegenden chemischen Kraftfeld ist. Dieser Umstand wird zunächst durch die Entwicklung eines modifizierten MSM-Modells behoben. Anschließend wird gezeigt, dass dieses Modell energetisch konsistent zum eingesetzten Kraftfeld ist. Um weitere Fortschritte mit dem gewählten molekularmechanischen Ansatz zu erzielen, wird dann ein verallgemeinertes MSM-Modell auf Basis eines fortschrittlichen chemischen Kraftfeldes entwickelt, das weitere Nachteile des ursprünglichen Ansatzes behebt und universeller einsetzbar ist. Das Modell wird dann zur Bestimmung der elastischen Konstanten von Armchair und Zig-zag CNTs eingesetzt und die erhaltenen Ergebnisse diskutiert.:1. Grundlagen 2. Modellbildung und Simulation einwandiger Kohlenstoffnanoröhren 3. Ergebnisse und Diskussion zum Zweck der Modellentwicklung 4. Ergebnisse und Diskussion der elastischen Parameter einwandiger CNTs 5. Zusammenfassung und Ausblick / For several years now, Carbon Nanotubes (CNTs) are seen as a promising new material for manifold applications in new technologies from different fields. The predicted excellent mechanical properties such as high strength and stiffness are of particual interest e.g. in lightweight structures. The nanoscopic propertiers are prone to lead to good mechanical properties also in macrosopic parts. This can be demonstrated for instance on the basis of a novel type of carbon fiber which is spun out of a multitude of individual carbon nanotubes. However, tests of the fibre show that the outstanding properties on the nanoscale are not fully transfered to the macroscale. In order to explain this effect as well as for designing structures made out of nanotubes (so called super structures) and other applications, models for simulations are needed. These models should be capable of reproducing the basic (elastic) mechnical properties of the nanotubes as well as to be capcable of dealing with a large number of participating nanotubes and hence atoms. Considering the additional aspect that multiple calculations of similar systems, e.g. for design purposes, are required, it is easy to understand, that for each calculation only a limited amount of computational effort is affordable. Hence, in the present work a mechanical model for the carbon nanotubes is developed which can fulfil the requested task in a much shorter time than quantummechanical or moleculardynamic calculations. The model is based on a molecular mechanics approach which creates a substitute model for the carbon nanotube based on beam elements. The parameters mandatory to define the beam elements in the beam framework are obtained on the basis of a chemical force field forming the foundation of the approach. The chemical force fields describes the properties of the covalent bonds in the carbon nanotube. As a result, the proposed model can be classified to be part of the molecular structural mechanics (MSM) approaches. Starting point of the present work is a well known MSM-model which is at first analyzed in order to identify its drawbacks. During this investigation it is found, that the model used so far is not consistent in terms of energy to its underlying chemical force field. This problem is fixed by the development of a modified MSM-approach. It is shown that this modified approach is now consistent to the underlying chemical force field in terms of energy. In order to further improve the method, a generalized, advanced MSM-framework is developed on the basis of a sophisticated chemical force field. This advanced framework resolves further drawbacks of the models and enables a more general application of the model. The obtained model is then used to calculate and discuss the elastic constants of Armchair and Zig-zag Carbon Nanotubes.:1. Grundlagen 2. Modellbildung und Simulation einwandiger Kohlenstoffnanoröhren 3. Ergebnisse und Diskussion zum Zweck der Modellentwicklung 4. Ergebnisse und Diskussion der elastischen Parameter einwandiger CNTs 5. Zusammenfassung und Ausblick

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We numerically analyze three mechanics problems described below. For each problem, the developed computational model is verified by comparing computed results for example problems with those available in the literature. Effective utilization of single wall carbon nanotubes (SWCNTs) and single layer graphene sheets (SLGSs) as reinforcements in nanocomposites requires their strong binding with the surrounding matrix. An effective technique to enhance this binding is to functionalize SWCNTs and SLGSs by covalent attachment of appropriate chemical groups. However, this damages their pristine structures that may degrade their mechanical properties. Here, we delineate using molecular mechanics simulations effects of covalent functionalization on elastic moduli of these nanomaterials. It is found that Young's modulus and the shear modulus of an SWCNT (SLGS), respectively, decrease by about 34% (73%) and 43% (42%) when 20% (10%) of carbon atoms are functionalized for each of the four functional groups of different polarities studied. A shell theory that gives results close to the solution of the corresponding 3-dimensional problem depends upon the shell geometry, applied loads, and initial and boundary conditions. Here, by using a third order shear and normal deformable theory and the finite element method (FEM), we delineate for a doubly curved shell deformed statically with general tractions and subjected to different boundary conditions effects of geometric parameters on in-plane and transverse stretching and bending deformations. These results should help designers decide when to consider effects of these deformation modes for doubly curved shells. Composite laminates are usually fabricated by curing resin pre-impregnated fiber layers in an autoclave under prescribed temperature and pressure cycles. A challenge is to reduce residual stresses developed during this process and simultaneously minimize the cure cycle time. Here, we use the FEM and a genetic algorithm to find the optimal cycle parameters. It is found that in comparison to the manufacturer's recommended cycle, for a laminate with the span/thickness of 12.5, one optimal cycle reduces residual stresses by 47% and the total cure time from 5 to 4 hours, and another reduces the total cure time to 2 hours and residual stresses by 8%. / Ph. D. / We analyze using computational techniques three mechanics problems described below. In the last three decades, two carbon nanomaterials (i.e., allotropes of carbon having length-scale of 10^-9 m), namely, single wall carbon nanotubes (SWCNTs) and single layer graphene sheets (SLGSs) have evolved as revolutionary materials with exceptional properties per unit weight that are superior to conventional engineering materials. A composite (i.e., a material made by combining two or more materials to attain desired properties which cannot be achieved by any of its constituents alone) made by using either of these carbon nanomaterials as reinforcements in a polymer could be a potential candidate for applications requiring high strength and light weight. However, the effective utilization of these composites for an application requires the strong binding between their constituents. An effective technique to enhance this binding is to modify the surface properties of SWCNTs and SLGSs by covalently bonding to them suitable chemical group that is usually called covalent functionalization. However, this damages their pristine structures that may degrade their mechanical properties. Here, it is found that the functionalization reduces elastic moduli of carbon nanomaterials, the reduction increases with an increase in the amount of functionalization and is essentially independent of the functionalizing chemical group. This study should help engineers interested in utilizing these materials to design novel nanocomposites. Composite laminates, made by stacking and binding together layers of fiber-reinforced composites, are widely used in aircraft, aerospace, marine, automobile, power generation, chemical and ballistic applications due to their high strength and stiffness per unit weight compared to that of conventional metallic materials. Shell theories are widely used to analyze deformations of composite laminates which reduces a 3-dimensional (3-D) problem to an equivalent 2-D problem by making certain assumptions related to the deformations of the laminate. This approach requires less computational effort to find a numerical solution (i.e., an approximate solution obtained using a computational technique) of the problem as compared to that needed for solving the full 3-D problem. However, the accuracy of the results predicted by a shell theory depends on the problem being studied, i.e., the shell geometry, applied loads, initial conditions (i.e., the motion of the laminate at the start of application of the load) and boundary conditions (i.e., constraints imposed on the deformations of the edges of the laminate). Here, we analyze effects of geometric parameters of the laminated shells on their deformations for different types of applied loads and various boundary conditions specified on the edges. The results should help designers find an optimal geometry of the composite laminates for a given mechanical application. Fiber-reinforced composite laminates are usually fabricated by curing (which involves heating and cooling in a prescribed manner under application of the pressure) resin preimpregnated fiber layers under prescribed temperature and pressure cycles. However, during this cure process the laminate deforms and the final product is not stress-free. Here, we find optimal parameters of the cure cycle that minimize stresses developed during the cure process as well as the time required to cure the laminate. It is found that for a laminate studied these optimal parameters reduce the stresses by 47% and the cure time from 5 to 4 hours in comparison to the standard cure cycle recommended by the laminate manufacturer. This study will provide manufacturing engineers with an approach to fabricate composite laminates of desired quality.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Les protéines-G hétérotrimériques, constituées des sous-unités α, β et γ, sont les premières actrices de la transduction du signal en interagissant directement avec les Récepteurs Couplés aux protéines-G (RCPG). Les protéines-G ont la capacité de lier soit une molécule de GDP lorsqu'elles sont inactives, soit une molécule de GTP quand elles sont activées par un RCPG. Cet échange de nucléotide va conduire à la dissociation de l'hétérotrimère avec d'une part la sous-unité α seule, et d'autre part le complexe βγ. Chacune de ces entités va ensuite propager le signal dans le compartiment intracellulaire. Les travaux effectués au cours de cette thèse ont pour but de mieux comprendre la dynamique des protéines-G hétérotrimériques et de leurs récepteurs par des techniques de mécanique moléculaire incluant la Dynamique Moléculaire (DM) et l'Analyse de Modes Normaux (AMN). Dans un premier temps une AMN nous a permis de décrire les possibles mouvements de larges amplitudes des protéine-G. Nous avons à l'occasion de cette étude mis au point une méthode de sélection de Modes Normaux (MN) pertinents que nous avons appelés modes représentatifs. Nous avons également développé une méthode d'extraction de ligand (ici le GDP) le long de ces MN. Ceci nous a permis de montrer qu'un mouvement concerté de toute la sous-unité α pouvait permettre l'ouverture de la poche et la sortie du GDP. Dans un deuxième temps, nous avons affiné nos résultats en reconstruisant des profils d'énergie libre le long de plusieurs chemins de sortie possibles pour le GDP. Ainsi nous avons pu proposer un mécanisme fin de sortie du ligand et plusieurs résidus clés impliqués dans cette sortie. Nous avons également étudié le processus de dissociation de l'hétérotrimère par la technique de la Dynamique Moléculaire Dirigée. Il a été possible, à l'issue de cette étude, de proposer un mécanisme à l'échelle moléculaire de la séparation des sous-unités α et βγ. Pour finir, nous avons également étudié le macro-complexe RCPG : protéine-G. Deux études traitent des mécanismes d'activation et de couplage des protéines-G à son récepteur. Nous avons notamment montré que l'hétérotrimère de protéine-G contraint très fortement les mouvements du récepteur. Un mouvement très largement retrouvé dans le complexe ainsi que dans plusieurs autres RCPGs dont les structures sont connues a été proposé comme étant le mouvement d'activation des RCPG une fois complexés à leurs protéines partenaires. / Heterotrimeric G-proteins, constituted of α, β and γ subunits are the first actresses of the intra-cellular signal transduction and interact directly with G-protein Coupled Receptors (GPCR). The heterotrimer is able to bind either a GDP molecule (inactive state) or a GTP molecule (active state). The nucleotide exchange is triggered by the interaction with an activated GPCR and leads to the dissociation of the whole heterotrimer into two independant entities : α and tightly bound βγ subunits. Both subunits further propagate the signal into the intracellular compartment. Goals of the present work were to better understand the mechanics of G-proteins and GPCR by combining several molecular mechanics techniques such as Molecular Dynamics (MD) and Normal Mode Analysis (NMA).Firstly, we described large amplitude motions of the whole G-protein heterotrimer. In this study we developped a method to select relevant Normal Modes (NM), we called representative NM. We also developped a method which consists to extract a ligand (in our case the GDP) out of its binding pocket along computed NM. With these two new methods, we showed that a concerted motion of the α subunit would promote the opening of the pocket and the release of the GDP.Secondly, to refine our results, we performed free energy profiles reconstructions along several putative exit pathways of the GDP. Thus, we proposed for the first time a fine-tuned mechanism of GDP exit at the molecular scale and putative key-residues. We proposed also a molecular scale mechanism for the dissociation of the heterotrimeric G-protein through the use of the Targeted Molecular Dynamics (TMD). Finally we were interested in the study of the GPCR:G-protein complex. We performed two studies related to the activation and to the coupling of the macro-complex. We showed that G-protein constrain drastically the GPCR motions. One over-represented motion in the complex that was also retrieved in other crystallized structures of several different GPCRs thus suggested that this motion could be the putative activation motion of a GPCR when complexed to its favorite protein partners.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Des études antérieures ont mis en évidence que le transfert de solutés neutres à travers des membranes est influencé par la présence d'ions en solution. Ainsi, la connaissance des interactions multiples à l'échelle nanométrique, entre le polymère, l'eau et les solutés (ions, espèces organiques) constituent un verrou pour l'amélioration des performances des procédés membranaires. Dans cette étude une approche multi-échelle fondamentale est proposée, combinant des outils théoriques et expérimentaux, afin d'obtenir les paramètres microscopiques et macroscopiques caractérisant les interactions étudiées pour différentes compositions ioniques. Plus précisément, il s'agit de comprendre comment les ions affectent le transfert d'un soluté organique. Dans un premier temps, certaines propriétés caractérisant l'hydratation des ions sont calculées et comparées aux flux de diffusions de sucres à travers des membranes de Nanofiltration et échangeuses d'ions obtenus pour différentes compositions ioniques. Dans un deuxième temps, des systèmes constitués d'une membrane échangeuse de cations (CMX) équilibrée avec différents cations ainsi que le glucose hydraté sont modélisés en utilisant une approche combinée Mécanique Quantique/ Mécanique Moléculaire. Cette approche a permis d'étudier la solubilité du sucre dans la matrice polymère ainsi que les interactions polymère-polymère comme l'énergie de cohésion. Enfin, l'influence des ions sur les caractéristiques physiques de la membrane CMX est étudiée en utilisant diverses méthodes expérimentales comme la détermination des angles de contacts et des spectres IR ou la mesure de la température de solidification par DSC. Les propriétés vibrationnelles sont également calculées dans le cadre de la théorie de la fonctionnelle de la densité (DFT). L'ensemble de ces données sont comparées avec les grandeurs de transport afin de valider les mécanismes moléculaires proposés. Ce travail montre que la nature des contre-ions de la membrane modifie l'énergie de cohésion entre les fragments de la membrane. Ainsi, l'énergie de cohésion influe sur la diffusion des composés organiques neutres à travers les membranes. / Previous works have shown that the transfer of neutral solutes through membranes is influenced by the presence of ions in solution. In the framework of process intensification, the knowledge of the molecular mechanisms involved is of fundamental importance to increase and predict the process performances. The aim of this Thesis is to use a combined quantum/molecular computational approach and experimental methodologies to better understand how ions can affect the solute flux. In the first part of the work, some properties of ions in solution are computed and compared with sugar fluxes through membranes for nanofiltration and electrodialysis. In the following, systems composed of Cation-exchange membrane equilibrated by different counter-ion and hydrated glucose are examined by Quantum Mechanics/Molecular Mechanics. This is done mainly to investigate the sugar solubility in the polymer matrix and diffusion related interactions like polymer chain-chain cohesion energy. In the last part, contact angle, differential scanning calorimetry and Infra-Red spectra are measured to characterize the physical properties of the membrane and possible influence of the counter-ion on cation exchange membrane. This work shows that the nature of the counter-ions modifies the cohesion energy between the membrane polymer fragments. In its turn, the cohesion energy affects the diffusion of neutral organic compounds through the membranes.

Made available in DSpace on 2016-08-18T18:19:27Z (GMT). No. of bitstreams: 1 Arivaldo Cutrim Carvalho.pdf: 4594687 bytes, checksum: c5792781a97f8174ae6dcd9a0bfbd359 (MD5) Previous issue date: 2010-10-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / With the aim of the development of new nanodevices, there is great interest in understand the electronic properties of nanostructured materials. Above all, how to modify the electronic properties of nanostructures already well known in a controlled manner. With this goal, many methodologies and experiments has been developed. We studied the an entirely through computer simulation atomistic interaction of amino glycine with the surface of graphene using two methods, classical and quantum, for both modules use Materials Studio (Accelrys), and the Forcit Dmol3 states that are Art in atomistic simulations. From the classical point of view, we used force fields universal to describe the interactions, and the quantum point of view, the method of density functional. The methodology consisted basically realize a scan with glycine in different orientations on the surface of the graphene sheet grid in a considerable build a 3D map of potential interaction that enables us to accurately define where are enough sites and orientations of the amino acid glycine to more energetically favorable for adsorption. From the selection of the best candidates obtained from calculations in classical mechanics, we performed electronic structure calculations using the method DFT (Density Functional Theory) to estimate the binding energy and in that regime adsorption occurs. In addition, we obtained the electron density of the system and did Mulliken population analysis as well. / Com a finalidade do desenvolvimentos de novos nanodispositivos, há um grande interesse em conhecer as propriedades eletrônicas de materias nanoestruturados. Sobretudo, como modificar as propriedades eletrônicas de nanoestruturas já bem conhecidas de forma controlada. Com este objetivo, muitas metodologias e experimentos tem sido desenvolvidos. Estudou-se de forma inteiramente atomística através de simulação computacional a interação do aminoácido glicina com a superfície do grafeno utilizando dois métodos , clássico e quântico, para tanto utilizamos os módulos do Materials Studio (Accelrys), o Forcite e o Dmol3 que são estados de arte em simulações atomísticas. Do ponto de vista clássico, utilizou-se campos de força universal para descrever as interações; e do ponto de vista quântico, utilizamos o método do funcional da densidade. A metodologia consistiu basicamente em realizarmos um "scan"com a glicina em diversas orientações sobre a superfície da folha de grafeno num grid considerável, construímos uma mapa 3D do potencial de interação que nos possibilita conhecer com precisão suficiente onde são os sítios e as orientações do aminoácido glicina que mais favoráveis energeticamente para a adsorção. A partir da seleção dos melhores candidatos obtidos através dos cálculos de mecânica clássica, realizamos cálculos de estrutura eletrônica utilizando o método DFT (Density Functional Theory) a fim de estimar a energia de ligação e em que regime ocorre a adsorção. Além disso, nós obtivemos a densidade eletrônica do sistema e fizemos uma análise populacional de Mulliken também.