Magister Scientiae - MSc (Chemistry) / This work involves the synthesis of copper telluride-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuTe@PPI) and copper antimonide-polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (CuSb@PPI) core-shell nanoparticles (NPs), using two-pots and one-pot synthesis methods, respectively. Their morphology was studied by X-ray diffraction spectroscopy (XRD), high resolution transmission electron microscopy (HRTEM) and high resolution scanning electron microscopy (HRSEM); while their structures were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis. Photophysical properties of the core-shell NPs were determined from ultraviolet-visible absorption spectroscopy (UV-Vis) and photoluminescence spectroscopy (PL). For core-shell NPs produced via two-pots method only CuTe@PPI exhibited ? ? ?* and n ? ?* which indicate that CuSb@PPI produced via two-pots method was unsuccessfully synthesized. The ? ? ?* and n ? ?* transitions indicate the presence of polypropylenimine tetra(5-(2-thienyl) salicylaldimine) (PPI) on the surface of CuTe NPs and CuSb NPs. FTIR confirmed coordination of PPI on the surface of CuTe NPs and CuSb NPs by showing a shift in wavenumber of C=N group bands from PPI. HR-TEM showed that the CuTe@PPI synthesized via one-pot method have a wide particles sizes distribution with an average particles size of 13.60 nm while for CuTe@PPI synthesized via two-pots it was impossible to determine the particles size due to aggregation. CuSb@PPI synthesized via twopots method and one-pot method has a wide particles sizes distribution with an average size of 7.98 nm and 11.61 nm respectively. The average particles sizes determined by HR-SEM were found to be 35.24 nm (CuTe@PPI two-pots method), 33.90 nm (CuTe@PPI one-pot method), 18.30 nm (CuSb@PPI two-pots method), and 16.18 nm (CuSb@PPI one pot method). / 2021-08-31

Le développement de nouveaux matériaux à hautes performances dépend fortement des procédés de frittage mis en oeuvre. La réduction de l’énergie libre de surface, force motrice de la densification, peut être activée en appliquant une pression extérieure et/ou en améliorant les processus de diffusion en phase solide ou liquide à l’aide de chauffages ultra rapides, les procédés associés requérant de hautes températures. Ainsi, le challenge est de permettre une densification à basse température afin de surmonter les verrous technologiques actuels (procédé peu coûteux et économe en énergie ; frittage de matériaux métastables, à basse température de décomposition et/ou nanométriques ; cofrittage de multimatériaux). Dans ce contexte, un procédé innovant de frittage hydrothermal inspiré des processus géologiques de densification a été développé : une contrainte uniaxiale est appliquée à une poudre en présence d’eau en conditions hydrothermales sur des durées relativement courtes. La force motrice principale réside dans les gradients de contrainte intragranulaires générant des phénomènes de dissolution-précipitation aux interfaces liquide/solide. Outre une optimisation du procédé, l’objectif principal a été la compréhension des mécanismes complexes spécifiques au frittage hydrothermal d’un matériau modèle, la silice nanométrique. Il a été montré que les effets mécano-chimiques à l’origine du fluage par dissolution sous contrainte sont assistés avec synergie par des effets chimiques de type polycondensation. L’influence de chaque paramètre de frittage (température, pression, durée de palier, rampe de montée en température, quantité de solvant, utilisation d’un co-solvant ou d’un agent minéralisateur) a été identifiée et a permis d’optimiser la densification de la silice (86-88% de compacité). De plus, du quartz-α massif polycristallin et nanométrique a pu être obtenu avec une densité relative de 98%. Enfin, le frittage hydrothermal a été mis en oeuvre pour la densification de multimatériaux complexes. Des nanocomposites de type 0-3 où des nanoparticules de pérovskite de manganèse sont dispersées dans une matrice de silice ont ainsi été obtenus. L’apport de cette nanostructuration sur les propriétés de magnétotransport a été évalué. / The development of new high performance advanced materials is strongly dependent on the mastering of sintering processes. The driving force for densification is the decrease of surface free energy, which can be promoted either by applying a pressure and/or by enhancing diffusional mechanisms in a solid or liquid phase with ultra-fast heating routes. High temperatures are then usually required in the as-involved processes. The challenge is to perform densification at low temperature in order to overcome current technological barriers (energy- and cost-efficiency of the process; sintering of metastable, low temperature decomposition and/or nanometric materials; cosintering of multimaterials). In this context, we have developed an innovative hydrothermal sintering process which is geologically-inspired: a powder mixed with water is externally and mechanically compressed under hydrothermal conditions over short time periods. The main driving force is the stress gradient within grains induced by external uniaxial compression which allows the activation of the dissolution/precipitation phenomenon at solid/liquid interfaces. Besides the technological development of the apparatus, our goal was to understand all the complex mechanisms involved in the hydrothermal sintering of a model material, nanometric silica. We have shown that the mechanical-chemical effects (pressure solution creep) were synergistically assisted by chemical ones (polycondensation). The influence of each parameter (temperature, pressure, time, heating rate, solvent amount, use of a co-solvent or of a mineralizer) were investigated. Consequently, the densification of silica was optimized, reaching 86-88% of relative density. In addition, bulk polycrystalline nanometric α -quartz with 98% of relative density was obtained. Finally, the hydrothermal sintering process has been implemented to densify complex multimaterials. In this way, 0-3 type nanocomposites where nanometric manganese perovskite are embedded in a silica matrix have been obtained. The advantage of nanostructuration on magnetotransport properties was evaluated.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Foram sintetizadas microesferas poliméricas com propriedades magnéticas e morfologia casca-núcleo por meio da técnica de polimerização em suspensão em duas etapas. O núcleo foi constituído por poli(estireno-co-divinilbenzeno) e magnetita modificada com ácido oleico. Foi avaliada a influência da velocidade de agitação e da concentração de iniciador sobre as características das microesferas utilizadas como núcleo (morfologia, tamanho de partículas, propriedades magnéticas e estabilidade térmica). A casca foi constituída por poli(estireno-co-divinilbenzeno) sem material magnético. Foi avaliado o método de adição da emulsão dos monômeros formadores da casca e o tempo de inchamento dos núcleos na emulsão. As microesferas casca-núcleo foram caracterizadas quanto ao seu aspecto morfológico e à estabilidade térmica. Os núcleos não apresentaram ciclos de histerese, estando assim próximos de um material com propriedades superparamagnéticas. O copolímero sintetizado com a maior velocidade de agitação e a menor concentração de iniciador foi o que apresentou o maior teor de ferro incorporado (3,317 %), a maior magnetização de saturação (2,99 emu/g) e o menor diâmetro médio de partículas (81 m). As microesferas casca-núcleo apresentaram apenas um estágio de degradação e as suas Tmáx foram menores do que a do núcleo. O mapa composicional de ferro confirmou a presença de magnetita na superfície das microesferas casca-núcleo / Polymeric microspheres with magnetic properties and core-shell morphology were synthesized by the technique of suspension polymerization in two stages. The core was constituted by poly(styrene-co-divinylbenzene) and magnetite coated with oleic acid. The influence of stirring rate and initiator concentration on microspheres characteristics (morphology, particle size, magnetic properties and thermal stability) was studied. The shell was constituted by poly(styrene-co-divinylbenzene) without magnetic material. It was also evaluated the methods of the monomers emulsion addition and the coress swelling time in the emulsion.. The microspheres were characterized by morphologic aspect and thermal stability. The cores did not presented hysteresis cycles, thus proving the superparamagnetic properties. Copolymer synthesized with the higher stirring rate and lower initiator concentration presented the higher incorporated iron content (3.317 %), higher saturation magnetization (2.99 emu/g) and the lower average particle diameter (81 m). Core-shell microspheres presented only one stage of degradation and the Tmáx were smaller than temperature of the cores. Iron compositional map confirmed the presence of magnetite on the surface of core-shell microspheres

Copolímeros casca-núcleo de poli(acrilato de butila) (núcleo) e poliestireno (casca) foram sintetizados por meio de polimerização em emulsão, conduzida em duas etapas. A adição de ácido itacônico como monômero funcional na polimerização do núcleo foi realizada para verificar seu efeito sobre suas propriedades mecânicas e de processamento. Os copolímeros foram caracterizados por espalhamento dinâmico de luz (DLS), microscopia eletrônica de transmissão (MET), cromatografia de exclusão por tamanho (SEC), espectrometria na região do infravermelho (FTIR) e calorimetria diferencial por varredura (DSC). A incorporação do monômero funcional foi confirmada por DSC e quantificada por titulação. A proporção de poli(acrilato de butila) e poliestireno influenciou diretamente o processamento e as propriedades mecânicas do polímero. Os copolímeros com teores de poliestireno acima de 50% foram processados por compressão e extrusão a temperatura ambiente, apresentando comportamento baroplástico. A presença do monômero funcional não alterou o processamento do polímero e melhorou significativamente sua resistência à tração, aumentando sua tenacidade / Core shell copolymers of poly(butyl acrylate) cores and polystyrene shells were synthesized in a two-stage emulsion polymerization. The addition of itaconic acid as functional monomer in the core polymerization was carried out in order to study its influence on polymer processing and mechanical properties. The copolymers were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The functional monomer incorporation was confirmed by DSC and quantified by titration. The poly(butyl acrylate) and polystyrene proportions had a direct influence on the polymer processing and mechanical properties. Copolymers with polystyrene contents higher than 50% presented baroplastic behavior and they could be processed at room temperature by compression molding and extrusion. The presence of itaconic acid did not affect polymer processing and significantly improved tensile resistance, increasing its toughness

Polímeros híbridos são caracterizados por conter numa mesma partícula dois tipos diferentes de polímero, normalmente incompatíveis e ligados por ligações covalentes. Neste trabalho, a síntese de um polímero híbrido acrílico-alquídico pelo processo de miniemulsão foi investigada. Com o objetivo de compreender como diferentes variáveis na fórmula afetam as características físico-químicas finais do látex, um planejamento experimental foi desenhado com o auxílio do software DX9 e os resultados analisados por modelos empíricos de superfície de resposta. Os fatores variados (variáveis independentes) foram: quantidade de co-estabilizante, surfactante e iniciador. Cada um dos fatores contribuiu para a alteração de pelo menos uma característica avaliada, e a sinergia entre eles afeta diretamente o teor de RDG, resposta indicativa do grau de grafitização entre as espécies. O coestabilizante contribuiu para o aumento do diâmetro médio de gotas e da polidispersidade de tamanho de partículas, enquanto reduziu a dureza do filme. A concentração de surfactante alterou o número de partículas por litro, reduzindo o diâmetro médio de partículas e aumentando a polidispersidade. O iniciador aumentou a conversão monomérica. Todas as dispersões de polímero híbrido produzidas apresentaram redução do tamanho médio de gotas para partículas, indicando evidência de nucleação secundária. As avaliações por microscopia eletrônica de transmissão (TEM) e microscopia de força atômica (AFM) mostraram que a morfologia obtida é do tipo núcleo-casca. / Hybrid polymers are defined by the presence of two or more different kinds of polymers in the same particle, typically incompatible and linked by covalent bonds. In this work, the synthesis of an alkyd-acrylic hybrid polymer produced by miniemulsion polymerization has been investigated. With the goal of understanding how different variables in the recipe affect the final characteristics of the latex, a design of experiment was planned using DX9 software and the results were analyzed by response surface methodology. The factors studied (independent variables) were: amount of co-stabilizer, surfactant and initiator. Each of these factors contributed to change at least one of the evaluated characteristics, and the synergy between them directly affects the RDG content, response of the polymer degree of grafting. Costabilizer contributed to increase the average droplets diameter and polydispersity of particle size, while reducing the film hardness. The amount of surfactant changed the number of particles, reducing the average particle size and increasing polydispersity. Initiator increased the monomer conversion. All hybrids dispersions presented a decrease in the droplet size to particle size, indicating evidence of secondary nucleation. Core-shell morphology was observed through transmission electron microscopy (TEM) and atomic force microscopy (AFM).

O objetivo do trabalho foi sintetizar e caracterizar nanopartículas de poliestireno e polimetacrilato de metila com morfologia núcleo-casca obtidas através da polimerização em emulsão sem emulsificante em dois estágios e através da fotopolimerização. Nos experimentos avaliou-se a influência de diferentes condições operacionais baseadas em fatores cinéticos sobre os dois estágios da polimerização. As condições avaliadas para o primeiro estágio de preparação de núcleos foram: a temperatura do processo, a concentração de iniciador e de monômero, e o efeito de agentes modificadores de superfície (sal, co-monômero e reticulante). Para o segundo estágio de formação da casca as condições avaliadas foram: a concentração e o regime de alimentação de monômero. Os resultados experimentais mostraram que uma relação de co-monômero e reticulante é a melhor alternativa para preparar núcleos estáveis e de diâmetros pequenos. Assim, o revestimento uniforme dos núcleos é obtido no segundo estágio e com isso a morfologia núcleo-casca em equilíbrio é alcançada. Dois sistemas experimentais (reator convencional e reator fotoquímico) foram propostos e avaliados na sua eficiência para obter partículas com este tipo de morfologia no segundo estágio (formação do revestimento). As diversas técnicas de caracterização indicaram a formação de morfologia núcleo-casca na maioria dos casos estudados para os dois sistemas propostos. Por sua vez, as técnicas espectroscópicas (NIR e Raman) permitiram o monitoramento do processo em tempo real e a elaboração de modelos de calibração que correlacionaram o crescimento do tamanho da partícula núcleo. Da mesma forma, os fatores termodinâmicos foram estudados para predizer a morfologia final esperada nos sistemas. Os resultados, experimental e predito são comparados e discutidos em termos de aspectos chave envolvidos no controle da morfologia da partícula. / This work aimed at studying the synthesis and characterization of core-shell nanoparticles of polystyrene and polymethylmethacrylate obtained in a two-stage emulsifier-free emulsion polymerization and photopolymerization. The influence of different operational conditions based on kinetic factors was experimentally evaluated. In the first stage (seed preparation) the process temperature, initiator and monomer concentrations and the effect of surface-modifier agents (acids, salts, co-monomers and crosslinker) were investigated; similarly in the second stage, the concentration and feeding regime of monomer were evaluated with respect to the shell formation. Experimental results showed that both, crosslinker and co-monomer are the best alternative to achieve a stable seed with small diameter; hence, with this core, uniform coating is obtained in the second stage and core-shell morphology is reached. In order to evaluate the efficiency of the preparation of core-shell particles, two experimental systems (conventional and photochemical reactor) were studied. Different characterization techniques indicated that in most of the cases studied particles with the desired core-shell morphology were formed. The use of spectroscopic techniques NIR and Raman were tested for the real-time monitoring of the process using adequate calibration models developed to correlate the average size of the growing core particle with the spectra. In the same way, thermodynamic factors were used to predict the expected final morphology of the particles. Experimental and predicted results were compared and discussed in terms of the key aspects involved in the control of the particle morphology.

Made available in DSpace on 2014-07-29T15:12:47Z (GMT). No. of bitstreams: 1 dissetacao michelly.pdf: 1756345 bytes, checksum: 8d8e696af201df577651d9a67b7d21e1 (MD5) Previous issue date: 2008-11-25 / Magnetic nanoparticles coated with several kinds of materials have been the focus of intense research due to their potential applications in biomedicine. This type of material contains a magnetic core that is involved by a shell of different composition. The coating of magnetic nanoparticles with amino-functionalized silica is particularly important because of its reactivity which allows the coupling with biological molecules such as fragments of DNA, antibodies, proteins, among others. In this work, maghemite nanoparticles were coated with silica and with amino-functionalized silica, the incorporation of them being achieved by using tetraethoxysilane (TEOS) and aminopropyltrimethoxysilane (APTS) as the precursors of the sol-gel system. Firstly, the magnetic fluids of maghemita were prepared and afterwards they were employed in the silica-coating procedures which were based on the Stöber method. Some experimental parameters were varied, such as the number of maghemita nanoparticles, pH of the reaction medium, time of reaction and amount of TEOS, with the aim of modulating the characteristics of the solids and obtaining stable aqueous suspensions of the materials. All products were analyzed by the following techniques: X- ray diffraction, diffuse reflectance infrared spectroscopy, dynamic light scattering, electrophoretic mobility (zeta potential) and transmission electron microscopy. / Nanopartículas magnéticas recobertas com diversos tipos de materiais tem recebido sido o foco de intensa pesquisa devido a suas aplicações potenciais na área biomédica. Este tipo de material contém um núcleo magnético ( core ) que é envolvido por uma coroa ( shell ) de composição diferente da nanopartícula. O recobrimento de nanopartículas magnéticas com sílica funcionalizada por grupos amino é particularmente importante dada à reatividade deste, possibilitando inclusive o acoplamento de moléculas biológicas, como fragmentos de DNA, anticorpos, peptídeos, dentre outros. Neste trabalho nanopartículas de maguemita foram recobertas com sílica e com sílica funcionalizada com grupos amina, sendo que a incorporação desses materiais ocorreu por meio dos precursores tetraetoxissilano (TEOS) e aminopropiltrimetoxissilano (APTS) utilizando-se de princípios do processo solgel. Primeiramente, foram preparados os fluidos magnéticos de maguemita e, em seguida, estes foram utilizados nos procedimentos de recobrimento da maguemita com sílica, os quais se basearam no método de Stöber. Alguns parâmetros experimentais foram variados, como o número de partículas de maguemita, pH do meio reacional, tempo de síntese e quantidade de TEOS, visando modular as características dos sólidos e obter suspensões aquosas estáveis dos materiais preparados. Os produtos obtidos foram analisados pelas seguintes técnicas: difratometria de raios-X, espectroscopia no infravermelho médio por refletância difusa, espalhamento dinâmico de luz, mobilidade eletroforética (potencial zeta) e microscopia eletrônica de transmissão.

O uso de pesticidas levou ao aumento da produtividade e qualidade dos produtos agrícolas, porém o seu uso acarreta na intoxicação dos seres vivos pela ingestão gradativa de seus resíduos que contaminam o solo, a água e os alimentos. Dessa forma, há a necessidade do monitoramento constante de suas concentrações nos compartimentos ambientais. Para isto, busca-se o desenvolvimento de métodos de extração e enriquecimento de forma rápida, com baixo custo, gerando um baixo volume de resíduos, contribuindo com a química verde. Dentre estes métodos destacam-se a extração por banho de ultrassom e a extração por ponto nuvem. Após o procedimento de extração, o extrato obtido pode ser analisado por técnicas de Cromatografia a Líquido de Alta Eficiência (HPLC) e a Cromatografia por Injeção Sequencial (SIC), empregando fases estacionárias modernas, tais como as monolíticas e as partículas superficialmente porosas. O emprego de SIC com coluna monolítica (C18, 50 x 4,6 mm) e empacotada com partículas superficialmente porosas (C18, 30 x 4,6 mm, tamanho de partícula 2,7 µm) foi estudado para separação de simazina (SIM) e atrazina (ATR), e seus metabólitos, desetilatrazina (DEA), desisopropilatrazina (DIA) e hidroxiatrazina (HAT). A separação foi obtida por eluição passo-a-passo, com fases móveis compostas de acetonitrila (ACN) e tampão Acetato de Amônio/Ácido acético (NH4Ac/HAc) 2,5 mM pH 4,2. A separação na coluna monolítica foi realizada com duas fases móveis: MP1= 15:85 (v v-1) ACN:NH4Ac/HAc e MP2= 35:65 (v v-1) ACN:NH4Ac/HAc a uma vazão de 35 µL s-1. A separação na coluna com partículas superficialmente porosas foi efetivada com as fases móveis MP1= 13:87 (v v-1) ACN: NH4Ac/HAc e MP2= 35:65 (v v-1) ACN:NH4Ac/HAc à vazão de 8 µL s-1. A extração por banho de ultrassom em solo fortificado com os herbicidas (100 e 1000 µg kg-1) resultou em recuperações entre 42 e 160%. A separação de DEA, DIA, HAT, SIM e ATR empregando HPLC foi obtida por um gradiente linear de 13 a 35% para a coluna monolítica e de 10 a 35% ACN na coluna com partículas superficialmente porosas, sendo a fase aquosa constituída por tampão NH4Ac/HAc 2,5 mM pH 4,2. Em ambas as colunas a vazão foi de 1,5 mL min-1 e o tempo de análise 15 min. A extração por banho de ultrassom das amostras de solo com presença de ATR, fortificadas com concentrações de 250 a 1000 µg kg-1, proporcionou recuperações entre 40 e 86%. A presença de ATR foi confirmada por espectrometria de massas. Foram realizados estudos de fortificação com ATR e SIM em amostras de água empregando a extração por ponto nuvem com o surfactante Triton-X114. A separação empregando HPLC foi obtida por um gradiente linear de 13 a 90% de ACN para a coluna monolítica e de 10 a 90% de ACN para a coluna empacotada, sempre em tampão NH4Ac/HAc 2,5 mM pH 4,2. Em ambas as colunas a vazão foi de 1,5 mL min-1 e o tempo de análise 16 min. Fortificações entre 1 e 50 µg L-1 resultaram em recuperações entre 65 e 132%. / The use of pesticides has led to increased productivity and quality of agricultural products, but its use brings the intoxication of living beings by the gradual intake of their residues that contaminate soil, water and food. Thus, there is the need for constant monitoring of their concentrations in environmental compartments. For this, there is a quest for development of fast extraction and enrichment methods, at low cost, generating a low volume of waste, contributing to green chemistry. These methods include the extraction assisted by ultrasound bath and the cloud point extraction. After the extraction, the extract obtained can be analyzed by techniques such as High Performance Liquid Chromatography (HPLC) and Sequential Injection Chromatography (SIC) employing modern stationary phases, such as monolithic and superficially porous particles. The use of SIC with either monolithic column (C18, 50 x 4.6 mm) or column packed with superficially porous particles (C18, 30 x 4.6 mm, 2.7 µM particle size) was studied for separating simazine (SIM) and atrazine (ATR), and metabolites, deethylatrazine (DEA), deisopropylatrazine (DIA) and hydroxyatrazine (HAT). Separation was obtained by stepwise elution, with a mobile phase consisting of acetonitrile (ACN) and 2.5 mM ammonium acetate / acetic acid (NH4Ac / HAc) pH 4.2 buffer. The separation was performed with two mobile phases: MP1 = 15:85 (v v -1) ACN: NH4Ac / HAc and MP2 = 35:65 (v v-1) ACN: NH4Ac / HAc at a flow rate of 35 µL s-1. The separation in the column with superficially porous particles was carried out with the mobile phases MP1 = 13:87 (v v-1) ACN: NH4Ac / HAc and MP2 = 35:65 (v v-1) ACN: NH4Ac / HAc at 8 µL s-1. The ultrasound bath extraction from a soil fortified with herbicides (100 and 1000 µg kg-1) resulted in recoveries between 42 and 160 %. Separation of DEA, DIA, HAT SIM and ATR was obtained by HPLC employing a linear gradient from 13 to 35% for the monolithic column and from 10 to 35% ACN in the column packed with superficially porous particles, with the aqueous phases consisting of 2.5 mM NH4Ac / HAc buffer pH 4.2. In both columns the flow rate was 1.5 mL min-1 and the analysis time was15 min. Ultrasonic extraction from a soil sample containing ATR, fortified with concentrations from 250 to 1000 µg kg-1, provided recoveries between 40 and 86%. The presence of ATR was confirmed by mass spectrometry. Fortification studies with ATR and SIM were carried out in water samples employing cloud point extraction using Triton-X114 surfactant. The separation was accomplished by HPLC using a linear gradient from 13 to 90 % of ACN for the monolithic column and from 10 to 90 % ACN for the packed column, always in 2.5 mM NH4Ac / HAc pH 4.2 buffer. In both columns the flow rate was 1.5 mL min -1 and the analysis time 16 min. Fortifications between 1 and 50 µg L-1 resulted in recoveries between 65 and 132%.

L'objectif de la présente thèse est d'améliorer les propriétés barrières du MetPET en conditions sévères. Plus précisément d'établir les corrélations structure-propriétés : structure du primaire d'accroche déposé en ligne sur le film PET avant métallisation et propriétés barrières et d'adhésion métal en condition sèche et humide du composite final obtenu (MetPET). L'approche choisie est la polymérisation en émulsion en régime semi-continu. Cette approche permettra d'apporter un caractère hydrophobe (phase 1-Coeur) favorisant la barrière à l'humidité et un caractère hydrophile (phase 2-Ecorce) en surface présentant des groupes fonctionnels favorables à l'adhésion métal en condition humide. De plus nous avons aussi investigué l'influences de divers additifs de formulations afin d'optimiser les propriétés du primaire d'accroche. Les latex présentant une structure de type cœur/écorce est très favorable pour la barrière à l'humidité. En effet ces latex présentent une perméabilité à l'eau sensiblement plus faible que les latex sans structure (100% composition de l'écorce). Concernant la perméabilité à l'oxygène celle-ci est principalement assurée par la couche métallique. La formulation de nos latex cœur/écorce par des agents mouillants s'est révélé indispensable pour assurer un bon étalement de nos latex sur le substrat PET. En effet nos latex issus de la synthèse ont été synthétisés avec un minimum de tensio-actif, ce qui a eu pour effet négatif d'obtenir une tension de surface trop élevée pour assurer leur mouillabilité sur le film PET. De plus cet agent mouillant a aussi montré une influence positive sur les performances d'adhésion de la couche métal sur le film PET traité avec le primaire d'accroche. La formulation de nos latex cœur/écorce par un réticulant a montré un influence positive sur les propriétés barrières aux gaz et sur les performances d'adhésion métal. Cet agent réticulant tend à favoriser la réticulation entre particule et ainsi favoriser la formation d'un film continu barrière à l'eau. De plus celui-ci apporte des fonctions de type azoté favorable aux interactions avec les atomes métallique (aluminium) / The proposition of this work is to improve the barrier property of a laminate aimed to be applied in the field of packaging for foodstuffs. This laminate is constituted by a PET substrate metallized with aluminum deposited under vacuum. Given that PET is not a strongly polar polymer it is necessary to improve its adhesion to metals. This study was carried out in order to understand if there is a way to make sure that the polar groups will be bonded to the metal without being disturbed by the conditions of the environment. Our approach was the semi-batch emulsion polymerization, followed by the restructuration of the initial system (regular spherical polymeric nanoparticles) by changing its original morphology. Finally, the latexes were formulated by using variable amounts of compounds that were expected to improve the properties of the final material in terms of barrier and metallic adhesion. A part of the latexes synthesized in the scope of this work was submitted to pilot trials in an industrial line of PET extrusion. The latexes were used to coat the PET inline. The core-shell nanoparticles presented a lower permeability to water than the particles synthesized in the absence of the seed. This was related to the tortuosity promoted by the core, which increases the pathway of a diffusing molecule. The permeability to oxygen was found to be mainly related to the metallic layer. Given that the latexes were synthesized with the minimum amount of tensioactive necessary to originate stable dispersions, the wettability agent was found to be indispensable for the proper spreading of the coatings onto the PET. Furthermore, this compound played an important role on the adhesion property of the films. The cure agent, in the correct concentration, promotes the adhesion to metallic substrates. Moreover, this compound helped to prevent the interaction of the wettability agent with the water, decreasing consequently the plasticization of the structure in conditions of high humidity

This thesis is devoted to the investigation of mechanical properties and self-assembly process of materials at the nanoscale. Various nanostructured materials such as nanoparticles, nanotubes, nanowires and thin films are used as constituent elements of nanodevices. Hence, knowledge of the mechanical properties of materials at the nanoscale is extremely important for understanding their functionality in nanodevices. Mechanical properties of nanostructured materials may significantly differ from those of their bulk counterparts due to the high surface to volume ratio in nanostruc-tures. We particularly focus on the role of the surface region on the stiffness of nanomaterials. We have shown that the stiffness of a nanomaterial can be tuned over a wide range by introducing appropriate coating on the nanostructure surface. We have also explored the effects of the surface region on the stability of various phases in a nanostructure. In the second part of this thesis, we have described the self-assembly process of nanostructures mediated by drendrimers. Self-assembly techniques are frequently used to decorate nanostructures into specific networks. The motivation of this study is to investigate the mechanisms which control the effective interaction and the inter-particle distance between nanoparticle-dendrimer compos-ites. Control over the inter-particle separation is very important since it has a strong influence on the electronic and optical properties of the nanostructures. In the following paragraphs, we sum-marize the results of our study. We start with a brief introduction to the mechanical properties and self-assembly process of nanostructures in the first chapter. A brief review of the work done on these topics in the recent past is presented in this chapter. We discuss the results and conclusions of various experimental and numerical studies on these topics. We also mention the motivation for the studies we have carried out. At the end, we briefly describe the numerical methods (molecular dynamics (MD) and density functional theory (DFT)) which have been used in our investigations. In the second chapter, we discuss the effects of the surface region on the mechanical properties of nanostructures. We have investigated the size and growth direction dependence of the mechanical properties of ZnS nanowires and thin films as a case study. We observe that the Young’s modulus of nanowires and thin films strongly depends on their size and growth direction. This size and growth direction dependence of the stiffness of nanostructured materials can be explained in terms of their surface modifications. Since the energy of the surface region is usually higher than that of the core region in a nanostructure, the surface atoms move their positions to minimize the surface energy. As a result, bond lengths at the surface region are usually different from their bulk values. We observe that in ZnS nanowires and thin films, the average bond length at the surface region is lower than that in the core region which remains unchanged from its bulk value. This decrease in the bond length (or equivalently increase in the bond energy) increases the effective stiffness of the entire nanostructure. As the size of the nanowire/thin film increases, the effect of the surface region gradually decreases and hence the Young’s modulus value converges to the bulk value. Since the surface region has a strong influence on the mechanical properties of nanostructures, the stiffness of a nanostructure can be tuned by modifying the surface region with other materials. In chapter three, we have shown that the stiffness of ZnS nanowires can be tuned by introducing a thin CdS shell on top of the ZnS surface. In general, the stiffness of a nanostructure can be increased (decreased) by coating the surface region with a stiffer (less stiff) material. However, the stiffness of the core/shell nanostructures strongly depends on the properties of the interface between the core and the shell. We observe that the binding energy between the core and shell regions is relatively low due to the lattice mismatch at the interface region of core/shell nanostructures. This lower binding energy strongly affects the stiffness of core/shell nanostructures. We have also shown that thermal properties such as thermal conductivity and melting temperature of core/shell structures can be tuned by changing the coating material. In chapter four, we discuss the effects of the surface region on the stability of various phases in a nanostructure. The surface atoms may stabilize a particular phase in a nanostructure which is not a stable phase in the bulk material. In this chapter, we investigate the stability of the h-MgO phase, an intermediate structure found during the wurtzite to rock salt transformation, in CdSe nanostructures. We observe that this five-fold coordinated phase is more stable at lower temperatures and smaller sizes of the nanowires. The appearance of this phase has not been observed till now in experiments. We show that this phase is not stable for larger CdSe nanocrystals on which the experiments have been done. In the rest of the thesis, we have presented the results of our studies of self-assembly of nanostructures mediated by DNAs and dendrimers. First we describe in chapter five the nature of the effective interaction between two PAMAM dendrimers. Dendrimers are frequently used to coat surfaces of nanoparticles to prevent the nanoparticles from aggregation. The interaction between such nanoparticle-dendrimer composites depends strongly on the nature of the effective interac-tion between dendrimers. We have used fully atomistic MD simulations to calculate the potential of mean force (PMF) between two PAMAM dendrimers. We show that the effective interaction strongly depends on the size (generation) and protonation level of the dendrimers. The PMF profiles of nonprotonated dendrimers show a global minimum which represents the attractive nature of the interaction between the dendrimers up to a certain center-to-center distance. On the other hand, the interaction between protonated dendrimers is repulsive throughout their interaction re-gion. The PMF profiles are fitted very well by a sum of an exponential and a Gaussian function. This observation is in contradiction with some of the results of existing coarse-grained simulations which predicted the effective interaction between dendrimers to be Gaussian. Our atomistic simulation which includes all the local fluctuations is expected to give more accurate results. Information about the effective interaction between two dendrimers helps in understanding how dendrimer molecules can be used to control the interaction strength and the preferred inter-particle distance between two nanostructures. In chapter six, we discuss the effective interaction between two dendrimer grafted gold nanoparticles. We observe that dendrimer molecules can get adsorbed spontaneously on the surface of a gold nanoparticle. These grafted dendrimers significantly alter the interaction between the gold nanoparticles. We have explored the effects of proto-nation level and the density of the grafted dendrimers on the effective interaction between two gold nanoparticle-dendrimer composites. We observe that these nanoparticle-dendrimer composites at-tract each other at low grafting density. However, the interaction strength and the inter-particle distance at the minimum of the potential are much lower and higher, respectively than those between two bare gold nanoparticles. Interestingly at higher grafting density, the nature of the interaction between the nanocomposites depends on the protonation level of the grafted dendrimers. Nanoparticles grafted with nonprotonated dendrimers still attract each other but with lower inter-action strength and higher inter-particle distance compared to the values for low grafting density. On the other hand, nanocomposites grafted with protonated dendrimers repel each other at high grafting density. Thus we show that the effective interaction and the optimal inter-particle distance between the nanostructures can be tuned over a wide range by using a suitable grafting density and protonation level of the dendrimers. In the seventh chapter, we describe a strategy to assemble dendrimers with the help of sin-gle stranded DNA (ssDNA). We attach an ssDNA to one dendrimer and a complementary ssDNA to a second dendrimer. These two complementary ssDNAs bind with each other through base pair formation to assemble the dendrimers into a single structure. The complementary ssDNAs form a dsDNA which is rigid enough to maintain the inter-dendrimer distance almost the same as the length of the DNA. The inter-dendrimer distance can be tuned by changing the DNA length. However, this method strongly depends on the protonation level of the dendrimers. It works well only for nonprotonated dendrimers. Since the protonated dendrimers are positively charged, they strongly interact with the negatively charged ssDNAs through electrostatic interaction. As a result, ssDNAs wrap the dendrimer surface and hence the inter-dendrimer distance can not be controlled. We have also verified that this method works for multiple nonprotonated dendrimers as well. In the final chapter of this thesis, we summarize the main results and conclude with a brief discussion of future directions of research on the problems considered in the thesis.