No description available.

No description available.

When many semicrystalline polymers are used as matrix materials in composites, a morphology known as the transcrystalline region is formed on the surface of the reinforcing material. This region introduces a new crystalline structure to the system that is different from that of the bulk matrix material. Whether this region is advantageous or detrimental to the mechanical performance of the composite has been debated. Therefore, efforts were made to control the size and structure of this region for a specific composite system, i.e., nylon 66 reinforced with high modulus carbon fibers. In many systems this additional phase can be avoided simply by altering the crystallization history of the matrix polymer. In this study, the interphase region is removed not by changing the crystallization history of the matrix, but by altering the crystallization kinetics of the matrix material by introducing a diluent which is known to induce such changes in blends of itself and the host polymer. The diluent in this study is poly(vinyl pyrrolidone) (PVP) which is a highly polar, uncrystallizable polymer, and the host polymer is nylon 66 which is a highly polar crystallizable polymer. Initially, microscopy studies were performed on blends of nylon and two molecular weights of PVP at very low diluent concentrations, i.e., < 7% by weight. Next, commercial high modulus carbon fibers were unsized by exposure to a benzene wash. In addition, sets of the unsized fibers were sized with various amounts and molecular weights of the diluent by exposure to dilute solutions. Both unsized and sized fibers were then embedded in the previously made blends, and an optical study of the morphological changes in the interphase is performed. Furthermore, preliminary studies of the fiber surfaces using x-ray photoelectron spectroscopy (XPS) were conducted. PVP dramatically reduced the nucleation density of spherulites and modified the lamellar organization in the spherulites (as evidenced by the occurrence of banding which is the twisting of lamella as they grow radially). Furthermore, in the presence of unsized fibers, the addition of small amounts of diluent to the matrix increased the size of the transcrystalline region. At slightly higher diluent concentrations, the nucleation density on the fiber surface was reduced. Only with sizing of the fibers with the diluent along with adding the diluent to the matrix was there a complete removal of the transcrystalline region. / Master of Science

In the effort to improve oil production riser performance, new materials are being studied. In the present case, a Polymer Matrix Composite (PMC) is being considered as a replacement for carbon steel in flexible risers manufactured by Wellstream Inc., Panama City, Florida. The Materials Response Group (MRG) at Virginia Tech had the primary responsibility to develop the models for long-term behavior, especially remaining strength and life. The MRG is also responsible for the characterization of the material system with a focus on the effects of time, temperature, and environmental exposure. The present work is part of this effort. The motivation to use a composite material in a non-bonded flexible riser for use in the offshore oil industry is put forth. The requirements for such a material are detailed. Strength analysis and modeling methods are presented with experimental data. The effect of matrix crystallinity on composite mechanical properties is shown. A new method for investigating matrix behavior at elevated temperatures developed. A remaining strength life prediction methodology is recalled and applied to the case of combined fatigue and rupture loading. / Master of Science

The present work describes a selection of strategies for the synthesis of functional aliphatic polycarbonates. Using an end-group functionalization strategy, a series of DNA-binding cationic poly(trimethylene carbonate)s was synthesized for application as vectors for non-viral gene delivery. As the end-group functionality was identical in all polymers, the differences observed in DNA binding and in vitro transfection studies were directly related to the length of the hydrophobic poly(trimethylene carbonate) backbone and the number of functional end-groups. This enabled the use of this polymer system to explore the effects of structural elements on the gene delivery ability of cationic polymers, revealing striking differences between different materials, related to functionality and cationic charge density. In an effort to achieve more flexibility in the synthesis of functional polymers, polycarbonates were synthesized in which the functionalities were distributed along the polymer backbone. Through polymerization of a series of alkyl halide-functional six-membered cyclic carbonates, semicrystalline chloro- and bromo-functional homopolycarbonates were obtained. The tendency of the materials to form crystallites was related to the presence of alkyl as well as halide functionalities and ranged from polymers that crystallized from the melt to materials that only crystallized on precipitation from a solution. Semicrystallinity was also observed for random 1:1 copolymers of some of the monomers with trimethylene carbonate, suggesting a remarkable ability of repeating units originating from these monomers to form crystallites. For the further synthesis of functional monomers and polymers, azide-functional cyclic carbonates were synthesized from the bromo-functional monomers. These were used as starting materials for the click synthesis of triazole-functional cyclic carbonate monomers through Cu(I)-catalyzed azide–alkyne cycloaddition. The click chemistry strategy proved to be a viable route to obtain structurally diverse monomers starting from a few azide-functional precursors. This paves the way for facile synthesis of a wide range of novel functional cyclic carbonate monomers and polycarbonates, limited only by the availability of suitable functional alkynes.

Controlling the micro-structure of organic materials is crucial for a variety of practical applications such as photonics, biomedicine or the rapidly growing field of organic electronics. Recent studies have shown a possibility of tailoring the polymer structure on the nanoscale using supramolecular self-assembly under spatial confinement. Despite extensive studies already performed in this field, many questions remain open. In particular, it will be important to understand how different structure formation processes such as crystallization, LC-phase formation, microphase separation, and others occur under confinement. In the present work, we address the effect of 1D- and 2D-confinement on the structure formation for a variety of systems including segmented poly(ether-ester-amide) (PEEA) copolymers, main-chain liquid-crystalline (LC) polymers belonging to the family of poly(di-n-alkylsiloxane)s and liquid-crystalline/semicrystalline block copolymers formed through complexation of poly (2-vinylpyridine-b-ethylene oxide) (P2VP-PEO) with a wedge-shaped ligand, 4'-(3'',4'',5''-tris(octyloxy) benzamido) propanoic acid. In order to reveal the morphological diversity of the studied systems under confinement, the work was carried out on bulk materials and on thin films employing a battery of experimental methods. The main experimental techniques operational in direct and reciprocal space applied in my work are described in chapter 2. [...]

Les isolateurs diélectriques utilisés dans les disjoncteurs haute tension développés par Areva, sont fabriqués à partir du PET (polyéthylène téréphtalate). Ce polymère semi-cristallin a remplacé, depuis quelques années, les résines époxy. Il a été choisi pour ses propriétés mécaniques et diélectriques, mais surtout pour sa recyclabilité. Le souci dans cette application, concerne l’évolution dans le temps de ses caractéristiques sachant les contraintes d’application. En effet, dans les conditions de travail, les isolateurs maintiennent des conducteurs électriques. Les pertes thermiques affectent certaines zones pouvant atteindre, voire dépasser, la température de transition vitreuse du matériau [70-80°C]. Par conséquence, les pièces isolatrices subissent un phénomène de vieillissement qui nécessite un suivi dans le temps afin d’étudier l’évolution de leur caractéristiques Dans ce contexte, nous avons étudié le vieillissement thermique du PET. Ainsi des échantillons ont été mis dans des étuves sous vide, chacune réglée à une température : 60, 80, 115 et 125°C pendant différentes durées (jusqu’à 12 mois de vieillissement), puis retirés et testés au fur et à mesure du vieillissement. Différentes techniques ont été employées pour analyser les propriétés du semi-cristallin en question, i.e. l’étude calorimétrique différentielle (DSC), les diffractions aux rayons X aux grands et petits angles (WAXS/SAXS). L’analyse thermomécanique (DMA) et finalement les essais de traction.Les résultats de DSC révèlent une augmentation du taux de cristallinité. Les analyses thermomécaniques ont montré une faible augmentation du module de Young qui pourrait être le résultat d’une évolution de la cristallisation. Les températures de fusion sont restées quasiment stables, par contre une augmentation des températures de transition vitreuse a été remarquée. Les analyses des spectres de diffraction aux rayons X aux grands angles, ont confirmé la croissance du taux de cristallinité. En outre les longues périodes calculées, diminuent. Nous avons ainsi vérifié l’apparition de cristallites dans phase amorphe. Par ailleurs un comportement de fragilité continue en fonction du temps et de la température du vieillissement, a été constaté. Les observations au MEB ont révélé la présence d’une importante quantité de particules supposées être des agents nucléants (talc, SiO2, MgO…) / For insulating application, AREVA has chosen PET (polyethylene terephthalate) to substitute the epoxy resin as material for insulators in High Voltage Gas Insulated Substation. The main problem of this application is the fact that in operating conditions, the temperature of the PET plates would reach even exceed its glass transition [70°C-80°C]. The material undergoes aging phenomena which affect the temperature-dependent properties. The current investigation aims at observing and analyzing the gradual evolution of the mechanical, morphological and dielectric properties during thermal aging. To reach this goal, semi crystalline PET samples have been aged under vacuum at different temperatures i.e. 60°C, 80°C, 115°C and 125°C for various periods of time (until 12 months). The characterizations have been performed using several techniques: Differential scanning calorimetric (DSC), wide and small angle X-ray scattering (WAXS/SAXS), thermo-mechanical analysis (DMA), tensile test and morphological observation.The DSC measurements show that the crystallinity ratio increases with temperature and time of aging. The glass transition has increased. However no significant changes have been seen on the melting temperature. The DMA results agree with the DSC measurement in so far as it has revealed an expected increase of the Young modulus for all the samples studied. Significant differences have been observed using X-ray scattering: while the crystallinity ratio did increase, the long period has decreased specially for the case of aging at 115 and 125°C. The DMA measurements showed an almost stable glass transition around 80°C but an increase for samples aged at 125°C. When the samples have been subjected to the tensile test, a significant brittleness rise has been noticed. In addition, the SEM has revealed the presence of important amount of nucleant agent (talc, SiO2, MgO …)

Ce travail a consisté en l’étude par Spectroscopie Diélectrique de la mobilité moléculaire dans trois systèmes nanostructurés et confinés à base de polymère. La dynamique moléculaire des films de copolymères greffés à base d’Acétate de Cellulose (chaîne principale, peu mobile) et de poly(méthyl (diéthylène glycol) méthacrylate) (chaîne greffée, très mobile) a été analysée. Nous avons dans ce cas, observé une augmentation de la mobilité de la chaîne principale et une réduction de la mobilité des greffons. Ensuite nous avons mis en évidence les effets de confinement induits par la phase cristalline sur la phase amorphe des films de Poly(éthylène naphtalène - 2,6 - dicarboxylate) (PEN) cristallisés à différents temps et températures de cristallisation, en fonction de la morphologie des matériaux. Une forte influence de la température de cristallisation sur la dynamique moléculaire du PEN a pu être observée : la mobilité des chaînes dans le cas des échantillons cristallisés aux hautes températures de cristallisation s’est révélée être plus élevée que celle des échantillons cristallisés aux basses températures. Le troisième système choisi consiste en des nanocomposites à base de Polyamide 6 (PA6) et de Montmorillonite (MMT) ont été les derniers matériaux étudiés. Aucune influence significative des feuillets de MMT sur la mobilité des chaînes de PA6 n’a été observée dans ces systèmes. Cependant, les deux relaxations interfaciales observées sont très sensibles au taux de charge. / The aim of this work was to study the molecular dynamics in several polymeric nanostructured and confined systems. Thus, by means of Dielectric Spectroscopy, the molecular mobility of cellulose acetate (rigid chain) grafted poly(methyl (diethylène glycol) methacrylate) (very mobile chains) copolymers were studied. In this case, an increase of the mobility of the main chain and a reduction of the mobility of the grafted moieties have been observed. The confinement effects induced by the presence of the crystalline lamellae on the mobility of the chains belonging to the amorphous region, was also studied as a function of the morphology, in the case of the amorphous films of Poly(ethylene naphtalene - 2,6 - dicarboxylate) (PEN) which were crystallized at different crystallization temperatures during different crystallization times. A strong influence of the crystallization temperature on the molecular dynamics of PEN has been evidenced: in the case of the samples crystallized at high temperatures, the mobility of the chains was higher than in the case of the samples crystallized at low temperatures. Finally, the confinement effects induced by the Montmorillonite (MMT) platelets on the Polyamide 6 (PA6) matrix were studied in the PA6/MMT nanocomposites. No significant influence of the filler on the molecular mobility of the PA6 chains was observed. Nevertheless, as expected, the two interfacial relaxations were very sensitive to the filler content.

No description available.

Ziel dieser Arbeit ist es, eine Verbindung zwischen der Mikrostruktur teilkristalliner Polymere und derer mechanischen Eigenschaften auf der Mikro- und Nanometerskala aufzubauen. Dazu wurden Methoden der Rasterkraftmikroskopie verwendet um sowohl orts- als auch zeitaufgelöst Kristallisations-, Deformations- und Diffusionsprozesse in der Mikrostruktur von elastomerem Polypropylen (ePP) abzubilden. Die mechanischen Eigenschaften wurden simultan mit Mikrozugversuchen bestimmt. So konnte beispielsweise ein Zusammenhang zwischen abnehmender Kristall-Kristall-Distanz und einem Ansteigen des Elastizitätsmoduls während der Kristallisation nachgewiesen werden. Weiterhin war es möglich die Veränderung der nano-mechanischen Eigenschaften während der Kristallisation einzelner kristalliner Lamellen in deren direkter Umgebung mit MUSIC-mode Rasterkraftmikroskopie zu untersuchen. Laterale Querexpansion (auxetisches Verhalten) konnte bei uniaxialen Zugversuchen für die Kreuzschraffur-Struktur elastomeren Polypropylens auf der Größenskala einiger Mikrometer nachgewiesen werden. Zusätzlich wurde eine Orientierungsabhängigkeit dieses Effekts beobachtet. Außerdem wurde die Diffusion einzelner Kristalle in der Mikrostruktur von ePP beobachtet. Die Heterogenität dieser Diffusion lässt auf eine kristallin-amorph Grenzschicht um alle Kristalle schließen.