Polymer brushes provide the responsive smart surfaces which can be used for fabrication of various devices. In this thesis work, adhesion, friction, and wear of polystyrene (PS) - poly(2-vinyl pyridine) (P2VP) and polystyrene - poly(acrylic acid) (PAA) binary brushes and corresponding monobrushes were investigated in dried state under controlled environment. Spin-coated films were also investigated for comparison. The aim was to explore possibilities to control/tune adhesion, friction, and wear between inorganic or polymeric surfaces by use of polymer brushes. Atomic force microscopy (AFM) with sharp silicon nitride tip and colloidal probes was employed to investigate the nanoscale adhesion and friction forces between different inorganic and polymeric surfaces. Adhesion and friction on the polymer brushes were comparable to that on the spin-coated films. Adhesion and friction force values were correlated, and were in accordance with the wettability of the brush surfaces for most of the samples. Switching in the adhesion and friction forces was observed for the PS+P2VP and PS+PAA binary brushes on treatment with selective solvents. Maximum switching in adhesion force and friction coefficient was by a factor of 2.7 and 5.4, respectively. Furthermore, switching of friction for mixed brush surface was observed during macroscale friction measurements using nanoindenter. Friction coefficients at macroscale were higher than those at the nanoscale. Moreover, adhesion and friction forces between the surfaces were significantly influenced by the humidity, grafting density of polymer brushes, chemical composition of top of the binary brush surface, and tip scan velocity. Nanowear studies were carried out with AFM using sharp silicon nitride tip while macrowear studies were carried out using nanoindenter. Nanowear on the surfaces was affected by molecular entanglements, adhesion and friction forces as well as shape and status of the tip. It was observed that the typical wear mode for PS brushes (treated with toluene) was ripple formation. In case of P2VP brushes (treated with ethanol) and PAA brushes (treated with pH 10 water), wear occurred via removal of the polymeric material. Wear mechanism observed for the monobrushes was similar to that observed for the spin-coated thick films of the same polymeric material. However, extent of the wear on the brush surfaces significantly differed from that on the spin-coated films. In case of PS+P2VP and PS+PAA binary brush samples, change in the wear mode was observed on treatment with the different selective solvents. On treatment with toluene (PS on the top), both of these binary brushes showed the wear by formation of the ripples. On the other hand, when these binary brushes were treated with selective solvent for P2VP or PAA, wear occurred mainly via removal of the polymeric material. The amount of wear increased with the number of scans for all the polymer brush samples. Moreover, wear on the polymer brush surfaces was also increased on increase in the applied load and decrease in the scan speed. Wear behavior on macroscale was averaged due to contact between surfaces at large number of asperities. Our results show that adhesion, friction, and wear of polymer surfaces can be controlled/tuned by the use of binary polymer brushes.

In the past decade there has been a growing interest in one-dimensional (1D) nanostructures, such as nanowires, nanotubes and nanorods, owing to their size-dependent optical and electronic properties and their potential application as building blocks, interconnects and functional components for assembling nanodevices. One of the ways to obtain such architectures is a template-directed synthesis which is practically a straightforward route to 1D nanostructures. In this approach, the template simply serves as a scaffold, within (or around) which a different material is generated in situ and shaped into a nanostructure with its morphology complementary to that of the template. It is generally accepted that template-directed synthesis provides a simple, high-throughput, and cost-effective procedure that also allows the complex topology present on the surface of a template to be duplicated in a single step. In the current work, utilization of the molecular bottle brushes as templates is proposed for the fabrication of conductive nanorods. Their non-spherical macromolecular geometries and lengths up to a few hundred nanometers allow the application of these structures in nanowire synthesis. The variety of molecular bottle brush architectures and their composition enables the adjustment of appropriate conditions for the preparation of conductive materials. Moreover, the ability of the brushes to assemble on a surface under certain conditions provides their usage as building blocks for the preparation of complex conductive networks. Here, the preparation, characterization, and applications of molecular bottle brushes are discussed. Two main goals were pursued. First, to deepen the knowledge in the synthesis of molecular bottle brushes, and to investigate their behavior on the surface. Second, to explore the application of the brushes as templates or building blocks for the formation of conductive nanowires. For the purpose, new ways of molecular brush synthesis by using the “grafting to” approach had to be developed. It was found that the reaction of nucleophilic addition based on pentofluorophenol chemistry and a coupling “click chemistry” reaction can be used to fabricate molecular brushes. Both methods showed efficient results and demonstrated high reactivity of the backbone with the end groups of the side chains. The “click chemistry” approach, however, demonstrated better results considering higher thicknesses of the brushes and, therefore, higher grafting density of the side chains. The “grafting to” together with the “grafting from” methods are very powerful synthetic tools, which can be used in the fabrication of any desired molecular bottle brush architectures. Additionally, complexation of oppositely charged bottle polymer brushes at a single-molecule level using AFM and CryoTEM was experimentally investigated. It was found that polyelectrolyte complexes have “scrambled-egg” morphology, where oppositely charged polymer chains are not oriented parallel to each other but cross each other. Furthermore, molecular bottle brushes were used as templates for the preparation of conductive nanowires. Three approaches for their fabrication were tested. It was found that brushes could easily be covered with various conductive materials, for example conductive polymers or metals. It was showed that for very small, tiny objects as molecular bottle brushes, one can use FIB in order to build up electrodes at its ends. The electrodes could be sputtered with an accuracy of 500 nm and further be used in the determination of the conductivity. The molecular bottle brushes covered with palladium showed the resistance of 50 MΩ, which, regarding the size of the brush, corresponds to a conductivity of one single molecule being ~1 S*cm-1. The obtained conductivity data were in good correlation with the data found in literature. We believe that the molecular bottle brushes have high potential applicability for the building of complex conductive networks. Future refinement of the synthetic methods, combined with improvements in structuring and positioning of objects at the nanoscale, could lead to their implementation in the construction of high-performance electronic devices.

No description available.

In the past decade there has been a growing interest in one-dimensional (1D) nanostructures, such as nanowires, nanotubes and nanorods, owing to their size-dependent optical and electronic properties and their potential application as building blocks, interconnects and functional components for assembling nanodevices. One of the ways to obtain such architectures is a template-directed synthesis which is practically a straightforward route to 1D nanostructures. In this approach, the template simply serves as a scaffold, within (or around) which a different material is generated in situ and shaped into a nanostructure with its morphology complementary to that of the template. It is generally accepted that template-directed synthesis provides a simple, high-throughput, and cost-effective procedure that also allows the complex topology present on the surface of a template to be duplicated in a single step. In the current work, utilization of the molecular bottle brushes as templates is proposed for the fabrication of conductive nanorods. Their non-spherical macromolecular geometries and lengths up to a few hundred nanometers allow the application of these structures in nanowire synthesis. The variety of molecular bottle brush architectures and their composition enables the adjustment of appropriate conditions for the preparation of conductive materials. Moreover, the ability of the brushes to assemble on a surface under certain conditions provides their usage as building blocks for the preparation of complex conductive networks. Here, the preparation, characterization, and applications of molecular bottle brushes are discussed. Two main goals were pursued. First, to deepen the knowledge in the synthesis of molecular bottle brushes, and to investigate their behavior on the surface. Second, to explore the application of the brushes as templates or building blocks for the formation of conductive nanowires. For the purpose, new ways of molecular brush synthesis by using the “grafting to” approach had to be developed. It was found that the reaction of nucleophilic addition based on pentofluorophenol chemistry and a coupling “click chemistry” reaction can be used to fabricate molecular brushes. Both methods showed efficient results and demonstrated high reactivity of the backbone with the end groups of the side chains. The “click chemistry” approach, however, demonstrated better results considering higher thicknesses of the brushes and, therefore, higher grafting density of the side chains. The “grafting to” together with the “grafting from” methods are very powerful synthetic tools, which can be used in the fabrication of any desired molecular bottle brush architectures. Additionally, complexation of oppositely charged bottle polymer brushes at a single-molecule level using AFM and CryoTEM was experimentally investigated. It was found that polyelectrolyte complexes have “scrambled-egg” morphology, where oppositely charged polymer chains are not oriented parallel to each other but cross each other. Furthermore, molecular bottle brushes were used as templates for the preparation of conductive nanowires. Three approaches for their fabrication were tested. It was found that brushes could easily be covered with various conductive materials, for example conductive polymers or metals. It was showed that for very small, tiny objects as molecular bottle brushes, one can use FIB in order to build up electrodes at its ends. The electrodes could be sputtered with an accuracy of 500 nm and further be used in the determination of the conductivity. The molecular bottle brushes covered with palladium showed the resistance of 50 MΩ, which, regarding the size of the brush, corresponds to a conductivity of one single molecule being ~1 S*cm-1. The obtained conductivity data were in good correlation with the data found in literature. We believe that the molecular bottle brushes have high potential applicability for the building of complex conductive networks. Future refinement of the synthetic methods, combined with improvements in structuring and positioning of objects at the nanoscale, could lead to their implementation in the construction of high-performance electronic devices.

A systematic study of the surface-initiated polymerisation kinetics of a relatively new type of atom transfer radical polymerisation (ATRP), activators regenerated by electron transfer (ARGET) ATRP, is first demonstrated in this report. Poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(methyl methacrylate) (PMMA) were successfully grown from silicon surfaces at room temperature by surface-initiated ARGET ATRP using a "3rd generation" cationic macroinitiator. The polymer films were analysed by ellipsometry, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). With the initial experiment showing that water accelerated conventional ATRP but made it less controlled, the effect of solvent on ARGET ATRP was also evaluated. The living character of ARGET ATRP was demonstrated by successfully reinitiating PHEMA-grafted silicon wafers to grow a second block of PHEMA. Initiator density was shown to have a great effect on the growth rate of PHEMA film thickness on silicon surfaces by comparing the ARGET ATRP growth of PHEMA films using two different initiators, "1st generation" and "3rd generation" cationic macroinitiators, which have different ratios of initiating groups to positive charge. Another type of initiator for ATRP systems, an amide silane, was then investigated as an alternative to polyelectrolyte macroinitiators to avoid degrafting. The effects of solvent, 2, 2′ bipyridyl (bpy) ligand concentration and different types of reducing agent on the growth of PHEMA film from amide-initiator coated silicon wafers by ARGET ATRP were then explored at room temperature. However, it was found that the swings in the uncontrolled laboratory ambient temperature caused inter-sample and inter-experiment variability and so could make the evaluations inaccurate or even wrong. An investigation of temperature on ARGET ATRP showed a dramatic effect on the polymerisation rate. The higher the temperature, the faster the polymerisation proceeded. Therefore, the effects of solvent, ratio of bpy to Cu and reducing agent on the ARGET ATRP growth of PHEMA brushes from amide initiator-coated silicon wafers were re-evaluated at a constant temperature, 30 °C. The development of a polydopamine-based initiator, which was designed to be able to be immobilised on a wide range of surfaces, is then presented in this report. Polydopamine was first shown to be able to deposit on various types of material surfaces by oxidative polymerisation in aqueous solution. Bromoester initiating groups for ATRP systems were incorporated into polydopamine coatings by reacting a fraction of the dopamine monomer with 2-bromoisobutyryl bromide (BIBB) before polymerisation. The modified polydopamine initiator film grew at a comparable rate to unmodified polydopamine, with a 45 nm being grown in 24 hours. Successful incorporation of initiator groups was confirmed by XPS and FTIR, and by the growth of PMMA and PHEMA polymer brushes by ARGET ATRP from the polydopamine initiator coatings. A PMMA brush with a thickness of 239 nm was grown in 72 hours, indicating that the grafting density is sufficiently high to be in the brush regime. This initiator was demonstrated to be able to deposit on a range of substrates, such as metals (steel) and polymers (polystyrene), and successfully initiate polymer growth, demonstrating its broad applicability. The assessment of ARGET ATRP as a simple and effective tool for interfacial shear strength improvement in cellulose-based fibre reinforced thermoplastic composites is finally presented. It was demonstrated by control experiments that grafting polystyrene on glass fibre surfaces via ARGET ATRP greatly improved the interfacial adhesion between glass fibres and a high-impact polystyrene (HIPS) matrix, although a specific value of interfacial strength was not obtained due to failure of the modified glass fibre composite samples in areas other than the interface. It was then demonstrated that PMMA was successfully grown from the surfaces of polydopamine initiator coated cotton fibre and BIBB-modified cotton fibre by ARGET ATRP. Polydopamine initiator was shown to be a better initiator for cotton fibre than BIBB, possibly since the adsorbed water on cotton fibres can react with BIBB. The improvement of interfacial adhesion between cotton fibres and a PMMA matrix by grafting PMMA on the cotton surface was assessed by peel testing of cotton fibres pressed into PMMA sheets. There is a clear trend in the relationship between the peeling force and growth time of PMMA on the cotton fibre by ARGET ATRP, although the inter-sample reproducibility is not good.

Dans une automobile, les composants électriques sont alimentés par un type de générateur électrique particulier : l’Alternateur. Entraîné par le moteur du véhicule, il lui prélève donc de la puissance mécanique. Dans les périodes de chasse au gramme de CO2 que nous connaissons, la puissance prélevée au moteur doit être minimisée pour ne pas pénaliser sa consommation. Or les besoins électriques du réseau de bord et des systèmes (éclairage, ventilation, servitudes, contrôle moteur, etc.) sont imposés par l’équipement du véhicule. Il est apparu que dans les cas où l’alternateur est fortement sollicité, des pertes de performances peuvent être imputées au circuit d’excitation de la machine. Il comporte deux contacts électriques glissants balai-bague qui ont été incriminés. Du point de vue théorique aucun élément ne permettait d’expliquer les augmentations brutales de résistance de ces contacts, nous avons donc développé une méthode de mesure originale destinée à rechercher des paramètres influents sur leurs variations. Il nous a été impossible de faire une mesure de l’épaisseur film de transfert et d’identifier les quantités de graphite, nature et quantité des oxydes en présence dans le film. Cependant, nous avons estimé la pertinence de l’hypothèse du rôle de l’oxydation et avons surtout pu isoler le rôle du film de transfert dans l’expression des résistances de contact. Nous avons, de plus, mis en évidence la corrélation existant entre les résistances de contact et le coefficient de frottement pour les deux contacts. / The Alternator is an electrical generator especially designed to supply current to the vehicle electrical components. It is powered by the engine and takes off a part of its power. Currently, car makers are more than willing to minimize the power losses of this device in order to save fuel and reduce CO2 emissions, but the electrical need depend only on the vehicle equipment and use. Sometimes when the alternator runs at full load, the performance could be diminished by unpredictable resistance changes in the electrical sliding contacts of the excitation circuit. There was no theory capable of explaining these phenomena. The analysis limits were the impossibility to measure transfer film thickness and to determine the amount of graphite and the nature and volume of oxides into the film. We developed an experimental method adapted to contact resistance and friction measurements with the aim to find influencing parameters. A dedicated software has been created to extract data from these measurements. We appreciated the relevancy of oxidation hypothesis and we added a study on transfer film, gauging its contribution to contact resistance. Moreover we found a significant correlation between contact resistance and friction, meaning that there are some influent hidden parameters between brush and disc/ring.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by David Alan Stephenson. / B.S.

New electrochemical storage and conversion materials hold promise as important additions to the world's energy supply, and the growing ability to control both sequestration and transfer of charge and matter via functionally responsive materials promises to transform the field. Already, new understanding of the role played by nano-scale morphology of materials in transport function has contributed to considerable material improvements, with functional polymers possessing specific chemistry and morphology playing a key role in the future of electrochemical material applications. However, many challenges to optimizing properties still exist due to incomplete descriptions of transport. In this work, fluorescence spectroscopy and single molecule spectroscopy experimental techniques and analysis are developed and employed to reveal details of the mechanisms underpinning ion transport in structurally ordered polyelectrolyte polymer-brush membranes. The studies reveal the existence and nature of heterogeneous transport mechanisms in these polymer films, and provide a description of the dynamic association of molecular ions with the brush. It is also shown that it is possible to tune charged ion transport characteristics in the thin films by controlling the solvent pH, with an effective switching of ion transport rates in these brushes past a threshold pH value. Additionally, Monte Carlo models designed to model molecular scale interactions that give rise to experimental observables are developed to provide additional insight into the physical nature of transport processes in these materials. These models provide additional support for the conclusions of the experimental work.

The work presented here has a multidisciplinary character, having as a common factor the characterization of self-assembled nanostructures through force spectroscopy. Exploring AFM as a tool for characterizing self-assembly and interaction forces in soft matter nanostructures, three different Bio and nonbiological systems where investigated, all of them share the common characteristic of being soft matter molecular structures at the nanoscale. The studied systems in question are: a) Polyelectrolyte – lipid nanocomposites. Single polyelectrolyte adsorption-desorption from supported lipid bilayers, b) Polyelectrolyte brushes and c) Virus-Like particles (VLPs). The scientific interest and industrial applications for each of these different nanostructures is broad, and their potential uses in the near future ranges from smart nanocontainers for drug and gene delivery, surface platforms for molecular recognition to the development of new nanodevices with ultrasensitive external stimuli responsiveness. These nano-structures are constructed following assembly of smaller subunits and belong to representative examples of soft matter in modern nanotechnology. The stability, behavior, properties and long term durability of these self-organized structures depends strongly on the environmental conditions to which they are exposed since their building mechanism is a balance between attractive noncovalent interactions and momentum transmitted collisions due Brownian motion of the solvent molecules. For example a set of long chain molecules firmly attached to one end to a surface will alter their conformation as the space between them is reduced or the environmental conditions are modified (i.e. ionic strength, pH or temperature). For a highly packed condition, this fuzzy surface known as a polyelectrolyte brush will then behave as a responsive material with tunable responsiveness. Thus the objective in the present case was to investigate the change in morphology and the mechanical response of a polyelectrolyte brush to external forces by application of AFM nanoindentations under different ionic strength conditions. The degree of penetration of the AFM tip through the brush will provide insights into the forces exerted by the brush against the tip. Compressions on the brush should aid to characterize its changes in compressibility for different salt concentrations. For the second chosen system, the interaction between two assembled interfaces was investigated at the single molecular level. A multilayered film formed by the consecutive assembly of oppositely charged polyelectrolytes and subsequently coated with a lipid membrane represents a fascinating soft composite material resembling more than a few structural components emerging in living organisms. The fluid bilayer, thus provide a biocompatible interface where additional functionalities can further be integrated (fusion peptides for instance). The smooth polymer cushion confers not only structural flexibility but also adaptability of the chosen substrate properties to be coated. This type of interface could be useful in the development of novel molecular biosensors with single molecule recognition capacities or in the fabrication of assays against pathogenic agents. The aim of this project was to study the molecular binding mechanism between the last polyelectrolyte layer and the lipid head group of the lower lipid leaflet. Understanding this adsorption mechanism between both interfaces, should likewise contribute to improve the fabrication of lipid coated polymeric nano/micro capsules with targeting properties. For example this could be critical in the field of nonviral gene therapy, where the improvement in the design of condensates of nucleic acids and other polymers with lipids (lipoplexes) are of main interest for its posterior use as delivery vectors. Finally, viral capsids were investigated. These naturally occurring assembled nanocontainers within living organisms stand for a remarkable example of nature’s morphological designs. These structures self-assemble from a small number of different proteins occurring in identical copies. The capsid as a self-assembled structure carries multiple functions: compaction of the genome, protection against external chemical threats, target recognition, structural support and finally facilitating the release of the genome into the host cell. It is highly interesting how these different functions are organized within the capsid which consists, for example, in the case of the norovirus of 180 identical copies of one single protein. Therefore, the mechanical stability and elastic properties of virus-like particles of Rubella and Norovirus were investigated by external application of loading forces with an AFM tip. The measurements were performed under conditions relevant for the virus infection mechanism. The applied compressions on these protein shells at pH values mimicking the virus life cycle will aid to learn about possible internal transitions among proteins which may be important for switching between the various functions of the capsid. The choice of two unrelated viral systems with different entry pathways into the cell and with different morphological architectures is expected to reveal crucial information about the stability and mechanical resistance to deformation of these empty membrane-coated and bare viral capsids. This last might provide clues on the stage of particle disassembly and cargo release during the final step of the infection process.

A non-crosslinking approach that covalently bonds nitroxide polymer brushes onto the ITO substrates via surface-initiated atom transfer radical polymerization (ATRP) was develpoed. Since the indium tin oxide (ITO)-silane covalent bonding providesvery strong chemical bonds to adsorb the nitroxide polymer brushes on ITO, it prevents polymers from dissolving into electrolyte solvent and thus improves its electrochemical properties. Moreover, micro-contact printing technology was used to pattern nitroxide polymer brushes on an ITO surface for the potential application in microbatteries. The morphology of electrodes was observed by atomic force microscopy.The electrochemical properties of the cathode were also studies.