Ces travaux de thèse contribuent à l’étude de matériaux hybrides à base d’un polymère semi cristallin, le polyfluorure de vinylidène (PVDF) connu pour ses propriétés piézoélectriques. Cependant, celles-ci dépendent du polymorphisme du PVDF et les phases polaires nécessaires à l’obtention de telles propriétés sont classiquement obtenues par étirage ou par incorporation de charges. L’objectif de ces travaux est la compréhension des comportements mis en jeu au cours d’essais d’étirage et l'évaluation de l’influence des charges sur les propriétés résultantes. L’originalité de ces travaux réside dans le suivi in-situ de l’évolution structurale durant l’étirage, permettant de conclure que la phase polaire β du PVDF se forme lors de la striction et que la relaxation cristalline joue un rôle primordial sur les mécanismes de déformation du PVDF. L’insertion de nanotubes de carbone (NTC) dans la matrice PVDF induit un changement partiel de phase cristalline apolaire α-->polaire γ. Le taux de cette phase induite passe par un maximum pour un taux de NTC proche du seuil de percolation. L’étude in-situ sous étirage de ces composites montre que la présence de NTC n’affecte pas la formation de la phase polaire β du PVDF induite mécaniquement. Pour améliorer les propriétés piézoélectriques du PVDF, des composites à base de céramiques piézoélectriques (BaTiO3) ont été élaborés. Cependant l’interface entre les deux constituants est peu cohésive. L’utilisation d’un dérivé de dopamine, la nitro-dopamine permet d’améliorer l’adhésion interfaciale mais n’engendre pas de changement de phase cristalline du PVDF. L’étude piézoélectrique des composites étirés souligne le potentiel de tels systèmes. / This work is a contribution to the study of hybrid materials based on a semi-crystalline polymer: poly(vinylidene fluoride), PVDF, well known for its piezoelectric properties. However these properties are dependent of the PVDF crystalline structure which must be in polar form. The most polar phase (β) is classically obtained by stretching the non-polar phase (α) or by incorporating fillers.The aim of this work is the understanding of structural changes and deformation mechanisms of PVDF during stretching. The impact of the incorporation of different (nano)fillers on PVDF structure and mechanical behavior is also studied.The distinctive feature of this work is the use of synchrotron radiation for in-situ strain-induced structural evolution studies. These experiments show that the polar β phase appears with necking and that the crystalline relaxation of PVDF plays a main role on plasticity mechanisms.The incorporation of carbon nanotubes (CNT) in PVDF matrix leads to a partial crystalline change from non-polar form α to a polar form γ. The ratio of this γ phase is maximal for a CNT loading close to the electric percolation threshold. The in-situ structural evolution shows that CNT's do not affect the formation of the mechanically-induced polar β phase.In order to improve the PVDF piezoelectric properties, composites based on piezoelectric ceramics (BaTiO3) are elaborated. Nevertheless, interactions between PVDF and BaTiO3 are too weak. The use of a dopamine derivative, nitro-dopamine improves the interfacial interactions but does not change the PVDF crystalline form. However a piezoelectric study of these stretched composites suggests interesting electroactive capabilities.

The aim of this project was to develop and compare microwave at 2.45 GHz and conventional heating processes to produce low molecular weight methyl methacrylate oligomers. The required level of conversion for this process to be successful was 40 %. Batch and flow processes were investigated for both heating methods, and conversions of up to 70 % in batch and 55 % in flow were achieved. A large number of publications conclude that microwave heating can lead to significant improvements in conversion, rate and product quality when compared to conventionally heated reactions. In many cases this has been attributed to a non-specific thermal effect due to material interactions with the electric field component of the microwave. In many cases these conclusions stem from inaccurate temperature measurement in a microwave environment, and comparisons with conventional heating being carried out under different conditions. In this work a robust methodology was developed to compare microwave and conventional heating for oligomer production. Initial work was carried out at 30-50 cm³ using a CEM Discover SP (300 W) microwave, a conventionally heated stirred oil bath and a single mode cavity. Investigation into the effects of temperature, heating rate and reaction time on conversion were carried out, and it was found that the target material could be produced to 70% conversion in less than 5 minutes. From the understanding gained using small scale batch reactions a set of design requirements were produced for larger scale flow systems using microwave and conventional heating, and these systems were constructed as part of this project. This apparatus allowed for repeatable production of the target material at flow rates of up to 36 kg/hour when operated using a 2 kW microwave generator with automated tuning. Conventional comparisons were carried out at in flow with the use of apparatus of the same physical geometry under the same conditions. This is the first work published to carry out direct comparisons between conventionally heated and microwave reactions under the same conditions. It was found that at all scales and under all the conditions investigated that there were no quantifiable benefits in conversion or product quality offered by the use of microwave heating. It was concluded that such benefits concluded elsewhere are likely to result from poor control and rigour in experimental comparisons.

No description available.

Nanofiltration has many potential applications as a separation technology for processes that use mixtures of aqueous and organic solvents, for example alcohol/water mixtures. Membrane systems are well established for separations carried out in aqueous media, however they have seen a much slower rate of uptake in non-aqueous processes or with aqueous/organic mixtures. This is because the interaction between membrane and solvent(s) dictates both the permeability and selectivity, and there is currently limited criterion for identifying the theoretical performance of a membrane based on the properties of a bulk polymeric material. The small numbers of commercial successes to date have arisen from empirical findings, with no agreed methodology by which new candidate membrane materials can be identified. New membrane materials are required to exhibit a high permeability along with the selectivity demanded by the application. Permeability can be relatively easily manipulated using engineering solutions such as large surface areas, or very thin active separation layers. Selectivity, however cannot be manipulated in such an intuitive fashion, with the mixture type and composition, pressure and polymer characteristics all reported to be major factors. This work investigates the factors which influence the inherent selectivity of polymeric materials, and the link to nanofiltration processes. The aims of this study are to investigate the effectiveness of current theoretical and predictive tools, and to establish a technique to evaluate polymeric materials without having to fabricate a membrane. Two polymers were chosen for study which are at opposing ends of the permeability/selectivity spectrum. Polydimethylsiloxane (PDMS) and poly (vinyl alcohol) (PVA) membranes have been previously investigated in several separations involving organic-water mixtures. The materials were characterized using GPC and ATR-FTIR techniques, with ATR-FTIR further used to quantify the crosslinking content of the polymers. The total swelling degree and the inherent separation that occurs upon swelling with solvent mixtures was studied for a range of model and industrially-relevant systems, using polymer materials fabricated under different conditions. It was found that the selectivity of the polymer was a highly non-linear function of mixture type and concentration. PDMS and PVA were shown to change their affinity toward mixture components depending on the concentration, and it was hypothesized that this is due to competing mechanisms based on both molecular size and polarity. Selectivity was shown to be less dependent on the applied pressure and the degree of crosslinking, with the polymer type and mixture composition the two most dominant factors. The Flory-Huggins model was evaluated and found to give an extremely poor prediction of the selectivity in all the polymer-solvent systems studied. Further analysis was carried out using chemical potential and activity coefficient models in order to establish the sorption coefficient for future comparison with membrane filtration data. One of the key outcomes of this work is the measurement of sorption coefficients at varying composition and pressure, which can subsequently be used with existing Solution-Diffusion and Pore-Flow filtration models with greater confidence than has been possible to date.

Fire is a continuous threat to life and property. The total annual UK fire loss is estimated to be 0.25% of its gross domestic product (GDP) (Goddard, 1995). According to fire statistics, more than 12 million fires break out every year in the United States, Europe, Russia, and China killing about 166,000 people and injuring several hundreds of thousands (Morgan and Wilkie, 2007). Polymers which take up 80% of the organic chemical industry, are known for their high flammability with the production of heat, corrosive toxic gases, and smoke (Bent, 2010). Improving the fire retardancy of polymeric materials is a major concern and also a major challenge. Nanotechnology could have a significant impact on polymeric materials through the achievement of polymer nanocomposites (PNs) with enhanced functional properties (Giannelis, 1996, Schartel and Batholmai, 2006). If this can be achieved, there will be an enormous increase in the use of improved flame retardant (FR) PNs in mass transportation, aerospace, and military applications where fire safety will be of utmost importance (Horrocks and Price, 2008). In this research project nanoparticles that could have a synergistic effect with traditional FR systems, or that could have a FR action (nano-fire extinguishers), were formulated and surface modified during continuous hydrothermal synthesis (CHS). The bespoke nanoparticles were developed in a structure that could be easily integrated and effectively dispersed into a polymeric matrix. A solvent blending approach for integrating and dispersing colloidal organic modified nanoparticles into polymeric matrices was developed. The impact of nanoparticles of different morphologies including nanospheres, nanoplates, and nanorods on epoxy mechanical, thermal, and flammability properties was evaluated. A laboratory based technique using a Bunsen, video footage, and image analysis was developed to quantify the nanocomposite's direct flame resistance in a repeatable fashion. A new self extinguishing epoxy nanocomposite was developed which showed an enhanced performance in extreme conditions and with good mechanical properties.

Photocuring of polymers and polymer composites, from epoxy resin based formulations, has been of growing interest over the past two decades. The photocuring occurs when an epoxy formulation is exposed to electromagnetic radiation, usually ultraviolet (UV) radiation. This process has been explored widely and it can be described as an open mould process by which the epoxy formulation is exposed directly to the radiation. However, for a closed mould process, thermal curing, rather than radiation curing, typically is employed. The potential of using photocuring for a closed mould process has not yet been investigated in detail. The challenge in photocuring of polymers and polymer composites in a closed mould is directing the radiation into the mould to activate the photocuring process, which is not possible using the conventional methods. Hence, for this reason the present work is focused on the development of a closed mould photocuring system using side emitting optical fibres. This photocuring system using side emitting optical fibres relies upon the optimisation of epoxy based resins. As a result, an extensive characterisation of different types of UV curable cationic epoxy resins is carried out using two pre-formulated commercial resins, formulations from bisphenol A/F, and formulations from cycloaliphatic epoxy. The formulations showed different reactivity and hardness. An important result is that the cycloaliphatic epoxy resin formulations cured much faster than the other bisphenol A/F formulations, having a more uniform hardness distribution and UV radiation transparency during the curing. Side emitting optical fibres are adopted to photocure epoxy in a closed mould. Different types of side emitting optical fibres are characterised to determine irradiation efficiency. The optical fibres had either a silica core or a PMMA core. The silica core fibres have a silicone cladding containing radiation scattering particles (either ZnO or Al2O3) and diffuser (either PA6 or ETFE). The PMMA core polymer optical fibres (PMMA POFs) have a PVDF cladding with micro-perforations as a side emission mechanism. Silica core fibres with the Al2O3 scattering particles and the PMMA core fibre are more suitable for the closed mould application as they transmitted efficiently in the UV radiation band. The high side emission characteristics of the PMMA POF compared to the silica core fibre showed higher potential for use in the closed mould photocuring process. As the polymerisation speed is influenced by the amount of flux density of the radiation source, a high flux lamp (Hg lamp, 40 W/cm2) is coupled to the side emitting optical fibres. This lamp caused thermal degradation to the PMMA POF at the launch point when in use. A cooling device is made to minimise the thermal degradation generated by radiation absorption. After improving the optical transmission stability of the PMMA POF, its side emission is enhanced by various treatments, such as permanent modification of the fibre geometry with adjusted bend radii as well as by mechanically embedding silica scattering particles into the fibre and applying micro-cuts. The developed, closed mould photocuring system consists of: enhanced side emitting PMMA POF, a cooling device, high emission Hg lamp, and a closed mould setup. 1.5 mm and 5 mm thick components, made from an optimised epoxy formulation (based on cycloaliphatic epoxy), are cured using the photocuring system. The 1.5 mm thick component (20 mm wide and 245 mm long) is cured in 45 minutes using a single PMMA POF treated with silica particles (side emission of ~81 % of the total launched emission). The 5 mm thick component (75 mm wide and 170 mm long) partly cured in 45 minutes by simultaneously using three PMMA POFs treated with silica particles and geometric modification (side emission of ~96 % of the total launched emission). This sample eventually cured with time (up to 36 hours) due to dark reaction. The efficiency of the developed closed mould photocuring system is validated by curing a 1.5 mm thick component made from a pre-formulated polyester resin formulation. This component cured in 7 minutes (30 mm wide and 245 mm long) using a single PMMA POF treated with silica particles.

The research work described in this thesis is concerned with the development of glassfibre reinforced plastics for structural uses in Civil Engineering construction. The first stage was primarily concerned with the design of GRP lamintes with structura1 properties and method of manufacture suitable for use with relatively large structural components. A cold setting, pressure moulding technique was developed which proved to be efficient in reducing the void content in the composite and minimising the exothermic effect due to curing. The effect of fibre content and fibre arrangement on strength and stiffness of the cornposite was studied and the maximum amount of' fibre content that could be reached by the adopted type of moulding technique was determined. The second stage of the project was concerned with the introduction of steel-wire "sheets" into the GRP cornposites, to take advantage of the high modulus of steel wire to improve the GRP stiffness and to reduce deformation. The experimental observations agreed reasonably well with theoretical predictions in both first and second stages of the work. The third stage was concerned with studying the stability of GRP flat rectangular plates subjected to uniaxial compression or pure shear, to simulate compression flanges or shear webs respectively. The investigation was concentrated on the effect of fibre arrangement in the plate on buckling load. The effect of the introduction of steel-wire sheets on the plate stability in compression was also investigated. The boundary conditions were chosen to be close to those usually assumed in built-up box-sections for both compression flanges and webs. The orthotropic plate and the mid-plane symmetric were used successfully in predicting the buckling load theoretically. In determining the buckling load experimentally, two methods were used. The Southwell plot method and electrical strain gauge method. The latter proved to be more reliable in predicting the buckling load than the former, especially for plates under uniaxial compression. Sample design charts for GRP plates that yield and buckle simultaneously under compression are also presented in the thesis. The final stage of the work dealt with the design and test of GRP beams. The investigation began by finding the optimum cross-section for a GRP beam. The cross-section which was developed was a thin walled corrugated section which showed higher stiffness than other cross-sections for the same cross-sectional area (i.e. box, I, and rectangular sections). A cold setting, hand layings technique was used in manufacturing these beams wbich were of nine types depending on the type of glass reinforcement employed and the arrangement of layers in the beam. The simple bending theory was used in the beam design and proved to be satisfactory in predicting the stresses and deflections. A factor of safety of 4 was chosen for design purposes and considered to be suitable for long term use under static load. Because of its relatively low modulus, GRP beams allowable deflection was limited to 1/120th of the span which was found to be adequate for design purposes. A general discussion of the behaviour of GRP composites and their place relative to the more conventional structural material was also presented in the thesis.

No description available.

No description available.

No description available.