Thesis (M.S.) in Mechanical Engineering--University of Maine, 2002. / Includes vita. Includes bibliographical references (leaf 108).

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 120-125). Also available in electronic version. Access restricted to campus users.

Electrically conductive polymer composites, i.e. polymers filled with conductive fillers, may display a broad range of electrical properties. A rational design of fillers, filler surface chemistry and filler loading can tune the electrical properties of the composites to meet the requirements of specific applications. In this dissertation, two studies were discussed. In the first study, highly conductive composites with electrical conductivity close to that of pure metals were developed as environmentally-friendly alternatives to tin/lead solder in electronic packaging. Conventional conductive composites with silver fillers have an electrical conductivity 1~2 orders of magnitude lower than that of pure, even at filler loadings as high as 80-90 wt.%. It is found that the low conductivity of the polymer composites mainly results from the thin layer of insulating lubricant on commercial silver flakes. In this work, by modifying the functional groups in polymer backbones, the lubricant layer on silver could be chemically reduced in-situ to generate silver nanoparticles. Furthermore, these nanoparticles could sinter to form metallurgical bonds during the curing of the polymer matrix. This resulted in a significant electrical conductivity enhancement up to 10 times, without sacrificing the processability of the composite or adding extraneous steps. This method was also applied to develop highly flexible/stretchable conductors as building block for flexible/stretchable electronics. In the second study, a moderately conductive carbon/polymer composite was developed for use in sensors to monitor the thermal aging of insulation components in nuclear power plants. During thermal aging, the polymer matrix of this composite shrank while the carbon fillers remained intact, leading to a slight increase in filler loading and a substantial decrease in the resistivity of the sensors. The resistivity change was used to correlate with the aging time and to predict the need for maintenance of the insulation component according to Arrhenius’ equation. This aging sensor realized real-time, non-destructive monitoring capability for the aging of the target insulation component for the first time.

Composite materials have been an area of active research in recent years due to the possibility of obtaining multifunctional structures. Viscoelastic layered composites with parallel plane layers consisting of a stiff constituent and a soft viscoelastic constituent are of particular interest as they have been shown to exhibit simultaneous high stiffness and high damping. Such materials would be useful in structural applications and in high vibration environments such as in a vehicle or machinery. They would provide the rigidity required while simultaneously dissipating mechanical energy. The finite deformation mechanics of parallel plane viscoelastic layered composites has not been extensively studied. Under compressive loads they are very susceptible to instabilities. Buckling, for example is an elastic instability seen in load bearing materials. Since viscoelastic materials are rate and time dependent, the buckling modes for these composites not only depend on these factors, but also on the volume fraction of the stiff constituent. Three different cases are identified in the buckling and post-buckling response of these composites: non-dilute (high volume fraction), transition (intermediate volume fraction) and dilute (small volume fraction) cases. Due to buckling from the application of prestrain, the stiffness and damping of these composites can be tuned by orders of magnitude. Adaptive and multifunctional materials can be designed taking advantage of this idea and the rate dependence of the modes of deformation.

M. Tech. Polymer Technology / Reduced carbon dioxide emissions, improved fuel economy and better performance are just but a few pressures that have continued to define the automotive industry, globally. Low-cost and light weight materials have continually been used to replace metals as conventional raw materials in the manufacturing of automotives. Polymers have been a material of choice for meeting these demands; mainly due to their versatility, ease of processing and giving the manufacturers an option of parts integration. The industry has experienced remarkable product improvement from metal-to-plastic conversion and hence, further the plastic-to-plastic refinement of vehicle parts, from the interior, to body panels and to engine parts. Glass-filled polyamide 66 (PA66) composites have been extensively used in the production of engine components since post World War II era, which has paved the way for glass-filled polyamide 6 (PA6) composites, which has a lower melting point than PA66, to be used in technical applications for vehicle engine parts. This work attempts to investigate the use of layered double hydroxide (LDH) clay, the so called "anionic" clays for the preparation of PA6/LDH composites. In this study, PA6/LDH composites were prepared and the effect of organic modification of magnesium (Mg)-aluminium (Al) LDH clay was investigated. The aim of this research was to investigate the effect of organically modified and "unmodified" commercial Mg-Al LDH on the thermal, mechanical and physical properties of PA6 composites.

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