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The objective of this research was to determine the effect of thermal cycling combined with mechanical loading on the development of microcracks in M40J/PMR-II- 50, the second generation aerospace application material. The objective was pursued by finding the critical controlling parameters for microcrack formation from mechanical stress-thermal cycling test. Three different in-plane strains (0%, 0.175~0.350%, and 0.325~0.650%) were applied to the composites by clamping composite specimens (M40J/PMR-II-50, [0,90]s, a unitape cross-ply) on the radial sides of half cylinders having two different radii (78.74mm and 37.96mm). Three different thermal loading experiments, 1) 23oC to –196oC to 250oC, 2) 23oC to 250oC, and 3) 23oC to -196oC, were performed as a function of mechanical inplane strain levels, heating rates, and number of thermal cycles. The apparatus generated cracks related to the in-plane stresses (or strains) on plies. The design and analysis concept of the synergistic stress-thermal cycling experiment was simplified to obtain main and interaction factors by applying 2k factorial design from the various factors affecting microcrack density of M40J/PMR-II-50. Observations indicate that the higher temperature portion of the cycle under load causes fiber/matrix interface failure. Subsequent exposure to higher stresses in the cryogenic temperature region results in composite matrix microcracking due to the additional stresses associated with the fiber-matrix thermal expansion mismatch.

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.

An investigation to study factors controlling the structure and properties of binary- and ternary-phase polypropylene (PP) composites containing ethylene-propylene rubber (EPR) and glass beads has been carried out. The composite structure was evaluated using various techniques including SEM, DSC, XRD and DMA. While the mechanical tests included tensile and impact measurements at ambient temperature, and a fracture toughness test based on the J-integral method carried out at -20 oC. EPR and glass beads were found to influence the structure and properties of polypropylene in different ways. Incorporation of EPR into polypropylene results in an improvement in impact strength and toughness, accompanied by a decrease in tensile strength and modulus. The opposite was found for composites containing glass beads. Polypropylene composites with balanced mechanical properties were achieved by physical blending of this polymer with both EPR and glass beads. The effect of composite structure, composition and processing variables on the properties of the ternary systems were analysed. A study of their morphology has shown that two kinds of phase structure can be formed, either a separate dispersion of the phases, or encapsulation of the filler by rubber. Factors controlling these structures are believed to be due mainly to the surface characteristics of the components. Modification of EPR by maleic-anhydride grafting results in composites with rubber encapsulation of the filler, with FTIR revealing a reaction between these phases. Composites containing unmodified EPR, on the other hand, show separate dispersion of the components. The former composites, with good adhesion at the rubber and filler interface, have noticeably higher impact strength and fracture toughness at and below ambient temperatures, while the latter variant is characterised by higher tensile strength and modulus, accompanied by a lower impact strength. Improvements in impact strength of the composites was also achieved by promoting adhesion between the polymer and filler interface using surfacecoated glass beads, or by increasing the number of rubber particles adhering to the glass bead surfaces using a two-step mixing technique. Results of the present study have thus shown that mechanical properties of ternary phase polypropylene composites can be adjusted, to a certain extent, by controlling their morphologies through the use of suitable functionalised materials and also by using an appropriate compounding methodology.

The objective of this research programme was to study the influence of filler incorporation on the rheological, mechanical and morphological properties of high density polyethylene. Several compositions of general moulding grade of HDPE filled with increasing concentrations of diatomaceous earth, an amorphous silica-based particulate mineral of high porosity and friable nature, were compounded in a single-screw extruder. Good filler dispersion into the polymer matrix was achieved with complete polymer permeation through the porous structure of the diatomite particles, and with no apparent filler aggregates or voids being observed on fracture surfaces from SEM photomicrographs of the composites. The specific volume of the filled polymer was a simple linear additive function of the specific volumes of the individual components. DSC measurements carried out on the polyethylene in the composite, indicated very minor influence of the filler incorporation on the crystallinity content of polyethylene and, therefore, the diatomite filler was considered as non-active filler for the polyethylene matrix used.

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In this work the potential of graphene-like particles for mechanical reinforcement is investigated. Different polymer processing methods are studied from traditional ones to more advanced techniques. The potential of graphene as a reinforcement for polymer composites is addressed as a result of polymer modifications and the morphology of the graphene like particles. First, a composites of polycarbonate (PC) and graphite nanoplatelets (GNP) are produced by a traditional melt-mixing method. The GNP composites present a low mechanical reinforcing efficiency which is believed to be due to a poor dispersion of the GNP and a weak interaction between the GNP and the matrix. Secondly, solution cast composites of polyvinyl alcohol (PVA) with very low loadings of graphene oxide (GO) are produced. The polymer morphology undergoes some modifications after the addition of GO. A strong increase of the Tg is observed after the addition of GO which is the result of a reduction in polymer mobility, while a dramatic increase of the mechanical properties is seen as well. Uni-axial drawing is applied in order to align the particles. No polymer modifications are observed between the drawn PVA and the drawn nanocomposites due to the strong alignment of the polymer chains during the drawing. Mechanical reinforcement is observed after addition of the GO showing real reinforcement. Finally, a more advanced processing method is investigated using spraying. The condition of spraying a layer of polymer and GO is studied. Finally a hierarchical composite of PVA - GO is produced by this spraying method. 150 bi-layers are deposited to create a film with improved mechanical properties at a loading of 5.4 wt.% GO. The Young’s modulus and strength of these films doubled or nearly doubled which is believed to be due to the high level of structural organization of the layered nanocomposite incorporating the 2D GO nanofiller, together with hydrogen bonding between the PVA and the GO sheets.

Transport Properties of Nanocomposites were studied in this work. A Monte Carlo technique was used to model the percolation behavior of fibers in a nanocomposite. Once the percolation threshold was found, the effect of fiber dimensions on the percolation threshold in the presence and absence of polymer particles was found. The number of fibers at the percolation threshold in the presence of identically shaped polymer particles was found to be considerably lower than the case without particles. Next, the polymer particles were made to be of different shapes. The shapes and sizes of the fibers, as well as the polymers, were made the same as those used to obtain experimental data in literature. The simulation results were compared to experimental results, and vital information regarding the electrical properties of the fibers and fiberfiber junctions was obtained for the case of two stabilizers used during composite preparation ? Gum Arabic (GA) and Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). In particular, the fiber-fiber connection resistances, in the case of these 2 stabilizers, were obtained. A ratio between the fiber path resistance and the total connection resistance, giving the relative magnitude of these resistances in a composite, was defined. This ratio was found through simulations for different fiber dimensions, fiber types and stabilizers. Trends of the ratio with respect to composite parameters were observed and analyzed, and parameters to be varied to get desired composite properties were discussed. This study can serve as a useful guide to choose design parameters for composite preparation in the future. It can also be used to predict the properties of composites having known fiber dimensions, fiber quality and stabilizing agents.