Neste trabalho, nanocompósitos de PBT, poli(tereftalato de butileno) e argila brasileira montmorilonita (MMT) modificada organicamente, foram obtidos com e sem agente tenacificante. Sais quaternários de amônio e fosfônio com estruturas químicas diferentes foram utilizados para modificar as argilas. Nanocompósitos de PBT com argilas comerciais, dos Estados Unidos, modificadas com de sais de amônio, foram obtidos para comparação das propriedades. As argilas e os polímeros foram misturados utilizando um misturador e uma extrusora dupla rosca, acoplados a um reômetro de torque. A qualidade da troca catiônica foi avaliada por difração de Raios-X (XRD), inchamento em solventes e análises termogravimétricas (TGA). O estado das argilas modificadas (OMMT) na matriz de PBT foi avaliado por XRD, microscopia ótica e microscopia eletrônica de transmissão (TEM). A dispersão do agente tenacificante foi avaliada por microscopia eletrônica de varredura (SEM). As propriedades mecânicas e de flamabilidade também foram avaliadas. Os resultados de flamabilidade foram explicados com ensaios de (TGA). Os resultados de espaçamento basal obtidos por XRD e inchamento em solventes foram dependentes da arquitetura do sal quaternário utilizado. Os espaçamentos basais ficaram maiores para os sais quaternários de longas cadeias alquílicas. Entretanto, o sal quaternário em excesso não foi eliminado na lavagem. A maior estabilidade térmica foi obtida com sais quaternários de fosfônio. Após a adição ao PBT, foi observado que a adição da argila organofílica na matriz polimérica não contribuiu para a significativa melhora das propriedades mecânicas que, em alguns casos, foram inferiores àquelas do PBT. Entretanto, a retardância a chama apresentou melhores resultados na presença de argila organofílica, sendo ainda melhores apenas na presença de sais quaternários de fosfônio. No sentido de melhorar as propriedades de flamabilidade do PBT sem perder em propriedades mecânicas, utilizou-se o agente tenacificante P(E-co-MA-co-GMA), copolímero etileno acrilato de metila metacrilato de glicidila. Desta forma, foram preservadas as propriedades mecânicas e retardância à chama. / In this work, nanocomposites of PBT, poly (butylene terephthalate) and Brazilian clay montmorillonite (MMT) organically modified were obtained with and without further addition of toughening agent. Quaternary ammonium and phosphonium salts with different chemical structures were used to organically modify the clays. PBT nanocomposites with commercial organoclays were also obtained for comparison. The materials were mixed using a mixer and a twin screw, coupled to a torque rheometer. The efficiency of cation exchange was evaluated by X-ray diffraction (XRD), swelling and thermogravimetric analysis (TGA). The dispersion of PBT with modified clay (OMMT) was evaluated by XRD, optical microscopy and transmission electron microscopy (TEM). The toughness dispersion was evaluated by scanning electron microscopy (SEM). The flammability and mechanical properties were also evaluated. Thermogravimetric analysis (TGA) of the OMMTs and PBT / OMMTs was also studied. The basal spacing obtained from XRD analysis were shown to depend on the architecture of the quaternary salt used and were larger for long alkyl chains. The quaternary salt excess was not removed during the washing step. The highest thermal stability was obtained with quaternary phosphonium salts. After adding the PBT, it was observed that the addition of organoclay to the polymer matrix did not contribute to a significant improvement of mechanical properties and in some cases even resulted in a decrease of mechanical properties. However, the flame retardancy showed best results in the presence of organoclay. The best results for the flammability properties were observed in the presence of only quaternary phosphonium salts. However, these materials were very fragile. In order to improve the flammability properties of PBT maintaining the mechanical properties, a toughening agent P(E-co-MA-co-GMA), copolymer ethylene methyl acrylate glycidyl methacrylate was used. In doing so both the mechanical and flame retardancy were preserved.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

The aim of this research was to prepare high performance and fully biodegradable polymer nanocomposites. The most representative polymers classified as biodegradable are poly(!-caprolactone) (PCL) (issued from petrochemistry) and polylactide (PLA) (issued from renewable bio-resources). Biodegradable nanoparticles purposely extracted from biomass were selected, namely Cellulose NanoWhiskers (CNW) and Starch NanoCrystals (SNC). CNW are rod-like nanoparticles with 2 nanometric dimensions while SNC consists in nanosheets, thus with 1 nanometric dimension. A 3 nanometric-dimension particle often considered as “silica- type nanocage” was selected to complete this study, namely Polyhedral Oligomeric Silsesquioxane (POSS). The addition of such nanoparticles was expected to enhance several properties of the filled polymer matrix, especially thermo-mechanical performances and extent of crystallinity. In this field, the quality of the nanoparticle dispersion throughout the matrix is an essential parameter to produce nanocomposite materials with largely improved properties. One of the most cited techniques to overcome nanofiller aggregation and even agglomeration relies upon the creation of strong chemical bonds between the nanoparticle and the polymer matrix, leading to the preparation of so-called nanohybrids. For that purpose, the surface of the nanoparticles was first modified by chemical grafting and polymerization reactions. The ring-opening polymerization (ROP) of e-caprolactone and L,L-lactide catalyzed by tin(II) 2- ethylhexanoate (tin octoate, Sn(Oct)2) was initiated from functional groups available on the nanoparticle surface. The grafting efficiency was demonstrated for the three investigated nanofiller/polyester systems. Different characterization techniques were approached depending on the nanofiller nature. In a second step, the so-formed nanohybrids were used as “masterbatches” and dispersed in their corresponding commercial polyester matrices, i.e. PCL and PLA, by melt-compounding using a mini-lab twin screw extruder. The nanocomposite materials were fully characterized, correlating morphological observations with thermal, mechanical and rheological properties. To highlight the beneficial effect of the surface covalent grafting, simple melt-blends, i.e., containing unmodified nanofillers and polyester matrices (PCL or PLA) were prepared. The level of property improvement was most of the time directly related to the degree of nanofiller dispersion, and proved systematically better in case of masterbatch-based materials. Keeping in mind the effect of the nanoparticle geometry, as well as its mechanical modulus, crystallinity or extent of dispersion within the polyester matrix, the rod-like 2D-nanofiller, namely cellulose nanowhiskers extracted from ramie, appeared as the most efficient candidate for polyester reinforcement. The incorporation of PCL chains surface-grafted onto CNW contributed to substantially increasing the overall thermo- mechanical properties, most likely due to the formation of a strong physical chain network between surface- grafted chains and chains composing the matrix. Additionally, CNW-based nanohybrids revealed their potential as both nucleating sites dramatically increasing the crystallization rate of PLA matrix and compatibilizing PCL/PLA immiscible blends.

Nanocomposites are a new class of mineral-field plastics that contain relatively small amounts (&lt / 10%) of nanometer-sized clay particles. The particles, due to their extremely high aspect ratios (about 100-15000), and high surface area (in excess of 750-800 m2/g) promise to improve structural, mechanical, flame retardant, thermal and barrier properties without substantially increasing the density or reducing the light transmission properties of the base polymer. Production of thermoplastic based nanocomposites involves melt mixing the base polymer and layered silicate powders that have been modified with hydroxyl terminated quaternary ammonium salt. During mixing, polymer chains diffuse from the bulk polymer into the van der Waals galleries between the silicate layers. In this study, new nanocomposite materials were produced from the components of recycled thermoplastic as the matrix and montmorillonite as the filler by using a co-rotating twin screw extruder. During the study, recycled poly(ethylene terepthalate), R-PET, was mixed with organically modified quaternary alkylammonium montmorillonite in the contents of 1, 2, and 5 weight %. Three types of clays were evaluated during the studies. For comparison, 2 weight % clay containing samples were prepared with three different clay types, Cloisite 15A, 25A, 30B. The nanocomposites were prepared at three different screw speeds, 150, 350, 500 rpm, in order to observe the property changes with the screw speed. Mechanical tests, scanning electron microscopy and melt flow index measurements were used to characterize the nanocomposites. The clay type of 25A having long alkyl sidegroups gave the best results in general. Owing to its branched nature, in nanocomposites with 25A mixing characteristics were enhanced leading to better dispersion of clay platelets. This effect was observed in the SEM micrographs as higher degrees of clay exfoliation. Nearly all the mechanical properties were found to increase with the processing speed of 350 rpm. In the studies, it was seen that the highest processing speed of 500 rpm does not give the material performance enhancements due to higher shear intensity which causes defect points in the structure. Also the residence time is smaller at high screw speeds, thus there is not enough time for exfoliation. In general, the MFI values showed minimum, thus the viscosity showed a maximum at the intermediate speed of 350 rpm. At this processing speed, maximum exfoliation took place giving rise to maximum viscosity. Also, the clay type of 25A produced the lowest MFI value at this speed, indicating the highest degree of exfoliation, highest viscosity, and best mechanical properties.

The current thrust towards the compaction of electrical power equipment, resulting in increased insulation electrical stress levels, necessitates new electrical insulating materials. In the last few decades, polymeric materials that exhibit light weight, excellent mechanical properties, low cost, and some with unique non-wetting surface characteristics, have surpassed the use of the conventional porcelain and glass insulating materials. Despite these advantages, polymeric materials are incapable of withstanding the high heat from surface arcing that is instigated by the synergism of pollution, moisture, and voltage. Surface arcing results in material loss due to heat ablation and/or the electrical tracking of polymeric materials. To overcome such issues, inorganic fillers are added to the base polymers to enhance their resistance to surface discharge activities and other performances. Since their addition can significantly reduce material costs, their use is compelling. Micron-sized fillers, here after defined as microfillers, have been used to acquire these desirable properties, but due to limitations in material processability, the further application of such fillers is limited. Consequently, nano-sized fillers, here after defined as nanofillers, have been viewed as replacements or assistant combinations to microfillers. Nanofillers are characterized by large surface areas, resulting in increased bond strengths that yield significant improvements in the various properties at fill levels well below that of microfillers. However, the primary problem of using nanofillers is their characteristic property of agglomeration due to their physical size and the forces between the fillers. Conventional mechanical mixing of nanofillers does not adequately separate the nanofillers, leading to behaviour similarly to that of microfillers. Therefore, the implementation of nanofillers is not completely effective. In chemical dispersion techniques, for example, the use of surfactants, are normally very elaborate and complicated. Due to the negative impact of agglomeration, the successful dispersion of nanofillers is pivotal in the further development of nanodielectrics for various insulation applications. In this thesis, electrospinning is proposed and realized as a new dispersal method for nanofillers in polymeric materials. This novel technique facilitates polymeric nanocomposites with improved properties due to the uniform distribution of fillers. Scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) clearly indicate that electrospun nanocomposites demonstrate a better filler distribution than nanocomposites, produced by conventional mechanical mixing. Also electrospinning introduces the possibility of separating different nanofillers in different base polymers. The mechanical properties: tensile strength and hardness; the electrical properties: permittivity, tracking, and erosion resistance; and the thermal properties: thermal conductivity, thermal degradation, and heat erosion resistance of electrospun nanocomposites are compared to those of conventional nanocomposites for silicone rubber and cycloaliphatic epoxy-based polymers. All the experimental studies in this thesis confirm that electrospun nanocomposites exhibit better thermal performances than the conventional composites which are attributed to the improved distribution of the nanofillers by the newly developed electrospinning process. Also in this investigation, a two-dimensional thermal model is developed in COMSOL MultiphysicsTM by using the finite element method (FEM) to theoretically address the benefits of using nanofillers and the effects of filler dispersion. The model confirms that electrospun nanocomposites have much more uniform temperature distribution than conventional nanocomposites. This thesis presents the possible mechanisms by which nanofillers improve the heat and erosion resistance of silicone rubber nanocomposites, and also addresses the possible mechanism by which electrospinning improves nanofiller dispersion.

Abstract Chapter 1 provides an overview of this study and a literature review. Emphasis is put on the materials used, the different processes available to synthesize polymer-clay nanocomposites, analytical methods to characterize nanophase materials and on the impact of the nanophase on the final physical properties of polymer-clay nanocomposites. Chapter 2 discusses PE-clay nanocomposites which were synthesized using metallocene and Ni-diimine catalysts through in-situ polymerization. Morphological studies were carried out by XRD, SEM, EDX, and TEM to investigate the intercalation and exfoliation mechanism. Prior to its injection into the polymerization reactor, montmorillonite (MMT) was treated with triisobutyl aluminum and undecylenyl alcohol (UOH). Triisobutyl aluminum (TIBA) can react with hydroxyl groups on the surface of MMT and UOH is able to react with TIBA on the MMT surface. An alkoxy bond is generated by the reaction of the hydroxyl groups of UOH with the TIBA on the surface of MMT. A single site catalyst was then supported on the MMT/TIBA/UOH support, generating a MMT/TIBA/UOH/CAT system. The free vinyl groups of the surface UOH molecules can be copolymerized with ethylene, leading to the formation of chemical bonds between the MMT surface and polyethylene (PE). Ethylene polymerizations with the MMT/TIBA/UOH/CAT system were compared with ethylene polymerization with unsupported catalysts. The resulting PE-clay nanocomposites were analyzed with electronic and optical microscopes to confirm the nanophase distribution of MMT platelets in the polymer matrix. TEM images showed that the exfoliated MMT layers appeared as single layers or aggregated layers in the polyethylene matrix. After Soxhlet extraction with boiling 1,2,4-trichlorobenzene, the morphology of the residue particles remaining the thimble showed polymer fibrils stemming from the MMT surface, providing direct evidence of the chemical bonds between MMT surfaces and polymer matrix. Some residue particles also show PE-clay hybrid fibers between the particles. Through SEM/EDX analysis, it was confirmed that the fiber’s composition possessed silicone atoms together with carbon atoms. Chapter 3 discusses the results of in-situ polymerizations in gas-phase. The same catalyst systems and polymerization conditions discussed in Chapter 2 for slurry polymerization were applied to the gas-phase polymerization in order to investigate the particle fragmentation mechanism. After gas-phase polymerization at atmospheric pressure, the surface morphologies were investigated by SEM and TEM. In the case of the MMT/TIBA/UOH/Cp2ZrCl2 system, small particles (< 10m) were shattered from the larger particles (> 100 m) in the early stages of polymerization. After 24-hours of continuous polymerization, polymer fibrils growing from the inside of the MMT particles were observed by SEM. After further investigation with TEM, the cross-section profile of the particles showed curved bundles of MMT platelets, which illustrates exfoliation starting from the edges of the MMT particles. The MMT/TIBA/UOH/Ni-diimine system shows a different surface morphology after polymerization. In the early stages of the polymerization, polymer films were generated from the inside of the particles. After further polymerization, the MMT particles shattered and formed aggregates of PE-clay nanocomposites, similar to the ones proposed in the multigrain model. Chapter 4 discusses the copolymerization of ethylene and acrylonitrile. Ethylene/acrylonitrile copolymers were produced in the presence of a Ni-diimine/EASC catalyst system without the use of supports. Polymerizations of ethylene and acrylonitrile showed comparable activities in low concentrations of acrylonitrile. However, in higher concentrations, acrylonitrile induced a reductive elimination of the alkyl groups in the activated nickel-diimine catalyst. Conclusively, GPC analyses showed that acrylonitrile behaves as a chain transfer agent, showing reductive elimination of alkyl groups in the catalytic active center. The polymerization product morphology was analyzed by SEM and TEM. Polyacrylonitrile domains were observed in the polyethylene matrix and confirmed its nanosize distribution in the polyethylene matrix. DSC analysis of ethylene/acrylonitrile copolymers shows that an exothermic reaction takes place from 300 C to 370 C. This exotherm band detected by DSC can be related to the cyclization and aromatization of the nitrile groups of polyacrylonitrile. Through IR analysis of the ethylene and acrylonitrile polymer under high temperatures, this cyclization and aromatization was confirmed to be the cause of the decrease of the nitrile band (at 2244 cm-1) and increase of the vinyl bands (at 1640 cm-1). In addition, thermal treatment in DSC and successive XRD analysis showed the formation of the lamellar structures in the polyethylene matrix, reported as lamellar formation of polyacrylonitrile due to cyclization and aromatization of nitrile groups. The decomposition temperatures measured by TGA increased up to 50 C due to the presence of the nitrile groups in the polymer matrix. Tensile testing showed that the modulus increased, together with the yield strength and elongation. This phenomenon supports that strong interfacial interactions exist between the polyethylene matrix and polyacrylonitrile domains, as confirmed by TEM and IR analysis. Chapter 5 introduces the idea of acrylonitrile as a clay surface modifier. MMT was treated with acrylonitrile, using the same modification method of MMT that was applied in the MMT/TIBA/UOH/CAT system in Chapter 2. The nitrile groups in PE-MMT/TIBA/AN/CAT composites were confirmed at 2244 cm-1 by IR analysis. DSC analysis of PE-MMT/TIBA/AN/CAT showed that an exothermic reaction takes place from 300 C to 375 C. Successive DSC analysis with the same sample showed a new glass transition temperature band, induced by the reduction of polymer chain mobility. The basal diffraction band disappeared due to the exfoliation of MMT. Tensile tests showed an increase in modulus, without sacrificing the yield strength and elongation of PE-clay hybrid composites. Through these analyses, it was confirmed that strong interfacial forces exist between the polyethylene matrix and MMT layers in these PE-clay nanocomposites.

Abstract Chapter 1 provides an overview of this study and a literature review. Emphasis is put on the materials used, the different processes available to synthesize polymer-clay nanocomposites, analytical methods to characterize nanophase materials and on the impact of the nanophase on the final physical properties of polymer-clay nanocomposites. Chapter 2 discusses PE-clay nanocomposites which were synthesized using metallocene and Ni-diimine catalysts through in-situ polymerization. Morphological studies were carried out by XRD, SEM, EDX, and TEM to investigate the intercalation and exfoliation mechanism. Prior to its injection into the polymerization reactor, montmorillonite (MMT) was treated with triisobutyl aluminum and undecylenyl alcohol (UOH). Triisobutyl aluminum (TIBA) can react with hydroxyl groups on the surface of MMT and UOH is able to react with TIBA on the MMT surface. An alkoxy bond is generated by the reaction of the hydroxyl groups of UOH with the TIBA on the surface of MMT. A single site catalyst was then supported on the MMT/TIBA/UOH support, generating a MMT/TIBA/UOH/CAT system. The free vinyl groups of the surface UOH molecules can be copolymerized with ethylene, leading to the formation of chemical bonds between the MMT surface and polyethylene (PE). Ethylene polymerizations with the MMT/TIBA/UOH/CAT system were compared with ethylene polymerization with unsupported catalysts. The resulting PE-clay nanocomposites were analyzed with electronic and optical microscopes to confirm the nanophase distribution of MMT platelets in the polymer matrix. TEM images showed that the exfoliated MMT layers appeared as single layers or aggregated layers in the polyethylene matrix. After Soxhlet extraction with boiling 1,2,4-trichlorobenzene, the morphology of the residue particles remaining the thimble showed polymer fibrils stemming from the MMT surface, providing direct evidence of the chemical bonds between MMT surfaces and polymer matrix. Some residue particles also show PE-clay hybrid fibers between the particles. Through SEM/EDX analysis, it was confirmed that the fiber’s composition possessed silicone atoms together with carbon atoms. Chapter 3 discusses the results of in-situ polymerizations in gas-phase. The same catalyst systems and polymerization conditions discussed in Chapter 2 for slurry polymerization were applied to the gas-phase polymerization in order to investigate the particle fragmentation mechanism. After gas-phase polymerization at atmospheric pressure, the surface morphologies were investigated by SEM and TEM. In the case of the MMT/TIBA/UOH/Cp2ZrCl2 system, small particles (< 10m) were shattered from the larger particles (> 100 m) in the early stages of polymerization. After 24-hours of continuous polymerization, polymer fibrils growing from the inside of the MMT particles were observed by SEM. After further investigation with TEM, the cross-section profile of the particles showed curved bundles of MMT platelets, which illustrates exfoliation starting from the edges of the MMT particles. The MMT/TIBA/UOH/Ni-diimine system shows a different surface morphology after polymerization. In the early stages of the polymerization, polymer films were generated from the inside of the particles. After further polymerization, the MMT particles shattered and formed aggregates of PE-clay nanocomposites, similar to the ones proposed in the multigrain model. Chapter 4 discusses the copolymerization of ethylene and acrylonitrile. Ethylene/acrylonitrile copolymers were produced in the presence of a Ni-diimine/EASC catalyst system without the use of supports. Polymerizations of ethylene and acrylonitrile showed comparable activities in low concentrations of acrylonitrile. However, in higher concentrations, acrylonitrile induced a reductive elimination of the alkyl groups in the activated nickel-diimine catalyst. Conclusively, GPC analyses showed that acrylonitrile behaves as a chain transfer agent, showing reductive elimination of alkyl groups in the catalytic active center. The polymerization product morphology was analyzed by SEM and TEM. Polyacrylonitrile domains were observed in the polyethylene matrix and confirmed its nanosize distribution in the polyethylene matrix. DSC analysis of ethylene/acrylonitrile copolymers shows that an exothermic reaction takes place from 300 C to 370 C. This exotherm band detected by DSC can be related to the cyclization and aromatization of the nitrile groups of polyacrylonitrile. Through IR analysis of the ethylene and acrylonitrile polymer under high temperatures, this cyclization and aromatization was confirmed to be the cause of the decrease of the nitrile band (at 2244 cm-1) and increase of the vinyl bands (at 1640 cm-1). In addition, thermal treatment in DSC and successive XRD analysis showed the formation of the lamellar structures in the polyethylene matrix, reported as lamellar formation of polyacrylonitrile due to cyclization and aromatization of nitrile groups. The decomposition temperatures measured by TGA increased up to 50 C due to the presence of the nitrile groups in the polymer matrix. Tensile testing showed that the modulus increased, together with the yield strength and elongation. This phenomenon supports that strong interfacial interactions exist between the polyethylene matrix and polyacrylonitrile domains, as confirmed by TEM and IR analysis. Chapter 5 introduces the idea of acrylonitrile as a clay surface modifier. MMT was treated with acrylonitrile, using the same modification method of MMT that was applied in the MMT/TIBA/UOH/CAT system in Chapter 2. The nitrile groups in PE-MMT/TIBA/AN/CAT composites were confirmed at 2244 cm-1 by IR analysis. DSC analysis of PE-MMT/TIBA/AN/CAT showed that an exothermic reaction takes place from 300 C to 375 C. Successive DSC analysis with the same sample showed a new glass transition temperature band, induced by the reduction of polymer chain mobility. The basal diffraction band disappeared due to the exfoliation of MMT. Tensile tests showed an increase in modulus, without sacrificing the yield strength and elongation of PE-clay hybrid composites. Through these analyses, it was confirmed that strong interfacial forces exist between the polyethylene matrix and MMT layers in these PE-clay nanocomposites.

Includes bibliographical references (p. 149-179).

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Tomorrow’s lightweight, high-performance composite systems will be made of structures built with materials that have unprecedented intrinsic properties for performing a wide range of functions, such as EMI shielding, thermal management, flame resistance, lightning strike protection, acoustic damping, and health-monitoring. Current structures require parasitic components, e.g., metal strips, copper wire meshes, strain gauges, and heat sinks to provide these functions. By eliminating parasitic components, future high-performance multifunctional systems can achieve the intended objectives, while maintaining optimum weight, reliability, cost, and fuel efficiency. With the continuing growth of polymer composites in industries, such as aerospace, automotive, and wind energy, research and development on lightweight, high-performance composites that possess extraordinary properties for future multifunctional systems has generated considerable interest and excitement. Recent advances in nanomaterial synthesis and functionalization have shown that tailored property combinations are possible with reduced parasitic content to achieve multifunctionality. Cyanate ester (CE), a class of high-performance thermosetting resins (high T [subscript g], >250°C), has received considerable attention due to its good mechanical properties, thermal stability, flammability properties, ease of process, and volatile-free curing process. Multiwall carbon nanotubes were selected due to their unique combination of excellent mechanical, electrical, and thermal properties. The principal objective of this work is to determine the extent to which several different processing techniques will affect the MWNT dispersion and corresponding nanocomposite properties, such as thermal, flammability, mechanical, and electrical properties. A processing-structure-property relationship, as well as performance of this class of carbon-based CE nanocomposite, will be established. Therefore, a major scientific contribution of this study will be the development and characterization of a novel, multifunctional CE nanocomposite. Different mixing instruments, such as high shear mixer, ultrasonicator, planetary centrifugal mixer, etc. were used to disperse the nanotubes in the cyanate ester resin matrix. Microstructural morphology characterizations by SEM, STEM, and TEM show that various degrees of dispersions of MWNTs were obtained by the different mixing techniques. An attempt to quantify the MWNT dispersion was made. Electrical resistivity of samples processed by both stand mixer and three-roll mill passes the ESD requirement; however, the MWNT percolation thresholds by the two techniques are different. Thermal analysis shows that the addition of the Fe³+ catalyst or the coupling agent lowers the glass transition temperature and degrades the mechanical properties (e.g., storage modulus, tangent of phase angle delta) of the CE resin. On the other hand, processing techniques only affect the mechanical properties of the resin. Thermal stability of CE is only slightly affected by different processing techniques, as well as the addition of MWNTs. Much more significantly, flammability characterization shows that the incorporation of either the Fe³+ catalyst or the coupling agent substantially increases the peak heat release rate (PHRR) relative to the neat CE resin value. / text