Conventional methods of imparting toughness to ordinarily brittle thermosetting resins involve the incorporation of a second, discreet phase. Traditionally, this phase has been either a functionalized butadiene-acrylonitrile based elastomer or an unreactive thermoplastic. This dissertation describes the preparation, characterization, and evaluation of new functionalized polysiloxane elastomer and thermoplastic modifiers and their morphological implications to the toughening and physical behavior of, principally, epoxy thermosetting systems. Secondary amine-terminated poly(dimethyl-co-diphenyl siloxane) oligomers were found to be comparable tougheners to acrylonitrile-butadiene rubbers for a bisphenol-A based epoxy resin. The system that imparted the highest toughness was comprised of statistically placed 40% diphenyl and 60% dimethylsiloxane units with Mn̅ of 5000 g/mole loaded at 15% w/w. This composition resulted in a discreet second phase consisting of l μm spherical particles which were evenly dispersed throughout the cured epoxy matrix. Amine-terminated poly(arylene ether ketone) and poly(arylene ether sulfone) thermoplastics were reacted into an EPON 828/4,4'·DDS system. However, the polyketones proved to be ineffective toughening agents due to an incompatibility resulting in macroscopic phase separation. In contrast, the functionalized polysulfones were shown to be effective toughening agents, with the resultant morphology primarily a function of percent incorporation. At ~15% w/w, the polysulfone separated as l-2μm discreet particles while a 30% loading level resulted in a bicontinuous “honeycomb” morphology. The amine endgroups were shown to be necessary in controlling morphology and maximizing toughness. The polysulfone oligomers were also incorporated into a graphite fiber reinforced epoxy composite. Although improved mechanical properties were achieved, the toughness values were not as high as predicted by the neat resin evaluation. The morphology was less definable due to the complex nature and dimensions of the carbon fiber/matrix interactions. Finally, melt processing experiments indicated that amine-terminated polysulfones may act as effective processing aids for brittle bismaleimide systems, by reacting with the BMI, possibly via a Michael addition. This results in a chain extension and higher molecular weight without premature gelation occurring. / Ph. D.

A series of reactive phenylethynyl endcapped imide oligomers has been prepared in either fully cyclized or amic acid precursor form. Soluble oligomers have been synthesized with controlled molecular weights ranging from 2- to 12 Kg/mol. Molecular weight characterization was performed using SEC (size exclusion chromatography) and 13C-NMR, revealing good agreement between the theoretical and experimental (Mn) values. Crosslinked polyimides were obtained by solution or melt processing the oligomers into films and gradually heating in a programmed temperature manner up to the appropriate reaction temperature for the phenylethynyl groups, which is approximately 350-400°C. Thermal analysis of the resulting films showed high glass transition temperatures (>300°C) and excellent thermal stability, comparable to those found for thermoplastic control polyimides. The crosslinked films also had exceptional solvent resistance as evidenced by a high gel fraction (greater than or equal to 95%) following extraction in common solvents for several days. This was in contrast to the amorphous thermoplastic controls, which quickly dissolved upon immersion in solvents. The monomers used for synthesizing the polyimide oligomers were varied systematically within the series to study the influence of both molecular structure and molecular weight on the physical and film-forming properties. The incorporation of fluorinated monomers, such as 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), reduced water absorption and lowered the dielectric constant relative to non-fluorinated polyimides in the series. When flexible ether linkages were incorporated in the repeat unit by using 4,4'-oxydianiline (ODA), relatively more ductile solvent-cast films were obtained from oligomers having Mn values as low as 10 Kg/mol. Additionally, oligomer Mn and the relative rigidity/symmetry of the repeat unit structure greatly influenced the solubility of polyimides in NMP. For example, even 6FDA/p-phenylenediamine based oligomers with Mn values targeted below 10 Kg/mol precipitated from NMP at 180°C during solution imidization. The relationship between solution viscosities of polyimide and poly(amic acid) thermosetting oligomers and wetting/spreading ability to form continuous films during spin casting was elucidated. Employing o-dimethoxybenzene (DMB) as a cosolvent with NMP improved the film-forming ability of the fully imidized 6FDA/ODA oligomer series. This was evidenced by a decrease in viscosity (via suppression of physical-type gel formation) and better overall coverage and clarity of the films. Humidity was found to have a detrimental effect, causing the polyimide oligomers to phase separate to form cloudy or porous films. When moisture was reduced, oligomers having Mn greater than or equal to 6 Kg/mol formed spin cast films of <20 micrometer thickness with good qualitative adhesion to several inorganic substrates. Dielectric constants (epsilon) were estimated for several of the polyimides by measuring the refractive indices (n) of the films and using Maxwell's relationship (epsilon at optical frequencies is equal to n raised to the second power). The apparent dielectric constants were low, ranging from 2.47 to 2.75. The novel combination of low dielectric constant, solvent resistance and isotropic physical properties inherent in the thermosetting polyimide oligomers makes these materials excellent candidates for use as thin film insulating layers in microelectronics packaging applications. / Ph. D.

Thermophysical characterization of anisotropic composite materials is extremely important in the control of today fabrication processes and in the prediction of structure failure due to thermal stresses. Accuracy in the estimation of the thermal properties can be improved if the experiments are designed carefully. However, on one hand, the typically used parametric study for the design optimization is tedious and time intensive. On the other hand, commonly used gradient-based estimation methods show instabilities resulting in nonconvergence when used with models that contain correlated or nearly correlated parameters. The objectives of this research were to develop systematic and reliable methodologies for both Experimental Design Optimization (EDO) used for the determination of thermal properties, and Simultaneous Parameter Estimation (SPE). Because of their advantageous features, Genetic Algorithms (GAs) were investigated for use as a strategy for both EDO and SPE. The EDO and SPE approaches used involved the maximization of an optimality criterion associated with the sensitivity matrix of the unknown parameters, and the minimization of the ordinary least squares error, respectively. Two versions of a general-purpose genetic-based program were developed: one is designed for the analysis of any EDO / SPE problems for which a mathematical model can be provided, while the other incorporates a control-volume finite difference scheme allowing for the practical analysis of complex problems. The former version was used to illustrate the genetic performance on the optimization of a difficult mathematical test function. Two test cases previously solved in the literature were first analyzed to demonstrate and assess the GA-based {EDO/SPE} methodology. These problems included the optimization of one and two dimensional designs for the estimation at ambient temperature of two and three thermal properties, respectively (effective thermal conductivity parallel and perpendicular to the fibers plane and effective volumetric heat capacity), of anisotropic carbon/epoxy composite materials. The two dimensional case was further investigated to evaluate the effects of the optimality criterion used for the experimental design on the accuracy of the estimated properties. The general-purpose GA-based program was then successively applied to three advanced studies involving the thermal characterization of carbon/epoxy anisotropic composites. These studies included the SPE of successively three, seven and nine thermophysical parameters, with for the latter case, a two dimensional EDO with seven experimental key parameters. In two of the three studies, the parameters were defined to represent the dependence of the thermal properties with temperature. Finally, the kinetic characterization of the curing of three thermosetting materials (an epoxy, a polyester and a rubber compound) was accomplished resulting in the SPE of six kinetic parameters. Overall, the GA method was found to perform extremely well despite the high degree of correlation and low sensitivity of many parameters in all cases studied. This work therefore validates the use of GAs for the thermophysical characterization of anisotropic composite materials. The significance in using such algorithms is not only the solution to ill-conditioned problems but also, a drastically cost savings in both experimental and time expenses as they allow for the EDO and SPE of several parameters at once. / Ph. D.

Cyanate ester or triazine networks are attaining increasing importance as potential candidates for high temperature adhesives and composite matrices. Low toughness is a major drawback with most crosslinked thermosetting materials, including the cyanate ester networks. Considerable attention has been devoted to the aspect of toughening such brittle networks in our laboratories. Reactive functional thermoplastics not only enhance toughness but also impart highly desirable stability to solvent stress cracking without seriously affecting the moderately high modulus. Various aspects of this technology, have earlier been successfully applied to epoxy and bismaleimide systems. Careful control of the heterophase morphological structure is necessary to achieve significant toughening. This thesis has focused on modifications of a specific cyanate ester network system based on Bisphenol-A with thermoplastic modifiers, which were systematically varied with respect to back-bone molecular weight and chemistry. Hydroxyl or cyanato functional Bisphenol-A based amorphous poly(arylene ether)s have been successfully utilized to toughen the cyanate ester networks. Blends of reactive and non-reactive Bisphenol-A based amorphous poly(arylene ether sulfone)s were also demonstrated to be useful tougheners, apparently by allowing phase size control. The use of Bisphenol-A based amorphous polyarylene ether ketones (which are of lower polarity relative to the Bisphenol-A based polyarylene ether sulfones) resulted in larger, well defined morphologies which in turn resulted in tougher networks. It was demonstrated that either hydroxyl or cyanato reactive end-groups could be effectively utilized. Both were superior to non-reactive systems in terms of mechanical performance as well as solvent stability. One of the major drawbacks of this effort was that 3-4 fold improvements in toughness were attained but this was at the expense of the upper use temperature which dropped to a significant extent. Hydroxyl functional phenolphthalein based amorphous poly(arylene ether)s have also been successfully utilized to toughen the cyanate ester networks. This is significant in that toughened multi phase networks were generated without a sacrifice in either the Tg or the moderately high modulus of the unmodified cyanate ester networks. It has been demonstrated that the heterophase morphological structure which strongly influences mechanical performance is in turn influenced by the back-bone chemistry, molecular weight and end-functionality of the thermoplastic modifier. In addition, the kinetics of network formation also significantly influences the microphase separated morphologies. Generation and control of such microphase separated morphologies employing both thermal and microwave radiation has been investigated. An interdisciplinary investigation was undertaken to explore the feasibility of hydroxy functionalized phenolphthalein based poly(arylene ether sulfone) modified cyanate ester networks as potential candidates for high performance adhesive and composite matrix applications. Investigations into composite matrix applications, involved establishing models for the experimentally determined time and temperature dependent kinetics of cure as well as melt rheology. It is expected that these models will consequently complement efforts in establishing an optimized cure protocol for the fabrication of composite panels. Preliminary studies concerning aspects of fiber-matrix interfacial adhesion and the viability of thermoplastic modified cyanate ester networks as a structural adhesive have been conducted. / Ph. D.

Chemical and physical changes occur during the processing of toughened thermosetting resins. A number of properties are related to the type and sequence of these changes. There is a need for the development of in-situ real-time sensors to follow these changes. Once these sensors are developed, they can be used to preferentially select networks and/or morphologies by feedback-controlled "intelligent" processing. A practical, durable, inexpensive Fourier Transform NearInfrared (FTNIR) fiber optic sensor was developed and the cure of a model toughened cyanate as well as a commercial paste adhesive was followed with this sensor In the near-infrared. The design was suitable for many applications. A mold was designed to incorporate the fiber sensor for composite applications. The growth of the normalized triazine (crosslink) peak appeared to follow second order kinetics. The normalized peak reflected chemical as well as physical changes. Analysis of the individual peaks showed significant physical effects. Conversion based on triazine concentration did not follow second order kinetics. / Ph. D.

A heightened interest in intelligent material systems has occurred in recent years due to their remarkable adaptive abilities. Intelligent materials systems, which contain sensors and actuators coupled by means of active control, frequently utilize composite materials as the skeletal structure. In order for composite materials to be utilized in intelligent material systems to their utmost capability, many material properties, including the interfacial shear strength between the embedded sensor or actuator and the matrix must be thoroughly understood.. Investigations were performed in order to examine the effects of different variables on the interfacial characteristics between a nitinol fiber and a composite matrix. First, rough, clean fiber surfaces were found to provide the best adhesion to the matrix due to the mechanical interaction of the matrix with the rough surface finish. Second, it was determined that the interfacial shear strength is not dependent upon embedded fiber length. Third, a very small diameter fiber will break before pulling out of the matrix, but overall, large fibers have a greater interfacial strength. Fourth, it was found that the initial prestrain on the fiber during processing had no effect on the interfacial shear strength of the fiber to the matrix. Fifth, it was determined that fatigue does not degrade the shear strength of any of the different initial pres trains. Finally, it was found that a coating that does not adhere well to the fiber neither macroscopically degrades nor enhances interfacial strength. / Master of Science

This research project is partnership project involving industrial, university and government collaborators. The overall objective is to develop and enhance tools for use in Resin Transfer Moulding (RTM) design technology in order to re-design existing metallic parts using composite materials. / The specific objective of this work is to present preliminary research findings of the development of an optimized design of a leading edge slat (horizontal stabilizer component) from the Bell Model 407 Helicopter. The results presented here focus on the static stress analysis and the structure design aspects. The findings will serve as a basis for future design optimization as well as further developments in the use of RTM technology in re-designing metallic aeronautic components and can be considered to be "semi-optimized". / This research is based on extensive finite element analysis (FEA) of several composite material configurations, with a comparison made with the original metallic design. Different key criteria of the part design such as ply lay-up, bracket geometry, angle and configuration are tested using FEA technology with the objective of selecting the design which is minimizing stress concentrations. The influence of the modification of model-related parameters was also studied. / Preliminary comparative studies show that the slat configuration with half brackets opened towards the inside with an angle of 70 degrees (angle between the top of the airfoil and the side of the bracket) is the best option according to minimum stress concentration and structural flexibility. This choice is confirmed by other factors such as material savings and ease of processing.

Thesis (Ph.D.) -- Mississippi State University. Department of Chemistry. / Title from title screen. Includes bibliographical references.

Submitted by José Eduardo Estevam da Silva (eduardoposmat@gmail.com) on 2018-04-15T21:56:51Z No. of bitstreams: 1 DISSERTAÇÃO VERSÃO FINAL.pdf: 4070557 bytes, checksum: fb263583bfb8f6573957500e76dce18a (MD5) / Approved for entry into archive by Minervina Teixeira Lopes null (vina_lopes@bauru.unesp.br) on 2018-04-16T19:34:27Z (GMT) No. of bitstreams: 1 silva_jee_me_bauru.pdf: 3974286 bytes, checksum: c0b61a56b4e52184bb3bdb58c2d99f52 (MD5) / Made available in DSpace on 2018-04-16T19:34:27Z (GMT). No. of bitstreams: 1 silva_jee_me_bauru.pdf: 3974286 bytes, checksum: c0b61a56b4e52184bb3bdb58c2d99f52 (MD5) Previous issue date: 2018-01-26 / O processo de fabricação de painéis MDF é muito dinâmico e a todo instante surgem novidades tecnológicas buscando não somente otimizar custos de processo, mas incrementar novas características ou melhorar a resistência mecânica e usinabilidade. As fibras de madeira de eucalipto e as resinas termofixas como a uréia-formaldeído desempenham papel importantíssimo no contexto industrial. Um grande número de pesquisas e estudos se direciona a aperfeiçoar os processos de fabricação por meio do aprimoramento da matéria-prima. Muitos materiais foram desenvolvidos e adicionados tanto as fibras como as resinas sendo muito comum a busca por melhorias na resistência mecânica nas chapas MDF. A resina MDI (metileno difenil diisocianato), por exemplo, possui muitas vantagens conhecidas como a isenção de emissão de formol e a alta resistência a água, porém, outras características relacionadas à interação química com a madeira são pouco estudadas. Nesse contexto tecnológico, o presente trabalho se dedicou ao uso das técnicas termoanalíticas TG/DTG-DTA e DSC para investigar e compreender melhor a interação química entre as fibras de madeira de eucalipto e as resinas sintéticas termofixas e por fim, sugerir melhorias ao processo produtivo buscando assim, reduzir custos de fabricação. Além das técnicas termogravimétricas, a espectroscopia vibracional de absorção na região infravermelho médio com transformada de Fourier também foi usada para auxiliar na proposta reacional de polimerização e principalmente na identificação da estrutura química dos produtos gerados. Investigou-se também o processo de polimerização térmica do monômero MDI bem como sua degradação térmica seguindo as recomendações do ICTAC. As curvas TG/DTG-DTA mostraram que a polimerização foi incompleta em todas as resinas indicando a necessidade de ajustes no processo. A proposta reacional para as resinas UF e MDI está de acordo com os dados das curvas TG e espectros de MIR. A partir da polimerização da resina MDI sugeriu-se a formação de carbodiimidas como produto final. Ao final da pesquisa, algumas melhorias foram propostas como o aumento da temperatura da prensa e a diminuição da concentração de resina visto que na situação atual, a polimerização é incompleta e deixa resíduos nas chapas MDF. Comprovou-se também que as chapas fora de especificação podem ser usadas como biomassa combustível nas caldeiras em substituição ao cavaco de eucalipto, pois liberam mais calor quando estão em combustão. / The process of manufacturing MDF panels is very dynamic and at every moment technological innovations are emerging, seeking not only to optimize process costs, but to increase new characteristics or improve mechanical strength and machinability. Eucalyptus wood fibers and thermoset resins such as urea-formaldehyde play a very important role in the industrial context. A great number of researches and studies are directed at perfecting the manufacturing processes through the improvement of the raw material. Many materials have been developed and added to both fibers and resins, and the search for improvements in mechanical strength in MDF sheets is very common. MDI resin (methylene diphenyl diisocyanate), for example, has many advantages known as formaldehyde emission exemption and high water resistance, but other characteristics related to the chemical interaction with wood are little studied. In this technological context, the present work was dedicated to the use of TG / DTG-DTA and DSC thermoanalytical techniques to investigate and better understand the chemical interaction between eucalyptus wood fibers and thermosetting synthetic resins and, finally, to suggest improvements to the production process thus seeking to reduce manufacturing costs. In addition to the thermogravimetric techniques, the vibrational absorption spectroscopy in the medium infrared region with Fourier transform was also used to aid in the polymerization reaction proposal and mainly in the identification of the chemical structure of the generated products. The thermal polymerization process of the MDI monomer as well as its thermal degradation following the recommendations of the ICTAC were also investigated. The TG / DTG-DTA curves showed that the polymerization was incomplete in all resins indicating the need for process adjustments. The reaction proposal for the UF and MDI resins is in agreement with the data of the TG curves and MIR spectra. From the polymerization of MDI resin the formation of carbodiimides was suggested as the final product. At the end of the research, some improvements were proposed, such as the increase in the temperature of the press and the decrease of the resin concentration, since in the current situation the polymerization is incomplete and leaves residues in the MDF sheets. It has also been proven that non-specification sheets can be used as biomass fuel in boilers instead of eucalyptus chips, as they release more heat when they are in combustion.

Les matériaux thermodurcissables sont naturellement des isolants électriques, limitant leurs applications dans certains domaines comme l’électronique ou l'aéronautique. Ce travail de thèse consiste à développer un nouveau matériau composite thermodurcissable en présence de charges inorganiques conductrices pour apporter des propriétés électriques sans pour autant changer notablement la viscosité du système avant la polymérisation afin de permettre l'utilisation des technologies d'infusion de résine de l'aéronautique. La stratégie des travaux est basée sur la génération d'une séparation de phase au sein du matériau et la localisation des charges conductrices aux interfaces.Cette étude est scindée deux objectifs principaux. Le premier objectif consiste à étudier un système TP/TD (thermoplastique/thermodurcissable) afin d’obtenir et contrôler une morphologie interpénétrée, selon le processus de séparation de phase induite par la polymérisation. Le second objectif consiste à étudier la localisation des charges conductrices dans un système TP/TD. Deux procédés de mise en œuvre ont été développés. Le premier procédé dit « one shot » permet de localiser les charges préférentiellement et de manière homogène dans la phase thermodurcissable et apporte une conductivité uniquement à forte concentration en particules conductrices.Une seconde méthodologie a été élaborée permettant d’obtenir un matériau biphasique dans lequel les charges sont localisées préférentiellement à l’interface du système Epoxy/thermoplastique. Des conductivités, à faible taux de charges (5 % massique), de l’ordre 10-1 S/m ont pu être atteintes avec cette méthodologie. / Thermosetting materials suffer from a lack of electrical conductivity. In order to overcome this barrier, a natural strategy is to introduce conductive fillers above the percolation threshold. However, addition of fillers usually leads to an increase of viscosity of the formulation which precludes infusing the resin through the porous bed of carbon fibers. In order to solve this problem, we aim at creating a two phase material and locate the fillers at the interface in order to decrease the percolation at very low values.With this view, this study is divided into two parts. The first one concerns the control of multiphase composites in order to get a co-continuous morphology by a strategy called Reaction Induced Phase Separation (RIPS). Phase diagram and influence of parameters have been studied.The second part is the formulation of a composite material (thermoplastic/thermoset) in presence of inorganic fillers. We developed two differents processes which allowed us to control fillers localisation in the blend. A process called “one shot” allows to locate homogeneously inorganic particles in epoxy phase. A second process called “premix” would preferentially locate conductive fillers at the interface of the interpenetrating system.Diffusion of particles at the interface was observed in situ during the curing of a biphasic thermoset material permitting to open the road of conducting materials and a conductivity around 10-1 S/m has been reached using as low as 5 wt% carbon black. The concept of localization of filler has been valided on several systems.