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Thesis (M.Eng.). / Written for the Dept. of Mechanical Engineering. Title from title page of PDF (viewed 2008/01/14). Includes bibliographical references.

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.

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.