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The effect of thermal-cycling-induced microcracking in fiber-reinforced polymer matrix composites is studied. Specific attention is focused on microcrack density as a function of the number of thermal cycles, and the effect of microcracking on the dimensional stability of composite materials. Changes in laminate coefficient of thermal expansion (CTE) and laminate stiffness are of primary concern. Included in the study are materials containing four different Thornel fiber types: a PAN-based T50 fiber and three pitch-based fibers, P55, P75, and P120. The fiber stiffnesses range from 55 Msi to 120 Msi. The fiber CTE's range from -0.50x10⁻⁶/°F to -0.80x10⁻⁶/°F. Also included are three matrix types: Fiberite's 934 epoxy, Amoco's ERL1962 toughened epoxy, and YLA's RS3 cyanate ester. The lamination sequences of the materials considered include a cross-ply configuration, [0/90]2s, and two quasi-isotropic configurations, [0/+45/-45/90]s and [0/+45/90/-45]s. The layer thickness of the materials range from a nominal 0.001 in. to 0.005 in. In addition to the variety of materials considered, three different thermal cycling temperature ranges are considered. These temperature ranges are ±250°F, ±150°F, and ±50°F. The combination of these material and geometric parameters and temperature ranges, combined with thermal cycling to thousands of cycles, makes this one of the most comprehensive studies of thermal-cycling-induced microcracking to date. Experimental comparisons are presented by examining the effect of layer thickness, fiber type, matrix type, and thermal cycling temperature range on microcracking and its influence on the laminates. Results regarding layer thickness effects indicate that thin-layer laminates microcrack more severely than identical laminates with thick layers. For some specimens in this study, the number of microcracks in thin-layer specimens exceeds that in thick-layer specimens by more than a factor of two. Despite the higher number of microcracks in the thin-layer specimens, small changes in CTE after thousands of cycles indicate that the thin-layer specimens are relatively unaffected by the presence of these cracks compared to the thick-layer specimens. Results regarding fiber type indicate that the number of microcracks and the change in CTE after thousands of cycles in the specimens containing PAN-based fibers are less than in the specimens containing comparable stiffness pitch-based fibers. Results for specimens containing the different pitch-based fibers indicate that after thousands of cycles, the number of microcracks in the specimens does not depend on the modulus or CTE of the fiber. The change in laminate CTE does, however, depend highly on the stiffness and CTE of the fiber. Fibers with higher stiffness and more negative CTE exhibit the lowest change in laminate CTE as a result of thermal cycling. The overall CTE of these specimens is, however, more negative as a result of the more negative CTE of the fiber. Results regarding matrix type based on the ±250°F temperature range indicate that the RS3 cyanate ester resin system exhibits the greatest resistance to microcracking and the least change in CTE, particularly for cycles numbering 3000 and less. Extrapolations to higher numbers of cycles indicate, however, that the margin of increased performance is expected to decrease with additional thermal cycling. Results regarding thermal cycling temperature range depend on the matrix type considered and the layer thickness of the specimens. For the ERL1962 resin system, microcrack saturation is expected to occur in all specimens, regardless of the temperature range to which the specimens are exposed. By contrast, the RS3 resin system demonstrates a threshold effect such that cycled to less severe temperature ranges, microcracking does not occur. For the RS3 specimens with 0.005 in. layer thickness, no microcracking or changes in CTE are observed in specimens cycled between between ±150°F or ±50°F. For the RS3 specimens with 0.002 in. layer thickness, no microcracking or changes in CTE are observed in specimens cycled between ±50°F.. Results regarding laminate stiffness indicate negligible change in laminate stiffness due to thermal cycling for the materials and geometries considered in this investigation. The study includes X-ray examination of the specimens, showing that cracks observed at the edge of the specimens penetrate the entire width of the specimen. Glass transition temperatures of the specimens are measured, showing that resin chemistry is not altered as a result of thermal cycling. Results are also presented based on a one-dimensional shear lag analysis developed in the literature. The analysis requires material property information that is difficult to obtain experimentally. Using limited data from the present investigation, material properties associated with the analysis are modified to obtain reasonable agreement with measured microcrack densities. Based on these derived material properties, the analysis generally overpredicts the change in laminate CTE. Predicted changes in laminate stiffness show reasonable correlation with experimentally measured values. / Ph. D.

Part I: This part of the dissertation describes a new approach to high molecular weight aromatic polyketones via soluble precursors derived from bis(a-aminonitrile)s. Bis(a-aminonitrile)s were easily synthesized from dialdehydes and secondary amines in very high yield by the Strecker reaction. Polymerization of bis(a-aminonitrile)s with activated dihalides using NaH as base in DMF yielded soluble, high molecular weight polyaminonitriles, which were hydrolyzed in acidic conditions to produce the corresponding polyketones. A novel approach to the synthesis of high molecular weight wholly aromatic polyketones without ether linkages or alkyl substituents in the polymeric backbones was demonstrated. These polyketones displayed excellent thermal properties and solvent resistance. A very efficient synthesis for diphenol and activated dihalide monomers containing keto groups was also developed based on a-aminonitrile chemistry. Novel activated dihalide monomers were obtained in quantitative yields. This method is suitable for any activated dihalide by reaction with 2 equivalents of 4-fluorobenzylaminonitrile and NaH, followed by hydrolysis to produce a new monomer with two more p-fluorobenzoyl units. For the synthesis of polyaminonitriles containing ether linkages in the polymeric backbone, only low to medium molecular weight polymers were obtained. The model studies proved that the carbanions of the aminonitriles can react with ether linkages to form more stable phenoxide anions and cause the termination of the polymerization. Part II: Functional hyperbranched poly(arylene ester)s were synthesized by thermal polymerization of 5-acetoxyisophthalic acid or 3,5-diacetoxybenzoic acid. Carboxylic terminated hyperbranched copolyesters were also synthesized by copolymerization of 5-acetoxyisophthalic acid and 3-hydroxybenzoic acid using different molar ratios of these two monomers. Both carboxylic acid and phenolic terminated hyperbranched polyesters were functionalized with different reactive groups. The carboxyl terminated hyperbranched poly(arylene ester)s were successfully used to modify inherently brittle epoxy resins. The hyperbranched polymers were chemically incorporated into the epoxy networks using triphenylphosphine (TPP) as a catalyst and 4,4'-diaminodiphenyl sulfone (DDS) as a curing agent. The chemistry and the proper formation of crosslinked networks were confirmed by solution 1H NMR, solid state CPMAS 13C NMR, kinetic FTIR spectroscopes and gel fraction analysis. Fracture toughness was improved without sacrificing thermal properties. The fracture toughness K1C values of the modified epoxies were found to be a function of the percentage loading, the molecular weights and the proportion of linear units of hyperbranched polyesters. Because the carboxylic acid terminated hyperbranched poly(arylene ester)s were immiscible with the commercially available epoxy EPON 828, the percentage loadings of hyperbranched modifiers were limited and the processibility of epoxy resins was difficult, especially at high percentage loadings of hyperbranched modifiers. These problems could be solved using phenolic terminated hyperbranched poly(arylene ester)s, which are more soluble in epoxy resins. / Ph. D.

Dimethacrylate oligomers diluted with styrene reactive diluents (so-called vinyl ester resins) are becoming increasingly important for composites in applications such as transportation vehicles, printed wiring boards and civil infrastructure. This research has focused on the generation and comparative analysis of glassy dimethacrylate networks as a function of oligomer structure, the type of reactive diluent, composition and curing conditions. A novel cycloaliphatic dimethacrylate was synthesized and its networks were compared to oligomeric structures containing dimethacrylates derived from epoxy terminated oligomers (from bisphenol-A and epichlorohydrin). Both types of dimethacrylates co-cured with methyl methacrylate exhibited increases in Mc and fracture toughness as the concentration of methyl methacrylate was increased. By contrast, networks prepared with a styrene diluent displayed reduced toughness with increasing styrene and Mc. Due to the need for long-term composite environmental durability, the effects of moisture and exposure to sunlight are important. Thus, these materials were exposed to ultraviolet light on a rotating drum for 225 days and the exposure was carefully monitored. Initial results from this study suggest that both the networks comprising the aromatic dimethacrylate/styrene structures as well as the cycloaliphatic analogues cured with methyl methacrylate undergo reductions in rubbery moduli upon aging under these conditions. X-Ray photoelectron spectroscopy shows higher levels of oxidation on the bisphenol-A vinyl ester networks cured with styrene, especially those containing dimethylaniline and cobalt naphthenate as accelerators. Scanning electron microscopy indicates a smooth surface for the cycloaliphatic networks cured with methyl methacrylate and a pitted surface for the aromatic networks cured with styrene. Water absorption is also higher for the cycloaliphatic vinyl ester; however, curing with a longer alkyl chain methacrylate (butyl methacrylate) decreases the water absorption to levels comparable to those cured with styrene. / Ph. D.