Young's modulus by 50%. These results were realized without compromise to the resin toughness.35

The inherent strength of individual carbon nanotubes (CNTs) offers considerable opportunity for the development of advanced, lightweight composite structures. Recent work in the fabrication and application of CNT forms such as yarns and sheets has addressed early nanocomposite limitations with respect to nanotube dispersion and loading and has pushed the technology toward structural composite applications. However, the high tensile strength of an individual CNT has not directly translated into that of sheets and yarns, where the bulk material strength is limited by intertube electrostatic attractions and slippage. The focus of this work was to assess postprocessing of CNT sheets and yarns to improve the macro-scale strength of these material forms. Both small-molecule functionalization and electron-beam irradiation were evaluated as means to enhance the tensile strength and Young's modulus of the bulk CNT materials. Mechanical testing revealed a 57% increase in tensile strength of CNT sheets upon functionalization compared with unfunctionalized sheets, while an additional 48% increase in tensile strength was observed when functionalized sheets were irradiated. Similarly, small-molecule functionalization increased tensile strength of yarn by up to 25%, whereas irradiation of the functionalized yarns pushed the tensile strength to 88% beyond that of the baseline yarn.

The hygrothermal aging characteristics of an epoxy resin were characterized over 1 year, which included 908 temperature and humidity cycles. The epoxy resin quickly showed evidence of aging through color change and increased brittleness. The influence of aging on the material's glass transition temperature (T g ) was evaluated by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). The T g remained relatively constant throughout the year long cyclic aging profile. The chemical composition was monitored by Fourier Transform Infrared Spectroscopy (FTIR) where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged. This property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions. This paper details the influence of processes such as: advancement of cure, chemical degradation, and physical aging on the chemical and physical properties of the epoxy resin. IntroductionEpoxy resins constitute an important class of matrix material for the aerospace industry. These materials are generally easy to process and can be tailored to provide a wide range of chemical and physical properties (Ref. 1). Composites based on epoxy resins are often used in structural applications where environmental conditions can include cyclic exposure to high heat and humidity. Repeated exposure to temperature/humidity cycles will be referred to as "hygrothermal aging" in this paper. Hygrothermal aging results in changes to the physical and mechanical properties of an epoxy resin (Ref. 2). When the epoxy resin is used as the matrix material in a composite, these changes in resin properties can contribute to microcracking and to reduced strength for matrix dominated composite failure modes.NASA/TM-2011-216999 2 Hygrothermal aging can induce permanent changes in an epoxy resin through non-reversible mechanisms that alter the chemical structure of the material. This process is called chemical aging. A change in the glass transition temperature (T g ) during aging is one indication of chemical aging in the form of continued cure or thermal degradation in the bulk resin. Chemical aging can also occur as oxidation (which is typically limited to exposed surfaces) and as hydrolysis (which is dependent on moisture absorption). There is a vast body of literature detailing the influence of thermal-oxidative aging on the chemistry of epoxy resins and epoxy based composites. It has been reported that thermal-oxidative environments can be detrimental to the integrity of the resin, and many papers report a reduction in T g with aging; attributed to oxidative chain scission within the structure (Refs. 3 and 4). The chemical reactions associated with oxidation have been extensively monitored by Fourier Transform Infrared Spectroscopy (FTIR), and a significant database of peak assignments has been collected (...

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