structure in all cases was found to be the terminal methyl group which was attributed to polymer chain scission reactions. Further evidence for chain scission was found through the presence of aldehyde and formamide structures upon prolonged UV exposure and heat aging in the presence of atmospheric air and terminal vinyl structures upon UV exposure. The substitution of OH adjacent to the amide group at the end of a cleaved alkyl chain may be caused by the presence of oxygen during UV exposure. They also observed the Terminal amide groups in the starting material and same concentration upon heat aging or UV exposure. In heat aged samples, no evidence was found for olefin or hydroxyl-containing structures in nitrogen, but hydroxyl structures were observed in the sample. A degradation mechanism was proposed to account for the structures observed in their research work. Auerbach [6] studied the tensile strength degradation of nylon 66 yarns at elevated temperatures and over a broad range of relative humidities. It is observed that degradation rates for nylon was initially slow, but increased swiftly suggesting the depletion of an inhibitor. It is concluded that degradation is governed by thermal-oxidative and moisture induced mechanisms. Rate relationships were developed in which contribution from each mechanism was considered. Calculated degradation from these relationships agreed well with observed degradation over a broad range of temperatures and humidities. The goals of the present study were aimed at identifying chemical, micro-structural changes caused by degradation, to evaluate these results in heat-aged and UV-exposed systems, and to use this information on these structures to understand the underlying degradation mechanisms. 2. EXPERIMENTAL DETAILS 2.1. Sample Preparation Nylon commercial fibers for the analysis were obtained by Providence Yarn Company, Inc., RI, USA. An unknown stabilizer had initially been used in the spinning process of fibers. The typical diameter of the fiber was 30 µm. The fibers were molded in an epoxy plug for AFM characterization. A two-part thermoset epoxy (Epothin, supplied by Buehler Ltd., IL, USA) was specifically chosen since it had the properties of room temperature curing with minimum heat generation during the curing process. 2.2. AFM Characterization 2.3. UV and Thermal Exposure of Fibers The nylon fibers were subjected to Ultraviolet (UV) exposure using a Q-panel UV (QUV) weatherometer (QUV, Q-Lab Corp. OH, USA). Fluorescent UV lamps generated radiation in the QUV chamber. The UV wavelength of 340nm was chosen as per recommendation stated in ASTM G 154 standard for simulation of direct solar radiation, which allows enhanced correlation with actual outdoor weathering. The cyclic exposure conditions were eight hours of UV exposure at 50oC and four hours of condensation at 48oC. The nylon fibers were exposed in the QUV weatherometer for a maximum period of 144 hours in increments of 24 hours. Thermal exposure of nylon fibers was carried out using a controlled temperature/humidity test chamber (AH-202XCC, Bryant Manufacturing Associates, MA, USA). The Figure 1: Indentation pattern across the radius of the fiber In this study, an atomic force microscope (XE-100, Park Systems Inc, CA, USA) was used in the topographic imaging and nanoindentation of nylon fibers. A contact mode scan was made prior to the indentation of the sample. A typical scanned image with indentation pattern is shown in Figure 1. This facilitates the user to locate the specific location for indentation. Nanoindentaions were performed on fiber cross sections in the radial direction. For each duration of UV and thermal exposure, more than 15 radiuses were selected for indentation and for each radius; 6 points were chosen as shown in Figure 1. The force-indentation depth diagrams were further processed using a previously established methodology to determine Young’s modulus [7]. 230
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