Figure 2: The schematic of tensile testing specimen. The blue dots represent the epoxy adhesive (L: Gage Length = 16mm). samples were exposed to a constant temperature of 175oC for a maximum duration of four weeks in increments of one week. A relative humidity level closer to zero was maintained throughout the test. 2.4. Tensile Testing 2.5. Wide Angle X-ray and Fourier Transform Infrared Spectroscopy Analysis WAXS was done using a set-up from Molecular Metrology, Inc. housed at University of Massachusetts, Amherst. It uses a 30w micro-source (Bede) with a 30x30 µm2 spot size matched to a MaxfluxR optical system (osmic) leading to a low divergence beam of monochromatic CuKα radiation (wavelength, λ = 0.1542). WAXS is performed using an image plate positioned in the sample chamber at a distance of 39 mm. The image plate (maximum resolution 50 µm) has a hole in its center. The whole system is evacuated while the test is conducted. Fourier transform infrared spectroscopy microscopy (Varian FTIR Bench and Microscope, CA, USA) was used to identify the chemical groups present in the fiber samples. The experiments were carried out in the reflectance mode. The percentages crystallinity of the fibers was calculated using the ‘Polar’ software. 3. RESULTS AND DISCUSSION 3.1. Nanoindentation of UV degraded nylon fibers The specimen for tensile testing was prepared as shown in figure 2. The fiber was held by two epoxy dots at the ends. The uniaxial tensile testing of nylon fibers was carried out using the INSTRON 5569 materials testing system. A load cell having a maximum value of 2.5 N was used in the experiments. The single fiber tensile testing method described in ASTM G3822 was followed. A constant extension rate of 2.54 mm/min was used. The crosshead displacement was used to determine axial strain of the fiber and recorded force values were used to determine the axial stress. The tests were continued until there were failure of fibers to determine tensile strength and percentage elongation at break. The typical force deflection diagrams of unexposed and UV exposed nylon fibers obtained for the indentation point close to the surface are shown in figure 3. The initial slope of the unloading portion of the curve is calculation of Young’s modulus. These curves testify the reduction of Young’s modulus for fibers with longer UV exposure time. Variation of Young’s modulus of ultraviolet exposed nylon fibers from the center to surface is shown in figure 4. Young’s modulus profile of the unexposed fibers remained almost the same from center to surface of the fiber. This proves that the initial stabilizes added to nylon melt during the melt spinning stage have worked well since the fiber has maintained the mechanical properties without deterioration. When the exposure hours are increased by increments of 24 hours up to 144 hours, the Young’s modulus was reduced. The decrease of modulus values at the surface is higher than the center of the fiber. This clearly shows that the mechanical deterioration of the nylon fibers happens more severely close to the surface than the center. The Young’s modulus values at the center also reduce down to ~2 GPa at 144 hours from ~3 GPa at unexposed level. This suggests a 33% reduction of modulus values at the center, while the reduction of modulus close to the surface is around 65%. The decrease of Young’s modulus values can be attributed to the decrease in crystallinity as discussed in section 3.2. This leads to the observed reduction of mechanical properties which is evident from the results of percentage elongation to break and tensile strength which report lowest values as discussed in section 3.3. 231
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