1 1.4 1.8 2.2 2.6 3 0 5 10 15 Young's modulua (Gpa) Radial distance from the center (µm) Week 4 Week 3 Week 2 Week 1 3. CONCLUSION Photo-oxidation during the manufacturing process has no significant effect on sub-micron scale measurement of Young’s modulus across the nylon fibers. Nanoindentation provided gradation of Young’s modulus values across the fiber cross section; however the macro-tensile testing provided no significant change of Young’s modulus as a function of UV and thermal exposure hours. It was evident that there is a significant decrease of Young’s modulus values near the surface than at the center as the UV exposure duration increases to 144 hours. FTIR spectra proved the formation of hydroxyl bonds with UV exposure. WAXS studies confirmed the destruction of crystalline structure of the nylon fibers when the exposure time increased. Nanoindentation on nylon fibers subjected to thermal exposure showed a similar pattern of decreasing Young’s modulus with exposure time increased. A gradual reduction of Young’s modulus from surface to the center as the thermal exposure time increased was also observed. Acknowledgements The authors greatly appreciate the financial support from the National Science Foundation through Grant CMS0618119 for acquiring an atomic force microscope. Also we would like to acknowledge the financial support from the National Textile Center through project number M08-MD13. References 1. Fuller CS, Baker NO, Paper NR, “Crystalline Behavior of Linear Polyamides. Effect of Heat Treatment”, Journal of American Chemical Society 62, 12, 3275, 1940 2. Bunn CW, Garner EV, “The Crystal Structures of Two Polyamides ('Nylons')”, Proceedings of Royal Society A189, 39, 1947 3. Mai YW, Head DR, Cotterell B, Roberts BW, “Mechanical properties of nylon 6 subject to photodegradation”, Journal of Material Science 15, 3057 – 3065, 1980 4. Mead JW, Mead KE, Auerbach I, Ericksen RE, “Accelerated aging of nylon 66 and Kevlar 29 in elevated temperature, elevated humidity, smog, and ozone”, Industrial and Engineering Chemistry, Product Research and Development 21, 2, 158–163, 1982 5. Krejsa MR, Udipi K, Middleton JC, “NMR analysis of UV and heat aged Nylon 66”, Macromolecules; 30, 16, 4695-4703, 1997 6. Auerbach I, “Kinetics for Tensile Strength Degradation for Nylon and Kevlar Yarns”, Journal of Applied Polymer Science 37, 2213-2227, 1959 0 100 200 300 400 0 10 20 30 Stress (Mpa) % Strain Unexposed Week 1 Week 2 Week 3 Week 4 Figure 8: Variation of Young’s modulus values across the radius of nylon fibers for different durations of thermal exposure Figure 9: Stress-strain curves for thermal exposed nylon fibers 234
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