The Combined Effect of NOx and Stress on Chain Scission and Mechanical Properties of Oriented Semi-Crystalline Nylon 6 Fibers

Prior work has shown that aggressive environments combined with stress can result in significant reductions in strength for polymers. The investigations to be reported in this paper focused attention on the molecular aspects of this observed behavior to better understand the relationship between atomic scale phenomena associated with this type of degradation and macroscopic behavior of the polymeric system. Oriented semi-crystalline nylon 6 fibers were placed in a NOx environment under sustained tensile loads. After exposure periods of 0 to 240 minutes in NOx (concentrations of 0% to 0.5% by volume) the fibers were removed and pulled to failure in a tensile testing machine coupled with an Electron Spin Resonance Spectrometer (ESR). Froo radicals produced as a result of bond rupturo wore detected and recorded with the ESR. The number of free radicals generated as a Function of sample elongation provide an indication of the number of tie chains ruptured during the final failure process and insight into the distribution in loading of the tie chains connecting the crystalline regions of the polymer. Elongation of the sample prior to the onset of free radical production is found to increase with increasing NOx concentration, the magnitude of the sustained stress, and duration of exposure to the NOx, while the number of free radicals produced during this final loading to failure decreased. Variations in the distribution in loading between the tie chains are reflected by differences in the mechanical properties of the polymer, i.e. a more uniform distribution results in higher strength. Changes, subsequent to sustained loading and environmental exposure, in strength, modulus, and enginering toughness are quantified. Variation in the distribution of effective chain length as a function of NOx concentration and exposure time was also investigated and correlated with these macroscopic properties.