Molecular modeling of macroscopic failure in polymers

The ability to predict service lifetimes and damage tolerance of polymeric systems depends significantly on the method used to evaluate the response of the system under investigation to accelerated testing regimes. The ability of the accelerated testing format to truly mimic the in service conditions without the addition of any extraneous mechanisms competing with the fundamental failure mode bear heavily on the correlation of the experimental data to that observed from in service failure specimens. As a consequence of the very nature of polymeric systems careful consideration must be given to the subtleties of the potential degradation processes that may be active during the accelerated testing process and their relationship to the actual failure mode(s) observed from in service failures. One specific application of polymeric systems is that of a corrosion inhibitor, This application has led many to assume, incorrectly, that polymeric systems are not subject to corrosion or degradation in a manner similar to those of the more traditional engineering materials. In this laboratory much effort has been devoted to understanding the modes in which polymeric systems degrade and the various competing mechanisms responsible for the observed degradation processes. Mechanical testing methods employed to evaluate specimens subjected to accelerated testing regimes may not provide sufficient sensitivity to elucidate the actual mechanism responsible for failure. In this laboratory Electron Spin Resonance (ESR) has been incorporated to study the failure of polymeric systems on a molecular level. One of the early polymers, nylon 6, has been well studied and has found broad application as an engineering material from engine parts to cabling. An advantage in using nylon 6 in this study is that the material lends itself well to the application of ESR spectroscopy. The half life of free radicals generated from pulling nylon samples to failure is on the order of thirty minutes which provides sufficient time for data collection without significant loss of signal. Test specimens have been exposed to aggressive environments for varying periods of time and under various loading conditions and then pulled to failure to determine the amount of residual strength residing in the polymeric system, At high concentrations of NOx gas (>0.5% by volume) and high sustained loads (>40% s(u)) a synergistic effect between the aggressive environment and the sustained load is observed. At concentrations of 0.1% NOx and 30% s(u) the residual strength of the system is equivalent to that of the virgin material yet the residual spin count is observed to fail for short time exposures (4hrs<) and then recover to that of the virgin material for times greater than 4 hours. For a concentration of 0.2% NOx the residual strength and spin count is found to slowly degrade with increasing time exposure. As the concentration of NOx is increased to 0.4% by volume degradation of the mechanical properties as well as the residual spin count is significant. At these lower loading conditions and concentrations of NOx a departure from previous observations has been manifest. Exposure under these somewhat less rigorous conditions there is no observable synergistic effect between the aggressive environment and the 30% s(u) sustained load. Free hanging samples exposed concurrently to the same conditions as the stressed samples exhibit the similar values as those observed for the loaded samples. Possible mechanisms to account for these observation will be proposed.