Thermal degradation mechanisms of Nylon 6 deduced from kinetic studies by pyrolysis-g.c.

The thermal degradation of Nylon 6 has been studied over the temperature range 350-500 degrees C by pyrolysis-gas chromatography. Samples in the microgram range were deposited as thin films on a pyrolysis filament with the capacity to reach the degradation temperature in ca. 20 ms, and under these conditions it was found that caprolactam monomer was produced almost exclusively. Kinetic measurements were made by a sequential pyrolysis method. The limiting yields, which are required for the kinetic plots, were not found by taking the degradation to total conversion, but were found by a computation procedure. Using these computed values, first-order log plots were drawn, and these revealed that the monomer is evolved by two independent processes, the faster of which can occur in only a minor portion of the sample. The rate constant for the faster process is two orders of magnitude greater than that of the slow process at 350 degrees C, and one order of magnitude greater at 500 degrees C. This large difference permitted the rate parameters for both processes to be evaluated by a simple kinetic analysis. The slower process was found to have the higher activation energy (170+/-20 compared with 100+/-20 kJ mol(-1)), and also has the higher A factor (2.2+/-0.3x10(10) compared with 2.5+/-0.7x10(6) s(-1)), so the rate of this process approaches that of the faster process as the degradation temperature increases. The associated difference in entropy of activation (75 J mol(-1) K-1) is qualitatively interpreted in terms of the change in configurational entropy between the reactant molecule and the transition state for each of the processes. It is proposed that the slow process involves loss of monomer randomly from within any polymer chain, but the faster process is associated with backbiting from the ends of only a restricted number of molecules which have labile ends, leading to sequential loss of monomer. It is suggested that NH2 groups are the active ends which are involved in promoting the fast process. This mechanism suggests the possibility that nylon 6 could be made more thermally stable by modifying or blocking these active ends. (C) 1999 Elsevier Science Ltd. All rights reserved.