Understanding thermal aging of non-stabilized and stabilized polyamide 12 using 1H solid-state NMR

Noted for its outstanding properties, polyamide 12 (PA12) is widely used in many long-term applications today. Even so, little is known about how exposure to various conditions, including elevated temperatures, induces changes in PA12. Since such information is crucial to ensure its proper function over a long timespan, this current study compares the thermal aging of stabilized and non-stabilized PA12 samples at elevated temperature. For this, a detailed morphological study was for the first time conducted using a combination of 1H solid-state Nuclear Magnetic Resonance (NMR) measurements at high- and low-magnetic field, Fourier-Transform Infrared (FT-IR) spectroscopy, Differential Scanning Calorimetry (DSC), and viscometry measurements. The morphology of both samples characterized in terms of phase composition and chain mobility changes as a function of exposure time, being the most prominent for the non-exposed sample. This study shows also that simple depth-dependent relaxation measurements performed truly non-destructively by single-sided NMR are sensitive enough to monitor the extent of thermo-oxidation in PA12. Supported by DSC, FT-IR, and viscosity data, the NMR results show a complex interplay between annealing and thermo-oxidation. In particular, the NMR results demonstrate that thermo-oxidation affects both the semi-rigid and mobile amorphous phase and they indicate a preferential start of oxidative chain scission in the semi-rigid phase even in the presence of the stabilizers. To our knowledge such effects have never been experimentally observed and reported before.