Exploration of dynamic mechanical analysis for studying degraded polymers: Dynamic mechanical changes after ultraviolet exposure

Abstract

Many techniques have been used to investigate the changes in the mechanical properties of polymers induced by photodegradation. In general, the observation of clear variations in bulk properties relies on destructive testing methods. This indicates the difficulties associated with distinguishing the effects of varying degradation levels using less-destructive approaches. This study employed dynamic mechanical analysis (DMA) as a less-destructive approach by applying a very low strain amplitude and narrowing the scanned temperature range. The temperature sweep with multi-frequency scans was limited from the glassy state to the lower portion of the glass transition. This approach was designed to minimize modification to the internal molecular structure of the samples while still capturing discernible changes in mechanical behavior. An amorphous polyester film was used with varying ultraviolet (UV) exposure. Different degrees of degradation were carried out on the polyester using NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure). After exposure, samples were stored in an ambient environment for different durations. The DMA results showed the exposed polyester had higher stiffness and less sensitivity to a high-frequency oscillatory strain relative to the unexposed polyester. Furthermore, the continuous chemical conversion of unstable intermediates to final photoproducts occurring in the samples after UV exposure was revealed. The chemical property was characterized using Fourier transform infrared (FTIR) spectroscopy. The impact of these chemical changes was effectively monitored using DMA by showing dynamic changes in the bulk mechanical properties of the samples during post-exposure storage. The dynamic mechanical changes facilitated the evaluation of the effects of varying degrees of degradation and the differences in internal structure between the exposed samples. This study demonstrated the potential of utilizing DMA to assess polymer degradation, including in severely degraded polyester samples, in a less destructive manner compared to test-to-failure methods.