Why Do Polymers Turn Yellow? Monitoring Polymer Aging with Time-Resolved Photoluminescence

Project Overview

In this feasibility study, we hypothesized that structural defects that lead to color change and degradation in polymers can be detected through time-resolved photoluminescence (PL) before the emergence of other visual or chemical cues. We discovered that structural defects can indeed be detected through the spectral and temporal characteristics of PL, even when the PL is excited at a visible wavelength (λ = 405 nm) at which the polymer is nominally transparent. As polymers were aged through exposure to either ultraviolet (UV) light or ozone, the PL emission intensity increases, the spectrum redshifts, and the lifetime decreases. These observations are consistent with a mechanism in which defect density, rather than specific defect electronic states, are responsible for the well-known yellowing of aged polymers. If confirmed, this mechanism would explain the universality of the polymer yellowing phenomenon.

The changes observed in the PL were not observable prior to the emergence of other degradation signatures through other conventional techniques such as vibrational spectroscopy, visible light absorption, or mechanical testing. However, PL has the advantage of being a non-contact, nondestructive test that does not require transmissive samples. Therefore, while it may not detect incipient damage prior to other techniques, PL may be a useful method for monitoring the polymers in the field.

Mission Impact

This project showed, for the first time, that polymer aging can be monitored through a PL technique. This development may allow for enhanced surveillance of polymeric components present in the field. Moreover, tying the aging behavior of polymers to defect density rather than defect type has profound implications for the field of polymer aging and stability as a whole. Material aging and reliability are central to the Lawrence Livermore National Laboratory mission and understanding the underlying science of this process in polymers could build toward mitigating aging phenomena in the future. The work advances the core competency of advanced materials and manufacturing, and the NNSA's stockpile stewardship mission.

Future work can focus on understanding this phenomenon quantitatively—can knowledge of PL spectral and temporal character, along with chemical structure analysis, lead to quantitative estimates of defect density? If so, it may be possible for advanced surveillance techniques to predict material failure, whether defined mechanically or otherwise, with a non-contact optical method.