PS undergoing a typical oxidative degradation pathway forms polar functional groups such as ketones and ester carbonyls which can be detected by FTIR. The higher intensity in carbonyl bands does not always imply a higher extent of degradation, so it is compared with a reference band such as the C-H band which is unaffected due to degradation. The extent of degradation is further quantified by calculating the carbonyl index defined as the ratio of carbonyl band (around 1720 cm -1 ) to aromatic C-H stretching (2850 cm -1 ) (CI = A 1720 /A 2850 ). At least 3 measurements were made for each degradation case and the residual absorbance is plotted with respect to blank with a standard deviation. The residual concentration (C or CI) is plotted over the initial concentration (C 0 , CI of blank PS), the results are presented in SI, Figure .