A nano-IR study on the mechanistic degradation of UV-exposed PET films
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The occurrence of nanoplastics has recently gained awareness as a potential environmental concern. This has emphasized the necessity to better understand their formation pathways. A relatively new nano-IR tool, photo-induced force microscopy (PiFM), can provide a way to investigate this; offering a combination of highly space-resolved topographic and chemical information. We developed a spectral degradation series using the surface sensitive grazing incidence attenuated total reflectance (GI-ATR)-IR spectroscopy, in order to better understand the information gathered from PiFM nano- IR hyper maps of UV-degraded PET films. Spectral features indicative for carboxylic acid end groups were found to be formed in the film upon UV exposure. These features were additionally found on the outer surface of the film with PiFM. Yet, their presence showed to be inconsistent over multiple PiFM scans. Also, initial indications were found in the PiFM scans suggesting differences in the spectral features that may arise from other (degradative) processes, such as through changes in crystallinity. These results demonstrate the usefulness of PiFM to differentiate chemical information at the nanoscale, and may help to unravel the formation process of nanoplastics. However, the observed variability, inconsistency, and proneness to many measurement variables also underline its complexity. Furthermore, the development of morphological features measured by the atomic force microscopy (AFM) scans suggested the initial presence and growth of spherulite-like structures upon UV exposure. Over time, and upon submergence in water, these morphologies were transformed into more lamellar-like ridge structures which were elongated in a preferred orientation, seemingly inherent to the structure of the film. For the assessment of the spectral relation to topography, a tangible methodology was developed. This revealed the heterogeneous distribution of spectral features and structural properties to the surface topography. In this regard, the most exemplary observation was a frequently observed asymmetrical distribution of spectral features with respect to the formed cracks.