Spectroscopically Evaluating Composition and Crystallinity in Pristine and UV-Exposed PET Films Using Polarization-Dependent Optical Photothermal Infrared Microscopy

Abstract In this study, we used polarization-dependent photothermal infrared (OPTIR) microscopy to extend infrared (IR)-based methods to characterize structurally and chemically modified polymer films. Because of the reported relationship with polymer crystallinity and stability, methods capable of monitoring and estimating changes in crystalline regions are paramount for understanding the performance and fate. To develop PTIR methods that could advance the understanding of plastic and polymer degradation, we examined UV-irradiated polyethylene terephthalate (PET) films using simulated photodegradation via a high-energy radiant exposure (SPHERE) weathering device. The well-studied UV-irradiated PET system provided a model to expand the IR characterization capabilities. Previously reported IR methods for determining crystallinity in bulk PET through glycol conformer fractions were combined with advanced structural determination methods to characterize thick and opaque cross-sectioned films using azimuthal angles and Hermans in-plane orientation functions, which extended the sample types available for analysis. To demonstrate the opportunity to decouple signatures from chemical and structural modifications in changing and chemically heterogeneous films, heterogeneity mapping was reported as a function of depth in cross-sectioned films using two-trace two-dimensional (2T2D) correlation analysis coupled with K-means clustering. Faster imaging procedures were also developed to limit possible effects from sample degradation and instrument drift during the acquisition.