In Situ Observation of the Methanolysis Reaction Proceeding on a PET Film Surface at Room Temperature: Physicochemical Viewpoints and the Role of Higher-Order Structure

According to a scheme of room-temperature transesterification [S. Tanaka et al. Green Chem. 2021, 23, 9412–9416.], a polyethylene terephthalate (PET) film was chemically decomposed, and its surface structure was inspected by using scanning electron microscopy (SEM) and X-ray grazing incidence small-angle scattering (XGSAS), covering small-angle scattering (SAXS) and diffraction (XRD). In the reaction solution mixed with dimethyl carbonate (DMC), the decomposed fragments of ethylene glycol (EG) are immediately converted to ethylene carbonate (EC) (chemical-trap effect) so the decomposition reaction continuously proceeds forward at room temperature. After 3 days of reaction, SEM elucidated that the amorphous regions are dominantly degraded, and stripes of crystalline bundles remain on the film surface. The degraded fragments, i.e., dimethyl terephthalate (DMT) monomer or its oligomers, immediately form the cauliflower-like granule (600 nm in diameter) composed of primary clusters (40 nm in diameter), which are connected in a line and emanate from a core of the granule (referred to as the “Pot & Sugar” structure). Thus, DMT cannot be backward polymerized again (physical trap effect). In an in situ or ex situ manner, X-rays irradiated at a grazing angle to the film surface elucidated that the crystallinity at the film surface apparently increases, and small-angle scattering originating from the primary clusters of DMT appears on the surface. Although methanol, a key reagent, is a poor solvent for PET, it diffuses into the amorphous region on the film surface and more easily from the edge surface than the film surface. The swelling behavior at the film surface was accelerated by decomposition (reaction absorption).