Polyethylene terephthalate (PET) optical properties deterioration induced by temperature and protective effect of organically modified SiO2-TiO2 coating

Polyethylene terephthalate (PET) is one of the most widely used polymeric substrates for flexible photonic applications. Like all polymers, its usage is limited by the modest environmental and temperature resistance. In this work, the optical properties deterioration of PET substrate, in the form of 175 ?m thick film, is discussed and the strategy to overcome this problem, by application of the sol-gel coating, is presented. The investigations of physicochemical changes occurring in PET after the thermal treatment at temperatures of 160 °C, 190 °C, and 210 °C were performed by various characterization techniques, such as thermal analysis (DSC - differential scanning calorimetry and TG - thermal gravimetry), structural and chemical composition analysis (XRD - X-ray diffraction, Raman spectroscopy, and XPS - X-ray photoelectron spectroscopy) and microscopic analysis (SEM-EDS - scanning electron microscopy with chemical composition analysis, AFM - atomic force microscopy). The 25-layer organically modified SiO2-TiO2 coating obtained by the sol-gel method is demonstrated to be an effective protection against the optical properties deterioration of PET. The protective effect is demonstrated by the comparison of surface morphology, and optical properties of PET, with and without the protective coating, after thermal treatment at different temperatures. The refractive index of materials and thickness of coatings have also been studied with M-line and white light reflectance spectroscopy (WLRS).