Controlled Hydrophilic–Hydrophobic Transition of PET Films via Fluorination and Drying

Polyethylene terephthalate (PET) films were modified by direct fluorination using fluorine gas at room temperature and 660 torr for reaction times ranging from 10 min to 5 h. Some of the fluorinated samples were dried at 70 °C for 7 days. FT-IR and XPS analyses confirmed the successful incorporation of fluorine into the PET structure, with the formation of -CHF- and -CF2- groups. The degree of fluorination increased with the reaction time, but excessive reaction led to the formation and loss of CF4. Drying further decreased the fluorine content due to the continued CF4 formation. XRD revealed that fluorination increased the crystallinity of PET owing to increased polarity, whereas drying decreased the crystallinity owing to increased crosslinking. The DSC results showed an increase in the glass transition temperature (Tg) after fluorination and drying, which was attributed to increased polarity and crosslinking, respectively. The surface hydrophilicity of PET increased significantly with fluorination time, and the water contact angle decreased to as low as 3.35°. This was due to the introduction of polar fluorine atoms and the development of a rough and porous surface morphology, as observed by AFM. Interestingly, drying of the fluorinated samples led to an increase in the water contact angle, with a maximum of 85.95°, owing to increased crosslinking and particle formation on the surface. This study demonstrates a simple and effective method for controlling the hydrophilicity and hydrophobicity of PET surfaces via direct fluorination and drying.