Effects of CF4 plasma treatment on the moisture uptake, diffusion, and WVTR of poly(ethylene terephthalate) flexible films

We studied the effects of CF4 plasma surface treatment on moisture uptake, diffusion, and the water-vapor-transmission-rate (WVTR) of poly(ethylene terephthalate) (PET) films using a CF4 plasma generated by a radio-frequency (13.56 MHz) reactive ion etcher at 60 W. After CF4 plasma treatment, moisture uptake in PET film was reduced with increasing treatment time due to (1) lower adsorption of water molecules onto the hydrophobic surface, as confirmed by contact angle measurement and XPS analysis, and (2) reduced diffusion coefficient through the denser fluorinated top-layer as detected by XRR. In addition, the WVTR of untreated PET is found to be 2.7 g/m2/day, while a significant reduction (84%) of WVTR to 0.43 g/m2/day, is achieved for CF4 plasma-treated PET film (60 W, 15 min), which alters the surface hydrophobicity (~ 107°) and simultaneously builds a denser, fluorinated top-layer (47 nm). The surface fluorinated layer has a diffusivity of to 8.7 × 10−12 m2/s and a WVTR of ~ 1.0 × 10−4 g/m2/day based on a series resistance model.