Gas barrier property of silica-based films on PET synthesized by atmospheric pressure plasma enhanced CVD

As the O2 flow rate increased to 500 mL/min, the oxygen transmission rate (OTR) of the films decreased to 8.2 cm3/m2/24 h/atm, which is approximately one third of that of the uncoated PET. On the other hand, the OTR increased when the O2 flow rate exceeded 500 mL/min. From the fourier-transform infrared (FT-IR) spectra, the intensity of Si-O-Si peaks increased and the shape of Si-O-Si peaks changed to be sharp with increasing the O2 flow rate, indicating that the structure of the films became dense. Oxygen molecules could not permeate through the highly dense films, so that the gas barrier property was improved. Using the atomic force microscope (AFM), many pinholes with a diameter of approximately 30 nm were observed on the film surface at the O2 flow rate over 500 mL/min. From the observation of the uncoated PET exposed to N2/O2 plasma, the surface texture hardly changed and kept its smooth surface at less O2 flow rate up to 500 mL/min, while the rugged texture were obviously observed at the O2 flow rate over 500 mL/min because of the etching effect in plasma. The holes were formed because the films were synthesized at the PET surface in the rugged shape. Therefore, the formation of the pinholes on the film surface could allow oxygen molecules to diffuse into the films.