Carbon Plasma-induced In-situ Growth of Amorphous Carbon Film on the Surface of Flexible PET Film

Polyethylene terephthalate (PET) films are among the most commonly used synthetic polymers in light-emitting diodes, touch screens, solar cells, and other applications owing to their high tensile strength, flexibility, and corrosion resistance. However, there are functional groups on the surface of flexible PET films that lead to low surface free energy, poor wettability, and poor biocompatibility. The optical transmittance and mechanical properties of flexible PET films are also affected by abrasion and degradation while in use. Surface modifications can be performed to broaden the application field of PET films and improve their service life. Plasma modification methods have attracted much attention owing to their lasting effect and environment-friendliness. Among these, carbon plasma modification not only has a bombardment effect but also a deposition effect that can be used to prepare amorphous carbon films on the sample surface. However, studies on the carbon plasma modification of flexible PET films remain few. Therefore, the effects of different carbon plasma bombardment currents (0.5 A, 1.0 A, 1.5 A, and 2.0 A) on the structure and properties of flexible PET films are studied. The surface cross-sectional structure, wettability, transmittance, and aging resistance of the samples were characterized by Fourier-transform infrared spectroscopy, Raman spectroscopy, dual-beam scanning electron microscopy, scanning probe microscopy, contact angle measurement, UV-Vis spectroscopy, a xenon lamp aging test chamber, and a multifunctional friction and wear testing machine. The growth mechanism of amorphous carbon films on the surface of flexible PET films was also analyzed. The experimental results show that the growth of carbon plasma on the surface of flexible PET films has three parts: the bombardment of the plasma that causes the organic polymer chains on the polymer surface to randomly break and activate the sample surface; the random reaction of fractured living active chains with carbon radicals to form an in-situ transition layer in which organic and inorganic chains are connected; and the epitaxy-like growth of amorphous carbon films. At a low current (0.5 A), the main role played by the carbon plasma is bombardment, which results in the random fracture and rearrangement of the organic carbonaceous structures on the surface of the PET films. The transmittance at 550 nm of the sample prepared at this current was 87.077%, which is 0.439% lower than that of the substrate. In addition, the samples prepared at this current were hydrophobic, aging-resistant, and tough. The films neither peeled nor cracked during bending. In contrast, the samples prepared at a high current (1.0 A, 1.5 A, and 2.0 A) had significantly improved surface hardness and wear resistance. The increased thickness of the amorphous carbon films significantly reduced the light transmittance of the sample surface. At the same time, the stress in the films increased as the carbon plasma treatment current increased. Therefore, the amorphous carbon film on the sample surface released the stress during the bending process, which caused the films to crack but not peel off. In conclusion, the existence of the in-situ transformation layer improved the bonding force of the amorphous carbon films on the surface of flexible PET films. The amorphous carbon films grown by the carbon plasma induction method can delay the degradation of flexible PET films while in service, and improve their mechanical properties.