Hydrophobic treatment on polymethylmethacrylate surface by nanosecond-pulse DBDs in CF₄ at atmospheric pressure

Highlights: • Increase in hydrophobicity on PMMA is achieved after the DBD treatment in CF₄, and the water contact angle can increase from 68° to 100° after treatment. • Nanosecond-pulse DBD is used for the surface treatment and the power density is about 114.8 mW/cm². • The effects of applied voltage, CF₄ flow, and time on plasma treatment are investigated. • Plasma treatment causes morphological change, significantly increases the roughness of the surface, and introduces fluorine-containing groups into the polymethylmethacrylate surface. • Hydrophobic behavior of the treated PMMA surface is slightly affected by the aging effect. - Abstract: Nanosecond-pulse dielectric barrier discharge (DBD) can provide non-thermal plasmas with extremely high energy and high density, which can result in a series of complicated physical and chemical reactions in the surface treatment of polymers. Therefore, in this paper, hydrophobic treatment of polymethylmethacrylate (PMMA) surface is conducted by nanosecond-pulse DBD in carbon tetrafluoride (CF₄) at atmospheric pressure. Investigations on surface morphology and chemical composition before and after the DBD treatment in CF₄ are conducted with the contact angle measurement, atomic force microscope, Fourier transform infrared spectroscopy, and X-ray photoelectron spectrometer. The effects of the applied voltage, CF₄ flow rate, and treatment time on the hydrophobic modification are studied. Results show that the contact angles of the treated PMMA surface increases with the applied voltage, and it could be greatly affected by the CF₄ flow rate and the treatment time. The water contact angle can increase from 68° to 100° after the treatment. Furthermore, both surface morphology and chemical composition of the PMMA samples are changed. Both the increase of the surface roughness and the occurrence of fluorine-containing functional groups on the PMMA surface treated by DBD in CF₄ lead to the hydrophobicity improvement of the PMMA surface after high CF₄ flow rate and long time treatment. Moreover, due to the small amount of oxygen in the DBD plasma, hydrophilic effect exists on the PMMA surface after small CF₄ flow rate treatment. Similar phenomenon occurs at short time treatment. It is because that -CH₂F, -CHF-, -CHF₂ groups, having hydrophilic property, are likely generated in the initial stage of hydrogen abstraction. In addition, because the residual groups (mainly -CF-, -CF₂- and -CF₃) on the PMMA surface have strong hydrophobic property, the hydrophobic behavior of the treated PMMA surface could maintain for 8-day aging period