Surface modification of polypropylene membrane by RF methane/oxygen mixture plasma treatment

The hydrophilic surface modification of micro-porous polypropylene (PP) membranes is achieved by low-pressure 13.56 MHz RF methane (CH ₄)/oxygen (O₂) gas mixture plasma treatment. The changes in surface wettability and surface free energy were examined by static contact angle analysis. The static water contact angle of the plasma modified membrane notably decreased with increases in treatment time and plasma power. The obvious increase in the surface energy of polypropylene membranes due to CH ₄/O₂ mixture gas plasma treatments was also observed. Optical emission spectroscopy (OES) was used to analyze the chemical species of CH₄/O₂ mixture gas plasma treatment. The variations in the surface morphology and chemical structure of the micro-porous PP membranes were confirmed by confocal laser scanning microscopy (CLSM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) measurements. XPS analysis showed significantly higher surface concentrations of oxygen functional groups for CH₄/O₂ mixture gas plasma-modified polypropylene membrane surfaces than for the originally unmodified polypropylene membrane surface. The experimental results show the important role of chemical species in the interaction between a CH ₄/O₂ mixture gas plasma and a membrane surface, which can be controlled by surface modification to tailor the hydrophilicity of the membrane to the requirements of various applications.