Optical emission spectroscopy studies during nitrogen plasma of polystyrene surfaces modification

Abstract

Nitrogen plasma treatment was successfully performed to modify polystyrene surfaces from hydrophobic into hydrophilic. The objective of the work was to achieve better wettability of the polystyrene surfaces. Various surface characters of the polystyrene were obtained by synthesising the polystyrene layer from different molecular weight (Mw) solution using a spin coating method. The solution prepared for the coating procedure was made by dissolving monomers which have Mw of 35,000 g/mol, 192,000 g/mol, and 280,000 g/mol into toluene solvent at the concentration of 6%. The polystyrene surfaces then treated by means of the nitrogen plasma using a 2 MHz plasma generator with the power of 40 watts and the gas flow of 20 ml/minutes for 2 minutes. The optical emission spectroscopy (OES) provided the measurement of plasma species and the calculated electron temperature (Te) and electron density (ne). The species N2+ ion was identified from the intensity emission of OES. The existence of the N2+ ion and corresponding to the condition of Te and ne in the plasma plays a key role in the polystyrene's surface changes as found in the polystyrene surface after plasma treatment. After plasma treatment, nitrile functional group (C≡N) was found in the polystyrene surface which is indicated by the Fourier Transform Infrared (FTIR) fingerprint of the nitrile functional group at the vibration peak of 2300-2400 cm-1. The existence of the C≡N represent the surface changed from hydrophobic to hydrophilic. This result was in agreement with a lower contact angle of the polystyrene surface.