The effect of molecular weight on the surface wettability of polystyrene treated with nitrogen plasma

The wettability of polystyrene thin film surface in the development of biosensor based on quartz crystal microbalance (QCM) is an important aspect in biomolecule immobilization. The polystyrene thin film was produced by a spin coating method. The wettability was achieved by modification of the polystyrene surface using a nitrogen plasma treatment. The aim of this work is to investigate the effect of polystyrene raw material molecular weight to the wettability of the polystyrene surface treated with the nitrogen plasma. A TMS-1200 micro profilometer was utilized to determine the surface roughness. The contact angle of the polystyrene surface was measured by a contact angle instruments to observe the surface wettability. The results showed a decrease in the surface roughness and the contact angle of polystyrene layer after the nitrogen plasma treatment, indicating the polystyrene surface changed to a higher wettability. Furthermore, the higher molecular weight of the polystyrene led to the decrease of the surface roughness and the increase of the wettability. The wettability depends on functional group in the polystyrene. The changes of polystyrene the functional group were determined by an FTIR spectrometry. The infrared spectroscopic measurement has confirmed the presence of polystyrene nitrile (C-N) group in the form of C≡N representing a polar group structure. Moreover, the intensity of the C≡N peak increased in the treated polystyrene with the higher molecular weight. The increase of the absorbance is related to the increase of the surface polarity and hence the wettability.