Effects of UV irradiation and plasma treatment on a polystyrene surface studied by IR-visible sum frequency generation spectroscopy

IR-visible sum frequency generation (SFG) spectroscopy was used to study su rface modification of polystyrene by its exposure to a UV light source or p lasma. It was found that the polystyrene surface underwent dramatic changes after exposure to these treatments, as evidenced by marked changes in the surface SFG spectra. Before the treatments, the surface spectrum showed a p ronounced peak at 3068 cm(-1) which is characteristic of the symmetric stre tch of the aromatic C-H of polystyrene. This peak decreased markedly, and o ther vibrational bands associated with the CH2 and CH3 groups appeared afte r the treatments. The observed spectral changes provided direct evidence of surface reactions involving the aromatic ring. In addition, our data showe d that the degrees of oxidation of the polystyrene surface were different w ith the two processes. The oxidation to a higher oxidation state, resulting in the formation of carbonyl/carboxyl species, was observed with plasma tr eatment but not with UV irradiation. This difference was also reflected in contact angle measurements. Before the treatments, the contact angle was 95 +/- 4 degrees. It decreased to 45 +/- 4 degrees and to less than 100 with UV irradiation and plasma treatment, respectively. The different pathways f or the two treatments are discussed. In addition, the kinetics of photooxid ation of the polystyrene surface was measured in situ, yielding a half-life of 15 min, which is much shorter than that of the bulk.