Surface characterization and ageing of ultraviolet-ozone-treated polymers using atomic force microscopy and x-ray photoelectron spectroscopy

Ultraviolet–ozone (UVO) treatment of poly(ethyleneterephthalate) (PET) films and polystyrene (PS) dishes of up to 10 min exposure has been studied. Surface polarity, oxygen chemisorption and topographical change were analysed by contact angle measurement, x-ray photoelectron spectroscopy and atomic force microscopy. Studies of the treated surface reveal the chemistry behind the increasing oxygen content. The oxidation process is shown to proceed via different mechanisms for the two polymers. Polyethyleneterephthalate appears to undergo a Norrish-type chain depolymerization reaction, whereas PS undergoes a much more random chain scission attack. Atomic force microscopy analysis shows an increase in the surface roughness with increasing exposure to UVO for both polymers, with grains of low-molecular-weight oxidized material (LMWOM) forming at the surface. This material can be removed by washing. Surfaces that remain after washing have a higher concentration of oxygen species than the native surfaces. Analysis of aged surfaces shows that for oxidized PET a relaxation process occurs, lowering the levels of surface oxygen. This appears to occur due to the diffusion of LMWOM into the PET bulk. Relaxation of the oxidized PS surface is less thermodynamically favourable owing to the apolar nature of the PS.