This work deals with the study of atactic and isotactic polystyrene (aPS and iPS, respectively) films treated by oxygen DC pulsed plasma of very low duty factor (1%). The main surface modifications were investigated using water contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). By using a "temporal afterglow", our plasma treatment leads to the grafting of hydrophilic moieties such as C-O, C=O and O-C=O, minimising the degradation and mechanical processes at the surface. Ageing studies showed a better stability of the treatment under our experimental conditions, compared to published studies made under radio frequency or microwave plasma conditions. Ageing studies have also highlighted a surface relaxation with time, depending on the degree of crystallinity of the polymer. The hydrophobic recovery commonly observed is restricted by an orderly packed structure of the crystallites in iPS, reducing the polymer chains motion towards the polymer bulk. This reorientation of polar group kinetics has also been studied with increasing the storage temperature in order to have a thermodynamic signature of the ageing phenomena.