Free radical kinetics in a plasma immersion ion implanted polystyrene: Theory and experiment

The kinetics of free radicals in polystyrene (PS) treated by plasma immersion ion implantation (PIII) in nitrogen plasma are investigated by means of electron paramagnetic resonance (EPR). EPR spectra are recorded for PS samples with various PIII treatment times ranging from 40 to 1600 s (corresponding to ion fluences of 5 × 1014–2 × 1016 ions/cm2) after storage times ranging from 45 min to 3 weeks (room temperature). An unexpected, non-linear behaviour at short treatment times is observed. As treatment time increased, the EPR signal first remains constant then shows a sharp increase and a subsequent saturation at long treatment times. An analytical model for the processes of formation and decay of free radicals in PIII treated polymers is proposed. The model describes the kinetics of short-lived and long-lived radicals using a system of linear differential equations. The model is in good agreement with experimental data.