Sputtering kilodalton fragments from polymers

The sputtering of large fragments from polystyrene oligomers (7.5 kDa) is investigated using classical molecular dynamics. Simulation results obtained with the new adaptive AIREBO potential including long-range van der Waals interactions are compared to calculations performed with the short-range Brenner potential functions. The model qualitatively accounts for the experimental mass spectra and kinetic energy distributions of heavy kilodalton fragments. Concerning the mechanisms, the simulations show that there exists a transition from an atomic cascade, where atoms collide much like billiard balls, to a molecular motion regime, where the energy is stored in the rotation and vibration modes of the molecule and the motions become collective. Large chain segments are sputtered during the molecular motion stage of the interaction