Cratering and plastic deformation in polystyrene induced by MeV heavy ions : dependence on the molecular weight

Abstract

Cratering and plastic deformation induced by individual MeV ions on the surface of polystyrene thin films of different molecular weights (Mw) (from 3250 to 2 X 10⁷u) are investigated using scanning force microscopy. 20 MeV, 85 MeV and 197 MeV gold ions are used to bombard the targets at grazing incidence (79° to the surface normal). Induced surface tracks consist of an elliptical crater followed by a hillock elongated in the direction of the ion incidence. For a given ion energy, the crater size is largest on the lowest Mw film. Crater dimensions are systematically reduced on films of heavier macro-molecules, up to a molecular weight of about 1.6 X 10⁵u. For Mw>1.6 X 10⁵u, the crater size remains approximately constant. The difference observed for the lateral dimensions of the craters are about 50% when comparing the lowest and the highest Mw films at a fixed energy. The observed saturation of the crater size for high Mw values coincides with the onset of entanglement effects in the polymer, which influences the viscosity and the compliance of the material. Moreover, the curve of the crater size versus Mw follows the same trend as the reciprocal viscosity (ηˉ¹) versus Mw , indicating that the viscosity is governing the final lateral dimensions of the craters. The hillock dimensions present a weak dependence on Mw , above a threshold at 3250u. The different behavior observed for craters and hillocks is discussed based on the viscoelastic properties of the polymer at different Mw and on the transient heating occurring close to the ion impact site.