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

Cratering and plastic deformation induced by individual MeV ions on the surface of polystyrene thin films of different molecular weights ${(M}_w)$ (from 3250 to $2\times {10}^{7}u)$ are investigated using scanning force microscopy. 20 MeV, 85 MeV and 197 MeV gold ions are used to bombard the targets at grazing incidence $(79\mathrm{}°$ 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 $M_w$ film. Crater dimensions are systematically reduced on films of heavier macro-molecules, up to a molecular weight of about $1.6\times {10}^{5}u.\mathrm{}$ For $M_w>1.6\mathrm{}\times {10}^{5}u,$ 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 $M_w$ films at a fixed energy. The observed saturation of the crater size for high $M_w$ 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 $M_w$ follows the same trend as the reciprocal viscosity $\left({\eta }^{-1}\right)$ versus $M_w,$ indicating that the viscosity is governing the final lateral dimensions of the craters. The hillock dimensions present a weak dependence on $M_w,$ 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 $M_w$ and on the transient heating occurring close to the ion impact site.