Surface tracks on polymers: A means to probe material properties at the nanometer scale?

Abstract

The morphology and the dimensions of nanometer-sized craters and crater rims induced by individual MeV heavy ions impacting polymer surfaces have been systematically investigated by means of scanning force microscopy. We demonstrate that the morphology of the surface tracks varies systematically with temperature, molecular weight ( M w) and tacticity of polymer thin films. Such parameters influence chain mobility and the characteristic relaxation times of the polymer, which appear to be key factors determining the final shape and size of observed holes and hillocks. Because of the correlation of the impact features with the thermal and rheological properties of polymers, the problem can be inverted and the surface track morphology may be used to identify physical parameters of the targets. This procedure has been applied to model polymer films. It was possible to identify a critical target temperature above which a sharp change in crater morphology is observed in the scanning force microscopy images. We propose this temperature to be the glass transition temperature. Also, crater size decreases steeply with increasing molecular weight up to a critical M w above which crater dimensions change very little. The transition region is around M c ′, the critical molecular weight at which entanglement coupling of macromolecular chains becomes important.