Atomic force microscopy characterisation of mechanically induced surface damage in ethylene-propylene diblock copolymeric materials

Abstract

Atomic force microscopy (AFM) in tapping mode has been used to characterise surface damage induced during the scratch test in ethylene - propylene diblock copolymers. The AFM enabled prediction of the deformation resistance of two different types of copolymer. The undeformed surface microstructure of the two copolymers (designated EP-M and EP-TC) was distinguished by differences in arrangement (regular or irregular) of fibrils, depending on their melt flow conditions. Type EP-M is a copolymer with long chains obtained by low melt flow rate, whereas EP-TC is a short chain copolymer with high melt flow rate. The microfibrils in both copolymers exhibited small kinks/nodules. Atomic force nanoscale images provided details of microfibrils containing molecular chains. The long chain EP-M copolymer exhibited non-uniformity in the alignment of molecular chains in comparison with short chain EP-TC. Surface deformation induced by the scratch test led to the formation of parabolic scratch tracks. The average surface height and peak - valley height of a track (considered a measure of the depth of the induced scratch) suggested that short chain EP-TC copolymer is more resistant to mechanically induced surface damage in comparison with long chain EP-M copolymer. Also, the average thickness of track was more for EP-TC, implying that the density of tracks/area is more in EP-M than in EP-TC, consistent with scanning electron microscope observations. Scanning electron microscopy investigations suggested localised plastic flow of material in the region surrounding the track, involving the formation of voids. Also, a comparative assessment of scratch damage was done, in terms of average surface height of the plastically deformed region, in relation to homopolymer and isotactic polypropylenes under identical test conditions.