Atomic force microscopy of scratch damage in polypropylene

Abstract

Atomic force microscopy (AFM) in tapping mode has been used to characterise surface damage on deformed polypropylenes induced during a scratch test. Atomic force micrographs revealed differences in microstructures that could be used to predict the deformation resistance of two different types of polypropylene. The undeformed surface of the two types of polypropylene (identified as polypropylene-L and polypropylene-R) was characterised by differences in arrangement (regular or irregular) of fibrils depending on their melt flow conditions. Polypropylene-L is a polymer with longer chains and with restricted flow, whereas polypropylene-R has shorter chains obtained by controlled rheology. The microfibrils in undeformed polypropylene-L bend, forming raised surface features of height in the region of 10- 50 nm. In comparison to polypropylene-L, the microfibrils in undeformed polypropylene-R exhibited surface features of relatively lower height (10 - 20 nm). 30 × 30 nm scan AFM images provided details of microfibrils containing chains of molecules of ~0.5 nm wide. Surface deformation induced by the scratch resulted in the formation of scratch tracks characterised by regions of quasi-periodic (consecutive) cracking. This type of deformation is attributed to higher applied loads or to higher contact strains. This is particularly important in semicrystalline polymers, where there is partial reorganisation of microstructure on the application of surface stresses because of their viscoelastic properties. Atomic force micrographs of mechanically deformed polypropylene-L and polypropylene-R at a scan size of 1 × 1 μm indicated a lesser amount of reorganisation of microstructure in polypropylene-L as compared with polypropylene-R. Surface profiles and section analysis of the AFM micrographs suggested that polypropylene-R is more scratch resistant in comparison to polypropylene-L under identical scratch test conditions, consistent with Raman spectroscopy observations of tensile deformed polypropylene.