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 (identi® ed as polypropylene-L and polypropylene-R) was characterised by differences in arrangement (regular or irregular) of ® brils depending on their melt¯ow conditions. Polypropylene-L is a polymer with longer chains and with restricted¯ow, whereas polypropylene-R has shorter chains obtained by controlled rheology. The micro® brils in undeformed polypropylene-L bend, forming raised surface features of height in the region of 10 ± 50 nm. In comparison to polypropylene-L, the micro® brils in undeformed polypropylene-R exhibited surface features of relatively lower height (10 ± 20 nm). 30630 nm scan AFM images provided details of micro® brils 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 161 l m indicated a lesser amount of reorganisation of microstructure in polypropylene-L as compared with polypropylene-R. Surface pro® les 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.MST/5253

Strain rate sensitivity of homopolymer polypropylenes and micrometric wollastonite-filled polypropylene composites

“…Strain rate has a complicated and dramatic effect on materials deformation processes because the energy expended during plastic deformation is largely dissipated as heat. This process was observed to be more prominent at higher loading rates that are associated with adiabatic drawing than during lower loading rates where isothermal drawing occurred [10,11].…”

Effect of wollastonite and talc on the micromechanisms of tensile deformation in polypropylene composites

“…Even though the region appears to be brittle, there are a large number of unbroken fibrils that continued to plastically deform as decohesion of fibrils progressed. It is also relevant to note here that atomic force microscopy of the brittle region indicated that it involves reorganization of microstructure signifying reduced stretching of fibrils resulting in an irregular surface morphology at the micrometric scale [26].…”

“…Misra and co-workers [23,24] believe that polymeric materials characterized by high modulus and yield strength, and a large elastic regime are preferred materials in applications where mechanically induced surface damage is the primary concern. Mechanically induced surface damage in the form of scratch or internal damage in the form of voids undermines the longevity of the polymeric components and limits their use [25,26]. The minimization of plastic deformation (or stress whitening) can be accomplished by increasing crystallinity [27], use of short chain polymers [28], compounding with additives, and a significant influence can be realized by reinforcement of polymers with micrometric size particles, such as wollastonite and talc minerals.…”

Influence of branch content, comonomer type, and crosshead speed on the mechanical properties of metallocene linear low‐density polyethylenes

“…This effect was observed to be more prominent at high crosshead speeds associated with adiabatic drawing rather than during small crosshead speeds where isothermal drawing occurs. 26,28 Termonia et al 29 reported that each molecular weight exhibits a different temperature or elongation window within which optimum drawing occurs. Within these windows, the rate of slippage of chains through entanglements reaches its optimum value.…”