Investigation of surface microstructural modification during tensile deformation in polypropylene using atomic force microscopy and Raman spectroscopy

Abstract

Surface microstructural modification during tensile deformation in polypropylenes has been studied by atomic force microscopy (AFM) operated in tapping mode and Raman spectroscopy. The atomic force micrographs revealed differences in microstructures of undeformed and deformed polypropylenes that were used to predict the deformation characteristics of the two polypropylenes, designated as PP-L and PP-R. The undeformed surface of the two polypropylenes was characterised by differences in arrangement of fibrils depending on their melt flow conditions. PP-L is a polymer with longer chains and with restricted flow, and PP-R has shorter chains obtained by controlled rheology. The microfibrils in undeformed PP-L bend, forming raised surface features of height ~10 - 50 nm, whereas microfibrils in undeformed PP-R exhibit 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. The large scan AFM micrographs of mechanically deformed PP-L and PP-R indicated that under identical conditions of tensile deformation, PP-R exhibits a very different deformed structure in comparison to PP-L. At low displacement rates of 7.5 cm min-1, both PP-L and PP-R exhibited splitting of molecular chains. However at high displacement rates of 50 cm min-1, splitting was absent in both. Raman spectroscopy provided an insight of the molecular deformation of polymeric materials. The observations indicated that the rate of shift of frequency of Raman band per percent strain is greater for PP-L and less for PP-R, implying that PP-L experiences higher molecular deformation in comparison to PP-R. It is possible that the shorter chains in PP-R are less likely to unravel and separate less easily than the longer chains in PP-L.