Elastic constants of lamellar and interlamellar regions in and mesomorphic isotactic polypropylene by AFM indentation

A study of the nanoscale mechanical properties of isotactic mesomorphic and semi-crystalline polypropylene (iPP) is presented. Two iPPs produced with metallocene and Ziegler-Natta catalyst polymerization are used. The resulting fibers are characterized by wide angle X-ray scattering, small-angle X-ray scattering (SAXS), and Raman spectroscopy. The spatial variability of the percentage crystallinity is evaluated based on the SAXS data. AFM indentation is performed to measure the elastic modulus of the fibers in the direction perpendicular and parallel to the fiber axis. Since the AFM probing is performed on a scale larger than the lamellar thickness, a statistical analysis of the AFM and SAXS data is necessary to infer the elastic moduli of the crystals and of the inter-lamellar regions. The elastic modulus of the crystalline lamellae in the direction perpendicular to the c-axis of the crystal probed in compression is estimated at approximately 3.3 GPa, while the effective modulus of the interlamellar regions ranges from 1.5 to 2.2 GPa. The method proposed can be applied to other material systems with similar layered structure to measure elastic moduli or hardness on length scales smaller than the resolution of the indentation test. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43649.