Intraspherulitic mechanical heterogeneities and microstructure of i-PP highlighted by nanoindentation and atomic force microscopy

Abstract

This study quantifies elastic modulus heterogeneity at the intraspherulitic scale in semi-crystalline i-PP polymer using nanoindentation. Prior to mechanical characterization, Atomic Force Microscopy (AFM) is used to characterize the topography of the local micro- and nanostructure of the spherulites. Small indents provide 3–5 μm resolution maps within a spherulite, remaining unaffected by local roughness. The major findings highlight a clear relationship between microstructure and mechanical properties at this scale. Firstly, elastic modulus decreases from the spherulite center to the edges. Secondly, regions along the main growth axes exhibit higher modulus values, while areas around these axes are less stiff. Moreover, similar elastic properties are found at the ends of spherulite branches measured in a transverse and longitudinal orientation with respect to the growth axis. The difference in measured mechanical properties is attributed to the local density and orientation of crystalline lamellae, which in turn depend on the spherulite's growth stage.