AFM Characterization of Freely Crystallized Isotactic Polypropylene Surface Morphology: From Spherulites Down to Lamellae

A multiscale morphological analysis of isotactic polypropylene (iPP) spherulites formed on the surface of thick samples prepared without chemical etching was conducted by using optical microscopy and atomic force microscopy (AFM). A dedicated protocol preserved the native three-dimensional microstructure, enabling the precise spatial localization of AFM measurements within the spherulites. This study provides a spatially resolved picture of lamellar organization on the surface of iPP spherulites across multiple length scales. The results reveal the presence of an intermediate structural level: micrometer-scale radial branches that are not considered in the classical two-scale descriptions (lamellae and spherulites). Within these branches, lamellae are arranged in a lath-like configuration with orthoradial orientation. A cross-hatching pattern is observed in the closure (“eye”) regions of the spherulites. The local lamellar period (L), which corresponds to the sum of the thickness of a crystalline lamella and its adjacent amorphous layer, was directly measured by AFM profiles. Its value is centered at 9.4 nm, which is lower than the typical SAXS or DSC values for α-iPP. This discrepancy could be attributed to surface-specific effects, including faster cooling and a possible retreat of the amorphous phase into the bulk.