Elucidating the Hierarchical Architecture of Polymer Spherulites via 4D Scanning Transmission Electron Microscopy

Hierarchical spherulite structures are ubiquitous in semicrystalline polymers and impact their properties. Elucidating these delicate and complex structures, which span from molecular-level chain folding to mesoscale spherulites, however, presents a formidable challenge. Here, we showcase low-dose four-dimensional (4D) scanning transmission electron microscopy (STEM) as a powerful technique for investigating the multiscale hierarchical structures of polymer spherulites. Applying it to poly(ε-caprolactone) and polyethylene (PE) spherulite films, we reveal the preferential orientation and growth direction of lamellar crystals, as well as the twisted lamella structure in PE banded spherulites. Notably, our observations reveal a non-radial twisting axis forming a spiral texture in PE films. With the enhanced spatial resolution of cryogenic 4D-STEM, we directly visualize individual lamellar crystals at the nanoscale, enabling the identification of chain tilt within a single lamella and the elucidation of lamella configurations at spherulite boundaries. These insights advance our understanding of polymer spherulite crystallization mechanisms and underscore low-dose 4D-STEM as a powerful tool for exploring the intriguing structures of soft materials.