Direct Evidence for the Preferential Staining of Amorphous Regions in Semicrystalline Polymers by Heavy-Metal Staining Agents

Detailed characterization of heterogeneously distributed lamellar crystals is essential for understanding and tailoring the physical properties of semicrystalline polymers. Although transmission electron microscopy (TEM) is commonly used for this purpose, it often involves staining with a heavy metal agent, such as RuO4. Such agents are generally believed to stain the amorphous regions more effectively owing to their lower density; however, this mechanism has not been directly confirmed experimentally until now. Recently, nanodiffraction imaging (NDI), a nanoscale-resolution mapping technique based on diffraction patterns obtained by TEM, has emerged as a valuable method for analyzing local crystalline structures in polymers. This study utilized energy-dispersive X-ray spectroscopy and NDI to examine the spatial relationship between Ru distribution and crystalline regions in RuO4-stained high-density polyethylene, demonstrating that RuO4 indeed preferentially stains amorphous regions of the polymer. This study provides strong evidence to support the previous hypothesis on the staining manner of polymer samples.