Stain-free mapping of polymer-blend morphologies via application of high-voltage STEM-EELS hyperspectral imaging to low-loss spectra

Polymer blends composed of multiple types of polymers are used for various industrial applications; therefore, their morphologies must be understood to predict and improve their physical properties. Herein, we propose a spectral imaging method based on scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy to map polymer morphologies with nanometric resolution as an alternative to the conventional electron staining technique. In particular, the low-loss spectra of the 5–30 eV energy-loss region were measured to minimize electron irradiation damage rather than the core-loss spectra, such as carbon K-shell absorption spectra, which require significantly longer recording times. Medium-voltage (200 kV) and high-voltage (1000 kV) STEM was used at various temperatures to compare the degrees of electron-beam damage resulting from various electron energies and sample temperatures. A multivariate curve resolution technique was used to isolate the constituent spectra and visualize their distributions by distinguishing the characteristic peaks derived from various chemical species. High-voltage STEM was more useful than medium-voltage STEM for analyzing thicker samples while suppressing ionization damage.