Nanodiffraction Imaging of Polymer Crystals

Nanodiffraction imaging (NDI) is a novel imaging technique based on scanning transmission electron microscopy (STEM); it uses a nanometer-size electron beam to scan across a specimen, and the electron diffraction (ED) pattern at each position is recorded onto a two-dimensional (2D) pixelated detector. In this study, NDI was used to image the nanoscale spatial distribution and orientation of lamellar crystals of polyethylene (PE) - one of the most popular and electron-beam-sensitive semicrystalline polymers - without any pretreatment (e.g., RuO4 staining). 2D ED patterns as many as 1282 probe positions (over a scanning area of 3.842 μm2) were obtained for two PE samples featuring significantly different crystal orientations (non-oriented and oriented samples). Spot-like diffractions, corresponding to orthorhombic PE 110 and 200 peaks, were observed in numerous ED patterns, the detailed analysis of which provides the spatial distribution and orientation of lamellar crystals at nanometer resolutions. No distinct morphologies were observed under conventional STEM. A substantial difference between non-oriented and oriented samples was identified: the lamellae orientations of non-oriented samples were random, whereas those of oriented samples were uniform, with the c-axis aligned in the stretching direction. Furthermore, the local orientation fluctuation of the lamellar crystals in the oriented sample was clarified by NDI. Such local structural information is key to understanding higher-level hierarchical elements (e.g., spherulite and shish kebab) but cannot be obtained by conventional diffraction/scattering methods.