Dark field imaging of semicrystalline polymers by scanning transmission electron microscopy

Abstract

Scanning transmission electron microscopy (STEM) has been suggested to have advantages over conventional transmission electron microscopy (CTEM) for the observation of diffraction contrast features and diffraction patterns from radiation sensitive crystalline polymers. Because of image intensification, control of illumination location and magnification independent focus, STEM operation for focusing, area selection and set up of optics permits a high yield of systematic data. Dark field (DF) imaging is most useful when employed in conjunction with scanning microarea diffraction. For convergent beam microdiffraction and efficient DF imaging of thin crystals the beam divergence should be less than 5×10−3 radians. For single beam DF, the reflection of interest is selected by the intermediate lens aperture. Use of a STEM annular detector to collect more than one reflection results in increased DF image intensity and resolution. Use of the entire azimuthal range of a single powder pattern reflection permits examination of crystal texture — in particular, images produced by chain axis reflections show the detailed arrangements of lamellae.