Visualization of polymer interfaces by phase contrast (’’defocus’’) electron microscopy

Abstract

A significant problem in studies of polymer microstructure is obtaining enough contrast in electron micrographs to visualize the system. Most techniques now used take advantage of amplitude changes produced by the interaction of the electron beam with the object. In this paper we investigate the possibility of using phase changes induced in the electrons by the object and by microscope defocus to produce usable image contrast. We use the standard linear transfer theory of microscope imaging to evaluate the images produced by several different model objects. The basic structure investigated is an alternating sequence of domains. These models correspond reasonably closely to the suggested microstructure of several polymer systems and the calculations should be directly applicable to them. Our calculations show that for reasonable inner potential changes and film thickness usable image contrast can be obtained. The technique is most sensitive to rapid phase changes and will be most successful with objects having sharp rather than diffuse domain boundaries. Defocusing produces fringes at the domain boundaries with the size of these fringes depending on both the object and the amount of defocus. The results indicate that defocusing is useful for polymer structure investigations. However, the defocused image will not correspond directly to the object structure, and some care must be taken in deducing the true object structure.