Low-voltage scanning electron microscopy of polymers

Micromorphological and microstructural characterizations of the type and degree of crystallinity and the relative dispersion of phases within polymers, as well as in the study of their surfaces and associated interfaces, offer a number of challenges in the field of materials science of polymers. Microscopy is a natural methodology for the acquisition of microstructural information, but for polymers there are few straightforward techniques. Conventional electron microscopy methods are limited in their ability to address fine surface details or to determine the bulk microstructure of multicomponent polymeric materials. Sometimes these problems can be overcome, but only within the practical restrictions associated with meticulous sample preparation. An extremely promising and efficient alternative to conventional approaches is the state-of-the-art, low-voltage scanning electron microscope (LVSEM). It is demonstrated here that straightforward operation of an LVSEM equipped with a field emission gun (FEG) source can produce topographical contrast secondary electron images of polymers at substantially higher magnifications than a conventional SEM, and with a resolution that rivals TEM. An added advantage is the capability of being able to produce contrast based on differences in chemical composition within the sample. The ability to produce quality images at low accelerating beam voltages minimizes beam damage to the sample, and affords an operating window (e.g. E₂) where the sample does not build up negative charge. This obviates the normal requirement to coat samples with a conductive layer. We also describe experimental and theoretical developments that can help us to understand the physics of interaction between low-voltage electron beams and polymer samples. This knowledge base, along with further theoretical and instrumental development and the subsequent applications to polymers, promises a whole new field of electron microscope methodology based on the LVSEM.