Polymer Microscopy

Microscopy is the study of the fine structure and morphology of objects with the use of a microscope. Microscopes range from optical microscopes, which resolve details on the micrometer level, to microscopes that can resolve individual atoms in suitable samples, but all form magnified images of the specimen. Some instruments give information about a surface and not the specimen interior, but preparation methods may create an internal surface that can be imaged. Apart from this, the size and visibility of the polymer structure to be characterized generally determines which instrument is to be used. For example, the size and distribution of spherulites can be observed by light-optical techniques, but a study of their internal structure requires a higher resolution method, such as transmission electron microscopy. Combinations of the various microscopy techniques generally provide the best insight into the morphology of polymer materials.

The basic optics of the optical microscope and the conventional TEM are similar. Condenser lenses illuminate the object to be imaged with a flood of radiation, and imaging lenses form the radiation leaving the object into a magnified image. Both electrons and light may be considered as particles or as propagating waves in space. The wave has an amplitude and a phase, though only the intensity which equals (amplitude)² can be directly observed. Although wave optics gives the most rigorous derivation, it is simpler to consider both geometrical and wave optics to understand the formation, contrast and resolution of microscope images.

Characterization of the microstructure of polymer fibers can provide insights into the fundamental structures present and into the relationship between structure and properties important for applications. Morphological characterization provides information to help understand the effects of processing history on mechanical and other physical properties. Microscopy techniques are used to observe features such as: fiber shape, diameter, structure (crystal size, voids, etc.), molecular orientation, size and distribution of additives, structure of yarn and fabric assemblages and failure mechanisms. These features are directly related to specific mechanical and thermal properties. Emphasis in this section is on assessment of the structure of polymer fibers as it relates to solving problems or evaluating the effect of process modifications. Fibers prepared from liquid crystalline polymers require special techniques and interpretation which will be described later (Section 5.6). It must be emphasized that any study of polymer fibers will be incomplete if the only technique applied is microscopy. X-ray diffraction, thermal analysis (DSC, TGA, heat shrinkage) and spectroscopy (IR, Raman, XPS) are among the many techniques which complement microscopy investigations (Section 7.4).

A wide range of techniques in microscopy has either appeared within the past few years, or has only recently been applied to the study of polymer systems. These new forms of microscopy are distinct from the continuing evolution of the microscope. This is currently rapid in the direction of ease of use, computer control and increased use of digital image storage and processing, even for optical microscopy [1, 2]. Some examples of these new forms of microscopy as they have been applied to polymers are included in the previous two chapters. In the case of novel types of scanning electron microscopy and high resolution transmission electron microscopy, the principles of microscope operation and image formation are described in Chapters 2 and 3, respectively.

The preceding chapters have provided a description of microscopy techniques, imaging theory and the specimen preparation methods required to investigate polymer structures. The theme of this chapter is to put all of this together within a useful framework. This framework might be a review to experienced microscopists (who likely have developed their own protocols), but it will provide useful information regarding problem solving ideas. A problem solving protocol will be developed that permits microscopy characterizations to follow an easy and short path to a solution. These characterizations will all be classified as ‘problems’ that require a solution. Problems can range from simple to complex and include, for example, determination of the phase structure in a polymer blend, the cause of failure of a composite or the complete and fundamental characterization of a new membrane, fiber, film, etc. Clearly, such problem solving will require a range of time and effort, but the protocols used to begin the characterization and to know when the problem is solved are similar overall. Generally more than one technique is required to solve problems relating to polymer morphology and thus complementary multidisciplinary techniques are important in conducting problem solving analyses. Interpretation of the images produced is of critical importance in evaluating polymer structures and thus the topic of artifacts will be included in this discussion. Finally, although structural characterizations cannot generally be accomplished without microscopy methods and techniques, there are other complementary analytical techniques that are often quite important in understanding polymer structures. The last section will be devoted to a short description of these techniques, including x-ray diffraction, thermal analysis, electron spectroscopy and others.