Darkfield TEM studies of thermoplastics

While thermoset polymers are amorphous, the structures of thermoplastic polymers vary from amorphous to semi-crystalline, depending on the nature of the molecule and the processing they have undergone to shape . In some cases, other types of order may be present, such as oriented fiber structure or mesomorphic orders. Because of the nanometric scale of ordered domains, transmission electron microscopy (TEM) is the tool of choice for studying the morphological features of these structures . One of the great advantages of the transmission electron microscope is that it can operate in different imaging and diffraction modes, switching easily from one to the other. It thus provides diverse and complementary information on the same microscopic area of the sample.

One of these modes, called the darkfield mode, is particularly attractive in the case of semi-crystalline thermoplastics for visualizing crystalline figures and analyzing their characteristics (shapes, sizes, distribution). This mode results from electron diffraction in periodic structures (crystals, fibers and liquid crystals). However, due to the alteration of the structure by the electron beam, it can only be used if the polymer is sufficiently resistant, and provided modest resolutions are achieved. Under these conditions, dark-field images of ordered domains in the polymer can be formed and recorded.

Here, we present several experimental cases in which dark-field TEM has made a valuable contribution to the determination of ordered structure. In thermoplastic polymer matrix composites, we were able to highlight the semi-crystalline morphology of the matrix and examine how fibers modify initial morphologies at the interface, as a function of shaping parameters. Other cases concern the structure of an oriented polyethylene film and that of a liquid-crystalline polymer.