Although one of the most readily accessible tools in modern analysis, optical microscopy only finds occasional use in modern polyurethane research. Modern techniques of optical microscopy are well covered in various books and reviews devoted to the subject [22-24]. Use of optical microscopy for the study of polyurethanes often exploits the ability to polarize the light from the light source and so illuminate crystal structures that may evolve as a result of soft segment crystallization or formation of mesophase (liquid crystalline) structures [25, 26]. The use of a hot stage is well applied in this work since it allows the slow melting/recrystallization of a material to be observed in real time . Optical techniques allow for a descriptive evaluation of the phase structure (i.e., spherulitic, lamellar, etc.) often accompanied by quantitative thermodynamic measurements obtainable by differential scanning calorimetry . Measurements such as these can be compelling especially when applied to highly systematic structural studies that can track the size of crystallites as a function of some molecular parameter [29, 30].
Classical optical microscopy can find uses as well for gross descriptions of polymer wear, aging, and forensic analysis. Other uses of optical microscopy can be frustrated by the mismatch of relevant scales of interest, poor contrast between phases, interference by light scattering off of a top surface, poor transmission of light through a polymer film, and light scattering off of the myriad surfaces offered by a foam specimen. Modern techniques in optical microscopy, such as optical phase contrast imaging, can provide more information than other optical techniques by increasing contrast between phases (Fig. 5.4) ; however, other techniques such as TEM and AFM can provide a higher degree of resolution and are more widely used.
Figure 5.4 Phase contrast optical microscopy of a cast polyurethane elastomer—dark areas are hard segment. See Figure 5.6 for comparative image quality and information content versus TEM. Image courtesy of Robert Cieslinski and Justin Virgili. (See insert for color representation of the Figure .)