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TEM is an electron microscopy that has established itself as a very useful tool in polyurethane material science [34]. Like SEM, the sample depends on the interaction of the sample with a thermally or field generated electron beam. Unlike SEM that bounces electrons off of a sample surface, TEM passes electrons through the sample. Since electrons readily interact with materials, the sample must be very thin, on the order of 10s of nanometers thick, to allow sufficient electrons to pass through the sample that a signal can be collected. The image transmitted is a function of the interaction of the electrons with different components of the sample. Although staining is not always required, image contrast can be improved if one phase or another has affinity for a heavy metal contrast agent such as osmium tetroxide or uranyl (uranium) acetate (Fig. 5.6).

The many applications and varieties of use for TEM can be found in resources dedicated to the subject. Among the most critical aspects obtaining a TEM of polyurethane is sample preparation [35, 36] and sampling bias [37]. The need to prepare exceptionally thin samples is complicated by the fact that many urethanes are relatively soft materials resulting in deformation during thin sectioning (microtoming). This may result in the need to perform the microtome at very cold temperatures. However, the utility of TEM for understanding polyurethanes (and for that matter many polymers) has resulted in standardization and commercial instruments able to minimize the complications. Another problem is one of systematic error related to the very small areal coverage of the measurement. This can lead to the search for interesting but rare structures within a sample. It is not uncommon for the rarity of a structure to be lost in the novelty. It is of course the role of the experimentalist to search for representative and novel structures. The last issue is the possibility of localized heating of the sample by the beam altering the morphology. This is often visible to the experimentalist and of course should not be ignored.

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