Observations with polarized-light microscopy

The depolymerization of crystalline cellulose in wood has been observed with polarized light microscopy (IT). A loss of birefringence was associated with early stages of brown rot. The attack on cellulose is rapid and difiuse throughout the wood. The cells appear to maintain their usual form, but will shatter and collapse because they lack the strength that cellulose provides the woody cell wall (Figure 6b). 

The starting system is achiral (plates at 90° with isotropic fluid between), but leads to the formation of a chiral TN structure when the fluid becomes nematic. In this case, enantiomeric domains must be formed with equal likelihood and this is precisely what happens. The size of these domains is determined by the geometry and physics of the system, but they are macroscopic. Though the output polarization is identical for a pair of heterochiral domains, domain walls between them can be easily observed by polarized light microscopy. This system represents a type of spontaneous reflection symmetry breaking, leading to formation of a conglomerate of chiral domains. 

Figure37. (a) Crystalline and right-handed helical fibersmadeofGlc-NC(12)CN-Glc (116, n= 12) observed using polarized light microscopy (at 25 C in water). Periodical structures of the fibers are denoted by arrows, (b) Polarized light micrographs of representative dehydrated and right-handed fibers from Glc-NC(12)CN-Glc (116, rt = 12), (top) photographed trough cross-polarized filters and (bottom) through plane-polarized filters. Reproduced from ref. 338 (Shimizu and Masuda, J. Am. Chem. Soc. 1997, 119,28)2) with permission of the American Chemical Society.

Figure 2. Development of mesophase during heat treatment of petroleum pitch as observed by polarized light microscopy of polished sections at room temperature. A 400°C, 2 h B 400°C, 6 h C 400°C, 12 h D 400°C, 20 h. (Reproduced with permission from reference 10. Copyright 1984.)...

As Fig. 2.6 reveals, optical microscopy with polarized light makes it possible to distinguish various intermediate products formed from a precursor of graphitic carbon at different stages of pyrolysis. The observed morphology is the so-called optical texture, which enables carbon sohds to be classified according to their degree of anisotropy. Besides optical microscopy, other techniques... 

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