RESINOGRAPHY 



Fig. 12. Experimental fibers of polyacrylonitrile showing variation in cross-sectional texture. Left: 

 Electron micrograph of ultrathin section cut after coating with a resilient polyacrylate latex (R) and 

 then mounting in methacrylate (4). Right: Light micrograph of a thin section showing that these ex- 

 perimental fibers varied in the number of rings. 



vertical illuminator and polarizer is used for 

 surfaces, opaque grains, polished thick sec- 

 tions, opaque inserts, and optical sections of 

 transparent objects (Figure 8, left). 



Along with other optical methods for en- 

 hancing contrast, darkfield illumination by 

 either transmitted or reflected illumination 

 is handy, if not required. Dark field by re- 

 flected light permits the distinction of pig- 

 ment or streak color from specular color. 

 In general, images by dark-field reflected 

 illumination are easier to interpret because 

 they are more natural than bright-field 

 images. 



Among high polymers and resins obser- 

 vation and measurement of optical proper- 

 ties are important adjuncts to descriptive 

 and interpretive morphology or texture. In 

 order to observe most optical properties the 

 light microscope must have variable aper- 

 tures from low to very high, in both objec- 

 tive and condenser, the polars must be 

 crossable and uncrossable, and their direc- 

 tions of vibrations must be known. There 

 should be a rotatable object-stage. 



With transmitted illumination specific or 

 critical refractive indices may be measured. 

 Double refraction can be detected and meas- 

 ured. Distinction can be made among the 



origins of the double refraction, such as 

 single crystals, crystal aggregates, strain 

 (Figure 9), deformation, orientation of aniso- 

 tropic particles of resin, or orientation of 

 tiny rods or plates of isotropic particles in 

 an isotropic medium of different refractive 

 index (2). The direction(s) of vibration of 

 slower component (s), the sign of elongation 

 or the optical sign can be determined di- 

 rectly. The two or three specific refractive 

 indices may be determined with a com- 

 pensator and crossed polars. 



With only one polar and knowing its 

 vibration direction, pleochoism and sign of 

 color absorption can be determined quali- 

 tatively and quantitatively in colorful, dyed 

 or stained materials such as fibers. 



Rotated between crossed polars the be- 

 havior of a polymeric specmien may be very 

 different from that of a single, unstrained 

 crystal. Examples are : crystalline aggregates, 

 such as spherulites; natural aggregates, such 

 as starch grains and natural fibers ; synthetic 

 aggregates such as special polarizing films, 

 wrapping foils and textile fibers. 



Crystals and other orientations of highly 

 polymeric particles have characteristic dif- 

 fraction patterns (interference figures) which 

 are separate sources of determinative and 



534 



