GENERAL MICROSCOPY 



have a considerable ei'tect on the unilorniity 

 of filler dispersion. Hence it is very impor- 

 tant to keep them intact. 



(5) If a mixture of two different synthetic 

 components is to be examined, correct ap- 

 praisal of the dispersion is very important. 

 Disturbance of the original distribution is 

 undesirable. 



Appraisal of Plastic Films with the 

 Polarizing Microscope 



Most plastic films have a molecular orien- 

 tation, even though they are not deliber- 

 ately elongated. Even rolling causes a certain 

 amount of double refraction. The intersec- 

 tion of molecular strands gives rise to a film 

 texture. Elongation in the main direction of 

 the strands soon causes a parallel structure, 

 but elongation perpendicular to this direc- 

 tion widens the angle of intersection of the 

 strands. In many films this film structure is 

 demonstrable with a phase contrast micro- 

 scope. In other cases, in which the refractive 

 index difference between strands and sur- 

 rounding material is slight, the phenomenon 

 will hardly be visible with a phase contrast 

 microscope. In these cases it can often be 

 rendered visible by staining with I2-KI solu- 

 tion or iodine tincture. The film is then ex- 

 amined in its elongated state under a polar- 

 izing microscope, the axis of elongation of 

 the specimen being rotated from the diago- 

 nal position to the orthogonal position. When 

 the axis of the specimen forms an acute an- 

 gle with the orthogonal position, the molecu- 

 lar strands of the one axis fade out while 

 those of the other axis do not. In this way 

 the microscope shows a "herringbone struc- 

 ture." This structure, which is regular in 

 most films, also occurs in nearly all synthetic 

 fibers, although the round or irregular sec- 

 tions of these fibers renders examination 

 difficult. Exact examination can therefore 

 only be made with longitudinal sections. 

 With synthetic fibers I2 • KI staining is also to 

 be recommended. The texture occurring 

 with the fibers may be: 



(1) Regular single heri'ingbone structure. 

 This corresponds completely to the film tex- 

 ture and can be further divided into coarsely 

 fibrillary and finely fibrillar. 



(2) Irregular single herringbone structure. 

 No clear arrangement of molecular strands. 

 Both the angle of intersection, strand thick- 

 ness and strand distance vary from place to 

 place. 



(3) Regular film structure. The angle of 

 intersection, distance of molecular fibrils and 

 strand thickness show a regular variation 

 from outside to inside, but from place to 

 place the herringbone structure may still be 

 regular. 



(4) Irregular film texture. The general 

 character of the fibrillar structure is differ- 

 ent on the outside of the fiber and in the 

 middle. But the structure is everywhere 

 irregular. 



(5) Mixed film texture. The outer layer 

 of the fiber shows a regular herringbone 

 structure, while the pith has an irregular 

 texture, or vice versa. 



(6) Branched film texture. When the fiber 

 is examined with its axis roughly correspond- 

 ing to the orthogonal position, a fine second- 

 ary film texture may sometimes be visible 

 in the light bands, having a slightly different 

 orientation from the main direction. The 

 secondary fibrils are noticeably thinner than 

 the principal fibrils. 



The above system applies in the first place 

 to synthetic fibers, but is also found with 

 films — though to a less extent. 



Investigation of Spherulite Structures 



For examining spherulite structures it is 

 usually advisa))le to use thin films. 



Many high polymers form spherulite 

 structures when they crystallize from the 

 melt. The number of spherulites formed de- 

 pends upon the number of crystallization 

 nuclei present and the speed of cooling. They 

 may be very suitably examined with phase 

 contrast and polarizing microscopes. The 



392 



