PRINCIPLES OF STRUCTURE 



53 



axis are, therefore, equivalent in all respects. Consequently, mesophases 

 are usually optically uniaxial, and as a rule no isotropic or biaxial mesophases 

 are observed (Zocher, 1925, 1931). In a polarization microscope between 

 crossed nicols, mesophases will therefore appear completely dark if we 

 observe in the direction of the sheaf axis, whereas they light up in all other 

 directions. According as to whether the refractive index parallel to the axis 

 is larger or smaller than that perpendicular to it, the mesophase is called 

 optically positive or optically negative (cf. p. 87). 



TABLE VII 



CRYSTALLINE LIQUID STATE (ACCORDING TO VORLANDER 1 95 6) 



Compare Fig. 45 





(1) 

 (2) 

 (3) 



(4) 



Solid phase 



Alesophase 



melt.pt. I 



Liquid phase 

 melt.pt. II 



Crystalline solid 



: amorphous liquid 



„ ^ crystalline liquid ^ „ ,, 

 ^ „ ,, (supercr.) -> decomposed 

 ,, (supercr.) -> decomposed, infusible 

 rising temperature 



The ease with which a mesophase is changed into an isotropic 

 liquid is a function of the chain length. This is apparent, in particular, 

 from Vorlander's researches (1936). With increasing chain length it 

 becomes increasingly difficult to attain the amorphous liquid state, 

 because finally the melting point II is such a high temperature that 

 the chain molecules are decomposed before the mesophase is converted 

 into a real liquid. With still greater chain lengths the substance does 

 not fuse at all, because the molecules are subject to degradation be- 

 fore becoming movable. In this case, therefore, the cohesive forces be- 

 tween the very long chains are stronger than the main valency bonds 

 in the chain molecule: the molecular structure breaks down before 

 the lattice disintegrates. Substances which cannot be changed into 

 the amorphous state, because the ////^r-molecular forces in the lattice 

 or in sheaves (mesophase) are larger than the /«/r<2-molecular binding 

 forces, are called super -crystalline (Vorlander). A survey is given in 

 Table VII; the substances (2) and (3) occur as mesophases at certain 

 intervals of temperature. 



Fig. 45 shows a series of molecules of increasing chain length which 



