52 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY 



molecules are released at a certain temperature and finally the crystal melts. 

 With increasing chain length, however, the disintegration of the lattice is 

 impeded. Although the mobility of the chain molecules is increased, their 

 parallel alignment is maintained, in much the same way as in a sheaf of 

 pencils in which each pencil can be turned about its axis and shifted with 

 respect to its neighbours, but cannot be turned out of its parallel position. 

 This state is evidently intermediate between the crystalline solid and the 

 amorphous liquid state, because the mobility of the molecules does not 

 refer to all directions in space but is restricted to a single one. We are, 

 hertefore, dealing with a state of matter which is designated as tnesophase 



(Friedel, 1922) or crystalline liquid 

 (VoRLANDER, 1 93 6). Since an align- 

 ment into loose sheaves is only pos- 

 sible with rod-shaped molecules, 

 only chain molecules can occur as 

 mesophases. If a crystal lattice of 

 isodiametric molecules is dissolved, 

 its molecules become at once inde- 

 pendently mobile. With a chain lattice 

 this is not always true, as the pattern 

 is often destroyed in two steps. The 

 first step frees the crystalline bonds 

 between the chain molecules; but 

 there remains some cohesion, which 

 maintains a certain parallelism of the 

 individual chains, which can rotate 

 and shift along each other as indicat- 

 ed above. If the rod-shaped molecules 

 can only rotate round their longi- 

 tudinal axis, their ends remaining in 

 definite planes (cf. Fig. 32a), the crystalline mesophase is in the so-called j'/^?^^//V 

 state; but if rotation and shifting in the direction of the molecular axis is 

 possible, so that the ends of the molecular rods no longer correspond (cf. 

 Fig. 32b, p. 35), the mesophase is said to be nematic. In many cases spindle- 

 shaped bundles are formed which are strongly birefringent (cf. Fig. 44). 

 It is only by a second step that the crystalline mesophase can be converted 

 into an isotropic amorphous liquid phase, where the molecules become com- 

 pletely mobile. 



The transformation of the chain lattice into a mesophase occurs at a well- 

 defined temperature (melting point I), whereas the conversion into an 

 amorphous liquid takes place at a given higher temperature (melting 

 point II). 



Compared to solid crystals, the optics of mesophases is simple. As all 

 molecules in the sheaf can be rotated about their axes, no order exists in 

 directions perpendicular to these axes. All directions perpendicular to the 



Fig. 44. Anisotropic liquid aggregates in 



a sol of benzopurpurin between crossed 



nicols (from Zocher, 1925). 



