I08 FUNDAMENTALS OF SUBMI C RO SCOP I C MORPHOLOGY I 



parallel arrangement is founds At some distance, however, the ar- 

 rangement becomes gradually more and more disturbed, so that all 

 possible orientations are found in a gel volume of even microscopic 

 dimensions. This is shown by Fig. 76. Hence, when considering 

 the dispersion of orientations in Fig. 51b (p. 66) or 54a (p. 70), it 



must be borne in mind that neighbouring 

 particles are almost parallel. An entirely dif- 

 ferent orientation is only found at a certain 

 submicroscopic distance as a result of grad- 

 ual changes in orientation. In the much 

 larger microscopic dimensions this means. 







>>\nOa\\ i/i,n\l 



Fig. 76. Short-range order of however, that all anisotropy effects are neu- 



short rod molecules (from ^ralized as if a random criss-cross arrange- 

 Hermans, 1941). . 



ment existed. 



The principle of short-range order would explain why it is that, 

 when stretched, gels of a low degree of swelling can behave as if their 

 particles were freely floating micelles. In fact, the movement of each 

 particle is very similar to its neighbour: there is no steric hindrance, 

 as would be the case if the arrangement were an irregular one. The 

 principle of short-range order does not suffice, however, to explain 

 altogether the behaviour of gels when stretched ; for, the extensibility 

 of these gels would have to be unlimited, and it should be possible 

 to deform them to fibres of arbitrary length, even in those cases 

 where the degree of swelling is low. 



In the cellulose fibres mentioned, prepared by Hermans, this is im- 

 possible. We are therefore forced to assume that the micelles are not 

 freely movable, but that they are interlinked by junctions (Frey- 

 Wyssling 1936a, 1936c) or hinges (Fig. 77). This assumption of 

 complete interlinking of the structural elements in the gel is designated 

 by Kratky as the second limiting case. Here again, there exists short- 

 range order, and the picture arrived at (Hermans, 1941) corresponds 

 more or less to the one given by us (compare Fig. 54, p. 70). In other 

 words, the orientation takes place as if chains consisting of rigid links 

 and movable but inextensible hinges were stretched by pulling at the 



^ The voluntar\'^ parallel arrangement of rod-shaped particles is not confined to colloid 

 raatter.lt occurs also in pure liquids and real solutions, where physicists speak of short- 

 range order (Zernike, 1939; Stuart, 1941; Peterlin and Stuart, 1943). Taking an 

 arbitrary molecule, its immediate neighbours are more or less orderly as regards distance 

 and orientation. 



