STUDIES IN GELS 



109 



ends. On this assumption a distribution function for the orientation 

 in network systems can be derived. A striking result of this theory is, 

 that a completely parallel arrangement of all micellar strands would be 

 reached at a degree of stretch 2 

 (100% stretch). This is not in 

 keeping with the observed facts, 

 seeing that Hermans's cellulose 

 fibres, especially when greatlv 

 swollen, can undergo a stretch 

 of several times 100",,. One 

 must, therefore, assume that in 

 reality neither the first nor the 

 second limiting case is realized. 

 The behaviour is intermediate 

 between those corresponding to 

 the two extreme cases calculated, 

 i.e., the micelles are not freely 

 suspended but they are inter- 

 linked to form micellar strands. 

 The junctions present, however, 



are not fixed indis soluble hinges which completely prevent the micellar 

 strands or parts of these strings from gliding past each other. In fact, the 

 cohesion must be due to forces which at certain points can be overcome 

 by the orientating forces, so that a "flow in small regions" takes place. 



c. Swelling of Gels 



If isotropic gels are immersed in a swelling medium, they swell 

 uniformly in all directions. If a certain orientation of the micellar 

 strands prevails, however, the swelling is anisotropic, i.e., different 

 in different directions. The anisotropy of swelling of starch grains 

 induced Nageli (1858) to consider the structural units (micelles) 

 of the gel as submicroscopic rodlets. 



Intermicellar swelling. According to Nageli, the swelling medium 

 penetrates between the rodlets which we now call the micellar strands. 

 In many cases X-ray analysis has confirmed this view, as often the 

 X-ray diagram does not change in the swelling process, so that ap- 

 parently the crystalline regions remain unaltered (e.g., plant cell walls 

 and cellulose gels). 



Fig. 77. Short-range order in a gel of inter- 

 linked micelles (from Hermans, 1941). 



