3 STRUCTURE OF GELS 6l 



In crystalline sodium pectate the screw axis is not twofold as in cellu- 

 lose, but threefold (Palmer and Hartzog, 1945). The crystallizing 

 tendency of pectic substances is much smaller than that of cellulose; 

 in the plant it occurs in the amorphous state only (Wuhrmann and 

 PiLNiK, 1945). 



The pentosans, which come partly within the hemicellulose class, 

 have a similar structure to that of the polysaccharides already de- 

 scribed, except for the absence of the side chains, i.e., the sixth C- 

 atom. If in cellulose or polygalacturonic acid this group is replaced by 

 H, we obtain the xylan chain or a polyarahinan. 



The polysaccharides demonstrate strikingly how slight morpholog- 

 ical variations of one and the same structural principle may give rise 

 to substances which behave quite differently from a physiological point 

 of view. 



Chain length of high polymers. According to Staudinger, all high polymer 

 chains terminate in end groups. Unfortunately, so far the terminal groups 

 of none of the high molecular weight natural substances are known; the 

 chains are therefore preferably written in "open" formulae (Fig. 50). 

 Contrarily, in comparatively short synthetic chains the end groups, hence 

 the molecular weights, of the products can be determined. If foreign atoms, 

 such as, for instance, iodine form the terminal groups, such determinations 

 can be easily performed. If, however, the chains are terminated by OH- 

 groups, the accuracy of this so-called end-group method diminishes rapidly 

 with increasing chain length. In polyoxymethylene dimethyl ether this 

 method can be successfully applied up to a degree of polymerization of 

 about 100. The methods of freezing point depression and rise of boiUng 

 point, commonly used in molecular weight determinations in substances 

 of low molecular weight, cannot be applied to high polymers, as the effects 

 are too small. 



On the other hand, the molecular weight, and thus the chain length of 

 high polymers, can be measured by osmotic means, in which case it must 

 be taken into account that Van 't Hoff's law does not apply rigorously to 

 molecules of so great a volume. Corrections similar to Van der Waals' 

 b-correction in the equation of state of gases must therefore ,be introduced 

 (ScHULZ, 1936). A method derived by Staudinger is based on the fact that 

 the specific viscosity of a solution of chain molecules (i.e., the viscosity 

 increase which is imposed upon the solvent by the solute), within a certain 

 range of molecular weights, is approximately a linear function of the chain 

 length. In addition to osmometry and viscometry we mention in particular 

 Svedberg's ultracentrifuge for the determination of the degree of poly- 

 merization of high polymer natural substances. X-ray analysis is not suitable 

 for this purpose (see p. 99). 



