370 BELL SYSTEM TECHNICAL JOURNAL 



required before direct results can be obtained on GR-S. An apparatus 

 employing electron multiplier tubes for measurement of the intensity of the 

 scattered Ught, which was developed in the Laboratories, is being used to 

 study this new technique. Its original form is illustrated in Fig. 6. Like- 

 wise, photographic determination of scattered light has established good 

 correlation with independent molecular weight evaluation of certain other 

 polymers". 



The distribution of molecule sizes in GR-S has a profound influence on its 

 properties. It is controversial still as to whether a uniform or non-uniform 

 distribution is desirable for all considerations. The presence of low molecu- 

 lar material favors ease of processing but depreciates properties. High 

 molecular material behaves the opposite. It is customary to regard the 

 viscosity, either of the rubber itself or the dilute solution viscosity, as a 

 measure of the average molecular weight. While this assumption is not 

 wholly true, it is partly justified because the shape of the distribution curve 

 as commonly measured for GR-S is roughly constant. When osmotic 

 measurements can be made sufficiently accurately, the number average 

 molecular weight together with the viscosity average provides a more 

 precise measure of the distributions present. 



Chain Structure of GR-S and Its Characterization 



As for simple polymers, the chain structure of GR-S is best considered from 

 two points of view : the micro-structure and the macro-structure. The micro- 

 stiucture, which has already been briefly discussed, is concerned with the 

 kinds of atoms forming the chain, their arrangement in space and the manner 

 in which they pack with the atoms of neighboring chain molecules. It is 

 this structure which is all-important in determining the nature of the forces 

 between molecules and, in turn, the intrinsic rubber-like properties of the 

 polymer. The micro-structure also determines the chemical properties of 

 the compound. The macro-structure, on the other hand, is not dependent 

 on the kind of atoms in the polymer or their immediate relation to each other 

 but with the length of the chain molecules, their general shape and the extent 

 to which they are joined with the other molecules (netted) or what were 

 originally other molecules in the material. It is the macro-structure which 

 plays the chief role in plasticity and viscosity of the rubber during process- 

 ing, its smoothness or roughness during extrusion, the extent to which it 

 elongates or creeps on stretching and the extent to which it swells in solvents. 



Let us approach the problem of the micro-chain structure of GR-S by 

 considering the possibilities from the organic structural point of view. In 

 the formation of GR-S about 6 butadiene molecules combine with each 

 styrene molecule. In (9) butadiene is shown in brackets and styrene residues 



