380 BELL SYSTEM TECHNICAL JOURNAL 



mixing operation. This phenomenon of the insolubilization of natural 

 rubber by carbon black has been known for some time.-- Only recently, 

 however, has its relation to the structural features of GR-S copolymer 

 become apparent.-^ Work is now underway to allow an estimation of both 

 types of gel in the presence of one another. When this is achieved the 

 analysis of the reactions occurring during compounding will be further 

 facilitated. 



Chain Structure and Polymer Properties 



We have now reviewed some of the molecular complexities which are 

 involved in the synthesis of GR-S synthetic rubber. It remains to discuss 

 more in detail the influence of chain structure on the properties we associate 

 with rubber-like behavior. We might begin by asking ourselves two 

 questions: (1) Wliat makes a polym.er exhibit rubber-like properties? (2) 

 \Miat composition and chain structure are desirable in a rubber? The first 

 question involves a discussion of the theor}^ of rubber-like elasticity. The 

 answer to the second involves an inquiry into the specific use to which the 

 material is to be applied. Since most of our rubber is employed in tires let 

 us consider the special requirements for that use. 



Taking up the first question, we fall immediately into the pit of having to 

 define what a rubber is and how it differs from a plastic. Originally rubber 

 meant "natural rubber". \\Tien synthetics with rubber-like properties 

 appeared we adopted the term "synthetic rubber" to describe them. Some 

 have objected (unsuccessfully) to the use of this term because it implies 

 synthetic natural rubber and have proposed the word "elastomer" instead. 

 Others have gone still further and suggested other terms (usually ending in 

 mer) for various plastics and rubber-like materials. 



All of these new names seem unnecessary. Polymer is the inclusive term. 

 The term rubber simply has come to mean a polymer which at ordinary 

 temperatures has properties like natural rubber. A plastic is a polymer 

 which at ordinary temperatures is hard and w^hich usually becomes soft and 

 deformable at higher temperatures. Such terms as rubber-like plastic or 

 glass-like plastic are frequently employed. This kind of terminology is 

 admittedly loose but it often tells just as much in familiar words as does the 

 newly proposed nomenclature. 



The significant fact is that there is a perfectly consistent and orderly 

 relationship between the properties of pol}TTiers and their chemical Composi- 

 tion and structure. Fundamentally, the major factor which determines 

 whether a long chain polymer will be a rubber or a plastic is the magnitude 

 of the forces acting between chain molecules. If the forces between pohmier 

 molecules are low, the polymer is a rubber; if they are high, it is a plastic. 

 And obviously since these forces can be regulated nicely there are all grada- 



