SYNTHETIC RUBBER RESEARCH 381 



tions from the hardest to softest pol>Tner. A rubber therefore may be 

 regarded simj^ly as a soft plastic — one in which the forces between chains 

 are very low— with one important distinction, namely that soft plastics to 

 show rubber-like j^roperties must be "vulcanized" i.e. a few very strong 

 inter-chain linkages must be established to prevent slippage. Some plastics 

 can be vulcanized, too, but here the inter-chain forces are high anyway and 

 the few additional strong bonds are not essential. However, the usual 

 inter-chain forces in plastics being of the van der Waals' type are very 

 susceptible to temperature. Consequently, if we weaken them by raising 

 the temperature we can, provided the plastic is ''vulcanized", cause it to 

 acquire rubber-like properties at the higher temperature. So the distinction 

 between rubbers and plastics is in the last analysis sUght. 



We still have not answered the question as to what causes a polymer to 

 exhibit rubber-like properties. In fact it was only during the last 10 years 

 or so-^ that the answer has been known, which is surprising, because it is 

 simply "temperature". Contrary to previous views, forces between atoms 

 in the same chain have little to do with the long range retraction phenomenon 

 shown by rubbers, at least at elongations up to about 200%. The stretched 

 polym.er returns because of the thermal heat motion in the mass which 

 seeks to restore the elongated chain molecules to their more stable, kinked-up 

 configurations. The molecules, through their vulcanization points, com- 

 municate their retraction behavior to the entire mass. Thus theory agrees 

 with experience that the chemical constitution of the polymer is of secondary 

 signiticance. As long as chain molecules are present which are capable of 

 kinking-up by rotations about chemical bonds, as long as the forces between 

 molecules are not large compared to the thermal energ\^ and as long as the 

 molecules are interconnected at points so as not to slip, we shall have a 

 rubber-like substance whether we call it a rubber or a plastic or an elastomer. 



Coming to our second question, it might now take the form: what composi- 

 tion and chain structure are desirable in a rubber for tires? We shall see 

 that the qualitative views expressed above must be altered if we are to 

 explain the more intimate properties of rubber involved in this application. 

 We have already seen from the sol-gel discussion what some of these reline- 

 ments are. There are certain dififerences between GR-S type synthetic 

 rubber and natural rubber, however, which go back even farther and involve 

 the manner in which the molecules pack and slip over one another during 

 deformation. These have geen discussed above under micro-structure. 



]SIan learns largely by imitation and our knowledge of rubber-like behavior 

 has been no exception. Natural rubber possesses amazing qualities which 

 no synthetic product has yet been able entirely to duplicate, although we 

 have found in many cases ways to overcome weaknesses in synthetic rubbers 

 by round-about means. For example, the hysteresis loss in vulcanized 



