SYNTHETIC RUBBER RESEARCH 383 



To return to our question and ask once more what structure we desire in 

 a rubber for tires we see that although we cannot quite write an order in 

 terms of a chemical formula we can state general requirements. We want 

 a i)olymer in which the interchain forces are as low as possible to give us low 

 hysteresis. At the same time we want regularity of chain molecules to 

 provide a minimum loss of cohesion with rising temperature and rising 

 elongation. To a chemist this sounds like an order for natural rubber and 

 the design of Buna S which we were forced to imitate in the emergency seems 

 wrong. If research can iron out some of this irregularity, a further improve- 

 ment in our product perhaps can be achieved. The chemist by the clever 

 trick of adding styrene to butadiene has provided himself a way he can 



Fig. 11. — X-Ray photographs of natural rubber, stretched (left) and synthetic 

 polyisoprene, stretched (right). 



regulate the interchain forces and therefore the degree of rubberiness of 

 Buna S. He is able to make it harder and stronger at will by increasing 

 the amount of styrene, something nature is unable to do. But his task is 

 not finished until he can control also the order and the packing of his mole- 

 cules or devise some equally clever way of getting the interchain forces to 

 behave. 



Conclusion 



We have attempted to review some of the problems arising out of the 

 effort to achieve the best possible Buna S type rubber for our war emergency 

 and to show how they have been attacked. We have also tried to give a 

 simple account of some of the theories underlying the behavior of polymers. 

 The story of synthetic rubber is of course much broader both in theory and 

 practice than we have indicated. 



