202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 



cules, and the elasticity of the rubber is explained by the thermal 

 motions of the molecules. \Vlien the rubber is relaxed, the thermal 

 agitation of these long threadlike molecules causes them to assume a 

 disordered, kinked condition. Small van der Waals forces permit 

 long-range extensibility. When the rubber is stretched, the molecules 

 are brought into alinement in the direction of the stress, and the 

 thermal agitation now makes itself evident by a tendency of the 

 rubber to retract and produce again a disordered, rather than an 

 orderly arrangement of the molecules. In other words, the entropy 

 of the rubber tends toward a maximum. It is further assumed that 

 when the rubber is stretched, partial crystallization occurs. This is 

 supported by the observation of Katz (9) in 1927 that stretched 

 rubber gives a definite X-ray diagram. This partial crystallization 

 results in an evolution of heat, and it is due to this effect that the 

 temperature coefficient of the stretching and relaxation is greater than 

 would be predicted on the assumption that no partial crystallization 

 took place. This explains the reversible Joule effect. 



THEORY OF VULCANIZATION 



A satisfactory theory of the structure of rubber should not only 

 account for the elastic properties of rubber in the unvulcanized con- 

 dition, but should also explain changes in the physical properties of 

 rubber produced by vulcanization. These changes were mentioned 

 early in this paper in connection with the work of Goodyear and Han- 

 cock. From a scientific point of view the most important effects of 

 vulcanization are a great increase in the temperature range over 

 which rubber exhibits elasticity, a lower permanent set or plastic 

 flow at a given temperature, and a marked decrease in swelling in 

 organic liquids. 



There has been a certain parallelism between various vulcanization 

 theories and contemporary chemical thought. Thus in 1851, Brande, 

 writing on the organic chemistry of rubber, stated that he could re- 

 move all of the sulfur after vulcanization, and that the rubber re- 

 mained vulcanized. He therefore assumed that the vulcanizate was 

 an allotropic form of rubber. Some of the later theorists regarded 

 vulcanized rubber as an indefinite combination of alloys of rubber 

 and sulfur. 



Weber concluded that the process of vulcanization involves the 

 formation of a series of addition products of hydrocarbon and sulfur; 

 that as the sulfur content of these compounds increases there is a 

 decrease in distensibility and an increase in rigidity; and that the 

 term of this series — i. e., the degree of vulcanization produced — is in 

 every case only a function of temperature, time, and proportion of 

 sulfur present. 



