314 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1952 



10,000 

 8000' 



6000 



4000 

 3000 



w 2000 



000 



800 



400 



200 



100 



100 



200 



I 

 10,000 



400 600 1000 2000 4000 6000 



FREQUENCY IN CYCLES PER SECOND 

 Fig. 3 — Viscosity and shear modulus of polyisobutyleue (cross-linked Butyl). 



emphasize that thinking about the mechanics of a particular chemical 

 structure must inchide the spatial relationships of groups within the 

 chains, as well as between them. 



The dynamic viscosities in Table I are also in accord with the se- 

 quence of structures. Their frequency dispersion again connotes varying 

 relaxation processes. Natural rubber's low inner friction, for both com- 

 pressional and shear waves is famous in its low hysteresis heating. 

 (This unique property is geopolitically crucial, because adequate truck 

 and bus tires cannot yet be made of any other rubber.) Indeed, it is 

 striking that at 100 cycles, a piece of gum rubber has a local viscosity 

 of only 350 poises. The silicone rubber gum also has high elastic efficiency, 

 and its temperature coefficient of viscosity is very low (see Fig. 2), like 

 the thermal coefficients for familiar silicone liquids. It is exciting to 

 speculate in Figs. 1 and 2, whether more precise measurements which 

 Hopkins is now undertaking will confirm the apparently negative tem- 

 perature coefficients of viscosity at some frequencies. "Kinetic theorj'- 



