382 BELL SYSTEM TECHNICAL JOURNAL 



natural rubber, i.e. the heat generated per cycle of reversible stretching, is 

 less than any synthetic rubber of comparable gum tensile strength. The 

 resistance to crack growth of vulcanized natural rubber, particularly in gum, 

 i.e. unfilled form, is better than any synthetic of comparable low-tempera- 

 ture flexibility. 



\\Tiat are the causes of these differences? We can make synthetic rubbers 

 from hydrocarbons (in fact from isoprene itself) which judged simply from 

 the composition should have the same interchain forces acting as in natural 

 rubber. That this is not so is shown by the fact that polyisoprene in all 

 the properties which would fit it for a tire rubber is much inferior to natural 

 rubber. We are forced to conclude that it is the form of the individual 

 isoprene units {cis or trans) and the way in which they are placed in the 

 chain that determines. 



In stretched natural rubber we are convinced that units are orderly 

 arranged in the cis configuration as follows: 



We might expect then that the interchain forces, which change so critically 

 with distance, are less of a match for the thermal energ>' in synthetic polyiso- 

 prene than in natural rubber. The high degree of molecular uniformity in 

 natural rubber, as X-rays show, gives rise to a crystallization on stretching 

 which is entirely absent from polyisoprene (see Fig. 11). Even before 

 crystallization as such has progressed far (it starts in nuclei and these multi- 

 ply throughout the mass) the interchain forces have prepared for and 

 compensated in an effective way for the increased effect of stress tearing 

 the chains apart and the increased thermal energy tending to weaken the 

 interchain forces. We may look upon natural rubber as a substance which 

 progressively and automatically transforms itself to a plastic as it is 

 elongated. It is these cr>'stallization phenomena which are responsible for 

 high gum tensile strength^^ and outstanding resistance to the growth of 

 cracks. 



