Polybutadiene is considered to be most promising elastomers in synthetic rubber industry as the result of remarkable improvement made on this material in recent years so that it contains now nearly all cis-1, 4 in its structure. The chemorheological study of this material is in this way becoming a very interesting subject.
For the measurement of stress relaxation two kinds of samples were used, the samples that were subjected to thoroughgoing extraction in aceton and benzene at room temperature after complete vulcanization, and therefore free from all chemical residue that might act as cross-linking agents, and the samples that were not subjected to extraction, and therefore retain chemical substances that might act as cross-linking agents. As was found by A.V. Tobolsky and others with natural rubber when f(t)/f(0) was plotted against log t, all cis-1, 4 polybutadiene showed the Maxwellian decay with approximate exactness in unextracted samples as is shown in Figs. 2 and 3, and with rough exactness in extracted samples as is shown in Figs. 4 and 5. It is clear from these figures that both cis 4 and Ameripol CB decay more rapidly in unextracted samples than in extracted samples at the temperature used in the present experiment. This phenomenon will be accounted for by the fact that the chemical impurities that remain in unextracted samples will accerelate the scission reaction. The samples shown in Figs. 2, 3, 4, and 5 are replotted in the form of log f(t)/f(0) vs. t, in Figs. 6, 7, 8, and 9, respectively.
From the slopes of the straight lines in Figs. 6, 7, 8, and 9, the values of k' can be obtained, the functions of T as the Maxwellian decay curve is expressed by equation (1).
f(t)/f(0)=e-k't (1)
As such functional dependence of k' on temperature is known to be expressed by the Arhenius equation (2), the values of Eact were obtained for the four samples, the unextracted cis 4, and Ameripol CB, the extracted cis 4 and Ameripol CB.
k'=Aexp(-Eact/RT) (2)
In this way the activation energy Eact for the four samples can be calculated, using the plotting in the form of log k' vs. 1/T as shown in Fig. 10.
The activation energy Eact of the four samples is roughly equal to the value (30±2)kcal/mol as was obtained by A.V. Tobolsky and others with natural rubber being approximately independent of the amounts of cis-1, 4 structure, and of the existence of chemical impurities.
Various samples of cis 4 and Ameripol CB of different initial cross-link density, i.e., different values of n(0) were prepared. The n(0) values for the samples were determined by measuring the initial stress f(0) at 25% on the equation of state for rubber as shown by equation (3).
f(0)=n(0)RT[(Lo/Lu)2-(Lu/Lo)] (3)
Here, Lo/Lu equals to 25% elongation.
The shapes of the stress relaxation curves of the various samples having different values of n(0) are shown in Fig. 11.
It is already known that if cleavage occurs randomly at the links of the network chains, the decay of the stress relaxation curves rapidly decrease in n(0).
According to our experiment shown in Fig. 11, however, the decay of the stress relaxation curves appears to be independent of the initial density of n(0). The fact that f(t)/f(0) is independent of n(0) indicates that cleavage takes place at or adjacent to the network junctures for all cis-1, 4 polybutadiene.
