Chemorheological Treatment of Cross-linked Polymers

I. The case when the scissions of both random chains and crosslinkages occur

Abstract

To make clear the degradation mechanism of cross-linked polymers is a very urgent work in connection with the evaluation of polymer stability and chemical antioxidants.
There have long been two extremely different opinions about the mechanism of scission of network chain polymers, especially about natural rubber vulcanizates. One is that only random chain scission of this vulcanizates occurs in the oxidation reaction from the experiments and theory by A.V. Tobolsky. Another is that only cross-link scission of them occurs in the oxidation reaction from the experiments and theory by Berry and Watson.
The recent results of cross-linked polymers inculding this vulcanizates show that both scissions may occur at the same time in many cases.
So the authors make an attempt to derive the general equation for both scissions for the purpose of the possibility for judging which scission occur preferably and of finding out the quantitative amounts of scission chains using the experimental results. We generalize the case that one cross-linkage is cut in the minimum model perfect chain network
As the results, the next equation was derived for vulcanizates both scission occur
(1)
The equation (1) in the both cases of k→0 and k→∞, conforms to those proposed bTobolsky. Assuming the more practically probable mechanism in the scission of cross- linked polymer, the following equation (2) obtained by modification of equation (1) is derived.
(2)
The theoretical relation between Q (t) /M₀ and N_t/N₀ is shown and discussed.
Then three kinds A, B and C of natural rubbers cured by sulphur were prepared, and D of that cured by peroxide was prepared. A, B and C have the different initial den- sities of cross-linking.
The actual density, N₀ for the four samples was obtained by using the equation of f (0) =N₀RT (α-α^-2) and by the swelling method using the equation of Flory-Rhener.
According to the results obtained already, both scissions of main chains and cross-link sites occur at the same time for A, B and C, but the only mainchain scission occur or the sample D.
So equation (3) is obtained for the sample D, because k=0 in the equation of
(3)
or
(4)
As the equation of is applicable for the samples of A, B and C, and q_m (t) is equal for the four samples under the same condition, the equation (5) is established by subsutituting the equation (4) into the above equation.
(5)
Using the experimental data of (f_t/f₀) A, (f_t/f₀) D and the known value of N_{0, A}, N_{0, D}, and M₀, K_A is calculated from the equation (5).
Similarly K_B, K_C are obtained.