Takae Kusano

Chemorheological Study of Vulcanized Rubbers under Mechanical Stress

Abstract Below the glass transition temperature, the cleavage of molecular chains of extended vulcanized rubbers will occur. The crosslink density of vulcanized rubbers, which were placed at room temperature for about 4 h in a nitrogen atmosphere, increased about 17 per cent after extension of 200 per cent at −76° C. This may be due to the recombination of cleft chains, and this can be shown by the Maxwellian type of curves resulting from a plot of crosslink density as a function of forced strain. The decreasing proportion of crosslink density in the presence of radical acceptors in the rubbers and the increasing crosslink density in the absence of radical acceptors, because of recombination of cleft chains, was ascribed to the recombination of unstable chains produced by the added mechanical stimulus.

Chemorheological Study on Vulcanized Rubbers under Mechanical-Curve Treatment

When vulcanized rubber is strained by external force at lower temperature, e. g. lower than its glass transition temperature, it is considered that there occurs cleavage in the network of polymer chains. When natural rubber has been strained by external force to 200% of its original bulk at -70°C, and is left in nitrogen at room temperature for about four hours, its cross-linking density increases about 17% as high. The relation between the cross-linking density and the rate of deformation due to the strain shows the Maxwellian type.Experiments have revealed the fact, on the other hand, that when vulcanized rubber is impregnated with radical acceptor its cross-linking density gets remarkably reduced in the rate of increment.It is conceivable, all things considered, that some mechanical reaction has taken place on the degraded rubber radicals to recombine.

Theory for Both the Scissions of Polymer Chains and Crosslink Sites of Crosslinked Polymers

In connection with the evaluation of polymer stability and chemical antioxidants it is a very urgent work to make clear the degradation mechanism of crosslinked polymers.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 their crosslink scission occurs in the oxidation reaction from the experiments and theory by Berry and Watson.The results of recent studies of crosslinked polymers including this vulcanizates show that both the scissions may occur at the some time in many cases.So the authors have made an attempt to derive the general equations for both the scissions for the purpose of finding out the quantitative amounts of scission chains using the experimental results and judging the relative estimation of the scissions.We have generalized the case where one cross-linkage is cut in the minimum model of perfect chain network, and the following equation has been derived for the vulcanizates in which both the scissions occur.Nt/N0=ft/f0={1-4kx/k+2x·Q(t)/M0}·e-2x2/k+2x·Q(t)/M0+2kx/k+2x·Q(t)/M0·e-4x2/k+2x·Q(t)/M0(1)The Eq. (1) in both the cases of k→0 and k→∞, conforms to that proposed by Tobolsky. Assuming the more practically probable mechanism in the scission of cross-linked polymer, the following Eq. (2) obtained by modification of Eq. (1) is derived.Nt/N0=ft/f0={1-8kx/k+2x·Q(t)/M0}·e-2x2/k+2x·Q(t)/M0+6kx/k+2x·Q(t)/M0·e-4x2/k+2x·Q(t)/M0-2kx/k+2x·Q(t)/M0·e-8x2/k+2x·Q(t)/M0(2)The theoretical relation between Q(t)/M0 and Nt/N0 is shown and discussed.

Application of the Theory to Vulcanizates of Natural Rubber

Four kinds of natural rubber were used for specimens, the kinds A, B and C were cured with sulphur and the kind D was cured with peroxide. The kinds A, B and C had different initial densities of crosslinking.The actual density, N0 for the four samples, was obtained by using the equation of f(0)=N0 RT (α-α-2) and by the swelling method using the equation of Flory-Rhener.According to the results obtained already, both the scissions of main chains and crosslink sites occurred at the same time for A, B and C, but the only main-chain scission occurred for the sample D.The Eq. (1) was obtained for the sample D because k=0 in the Eq. (3) of[ft/f0={1-2k/M0·qm(t)}·e-x/M0·qm(t)+k/M0·qm(t)·e-2x/M0·qm(t)](ft/f0)D=e-xD/M0·qm(t) (1)orqm(t)=-N0, Dln(ft/f0)D (2)As the Eq. (3) of [ft/f0={1-2k/M0·qm(t)}·e-x/M0·qm(t)+k/M0·qm(t)·e-2x/M0·qm(t)] is applicable to the samples of A, B and C, and qm (t) is equal to the four samples under the same condition, the Eq. (4) is established by substituting the Eq. (2) into the Eq. (3).-N0, Dln(ft/f0)D=(ft/f0)A-(ft/f0)DN0, D/N0, A/(ft/f0)DN0, D/N0, A-2·M0/kA·1/(ft/f0)N0, D/N0, A (4)Using the experimental data of (ft/f0)A, (ft/f0)D and the known value of N0, A, N0, D and M0, kA is calculated from the Eq. (4).Similarly kB, kC and kD are obtained.