抄録
One of the elastomers recently developed is ethylene-propylene co-polymer which was first synthesized by Natta and his co-workers using organo-metallic catalysts. The structure of this polymer is amorphous, or partially crystallitne, depending upon its ratio of co-polymers.
The characteristics of this polymer are high-resistance to high temperature, degradation, and chemicals. The chemorheology, one of the new research fields, was here thought to be one of the effective means to elucidate the changes of the structure of ethylene-propylene co-polymers under a given condition. The elastomer whose ratio of ethylene to propylene is 70 to 30 was offered to us by Prof. Tobolsky at Princeton University by his courtesy. This elastomer was then irradiated at 1.26MeV electron by the apparatus of Van de Graaf for 15sec., 30sec., and 150sec., respectively, under the room temperature (25°C) in the air. As the results, three kinds of elastomers with different initial densities, n(0) were obtained. Such initial densities, the mole numbers of total network chains in a unit volume were calculated from the stress relaxation measuremet as shown by Table 1.
When n(0) is plotted against the radiation time, EPR-2, EPR-1, and EPR-3 are all on the same straight line exactly. From these data, the G value in this case was calculated to be 1.34.
(I) In the case where the cross-linking chains cut alone or prior to the main chains, the relation between f(t)/f(0) and log t is independent of the initial densities n(0), but that between q(t) and t is dependent on the initial densities n(0).
These tendencies are shown by Fig. 2 (a).
(II) In the case that the main chains cut alone or preferably to the cross-linking chains, the relation between f(t)/f(0) and log t is dependent on the initial densities n(0), but that between q(t) and t is independent of the initial densities n(0).
The basic theory through the above results is described in the recent textbook by Prof. Tobolsky.
The stress relaxation of the ethylene-propylene co-polymer was studied at the elongation 15%, and 140°C. Fig. 3 shows that the relation between f(t)/f(0) and log t for three samples indicate the same tendency. On the other hand, the relation between q(t) and t for three samples shows different curves as indicated by Fig. 4. From the above results obtained, the change of the structure of the samples due to the mechanism (I) seems to occur, in another words, the chanis of cross-linking cut alone or preferably to the main chains for the ethylene-propylene co-polymer.
As the facts that when natural rubbers by irradiation cure are exposed to the oxidation under the air of high temperature, the scission of the main chains occurs preferably to the cross-linking chains have been observed by A.V. Tobolsky and his co-workers, our results are quite noticeable when compared with their results.
The stress decay curves by the intermittent and continuous method for ethylene-propylene co-polymer are shown by Fig. 5, Fig. 6 and Fig. 7. It is clear that for the sample of EPR-2 of which initial density n(0) is the lowest among the three samples, the interval between the curves by both methods is increasing gradually with the length of the time. In another words, the amounts of cross-linking chains are gradually increasing.
In EPR-1, whose density is the midst among the three, the amounts of cross-linking chains created are increasing with increasing of the time at first, then they are decreasing gradually with further increasing of the time. In EPR-3, whose initial density is the highest among the three, the amounts of cross-linking chains created by the oxidation are not so remarkable as in the other two samples, and the two curves by both methods become overlapped in the long time range
