Rubber natural, stress-temperature curves

Figure 6-1. Stress-temperature curves of natural rubber. Extension ratios X are indicated on the right of the figure. [After M. Shen, D. A. McQuarrie, and J. L. Jackson, J. Appl. Phys., 38, 791 (1967), by permission of the American Institute of Physics.]...

Fig. 10. Stress-temperature curves for sulfur-vulcanized natural rubber (99,103). Courtesy of the American Chemical Society.

That crystallization increases the elastic stress has already been demonstrated in Figure 6-8, in which the Mooney-Rivlin plot shows a rise at high extension ratios. However, it should be remembered that part of this increase is due to finite extensibility of network chains. In Figure 6-13 we show the stress-strain curves of natural rubber at two temperatures. At 0 °C there is considerable strain-induced crystallization, and we observe a dramatic rise in the elastic stress above X = 3.0. Wide-angle X-ray measurements show the appearance of crystallinity above this strain. At 60 °C there is little or no crystallization, and the stress-strain curve shows a much smaller upturn at high strains. The latter is presumably due only to the finite extensibility of the polymer chains in the network. 

This equation shows that the ratio of the birefringence to the true stress should be independent of stress. The expression on the RHS of equation (11.13) is known as the stress-optical coefficient. A test of equation (11.13) can be made by plotting An against cr, when a straight line should be obtained. Such plots for a vulcanised natural rubber at various temperatures are shown in fig. 11.5. The hysteresis shown in the curves for the lower temperatures is interpreted as being due to stress crystallisation, with the crystallites produced being oriented in the stretching direction and... 

Fig. 8.1 Stress-elongation curve for natural rubber in the vicinity of room temperature. (From Mark (5))...

At one time the pronounced upward sweep in the tensile stress-strain curve of a rubber vulcanizate was considered to be due to orientation-induced crystallization. It has however since been observed that the stress-strain curve for natural rubber does not greatly change in shape at temperatures as high as 100 C when such orientation-induced crystallinity would be very much reduced. In addition... 

Figure 11-15. Stress/strain diagram of a natural rubber (above) and an it-poly(styrene) (below) at room temperature. Left experimental curves right true stress curves. Numerical values were not given for the lower left figure in the original work. (After P. I. Vincent.)...

---