INTRINSIC DEFECTS AND THE FAILURE PROPERTIES OF CIS-1,4-POLYISOPRENES

Inherent flaw sizes were determined from fatigue lifetimes, and from t he crack length dependence of the strain energy to break, for four cis -1,4-polyisoprenes compounded to have the same crosslink density and l ow strain hysteresis. Both techniques indicated that the flaws intrins ic to guayule rubber (GR), and to a lesser extent conventional natural rubber, are larger than those found in deproteinized NR. This result may not be surprising; however, the failure properties of the elastome rs, expected to depend on flaw size, were surprising. The guayule rubb er and a natural rubber of relatively low purity (SMR-10) had the high est tensile strengths, tear strengths, breaking energies, and fatigue lifetimes, while DPNR exhibited the worst failure properties. Such an inverse correlation between flaw size and failure performance is due t o the dominant effect of strain-induced crystallization. GR and SMR-10 have the highest propensity for strain-induced crystallization, while DPNR is the least strain-crystallizable. Interestingly, all rubbers e xhibited the same isotropic crystallization behavior.