Role of Strain Crystallization in the Fatigue Resistance of Double Network Elastomers

Double networks were prepared from guayule rubber (GR), deproteinized natural rubber (DPNR), and styrene-butadiene rubber (SBR), and their properties compared to conventional “single networks” having the same crosslink density. Substantial residual strains (> 150%) were obtained in all double networks, whereby the modulus parallel to the residual strain was enhanced. For the two strain-crystallizing elastomers, the fatigue resistance of the double networks (for extensions parallel to the residual strain) was higher than for their single network counterparts. Moreover, the guayule rubber, which is more strain-crystallizable than DPNR, exhibited the greater enhancement. For the amorphous SBR, on the other hand, the network structure had an insignificant effect on the fatigue life. These results demonstrate that longer mechanical fatigue lifetimes in double network rubbers are a consequence of their intrinsic orientation. This provides the capacity to retain crystallinity at the front of growing cracks, even in the absence of stress. The origin of the improved fatigue resistance is similar to the mechanism responsible for the better performance of strain-crystallizing rubbers subjected to non-relaxing cyclic deformations.