Strain-crystallizing elastomers between

ISO 8013 is confined to static strain conditions and can seriously underestimate the creep that occurs under dynamic loading. The creep rate in cycled rubber is higher than that predicted by a simple Boltzmann superposition, but linearity is still observed between creep and logarithmic time or the logarithm of the number of cycles, as long as a physical mechanism applies [39,40]. The increase is most striking with strain-crystallizing elastomers such as natural rubber. 

A more striking difference is found between strain-crystallizing and noncrystallizing elastomers when the stress is not relaxed to zero during each cycle. As shown in Fig. 33, the fatigue life of a natural rubber vulcanizate is greatly increased when the minimum strain is raised from zero to, say, 100% because the crystalline barrier to tearing at the tip of a crack does not then... 

Elastomers are necessarily characterized by weak intermolecular forces. Elastic recovery from high strains requires that polymer molecules be able to assume coiled shapes rapidly when the forces holding them extended are released. This rules out chemical species in which intermolecular forces are strong at the usage temperature or which crystallize readily. The same polymeric types are thus not so readily interchangeable between rubber applications and uses as fibers or plastics. 

In some cases, double networks have shown increases in orientability and strain-induced crystallization, as well as improved fatigue resis-tance. ° In fact, some results show that there maybe less of a compromise between failure properties in general and the modulus, which may be due in part to the decreased hysteresis observed for some of these elastomers. There have even been reports of improved thermal stabil-ity, although it is hard to visualize how this would occur. Finally, electrical resistivity is more sensitive to strain in carbon-black reinforced double networks. Better molecular understanding of these observations is being sought with, for example, extensive studies of residual strains and birefringence. ... 

Ultimate strength, as represented by the modulus at rupture, shown as a function of the molecular weight between cross links for a unimodal and bimodal elastomer compared at two temperatures. The improvement is larger at the lower temperature, presumably due to enhanced strain-induced crystallization. 

An orientation process to enhance the elastic recovery and stiffness of propylene-based elastomers is described. Elastomers fabricated during this process have virtually complete elastic recovery, and are up to ten times stiffer than unoriented controls. The structural development during orientation is explored with WAXS. A relationship between crystal structure, strain recovery, and modulus is described. 

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