Strain-crystallizing elastomer

E.H. Andrews, Crack propagation in a strain-crystallizing elastomer, J. Appl. Phys., 32, 542-548, 1961. 

The cohesive fracture of conventional, non-strain crystallizing, unfilled elastomers is sensitive to rate and temperature 32.4-1.48-53) exhibiting increased values of 2J with increasing rate and decreasing temperature. The basic viscoelastic nature of the fracture of these materials is evidenced by the fact that it can be described over wide ranges of temperature and rate by time-temperature superposition as described by the WLF Equation... 

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. 

FIGURE 3.27 The time over which stress decay was observed for four NR elastomers. Initially viscoelasticity governs the relaxation time at higher strains crystallization commences (Choi and Roland, 1997). 

Absent mechanisms such as strain crystallization or the finite extension of the chains, uncrosslinked rubbers usually strain-soften, which means thatg( ) decreases witli strain. A damping function that has been shown to successfully describe elastomers for tensile strains of a few hundred percent is (Roland, 1989)... 

As shown in Figure 10.15, the tear strength of strain-crystallizing elastomers is greatly enhanced over the range of tear rates and temperatures at which... 

FIGURE 10.15 Fracture energy, G, for a strain-crystallizing elastomer, natural rubber, as a function of temperature, T, and rate of tearing, R. (From Greensmith and Thomas (1955).)... 

Although amorphous elastomers are found to tear steadily, at rates controlled by the available energy for fracture, G (as shown in Figures 10.13 and 10.14), strain-crystallizing elastomers do not tear continuously under small values of G, of less than about 10" J/m for natural rubber, for example (see Figure 10.15). Nevertheless, when small stresses are applied repeatedly, a crack will grow in... 

Nishikawa E, Finkelmann H. 1999. Smectic A liquid single crystal elastomers strain induced break down of smectic layers. Macromol Chem Phys 200 312 322. 

The different crack growth laws for strain-crystallizing and noncrystallizing elastomers thus lead to quite different fatigue life relations. For a noncrystallizing elastomer, the fatigue life is much more dependent on the size of... 

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... 

The incorporation of carbon black into 50/50 elastomer preblends shows carbon black affinity decreases in the order BR, SBR, CR, NBR, EPDM, and butyl mbber. Poor carbon black dispersion causes an increase in hysteresis and a decrease in fatigue resistance. The effect ofvolumeloading ofafiBer such as carbon black on the properties of the vulcanizate depends on whether the elastomer is strain-crystallizing or not. 

The crystallization of PDMS in the undeformed state has long been of interest, including unfilled elastomers - and block copolymers reinforced with silica. Of greater interest, however, is PDMS, generally in the filled state, in elongation - - where strain-induced crystallization provides considerable reinforcement of the elastomer. Strain-induced crystallization is of practical as well as fundamental importance. - Theoretical models have been developed to characterize this type of crystallization. - ... 

Santangelo, P. G. Roland, C. M., Role of Strain Crystallization in the Fatigue Resistance of Double Network Elastomers. Rubber Chem. Technol. 2003,76,892-898. 

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