Rubber stress crystallisation

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

In a (stereo-speeifie) rubber spontaneous crystallisation occurs under strain here also small regions in whieh ehain parts lie parallel, aet as nuclei.These nuelei, which improve the strength eonsiderably, do, however, not grow out into a continuous phase they disappear upon stress release. 

Neither SBR nor cis-PB contain any appreciable amount of crystallinity and in this respect they differ from natural rubber, which crystallises slowly at ambient temperatures. Crystalhsation is a disadvantage before vulcanisation but is a maj or advantage when subjected to stress and strain in a tyre. The higher the apphed stress, the greater is the crystallinity and... 

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... 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of crystallisation in both the unstressed and the stressed state. 

Rubber thus frozen in the extended state does not recover elastically upon release of stress it has a permanent set . In other words, retraction is blocked by cohesive forces On warming up, however, retraction occurs at once Under ordinary conditions, at room temperature, however, the lattice forces of the crystallites formed upon extension are not strong enough to prevent the rubber from retraction. The entropy factor then surpasses the work of crystallisation. 

The creep behaviour of various vulcanised rubbers under cyclic conditions has been compared with that under constant loading. Cyclic loading is found to produce an enhancement of creep rates thought to be due to crosslinks breaking under stress concentrations caused by crystallisation. Half of the cyclic creep can be accounted for in this way visco-elasticity accounts for the remainder. 6 refs. 

In addition to the slower rate of crystallisation of synthetic polyisoprenes in the raw state relative to natural rubber, which was mentioned earlier, the effect persists in vulcanisates. It has been shown by measuring the times ( 1/4) for 25 % stress relaxation at — 26 °C of vulcanisates at 150 % extension that crystallisation is an order of magnitude slower for the synthetic rubber (Table 9). 

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