Stress-strain cross-linked elastomers

Basically, four types of tensile stress-strain curve are found for cross-linked elastomers deformed below Tg. These are shown schematically in Fig. 20 where pre-... 

Fig. 20. Types of stress-strain behaviour observed in cross-linked elastomers below Tg. Origins curves indicate amount of (ve-extetKion (sd matic)...

Solution cross-linked elastomers also exhibit stress-strain isotherms in elongation that are closer in form to those expected from the simplest molecular theories of rubberlike elasticity. Specifically, there are large decreases in the Mooney-Rivlin 2C correction constant described in... 

Erman, B. Mark, J. E., Interpretation of Stress-Strain Isotherms for Elastomers Cross-Linked in Solution. Macromolecules 1987,20, 2892-2897. 

Stretching a rubber band makes a good demonstration of the stress-strain relationships of cross-linked elastomers. The time required is about 30 minutes. 

A typical stress-strain isotherm obtained on a strip of cross-linked elastomer, such as natural rubber is shown schematically in Fig. 6.2 [1-3]. 

The viscoelastic properties of the polysulfide polymers are governed by the interchange reactions of the polysulfide linkages in the chain. On rapid application of stress, the cross-linked rubbers behave as conventional elastomers. Under a fixed strain, the following interchange processes set in to relieve the... 

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

The information on physical properties of radiation cross-linking of polybutadiene rubber and butadiene copolymers was obtained in a fashion similar to that for NR, namely, by stress-strain measurements. From Table 5.6, it is evident that the dose required for a full cure of these elastomers is lower than that for natural rubber. The addition of prorads allows further reduction of the cure dose with the actual value depending on the microstructure and macrostructure of the polymer and also on the type and concentration of the compounding ingredients, such as oils, processing aids, and antioxidants in the compound. For example, solution-polymerized polybutadiene rubber usually requires lower doses than emulsion-polymerized rubber because it contains smaller amount of impurities than the latter. Since the yield of scission G(S) is relatively small, particularly when oxygen is excluded, tensile... 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

Equation (4-46) predicts that the stress-strain properties of an elastomer that behaves like an entropy spring will depend only on the temperature, the density of the material, and the average molecular weight between cross-links. In terms of nominal strain this equation is approximately... 

The distinguishing feature of the A-B-A thermoplastic elastomer structure is that a three-dimensional network is established by the dispersed domains serving as cross-link junctions of high functionality (see Figure 1). With Increase of the proportion of A, the stress-strain response changes and successively approximates that of a nonreinforced vulcanizate, a reinforced vulcanlzate, a flexible thermoplastic, and a toughened thermoplastic. 

The equilibrium small-strain elastic behavior of an "incompressible" rubbery network polymer can be specified by a single number—either the shear modulus or the Young s modulus (which for an incompressible elastomer is equal to 3. This modulus being known, the stress-strain behavior in uniaxial tension, biaxial tension, shear, or compression can be calculated in a simple manner. (If compressibility is taken into account, two moduli are required and the bulk modulus. ) The relation between elastic properties and molecular architecture becomes a simple relation between two numbers the shear modulus and the cross-link density (or the... 

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