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Anelastomer is apolymer withviscoelasticity (i.e. bothviscosity andelasticity) and with weakintermolecular forces, generally lowYoung's modulus (E) and high failure strain compared with other materials.[1] The term, aportmanteau ofelastic polymer,[2] is often used interchangeably withrubber, although the latter is preferred when referring tovulcanisates.[3] Each of themonomers which link to form the polymer is usually a compound of severalelements amongcarbon,hydrogen,oxygen andsilicon. Elastomers areamorphous polymers maintained above theirglass transition temperature, so that considerablemolecular reconformation is feasible without breaking ofcovalent bonds.[citation needed]
Rubber-like solids with elastic properties are called elastomers. Polymer chains are held together in these materials by relatively weakintermolecular bonds, which permit the polymers to stretch in response to macroscopic stresses.
Elastomers are usuallythermosets (requiring vulcanization) but may also bethermoplastic (seethermoplastic elastomer). The long polymer chainscross-link during curing (i.e., vulcanizing). The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links. The elasticity is derived from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-linkages ensure that the elastomer will return to its original configuration when the stress is removed.
Unsaturated rubbers that can be cured by sulfur vulcanization:
Saturated rubbers that cannot be cured by sulfur vulcanization:
Various other types of elastomers:
Crosslinking most likely occurs in an equilibrated polymer without any solvent. The free energy expression derived from theNeo-Hookean model of rubber elasticity is in terms of free energy change due todeformation per unit volume of the sample. The strand concentration, v, is the number of strands over the volume which does not depend on the overall size and shape of the elastomer.[5] Beta relates the end-to-end distance of polymer strands across crosslinks over polymers that obey random walk statistics.
In the specific case of shear deformation, the elastomer besides abiding to the simplest model of rubber elasticity is also incompressible. For pure shear we relate the shear strain, to the extension ratios lambdas. Pure shear is a two-dimensional stress state making lambda equal to 1, reducing the energy strain function above to:
To getshear stress, then the energy strain function is differentiated with respect to shear strain to get the shear modulus, G, times the shear strain:
Shear stress is then proportional to the shear strain even at large strains.[6] Notice how a low shear modulus correlates to a low deformation strain energy density and vice versa. Shearing deformation in elastomers, require less energy to change shape than volume.
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