Elastomer

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An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. [1] The term, a portmanteau of elastic polymer, [2] is often used interchangeably with rubber , although the latter is preferred when referring to vulcanisates. [3] Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature, so that considerable molecular reconformation is feasible without breaking of covalent bonds. At ambient temperatures, such rubbers are thus relatively compliant (E ≈ 3 MPa) and deformable.[ citation needed ]

Contents

IUPAC definition for an elastomer in polymer chemistry IUPAC definition for an elastomer in polymer chemistry.png
IUPAC definition for an elastomer in polymer chemistry

Rubber-like solids with elastic properties are called elastomers. Polymer chains are held together in these materials by relatively weak intermolecular bonds, which permit the polymers to stretch in response to macroscopic stresses.

(A) is an unstressed polymer; (B) is the same polymer under stress. When the stress is removed, it will return to the A configuration. (The dots represent cross-links) Polymer picture.svg
(A) is an unstressed polymer; (B) is the same polymer under stress. When the stress is removed, it will return to the A configuration. (The dots represent cross-links)

Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer). The long polymer chains cross-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.

Crosslinking most likely occurs in an equilibrated polymer without any solvent. The free energy expression derived from the Neohookean model of rubber elasticity is in terms of free energy change due to deformation 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. [4] Beta relates the end-to-end distance of polymer strands across crosslinks over polymers that obey random walk statistics.

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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 get shear 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. [5] 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.

Examples

Unsaturated rubbers that can be cured by sulfur vulcanization:

Saturated rubbers that cannot be cured by sulfur vulcanization:

Various other types of elastomers:

See also

Related Research Articles

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In engineering, deformation may be elastic or plastic. If the deformation is negligible, the object is said to be rigid.

<span class="mw-page-title-main">Poisson's ratio</span> Measure of material deformation perpendicular to loading

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<span class="mw-page-title-main">EPDM rubber</span> Type of synthetic rubber

EPDM rubber is a type of synthetic rubber that is used in many applications.

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<span class="mw-page-title-main">Delamination</span> Mode of failure for which a material fractures into layers

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Elastic energy is the mechanical potential energy stored in the configuration of a material or physical system as it is subjected to elastic deformation by work performed upon it. Elastic energy occurs when objects are impermanently compressed, stretched or generally deformed in any manner. Elasticity theory primarily develops formalisms for the mechanics of solid bodies and materials. The elastic potential energy equation is used in calculations of positions of mechanical equilibrium. The energy is potential as it will be converted into other forms of energy, such as kinetic energy and sound energy, when the object is allowed to return to its original shape (reformation) by its elasticity.

<span class="mw-page-title-main">Acrylate polymer</span> Group of polymers prepared from acrylate monomers

An acrylate polymer is any of a group of polymers prepared from acrylate monomers. These plastics are noted for their transparency, resistance to breakage, and elasticity.

Rubber elasticity refers to the ability of solid rubber to be stretched up to a factor of 10 from its original length, and return to close to its original length upon release. This process can be repeated many times with no apparent degradation to the rubber.

Membrane roofing is a type of roofing system for buildings, RV's, Ponds and in some cases tanks. It is used to create a watertight covering to protect the interior of a building. Membrane roofs are most commonly made from synthetic rubber, thermoplastic, or modified bitumen. Membrane roofs are most commonly used in commercial application, though they are becoming increasingly common in residential application.

The Ogden material model is a hyperelastic material model used to describe the non-linear stress–strain behaviour of complex materials such as rubbers, polymers, and biological tissue. The model was developed by Raymond Ogden in 1972. The Ogden model, like other hyperelastic material models, assumes that the material behaviour can be described by means of a strain energy density function, from which the stress–strain relationships can be derived.

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In continuum mechanics, an Arruda–Boyce model is a hyperelastic constitutive model used to describe the mechanical behavior of rubber and other polymeric substances. This model is based on the statistical mechanics of a material with a cubic representative volume element containing eight chains along the diagonal directions. The material is assumed to be incompressible. The model is named after Ellen Arruda and Mary Cunningham Boyce, who published it in 1993.

The acoustoelastic effect is how the sound velocities of an elastic material change if subjected to an initial static stress field. This is a non-linear effect of the constitutive relation between mechanical stress and finite strain in a material of continuous mass. In classical linear elasticity theory small deformations of most elastic materials can be described by a linear relation between the applied stress and the resulting strain. This relationship is commonly known as the generalised Hooke's law. The linear elastic theory involves second order elastic constants and yields constant longitudinal and shear sound velocities in an elastic material, not affected by an applied stress. The acoustoelastic effect on the other hand include higher order expansion of the constitutive relation between the applied stress and resulting strain, which yields longitudinal and shear sound velocities dependent of the stress state of the material. In the limit of an unstressed material the sound velocities of the linear elastic theory are reproduced.

<span class="mw-page-title-main">Charles Goodyear Medal</span> Award

The Charles Goodyear Medal is the highest honor conferred by the American Chemical Society, Rubber Division. Established in 1941, the award is named after Charles Goodyear, the discoverer of vulcanization, and consists of a gold medal, a framed certificate and prize money. The medal honors individuals for "outstanding invention, innovation, or development which has resulted in a significant change or contribution to the nature of the rubber industry". Awardees give a lecture at an ACS Rubber Division meeting, and publish a review of their work in the society's scientific journal Rubber Chemistry and Technology.

References

  1. De, Sadhan K. (31 December 1996). Rubber Technologist's Handbook, Volume 1 (1st ed.). Smithers Rapra Press. p. 287. ISBN   978-1859572627. Archived from the original on 2017-02-07. Retrieved 7 February 2017.
  2. Gent, Alan N. "Elastomer Chemical Compound". Encyclopædia Britannica. Archived from the original on 2017-02-07. Retrieved 7 February 2017.
  3. Alger, Mark (21 April 1989). Polymer Science Dictionary. Springer. p. 503. ISBN   1851662200. Archived from the original on 2017-02-07. Retrieved 7 February 2017.
  4. Boczkowska, Anna; Awietjan, Stefan F.; Pietrzko, Stanisław; Kurzydłowski, Krzysztof J. (2012-03-01). "Mechanical Properties of Magnetorheological Elastomers under Shear Deformation". Composites Part B: Engineering. 43 (2): 636–640. doi:10.1016/j.compositesb.2011.08.026. ISSN   1359-8368.
  5. Liao, Guojiang; Gong, Xinglong; Xuan, Shouhu (2013-09-01). "Influence of Shear Deformation on the Normal Force of Magnetorheological Elastomer". Materials Letters. 106: 270–272. Bibcode:2013MatL..106..270L. doi:10.1016/j.matlet.2013.05.035. ISSN   0167-577X.