Citizendia
Your Ad Here

An elastomer is a polymer with the property of elasticity. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. Vulcanization (or Vulcanisation refers to a specific curing process of Rubber involving high heat and the addition of Sulfur or other equivalent curatives Each of the monomers which link to form the polymer is usually made of carbon, hydrogen, oxygen and/or silicon. A monomer (from Greek mono "one" and meros "part" is a small Molecule that may become chemically bonded to other Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. The glass transition temperature, T g is the temperature at which an Amorphous solid, such as Glass or a Polymer, becomes brittle At ambient temperatures rubbers are thus relatively soft (E~3MPa) and deformable. Room temperature (also referred to as ambient temperature) is a common term to denote a certain Temperature within enclosed space at which humans are accustomed In Solid mechanics, Young's modulus (E is a measure of the Stiffness of an isotropic elastic material Their primary uses are for seals, adhesives and molded flexible parts. A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage (e Glue or adhesive is a compound that adheres or bonds two items together

Contents

Background

A is a schematic drawing of an unstressed polymer. The dots represent cross-links. B is the same polymer under stress. When the stress is removed, it will return to the A configuration.
A is a schematic drawing of an unstressed polymer. The dots represent cross-links. B is the same polymer under stress. When the stress is removed, it will return to the A configuration.

Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer). Thermosetting plastics thermosets are Polymer materials that irreversibly cure form Vulcanization (or Vulcanisation refers to a specific curing process of Rubber involving high heat and the addition of Sulfur or other equivalent curatives A thermoplastic is a Plastic that Melts to a liquid when heated and freezes to a Brittle, very Glassy state when cooled sufficiently Thermoplastic elastomers ( TPE) sometimes referred to as thermoplastic rubbers, are a class of Copolymers or a physical mix of polymers (usually a plastic The long polymer chains cross-link during curing. Cross-links are bonds that link one Polymer chain to another They can be Covalent bonds or Ionic bonds "Polymer chains" can refer 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. As a result of this extreme flexibility, elastomers can reversibly extend from 5-700%, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.

Temperature effects are also present in the demonstrated elasticity of a polymer. Elastomers that have cooled to a glassy or crystalline phase will have less mobile chains, and consequentially less elasticity, than those manipulated at temperatures higher than the glass transition temperature of the polymer.

It is also possible for a polymer to exhibit elasticity that is not due to covalent cross-links, but instead for thermodynamic reasons. Rubber elasticity, also known as hyperelasticity, describes the mechanical behavior of many polymers especially those with crosslinking

Mathematic justifications

Using the laws of thermodynamics, stress definitions and polymer characteristics (complete derivation in [1], pages103-105), we find ideal stress behavior:

\sigma\ = n k T [ \lambda\ _ 1 ^ 2 + \lambda\ _ 1 ^ {-1} ]

where n is the number of chain segments per unit volume, k is Boltzmann's Constant, T is temperature, and \lambda\ _ 1 is distortion in the 1 direction. Bridge from macroscopic to microscopic physics Boltzmann's constant k is a bridge between Macroscopic and microscopic physics

These findings are accurate for values of up to approximately 400% strain. At this point, alignment between stretched chains begins to result in crystallization from noncovalent bonding. A noncovalent bond is a type of Chemical bond, typically between Macromolecules that does not involve the sharing of pairs of electrons but rather involves more dispersed

While Young's Modulus does not exist for elastomers due to the nonlinear nature of the stress-strain relationship, a "secant modulus" can be found at a particular strain. In Solid mechanics, Young's modulus (E is a measure of the Stiffness of an isotropic elastic material

Examples of elastomers

Unsaturated rubbers that can be cured by sulfur vulcanization:

(Note that unsaturated rubbers can also be cured by non-sulfur vulcanization if desired).

Saturated Rubbers that cannot be cured by sulfur vulcanization:

Various other types of elastomers:

References

Dictionary

elastomer

-noun

  1. any polymer having the elastic properties of rubber
© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
 Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic