An
amorphous rubber, well-represented as a mostly
random mixture of
polymer chains. If a
stress is applied to the material, and
Young's Modulus is
evaluated, the
elastic response is noted to be
nonlinear. This is because until a
very large deformation occurs, the polymer chains simply unbend, and only
hydrogen bonds are stressed or broken. Due to this uncoiling and increase in
order, heat is released during stetching and elasticity varies proportionally with temperature.
Thermodynamic Properties
Some of the macroscopic properties of elastomers due to
thermodynamics are:
- they may be stretched easily to many times their original length
- They retract rapidly when the force is released to their original length, with very little heat transfer.
- They do not suffer permanent deformation as a result of extension
- When fully extended they have very high tensile strength and stiffness.
Microscopically, things are much more interesting. A high molecular weight of the polymeric material is required, with weak inter-chain interactions. As previously mentioned it must not easily crystallize, meaning it is amorphous. Furthermore, to display elastomeric behavior, the material must have a temperature greater than its glass transition temperature, denoted Tg. At this temperature, specific volume, thermal coefficient of expansion, specific heat capacity, and refractive index change. There may be small local rotations about bonds in the polymers, but the chains will be unable to move as they will be fixed in their positions. This will make the material hard and brittle. Once above that temperature, however, the molecular bits of the solid act like liquids.
As the chains are just kind of sitting where they are (recall, they most have low intermolecular attractions) they are easy to extend. This extension decreases the entropy of the system, and so retracting to the original state is thermodynamically favored. The original dimensions are not regained unless there is a network of some kind, usually formed by vulcanization.
As mentioned above, cross-linking between polmeric strands in the material gives structure to the material. Vulcanization does this, in tires most often with sulfur.
High modulus at full extension derives from the formation of crystalline structures when the molecules are fully stretched out and therefore aligned. These structures act as cross-links and increase stiffness.
A little bit of this came from my Materials Science and Engineering notes, but a lot of it came from a page by Prof. Kathryn R. Williams of University of Florida's Department of Chemistry.