High-Elastic State

High-Elastic State 

one of the three physical states of amorphous polymers. It is manifested in the temperature interval between the glass-transition temperature and the yield temperature of polymers whose macromolecules have a chain structure and are sufficiently flexible. The high-elastic state is also observed in polymers whose macromolecules are firmly bonded into a three-dimensional network with sufficiently long and flexible segments of the chain structure between nodes. Polymers in the high-elastic state are distinguished by the ability to undergo enormous reversible tensile strains (up to several hundred percent), by low values of the modulus of elasticity (0.1-10 meganewtons per sq m, or 1-100 kilograms-force per sq cm), by the liberation of heat upon elongation, and by an increase of the equilibrium modulus of elasticity with temperature. The most characteristic representatives of high-elastic materials are raw and cured rubbers.

The high-elastic state arises because of the ability of chain molecules to undergo changes in shape. Under the influence of thermal motion, the flexible chain molecules continuously change their form—that is, they assume a series of different conformations. When the molecules are sufficiently long, the number of permissible coiled conformations is extremely large. The action of stretching forces straightens the macromolecules; upon cessation of the action of the forces, the macromolecule recoils because of the random character of thermal motion. Thus, the resistance of a polymeric body toward a change in shape is basically caused by an increase in the number of straighter conformations, which are less probable, rather than by a change in internal energy, as in crystal-line materials. Therefore, the isothermal deformation of an ideal high-elastic polymer is associated with a decrease in entropy and, in this sense, is analogous to isothermal compression of the ideal gas. Correspondingly, for a ther-modynamically balanced high-elastic deformation, the force tending to shorten the polymeric body being stretched by external forces is determined from the equations

where S is entropy, l is the length of the elongated sample, and T is the absolute temperature. According to the statistical theory of thermodynamically balanced high-elastic polymer deformations, all of the special features of the high-elastic deformation are a consequence of the thermal motion of long and flexible chain molecules. During sufficiently rapid deformations, when the chain molecules are no longer able to change their form, as well as during very large deformations, when subsequent straightening of the molecules is difficult, polymers lose the capability for high-elastic deformation and behave as ordinary solid bodies.

The high-elastic state is distinguished by a peculiar combination of the properties of elastic solids (the ability to restore the initial shape of the body), the elastic properties of gases (the kinetic nature of elasticity), and the general properties of liquids (values for the coefficients of thermal expansion and compressibility).

REFERENCES

Kargin, V. A., and G. L. Slonimskii. Kratkie ocherki po fiziko-khimii polimerov, 2nd ed. Moscow, 1967.
Tager, A. A. Fiziko-khimiia polimerov, 2nd ed. Moscow, 1969.

G. L. SLONIMSKII