Amorphous State

Amorphous State

 

a solid state of matter with two characteristics: (1) the properties of a substance in the amorphous state—mechanical, thermal, electrical, and so forth—are ordinarily independent of the direction of measurement in the substance (isotropy) and (2) with increased temperature, the substance softens and enters the liquid state only gradually. (In other words, there is no definite melting point in the amorphous state.)

These characteristics result from the absence of long-range order in the amorphous state. Such long-range order is present in crystals, which exhibit strict periodicity in all directions of one and the same structural element—atom, atom group, molecule, and so forth—through hundreds and thousands of periods. At the same time, matter in the amorphous state possesses short-range order—that is, regularity in the position of neighboring particles (the order observed at distances comparable to the molecular dimensions). With distance this agreement diminishes, and after 0.5–1 nanometer it disappears.

Short-range order is also characteristic of liquids. In the case of liquids, however, there is an intensive exchange of positions between neighboring particles; this exchange is retarded with increase in viscosity. For this reason, on the one hand, a solid body in the amorphous state can be regarded as a supercooled liquid with a very high coefficient of viscosity, and on the other hand, the concept of the amorphous state embraces that of the liquid.

Isotropy of properties is also characteristic of the poly-crystal state; this, however, is characterized by a strictly defined fusion temperature, and it is this fact which justifies separating it from the amorphous state. The structural difference between the amorphous and the crystal states is readily detectable on X-ray diagrams. Monochromatic X-rays scattered on crystals form a diffraction picture consisting of distinct lines or spots; this is not characteristic of the amorphous state.

The crystal state is the stable solid state of matter at low temperatures. However, depending on the particular properties of the molecules, the time for crystallization may be long; the molecules must first manage to align themselves in crystalline order during the cooling of the matter. This sometimes takes a very long time, with the result that the crystalline state is not realized. In other cases, the amorphous state is reached by accelerating the cooling process. For example, by fusing a quartz crystal and then rapidly cooling the melt, one can obtain amorphous quartz glass. Many silicates behave in the same way, yielding ordinary glass upon cooling. For this reason, the amorphous state is often referred to as the vitreous state. However, in most cases even very rapid cooling does not prevent the formation of crystals, and hence attainment of the amorphous state is impossible for most substances. The amorphous state occurs less frequently in nature than the crystalline state. It is found in opal, obsidian, amber, natural resins, and bitumens.

The amorphous state is not limited to substances consisting of individual atoms and ordinary molecules, such as glass and liquids (low-molecular compounds), but may also be found in substances formed from long-chain macro-molecules—the so-called high-molecular compounds, or polymers. The structure of amorphous polymers is characterized by short-range order in the position of the chains, or segments, of the molecules. This order quickly disappears as the distance of the chains from one another increases. Polymer molecules form clusters, as it were, whose lifetime is very long because of the very high viscosity of polymers and the large diameters of their molecules. For this reason, in many cases clusters remain practically unchanged for indefinite periods.

Depending on temperature, amorphous polymers exist in three states, according to temperature. These three states differ in the character of thermal motion—vitreous, su-perelastic, and liquid (viscofluid). At low temperatures, the molecular segments possess no mobility, and the polymer in that case behaves as an ordinary solid in the amorphous state. At sufficiently high temperatures, the energy of thermal motion becomes great enough to produce a shift of the molecular segments, though not of the molecules as a whole. A superelastic state sets in, characterized by the ability of the polymer to be readily stretched or compressed. Transition from the superelastic to the vitreous state is called vitrification. In the viscofluid state, not only molecular segments but whole macromolecules may be shifted. Polymers then become fluid, although, distinct from ordinary liquids, their flow is always accompanied by the appearance of superelastic deformation.

REFERENCES

Kitaigorodskii, A. I. Poriadok i besporiadok v mire atomov. Moscow, 1966.
Kobeko, P. P. Amorfnye veshchestva. Moscow-Leningrad, 1952.
Kitaigorodskii, A. I. Rentgenostrukturnyi analiz melkokristal-licheskikh i amorfnykh tel. Moscow-Leningrad, 1952.
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