In a plastic crystal (or plastically crystal ) is a crystal consisting of molecules , between which there is only weak attractive forces, so that the molecules a certain degree of freedom with respect to the orientation or conformation have. The name plastic crystal refers to the mechanical softness of such phases : they resemble wax and are easy to deform. When the internal degree of freedom of the molecules consists in a rotation , the term rotor phase is also used.
In addition to plastic crystals, which are made up of molecules, there are also ionic plastic crystals (see also ionic liquid ), in particular organic ionic plastic crystals (OIPC) and protic organic ionic plastic crystals (POIPC).
When cooling, the excited internal degrees of freedom can freeze in some cases in a disorderly manner, e.g. B. in two different orientations along a bond . In English, such a solid with frozen orientational disorder is called orientational glass .
Plastic crystals versus liquid crystals
Another thing they have in common is that order and disorder are present at the same time: in the case of plastic crystals, the long-range positional order is retained, but the long-range orientation is partially or completely eliminated. Thus, e.g. B. spherical molecules such as methane , adamantane , neopentane , tetramethylbutane or cubane additional degrees of freedom of rotation at their lattice positions. In the case of liquid crystals, on the other hand, the long-range position order is first given up, whereupon the oriented smectic and / or nematic phases are formed.
In 1938 J. Timmermans discovered the plastic crystals due to their abnormally low entropy of fusion . He found that organic substances with a melting entropy of less than about 17 J · K −1 · mol −1 (~ 2Rg) have special properties. Timmermans gave them the name molecular globulare .
In 1948, Michils showed that these organic compounds are extremely malleable, which is why he named them plastic crystals ( French cristaux organiques plastiques ). For example, perfluorocyclohexane is so plastic that it begins to flow under its own weight . As with high-temperature creep of, for example, copper and gold, the plastic deformability disappears below a certain temperature.
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