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A plastic crystal is a
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. The name plastic crystal refers to the mechanical softness of such phases: they resemble waxes and are easily deformed. If the internal degree of freedom is molecular rotation, the name rotor phase or rotatory phase is also used. Typical examples are the
modification Modification may refer to: * Modifications of school work for students with special educational needs * Modifications (genetics), changes in appearance arising from changes in the environment * Posttranslational modifications, changes to prote ...
s Methane I and Ethane I. In addition to the conventional molecular plastic crystals, there are also emerging ionic plastic crystals, particularly organic ionic plastic crystals (OIPCs) and protic organic ionic plastic crystals (POIPCs). POIPCs are solid protic organic salts formed by proton transfer from a Brønsted acid to a Brønsted base and in essence are protic ionic liquids in the molten state, have found to be promising solid-state proton conductors for high temperature
proton-exchange membrane fuel cell Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable ...
s. Examples include 1,2,4- triazolium perfluorobutanesulfonate and
imidazolium Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-a ...
methanesulfonate In organosulfur chemistry, a mesylate is any salt or ester of methanesulfonic acid (). In salts, the mesylate is present as the anion. When modifying the international nonproprietary name of a pharmaceutical substance containing the group o ...
. If the internal degree of freedom freezes in a disordered way, an
orientational glass In solid-state physics, an orientational glass is a molecular solid in which crystalline long-range order coexists with quenched disorder in some rotational degree of freedom. An orientational glass is either obtained by quenching a plastic cryst ...
is obtained. The orientational degree of freedom may be an almost free rotation, or it may be a jump diffusion between a restricted number of possible orientations, as was shown for
carbon tetrabromide Tetrabromomethane, CBr4, also known as carbon tetrabromide, is a carbon bromide. Both names are acceptable under IUPAC nomenclature. Physical properties Tetrabromomethane has two polymorphs: crystalline II or β below 46.9 °C (320.0  ...
. X- ray
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
patterns of plastic crystals are characterized by strong diffuse intensity in addition to the sharp Bragg peaks. In a powder pattern this intensity appears to resemble an amorphous background as one would expect for a liquid, but for a single crystal the diffuse contribution reveals itself to be highly structured. The Bragg peaks can be used to determine an average structure but due to the large amount of disorder this is not very insightful. It is the structure of the diffuse scattering that reflects the details of the constrained disorder in the system. Recent advances in two-dimensional detection at synchrotron beam lines facilitate the study of such patterns.


Mechanical properties

Plastic crystals behave like true plastic metals under mechanical stress. For example, closer to melting, plastic crystals show high ductility and/or malleability. Plastic crystals can flow through a hole under pressure. For example, aminoborane plastic crystals bend, twist and stretch with characteristic necking, under appropriate stress. These crystals can be literally shaped into any possible way, like copper or silver metals. This way, they are very unique compared to other molecular crystals, which are generally brittle and fragile.


Plastic crystals versus liquid crystals

Like
liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. Th ...
s, plastic crystals can be considered a transitional stage between real solids and real liquids and can be considered ''soft matter''. Another common denominator is the simultaneous presence of order and disorder. Both types of phases are usually observed between the true solid and liquid phases on the temperature scale: : : The difference between liquid and plastic crystals is easily observed in
X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. Plastic crystals possess strong long range order and therefore show sharp Bragg reflections. Liquid crystals show none or very broad Bragg peaks because the order is not long range. The molecules that give rise to liquid crystalline behavior often have a strongly elongated or disc like shape. Plastic crystals consist usually of almost spherical objects. In this respect one could see them as opposites. Certain liquid crystals go through plastic crystal phase before melting. In general, liquid crystals are closer to liquids while plastic crystals are closer to true crystals. :


History

Plastic crystals were discovered in 1938 by J. Timmermans by their anomalously low
melting entropy Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increa ...
. He found that organic substances having a melting entropy lower than approximately 17 J·K−1·mol−1 (~2Rg) are having peculiar properties. Timmermans named them '. Michils showed in 1948 that these organic compounds are easily deformed and accordingly named them, ''plastic crystals'' ().
Perfluorocyclohexane Perfluorocyclohexane or dodecafluorocyclohexane is a chemical which belongs to the class of fluorocarbons, sometimes referred to as perfluorocarbons or PFCs. Fluorocarbons and their derivatives are useful fluoropolymers, refrigerants, solvents, ...
for example is plastic to such a degree that it will start to flow under its own weight.


References

{{Reflist Crystallinity, plastic Physical quantities Soft matter Phases of matter