Sofia Martinez
Plastic crystals were discovered in 1938 by Belgian chemist Jean Timmermans, who found that certain organic substances had peculiar properties. These plastic crystals are composed of weakly interacting molecules that possess some degree of freedom in their orientation or conformation. They are easily deformed and can be considered a transitional stage between solids and liquids, exhibiting both order and disorder. While the term plastic crystal refers specifically to this type of crystalline structure, the more general term plastic can be used to describe polymeric materials that can be molded or shaped, typically through the application of heat and pressure. These plastics, or polymers, can be semi-crystalline or amorphous, with the former having a highly ordered molecular structure and distinct melt points. The crystallization of polymers can be influenced by various factors, including stretching, temperature, and the presence of additives.
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What You'll Learn
Plastic crystals' discovery and properties
Plastic crystals, also known as molecular globulare, were discovered in 1938 by Belgian chemist Jean Timmermans. Timmermans discovered that certain organic substances with a low entropy of fusion had peculiar properties. Michils showed in 1948 that these substances were easily deformed and, thus, named them plastic crystals (cristaux organiques plastiques).
Plastic crystals are composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. They are closely related to condis crystals, which are conformationally disordered crystals that possess translational and rotational order. Plastic crystals can be considered a transitional stage between real solids and real liquids and can be moulded into various shapes. They can be distinguished from liquid crystals by their strong long-range order, which is evident in sharp Bragg reflections in X-ray diffraction patterns.
Plastic crystals exhibit unique behaviours under mechanical stress, such as bending, twisting, and stretching. They can flow through a hole when subjected to pressure and can be moulded like metals such as copper or silver. Perfluorocyclohexane, for example, is so plastic that it begins to flow under its own weight.
The creation of plastic crystals has been achieved by researchers using rod-shaped colloidal particles. These particles have a size ranging from one to a thousand nanometres. By applying an external electric field, the rotations of the rods can be halted, forming a three-dimensional crystal. This discovery opens up new possibilities for investigating the glass transition and developing novel materials, such as colour monitor screens based on electronic ink.
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Plastic crystals vs molecular crystals
Plastic crystals are composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. They are mechanically soft and easily deformed, resembling waxes. Examples include Methane I and Ethane I. Certain liquid crystals go through a plastic crystal phase before melting. Plastic crystals exhibit behaviour similar to ductile metals such as lead, gold, silver, or copper when subjected to mechanical stress. They can be molded into various shapes and can flow through a hole under pressure.
Molecular crystals, on the other hand, are formed by the packing of giant planar macromolecules. They are typically much simpler than plastic crystals, as intermolecular forces are usually much weaker than intramolecular ones. As a result, the structure of the molecules in a molecular crystal is almost the same as in the gas phase. Examples of materials that form molecular crystals include inert gases, diatomic halogens, closed-shell molecules like methane, and planar aromatic molecules like benzene. Molecular crystals are generally poor conductors of electricity.
The distinction between plastic crystals and molecular crystals lies in their structure and behaviour. Plastic crystals exhibit a simultaneous presence of order and disorder, characteristic of transitional stages between real solids and liquids. They possess strong long-range order, evident through sharp Bragg reflections in X-ray diffraction patterns. On the other hand, molecular crystals are generally associated with weaker intermolecular forces compared to intramolecular forces.
Plastic crystals were discovered in 1938 by Belgian chemist Jean Timmermans, who found that certain organic substances had peculiar properties due to their low entropy of fusion. These substances were later named "plastic crystals" by Michils in 1948 because of their deformable nature. The discovery of plastic crystals and their unique properties opened up new avenues in materials science, particularly in understanding the transitional states between solids and liquids.
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Polymer crystallization
The degree of crystallization in polymers can be determined using techniques such as X-ray diffraction (XRD) and differential scanning calorimetry (DSC). XRD is a gold-standard indicator of polymer crystallization, while DSC measures the crystallization enthalpy, which is the heat released during crystallization. These techniques provide valuable information for optimizing production processes and enhancing the quality of polymer products.
The process of crystallization itself can occur through various methods, including melting, extrusion, solution evaporation, and stretching. During melting, polymers are heated to their melting point and then molded or extruded, which can induce crystallization. Extrusion, used in the production of fibers and films, forces the polymer through a nozzle, creating tensile stress that aligns its molecules. Solution evaporation involves increasing the concentration of a dilute polymer solution, promoting interaction and potential crystallization between molecular chains. Additionally, some polymers that do not crystallize from the melt can be partially aligned by stretching.
Understanding the crystallization behavior of polymers is essential for tailoring their properties to specific applications. For example, crystallization improves dimensional stability and resistance to mechanical abrasion in semi-crystalline polymers. It also affects their optical, mechanical, thermal, and chemical properties. By controlling the degree of crystallization, manufacturers can enhance the performance and functionality of polymer-based products.
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Semi-crystalline polymers
Plastic is a polymeric material that can be moulded or shaped, typically using heat and pressure. Plastics can be divided into two categories based on their chemical composition: those made up of polymers with only aliphatic (linear) carbon atoms in their backbone chains, and those that are not.
Some plastics have a crystalline structure, and some do not. Amorphous plastics have a more disorganized molecular structure, with molecules oriented randomly and intertwined. This means they have a range of melting temperatures. In contrast, semi-crystalline polymers have a highly ordered and tightly packed molecular structure, resulting in a defined melting point. The areas of crystallinity in semi-crystalline polymers are called spherulites, and they vary in shape and size, with amorphous areas existing between them.
When tensile stress is applied to a semi-crystalline polymer, the molecular chains of the amorphous phase stretch, leading to plastic deformation of the crystalline regions. This deformation occurs through dislocation motion, resulting in coarse or fine slips in the polymer and ultimately, crystalline fragmentation. The addition of particles to semi-crystalline polymers can change their mechanical properties, either strengthening or weakening the material.
In summary, semi-crystalline polymers are a type of plastic with a highly ordered molecular structure and a sharp melting point. They exhibit improved chemical and heat resistance, superior performance against wear, and good strength. The degree of crystallinity within these polymers can impact their characteristics, and the specific application requirements should be considered when selecting this type of polymer.
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Plastic crystals' X-ray diffraction patterns
Plastic crystals are composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. They were discovered in 1938 by Belgian chemist Jean Timmermans, who found that certain organic substances with a low entropy of fusion exhibited peculiar properties. X-ray crystallography is a powerful technique used to determine the atomic and molecular structure of crystals, including plastic crystals. By shining a beam of X-rays through a crystal, the diffraction pattern that emerges contains valuable information about the crystal's structure.
The process of X-ray crystallography involves several steps. First, the crystal is placed in an intense beam of X-rays, typically of a single wavelength, resulting in a pattern of reflections. The angles and intensities of these diffracted X-rays are carefully measured, and the crystal is gradually rotated to capture additional reflections. Multiple data sets may be required, especially if the crystal degrades due to radiation damage.
In the next step, the collected data is processed computationally, combining it with complementary chemical information. This allows for the creation and refinement of a model of the arrangement of atoms within the crystal, including their positions, bond lengths, and bond angles. The final model represents the crystal structure and is often shared publicly.
X-ray crystallography has been applied to plastic crystals, providing insights into their unique properties. Plastic crystals exhibit mechanical softness, resembling waxes, and can be easily deformed. Some plastic crystals, like aminoborane, exhibit ductile behaviour similar to metals under mechanical stress. They can be moulded into various shapes and exhibit bending, twisting, and stretching.
The X-ray diffraction patterns of plastic crystals are characterised by strong diffuse intensity, resembling an amorphous background in powder patterns. However, in single crystals, the diffuse contribution reveals a highly structured nature. The Bragg peaks observed in plastic crystals provide information about the average structure, but the details of the constrained disorder are reflected in the structure of the diffuse scattering.
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Frequently asked questions
Plastic crystals are composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. They resemble waxes and are easily deformed.
Plastic crystals are formed by proton transfer from a Brønsted acid to a Brønsted base.
Examples of plastic crystals include methane I, ethane I, and aminoborane.
