Polymeric materials,their classification and properties of compositions

Polymeric materials ,their classification and properties of compositions

Polymeric materials are chemicalhigh-molecular compounds, which consist of numerous small-molecular monomers (links) of the same structure. Often the following monomeric components are used for the manufacture of polymers: ethylene, vinyl chloride, vinylidene chloride, vinyl acetate, propylene, methyl methacrylate, tetrafluoroethylene, styrene, urea, melamine, formaldehyde, phenol. In this article, we will consider in detail what polymeric materials are, what are their chemical and physical properties, classification and species.

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Types of polymers
A feature of the molecules of this material isa large molecular mass, which corresponds to the following value: M> 5 * 103. Compounds with a lower level of this parameter (M = 500-5000) are commonly called oligomers. In low-molecular compounds, the mass is less than 500. The following types of polymeric materials are distinguished: synthetic and natural. The latter include natural rubber, mica, wool, asbestos, cellulose, etc. However, the main place is occupied by synthetic polymers, which are obtained as a result of the process of chemical synthesis from compounds of low molecular weight. Depending on the method of manufacturing high-molecular materials, polymers are distinguished, which are created either by polycondensation or by the addition reaction.
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Polymerization
This process is an associationlow-molecular components into high-molecular ones with the formation of long chains. The magnitude of the polymerization level is the number of "measures" in molecules of a given composition. Most often, polymeric materials contain from one thousand to ten thousand of their units. The following commonly used compounds are obtained by polymerization: polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polystyrene, polybutadiene, etc.
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Polycondensation
This process is a stepwisea reaction which consists in the combination of a large number of monomers of the same type, or a pair of different groups (A and B) into polycondensors (macromolecules) with the simultaneous formation of the following by-products: methyl alcohol, carbon dioxide, hydrogen chloride, ammonia, water, etc. By polycondensation Silicones, polysulfones, polycarbonates, aminoplasts, phenolic, polyesters, polyamides and other polymeric materials are produced.
Polyaddition
Under this process is understood the formationpolymers as a result of the reactions of multiple addition of monomeric components that contain limiting reaction pools to monomers of unsaturated groups (active cycles or double bonds). Unlike polycondensation, the polyaddition reaction proceeds without isolation of by-products. The most important process of this technology is the curing of epoxy resins and the production of polyurethanes.
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Classification of polymers
By composition, all polymer materials are divided intoinorganic, organic and organoelemental. The first of them (silicate glass, mica, asbestos, ceramics, etc.) do not contain atomic carbon. They are based on oxides of aluminum, magnesium, silicon, etc. Organic polymers form the most extensive class, they contain atoms of carbon, hydrogen, nitrogen, sulfur, halogen and oxygen. Organoelemental polymeric materials are compounds that, in addition to the listed chains, contain silicon, aluminum, titanium and other elements capable of combining with organic radicals. In nature, such combinations do not arise. These are exclusively synthetic polymers. Typical representatives of this group are compounds on a silicone-based basis, the main chain of which is constructed from oxygen and silicon atoms.
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To obtain polymers with the necessary propertiesin technology often not using "pure" substances, but their combination with organic or inorganic components. A good example are polymeric building materials: metal plastics, plastics, fiberglass, polymer concrete.

Structure of polymers
The peculiarity of the properties of these materials is due totheir structure, which, in turn, is divided into the following types: linear-branched, linear, spatial with large molecular groups and very specific geometric structures, and also a ladder. Let us consider briefly each of them.
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Polymeric materials with a linear-branched structure, besides the main chain of molecules, have lateral branches. Such polymers include polypropylene and polyisobutylene.
Materials with a linear structure have longZigzag or spiral-twisted chains. Their macromolecules are primarily characterized by repetitions of sites in one structural group of the link or chemical unit of the chain. Polymers with a linear structure are distinguished by the presence of very long macromolecules with a significant difference in the character of bonds along the chain and between them. There are intermolecular and chemical bonds. The macromolecules of such materials are very flexible. And this property is the basis of polymer chains, which leads to qualitatively new characteristics: high elasticity, as well as the absence of brittleness in the hardened state.
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And now we learn what polymer materials are likespatial structure. These substances form strong chemical bonds in the transverse direction when the macromolecules join together. As a result, a network structure is obtained, in which the heterogeneous or spatial basis of the grid. Polymers of this type have greater heat resistance and rigidity than linear polymers. These materials are the basis of many non-metallic structural materials.
Molecules of polymeric materials with a ladderstructure consist of a pair of chains that are connected by a chemical bond. These include organosilicon polymers, which are characterized by increased rigidity, thermal stability, in addition, they do not interact with organic solvents.
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Phase composition of polymers
These materials are systems,which consist of amorphous and crystalline regions. The first of them helps to reduce stiffness, makes the polymer elastic, that is capable of large deformations of a reversible nature. The crystalline phase contributes to an increase in their strength, hardness, modulus of elasticity, as well as other parameters, while reducing the molecular flexibility of the substance. The ratio of the volume of all such regions to the total volume is called the degree of crystallization, where the maximum level (up to 80%) are polypropylenes, fluoroplastics, high density polyethylenes. The lower level of crystallization is possessed by polyvinyl chlorides, low-density polyethylenes.
Depending on how the polymeric materials behave when heated, they are usually divided into thermosetting and thermoplastic.
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Thermosetting polymers
These materials primarily have a linearstructure. When heated, they soften, but as a result of the occurrence of chemical reactions in them, the structure changes to spatial, and the substance turns into a solid. In the future, this quality is preserved. On this principle, polymeric composite materials are constructed. Subsequent heating does not soften the substance, but only leads to its decomposition. The finished thermosetting mixture does not dissolve and does not melt, therefore its recycling is unacceptable. This type of material includes epoxy silicone, phenol-formaldehyde and other resins.

Thermoplastic polymers
These materials, when heated, first softenand then melt, and with subsequent cooling hardens. Thermoplastic polymers under such treatment do not undergo chemical changes. This makes this process completely reversible. Substances of this type have a linear-branched or linear structure of macromolecules, between which small forces act and there are absolutely no chemical bonds. These include polyethylenes, polyamides, polystyrenes, etc. The technology of polymeric materials of thermoplastic type provides for their manufacture by injection molding in water-cooled forms, pressing, extrusion, blowing and other methods.
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Chemical properties
Polymers can be in the following states: solid, liquid, amorphous, crystalline phase, as well as highly elastic, viscous and glassy deformation. The wide use of polymeric materials is due to their high resistance to various aggressive media, such as concentrated acids and alkalis. They are not subject to electrochemical corrosion. In addition, as the molecular weight increases, the solubility of the material in organic solvents decreases. And polymers that have a spatial structure are generally not affected by these liquids.
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Physical properties
Most polymers are dielectrics,in addition, they belong to non-magnetic materials. Of all the structural substances used, only they have the least thermal conductivity and the maximum heat capacity, and also the thermal shrinkage (about twenty times greater than that of the metal). The cause of loss of sealing by various sealing units under conditions of low temperature is the so-called glass transition of rubber, as well as a sharp difference between the expansion coefficients of metals and rubbers in the vitrified state.
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Mechanical properties
Polymeric materials are distinguished by a widea range of mechanical characteristics that strongly depend on their structure. In addition to this parameter, various external factors can exert a great influence on the mechanical properties of the substance. These include: temperature, frequency, duration or rate of loading, type of stress state, pressure, nature of the environment, heat treatment, etc. A special feature of the mechanical properties of polymeric materials is their relatively high strength with very low stiffness (compared to metals).
Polymers are usually divided into solid, moduleelasticity of which corresponds to E = 1-10 GPa (fibers, films, plastics), and soft highly elastic substances, the elastic modulus of which is E = 1-10 MPa (rubber). The patterns and mechanism of destruction of both are different.
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Polymer materials are characterized by brightpronounced anisotropy of properties, as well as a decrease in strength, the development of creep under the condition of prolonged loading. Together with this they have quite a high resistance to fatigue. Compared with metals, they are characterized by a more pronounced dependence of mechanical properties on temperature. One of the main characteristics of polymeric materials is deformability (compliance). According to this parameter, it is common to assess their main operational and technological properties over a wide temperature range.

Polymeric materials for flooring
Now let us consider one of the practicalThe use of polymers, revealing the full range of these materials. These substances have found wide application in construction and repair and decoration works, in particular in the flooring. Huge popularity is due to the characteristics of the substances under consideration: they are resistant to abrasion, low heat conductivity, have little water absorption, are sufficiently strong and hard, have high paint and varnish qualities. The production of polymeric materials can be divided into three groups: linoleums (rolls), plate products and mixtures for the device of seamless floors. Now let us briefly examine each of them.
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Linoleum is made on the basis of different typesfillers and polymers. They can also include plasticizers, processing aids and pigments. Depending on the type of polymer material, polyester (glyptal), polyvinyl chloride, rubber, colloxylin and other coatings are distinguished. In addition, in structure, they are divided into non-basic and sound-insulated base, single-layered and multilayered, with a smooth, fleecy and corrugated surface, as well as single and multi-color.
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Tile materials made on the basis ofpolymer components, have very low abrasion, chemical resistance and durability. Depending on the type of raw materials, this kind of polymer products are divided into kumaronopolivinyl chloride, coumarone, polyvinyl chloride, rubber, phenolite, bituminous tiles, as well as chipboard and wood fiber boards.
Materials for seamless floors are the most convenient and hygienic in operation, they have high strength. These mixtures are usually divided into polymer cement, polymer concrete and polyvinyl acetate.