17DOI: 10.24412/2181-1385-2022-1-517-522
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USING OF POLYMERS AS ENGINEERING MATERIALS
Javokhir Abdirakhimov
bachelor student
Alimjon Akhmadjanovich Riskulov
DSc, Prof., Scientific advisor Tashkent State Transport University riskulov2008@mail.ru
High specific strength, corrosion resistance, thermal and electrical conduction as well as a combination of other advantages of metallic materials cannot completely meet requirements of experts in development of brand new technical equipment and technologies. Moreover, developers and technologists have to take into consideration depletion of raw stocks of traditional machine-building materials and increased power inputs and efforts related to their exploration, output, transportation and processing.
Therefore the key problems of up-to-date material science cover development of structural materials using new types of raw materials, more integral application of traditional and secondary resources and optimization of material structures so as to impart them a complex of unusual and, often, contradictory properties. A topical orientation in solution of these problems is development of machine-building materials based on synthetic natural and artificial binding materials. Plastics, rubbers, wood plastics and ceramic materials are among the most common and promising materials.
Plastics, materials based on polymers, are capable of acquiring a specified form on heating under pressure and maintaining it after cooling. Depending on the designation and conditions of operation plastics can contain auxiliary materials: filling compounds, plasticizers, stabilizers, pigments, lubricants, etc.
Classification of polymers. Polymers are high-molecular-weight compounds (HMC), with the molecules (macromolecules) containing a great number of replicated units bound by chemical links. There are several classifications of polymers according to the characteristics of the origin, chemical composition, macromolecular structure and shape, temperature change in structure, supramolecular structure, etc.
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DOI: 10.24412/2181-1385-2022-1-517-522
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According to their origin polymers are subdivided into natural, synthetic and artificial. Typical representatives of natural polymers or biopolymers are cellulose, starch, natural rubber, protein substances. Cells of all living organisms are built of biopolymers. The range of natural polymers is sufficiently limited. Human beings turned out to be much more inventive and developed a huge amount of synthetic polymers.
Synthetic polymers are produced by polymerization or polycondensation during chemical synthesis - targeted synthesis of compounds (polymers) from simple substances (monomers). The process is based on the molecular structure and reactivity of the latter. The nomenclature of synthetic polymers is constantly extending.
Artificial polymers are obtained by treatment (modification) of natural polymers, for example, the artificial polymer nitrocellulose is obtained by cellulose nitration.
According to the macromolecular structure into which monomer units are embedded we distinguish polymers of linear, branched and grid (spatial) structures (Fig.1). Molecules of linear polymers consist of long unbranched molecular chains which contain similar or different atomic groups. Macromolecules of branched polymers have the main (basic) molecular chain and side chains. Macromolecules of grid polymers are connected to each other by chemicals bonds, forming a spatial structure. In this case, the notion of molecule loses its meaning since a sample of cross-linked polymer represents a huge macromolecule.
(7
b
c
Fig.1. Schematic representation of structures of polymer macromolecules:
a, linear; b, branched; c, grid
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If a main chain of a polymer has two or more different monomers, it is a copolymer in contrast to a homopolymer made of replicated units of one monomer.
The structure of copolymers can be sufficiently complicated (Fig.2). If the units of two monomers are bound randomly in a macromolecule, the polymer consisting of such macromolecules is termed statistic. If a macromolecular chain has an ordered alternation of monomer units and the site containing one monomer is sufficiently lengthy, such a copolymer is referred to as a block copolymer.
Graft copolymers have the most complicated structure, with the blocks of one of the monomers attached as side branches to the main chain of the macromolecule composed of units of other monomer.
Fig.2. Schematic representation of structures of copolymer macromolecules:
statistic copolymers (a), block copolymers (b) and graft copolymers (c)
According to the chemical composition of macromolecules polymers are classified by organic, elementorganic and inorganic polymers.
Organic polymers include compounds whose molecules contain atoms of carbon, nitrogen, oxygen, sulfur and halogens that are incorporated into the main chain and the side groups.
The term heteroatom will be used further to designate noncarbon atoms.
The main molecular chain of an elementorganic polymer contains atoms of carbon and other inorganic elements of heteroatoms (except for nitrogen, oxygen and sulfur) which are directly connected with chain atoms.
Inorganic polymers are compounds which do not contain carbon atoms in macromolecules.
Organic polymers are subdivided into carbon-chain and heterochain. The main chain of carbon-chain polymers contains only carbon atoms. The chain composition of heterochain polymer macromoleculs contains heteroatoms (O,
a
•••••••XOxoo*«"«
N, S).
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The names for carbon-chain and heterochain polymers are formed from the names for the chemical class of monomers, constituents of macromolecules. For example, the class of carbon-chain polymers includes substances whose molecules are composed of the corresponding carbohydrate monomers (saturated or unsaturated), such as polyethylene, polypropylene, polyisobutylene, polybutadiene, etc; halogen-containing monomers - polyvinyl chloride, polychloroplen, etc., monomers, alcohol and aldehyde derivatives, polyvinylacetate, polymethylacrilate, etc.
The group of heterochain polymers includes polyesters, polyamides, polyacetal resins, polysulfides, etc.
The family of hetero-organic polymers incorporates polymers containing elements of all groups from the periodic system. Of the most practical significance are organisilicic polymers, with the molecules consisting of silicon and carbon atoms.
Carbohydrate macromolecules of organotitanic polymers contain titanium atoms, whereas organophosphorous polymers include phosphorous atoms. The group of organophosphorous polymers contains polyphosphazenes - polymers containing
I
phosphate groups—P = N.
I
The group of other elementoorganic polymers is formed by organoboron polymers, polymers containing Al and Sn as well as coordination polymers, with the molecules incorporating Fe, Co, Ni, Cu, Mn and Zn.
Inorganic polymers are subdivided into two groups.
The first group has mascromolecules consisting of homoatomic chains (i.e. those formed by the same atoms).
The second group has macromolecules with a heteroatomic structure, i.e. a combination of different atoms.
Polymerization is a process of chemical unification of polymer molecules which is not accompanied by release of low-molecular weight by-products. In this reaction, the element compositions of the monomer and the resulting polymer are the same.
Polycondensation is a method for polymer synthesis when the interaction of monomer molecules is generally accompanied by a release of collateral low-molecular-weight compounds. On polycondensation, the chemical composition of a
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polymer differs from the composition of initial monomers since this reaction gives rise to additional low-molecular-weight products (water, ammonia, hydrogen chloride etc.).
And at last the third method for polymer synthesis, the method of chemical modification, is realized by substitution of atoms of hydrogen or other elements by new atoms and groups in the main chain of a macromolecule or its branch. This is one of the promising methods for production of polymers with new properties.
Chlorination of polymers, which results in chlorinated polyethylene, chlorinated polyvinylchloride, etc., is widely used in chemical industry.
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