The physical and phase states in which the materials are located during operation have essential for their characteristics.
Physical states of polymers
The physical state of matter is determined by the packing density of atoms and molecules, which determines the nature of their thermal motion.
The states of a substance differ in its ability to have and maintain a given shape and volume at a constant temperature. Solid, liquid and gaseous states of low molecular weight substances are known. The transitions of substances from one state to another are accompanied by a change in many physical properties, which is explained by a change in the nature and level of thermal motion and the interaction of their molecules.
IN solid state, a substance is able to have a constant volume and retain the shape given to it; in liquid state, the substance also has a constant volume, but it is not able to maintain its shape, since it loses it even under the influence of the earth's gravity. Finally, in gaseous state, a substance is not capable of having either a constant volume or a constant shape.
Polymers can only exist in condensed states: solid and liquid.
The type of the physical state of the polymer depends on the ratio of the energies of intermolecular interaction and thermal motion. In cases where the energy of intermolecular interaction is much greater than the energy of thermal motion of macromolecules, the polymer is in a solid state. The liquid state is realized when both energies are comparable in their magnitude. In this case, the thermal motion of macromolecules is able to overcome the intermolecular interaction, and the polymer can exhibit the properties of a liquid.
The impossibility of the existence of polymers in the gaseous state is explained by the fact that the total energy of intermolecular interaction due to the large length of macromolecules is always higher than the energy of the strongest chemical bond in them. It follows from this that before the intermolecular interaction weakens so much that the polymer passes into a gaseous state, the chemical bonds within the macromolecule break and it degrades.
Another fundamental difference between polymers and other substances is their ability to exist in two solid states: glassy and highly elastic. The highly elastic state is present only in polymers; it is unknown for other materials.
Thus, polymers can exist in three physical states: vitreous, highly elastic And viscous. Transitions from one state to another occur in a certain temperature range (Fig. 2.1). For convenience, a fixed temperature is used, which is calculated from experimental data.
Rice. 2.1. Typical thermomechanical curve of a linear amorphous polymer: T s- glass transition temperature; T t- pour point; I, Nor III - temperature regions of three physical states (glassy, highly elastic and viscous, respectively)
Shown in fig. 2.1 the curve is called thermomechanical. It has three regions in which the state and behavior of the polymer are different: the region / corresponds to the glassy state, II - highly elastic and III - viscous state of the polymer. In each of these states, the polymer has its characteristic properties. The transition from the glassy to the highly elastic state occurs at the glass transition temperature T s, and the transition from a highly elastic state to a viscous flow - at a fluidity temperature T t. Glass transition and fluidity temperatures are the most important characteristics of polymers; at these temperatures, cardinal changes occur in most of their physical properties. Knowing these temperatures, it is easy to establish the temperature conditions for the processing and operation of polymeric materials. By purposefully changing them, it is possible to reduce the processing temperature or expand the temperature range in which the operation of products from this polymer is allowed.
Changes in the mechanical, electrical, thermophysical, and other properties of polymers at temperatures of transition from one state to another occur smoothly, which is explained by a gradual change in the interaction of sections of macromolecules: links, segments, blocks.
From fig. 2.1 it can be seen that above the pour point, the deformation of the polymer is very large, i.e., it flows like a liquid. As a rule, polymers are processed precisely in a viscous state or close to it.
The flow of polymers, like other processes, has its own characteristics that distinguish these materials from other substances. In contrast to low-molecular high-viscosity liquids, the viscosity of which does not change during flow, the viscosity of polymers increases during flow, which is associated with some straightening of chain macromolecules.
This phenomenon is widely used in the processing of polymers. Thus, the processes of fiber formation and the production of films from polymers under isothermal conditions are based on an increase in the viscosity of the polymer in the process of flowing through a spinneret.
The viscous-flowing state is a consequence of the intensification of the thermal motion of macromolecules with increasing temperature. As a result, at a certain temperature, it becomes possible for them to move relative to each other.
When the temperature of the polymer decreases below the pour point, it passes from the viscous to the highly elastic state. The process of deformation of polymers in the highly elastic state is reversible, and the amount of deformation does not depend on temperature. This property of polymeric materials is widely used. The most characteristic example of the use of the reversibility of the deformation of polymers and the independence of its magnitude from temperature is the widespread use of rubbers and rubbers. Their ability to large reversible deformations is well known.
The possibility of finding polymers in a highly elastic state distinguishes them from all other materials that cannot be in this state under any conditions.
It is no secret that other materials, such as plasticine, are also capable of large deformations. However, they all deform irreversibly. You can pull a rod out of a piece of plasticine, and it will retain its shape.
The polymer material in the highly elastic state can also be stretched, but after the load is removed, it will return to its original state, i.e., the polymer in the highly elastic state deforms reversibly. In this case, long chain macromolecules make a transition from one conformational state to another due to the movement of their individual sections.
Highly elastic deformation is a consequence of the flexibility of macromolecules and the mobility of their individual parts. The return of the polymer to its original state after removing the load occurs in a noticeable period of time, i.e., it can be observed and thus studied relaxation characteristics polymer.
In the highly elastic state, polymers have another feature that distinguishes them from all other solid materials. In this state, with increasing temperature, the elastic modulus of polymers increases, while for other materials it decreases. The fact is that due to the thermal motion of macromolecules and their links in the highly elastic state, they are twisted, which prevents the deformation of the polymer. This resistance is the greater, the higher the temperature, since with increasing temperature, the thermal motion of macromolecules becomes more intense.
The nature of the deformation of polymers in the highly elastic state depends on the rate of deformation, i.e., the rate of application of the load. Since the manifestation of high elasticity requires time to overcome the forces of intermolecular interaction, then at a high strain rate, high elasticity does not have time to manifest itself, and the material behaves like a glassy body. This must be taken into account when using polymers for the manufacture of products that must retain elasticity under operating conditions at dynamic loads and low temperatures.
When the temperature of the polymer is lowered below the glass transition temperature, no mechanical action is exerted on it, as can be seen from Fig. 2.1, deformation changes. At such a temperature, macromolecules are not capable of conformational changes, and the polymer loses its ability not only to viscous flow, but also to highly elastic deformation. This means that the polymer is in a glassy state.
The difference between the glass transition processes of polymers and low molecular weight substances should be noted. The glass transition of a low molecular weight liquid occurs when the entire molecule loses its mobility. For the transition of the polymer to the glassy state, it is sufficient to lose mobility even by segments of the macromolecule. In low molecular weight liquids, the glass transition and brittleness temperatures practically coincide, while in polymers they are different, which is explained by the preservation of some macromolecules of their mobility in the glassy state.
It is not uncommon for a polymer in a glassy state to be capable of significant deformations (sometimes up to several hundred percent). This is the so-called forced highly elastic deformation, it is associated with a change in the shape of flexible macromolecules, and not with their movement relative to each other. Such a deformation, being forced, disappears when the polymer is heated, when, at a temperature above the glass transition temperature, the mobility of macromolecules increases and they return to their original conformational state.
A comparison should be made between the forced elasticity of polymeric materials and the cold flow of metals. Both processes take place when the materials are in the solid state. However, a polymer sample that exhibited forced high elasticity restores its shape and dimensions upon heating. This is the basis for the creation of "smart" shape memory polymers. Unlike polymers, heating metals that have been drawn in a cold state, i.e., those that have exhibited cold flow, does not make it possible to restore their shape and dimensions.
It should be noted that for some polymers it is impossible to detect the pour point, and sometimes the glass transition temperature, since when heated, the thermal destruction of such polymers occurs before they have time to pass into a viscous-flow or highly elastic state. Such polymers can exist only in the glassy state. An example is the natural polymer cellulose, as well as a number of esters based on it (in particular, such a technically important one as nitrocellulose, which is the basis of ballistic powders).
Modern science makes it possible to control the glass transition and flow temperatures of polymers. Thus, the plasticization of nitrocellulose with the help of nitroglycerin reduces the glass transition and fluidity temperatures and creates conditions for processing this polymer into products of a given shape and size.
viscous state they are used primarily for the processing of polymers by extrusion, casting, pneumatic molding, etc. From the molecular-kinetic point of view (see subsection 4.2.2), in the viscous-flowing state, irreversible flow deformation develops in polymers, due to the mutual displacements of macromolecular coils. In practice, in addition to flow deformation in polymeric liquids or melts, there are highly elastic and elastic deformations, the flow of which during the processing of the polymer leads to a decrease in the dimensional stability of the final products and their consumer properties.
For amorphous polymers, the transition to a viscous-flow state is observed at the pour point T t(see subparagraph 4.2.3), the value of which depends on the molecular weight of the polymer (see figure 4.7). For a number of polymers, with an increase in molecular weight, the pour point begins to exceed the temperature of thermal degradation of the material, which makes it impossible to process it. A typical example is poly(methyl methacrylate), for which depolymerization processes already occur at temperatures around 200°C. In this regard, extrusion and casting methods are applicable only for polymethyl methacrylate with a sufficiently low (no more than 200,000) molecular weight, for which T t 200°C.
The physical and mechanical behavior of polymers in a viscous-flow state satisfactorily describes Newton's law (see expression (4.2)), in which the proportionality coefficient r) (viscosity) characterizes the resistance of the polymer to external force. In the general case, liquid media obeying Newton's law are called Newtonian. However, the behavior of melts of real polymers is more complex.
For a polymer in a viscous-flowing state, the dependence of viscosity on stress is shown in Fig. . 4.23. In regions I (region the greatest new-
Rice. 4.23
new viscosity) and III (area lowest Newtonian viscosity) the polymer flow obeys Newton's law (see equation (4.2)). In region II (region viscosity anomalies) viscosity is highly dependent on stress, i.e. to describe the viscous flow in this stress range, Newton's law is inapplicable.
The observed viscosity anomaly is associated with a complex of structural rearrangements caused by the applied stress. Such changes in the structure include, first of all, the destruction of the fluctuation network (see subparagraph 4.21), stabilized by intermolecular and intersegmental physical interactions. In other words, the region of the highest Newtonian viscosity corresponds to the flow of a "structured" polymeric fluid, and the region of the lowest Newtonian viscosity corresponds to the flow of a polymeric fluid with a destroyed fluctuation structure.
The dependence of viscosity on temperature is described by an exponential dependence taking into account the activation energy of the viscous flow E a. As the molecular weight increases M the activation energy of the viscous flow increases. However, when the critical value of the molecular weight, which is comparable with the size of the segment, is exceeded, the activation energy reaches the limit value and ceases to depend on the molecular weight. This behavior indicates that, during the flow, the mutual displacement of macromolecular coils or the relative displacement of their centers of mass is carried out by means of correlated displacements of polymer chain segments.
Naturally, the activation parameters of the elementary act of flow associated with the translational movement of the segments do not depend on the molecular weight of the macromolecule. However, the absolute value of viscosity significantly depends on it. For an irreversible displacement of the center of mass of a macromolecular coil, a coordinated displacement of a number of segments is necessary. The longer the chain, the greater the number of such movements is required for this.
Theoretical calculations and experimental data show that the overall dependence of viscosity on molecular weight is divided into two sections. At low molecular weights r ~ M. Upon reaching a certain critical value, the molecular weight has a stronger effect on the viscosity, u-M 3.5. One of the reasons for the observed behavior is that with an increase in the length of macromolecules, a network of entanglements is formed (see subsection 4.2.1) with the formation of a generalized coil.
Non-metallic materials
The structure and structure of polymers
Polymers are compounds in which a large number of identical or unequal atomic groups alternate more or less regularly, connected by chemical bonds into linear or branched chains, as well as into spatial networks.
Repeatedly repeating groupings are called monomer units, and a large molecule composed of units is called a macromolecule or polymer chain. The number of links in the chain is the degree of polymerization and is denoted by the letter “n”. The name of the polymer consists of the name of the monomer and the prefix "poly".
Polymers built from the same monomers are called homopolymers.
Compared with low molecular weight compounds, polymers have a number of features: they can only be in a condensed solid or liquid state; polymer solutions have a high viscosity; when the solvent is removed, the polymers are separated not in the form of crystals, like low molecular weight compounds, but in the form of films; polymers can be converted into an oriented state; many polymers are characterized by large reversible deformations, etc.
The specific properties of polymers are due to the peculiarities of their structure, the knowledge of the main parameters of which is necessary to create scientifically based methods for their regulation.
Types of macromolecules
The peculiarity of the properties of polymers is due to the structure of their macromolecules. According to the shape of macromolecules, polymers are divided into linear, branched, ladder, and network.
Linear macromolecules are long zigzag or spiral chains.
branched macromolecules are distinguished by the presence of side branches.
Stair macromolecules consist of two chains connected by chemical bonds.
Spatial polymers are formed by cross-linking macromolecules with each other in the transverse direction by chemical bonds.
Distinctive feature polymer molecules is flexibility. The flexibility of a chain is its ability to change its shape under the influence of the thermal movement of the links or the external field in which the polymer is placed. It characterizes the ability of polymers to crystallize, determines the melting temperature range, elastic, elastic and other properties.
According to their structure and relation to temperature, polymers are divided into thermoplastic and thermoset.
thermoplastic- polymers in which, when heated, no cross-linking of chemical bonds is formed and which, at a certain temperature, soften and pass from a solid to a plastic state.
thermoset- polymers that at the first stage of formation have a linear structure, and then due to the flow chemical processes form spatial networks, harden and pass into an infusible and insoluble state.
Synthetic polymers are obtained from low molecular weight substances (monomers) by polymerization, polycondensation, copolymerization reactions, as well as by chemical transformations of other natural and synthetic polymers.
Polymerization- the process of combining several monomers, which is not accompanied by the release of by-products and proceeds without changing the elemental composition. Polymerization produces such polymers as polyethylene, polystyrene, polyvinyl chloride, etc.
polycondensation- the process of combining several monomers, accompanied by the release of the simplest low molecular weight substances (H 2 O, Hcl, etc.). Polycondensation produces phenol-formaldehyde resins.
copolymerization- polymerization of two or more monomers of various structures. Copolymerization produces ethylene-propylene copolymers.
Phase states of polymers
Polymers can be in two phase states: crystalline and amorphous (liquid).
Polymers cannot be in the gaseous phase state, since the boiling point is much higher than the decomposition temperature.
crystalline the phase state is characterized by the presence of a three-dimensional long-range order in the arrangement of atoms and molecules. Long-range order - an order observed at distances exceeding the size of molecules by hundreds and thousands of times.
Liquid (amorphous) the phase state is characterized by the absence of a crystalline structure. In the amorphous state, short-range order is observed - an order that is observed at distances commensurate with the size of the molecules. Near a given molecule, its neighbors can be located in a certain order, but at a short distance this order is absent.
CRYSTAL STATE OF POLYMERS, a phase state characterized by the existence of a long-range three-dimensional order in the arrangement of atoms, links and chains of macromolecules. The possibility of transition to a crystalline state is inherent in stereoregularly constructed macromolecular chains with a degree of flexibility sufficient for a conformational rearrangement of the chain, leading to an ordered arrangement. A necessary condition for the transition is also the absence of bulky side substituents or side branches in the macromolecule. The presence of polar groups usually promotes the transition to the crystalline state due to increased intermolecular attraction. The crystalline state exists in such industrially important polymers as polyethylene, polypropylene, polyamides, polyethylene terephthalate, polytetrafluoroethylene, etc. Some polymers can go into a liquid crystalline state (see Liquid crystalline polymers).
The transition of polymers to a crystalline state - crystallization - occurs when polymer melts are cooled or during their precipitation from solutions, as well as during uniaxial tension of elastomers. Crystallization is a first-order phase transition with the temperature and heat of transition inherent in each polymer; these characteristics are determined by calorimetric methods. Crystallization from melts is carried out in a wide temperature range - from the glass transition temperature to the equilibrium melting temperature; the dependence of the rate of crystallization from the melt on temperature is expressed by a curve with a maximum.
During the crystallization of polymers, regions with a disordered (amorphous) structure are always preserved; therefore, the concept of the degree of crystallinity is used to characterize polymers. The degree of crystallinity shows the volume ratio of inseparable amorphous and crystalline phases, depends on the nature of the polymer and the structure of its chain, crystallization conditions and external influences. For example, the degree of crystallinity increases upon annealing of the polymer or upon uniaxial tension. The degree of crystallinity of polymers is usually 20-80% (less than 10% for polyvinyl chloride, about 80% for polyethylene). The degree of crystallinity is usually judged by the density of the polymer.
The simplest element of the polymer structure in the crystalline state is a crystalline cell (up to 5 nm in size). X-ray methods structural analysis make it possible to determine the cell parameters and the conformations of the macromolecules included in the crystal for all known polymers. The crystalline state of polymers is characterized by the possibility of polymorphism, i.e., depending on the conditions of crystallization, unit cells of various types are formed.
The crystalline state of polymers is characterized by a high degree of defectiveness of the crystals. The same macromolecular chain can enter both into crystallites - highly ordered crystalline regions up to 50 nm long, and into amorphous regions. In most cases, the polymer chains enter the crystallites in the form of a helix; the identity period may include several turns.
In the crystalline state of polymers, various supramolecular structures are formed. The most common lamellar and fibrillar structures. Lamellas (plates) are characterized by a folded conformation of macromolecules, polymer chains are located perpendicular to the surface of the lamella, and the thickness of the lamellas reaches 25–100 µm. Lamellar crystals are usually obtained by slow crystallization. A fibril is a supramolecular formation with alternating crystalline and amorphous regions in the form of a thread or ribbon up to 10 μm long and with a cross section approximately the same as the size of crystallites. Macromolecular chains are oriented parallel to the fibril axis. The fibrillar form of crystals is inherent in the oriented state of polymers and is characteristic of the secondary structure of some biopolymers.
More complex supramolecular structures of polymers, such as single crystals and spherulites, are built from lamellas and fibrils. Single crystals are formed during precipitation from dilute polymer solutions. Single crystals are usually built from lamellae 10–20 nm thick. The largest structural formations of polymers in the crystalline state (up to several mm in size) - spherulites - spherically symmetrical formations, are typical polycrystals, usually built from fibrils. Due to radial symmetry, spherulites have an anisotropy optical properties. Spherulites are usually formed during crystallization from highly viscous melts.
Some biopolymers can form globular crystals, in which the crystal lattice sites are formed by individual macromolecules in folded (globular) conformations.
The levels of ordering of polymers in the crystalline state are studied using electron microscopy or structural analysis methods, in particular, small and wide-angle scattering (waves of various lengths - from the X-ray to the optical range), which make it possible to estimate the sizes of various types of structural elements. To determine the spatial structure of macromolecules in the crystalline state of polymers, NMR, mechanical and dielectric spectroscopy methods are used.
The degree of crystallinity affects the physical properties of polymers (density, hardness, permeability, etc.). The properties of a polymer in a crystalline state are determined by a combination of properties inherent in its crystalline and amorphous phases. As a result, crystalline polymer materials have high strength along with the ability to large deformations.
Lit .: Wunderlich B. Physics of macromolecules. M., 1976. T. 1; Bartenev G. M., Frenkel S. Ya. Physics of polymers. L., 1990.
According to their physical state, polymers are divided into amorphous and crystalline. The amorphous state is characterized by the lack of order in the arrangement of macromolecules. Branched and reticulated polymers are generally amorphous.
Under the crystallinity of polymers understand the ordered arrangement of individual sections of macromolecules. Only stereoregular linear polymers have the ability to crystallize. The properties of crystalline and amorphous polymers differ significantly. For example, crystalline polymers, unlike amorphous ones, have a specific melting point. Amorphous polymers are characterized by a region of softening temperatures, i.e., a region of gradual transition from a solid to a liquid state. Thus, amorphous linear polymers, when heated, first soften, forming a viscous liquid. A further increase in temperature leads to destruction, the destruction of the polymer. For amorphous polymers, depending on the temperature (and the magnitude of mechanical stress), three physical (deformation) states are possible: glassy, viscous And highly elastic The practical use of polymers is determined by which of these states the given polymer is in at the temperature of its use. Glassy polymers are characterized by relatively small elastic (reversible) deformations (1–10%). Polymers in the glassy state are used in the production of plastics. The viscous-flowing state is usually realized at elevated temperatures and is used to process polymers into products. All rubbers are in a highly elastic state under operating conditions. This state is typical only for polymers capable of reversibly deforming by hundreds of percent.
Reticulated polymers sharply differ in properties from linear and branched ones. They do not crystallize, do not dissolve in solvents, do not melt without decomposition, and have high mechanical strength. This is explained by the fact that their macromolecules are connected by a large number of chemical bonds, the breaking of which requires a lot of energy.
Temperature effect
In relation to heating, polymers are divided into two groups - thermoplastic and thermosetting. Thermoplastic polymers are able to soften when heated and harden when cooled, retaining all of their physical and chemical properties. Such polymers include polyethylene, polystyrene, polyvinyl chloride, polyamides. Thermosetting polymers cannot be transferred to a plastic state, because. when heated, they either completely collapse, or they repolymerize with the formation of new, even stronger and more rigid structures.
Examples of thermoset polymers are phenol-formaldehyde, urea and polyester resins.
When using polymers and materials based on them, their mechanical properties are important. The mechanical strength of polymers increases with an increase in their molecular weight, during the transition from linear to branched and network structures. Stereoregular polymers are characterized by higher strength than irregular ones. An increase in strength occurs during the transition of the polymer to the crystalline state. Increase the mechanical strength of the polymer by introducing fillers (for example, chalk, carbon black, graphite, metal, etc.) and receive various plastics.
Methods for obtaining polymers
Polymers are obtained by polymerization, polycondensation, and also using chemical transformations of macromolecules.
Polymerization– this is a polymer formation reaction by sequential connection of monomer molecules to a growing chain by rearranging covalent bonds.
Polymerization is characteristic mainly for compounds with multiple (double or triple) bonds or cyclic compounds. In the process of polymerization, multiple bonds are broken or cycles are opened in monomer molecules, followed by the formation of chemical bonds between these molecules to form polymers. According to the number of monomers involved, they distinguish homopolymerization(polymerization of one type of monomer) and copolymerization(copolymerization of two or more different monomers). An example of a homopolymerization reaction is the production of Teflon (fluoroplast):
nCF 2 \u003d CF 2 (-CF 2 -CF 2 -) n
tetrafluoroethylene polytetrafluoroethylene
Copolymers combine the properties of polymers derived from each individual monomer. Therefore, copolymerization is an efficient method for the synthesis of polymers with desired properties.
Styrene-butadiene rubber is produced by a copolymerization reaction:
| 2 |
butadiene-1,3
styrene butadiene styrene rubber
polycondensation– this is the reaction of formation of polymers from monomers having two or more functional groups, accompanied by the release of low molecular weight products (H 2 O, NH 3 , HCl, etc.) due to these groups.
Polycondensation is the main method for the formation of natural polymers in vivo. During polycondensation, the chain grows gradually: first, the initial monomers interact with each other, then the resulting compound reacts with molecules of the same monomer, eventually forming a polymer, while (n-1) molecules of low molecular weight products are released from n monomer molecules.
Copolycondensation of hexamethylenediamine H 2 N–(CH 2) 6 –NH 2 and dibasic adipic acid HOOC–(CH 2) 4 –COOH gives an anide or nylon: nH 2 N–(CH 2) 6 –NH 2 + nHOOC–(CH 2 ) 4 –COOH →
→ [–NH–(CH 2) 6 –NH–CO–(CH 2) 4 –CO–] n + (n-1)H 2 O
Anid (nylon or perlon)
Polycondensation differs from polymerization in that it is based on a substitution reaction, and in the process of polycondensation, along with high-molecular compounds, low-molecular products are formed. As a result, the elemental compositions of the initial monomer and the resulting polymer differ by a group of atoms released as a low molecular weight product (in this example, H 2 O).
Polymer types
By origin, macromolecular substances are divided into natural, or biopolymers (proteins, nucleic acids, polysaccharides) and synthetic(polyethylene, phenolic resins).
According to the type of elements included in the composite link, organic, inorganic and organoelement polymers are distinguished.
organic polymers. Organic polymers, by origin, are divided into three groups:
– natural, are found in nature (natural rubber, starch, cellulose, proteins, nucleic acids);
– artificial, which are obtained by chemical modification of natural polymers (trinitrocellulose, acetate and viscose fiber, chlorinated natural rubber, rubber);
– synthetic obtained by synthesis (polyethylene, polypropylene, capron, polystyrene, phenol-formaldehyde resin).
According to the chemical composition of the main macromolecular chain, organic polymers are divided into homochain, whose chains are built from identical atoms, and heterochain, containing carbon atoms and atoms of other elements in the main chain, for example, polycarbonate [−O−R−O−CO−] n, cellulose (C 6 H 10 O 5) n, nylon [−NH−(CH 2) 5 −CO− ] n . Among homochain polymers, the most common carbon chain, whose macromolecule chains contain only carbon atoms, for example, polyethylene [-CH 2 -CH 2 -] n, polystyrene [-CH 2 -CH (C 6 H 5) -] n, polytetrafluoroethylene [-CF 2 -CF 2 -] n.
inorganic polymers. Many inorganic substances are polymers. All metals, some non-metals (plastic sulfur, black and red phosphorus, carbon in the form of diamond, graphite, charcoal and coal), silicic acids, silicates, aluminosilicates, silicon dioxide, polysilane, etc. have a polymeric structure. An important distinguishing property of many inorganic polymers is their thermal and chemical resistance. They can have a linear structure (plastic modification of sulfur chains ….- S-S -S -.... coiled into spirals), layered (mica, talc), branched or three-dimensional structure (silicates). Even more complex formations, zeolites - copolymers of silicates and aluminates of metals, form polyhedra containing cavities and channels inside, in which ions are located that can be exchanged for others (during water purification, for example).
organoelement polymers. These are polymers that in the main chain do not contain atoms of carbon, but of other elements (silicon, aluminum, oxygen, phosphorus). Side chains in such polymers are represented by organic radicals.
Biopolymers
Biopolymers are natural macromolecular compounds. These include polysaccharides, polyisoprenes, polypeptides, proteins, nucleic acids.
Polysaccharides – These are biopolymers whose macromolecules consist of monosaccharide residues. The most important representatives of polysaccharides are cellulose, starch, inulin, glycogen. Having general formula(C 6 H 10 O 5) n, polysaccharides differ in the structure of the macromolecule. Starch and glycogen consist of α-glucose residues, cellulose consists of β-glucose residues, and inulin consists of fructofuranose residues. Polysaccharides undergo hydrolysis under the catalytic influence of acids. The final product of the hydrolysis of starch, glycogen and cellulose is glucose, inulin - fructose. Cellulose is one of the most rigid-chain polymers, in which practically no flexibility of macromolecules.
Squirrels are biopolymers consisting of α-amino acid residues connected by peptide (amide) bonds. In protein molecules, the so-called peptide group of atoms −CO−NH− is repeated many times. Compounds consisting of many interconnected peptide units are called polypeptides. Accordingly, proteins are classified as polypeptides. The number of amino acid residues included in the peptide chain is very large, so the molecular weights of proteins can reach several million. Common proteins include hemoglobin (in human blood), casein (in cow's milk), albumin (in chicken eggs).
Proteins are the most important biological substances: They are necessary for the life of organisms. The synthesis of proteins in the body is carried out through polycondensation reactions:
nH 2 N-CHR-COOH ↔ [-NH-CHR-CO-] n + (n-1)H 2 O. When two molecules of α-amino acids interact, a reaction occurs between the amino group of one molecule and carboxyl group– the other, which leads to the formation of water.
According to their composition, proteins are divided into simple (proteins) and complex (proteins). During the hydrolysis of simple proteins, only α-amino acids are formed, during the hydrolysis of complex proteins, α-amino acids and non-protein substances are formed.
There are four levels of structure in proteins:
– primary structure proteins is the structure of the peptide chain, i.e. a set of amino acid residues and the sequence of their connection with each other in a protein molecule.
– secondary structure is determined by the peculiarities of the twisting of the polypeptide chains of protein molecules into a helix due to the occurrence of hydrogen bonds between the −СО− and NH− groups.
– tertiary structure is determined by the spatial arrangement of protein helices due to the occurrence of hydrogen, amide and disulfide bonds.
– Quaternary structure is determined by the spatial arrangement of macromolecules, which include several polypeptide chains.
Nucleic acids – natural biopolymers built from monomers: nucleotides repetitive nucleic acid fragments. Nucleotides are composed of three components: heterocyclic bases, monosaccharides, and phosphoric acid residues, by which mononucleotides are linked together in a polymer molecule. There are two types of nucleic acids: ribonucleic acids (RNA) and deoxyribonucleic acids (DNA). All living organisms necessarily contain both types of nucleic acids.
Application of polymers
One of the important applications of polymers is the manufacture of fibers and fabrics. The classification of fibers is shown in the diagram:
Fibers for the production of which are used chemical methods, constitute a group of chemical fibers. They are divided into artificial and synthetic. Artificial fibers are obtained by chemical modification of natural materials (cotton, wool), while only synthetic materials - polymers - are used for the production of synthetic fibers. The most important synthetic fibers are lavsan And nylon.
Lavsan obtained by polycondensation of ethylene glycol and terephthalic (benzene-1,4-dicarboxylic) acid:
The resulting linear polymer is a polyester, the elementary unit of which is as follows:
Fiber made from lavsan (other names for this polyester are terylene, dacron) has good strength, heat resistance, and is resistant to dilute acids and alkalis.
Nylon – polyamide fiber, which is obtained by poly-
condensation of hexamethylenediamine H 2 N (CH 2) 6 NH 2 and adipic acid HOOC (CH 2) 4 COOH:
The elementary link of nylon has the form:
Nylon and other polyamide fibers are characterized by high strength and abrasion resistance. Their disadvantages are high electrification and instability when heated. Therefore, nylon clothing should not be ironed with a hot iron.
Rubbers
natural rubber. Natural rubber is obtained from latex, the juice of some tropical plants. Its structure can be determined by chemical properties: rubber adds bromine, hydrogen bromide and hydrogen, and when heated without air, it decomposes to form isoprene (2-methylbutadiene). This means that rubber is an unsaturated polymer - polyisoprene.
The molecular weight of rubber varies from 100 thousand to 3 million. Each elementary unit in polyisoprene can exist in cis- and trans-forms. In natural rubber, almost all links have a cis configuration:
This means that natural rubber has a stereoregular structure, which determines its valuable properties.
The most important physical property of rubber is elasticity, i.e., the ability to reversibly stretch under the action of even a small force. Another important property is impermeability to water and gases. The main disadvantage of rubber is its sensitivity to high and low temperatures. When heated, the rubber softens and loses elasticity, and when cooled, it becomes brittle and also loses elasticity.
To reduce plasticity and increase strength, wear resistance, resistance to aggressive environments, rubber is subjected to vulcanization by heating in the presence of sulfur with various fillers (soot, chalk, zinc oxide, etc.). During vulcanization, linear rubber macromolecules are crosslinked by disulfide bridges (–S–S–) and a spatial polymer is formed – rubber.
Rubber has a branched spatial structure and is therefore less elastic than natural rubber, but has a much greater strength. The production of rubber is based on the processes of polymerization and vulcanization.
Synthetic rubbers
The first synthetic rubber was obtained in Russia in 1931 by Professor S.V. Lebedev by polymerization of butadiene obtained from ethyl alcohol by a radical mechanism in the presence of metallic sodium:
Butadiene rubber has good water and gas impermeability, but is less elastic than natural rubber because it has an irregular structure. In its chain, cis- and trans-units are distributed randomly. In addition, polymerization proceeds not only as a 1,4-, but also as a 1,2 addition, and a polymer with a branched structure of the type
Technologies have been developed for the production of synthetic isoprene and butadiene rubber with a linear stereoregular structure (the latter is called divinyl). Some synthetic rubbers are made using a copolymerization process. For example, styrene-butadiene rubber is synthesized by the reaction
The advantage of the copolymerization method is that, by varying the ratio between the components, it is possible to control the properties of the rubber.
plastics
plastics Polymer-based materials are called materials that can change their shape when heated and retain a new shape after cooling. Due to this property, plastics are easily machined and used to produce products with a given shape. In addition to polymers, plastics include plasticizers, dyes, and fillers that improve the physical and mechanical properties of polymers. Plastics come in two main types: thermoplastic And thermosetting.
thermoplastic Plastics can change their shape many times when heated and then cooled. These include polymers with linear chains. The ability of such polymers to soften when heated is due to the lack of strong bonds between different chains. These are materials based on polyethylene, polytetrafluoroethylene, polyvinyl chloride, polyamides, polystyrene and other polymers.
thermoset When heated, plastics also change their shape, but at the same time they lose plasticity, become hard and can no longer be further processed. This is due to the fact that as a result of the formation of cross-links, a grid spatial structure is irreversibly formed, which cannot be turned into a linear one. Thermosetting polymers are used as the basis for adhesives, varnishes, ion exchangers, and plastics. Plastics based on phenol-formaldehyde resins are called phenolic plastics, those based on urea-formaldehyde resins are called aminoplasts. The fillers in them are paper, cardboard, fabric (textolite), quartz and mica flour, etc. Phenoplasts are resistant to water, solutions of acids, salts and bases, organic solvents, slow-burning, weather-resistant, and are good dielectrics. Aminoplasts, in addition to the listed properties, are resistant to light and UV, can be dyed in different colors. Therefore, plastics are widely used in electrical engineering. electronics, mechanical engineering, automotive, construction and the scope and scope of their application is constantly increasing.
Examples of problem solving
Example 15.1. What group of atoms is the structural unit of the polyethylene macromolecule? Write the reaction for obtaining a polymer. Calculate the molecular weight of the polymer if it is known that N polymer molecules have a molecular weight of 28,000 and 3N molecules have a molecular weight of 140,000.
Find the number average degree of polymerization.
Solution. Polyethylene is produced by the ethylene homopolymerization reaction:
nCH 2 \u003d CH 2 → (-CH 2 -CH 2 -) n
ethylene polyethylene
In the process of polymerization, multiple bonds are broken in the molecules of the monomer - ethylene, and chemical bonds are formed between the molecules, which leads to the formation of macromolecules.
Structural unit of the polyethylene macromolecule: -CH 2 -CH 2 -.
We find the average (numerical) value of the molecular weight of the polymer:
We find the relative molecular weight of the structural unit: M(C 2 H 4)=28.
The number average degree of polymerization n cf in this case is equal to.