Polymer chain flexibility
The physical properties of a substance depend on its chemical structure. The relationship between the physical properties of polymers and their chemical structure is very complex and manifests itself, among other things, through flexibility macromolecules, determined by their chemical structure and macrochain length. A macromolecule acquires flexibility when its MM becomes equal to or exceeds the value of the Kuhn segment (M k). With MM < М To - the macromolecule acts as a rigid rod; at MM >> MTo - the macromolecule becomes flexible. It is capable of changing its geometric shape, capable of folding or folding. This quality is characteristic of the vast majority of thermoplastics.
The flexibility of a polymer chain is not associated with changes in bond angles or distances between its constituent atoms, but is determined by the ability of atoms or atomic groups to rotate around the connecting them chemical bonds. In real chain polymer molecules, the rotation of atoms or atomic groups is not free, since the position of each subsequent link in the main chain turns out to be dependent on the position of the previous one. This retardation of rotation leads to the fact that the potential energy of the macromolecule continuously changes, and each of its values corresponds to a certain shape of the macromolecule.
The energy required for a molecule to move from a position with a minimum reserve of potential energy to a position with its maximum value is called potential barrier to internal rotation.
A change in the shape of molecules under the influence of thermal motion (or under the influence of an external field), not accompanied by the breaking of chemical bonds, is called conformational transformation, the new forms of the molecule themselves - conformations.
Thermal movement and rotations around bonds in polymers are performed not only by atoms and atomic groups, but also by individual sections of macromolecules without changing the location of more distant sections of the chains. Thus, macromolecules are capable of changing their geometric shape, bending, twisting and unfolding, in accordance with random thermal impulses acting on individual sections of the macromolecule. Such movements usually occur in melts and solutions. The dimensions of the moving sections of macrochains are not strictly defined. In Fig. Figure 1.3 schematically shows a part of a flexible macromolecule, to different parts of which unequal thermal pulses are applied. They cause movement of sections of different lengths. The average section of a macromolecule that moves as a whole in an elementary act of thermal motion is called segment.
Very small intramolecular interactions and energies of conformational transitions (4.2-25.1 kJ/mol) make it possible to classify nonpolar polyethylene, polypropylene, and polyisobutylene as flexible polymers the statistical segment of which is 10-40 elementary units. The introduction of polar substituents into macromolecules leads to an increase in intra- and intermolecular interactions, therefore polyvinyl chloride and polyvinyl alcohol are rigid-chain polymers. The statistical segment of such polymers can reach 100 or more repeating units. Polyesters and polyamides are rigid-chain, as well as polyimides, cellulose and polysaccharides containing atoms capable of forming strong intermolecular hydrogen bonds. The rigidity of the chains increases if the macromolecules contain large volume and mass substituents. Conformational transitions in such macromolecules require significant energy and are long-lasting. At low temperatures they are practically absent, and at high temperatures they appear due to an increase in the overall kinetic flexibility of the chains.
The presence of chemical bonds between macromolecules significantly limits their flexibility. In network and densely networked polymers with a developed spatial structure, the flexibility of the chains degenerates.
Physical and phase states of polymers
There are three main known physical condition substances - solid, liquid and gaseous. This classification is based on the ability of bodies to maintain their volume and shape, as well as the ability to resist external forces. The chain structure (MM) and strong intermolecular interaction of macromolecules are the reason that polymers can only exist in a liquid or solid state.
From a thermodynamic point of view, there are phase states substances.
Polymers can contain both crystalline and amorphous phases. There is a direct connection between the phase state and the structure of a substance.
Crystalline polymers can be in solid and liquid (liquid crystalline) states. Amorphous polymers, in addition to solid and liquid states, can be in a specific - highly elastic condition.
A polymer changes from one physical state to another when the temperature changes, which affects the thermal energy reserve of macromolecules and causes changes in the mechanical and deformation properties of polymers.
All physical states of amorphous polymers can be observed by analyzing thermomechanical curves, showing the dependence of the deformation of a loaded polymer on temperature (Fig. 1.4). Each physical state has its own nature and characteristics.
Amorphous state of polymers
Glassy state An amorphous polymer is usually compared to the state of a supercooled liquid, the high viscosity of which prevents its free flow, turning it into a solid physical body. The glassy state of polymers is observed when their segments are “frozen,” that is, devoid of mobility. This can be achieved by lowering the temperature. During glass transition, no new types of bonds arise between macromolecules. In the hardened polymer, short-range order is observed in the arrangement of individual parts of macromolecules.
Glassy polymer - this is a solid, brittle material, in the macromolecules of which only atoms or their groups perform oscillatory movements near equilibrium positions.
With increasing temperature, the influx of thermal energy may be sufficient to begin the movement of larger fragments - segments. Outwardly, this is manifested in the fact that there is a gradual transition from the properties of a hard, brittle material to the properties of a softer plastic body. The average value of a certain temperature range in which segmental mobility of macromolecules occurs is called glass transition temperature TWith(see Fig. 1.4). Since chain flexibility and segment size are interrelated and depend on intra- and intermolecular interactions in the polymer, the factors responsible for its increase will increase TWith and, conversely, TWith will shift to lower temperatures as intermolecular forces weaken:
For linear polymers, the glass transition temperature depends on the molecular weight, increasing with its growth. In network polymers, the formation of a cross-linked structure leads to an increase TWith, the larger the denser the spatial grid.
The glass transition process is accompanied by a change in many properties of the polymer - thermal conductivity, electrical conductivity, dielectric constant, refractive index, and these properties change abruptly with TWith.
When the temperature drops below TWith in the polymer, a further decrease in the thermal motion of the kinetic fragments of macromolecules is observed. To now cause even slight deformation of the vitrified polymer, it is necessary to apply a large mechanical load to it. In this case, the polymer behaves like an elastic or elastic-viscous body. With a further decrease in temperature, the polymer collapses as a brittle body with practically disappearing deformation. The temperature at which brittle fracture of a polymer occurs is called brittleness temperature T xr . Polymers, as a rule, are operated in a glassy state, which corresponds to section I on the thermomechanical curve.
Highly elastic state (WPP) The polymer is characterized by relatively high mobility of segments of macromolecules. It manifests itself only when the macromolecules have a significant length (high molecular weight) and is especially characteristic of flexible-chain polymers, characterized by small intermolecular interaction forces.
In the case of significant intermolecular interactions (dipoles, hydrogen bonds), HEAs are observed at elevated temperatures, that is, when the effect of intermolecular forces weakens. The relative ease of a macromolecule adopting a wide variety of conformations under the influence of external mechanical stress explains the large deformations above TWith(hundreds of percent). After the load is removed, due to the thermal movement of the segments, the macromolecules return to their original conformations and the achieved highly elastic deformation disappears, that is, it is reversible. If the process of deformation of a linear polymer is carried out slowly, so that the macromolecules have time to move from one equilibrium conformation to another, instead of a highly elastic state, the polymer will find itself in a viscous flow state.
In thermoplastics, a highly elastic state is observed in the temperature range T c - TT, Where TT- pour point(melting) of the polymer (Fig. 1.4, section II).
IN viscous-flow state A thermoplastic polymer is a liquid and is capable of irreversibly flowing under the influence of relatively small external forces, that is, exhibiting plastic deformation. During flow, individual macromolecules move relative to each other. Deformation in a viscous-flow state can develop indefinitely and is irreversible. The viscous-flow state corresponds to section III in Fig. 1.4.
Some network polymers are also capable of transforming into HEAs. However, as the temperature rises above TWith they first soften and then are irreversibly destroyed.
Crystalline state of polymers
Many thermoplastic polymers can exist in a crystalline state. Thus, polyethylene, polypropylene, and polyamides can form microscopic crystals.
Polymers transform into a crystalline state from a liquid (melt, solution) as the temperature decreases. Crystallization occurs as a result of fixation of the position of individual segments and the appearance of elements of long-range three-dimensional order in their location.
To carry out the crystallization process in polymers, certain conditions must be met, each of which is necessary but not sufficient.
Firstly, to build a crystalline structure, it is necessary that the polymer molecules be regular, that is, they have a linear chain structure with a certain alternation of links and their uniform arrangement in space relative to the main chain.
Secondly, during a phase transformation, the mutual stacking of chains or segments should occur according to the principle of close packing. Packing coefficients (the ratio of the intrinsic volume of macromolecules to the true volume of the body) for most crystallized polymers lie in the range of 0.62-0.67 and are close to the packing coefficients of conventional solids. It is clear that close packing is difficult for macromolecules containing branches and bulky side substituents, which create steric hindrances.
Thirdly, for crystallization to occur, the polymer molecules must have a certain mobility so that the chains can move and fit into the crystal structure. In practice, crystallization can occur near and below the melting temperature Tmelt. Liquid crystalline polymers retain their crystalline organization even when T>Tmelt.
But even when all these conditions are met, polymers are not completely crystalline.
Along with crystalline ones, polymers always contain amorphous regions, which is why they are often called crystallizing. Thus, the content of the crystalline phase in high-density polyethylene reaches 75-90%, and in low-density polyethylene does not exceed 60%. Crystal structures, in turn, are always morphologically defective.
Unlike low-molecular compounds, the melting of polymers does not occur at a specific temperature, but in a temperature range determined by their chemical structure, molecular weight, and kinetic characteristics. Some average temperature of this interval is taken as the melting temperature.
The degree of crystallinity, the morphology of crystal structures and the melting temperature range of the polymer are associated with the temporary, relaxation nature of the crystallization process.
If the temperature is lowered slowly, more diverse crystal structures are formed.
Below are the average melting temperatures of some polymers:
From these data, in particular, it is clear that Tpl increases with an increase in the polarity of the elementary units of polymers, the regularity of their structure, and with a decrease in the flexibility of macromolecules.
Supramolecular structure of polymers
Supramolecular structure(NMS) reflects the physical organization of polymer macrochains and is characteristic of all polymers, regardless of their physical and phase state. The reason for the occurrence of NMS is the intermolecular interaction of macrochains. Morphologically, the supramolecular structure of polymers is complex, spatially distinct aggregates of different sizes and shapes, created by the stacking of macromolecules in a certain way. The creation of supramolecular structures reveals a fundamental property of a flexible chain - the ability to fold into folds (packs) or roll up into balls “on themselves.”
Flexible macromolecules can take the form of coils. The stability of this form is determined by the lowest values of surface and surface energy. A coil consists of one or more macromolecules, with individual sections of the chain inside it arranged randomly. This supramolecular organization is typical of most amorphous polymers and is formed during their preparation.
In polymers with MW > 10 4, structures that usually arise at the melt or solution stage as a result of the action of intermolecular forces either during the folding of one macromolecule or its segments, or during the bringing together of linear fragments of neighboring macromolecules, are also widespread. Folded formations (packs) can form larger and morphologically complicated structural aggregates - fibrils(Fig. 1.5, a, b). In synthesized polymers bundle-fibrillar structure(Fig. 1.5, V) precedes the formation of more developed supramolecular structures - lamellas(Fig. 1.5, d).
Depending on the crystallization conditions, the supramolecular structure can remain fibrillar or transform into lamellar(plate) or spherulitic(Fig. 1.6). The latter arise from fibrils that develop from one center in the shape of a sphere and are held by the so-called pass-through chains, that is, sections of macromolecules that are part of neighboring spherulites. Run-through chains form amorphous regions in a crystalline polymer. Spherulites can be created not only by laying fibrils, but also by lamellae.
The density of the polymer in crystals, due to the denser packing of macromolecules, turns out to be higher than in interstructural zones filled with disordered running macrochains, and higher than in amorphous regions. The average density of some polymers (p), the density of the crystalline (p cr) and amorphous (p am) components are given below (in kg/m 3): 
The higher the content of the crystalline phase in the polymer, the higher its Tm, resistance to impact loads, strength and deformation characteristics. Thus, polyethylene, being a crystalline polymer, depending on the amount of crystalline (or, conversely, amorphous) phase it contains, is known as HDPE and LDPE. 
The structure formation of crystalline polymers is usually limited to spherulites. Therefore, the crystalline phase state usually corresponds to a spherulitic structure. However, under special conditions, even single crystals can form in crystalline polymers (for example, in polyethylene, when crystallization is carried out from very dilute solutions and very slowly).
The type and size of supramolecular structures of polymers are determined using electron and optical microscopy, X-ray diffraction analysis and other methods. The smaller and more uniform in size the structure, the higher the physical and mechanical properties the polymer exhibits (Table 1.1).
NMS is also formed in network polymers. Morphologically, it is represented by isometric formations (globules) with a denser spatial network. The crosslinking density between the globules is lower, so the destruction of such polymers also occurs along the boundaries of the NMS.
Plasticization of polymers
Plasticization is a physicochemical process consisting of a change in the intermolecular interaction of macrochains due to the filling of the nanospace between them with organic substances, called plasticizers, if they reduce the interaction forces, and antiplasticizers, if the effect is the opposite. The effect of plasticization is determined not only by the supramolecular organization of the polymer, but also by the chemical properties of the components of this process.
Plasticizers in most cases are low molecular weight liquid organic compounds with a high boiling point and low vapor pressure. They are easily combined with polymers without, as a rule, entering into chemical reactions with them. The plasticizer molecules push the polymer chains apart and surround them, creating an intermediate layer. The appearance of an intermediate layer in the polymer facilitates the movement of chains, as a result of which the melting point always decreases and the plasticity (fluidity) of the polymer increases, which facilitates the technology of its processing. The amount of plasticizer that is added to the polymer is limited by the mutual solubility of these substances. If solubility is unlimited, a thermodynamically stable system is formed. If the solubility is very low or if the plasticizer is not combined with the polymer at all, then a colloidal system is formed that can be destroyed over time due to migration (“sweating”) of the plasticizer onto the surface of the polymer.
Surrounding macromolecules, the plasticizer screens certain groups in them. This affects the intramolecular interaction of the units of each macromolecule and their potential barriers to internal rotation. If the potential barrier to rotation decreases, the polymer chains become more flexible. As a result of plasticization, the ability of the material to large highly elastic and forced elastic deformations increases, frost resistance increases, but, as a rule, the elastic modulus and strength decrease, heat resistance decreases, etc.
Typically, the amount of plasticizer is several tens of percent by weight of the polymer. Both individual organic compounds (for example, esters) and various technical mixtures (for example, petroleum and mineral oils with varying contents of aromatic, naphthenic and paraffin hydrocarbons) are used as plasticizers. For most polymer materials, mixtures of two or three or more plasticizers of various types are used simultaneously. The most important plasticizers for plastics include esters of orthophthalic and sebacic acids, esters of phosphoric acid, polyesters, epoxidized compounds, etc. (Table 1.2).
A variation of this process is the so-called structural plasticization, when the effect is achieved by introducing very small amounts (up to 1%) of a plasticizer into the polymer. In this case, its molecules, as a rule, are located at the interfaces of the supramolecular structures of the polymer, which causes, as happens with conventional plasticization, a change in the physical and physicomechanical properties of the polymer.
LECTURE 3. Preparation of polymers. Polymerization. Radical polymerization. Ionic polymerization. Cationic polymerization. Anionic polymerization. Coordination-ionic (stereospecific) polymerization. Step polymerization
ORIENTED STATE OF POLYMERS
the state of bodies made of linear polymers, characterized by the fact that the axes of fairly extended straightened sections of chain macromolecules that make up these bodies are located predominantly. along certain directions - orientation axes. So, in polymer films Types of planar orientation can be realized: biaxial, radial. The simplest and most A common type of orientation for linear polymers is uniaxial orientation.
Landmark. widely distributed in plants. the world (eg cotton, flax) and animals (tendons, muscle tissue, wool, etc.). Almost everywhere in nature where strong and flexible structural elements are required, they are formed from landmarks. polymers.
A landmark in technology. receive basically orientation by stretching (by tens to thousands of percent) isotropic polymer bodies heated above the glass transition temperature. As a result, chain macromolecules, chaotically (statistically) oriented in the original body, under the influence of external influences. directional tensile force acquire one or another degree of orientation. In an amorphous flexible chain polymer, a landmark. the state is nonequilibrium and, in order to fix it, it is necessary to cool the polymer below the glass transition temperature without removing the tensile stress. In the case of flexible-chain crystallizing polymers, O. s. The point can be considered equilibrium below the melting temperature of the crystallites and the removal of tensile stress at the drawing temperature does not lead to misorientation, since the crystallites form a landmark. frame that preserves the amorphous sections of the polymer body in O. s. p.
Upon receipt of the orientation. flexible-chain polymers, using a two-step method, first the solution or polymer melt is oriented. This is achieved by creating flows with velocity gradients (transverse or longitudinal), as a result of which long chain molecules are oriented preferentially. along the direction of flow. What happens in this case fixes the achieved state, which leads to the formation of a landmark. polymer. Afterbirth. Solid phase drawing brings the polymer material (or product) to an ultra-high orientation. condition.
For rigid chain polymers O. s. the point is equilibrium and is achieved by a two-stage method: first, at a relatively moderate temperature, a landmark is formed by pulling it out of the solution. "workpiece", followed by heat treatment at elevated temperatures. t-re, leading to means. increasing orientation order in the polymer (a phenomenon such as directional crystallization).
Landmark. polymers contain characteristic supramolecular formations - fibrils - with a transverse size of ~ 10-100 nm and a length of at least ~ 1-10 µm.
Single-axis reference. polymer bodies are characterized by high anisotropy mechanical, acoustic, optical, electrical. and other saints. Therefore, anisotropy-sensitive methods (eg, diffractometry, NMR, ESR, IR, acoustic spectroscopy, birefringence measurements) are effective in studying landmarks. polymers. The latter are also characterized by a characteristic thermal anomaly. extensions: negative coefficient expansion along the orientation axis. This is due to transverse vibrations of straightened sections of chain molecules, the amplitude of which is much greater than longitudinal vibrations, as well as conformation. "twisting" guideline. areas of macromolecules in amorphous regions, which leads to a reduction in the size of these regions along the polymer orientation axis. Important technical holy landmark polymers - increased in tension and rigidity along the orientation axis while maintaining sufficient flexibility. This is due to the fact that the heads work along the orientation axis. arr. chem. bonds in the perpendicular direction are intermolecular. Yes, theoretically. the values and modulus of longitudinal elasticity for the fiber are respectively. 20-30 and 250 GPa; for tech. landmark. polymer fibers 0.5-1.0 GPa, 20-50 GPa; for highly oriented fibers 5-10 GPa, 100-150 GPa, which is close to the theoretical. values and is a big tech. achievement.
Tall fur. characteristics combined with low density, chemical. and thermal resistance (this is what distinguishes rigid-chain polymers; they contain cyclic groups in the main chains of macromolecules) determine the increasingly widespread use of landmarks. polymer fibers: cables, ropes, fabrics, reinforcing elements in various compositions. materials, etc. In technology, for example, polyamide, polyolefin, polyester, polyimide, and polyacrylonitrile fibers are widely used. See also Chemical fibers, Molding of chemical fibers.
Lit.:
Marikhin V. A., Myasnikova L. P., Supramolecular structure of polymers. L., 1977; Ultra-high modulus polymers, ed. A. Ciferri, I. Ward, trans. from English. L., 1983. A. I. Slutsker.
Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .
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Polymers (from the Greek polymeres - consisting of many parts, diverse), chemical compounds with a high molecular weight (from several thousand to many millions), the molecules of which (macromolecules) consist of a large number of repeating groups (monomeric units). The atoms that make up macromolecules are connected to each other by forces of principal and coordination valences.
Based on their origin, polymers are divided into natural (biopolymers), such as proteins, nucleic acids, natural and synthetic resins, for example polyethylene, polypropylene, phenol-formaldehyde resins. Atoms or atomic groups can be located in a macromolecule in the form of: an open chain or an elongated sequence of cycles; chains with branching; three-dimensional grid. Polymers whose molecules consist of identical monomer units are called homopolymers, for example polyvinyl chloride, polycaproamide, cellulose.
Macromolecules of the same chemical composition can be built from units of different spatial configurations. If macromolecules consist of the same stereoisomers or of different stereoisomers alternating in the chain at a certain periodicity, the polymers are called stereoregular.
Polymers whose macromolecules contain several types of monomer units are called copolymers. Copolymers in which units of each type form sufficiently long continuous sequences that replace each other within the macromolecule are called block copolymers. One or more chains of another structure can be attached to the internal (non-terminal) links of a macromolecule of one chemical structure. Such copolymers are called graft copolymers.
Polymers in which each or some stereoisomers of a unit form sufficiently long continuous sequences that replace each other within one macromolecule are called stereoblock copolymers.
Depending on the composition of the main chain, polymers are divided into: heterochain, the main chain of which contains atoms of various elements, most often carbon, nitrogen, silicon, phosphorus, and homochain, the main chain of which is built from identical atoms. Of the homochain polymers, the most common are carbon chain polymers, the main chains of which consist only of carbon atoms, for example polyethylene, polymethyl methacrylate, polytetrafluoroethylene. Examples of heterochain polymers are polyesters (polyethylene terephthalate, polycarbonates, etc.), polyamides, urea-formaldehyde resins, proteins, and some organosilicon polymers. Polymers whose macromolecules, along with hydrocarbon groups, contain atoms of inorganogenic elements are called organoelement. A separate group of polymers is formed by inorganic polymers, for example plastic sulfur and polyphosphonitrile chloride.
Linear polymers have a specific set of physicochemical and mechanical properties. The most important of these properties: the ability to form high-strength anisotropic highly oriented fibers and films; ability to large, long-term reversible deformations; the ability to swell in a highly elastic state before dissolving; high viscosity of solutions. This set of properties is due to the high molecular weight, chain structure, and flexibility of macromolecules. When moving from linear chains to branched, sparse three-dimensional networks and, finally, to dense mesh structures, this set of properties becomes less and less pronounced. Highly cross-linked polymers are insoluble, infusible and incapable of highly elastic deformations.
Polymers can exist in crystalline and amorphous states. Prerequisite crystallization - regularity of sufficiently long sections of a macromolecule. In crystalline polymers, various supramolecular structures can arise, the type of which largely determines the properties of the polymer material. Supramolecular structures in non-crystallized polymers are less pronounced than in crystalline ones.
Uncrystallized polymers can be found in three physical conditions: glassy, highly elastic and viscous. Polymers with a low (below room) temperature of transition from a glassy to a highly elastic state are called elastomers, while those with a high temperature are called plastics. Depending on the chemical composition, structure and relative arrangement of macromolecules, the properties of polymers can vary within very wide limits. Thus, 1,4-cis-polybutadiene, built from flexible hydrocarbon chains, at a temperature of about 20C is an elastic material, which at a temperature of -60C transforms into a glassy state; polymethyl methacrylate, built from more rigid chains, at a temperature of about 20C is a solid glassy product that turns into a highly elastic state only at 100C. Cellulose, a polymer with very rigid chains connected by intermolecular hydrogen bonds, generally cannot exist in a highly elastic state before its decomposition temperature. Large differences in the properties of polymers can be observed even if the differences in the structure of macromolecules are, at first glance, small. Thus, stereoregular polystyrene - crystalline substance with a melting point of about 235C, and non-stereoregular (atactic) is not able to crystallize at all and softens at a temperature of about 80C. Polymers can enter into the following main types of reactions: the formation of chemical bonds between macromolecules (so-called cross-linking), for example, during the vulcanization of rubbers and tanning of leather; decomposition of macromolecules into separate, shorter fragments, reactions of side functional groups of polymers with low molecular weight substances that do not affect the main chain (so-called polymer-analogous transformations); intramolecular reactions occurring between functional groups of one macromolecule, for example intramolecular cyclization. Cross-linking often occurs simultaneously with destruction. An example of polymer-analogous transformations is the saponification of polyvinyl acetate, leading to the formation of polyvinyl alcohol. The rate of reactions of polymers with low molecular weight substances is often limited by the rate of diffusion of the latter into the polymer phase. This is most obvious in the case of cross-linked polymers. The rate of interaction of macromolecules with low-molecular substances often significantly depends on the nature and location of neighboring units relative to the reacting unit. The same applies to intramolecular reactions between functional groups belonging to the same chain.
Some properties of polymers, such as solubility, viscous flow, and stability, are very sensitive to the action of small amounts of impurities or additives that react with macromolecules. Thus, in order to transform a linear polymer from soluble to completely insoluble, it is enough to form 1-2 cross-links per macromolecule. The most important characteristics of polymers - chemical composition, molecular weight and molecular weight distribution, degree of branching and flexibility of macromolecules, stereoregularity, etc. The properties of polymers significantly depend on these characteristics.
Natural polymers are formed during the process of biosynthesis in the cells of living organisms. Using extraction, fractional precipitation and other methods, they can be isolated from plant and animal raw materials. Synthetic polymers are produced by polymerization and polycondensation. Carbochain polymers are usually synthesized by polymerization of monomers with one or more multiple carbon-carbon bonds or monomers containing unstable carbocyclic groups. Heterochain polymers are obtained by polycondensation, as well as polymerization of monomers containing multiple carbon-element bonds (for example, C = O, C є N, N = C = O) or weak heterocyclic groups (for example, in olefin oxides, lactams). Due to their mechanical strength, elasticity, electrical insulation and other valuable properties, polymer products are used in various industries and in everyday life.
They also tried to use other unsaturated hydrocarbons to produce artificial rubber. This is how new elastic polymers-elastomers were obtained. Polymer chains consist of units that, due to the presence of simple carbon-carbon or other chemical bonds between them, are capable of intramolecular rotation, which leads to a set of different conformations. The most important physical property of long chain macromolecules is their flexibility, due to which the high elasticity of polymers is manifested. The choice of the reference state is quite arbitrary; when analyzing the effect of temperature on the permeability of an elastic polymer, the glass transition temperature is usually used as G; when assessing the influence of composition and pressure, fg = 0 P = 0.
It has been established that the most elastic polymers are those whose macromolecules consist of carbon and hydrogen atoms. This is due to the weak interaction of --CHg-- groups with each other; the potential barrier in such molecules is relatively small. The low potential barrier of high molecular weight hydrocarbons (natural and synthetic rubbers, polyisobutylene, polyethylene) ensures high flexibility of their chains, which is why they are the most elastic of all polymers. Not so long ago, it was accepted that in the amorphous state, polymers are a system of chaotically entangled macromolecules. However, work in recent decades has shown that in fact, already in the amorphous state, polymers exhibit some structural order. This ordering naturally increases sharply during crystallization. According to V. A. Kargin and G. L. Slonimsky, in amorphous polymeric substances, just like in ordinary liquids, there are regions of short-range order in which the molecules are oriented parallel to each other, forming bundles or packs of sufficient length. The existence of such packs does not in any way contradict the high elasticity of polymers, since macromolecules can take on different conformations even when they form packs. Molecules can be arranged differently in packs, and the packs themselves can take on a wide variety of shapes.
Currently, there are several ways to produce polyethylene: 1) producing it under high pressure (more than 100 MPa) at a temperature of about 200 ° C in the presence of initiators of the polymerization process - oxygen, etc. This produces an elastic polymer in which the macromolecules of the product are highly branched structure.
This phenomenon is called rotation. In the gaseous state, a substance can be more or less free, but in the general case it often experiences energetic or spatial constraints caused by the interaction of polar groups contained in the molecule or other reasons. It is internal rotation gives flexibility to molecules and underlies the elasticity of polymers.
To obtain synthetic polymer materials with desired properties, scientifically based methods for their processing are needed, i.e., methods for forming optimal molecular structures that provide increased strength, low fragility, and high elasticity of polymers. To increase the service life of polymer materials, special additives are introduced into them to increase heat resistance, dynamic endurance and other important properties. In the manufacture of products from polymer materials great value have the choice and implementation of the optimal product design, which most appropriately takes into account the specifics of the material.
When diene elastomers are heated in the presence of acids or metal chlorides of variable valence, intramolecular cyclization occurs with the formation of six-membered rings. In this case, the elastic polymer loses its basic quality and becomes hard and brittle. Most macromolecules of a high-molecular substance in a state of equilibrium are in a bent position, since this corresponds to a minimum free energy. By the way, B is based on the most important position modern theory The elasticity of polymers underlies these ideas about the flexibility of macromolecules. Under the influence of irradiation, intense hydrogen evolution and structuring of polyolefins occurs. The resulting polymacroradicals combine to form a spatial polymer. As the irradiation dose increases, the number of cross-links increases, which reduces the elasticity of the polymer and increases its hardness and fragility. Internal plasticization is copolymerization, i.e., the polymerization of a mixture of monomers, one of which should produce a very elastic polymer. In the chain of the resulting copolymer, chain links of both monomers alternate, which determines its elasticity. The more regular the structure of macromolecules, the higher the strength of the polymer. Characteristic of polymers is a sharp difference in the type and strength of bonds between links along the macromolecule chain and bonds between chains. The most elastic polymers are those whose macromolecules consist of carbon and hydrogen atoms - such as polybutadiene.
Dependence of polymer elasticity on molecular weight, length and configuration of the macromolecule is due to the difference in the longitudinal and transverse dimensions of the macromolecular chains. The length of macromolecules exceeds their transverse dimensions by several thousand times. This can be compared with a steel wire having, for example, a length of 5 and a thickness of 0.5 mm. Despite the hardness of steel, the wire with this ratio of length and thickness will be quite flexible. In addition, macromolecules are almost always curved and often have a helical configuration. This state can be likened to twisting a steel wire into a spring. Just as a coiled spring is more flexible than a straight wire, so a long curved macromolecule is much more flexible than a straight one. However, it must be remembered that the different effects of molecular weight can only be compared within the same class of polymer.
The lower and the higher the temperature, the more likely the rearrangement of macromolecules, the more elastic the polymer. If the value is large and the temperature is low, then chain macromolecules manifest themselves as rigid systems. The structural units from which polymers are formed are bundles consisting of a large number of chain macromolecules. Depending on the degree of ordering of molecules in packs, polymers can exist in crystalline and three amorphous (glassy, highly elastic and viscous-flowing) states. Each of them is determined by a set of physical and mechanical properties associated with the structure and strength of bonds along the molecular chain and between chains. Crystalline polymers are elastic and have anisotropic properties; amorphous polymers are elastic and isotropic.
The data presented indicate that the modulus of high elasticity of polymers is 6 orders of magnitude less than the modulus of elasticity of solids. This difference is due to the fact that the nature of elasticity is energetic, while elasticity is entropic. Direct mechanical impact on rubber during plasticization leads to the destruction of the globular structure of the rubber and to the breaking of polymer chains, i.e. to mechanical destruction. The possibility of mechanical destruction of rubber is confirmed by an increase in plasticity during mechanical processing on cold rollers of elastic polymers such as polyisobutylenes, which, due to the absence of double bonds, are not subject to oxidative destruction.
Of the rubbers studied, the polymer obtained from polytetrahydrofuran with a molecular weight of 1000 has the best elastic properties over a wide temperature range. For this composition, the effect of the polydispersity of the polymerdiol on the properties of the rubber and its vulcanizates was studied. Naturally, what is more high level Polymers containing a significant amount of high molecular weight fractions have elasticity. In the region of positive temperatures, rebound elasticity is a function of the polydispersity of the polyester (Fig. 2). The decrease in the elasticity of polymers with increasing polydispersity coefficient is explained by the increasing irregularity in the distribution of urethane groups along the chain. For polymers obtained from a mechanical mixture of rubbers, two transition regions characteristic of block polymers appear on the temperature dependence of rebound elasticity. Irregularity of physical knots and chemical cross-links at values. Quite a lot of such elastic polymers of acrylic and methacrylic acids are known under the name hikars of different brands. Their latexes with fillers and vulcanizers are easily converted into rubber-like materials that are inert to oils at 150° and have high thermal and ozone resistance.
Crystalline polymers can also be subjected to orientation. The mechanism of orientation of crystalline polymers has not yet been established. Perhaps, in this case, melting of crystallites and their subsequent recrystallization with simultaneous orientation in the direction of stretching, its forces, are observed (Fig. 23). Oriented crystalline polymers acquire anisotropy, which increases with increasing degree of orientation. In the direction of orientation, mechanical strength increases noticeably, elasticity decreases, and the polymer becomes harder and less elastic.
In chlorinated rubber, the amount of chlorine ranges from 64 to 65%. The absence of unsaturated groups in the macromolecules of chlorinated rubber gives it higher weather resistance, increases its thermal stability and resistance to solutions of acids and alkalis. Chlorinated rubber films compare favorably with films of unsaturated polymers by also having good adhesion to metal surfaces. Due to its high polarity, chlorinated rubber is brittle and hard, although it retains its film-forming properties. To give chlorinated rubber elasticity, e1o is combined with elastic polymers, oils or plasticizers.
The second stage of the reaction occurs only with an excess of dicarboxylic acid and at a temperature of about 200°. The reaction consists of esterification of the resulting hydroxyl groups of the linear polymer. This can lead to the formation of an insoluble polymer with a network structure. With increasing radical length. Dicarboxylic acid increases the elasticity of the polymer. In the second stage, the reaction is accompanied by the release of water. If between the methylene units of polyamides there are ether groups --O-- or thioether groups --5--, the macromolecules become more flexible and the elasticity of the polymer increases. When a hydrogen atom of a secondary amide group is replaced, it becomes a tertiary group. The strength of polyphosphonitrile chloride is similar to that of natural rubber vulcanizates, and the phosphorus-containing polymer is significantly more heat-resistant. Up to 110, elastic deformations are still fully retained in the polymer. Even at 160°, after 3 hours of exposure of the sample to a load of 1 kg cm, the reversible deformations of the polymer account for 90% of all deformations. In a humid atmosphere, the elasticity of the polymer decreases. This phenomenon is obviously associated with the gradual hydrolysis of the polymer and its transformation into a network polymer with oxygen cross-bridges between the chains. Some polyester polymers bond fiberglass with asbestos-cement boards, wood fiber boards, honeycomb plastics, and with each other. They are used in the manufacture of some putty mixtures used for hydro- and vapor barrier of concrete and self-leveling floors, which after curing acquire high impact strength and resistance to abrasion, water and aggressive environments. By adding pastes of some organic dyes in dioctyl phthalate, colored monolithic floors can be obtained. Sometimes, in the manufacture of self-leveling floors, polyester-coumaron mastic compositions with mineral fillers are used. The combination of polyester elastic polymers with brittle coumaron polymers makes it possible to create floor coverings with high performance properties. Glass fabric or glass fiber impregnated with solutions of polyesters in styrene is converted into fiberglass plastics that are not inferior in strength to steel, but with a significantly lower density. From such material it is possible to obtain various sanitary products of increased strength (baths, pipes, etc.).
Viscoelastic deformation, which occurs during viscous flow of polymers, affects their rheological behavior. The role of the highly elastic component of viscous flow is very significant in the unsteady stage of polymer flow, as well as when normal stresses arise. The high elasticity of polymers in a fluid state is associated with the presence of supramolecular structures, which undergo destruction when deformed. Since supramolecular structures hinder the development of deformation, after its destruction processes of structural relaxation begin to occur in polymers, upon completion of which a steady flow is achieved.
In accordance with Hooke's law a=Ee, where E is the tensile modulus (Young's modulus). If the deformation is strictly proportional to the stress, then the modulus is a coefficient of proportionality and has a single value for a given material. In Fig. Figure 8.3 shows a typical stress-strain curve for an elastic polymer. There is no proportionality between o and e. Therefore, the modulus is defined as the tangent of the angle of inclination of the tangent to the curve drawn from the origin. This is the initial, or conditionally instantaneous, module. Formally, the modulus for a given sample can be determined at any deformation as the derivative of stress with respect to deformation = da/de.
The elasticity of the polymer is reduced either by increasing the processing temperature, or by decreasing the molecular weight, or by formulation factors, for example, by introducing an inelastic (chalk powder) filler, which reduces the elasticity of the system as a whole. The pour point can also be significantly reduced by introducing a plasticizer.
When studying the mechanical properties of polymers (and other materials), they are usually tested on tensile testing machines, or dynamometers. In this case, a gradual change in the sample length l and a load increasing over time are recorded. The latter is related to the cross-sectional area of the sample and the stress value is obtained. During deformation, the cross-section of the sample decreases, so the stress can be calculated on the initial cross-section or on the cross-section at a given time S. This cross-section is calculated under the assumption of constant volume during deformation, i.e. from the condition (where and l is the length of the undeformed and deformed samples).
The stress-strain relationship is expressed by the so-called deformation curve, the shape of which depends on the phase and physical state of the polymer. The glassy state of polymers is characterized by small deformations at low stresses. However, in contrast to simple low-molecular glasses (rosin, silicate glass, etc.), glassy polymers retain the ability to undergo significant deformations, sometimes reaching hundreds of percent, when applying great forces. High-molecular glasses often become brittle at temperatures many tens of degrees below the glass transition temperature. The ability of glassy polymers to be significantly deformed without destruction is what makes them so widely used.
For a long time it was believed that large deformations caused by large forces were the result of flow processes called “cold flow”. However, the current, i.e. mutual movement of macromolecules in a glassy state is unlikely. Thus, it was shown that a sample of polymethyl methacrylate, which has a certain residual deformation at a temperature below the glass transition temperature, after heating above it acquires its original shape and size. The reversible nature of large deformations observed in high molecular weight glasses suggests that the glassy state is characterized by the same regularities as the highly elastic one.
Large deformations that develop in glassy polymers only under the influence of significant stresses, but are close in nature to highly elastic ones, are called forced elasticity, and the phenomenon itself is called forced elasticity.
Forced elastic deformations can only appear under the influence of high stresses. After the stress ceases, the rate of disappearance of forced elastic deformations is very small, and at temperatures below they are not removed. At higher temperatures, the sample completely restores its size. Thus, the deformation of glassy polymers is always reversible.
When many glassy polymers (polystyrene, polymethyl methacrylate, polyvinyl chloride, etc.) are deformed at a certain value of stress, a section with a significantly reduced cross-section, called “necks,” is formed (abruptly) in the deformed sample. Tensile deformation accompanied by necking.
During forced elastic deformation of some glassy polymers (for example, cellulose acetate and nitrate), necking is not observed. In such cases, there is no maximum on the deformation curve. The curve can be divided into several sections that characterize different stages of the deformation process. The initial region (region Oa), which is a straight section, corresponds to a deformation that formally obeys Hooke's law. In the region ab, the tangent of the angle of inclination of the curve to the abscissa axis decreases with increasing voltage. This is due to the beginning of the development of forced elasticity in the sample. With increasing stress, the rate of development of forced elastic deformation rapidly increases, which leads to a further decrease in the slope of the deformation curve. At the maximum or plateau region of the curve, the tangent to the curve is horizontal, i.e. the rate of forced elastic deformation becomes equal to the full rate of deformation specified by the device. The stress at which this occurs is called the forced elasticity limit.
In the region of maximum and in the region of voltage decline, the beginning of neck formation is observed. By the end of the voltage decline, neck formation is complete.
Region cd is part of the curve, parallel to the axis abscissa - corresponds to the elongation of the neck due to the contraction of adjacent, slightly deformed parts of the sample (=const). At point d, neck growth stops, i.e. the thickness of the entire sample becomes equal to the thickness of the neck. Region cd corresponds to further deformation of the sample with a reduced cross-section. Forced elasticity, like high elasticity, depends on the rate of deformation, which indicates its relaxation nature. How more speed deformation, the greater the stress causing forced elasticity. This means that the limit of forced elasticity increases with increasing strain rate.
Typical dependence curves for glassy polymers at different temperatures are presented. It is clear from the figures that as the temperature decreases, the forced elasticity limit naturally increases.
As the temperature decreases, it increases, since greater voltages are required to rearrange the circuits. At some sufficiently low temperature, the stress reaches the value of brittle strength (), and brittle fracture of the material occurs.
The temperature below which the polymer breaks down under the influence of this stress is called the brittleness temperature (). At the brittle temperature, the forced elasticity limit is equal to the brittle strength. The brittleness temperature of a polymer can be determined graphically from the temperature dependence of the brittle strength, i.e. the strength of the polymer during its brittle fracture, and the limit of forced elasticity. The forced elasticity limit increases with decreasing temperature, and the tangent of the slope of the curve is always greater than the tangent of the slope of the curve. The point of intersection of these two curves, at which, determines the brittleness temperature of the material.
It should be noted that the brittleness temperature is even more conditional than the glass transition temperature, since its position depends not only on the loading rate, but also on the type of deformation (compression, tension, shear).
For high-molecular glasses, the brittleness and glass transition temperatures determined at the same strain rates do not coincide (the first is always lower than the second). The difference determines the temperature range of forced elasticity. If the upper limit of the temperature range of plastic operation depends on the glass transition temperature, then the brittleness temperature in many cases determines its lower limit. Below this temperature, when exposed to high stresses, the polymer breaks down brittlely. It is most advantageous to operate glassy polymers in the temperature range from to. Therefore, a large temperature range of forced elasticity is a very valuable property of the polymer.
The properties of a polymer depend not only on the chemical composition of the polymer and the shape of the macromolecule, but also on their relative position. Macromolecules of different polymers have different chemical compositions, lengths, shapes and degrees of flexibility. On the flexibility of macromolecular chains significant influence exert intermolecular interaction forces. These forces limit to a certain extent the freedom of movement of individual chain links.
The nature of the rotation of the chain is determined by the kinetic energy of the macromolecule, and to change both the nature of the rotation and the shape of the chain, it is necessary to impart to it a certain amount of energy (for example, thermal), which is called the energy barrier of the macromolecule. Depending on the spatial arrangement of the macromolecule relative to each other, the degree of their flexibility and the elasticity of the polymer change, which, in turn, determines the nature of the deformation of the material under mechanical influence.
Based on the degree of order in the arrangement of macromolecules, two types of phase states of polymers are distinguished: amorphous and crystalline. Amorphous the phase is characterized by a chaotic arrangement of the macromolecule in the IMC with some ordering of the structure, observed at relatively short distances commensurate with the size of the macromolecule. Crystalline the phase is characterized by an ordered arrangement of macromolecules in the polymer, and orderliness is maintained at distances exceeding the size of the macromolecule by hundreds and thousands of times (Fig. 1).
Crystalline zone
Amorphous zone
Rice. 1. Schematic representation of a polymer globule
Amorphous and crystalline polymers differ significantly in their properties.
Amorphous polymers with a linear or branched macromolecule structure can exist in three physical states:
1. glassy. This state is characterized by the strongest bonding forces between molecules and, as a consequence, the least flexibility of the macromolecule. The lower the temperature of a polymer in a glassy state, the fewer units have mobility, and at a certain temperature, called the brittle temperature, glassy polymers collapse without deformation (or small deformation), like low-molecular-weight glasses.
2. Highly elastic the state is characterized by less strong bonding forces between macromolecules, their greater flexibility and, as a consequence, the ability of long chain molecules to continuously change their shape. In a highly elastic state, small stresses cause a rapid change in the shapes of the molecule and their orientation in the direction of the force. After the load is removed, the macromolecules, under the influence of thermal movements, take on the most energetically favorable forms, as a result of which the original dimensions of the polymer are restored (reversible deformation). In this case, the position of only individual links and sections of the chains changes, and the macromolecules themselves do not perform forward movement each other relative to each other. Polymers whose amorphous phase is in a highly elastic state over a wide temperature range are called elastomers or rubbers(for example, the temperature range of the highly elastic state of natural rubber is from –73 to +180 °C, organosilicon rubber is from –100 to +250 °C).
3. Viscous the state is characterized by the disappearance of bonding forces between macromolecules, as a result of which they are not able to move relative to each other. This can occur when the polymer is heated to a certain temperature, after which the highly elastic (or glassy) state is replaced by a viscous flow state. Highly elastic state – characteristic feature Navy.
Crystalline polymers are distinguished by the fact that they contain, along with the crystalline phase, an amorphous phase. Due to the very large length of the molecules and the likelihood of weakening the forces of intermolecular interaction in individual sections of the chains in the polymer, as a rule, a continuous crystalline phase cannot form. Along with the ordered sections of the chains, sections with randomly located links appear, which leads to the formation of an amorphous phase in the crystalline polymer. The main condition that determines the possibility of crystallization of polymers is the linear and regular structure of macromolecules, as well as a sufficiently high mobility of units at the crystallization temperature. If the substituting atoms are small, then polymers can crystallize even if they are randomly arranged, for example, fluorine atoms in polyvinyl fluoride
(−CH 2 −CH−) n
In the presence of lateral, substituting hydrogen atoms of groups (C 6 H 5 ~, CH 3 ~, etc.), crystallization is possible only if the macromolecules have a folded shape, their orientation relative to each other is difficult and crystallization processes require dense packing of molecules , do not leak – the polymer is in an amorphous state.
For the formation of a crystalline phase, it is necessary that the macromolecules have a relatively straightened shape and have sufficient flexibility; in this case, the orientation of the macromolecules occurs and their dense packing is achieved. Polymers whose macromolecules lack flexibility do not form a crystalline phase.
Crystallization processes develop only in polymers that are in a highly elastic and viscous flow state. The following types of polymer crystal structures exist:
Lamellar,
Fibrillar,
Spherulitic.
Lamellar crystal structures are a multilayer system of flat thin plates, the macromolecules of which are folded many times. Fibrils, consisting of straightened chains of macromolecules, have the shape of a ribbon or thread . Spherulites- more complex crystalline structures, built from fibrillar or lamellar structures growing radially at the same speed from one center. As a result of this growth, the crystal takes the shape of a sphere ranging in size from tenths of a micron to several millimeters (sometimes up to several centimeters).
Crystalline polymers include polyethylene (low pressure), polytetrafluoroethylene, stereoregular polypropylene and polystyrene, and a number of polyesters.
Crystalline polymers have greater strength than amorphous ones. Crystallization imparts rigidity to the polymer, but due to the presence of the amorphous phase, which is in a highly elastic state, crystalline polymers are elastic.
When heated to a certain temperature, crystalline polymers transform directly into the viscous flow state of amorphous polymers.
The considered patterns of phase states of polymers relate to polymers with a linear or branched structure of macromolecules.
In IMCs with a spatial structure, the phase states are determined by the frequency of cross-linking (the number of valence bonds between macromolecules).
Polymers with highly interlinked (three-dimensional) polymers are rigid and under all conditions form an amorphous phase, which is in a glassy state. IUDs with rare cross-links (mesh) form an amorphous phase, which is mainly in a highly elastic state.
Exceeds 10 12 -10 13 n sec/m 2 (10 13 – 10 14 poise) , A - 10 3 -10 4 Mn/m 2(10 4 -10 5 kgf/cm 2) .
The transition of polymers from a viscous or highly elastic state to a glassy state is called glass transition. The glassy state is also realized as a result of processes that are usually not classified as glass transition:
- drawing or cross-linking of polymers in a highly elastic state;
- evaporation of polymer solutions or drying of gels at temperatures below ( T s) or melting point respectively.
The main feature of the glassy state of polymers is its thermodynamic nonequilibrium. The relationship between the liquid, crystalline and glassy states of polymers can be explained using a diagram volume – temperature (Figure 1).
As the polymer melt cools, its volume continuously decreases due to the fact that, as a result of molecular rearrangements, the melt passes from one equilibrium state to another. If the cooling rate is low enough, at some temperature T k crystallization occurs, accompanied by an abrupt decrease in volume (line AB on Figure 1). For many polymers, at a high cooling rate, crystallization does not have time to occur, and the substance remains in a supercooled liquid state, nonequilibrium with respect to the crystalline state (line AB on Figure 1). At T s the molecular movement becomes so slow that even after a very long experiment, rearrangements do not have time to occur, that is, the substance vitrifies and hardens. At temperatures below T s the glassy state is not in equilibrium with respect to both the equilibrium liquid state (dashed line VD on Figure 1), and to the crystalline state.
The thermodynamic nonequilibrium of the glassy state leads to the fact that at a constant temperature T otzh over time, the glass structure changes, tending to equilibrium (the phenomenon of structural relaxation), with a corresponding change in properties (line GD on Figure 1). Achieving an equilibrium structure is practically possible only in a narrow temperature range, when T otzh less T s at 15-20⁰С.
In the glassy state, segmental mobility is greatly limited, however, relaxation processes occur associated with the rotation of end or side groups, reorientation of small sections of the molecular chain in the area of structural defects, for example, on the surface of microcracks. The corresponding relaxation transitions can be observed by the appearance of maxima in the temperature dependences of physical properties, for example, mechanical and dielectric losses.
Mechanical behavior: glassy state can be divided into fragile, which is realized at temperatures below brittleness temperature, And non-fragile. The non-brittle glassy state is characterized by the fact that with sufficiently slow stretching at stresses exceeding the limit, the polymer is drawn out. The molecular orientation that arises in this case remains after unloading for an almost indefinitely long time at T<Т с . Charged polymer glasses spontaneously crack over time.
References:
Kobeko P.P., Amorphous substances, M.-L., 1952;
Kargin V.A., Slonimsky G.L., Brief essays on the physical chemistry of polymers, 2nd ed., M., 1967;
Ferri J., Viscoelastic properties of polymers, trans. from English, M., 1963
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