It represents an adsorption decrease in strength - a change in the mechanical properties of solids due to physicochemical processes causing a decrease in the surface (interphase) energy of the body. In the case of a crystalline solid, in addition to reducing the surface energy, for the Rehbinder effect to manifest itself, it is also important that the crystal has defects in the structure necessary for the initiation of cracks, which then propagate under the influence of the environment. In polycrystalline solids, such defects are grain boundaries: 350. Manifests itself in a decrease in strength and the appearance of fragility, a decrease in durability, and easier dispersion. For the Rebinder effect to occur, the following conditions are necessary:
- Contacting a solid with a liquid medium
- Presence of tensile stresses
The main characteristics that distinguish the Rehbinder effect from other phenomena, such as corrosion and dissolution, are the following:337:
- rapid appearance - immediately after contact of the body with the environment
- the sufficiency of a tiny volume of a substance acting on a solid body, but only with an accompanying mechanical effect
- returning the body to its initial characteristics after removing the medium
Examples of the Rebinder effect
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Notes
Literature
- Getsov G.G. A drop chisels a stone // Chemistry and life. - 1972. - No. 3. - pp. 14-16.
- S.V. Grachev, V.R. Baraz, A.A. Bogatov, V.P. Shveikin. "Physical Materials Science"
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Excerpt describing the Rebinder Effect
– “To our mother-throne capital, Moscow.The enemy entered Russia with great forces. He is coming to ruin our dear fatherland,” Sonya diligently read in her thin voice. The Count, closing his eyes, listened, sighing impulsively in some places.
Natasha sat stretched out, searchingly and directly looking first at her father, then at Pierre.
Pierre felt her gaze on him and tried not to look back. The Countess shook her head disapprovingly and angrily against every solemn expression of the manifesto. She saw in all these words only that the dangers threatening her son would not end soon. Shinshin, with his mouth folded into a mocking smile, was obviously preparing to mock the first thing presented for ridicule: Sonya’s reading, what the count would say, even the appeal itself, if no better excuse presented itself.
Having read about the dangers threatening Russia, about the hopes placed by the sovereign on Moscow, and especially on the famous nobility, Sonya, with a trembling voice that came mainly from the attention with which they listened to her, read the last words: “We will not hesitate to stand among our people.” in this capital and in other places of our state for consultation and guidance of all our militias, both now blocking the paths of the enemy, and again organized to defeat him, wherever he appears. May the destruction into which he imagines throwing us fall upon his head, and may Europe, liberated from slavery, exalt the name of Russia!”
- That's it! - the count cried, opening his wet eyes and stopping several times from sniffling, as if a bottle of strong vinegar salt was being brought to his nose. “Just tell me, sir, we will sacrifice everything and regret nothing.”
Shinshin had not yet had time to tell the joke he had prepared for the count’s patriotism, when Natasha jumped up from her seat and ran up to her father.
- What a charm, this dad! - she said, kissing him, and she again looked at Pierre with that unconscious coquetry that returned to her along with her animation.
- So patriotic! - said Shinshin.
“Not a patriot at all, but just...” Natasha answered offendedly. - Everything is funny to you, but this is not a joke at all...
- What jokes! - repeated the count. - Just say the word, we’ll all go... We’re not some kind of Germans...
“Did you notice,” said Pierre, “that it said: “for a meeting.”
- Well, whatever it is for...
At this time, Petya, to whom no one was paying attention, approached his father and, all red, in a breaking, sometimes rough, sometimes thin voice, said:
“Well, now, daddy, I will decisively say - and mummy too, whatever you want - I will decisively say that you will let me into military service, because I can’t ... that’s all ...
The Countess raised her eyes to the sky in horror, clasped her hands and angrily turned to her husband.
- So I agreed! - she said.
But the count immediately recovered from his excitement.
“Well, well,” he said. - Here’s another warrior! Stop the nonsense: you need to study.
- This is not nonsense, daddy. Fedya Obolensky is younger than me and is also coming, and most importantly, I still can’t learn anything now that ... - Petya stopped, blushed until he sweated and said: - when the fatherland is in danger.
- Complete, complete, nonsense...
- But you yourself said that we would sacrifice everything.
“Petya, I’m telling you, shut up,” the count shouted, looking back at his wife, who, turning pale, looked with fixed eyes at her youngest son.
- And I’m telling you. So Pyotr Kirillovich will say...
“I’m telling you, it’s nonsense, the milk hasn’t dried yet, but he wants to go into military service!” Well, well, I’m telling you,” and the count, taking the papers with him, probably to read them again in the office before resting, left the room.
- Pyotr Kirillovich, well, let’s go have a smoke...
Pierre was confused and indecisive. Natasha's unusually bright and animated eyes, constantly looking at him more than affectionately, brought him into this state.
- No, I think I’ll go home...
- It’s like going home, but you wanted to spend the evening with us... And then you rarely came. And this one of mine...” the count said good-naturedly, pointing at Natasha, “she’s only cheerful when she’s with you...”
“Yes, I forgot... I definitely need to go home... Things to do...” Pierre said hastily.
“Well, goodbye,” said the count, completely leaving the room.
- Why are you leaving? Why are you upset? Why?..” Natasha asked Pierre, looking defiantly into his eyes.
In addition to the action of chemical processes that affect surface properties and frictional interaction between solids, there is an open and studied P.A. Rebinder is a similar lubricant, due to the purely molecular interaction of the lubricant with solid surfaces, called the “Rebinder effect”.
Real solids have both surface and internal structural defects. As a rule, such defects have excess free energy. Due to the physical adsorption of molecules of surfactants (surfactants), the level of free surface energy of the solid body at the sites of their landing occurs. This reduces the work function of dislocations reaching the surface. Surfactants penetrate into cracks and into the intercrystalline space, exerting a mechanical effect on their walls and, pushing them apart, lead to brittle cracking of the material and a decrease in the strength of the contacting bodies. And if such processes develop only on the protrusions of the contacting bodies, reducing the shear resistance of the irregularities of this material, then in general this process leads to smoothing of the surface, a decrease in the specific pressure in the contact zone and in general
reducing friction and wear of rubbing bodies. But if normal friction loads increase significantly, high specific pressures spread over the entire contour area, softening of the material occurs over a large area of the surface and leads to its very rapid destruction.
The Rehbinder effect is widely used both in the development of lubricants (for this, special surfactants are introduced into the lubricant), and to facilitate the deformation and processing of the material in the manufacture of machine parts (for this, special lubricants and emulsions in the form of cutting fluids are used).
The Rebinder effect occurs on a wide variety of materials. These include metals, rocks, glass, elements of machinery and equipment. The medium causing a decrease in strength can be gaseous or liquid. Often molten metals can act as surfactants. For example, copper released when a sliding bearing melts becomes a surfactant for steel. Penetrating into cracks and the intercrystalline space of carriage axles, this process causes brittle destruction of the axles and causes accidents in transport.
Without paying due attention to the nature of the process, we often began to encounter examples where ammonia causes cracking of brass parts, gaseous combustion products sharply accelerate the process of destruction of turbine blades, molten magnesium chloride acts destructively on high-strength stainless steels and a number of others. Knowledge of the nature of these phenomena opens up opportunities to specifically address issues of increasing wear resistance and destruction of critical parts and assemblies of machines and equipment, and with proper use of the Rehbinder effect, to increase the productivity of processing equipment and the efficiency of using friction pairs, i.e. to save energy.
The phenomena of wettability were considered for the equilibrium state of the system. Under reservoir conditions, unstable processes occurring at the interface are observed. Due to the displacement of oil by water, a moving three-phase wetting perimeter is formed. The contact angle changes depending on the speed and direction of movement of the liquid (liquid meniscus, Fig. 5.5) in channels and cracks.
Figure 5.5 – Scheme of changes in wetting angles when changing the direction of movement of the meniscus in the capillary channel: 1 – advancing, 2 – retreating wetting angles when the water-oil meniscus moves in a cylindrical channel with a hydrophilic surface ( – static wetting angle)
Kinetic wetting hysteresis it is customary to call the change in the contact angle when moving along a solid surface of the three-phase wetting perimeter. The amount of hysteresis depends on:
on the direction of movement of the wetting perimeter, i.e. on whether water is displaced from a solid surface by oil or oil by water;
the speed of movement of the three-phase interface on a solid surface;
roughness of a solid surface;
adsorption on the surface of substances.
Hysteresis phenomena occur mainly on rough surfaces and are of a molecular nature. On polished surfaces, hysteresis is weak.
5.6 Properties of surface layers of formation fluids
There are various assumptions about the structure of the surface layer.
Many researchers studying the structure and thickness of thin layers of liquid associate the formation of wall layers with the polarization of molecules and their orientation from the surface of a solid to the internal regions of the liquid with the formation of solvation 1 layers.
Oil layers in contact with formation rocks have a particularly complex structure, since the interaction of surfactants with minerals is very diverse.
It has been noted, for example, that reagents used in flotation technology can be fixed on the surface of a mineral both in the form of ordinary three-dimensional films that form an independent phase on the surface of mineral particles, and in the form of surface compounds that do not have a specific composition and do not form a separate independent phase.
Finally, reagents can concentrate in the diffusion part of the electrical double layer, and not on the phase interface itself.
Surfactant components appear to always be concentrated not only on the surface, but also in the three-dimensional volume near the interface.
Many researchers have made attempts to measure the film thickness of various liquids on solids. For example, according to the results of measurements by B.V. Deryagin and M.M. Kusakov, the thickness of wetting films of aqueous salt solutions on various solid flat surfaces is about 10 -5 cm (100 im). These layers differ from the rest of the liquid in structure and mechanical properties - shear elasticity and increased viscosity. It has been established that the properties of the liquid in the surface layer also change due to its compression. For example, the density of water adsorbed by silica gel, according to some measurements, is 1027-1285 kg/m3.
Adsorption and associated solvation shells at phase interfaces in an oil reservoir also have special properties. Some components of oil can form gel-like structured adsorption layers (with unusual - anomalous properties) with high structural viscosity, and at high degrees of saturation of the adsorption layer - with elasticity and mechanical shear strength.
Research shows that the composition of the surface layers at the oil-water interface includes naphthenic acids, low molecular weight resins, colloidal particles of high molecular weight resins and asphaltenes, paraffin microcrystals, as well as particles of mineral and carbonaceous suspensions. It is assumed that the surface layer at the oil-water interface is formed as a result of the accumulation of mineral and carbon particles, as well as paraffin microcrystals under the influence of selective wetting of hydrophilic areas of their surface with the aqueous phase. Asphalt-resinous substances adsorbed on the same interface surface, turning into a gel-like state, cement particles of paraffin and minerals into a single monolithic layer. The surface layer thickens even more due to the solvatization of gels of asphalt-resinous substances from the oil phase.
The special structural and mechanical properties of surface layers determine the stabilization of various systems and, in particular, the high stability of some water-oil emulsions.
The existence of adsorption layers at the residual water-oil interface also apparently has some retarding effect on the processes of miscibility of water injected into the reservoir with residual water.
5.7 Wedging effect of thin layers of liquid.
Deryagin's experiments. Rebinder effect
A liquid that wets a solid body, penetrating into thin cracks, can play the role of a wedge and push its walls apart, i.e. thin layers of liquid have a wedging effect 2. This property of thin layers also manifests itself when solid surfaces immersed in a liquid approach each other. According to the research of B.V. Deryagin, the wedging effect occurs provided that the layer thickness h fluid pushing apart the surface of the crack is less than a certain value h cr. At h > h cr the wedging effect is zero and at h < h cr it increases with decreasing thickness of the liquid layer, i.e. from the moment h ≤ h cr To bring the particle surfaces closer together, an external load must be applied to them.
The factors that create the wedging effect are forces of ion-electrostatic origin and the special state of aggregation of polar liquids near the boundary surfaces.
It was previously mentioned that the properties of the solvation layer on the surface of a solid differ sharply from the properties of the rest of the liquid. This (solvate) layer can be considered as a special boundary phase. Therefore, when particles approach to distances less than twice the thickness of the solvation layers, an external load must be applied to the particles.
Disjoining pressure of ion-electrostatic origin arises due to changes in the concentration of ions in the layer separating the particles and in the solution surrounding them.
According to the results of the experiment, the stronger the bond between the liquid and the surfaces of the solid body, the greater the wedging effect. It can be enhanced by introducing surfactants into the liquid that are well adsorbed by the surface of the solid. The Rebinder effect is based on this phenomenon. Its essence lies in the fact that small amounts of surfactants cause a sharp deterioration in the mechanical properties of the solid. The adsorption decrease in the strength of solids depends on many factors. It intensifies if the body is subjected to tensile forces and if the liquid wets the surface well.
The effect of adsorption reduction in strength is used in well drilling. When using solutions containing specially selected surfactants as flushing fluids, drilling in hard rocks is noticeably easier.
REBINDER Petr Aleksandrovich (03.X.1898-12.VII.1972), Soviet physical chemist, academician of the USSR Academy of Sciences since 1946 (corresponding member since 1933), born in St. Petersburg. Graduated from the Faculty of Physics and Mathematics of Moscow University (1924). In 1922-1932 worked at the Institute of Physics and Biophysics of the USSR Academy of Sciences and at the same time (in 1923-1941) at the Moscow State Pedagogical Institute named after. K. Liebknecht (from 1923 - professor), from 1935 - head of the department of disperse systems at the Colloid-Electrochemical Institute (from 1945 - Institute of Physical Chemistry) of the USSR Academy of Sciences, from 1942 - head of the department of colloid chemistry at the Moscow university.
Rehbinder's works are devoted to the physical chemistry of disperse systems and surface phenomena. In 1928, the scientist discovered the phenomenon of a decrease in the strength of solids due to the reversible physico-chemical influence of the environment on them (Rehbinder effect) and in the 1930-1940s. developed ways to facilitate the processing of very hard and difficult-to-cut materials.
He discovered the electrocapillary effect of plasticization of metal single crystals during the process of creep during the polarization of their surface in electrolyte solutions, studied the features of aqueous solutions of surfactants, the influence of adsorption layers on the properties of disperse systems, identified (1935-1940) the main principles of the formation and stabilization of foams and emulsions, as well as the process of phase reversal in emulsions.
The scientist found that the cleaning action includes a complex set of colloidal chemical processes. Rebinder studied the processes of formation and structure of micelles of surfactants, developed ideas about a thermodynamic stable micelle of soaps with a lyophobic inner core in a lyophilic environment. The scientist selected and justified the optimal parameters for characterizing the rheological properties of disperse systems and proposed methods for their determination.
In 1956, the scientist discovered the phenomenon of adsorption reduction in the strength of metals under the influence of metal melts. In the 1950s Scientists created a new field of science - physical and chemical mechanics. As Rehbinder himself wrote: “The ultimate task of physical-chemical mechanics is to develop the scientific basis for obtaining solids and systems with given structures and mechanical properties. Consequently, the task of this area includes the creation of an optimally targeted technology for the production and processing of essentially all building and structural materials of modern technology - concrete, metals and alloys, especially heat-resistant ones, ceramics and metal-ceramics, rubbers, plastics, lubricants.”
Since 1958, Rebinder has been chairman of the Scientific Council of the USSR Academy of Sciences on problems of physical and chemical mechanics and colloid chemistry, then (since 1967) chairman of the USSR National Committee under the International Committee on Surfactants. From 1968 to 1972 he was editor-in-chief of the Colloid Journal. The scientist was awarded two Orders of Lenin, had the title of Hero of Socialist Labor (1968), laureate of the USSR State Prize (1942).
The Rehbinder effect, the effect of adsorption reducing the strength of solids, facilitating the deformation and destruction of solids due to the reversible physico-chemical influence of the environment. Discovered by P. A. Rebinder (1928) while studying the mechanical properties of calcite and rock salt crystals. Possible when a solid body in a stressed state comes into contact with a liquid (or gas) adsorption-active medium. The Rebinder effect is very universal - it is observed in solid metals, ionic, covalent and molecular mono- and polycrystalline solids, glasses and polymers, partially crystallized and amorphous, porous and solid. The main condition for the manifestation of the Rehbinder effect is the related nature of the contacting phases (solid body and medium) in chemical composition and structure. The form and degree of manifestation of the effect depend on the intensity of interatomic (intermolecular) interactions of contacting phases, the magnitude and type of stress (tensile stress is required), strain rate, and temperature. A significant role is played by the actual structure of the body - the presence of dislocations, cracks, foreign inclusions, etc. A characteristic form of manifestation of the Rehbinder effect is a repeated drop in strength, an increase in the fragility of the solid body, and a decrease in its durability. Thus, a zinc plate soaked in mercury does not bend under load, but breaks brittlely. Another form of manifestation is the plasticizing effect of the medium on solid materials, for example, water on gypsum, organic surfactants on metals, etc. The thermodynamic Rebinder effect is caused by a decrease in the work of formation of a new surface during deformation as a result of a decrease in the free surface energy of a solid under the influence of the environment . The molecular nature of the effect is to facilitate the rupture and rearrangement of intermolecular (interatomic, ionic) bonds in a solid in the presence of adsorption-active and at the same time sufficiently mobile foreign molecules (atoms, ions).
The most important areas of technical application are facilitating and improving the mechanical processing of various (especially highly hard and difficult to machine) materials, regulating friction and wear processes using lubricants, effectively obtaining crushed (powdered) materials, obtaining solids and materials with a given dispersed structure and the required combination of mechanical and other properties by disaggregation and subsequent compaction without internal stresses. An adsorption-active environment can also cause significant harm, for example, reducing the strength and durability of machine parts and materials under operating conditions. Elimination of factors contributing to the manifestation of the Rebinder effect in these cases makes it possible to protect materials from undesirable environmental influences.
Even the strongest bodies have a huge number of defects, which weaken their resistance to load and make them less strong compared to what theory predicts. During the mechanical destruction of a solid body, the process begins from the place where the microdefects are located. An increase in load leads to the development of microcracks at the defect site. However, removing the load leads to the restoration of the original structure: the width of the microcrack is often insufficient to completely overcome the forces of intermolecular (interatomic) interaction. Reducing the load leads to “shrinking” of the microcrack, the forces of intermolecular interaction are restored almost completely, and the crack disappears. The point is also that the formation of a crack is the formation of a new surface of a solid body, and such a process requires the expenditure of energy equal to the surface tension energy multiplied by the area of this surface. Reducing the load leads to “shrinking” of cracks, since the system tends to reduce the energy stored in it. Therefore, to successfully destroy a solid, it is necessary to coat the resulting surface with a special substance called a surfactant, which will reduce the work of overcoming molecular forces when forming a new surface. Surfactants penetrate into microcracks, cover their surfaces with a layer just one molecule thick (which makes it possible to use very small amounts of additives of these substances), preventing the “collapse” process, preventing the resumption of molecular interaction.
Surfactants, under certain conditions, facilitate the grinding of solids. Very fine (down to the size of colloidal particles) grinding of solids is generally impossible to achieve without the addition of surfactants.
Now it remains to remember that the destruction of a solid body (i.e., the formation of new microcracks) begins precisely from the place where the defect in the structure of this body is located. In addition, the added surfactant is also adsorbed predominantly at the locations of defects - thus facilitating its adsorption on the walls of future microcracks. Let us quote the words of Academician Rebinder: “The separation of a part occurs precisely at these weak points [location of defects], and, consequently, the small particles of the body formed during grinding no longer contain these most dangerous defects. To be more precise, the probability of encountering a dangerous weak point becomes less, the smaller its size.
If, by grinding a real solid body of any nature, we reach particles whose dimensions are approximately the same as the distances between the most dangerous defects, then such particles will almost certainly not contain dangerous structural defects, they will become much stronger than large samples of the same the body itself. Consequently, one has only to crush a solid into small enough pieces, and these pieces of the same nature, the same composition will be the most durable, almost ideally strong.”
Then these homogeneous, defect-free particles must be combined, a solid (high-strength) body of the required size and shape must be made from them, the particles must be forced to pack tightly and unite very firmly with each other. The resulting machine or building part must be much stronger than the original material before grinding. Naturally, it is not as strong as a separate particle, since new defects will appear at the points of fusion. However, if the process of combining particles is carried out skillfully, the strength of the original material will be surpassed. This requires small particles to be packed especially tightly so that intermolecular interaction forces arise between them again. Typically, this is done by compressing particles by pressing and heating. The fine-grained aggregate obtained by pressing is heated without bringing it to melting. As the temperature increases, the amplitude of thermal vibrations of molecules (atoms) in the crystal lattice increases. At points of contact, the vibrating molecules of two neighboring particles come closer and even mix. The adhesion forces increase, the particles are pulled together, leaving virtually no voids or pores, and defects at the points of contact disappear.
In some cases, the particles can be glued or soldered to each other. In this case, the process must be carried out in such a way that the layers of glue or solder do not contain defects.
A radical improvement in the process of grinding solids, based on the practical application of the Rehbinder effect, has proven to be very useful for many industries. The technological processes of grinding have significantly accelerated, while energy consumption has noticeably decreased. Fine grinding has made it possible to carry out many technological processes at lower temperatures and pressures. As a result, higher quality materials were obtained: concrete, ceramic and metal-ceramic products, dyes, pencil masses, pigments, fillers and much more. Mechanical processing of refractory and heat-resistant steels is facilitated.
This is how he himself describes the method of applying the Rehbinder effect: “Building parts made of cement concrete can be reliably combined into a monolithic structure by gluing with cement vibrocolloidal glue... Such glue is a mixture of finely ground cement (part of which can be replaced with finely ground sand) with an extremely small amount of water and addition of a surfactant. The mixture is liquefied by extreme vibration during application to the bonded surfaces in the form of a thin layer. After rapid hardening, the glue layer becomes the strongest point in the structure.”
The use of Academician Rehbinder's ideas regarding facilitating the process of grinding solids is of great practical importance, for example, for developing a method for reducing the strength of minerals in order to increase the efficiency of drilling in hard rocks.
Reduction of the strength of metals under the influence of metal melts. In 1956, Rehbinder discovered the phenomenon of a decrease in the strength of metals under the influence of metal melts. It has been shown that the greatest decrease in the surface energy of a solid (metal) to almost zero can be caused by molten media that are close to the solid in molecular nature. Thus, the tensile strength of zinc single crystals was reduced tens of times by applying a layer of liquid tin metal 1 micron or less thick to their surface. Similar effects for refractory and heat-resistant alloys are observed under the influence of liquid low-melting metals.
The discovered phenomenon turned out to be very important for improving methods of metal forming. This process is impossible without the use of lubricant. For materials of new technology - refractory and heat-resistant alloys - processing is especially significantly facilitated by the use of active lubricants that soften thin surface layers of the metal (which, in fact, occurs under the influence of small quantities of metal melts). In this case, the metal seems to lubricate itself - the harmful excess deformation that occurs during processing, which causes the so-called hardening - an increase in strength that interferes with processing, is eliminated. New possibilities for processing metals by pressure at normal and elevated temperatures are opening up: the quality of products increases, the wear of the processing tool and the energy consumption for processing are reduced.
Instead of converting expensive metal into chips during the process of manufacturing a product by cutting, you can use a plastic change in shape: pressure processing without loss of metal. At the same time, the quality of the products also increases.
A sharp decrease in the strength of the surface layer of metals plays a significant role in improving the performance of friction units. An automatically operating wear control mechanism arises: if there are random irregularities on the rubbing surfaces (burrs, scratches, etc.), high local pressure develops in the places of their dislocation, causing surface flow of metals, significantly facilitated under the action of adsorbed melts (melt-wetted surface layer metal loses strength). Rubbing surfaces can be easily ground or polished. The introduced “lubrication” causes accelerated “wear” of the irregularities, and the speed of running-in (run-in) of the machines increases.
Active impurity melts can be used as modifiers of the crystallization process. Adsorbed on the seed crystals of the released metal, they reduce their growth rate. Thus, a fine-grained metal structure with higher strength is formed.
A process for “training” metal in a surface-active medium has been developed. The metal is subjected to periodic surface impacts that do not lead to destruction. Due to the relief of plastic deformations in the surface layers, the metal in the internal volume seems to “knead”, and the crystal lattice of grains is dispersed. If such a process is carried out at a temperature close to the temperature at which the metal begins to recrystallize, a fine-crystalline structure with a much higher hardness is formed in a surface-active medium. And the grinding of metals to obtain fine powder cannot be accomplished without the use of surface-active melts. Subsequently, products are produced from this powder by hot pressing (in full accordance with the process of hardening materials from powders described above).
REBINDER EFFECT IN POLYMERS. The outstanding Soviet physical chemist Academician Pyotr Aleksandrovich Rebinder was the first to try to influence the work of destruction of a solid. It was Rebinder who managed to understand how this could be done. Back in the 20s of the last century, he used for this purpose the so-called surface-active, or adsorption-active, substances, which are able to effectively adsorb on the surface even at low concentrations in the environment and sharply reduce the surface tension of solids. Molecules of these substances attack intermolecular bonds at the tip of a growing fracture crack and, adsorbed on newly formed surfaces, weaken them. By selecting special liquids and introducing them onto the surface of a destructible solid, Rebinder achieved a striking reduction in the work of fracture under tension (Fig. 1). The figure shows the stress-strain curves of a zinc single crystal (a plate about a millimeter thick) in the absence and presence of a surfactant liquid. The moment of destruction in both cases is marked by arrows. It is clearly seen that if you simply stretch the sample, it breaks at more than 600% elongation. But if the same procedure is carried out by applying liquid tin to its surface, destruction occurs at only ~10% elongation. Since the work of destruction is the area under the stress-strain curve, it is easy to see that the presence of liquid reduces the work not even by times, but by orders of magnitude. It was this effect that was called the Rehbinder effect, or adsorption decrease in the strength of solids.
Fig.1. Dependence of stress on deformation of zinc single crystals at 400°C: 1 - in air; 2 -- in molten tin
The Rehbinder effect is a universal phenomenon; it is observed during the destruction of any solids, including polymers. However, the nature of the object introduces its own characteristics into the destruction process, and polymers are no exception in this sense. Polymer films consist of large, whole molecules held together by van der Waals forces, or hydrogen bonds, which are noticeably weaker than the covalent bonds within the molecules themselves. Therefore, a molecule, even being a member of a collective, retains certain isolation and individual qualities. The main feature of polymers is the chain structure of their macromolecules, which ensures their flexibility. Flexibility of molecules, i.e. their ability to change their shape (due to deformation of bond angles and rotations of links) under the influence of external mechanical stress and a number of other factors underlies all the characteristic properties of polymers. First of all, the ability of macromolecules to mutually orientate themselves. However, it must be noted that the latter applies only to linear polymers. There are a huge number of substances that have high molecular weight (for example, proteins and other biological objects), but do not have the specific properties of polymers, since strong intramolecular interactions prevent their macromolecules from bending. Moreover, a typical representative of polymers - natural rubber - being “cross-linked” with the help of special substances (vulcanization process), can turn into a solid substance - ebonite, which does not show any signs of polymer properties at all.
In polymers, the Rehbinder effect manifests itself in a very unique way. In an adsorption-active liquid, the emergence and development of a new surface is observed not only during destruction, but much earlier - even during the process of polymer deformation, which is accompanied by the orientation of macromolecules.
Fig.2. Appearance of polyethylene terephthalate samples stretched in air (a) and in an adsorption-active medium (n-propanol) (b).
rebinder polymer metal strength
Figure 2 shows images of two lavsan samples, one of which was stretched in air, and the other in an adsorption-active liquid. It is clearly seen that in the first case a neck appears in the sample. In the second case, the film does not narrow, but becomes milky white and not transparent. The reasons for the observed whitening become clear upon microscopic examination.
Fig.3. Electron micrograph of a polyethylene terephthalate sample deformed in n-propanol. (Zoom 1000)
Instead of a monolithic transparent neck, a unique fibrillar-porous structure is formed in the polymer, consisting of thread-like aggregates of macromolecules (fibrils) separated by microvoids (pores). In this case, the mutual orientation of macromolecules is achieved not in a monolithic neck, but inside the fibrils. Since the fibrils are separated in space, such a structure contains a huge number of microvoids, which intensely scatter light and give the polymer a milky white color. The pores are filled with liquid, so the heterogeneous structure is maintained even after the deforming stress is removed. The fibrillar-porous structure appears in special zones and, as the polymer is deformed, it captures an increasing volume. Analysis of microscopic images made it possible to establish the features of structural rearrangements in the polymer subjected to crazing (Fig. 4).
Fig.4. Schematic representation of the individual stages of polymer crazing: I - initiation of crazes, II - growth of crazes, III - broadening of crazes.
Having originated on any defect (structure inhomogeneity), which are abundant on the surface of any real solid, crazes grow through the entire cross-section of the stretched polymer in the direction normal to the tensile stress axis, maintaining a constant and very small (~1 μm) width. In this sense, they are similar to true fracture cracks. But when the craze “cuts” the entire cross-section of the polymer, the sample does not break up into separate parts, but remains a single whole. This is due to the fact that the opposite edges of such a peculiar crack are connected by the thinnest threads of oriented polymer (Fig. 3). The dimensions (diameters) of fibrillar formations, as well as the microvoids separating them, are 1–10 nm.
When the fibrils connecting the opposite walls of the crazes become long enough, the process of their fusion begins (in this case, the surface area decreases, Fig. 5). In other words, the polymer undergoes a peculiar structural transition from a loose structure to a more compact one, consisting of densely packed aggregates of fibrils, which are oriented in the direction of the stretching axis.
Fig.5. Diagram illustrating the collapse of the polymer structure, which occurs at large values of deformation in an adsorption-active liquid, at various stages of stretching
There is a method for separating molecules by adsorption from solution of those that are able to penetrate into pores of a given size (molecular sieve effect). Since the pore size can be easily adjusted by changing the degree of polymer extension in the adsorption-active medium (using the Rebinder effect), selective adsorption is easy to achieve. It is important to note that the adsorbents used in practice are usually a kind of powder or granulate, which is filled with various types of containers (for example, the sorbent in the same gas mask). Using the Rehbinder effect, it is easy to obtain a film or fiber with through nanometric porosity. In other words, the prospect opens up to create a structural material that has optimal mechanical properties and at the same time is an effective sorbent.
Using the Rehbinder effect, in an elementary way (by simply stretching a polymer film in an adsorption-active medium), it is possible to make porous polymer films based on almost any synthetic polymers. The pore sizes in such films can be easily adjusted by changing the degree of deformation of the polymer, which makes it possible to produce separation membranes to solve a wide variety of practical problems.
The Rehbinder effect in polymers has great applied potential. Firstly, by simply extracting a polymer in an adsorption-active liquid, it is possible to obtain a variety of polymer sorbents, separation membranes and polymer products with a transverse relief, and, secondly, the Rehbinder effect gives the process chemist a universal, continuous method for introducing modifying additives into polymers.
List of materials used
- 1. www.rfbr.ru/pics/28304ref/file.pdf
- 2. www.chem.msu.su/rus/teaching/colloid/4.html
- 3. http://femto.com.ua/articles/part_2/3339.html
- 4. Great Soviet Encyclopedia. M.: Soviet Encyclopedia, 1975, vol. 21.
- 5. http://him.1september.ru/2003/32/3.htm
- 6. http://slovari.yandex.ru/dict/bse/article/00065/40400.htm
- 7. http://www.nanometer.ru/2009/09/07/rfbr_156711/PROP_FILE_files_1/rffi4.pdf
- 8. http://ru.wikipedia.org/wiki/Rebinder_Effect
This novel is a “collection of motley chapters,” where each chapter is named with a line from Pushkin and is an independent story about one of the heroes. And there are many heroes in the novel - a gifted musician of the post-war period, a “sweet womanizer”, and a homely, exemplary schoolgirl of the mid-50s, in whose soul passions that are invisible to the world burn - envy, jealousy, forbidden love; an orphanage boy, a nuclear physicist, the son of a repressed commissar and a village fire victim, a witness of the Gulag, and many, many others. Private stories grow into a picture of Russian history of the 20th century, but the novel is not a historical canvas, but rather a multifaceted family saga, and the further the narrative develops, the more the fates of the heroes become intertwined around the mysterious Katenin family, descendants of “that same Katenin,” Pushkin’s friend. The novel is full of mysteries and secrets, passions and grievances, love and bitter losses. And increasingly, an analogy arises with the narrowly scientific concept of the “Rehbinder effect” - just as a drop of tin breaks a flexible steel plate, so an insignificant, at first glance, event completely changes and breaks a specific human life.
“Short stories, elegantly strung, like beads on a thread: each of them is a separate story, but suddenly one plot flows into another, and the fates of the heroes intersect in the most unexpected way, the thread does not break. The entire narrative is deeply melodic, it is permeated with music - and love. Some people are spoiled by love all their lives, others struggle painfully for it. Classmates and lovers, parents and children, a strong and indestructible unity of people, based not on blood kinship, but on love and human kindness - and the thread of the plot, on which a few more beads have been added, is still strong... This is how human relationships stand the test of Stalin's time, the “thaw” and the hypocrisy of “developed socialism” with its peak - the Chernobyl disaster. The thread does not break, almost contrary to Rehbinder’s law.”
Elena Katishonok, laureate of the Yasnaya Polyana Prize and finalist of the Russian Booker
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