by Acad. Alexander BERLIN Director of the RAS Institute of Chemical Physics named after N. N. Semyonov
Materials used by man in his activities have always been playing an important and even decisive role in the development of the civilization. It is not accidental that they gave names to the whole historical epochs: Stone, Bronze, Iron Age... Modern epoch can be called the Polymer Age. While creating material world, people have always turned to the history of nature. To come closer to its best specimens is very exciting but still a challenging task.
FROM METALS TO POLYMERS
The key function of materials called constructional is to bear mechanical load. That is why strength is a principal requirement. The product shall not be destroyed at high static load, shall not be brittle, i.e. crumble at blow, in other words, formation of cracks in them must be blocked. The first property is quantitatively characterized by static strength (hereinafter-strength), the second property-crack resistance.
Conventionally, there are four principal types of constructional materials created by nature and man: inorganic (minerals, ceramics, glass, etc.), metals, organic polymers and composites. From the point of view of their use, all these materials have their own advantages and disadvantages.
Thus, metals combine strength and crack resistance, i.e. endure high load and are not brittle (though the higher their strength, the lower is plasticity). But products made of such materials are heavy, as their specific weight is very high (for steel - about 7.8 g/cm3 ). Production and processing require considerable power consumption and labor costs, in particular, in case of high-tensile alloys.
Ceramics, inorganic glass and glass ceramic materials are characterized by high strength and stiffness, i.e. deformation resistance (modulus of elasticity) and heat-resistance, possibility to operate at high temperatures, and comparatively small specific weight (2 - 4 g/cm3 ). But such materials are very brittle, while manufacturing process is rather long, requires high temperatures, and sometimes even pressures.
Main advantages of organic polymers are low specific weight (1 - 1.5 g/cm3 ), easiness of their processing into a product, good combination of strength and crack resistance. Disadvantages are low thermal, oxi-
It is difficult to find a compromise between conflicting groups of properties while developing new constructional materials.
Dependence of melting temperature of single-element substances (A) and polyoxides (B) on chemical bond energy.
dizing and radiation resistance, combustibility.
Finally, composites made on the basis of fibers and matrixes of different types (binders) have a unique combination of properties: high strength and crack resistance, but their production requires complex technical solutions and considerable period of time in the course of processing into products.
So, the process of development of constructional materials is always a compromise between conflicting groups of properties.
Production of materials and their processing into products are considered to be a field of activity where people have left behind the nature in relation to process effectiveness and speed. The nature was never in a hurry figuratively speaking; it made everything in one phase, producing the "product", at once, without preliminary "production" of a intermediate product, i.e. material. Recycling also requires much time-as a result of complete biological decomposition.
The man should make a lot during his short life. Initially, he used finished materials created by the nature-broke tree branches and made, for example, spears or bows, treated stones. Later, he learned to work with clay and metals.
Polymers gave birth to extremely effective and energy saving methods of processing-casting, extrusion*, chemical molding etc. All that, taking into account good physico-mechanical properties of these materials, contributed to universal
Boric oxide macromolecule: horizontal projection(A), side projection (B).
* Extrusion - method of manufacture of shaped products from plastics and rubber, consisting in continuous spinning of softened material using worm conveyer or pommel through a shaped hole (matrix) on a special machine-extruder. - Ed.
Dependence of boric oxide glass-transition temperature on modifying agent concentration (oxides of monad and dyad metals and caprolactam).
application of polymers in various spheres of human activity. At the present moment, weight volumes of polymer and metal production are equal.
For melting of three-dimensional (diamond, quartz) and two-dimensional (layered structures of graphite, molybdenum disulfide) polymers representing single macromolecules, it is necessary to break chemical bonds, that is why required temperature should be in proportion to binding energy. As for linear polymers (sulfur, selenium, tellurium), the same process requires lower temperatures and depends on intermolecular interactions of energy and macromolecular chain rigidity. If we change these two parameters, we can increase or reduce heat-resistance of the substance which is only limited by strength of chemical bonds.
At the same time thermal stability (temperature of decomposition to low-molecular products) depends on, first, binding energy and, second, oxidation susceptibility (materials are generally used in the open air). From this point of view, inorganic oxides are more preferable as they do not oxidize and burn. Bond energy of these materials is higher than that of single-element substances and majority of organic polymers. (For reference: carbon-carbon bond energy is about 80 kkal/mole, carbon-hydrogen bond energy - 81 kkal/mole while boron-oxygen, carbon-fluorine energy is 120 kkal/mole and aluminum-oxygen bond energy is about 150 kkal/mole. By the way, high bond energy of carbon-fluorine determines advantages of fully fluorinated polymers such as Teflon and others as compared with conventional hydrogenous polymers.)
Another disadvantage of organic polymers is formation of carbon oxide and dioxide during oxidation and decomposition; these substances are gases while oxides of other elements in normal conditions are solids (except sulfur and nitrogen).
RAS Institute of Chemical Physics initiated research works in this field starting from phosphate glass. Processing temperature of this material can greatly differ, depending on the origin and metal oxide concentration. In particular, scientists obtained relatively stable (as to their hydrolytic characteristics) and heat-resistant (decomposition begins at 600°C and over) materials with glass melting temperatures from 125 to l,000°C and higher. Blends of phosphate glasses with organic polymers are noncombustible and demonstrate other good characteristics: higher stiffness, lower frictional wear. Water resistance of polyphosphates also depends on their origin and metal ions ratios. Gradual decomposition in a humid medium is beneficial from environmental point of view (for example, creation of packing materials).
Boric oxide was used as a basis to create another type of inorganic materials. It has comparatively low softening temperature, which can be both lower and higher as a result of modifications using different organic and inorganic compounds including polymers.
STIFFNESS AND BRITTLENESS
Organic polymers and metals are characterized by combination of stiffness and crack resistance but the nature of their plasticity (fracture toughness) is different. This characteristic of metals proceeds from free dislocation movements in the crystal, that, in turn, depends on metallic bond nature - its nondirectionality. Linear polymers become stronger at large deformations in the crack top due to unfolding of coiled macro-molecules, the same as for formation of synthetic fiber. Thanks to this specific mechanism, in place of regular cracks so called crazes are formed the faces of which are interconnected by polymer fibers (fibrous filaments).
As a result they support load that is not lower than the initial solid material load. At the same time, crazes allow the polymer to expand in one direction without compression of others and without loss of load-carrying capacity. That is why it is easier to use it in a complex stress state as a composite matrix. On the other hand, the higher is molecular weight of a linear polymer, the more effective is the process of crazes formation, the higher is fracture toughness and the lower is brittleness. All the above-mentioned factors determine increased shock resistance of composite materials with linear polymer
as binder, their crack resistance at static, cyclic fatigue and dynamic loads, increased postimpact properties, etc.
Polyoxides are the most interesting materials among inorganic polymers. They have a set of properties: high chemical bond energy, heat and radiation resistance, incombustibility, oxidation stability, good mechanical characteristics-consequently, polyoxides can be used in extreme environments. The advantages include availability of raw materials, absence of volatile decomposition products at heating, possibility to process at relatively low temperatures. As you realized on the example of phosphor and boron oxides, the latter characteristic can be greatly varied in the presence of oxides of different metals, organic and inorganic compounds.
Polyoxides are an alternative to organic polymers, which are in the high of their operational functionality. We would like to point out that plasticity is one of the main problems during the process of synthesis of these inorganic substances. If we know how to make them plastic, we shall be able to make plastic ceramics. Its construction perspectives can de defined in the following order: production of linear inorganic polymers; plasticization using low-molecular inorganic compounds; modification by low-molecular organic compounds; creation of mixtures with inorganic rubber or organic polymers; occurrence of composites, nanocomposites with fiber or lamellate fillers.
It is worth mentioning that modification of inorganic polymers using linear organic polymers as their structure is similar (though more complex) to our bones, teeth, mussels and other natural composite materials.
Notwithstanding relatively low plasticity, modified polyoxides can be used as incombustible binders for composites, for example, carbon fiber-reinforced plastics, instead of usual organic binders materials such as epoxide and polyester.
In the majority of homogeneous structures including metals, ceramics, glass, and polymers there is an antagonism between heat-resistance, on the one hand, and plasticity, on the other hand. For example, the more strong material we make, the less crack resistance capacities it has.
Living nature (it lacks homogeneous materials) has found the way to partially solve this contradiction using composites which combine two or more substances, say, fibers, interconnected in a monolithic structure-it refers to trunks of trees and plants, muscles, etc.
The first synthetic composite materials appeared at the cradle of civilization. Reinforced construction materials are mentioned in the Bible. In Egypt and Mesopotamia people used boats made of bituminized reed (a prototype of modern glass fiber reinforced plastic
Natural cellular materials: a) corkwood, b) balsa, c) sponge, d) trabecular bone, e) coral, f) cuttlefish bone, g) iris leave, h) plant stem.
boats and mine sweepers). Egyptian mummies are the first example of ribbon winding method application (mummies were wrapped around with a resinated material). And all this happened thousand years before the new era.
A real boom in the development of modern materials technology started in the mid-20th century, when strong and light glass fiber and glass reinforced plastics appeared. These materials were used to produce gliders and other things. Later people developed carbon, boric, silicon-carbide fibers, other organic polymer fibers, and a wide range of binder materials.
At present, aviation, rocket and space technology, marine and mechanical engineering cannot exist without polymer composites (reinforced plastics). Many of them are lighter and stronger than the best metal (aluminum and titanium) alloys and make possible to reduce weight of the "product" (aircraft, rocket, space ship) and decrease fuel consumption. High-speed aircraft (by weight) are manufactured from polymer composites about 7 - 25 percent; reducing total weight of machines by 5 - 30 percent. It is also worth mentioning that after manufacture of polymer composite components waste level is about 10 - 30 percent of material, while waste volumes at production of analogous elements from high-strength alloys of aluminum and titanium can exceed the product mass by 4 - 12 times.
Experience shows that the maximum benefit from polymer composites usage can be obtained by taking a creative approach to the design of respective machines taking into account specific characteristics of reinforced plastics and production technology. Here is a simple example. We all know that metal is an isotropic material, its capacities are equal in all directions, while reinforced plastic has anisotropic characteristics (its strength along fibers is higher than that of cross fibers). Nevertheless, not each product requires isotropic materials. Suppose, longitudinal stress in the cylindrical pipe is two times higher than lateral stress. That is why it is more reasonable to place more cross fibers (because of higher stress) than linear ones. This construction is called full-strength and saves material.
Besides, manufacture of details from polymer composites is cost and labor effective, reduces number of production cycles, allows to produce one big part instead of a great number of small parts later to be joined with screw bolts or welding.
In this respect mankind is ahead of nature taking into consideration absolute strength values of materials and effectiveness of production and processing methods. The best results have been achieved using composite materials based on organic polymeric fibers and organic binders; in particular, it refers to strength-to-weight ratio (absolute strength divided into specific weight).
Organic polymers, metals and ceramics serve as binder materials for composites. Metals and ceramics considerably increase operating
temperatures, but there occur a great number of technological problems which limit practical applications of metal and ceramic composite materials as compared with reinforced plastics.
It is worth mentioning that the structure of high-strength heat-resistant metal alloys is, in fact, similar to the structure of composite materials including reinforced and binder components.
The majority of natural materials have extremely high specific mechanical-and-physical properties (strength, low specific weight, etc.) and form part in structures possessing unique operating characteristics (continuous alternating load resistance, modification under environmental conditions). They are more gradient than artificial materials created by people. In other words, any structure made of these materials has heterogeneous composition and properties.
Along with other functions, thorns of roses and cacti perform the most important duty-protect the plant from casual or intentional intrusion into its life. Spines of animals (hedgehogs, porcupines and fish) play the same role. If we try to measure critical force value through pressing which a very thin and sharp edge of the thorn of the cactus, we shall get a very high characteristic which is inaccessible to homogeneous spikes made of relatively strong structural metals.
Chemical composition of feathers, prickles and claws of wild animals are rather similar: the principal element is keratin. But scientists pay attention to highly heterogeneous composition of elements forming complex structures of different feather zones. Besides, specific strength of these materials can be compared to that of the premium aviation materials (aluminum-magnesium alloys, etc). These materials possess premium properties as for alternating strain resistance. In this connection, there is another very exciting problem. Feather tips act as specific directional and air flow velocity detectors at the wing boundary layer: superflexible material and construction of the wing itself ensure large deflection in terms of negligibly small change of pressure from different sides of feathered surface. Feather tip so deflected slows down expansion of reverse flows at the wing surface at a high angle of attack, that is why birds possess unique flight characteristics inaccessible to modern gliders and airplanes (supercritical attack angles, landing and take off without additional run). It goes without saying that we are far behind the nature in this field and have a great potential of improving operational characteristics of synthetic articles.
LIGHT CELLULAR MATERIALS
Nature created a great number of composite materials with micro- and macropores (cellular materials) in their structure. Core part of bones, in particular bird bones, includes large and small pores, cork tree bark is a classical example of light cellular material, structure of sea sponges and scapes represent combination of penetrating open pores. People also learnt to produce cellular materials using organic polymers, metals, and ceramics. Light thermal insulation materials are used in construction, freezing installations and refrigerators, radio engineering and electronics to ensure electrical insulation of mechanical units, seal off parts, coaxial cables. We all know lightened floating equipment: buoys, beacons, buoyant tanks, windsurfing boards, etc. Three layer cellular materials light and stiff on bending, are widely used in aviation. Structure of these materials is similar to that of bird bones.
After uniaxial compression the overwhelming majority of natural and synthetic materials dilate transversely and have a positive Poisson coefficient (ratio of transverse and longitudinal deformation with a reverse sign). Resin, for example, preserves its volume; it means that the above-named ratio is ~0.5. There is only one isotropic material with a zero ratio-natural corkwood. It does not compress at stretching, that is why it is so valuable for bottle cork production. Man managed to create a special cellular material the structure of which is unknown in the nature, has a negative Poisson coefficient (about - 1), i.e. it transversely dilates at uniaxial tension. But it is not yet used in human activity...
Having summarized all above-said, let's make some conclusions. The man is ahead of the nature in many fields relating to creation of new materials. He creates number metals and alloys resistant to various loads. We use oil as a raw material and synthesize a great number of organic polymers and high-strength fibers. We created composites with record strength and heat-resistance.
At the same time, there is a considerable lag in the development of gradient structures.
Plastic ceramics is still a dream of the mankind. Inorganic polymers, inorganic-organic composite materials (widespread in the nature) seem to be our future. They shall be easy to process, shall have thermal, chemical and water resistance.
Finally, we need a material, which could be used instead of oil and gas for synthesis of organic polymers and composites-it could be natural polysaccharides, cellulose, chitin, and proteins. It is necessary to develop new polymer processing methods that have less environmental impact.
One more important problem (especially from ecological point of view) is reprocessing of constructional materials and subsequent use of such products.
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