The nature of rubber-like elasticity

Unit 10

GRAMMAR: Participles, Gerund.

1. a) Using a dictionary translate the following words, word combinations and chemical terms into Russian:

rubber-like elasticity, extensibility, stretched, weight, flexible, fibrous, gelatin, substance, sulphur, polyvinyl alcohol, molecular chains, linkage, condition, force.

b) Find (in the list given below) synonyms to the following words. Translate these words into Russian:

can, certain, complete, contracted, to detect, different, evident, lightly, motion, obtain, property, shape, substance, sufficient, total, violent.

(be able to, behaviour, compressed, definite, enough, to find out, form, full, get, matter, movement, obvious, slightly, strong, various, whole).

c) Translate the following sentences paying attention to the

-ing- forms:

1. On reheating, “melting” occurs and rubber increases in volume.

2. Reinforcing agents harden the rubber and make it more wear resistant.

3. Katz showed that ordinary, unstretched rubber has a disordered structure, resembling that of a liquid.

4. In an ideal rubber-like substance no energy is used in separating chains and in increasing their separation during stretching.

5. The highly coiled and folded condition of the rubber chains permits their being extended up to seven times their original length.

6. New ways of modifying the properties of existing products are being discovered widening the field of application of the rubber-like state.

7. When parts of the long molecules of natural rubber arrange themselves in an ordered state crystallizing they are assumed to exhibit a first order transition.

8. The problems regarding the use of pigments in latex are similar to those in other types of coatings and finishes.

2. Choose the correct forms:

1. Latex may be used for (impregnating, impregnated) paper, leather, or cloth, the rubberized product being water proof.

2. Synthetic rubbers are high polymeric elastic substances (manufactured, manufacturing) from a wide range of chemical compounds.

3. In an ideal rubber-like substance no energy is used in (separating, being separating) chains and (being increasing, increasing) their separation during stretching.

4. This result was clearly demonstrated by Katz (being used, using) X-ray diffraction to detect the crystallinity.

5. On (being reheated, reheating), “melting” occurs and rubber increases in volume.

3. Read the text and translate it using a dictionary.

Text A

The characteristic properties of rubber, its extensibility and complete recovery after even very large deformations, is shown also by many other substances. Of these we may mention supercooled molten sulphur and selenium, gelatin, muscle fibrils, substances built from long chain molecules such as polyvinyl alcohol, etc. These substances are very different chemically, but their common feature is a long flexible molecule. There is a rubber-like state which many substances made from long molecules may assume under suitable conditions.

The two factors which are necessary if perfect rubber-like elasticity is to be obtained are, firstly, that whole molecules must not be able to slip past each other under the action of deforming forces, and, secondly, that these forces shall meet with little resistance in straightening out the coiled molecular chains. In lightly vulcanized rubber, the long chains are connected across at certain points by the strong sulphur linkages. The presence of only a few such points of linkage is sufficient to prevent slipping of the whole molecules, which would result in plastic flow. The atoms of the molecule share in the general thermal motion at any temperature, so that a free molecule would be continually coiling, twisting and changing its shape. There are many more ways in which such molecules can be arranged to give a crumbled chain.

The lengths of chain between sulphur crosslinks behave in essentially the same way as the free rubber molecule, so that the vast majority of them are in a contracted form, which changes momentarily with the thermal motion. This freedom comes from the weakness of the van der Waals forces between the chains, which are not strong enough to hold them permanently in position side by side.

When we apply a force to the rubber, the flexible chains are slightly straightened, but are always attempting to return to their folded condition. It is evident that, the more violent the thermal motion, the greater the tendency of the chains to return to their normal positions. If a rubber band is stretched by means of a weight, it contracts on heating owing to the effect of the increased thermal motion. This is contrary to the behaviour of normal substances, which deform more easily at high temperatures.

The highly coiled and folded condition of the rubber chains permits their being extended up to seven times their original length. Long before this, however, some of the chains will have been pulled approximately parallel. When this occurs, the attractive forces between them become sufficiently strong to bind them together in a regular arrangement. Thus the rubber is crystallized by tension. This result was clearly demonstrated by Katz using X-ray diffraction to detect the crystallinity. He showed that ordinary, unstretched rubber has a disordered structure, resembling that of a liquid. Sufficient stretching gives an X-ray diffraction picture similar to that shown by fibrous materials. If the rubber is cooled, while under tension, to a low temperature, it does not contract when the tension is removed, and still gives a crystalline X-ray diffraction pattern. On reheating, “melting” occurs and the rubber contracts. If the frozen stretched rubber is pulverized when cold, it splits up into fibrous pieces, owing to the parallel orientation of the chains.

If unstretched rubber is cooled, a slow crystallization takes place, giving a harder and less extensible material. On warming “melting” again occurs, but unlike that of ordinary crystalline substances, it takes place over a range of some 10 ºC, in temperature. These effects may be observed in crepe soles kept for some exposed to very cold weather.

In an ideal rubber-like substance no energy is used in separating chains and increasing their separation during stretching. As a result there is no change in the total volume of the substance when extended. This condition is not fulfilled by most rubber-like substances, so that their properties only partially correspond to those of the ideal substance.

Commercial rubbers are very complex systems, in which variation of the proportions of the constituents can give an immense range of products.

4. Answer the questions:

1. What substances are known to display the characteristic properties of rubber?

2. What factors is perfect rubber-like elasticity due to?

3. What happens if one applies a force to rubber?

4. In what way did Katz succeed in detecting crystallinity?

5. In what case does the process of slow crystallization take place?

5. Complete the following sentences using some active words and word combinations (see below):

1. The characteristic properties of rubber, its extensibility and complete recovery after even very large deformations, is shown also by many other....

2. The common feature of these substances is a long flexible ….

3. In lightly vulcanized rubber, the long chains are connected across at certain points by the strong … linkages.

4. The lengths of chain between sulphur crosslinks behave in essentially the same way as the free … molecule.

5. If a rubber band is stretched by means of a weight, it contracts on heating owing to the effect of the increased ….

6. The rubber is crystallized by ….

7. In an ideal rubber-like substance no energy is used in separating chains and increasing their separation during ….

8. Commercial rubbers are very complex ….

Tension, stretching, thermal motion, substances, rubber, molecule, sulphur, systems

6. Retell the text using the scheme (see Un.1).

Text B

1. Match the English words and word combinations in A with their Russian equivalents in B.

A B

acidity	агент сливкоотделения
ascending	частица
bacterial	коагулировать
carbohydrate	восходящий
centrifuge	кислотность
coagulate	растение
excess	подходящий
plant	бактериальный
suitable	эмульсия
particle	жидкость
emulsion	центрифуга
creaming agent	углевод
liquid	излишний, избыточный
layer	осадок
sediment	слой

2. Read the text trying to understand its main idea.

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