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the nature of rubber - like elasticity.

The characteristic property 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 ancohol, 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 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. These 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 the free molecules would be continually coiling, twisting, and changing its shape. There are many more ways in which such molecules can be arranged to give a crumpled 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 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 100C in temperature. These effects may be observed in crepe soles kept for some time exposed to very cold weather.

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

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

Exercises

I. Answer the following 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?

II. Find (in the list given below) synonyms to the following words. Translate these words in to Vietnamese:

can

certain

complete

contracted

to detect

different

evident

lightly

motion

obtain

property

shape

substance

sufficient

total

violent

to be able to

behaviour

compressed

definite

enough

to find out

form

full

get

matter

movement

obvious

slightly

strong

various

whole

III. What do you know about:

a branched chain

a coiled chain

a crumpled chain

a flexible chain

a main chain

a molecular chain

an oriented chain

a side chain

a straight chain

IV. Translate the following sentences paying attention to the words in bold type:

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

2. There is a rubber like state, which many substances made from long molecules may assume under suitable conditions.

3. Mendeleyev's theoretically assumed value of 240 for the atomic weight of uranium was confirmed by the scientists.

4. Synthetic rubber produced from isoprene was presumed to a long chain structure built up from isoprene units by 1-2, 1-4 linkages.

V. Fill in the blanks with prepositions:

1. The characteristic property ... rubber, its extensibility and complete recovery .... even large deformations, is shown ... many substances.

2. The lengths ... chain sulphur crosslinks behave ... essentially the same way as the free rubber molecule, which changes ... thermal motion.

3. 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 ... each other ... the action .... deforming forces, and, secondly, that these forces shall meet ... little resistance ... straightening ... the coiled molecular chains.

VI. 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.

VII. Translate into English

1. GÇn 25 n¨m sau mét sè nhµ b¸c häc ®• ®­a ra hµng lo¹t lý thuyÕt vÒ tÝnh ®µn håi cña cao su.

2. Sù ph¸t triÓn nh÷ng lý thuyÕt chung vÕ tÝnh ®µn håi lµ mét sù kiÖn quan träng trong khoa häc.

3. C¸c nhµ khoa häc ®• x¸c ®Þnh lµ c¸c ph©n tö cao su bÞ biÕn d¹ng khi chuyÓn ®éng nhiÖt.

4. Ng­êi ta còng ®• chøng minh ®­îc r»ng cã sù phô thuéc trùc tiÕp gi÷a lùc co l¹i vµ nhiÖt ®é tuyÖt ®èi.

5. CÇn ph¶i nhí r»ng cao su ch­a l­u ho¸ bao gåm mét sè lín c¸c ph©n tö.

6. Theo thuyÕt Brown th× bÊt kú mét m¹ch cao su nµo còng ë tr¹ng th¸i chuyÓn ®éng nhiÖt vÜnh cöu.