Natural rubber is obtained by coagulation of a latex from a tree called Hevea
Brasiliensis. It consists predominantly of cis-1,4-polyisoprene. Fossilised natural
rubber discovered in Germany in 1924 stems back about 50 million years. Columbus
learned during his second voyage to America about a game played by the natives of
Haiti in which balls of an elastic ‘tree-resin’ were used. The word ‘rubber’ is derived
from the ability of this material to remove (rub off) marks from paper, which was
noted by Joseph Priestley in 1770. Rubber materials are not restricted to natural
rubber. They include a great variety of synthetic polymers of similar properties. An
elastomer is a polymer that exhibits rubber elastic properties, i.e. a material that can
be stretched to several times its original length without breaking and which, upon
release of the stress, immediately returns to its original length. Rubbers are almost
elastic materials, i.e. their deformation is instantaneous and they show almost no
creep.
The unique character of rubber was discovered in 1305 by John Gough who
described his experiments and findings as follows:
‘Hold one end of the slip of rubber …. between the thumb and forefinger of each
hand; bring the middle of the piece into slight contact with the lips; …. extend the
slip suddenly; and you will immediately perceive a sensation of warmth in that part
of the mouth that touches it … For this resin evidently grows warmer the further it
is extended; and the edges of the lips possess a higher degree of sensibility, which
enables them to discover these changes with greater facility than other parts of the
body. The increase in temperature, which is perceived upon extending a piece of
Caoutchouc, may be destroyed in an instant, by permitting the slip to contract
again; which it will do quickly by virtue of its own spring, as oft as the stretching
force ceases to act as soon as it has been fully exerted.’ Gough made the following
comment about a second experiment: ‘If one end of a slip of Caoutchouc be
fastened to a rod of metal or wood, and a weight be fixed at the other extremity
….; the thong will be found to become shorter with heat and longer with cold.’
To convince yourself, please make the experiment. You will only need a strip of
rubber, a weight and a hair-dryer. Gough presented no good explanation to the
unexpected findings, i.e. that the stiffness increases with increasing temperature and
that heat is evolved during stretching. It took almost 50 years before the
thermodynamics of the rubber elasticity was formulated.