Chapter3-RubberElasticity_Gedde - Chapter 3. Rubber...

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59 Chapter 3. Rubber elasticity 3.1. Introduction 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.
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60 Figure 3.1. Entropy-driven elasticity of rubber materials Rubbers exhibit predominantly entropy-driven elasticity. This was concluded already by William Thomson (Lord Kelvin) (1857) and James Prescott Joule (1859) through measurements of force and specimen length at different temperatures. They discovered the thermo-elastic effects: (a) a stretched rubber sample subjected to a
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.

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Chapter3-RubberElasticity_Gedde - Chapter 3. Rubber...

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