as discussed in the Sperling’s book (Sperling 2006 ). Recent development about the
rubber elasticity theory has been well introduced in the textbook written by
Rubinstein and Colby (Rubinstein and Colby 2003 ).
In 1805, the blind philosopher Gough reported two experimental observations
(Gough 1805 ). In the first experiment, he found by his lips that the temperature rises
upon a fast stretching of a natural rubber. In the second experiment, he observed
that the stretched rubber strip retracts upon heating and elongates upon cooling.
After 50 years, Joule proved his observations (Joule 1859 ). Therefore, the above
phenomena are also calledthe Gough-Joule effect. The first observation is related to
the releasing of latent heat due to the crystallization of the rubber upon fast
stretching, while the second observation can be interpreted according to (3.30)
with the elastic modulus proportional to the temperature, revealing the entropic
nature of the rubber elasticity. The phenomenon of contraction-on-heating and
expansion-on-cooling in the stretched rubbers is right opposite to that of expan-
sion-on-heating and contraction-on-cooling in conventional materials. In Deutsches
Museum at Munich, there exhibited an experiment for such a phenomenon. As
demonstrated in Fig.3.5, if you press the button at the bottom of the exhibition
window, the light will be switched on to heat the rubber strip. Then, the elastic
modulus of the rubber will increase, and the electronic balance at the bottom will
display a reduced weight compared to before. After a few seconds, the light is
automatically switched off, and then the previous display of the weight will be
gradually recovered.
Question Sets
- Why do we need the statistical analysis to treat chain conformation?
- Why is the Gaussian function successful in the statistics of chain conformation?
- Why is the high elasticity of the rubber mainly sourced from the entropy change?
- Why does the stretched rubber show the hot-retraction and cold-expansion
phenomena?
Fig. 3.5 Illustration of the
experiment demonstrating the
Gough-Joule effects. The
light will heat the rubber strip,
increases the latter’s elastic
modulus and then decreases
the displayed weight in the
electronic balance. Switched-
off the light, the displayed
weight will be gradually
recovered
40 3 Conformation Statistics and Entropic Elasticity