Since dw = tdl, where w is work, the basic equation for the internal energy of rubber is dU = TdS + tdl, where T is the temperature and S is the entropy (indicator of the disorder of systems).
1) On the assumption that the tension required to keep a sample at a constant length is proportional to the temperature (t= aT) show that the tension can be ascribed to the dependence of the entropy on the length of the sample.