Thermoelastic Behavior of Natural Rubber

Stress—temperature measurements of natural rubber were carried out up to the elongation ratio, α, of 2.0. An automatic stress relaxometer was constructed for this purpose which can be completely enclosed in a controlled environment. Experiments were so conducted as to minimnze possible chemical effects and nonequilibrium conditions. Relative internal energy contribution to stress, fe/f, is calculated as a function of α in terms of statistical and thermodynamic theories. Both of these yield similar results. It is shown that in the region of low strains (1.0<α<1.5),fe/f decreases rapidly with increasing α, but appears to remain constant at 1.5<α<2.0. This observation is not in agreement with the prediction of the current statistical theory of rubber elasticity, which stipulates that the energy effects are intramolecular and independent of deformation. Implications of these findings are discussed. It is suggested that perhaps at low strains the intermolecular interactions are large in comparison with intramolecular energies, but become relatively insignificant at higher elongation ratios. The temperature coefficient of unperturbed chain dimensions is also calculated from thermoelastic data. It is constant only in the region 1.5<α<2.0. Finally, a new, more exact derivation of the Elliott—Lippmann anisotropy factor in terms of the statistical theory is given in the Appendix.