The strain energy density function of isoprene rubber vulcanizate has been revealed recently by the experiments using biaxial extension method. The functional form of the strain energy density function, W, obtained is
W=cT(I1-3)+β(I1, I2)
where c : constant, T : temperature, I1 and I2: invariants of deformation tensor, and β: function of I1 and I2. The first term of the right hand side of the equation is presumed to be the term based on entropy elasticity and the second term is on energetic elasticity of chain network on the basis of a thermodynamic analysis for the biaxial deformation. A concentrated discussion is made in this paper on the characteristics of the β function to clarify its origin based on some experimental results obtained recently. The value of the β function decreases with increasing degree of swelling of rubber and increases with approaching to the glassy state from the rubbery state. These results lead us to consideration of the effect of inter-molecular force on strain energy density function. A simple network model is presented to interpret the β function, where the three forces are considered i.e., attractive and repulsive forces between molecules and the forces of entropy elasticity acting at crosslinked points.
The derived form of the strain energy density function is similar to the form of the functional form observed by experiments. It is concluded that the β function may be related with the inter-molecular forces between network chains.
