The true stress vs. strain curves for unfilled natural rubber
matrix and up to 8.3 wt% carbon nanotubes NR composites at 20 C are represented in Fig. 2. In all cases the typical non-linear ‘‘S curve’’ described in the kinetic theory of rubber
elasticity [1] is observed. It is useful in that case to plot a socalled reduced (or Mooney) stress ½r ¼ r=ða 1 a2Þ as a function of the extension ratio a, or a1 (Fig. 3)modulus,
which is proportional to the cross-link density and to the
temperature [1]. In the Mooney–Rivlin model for large deformation,
[r*] varies linearly with a1, according the expression
½r ¼ 2C1 þ 2C2a1 [1]. Although many refinements have been
developed to describe the functional dependence of r vs. a
[38,39], we limit our analysis within the Mooney–Rivlin representation,
which is often used by rubber scientists to discuss stress–strain curves.