Mechanical and optical behavior of double network rubbers

Stress, strain, and optical birefringence were measured for a series of peroxide-cured natural rubbers, having both isotropic and double network structures. The residual stretch (permanent set) for the latter ranged from 2.0 to 4.5, with elastic moduli that were an increasing function of this residual strain, as found in previous works. A small birefringence, ca. 10(-5), was observed for the unstressed double networks, and its magnitude increased with increasing residual strain. The sign of this birefringence corresponded to extension of the (undeformed) double networks. Under stress, the birefringence followed the stress optical law. The double network properties were interpreted using the constrained-chain model of rubber elasticity, with the assumption of independent, additive contributions from the two component networks. The calculated results differed from the experimental findings, in particular underestimating the residual strain. This failure is a consequence of the overprediction of the stresses during compression, a limitation common to molecular theories of rubber elasticity. The modeling of the double networks does account qualitatively for the sign of their unstressed birefringence, which is due to the stress-optical coefficient being larger in tension than in compression. This particular deviation from the stress-optical law is known from both theory and experiment.