Properties of a Highly Crosslinked Elastomer

Summary

The properties of a highly crosslinked polyurethane elastomer, based on ricinoleic acid (castor oil) as the backbone, have been measured. The rubber, termed Galcit I, has been proposed as a standardized, highly birefringent, nearly elastic rubber for use in testing new apparatus on a material with known properties, and in checking or calibrating existing instruments. Various characterizations have been carried out and the results will be presented briefly. These include stress relaxation and dynamic shear modulus, uniaxial and uniform biaxial tests to failure, some additional biaxial tests in combined shear and tension, and swelling. in several liquids. The tensile modulus at 25°C is high, about 37 bar (535 psi). M_c ≈ 2300g/mole as determined from swelling and ultimate properties. This corresponds to 112 carbon atoms between crosslinks, which would be equivalent to M_c ≈ 1500 in a polybutadiene or 2000 in natural rubber. This is a high enough crosslink density to virtually eliminate the effects of chain entanglements on the time dependence of the modulus and to render the elastomer almost brittle. The stress/strain response is neo-Hookean, i.e. at 25°C and above the material appears to obey the classic, Gaussian-chain theory of rubberlike elasticity for strains up to rupture, though these are only of the order of 60%. At very small strains in biaxial tests, however, there appears to be a departure from the simple kinetic theory response.