Experimental Tests of Entanglement Models of Rubber Elasticity. 3. Biaxial Deformations

The relative abilities of eight molecular models to describe the response of a cross-linked polymer network to general biaxial deformations are tested. The reduced two-parameter models of Flory and Erman and Ball, Doi, Edwards, and Warner and, to a more limited extent, the reduced one-parameter Gaylord model are found to be able to account for large portions of the published experimental data. However, none of the eight models can reproduce the negative W₂values that are observed at very small strains. It is shown that the reduced curve method of Treloar for the representation of biaxial data is inferior to the traditional Rivlin-Saunders method.