Biaxial stress-strain behavior of chemical and physical gels of poly(vinyl alcohol)

In a recent paper, Urayama K, Ogasawara S, Takigawa T [Polymer 2006;47:6868-73.] found significant differences in the pure shear behavior of the poly(vinyl alcohol) gels with similar initial modulus but with different types of crosslinks, physical crosslinks formed by microcrystallites and chemical crosslinks made of covalent bonds. The non-Gaussian three-chain model was found to give but a limited explanation of the data. In this paper we show that the constitutive equation that we proposed and tested previously (Polymer 2006) for filler-reinforced rubber networks gives equally good description and reasonable interpretation of the stress-strain data on poly(vinyl alcohol) gels in different geometrical modes. The Arruda-Boyce eight-chain model combined with the Gaylord-Douglas theory of tube-like topological constraints describes well the stress-strain properties of chemical gels in pure shear and in uniaxial and equibiaxial extensions. The pure shear behavior of physical gels can be reasonably explained and described by taking into account the amplification of local strain in the presence of inextensible particles (crystalline domains).