Macromolecules, Vol.54, No.5, 2353-2365, 2021
Nonlinear Elasticity of Ultrasoft Near-Critical Gels with Extremely Sparse Network Structures Revealed by Biaxial Stretching
Unequal biaxial stretching experiments reveal the unique nonlinear elasticity of ultrasoft near-critical gels with an extremely sparse infinite network that are obtained slightly beyond the gel point. The explicit strain cross-effect (excepting the implicit one resulting from volume conservation) vanishes in near-critical gels, although it is finite in matured gels. The strain cross-effect vanishes for sufficiently sparse infinite networks in which the weight fraction of elastic backbone is comparable to or less than 0.1. The stress-strain relationships for the two types of near-critical gels with different chemical structures in four different types of stretching are satisfactorily reproduced using the simple Gent hyperelastic model with a single set of two parameters. These exceptional features for near-critical gels are attributed to the absence of trapped entanglement effect and the extremely sparse network backbone, which is mostly composed of singly connected chains.