Nonlinear Elasticity of Ultrasoft Near-Critical Gels with Extremely Sparse Network Structures Revealed by Biaxial Stretching

Abstract

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.

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