Fracture and large strain behavior of self-assembled triblock copolymer gels

Abstract

International audienceThe rate dependence of fracture has been studied in a series of physically associating triblock copolymer gels that have a well-defined molecular structure. Compressive experiments were performed to develop a strain energy function that accurately captures the strain hardening behavior of these materials. This same strain energy function was utilized in a finite element model of the crack tip stresses, which become highly anisotropic at stress values below the failure strength of the gels. The rate dependence of the energy release rate, G, is independent of the gel concentration when G is normalized by the small strain Young's modulus, E. The gels exhibit a transition from rough, slow crack propagation to smooth, fast crack propagation for a well-defined value of the characteristic length, G/E

Rate-Dependent Stiffening and Strain Localization in Physically Associating Solutions

“…13 Linear and non-linear mechanical behavior was thoroughly investigated as well. 1,13,14 The structure and mechanical properties of the physically associating solutions are dependent on temperature, as described in detail elsewhere. [14][15][16] In summary, at high temperatures (> 70°C) the triblock copolymer is fully dissolved in the solvent, forming a freely flowing, low-viscosity fluid.…”

“…At larger strains, the finite extensibility of the network strands and the subsequent strain-stiffening behavior is accurately described by a single fitting parameter, J*. This expression has been employed previously to describe the large strain behavior of elastic, self-assembled triblock copolymer gels deformed in uniaxial compression 1 and shear 14 and equivalent functions have been applied to describe the non-linear elasticity of biological systems 33 and recently stiff polymer networks. 34 The physically associating solution is assumed to deform affinely, so that the local extension ratios We assume that strain energy is stored in deformed 'network strands' or 'bonds', which in our case correspond to bridging midblocks that span different endblock aggregates.…”

“…These values are consistent with fracture energies obtained from conventional Mode I fracture tests of similar acrylic triblock copolymer gels. 1 Based on these similarities and the damage accumulation observed at γ > 400% in step-strain experiments, we believe that the rapid strain softening taking place in our solutions for Γ > 1 is an indication of shear-induced breakdown of the macromolecular network.…”

“…Introduction When soft polymeric materials are deformed in a non-linear manner (e.g., to large strains or at high strain rates), the resulting mechanical behavior is commonly a function of the formation and evolution of a wide variety of instabilities, such as ductile and brittle fracture [1][2][3] and shear banding [4][5][6] in polymeric and micellar solutions and gels. There is much evidence to suggest that the type of instability developed in soft materials is strongly influenced by the structure of the material and the nature of the imposed deformation.…”

Effects of finite chain extensibility on the stress fields near the tip of a mode III crack

“…Some of these applications have stimulated interest in understanding the fracture mechanics of soft materials (e.g., Baumberger et al 2006;Bouchbinder et al 2008;Krishnan et al 2008;Seitz et al 2009;Cristiano et al 2010;Livne et al 2010). However, in comparison with the vast literature on fracture of stiff solids, such as ceramics and metals, fracture of highly compliant soft elastic solids received much less attention and is not well understood.…”