(670d) Toughening Mechanisms and Fracture Behavior of Model Polymer Gels

The relationship between the viscoelastic behavior and fracture response of polymer gels is not well understood. In these experiments we examine two different mechanisms for toughening model polymer gels. In one system we exploit the viscoelastic nature of polymer chains by adding an uncrosslinked sol fraction to a crosslinked silicone gel. The other system capitalizes on the load-transfer mechanisms of double network gels: a highly crosslinked silica gel is interpenetrated by a thermoreversible acrylic triblock copolymer. The model silicone gels have varying sol fractions and sol molecular weights that allow us to better understand how the energy release rate is affected by changes in molecular weight, sol fraction, and viscoelastic properties of these gels. Axisymmetric indentation experiments have shown that the presence of a sol fraction appears to toughen the gel while simultaneously decreasing the modulus. The fracture behavior of these gels can be observed in a mode I tensile tear test. The tear tests provide a well-defined geometry that allows us to determine the energy release rate of the gels as a function of crack velocity. Results highlight the role of viscoelastic loss in the bulk gel as a method of increasing toughness. The fracture process does not change with the presence of sol fraction, but the sol provides an additional mechanism for energy dissipation. Likewise, the presence of an acrylic triblock copolymer in hybrid silica-triblock gels provides an energy dissipation mechanism that allows the samples to support small fractures. These experiments provide a basis to model the gels' behavior over a wide range of deformations.