Effect of Polymer Volume Fraction on Fracture Initiation in Soft Gels at Small Length Scales

The influence of polymer volume fraction, ϕ on fracture initiation via puncture is studied in self-assembled triblock copolymer gels. Spherically tipped indenters of radii varying over a wide range were used to characterize puncture at length scales on the same order of magnitude as the elasto-capillary length (∼μm) and significantly below the elasto-fracture length (∼mm) for ϕ = 0.12-0.53. Critical energy release rate, G_c for ϕ = 0.12-0.30 was found to be in agreement with the predicted scaling from the classical Lake-Thomas model modified for gel fracture via the failure mechanism of chain pull-out and plastic yielding of micelles (G_c ∼ ϕ^2.2). Interestingly, we demonstrate that fracture initiation energy, $$_o, from puncture scales as $$_o ∼ ϕ, thus, indicating the role played by different fundamental mechanisms governing fracture initiation in soft gels. Additionally, gels with ϕ = 0.53 show deviation from experimental scalings for G_c and $$_o, likely due to a change in micellar morphology leading to anomalous fracture behavior.