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

The influence of polymer volume fraction, ϕv 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 ϕv = 0.12–0.53. Critical energy release rate, Gc for ϕv = 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 (Gc ∼ ϕv2.2). Interestingly, we demonstrate that fracture initiation energy, Γo, from puncture scales as Γo ∼ ϕv, thus, indicating the role played by different fundamental mechanisms governing fracture initiation in soft gels. Additionally, gels with ϕv = 0.53 show deviation from experimental scalings for Gc and Γo, likely due to a change in micellar morphology leading to a...