EXTREME INDENTATION AND FRACTURE OF SOFT POLYMER GELS

The mechanical properties of conventional hard materials, such as metals and ceramics, have received widespread attention in the past several decades; however mechanical characterization, failure in particular, of soft materials, such as polymer gels, elastomers, and biological tissues and organs, has largely been ignored. While practical issues such as difficulty in handling, processing, and slippage offer complexities in characterization, the breakdown of the fundamental assumptions of linear elastic fracture mechanics due to large strains prior to failure, significant energy dissipation ahead of a crack tip and rate and time dependent effects makes understanding of failure in soft materials even more challenging. Moreover, most research efforts have primarily focussed on understanding crack propagation in soft polymer materials, and not much attention has been given to the critical force and energy involved in the nucleation of a crack in soft solids. Understanding the mechanics behind crack nucleation is not only vital for advancing soft material fracture mechanics, but is also relevant to numerous healthcare applications such as percutaneous needle insertion (biopsies, blood sampling, and anesthesia); robot-assisted surgeries, and design of advanced surgical instruments. Here, we focus on characterizing the process of crack nucleation via puncture with spherically tipped indenters. This simple method offers ease of implementation without requiring special sample preparation, while also yielding interesting insight into the process of crack nucleation, or fracture initiation, in soft polymer networks. First, we study puncture in a series of acrylic triblock copolymer gels with varying gel concentration at characteristic length scales relevant to intrinsic material properties and network structure. Failure properties namely, fracture initiation energy were characterized via puncture mechanics, and crack propagation energy, was measured via traditional pure shear tests. Crack propagation energy was shown to scale quadratically with polymer volume fraction. The observed scaling is in excellent agreement with the predicted scaling by modifying the classical Lake-Thomas model to incorporate fracture of triblock copolymer gels via chain pull-out and plastic yielding of micelles. Interestingly, our results demonstrate a linear dependence of fracture initiation energy on polymer volume fraction, thus indicating the role played by different fundamental mechanisms governing crack nucleation process in soft gels. Thus, our experimental results proved a new insight into the process of fracture initiation in soft polymer gels. The analysis of puncture tests in gels at the highest concentration showed interesting failure properties, and no longer obeyed the experimentally established scalings for lower gel concentrations. Based on our current understanding, we hypothesize significant enhancement in failure properties due to a change in micelle morphology as indicated by the small-angle X-ray scattering data. Understanding the role played by polymer volume fraction on fracture initiation will significantly contribute towards the molecular design of tough polymer gels for a plethora of innovative applications in soft robotics, tissue engineering, wearable electronics, and soft actuators. Next, we examine the effect of machine loading compliance (or far-field compliance) on the critical force to puncture soft polymer gels. Our results have shown that incorporation of a compliant spring beam in series with the indenter can lead to a reduction in critical puncture force by ~60% when the spring beam compliance is ten times greater than the effective gel-indenter compliance at the point of puncture. The reduction in critical puncture force with increasing beam compliance was varied at deformation rates varied over three orders of magnitude. It was found that varying the spring beam compliance alters the local deformation rate during puncture, thereby affecting fracture initiation in an analogous manner to widely studied crack propagation in soft gels. This work not only contributes towards the mechanistic knowledge of fracture initiation in soft polymer gels but also has an important technological application in the form of a reduced-pain medical device. Lastly, we adopt resilin-based hydrogels as a material system to understand the influence of polymer concentration above and below overlap concentration, c* on fracture initiation via puncture at length scales relevant to biological cells(~10-50 μm). Low strain oscillatory shear rheology enabled bulk mechanical characterization, whereas micro-indentation was used to characterize energy storing capability or resilience as a function of applied strain via micro-indentation at ~mm length scales. A significant enhancement in mechanical properties is observed at higher concentrations reflecting increased network homogeneity above c*. Moreover, the gel at the highest concentration possessed exceptionally high fracture initiation toughness possibly due to the formation of nanoaggregates toughening via a reinforcement mechanism. The lessons learned from this research will significantly advance the current synthetic strategies in the design of soft polymer gels for tissue engineering applications