Fracture of Heterogeneous Soft Materials

Abstract

Soft materials are often heterogeneous, or composed of multiple parts. The heterogeneous structure of a soft material alters its behavior, and recent works have leveraged this fact to develop rubbers and hydrogels with exceptional mechanical properties. However, a material’s structure also alters how it fails and fractures. In this thesis, we categorize soft materials by the length scale of their structure, either microscopic or macroscopic, and present methods to characterize fracture in each. Specifically, this thesis explores fracture of heterogeneous soft materials of three types: (i) particle-filled rubbers, (ii) soft materials with corners, and (iii) bonded hydrogels.

For particle-filled rubbers, we engineer the micro- and nanostructure to increase both modulus and fatigue threshold. We synthesize filled rubbers with long and highly entangled polymer chains, which interlink with rigid nanoparticles through strong bonds. For such materials, the feature size is microscopic, and we use continuum methods to characterize the mechanical properties. We measure the mechanical properties as functions of both the polymer chain length and the particle volume fraction. Dense entanglements scale the modulus of the composite, with the scaling factor increasing steeply when the particles percolate. At a crack tip, stress deconcentrates through not only long polymer chains, but also clusters of particles. Upon fracture, stress in a large volume of the composite is released, leading to a high fatigue threshold. We develop a mechanical model of the fatigue fracture process, and its predictions align with our experimental observations. With this material, we demonstrate a rubber gripper that bears high loads and resists crack growth during repeated operation.

We present methods to correlate the conditions for fracture of soft materials with corners. For these materials, the feature size is larger than any crack, and the approach of classical fracture mechanics must be reconsidered. We develop an asymptotic solution to the elastic field near the corner of a neo-Hookean material, and show that the elastic field near a 90-degree corner is finite. We then use the elastic field near the corner to predict the failure conditions of hydrogels with corners. Experimentally, we test hydrogels of two types: ones that stretch modestly before fracture, and ones that stretch greatly before fracture. In both cases, we use the near-corner elastic field to correlate the conditions for fracture.

We present methods to bond hydrogels without functional groups for chemical coupling. Although the feature size of such a system is macroscopic, we use classical fracture mechanics when fracture occurs along the adhesive interface. We design adhesives that crosslink into a polymer network that bridges the interface between hydrogels, such that no covalent bonds are formed between the adhesive and either adherend. By forming an adhesive polymer network with covalent crosslinks, we create adhesion that is stable in corrosive environments and under sustained loads. Furthermore, we show that when the adhesive is triggered by a change in pH, the rate of adhesion is set by the rate of diffusion across the interface. By reducing the thickness of the interface, we reduce the time to adhere by 100 times.

Throughout this work, we summarize our understanding and discuss future challenges for each topic. It is hoped that the techniques developed in this thesis will assist future scientists in making heterogeneous soft materials resist fracture.