Soft materials with high toughness and high fatigue resistance have found important applications in diverse areas, paving the way for reliable and durable soft machines. Representative examples include tough hydrogels for underwater actuators, fatigue-resistant hydrogels for gastro-retentive devices, and robust hydrogel adhesives for wound closure and biomedical implantation.

These applications aside, understanding fracture of polymer networks is a fundamental and challenging topic in the field of mechanics of soft materials. Although intensive efforts have been made on applications of soft materials, many intriguing phenomena related to fracture of polymer networks remain dilemma. For example, fatigue thresholds of elastomers and gels have been qualitatively attributed to the energy for fracturing a single layer of polymer strands at the crack path. However, it is still not clear how to quantitatively relate the topological structure to the fatigue thresholds of soft materials. Imperfections of polymer networks have been recently investigated using a nearly elastic hydrogel (i.e., polyacrylamide). Since it is challenging to engineering network imperfections in polyacrylamide hydrogels, it is still not well understood how the synergy of chain-length heterogeneity and chain entanglement gives rise to its extraordinary high fracture energy.

The aim of this letter is to develop a physics-based model that is capable of quantitatively capturing the effects of various major topological defects on fracture energy of polymer networks. The model is based on force transfer in affine polymer networks with crosslinks fixed in space. Different from the models for volumetric entropic elasticity of polymer networks, our model deals with localized non-uniform bond energy distributions at the vicinity of a crack tip. We first analyze the bond energy distributions around the crack path in idealized tree-like polymer networks with no topological defects. We further implement the model to analyze the network structure containing cyclic loops and dangling chains, the most two common topological defects in real soft materials. Using the model, we will quantitatively show that the presence of topological defects weakens the material by introducing inactive polymer strands, while toughens the material by increasing the chain length of polymer strands. The synergy of reduction of active polymer strands and increase of chain length leads to decreased fracture energy when dangling chains are introduced, or even increased fracture energy when secondary loop strands are introduced. Our model implies that, although the presence of topological defects inevitably decreases elastic modulus, it can intriguingly enhance fracture energy of soft materials considerably by introducing secondary loop strands while suppressing dangling chains. With recent advances in engineering defects in soft materials, our model is anticipated to open new design spaces for soft materials with desirable mechanical properties.


Universal Fracture of Polymer Networks With Topological Defects

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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