S4E7

Abstract

In gels and elastomers, how entanglements affect deformation has long been studied, but how entanglements affect fracture, fatigue, and friction has not. Here we synthesize gels and elastomers in which entanglements greatly outnumber crosslinks. We demonstrate that such polymers resolve a long-standing conflict: crosslinks stiffen polymers but embrittle them. When a polymer of dense entanglements and sparse crosslinks is stretched, before a polymer chain breaks, tension transmits in the chain along its length, and to many other chains through entanglements. This distribution of tension leads to high toughness, strength, and fatigue resistance. Upon swell in water to equilibrium, the hydrogels exhibit low hysteresis, low friction, and high wear resistance. Such an exceptional combination of properties opens doors to broad and immediate applications.

Introduction of speaker

Junsoo Kim is a graduate student in the Zhigang Suo’s group at Harvard University since 2017. He received his BS and MS in mechanical engineering from Seoul National University in 2011 and 2013, and worked as a researcher at Electronics and Telecommunication Research Institute in Korea from 2013 to 2017. He is currently studying the mechanical properties of polymer networks, including synthetic elastomers, synthetic hydrogels, natural hydrogels, and composites.

Abstract

Nature provides countless examples of the use of material heterogeneity to enhance the failure properties of materials. Many biological materials, such as bone, marine shells, and fish scales, are extremely resilient to fracture and failure. These often consist of regions that are highly mineralized and stiff and regions of biopolymers that are extremely soft. Such principle is widely applied in engineering materials. First, I will provide a mechanistic explanation for toughness enhancement in elastic dissipator, a class of material combining soft phases with contrasting moduli and strong adhesion. I use finite deformation theory to characterize deformation fields around crack tips in soft bi-material interface, validated with finite element analysis as well as experimental studies in Polydimethylsiloxane (PDMS) systems with variable cross-linker ratio.

In practice, combining such disparate materials in synthetic systems is fraught with difficulties, such as poor interfacial adhesion. I will show geometric heterogeneity can lead to similar enhancements to failure characteristics. Inspired by the microstructure of the dactyl club of the Mantis shrimp, I show how geometric defects that are intrinsic to extrusion-based additive processes (voids and weak interfaces) can be spatially arranged in a helical (Bouligand) pattern to produce complex crack patterns and enhanced energy absorption.

Introduction of speaker

Chengyang Mo is currently a fifth year PhD candidate at University of Pennsylvania in the department of Mechanical Engineering and Applied Mechanics advised by Prof. Jordan Raney. His researches primarily focus on 3D printing, mechanics of materials, and material design. Prior to joining Penn, he received BS/MS accelerated degree both in mechanical engineering and a minor in material science from Drexel University in 2017.