S1E8

Abstract:

Complex 3D functional architectures are of widespread interest due to their potential applications in biomedical devices, metamaterials, energy storage and conversion platforms, and electronics systems. In this talk, I will discuss exploiting structural buckling to create flexible, multifunctional 3D microstructures and electronics from thin-film materials including polymers, metals, semiconductor silicon, and a heterogeneous combination of those materials, which are inaccessible with existing technologies. Both the fundamental buckling mechanics and a wide range of assembled 3D functional thin structures, including shape-programmable 3D structures and 3D electronic scaffolds with sensing and stimulation capabilities, will be presented. I will conclude my talk by briefly discussing new opportunities in the development of multifunctional materials and structures for many applications.

Abstract:

Shape memory polymers (SMPs) are a class of smart materials that can remember temporary shapes and recover to permanent shapes responding with external stimuli. SMPs have a wide range of applications in a variety of fields, including aerospace, biomedicine, flexible electronics, and other fields. Due to the limited species and applications of SMP materials, the development of the SMP has been in a state of stagnation for a long time. This research mainly studies the new properties brought by the introduction of reversible bonds to SMPs and expands the species and applications of SMPs.

Reversible bonds are a class of bonds that can be broken and reconstructed in particular states. In this work, temperature sensitive reversible bonds are introduced into SMP systems. Thus, this novel SMP has another reversible bond activation temperature range besides the phase transition temperature of conventional SMPs. Within this reversible bond activation temperature range, the mechanical properties of the material are greatly affected by temperature. By introducing suitable reversible bonds and adjusting the network structure, the novel SMP exhibits the elastic behavior of conventional SMPs below the reversible bond activation temperature and exhibits the plastic behavior in the reversible bond activation temperature range.

Abstract:

The double network concept has been revolutionary in its ability to turn soft, brittle hydrogels into tough, robust materials with mechanical properties that match the best synthetic elastomers. Double network hydrogels consist of two interpenetrating networks, where each network has a specific mechanical response: the “first network” acts as a sacrificial network, consisting of a rigid, extended network, and the “second network” is a globally percolated, stretchable network. When a double network hydrogel is stretched, covalent bonds of the first network break, dissipating energy; this process continues with increasing strain, until the sacrificial network is completely broken and the second network ruptures. The goal of this research is to demonstrate that the “sacrificial bond concept” is applicable at length-scales beyond the molecular scale. We aim to incorporate this design concept universally for application in structural and medical devices.

Like double network hydrogels, our system consists of a rigid “first network,” 3d printed polyurethane/polyacrylate grids, embedded in a soft and stretchable “second network,” silicone rubber. We found that when the strength of the matrix exceeds the strength of the grid, local fracture occurs in the grid, and stretching is isolated to the rubber in the fractured region. As stretching increases, the force increases, and when the local force exceeds the global strength of the grid, fracture will occur elsewhere in the composite. This process continues sequentially throughout the sample until all grid fracture sites are exhausted, and the matrix ruptures. By tuning the stiffness of the grid, we can independently control the yield strength and fracture strain of the composite, until a point where the grid strength exceeds the matrix strength, and the multiple fracture process no longer occurs.

We also systematically studied the interfacial interactions between the matrix and the reinforcing grid. Both interfacial adhesion as well as topological interlocking are important towards developing a robust composite. By adhesive interactions alone, only minimal fracture of the reinforcing phase occurs; topological interlocking is required to maximize fracture. Based on this result, we systematically change the grid size to modify the number of fracture events. In the optimized form, an increase in work of extension of ~50% over the neat matrix was achieved, representing a ~70% toughening efficiency versus the calculated maximum toughness. These results demonstrate that macroscale double networks can dramatically increase the toughness of soft materials.