Figure 1. Mechanochemistry modes of activation.
At the intersection of mechanics and chemistry, mechanochemistry is a subject that embraces many everyday phenomena including wear and abrasion, friction and lubrication, and stress-accelerated degradation of materials. Through collaborative research at the University of Illinois Beckman Institute, we are testing the "mechanophore hypothesis" which states that force drives chemical change in selective and productive ways. The goal is to invent materials that have new functionality, such as the ability to repair themselves when damaged. Such materials promise to be safer and last longer than conventional materials. Our concept of a mechanophore is a stress or strain activated molecular unit that is inserted into a polymeric material to provide a molecular-scale reading of the local mechanical state or to transform materials properties in response to the local mechanical environment. Molecular designs and research activities are founded on fundamental principles borrowed from polymer science and physical organic chemistry. Building on the mechanophore concept, research encompasses a variety of goals and challenges. Most of our research in mechanchemistry falls into one of four categories: Sustainable materials, Mechanically-induced depolymerization, Mechanoacids and Shock wave energy dissipation (Fig 1.)
Shock wave energy dissipation (SWED) by Mechanochemically Active Materials
Dissipating shock wave energy from detonation is necessary to protect soldiers from traumatic brain injury. Shock waves cause a sudden spike in pressure and temperature when passing through a system. We are developing mechanochemically active materials that respond to these high pressures and undergo chemical transformations that dampen the shock wave energy. This project involves: i) designing and synthesizing molecules that can withstand impinging shock waves; ii) identifying and developing chemical transformations that are activated by shock waves, dissipate energy, and are potentially reversible; and iii) identifying the effect of chemical bonds and atoms involved in chemical transformations on capacity of SWED.
Autonomically adaptive materials
Materials systems with reusable building blocks are attractive for autonomically adaptive structures that remodel themselves in response to aging or stress. Nature uses depolymerization and repolymerization cascades to recycle monomeric building blocks in biomaterials. For synthetic materials, remodeling has the potential to extend device lifetime by removal and replacement of damaged regions or by restructuring parts to meet changing demands of their use. The group has initiated work aimed at utilizing mechanically-triggered depolymerization of metastable polymers followed by their repolymerization towards the goal of autonomically adaptive polymeric materials. Mechanically-triggered depolymerization has many other potential applications (e.g. microcapsules) where mechanical force can initiate the breakdown of shell walls to release stored cargo.
We are learning to design mechanoacids that are selective to mechanical stress and to ultimately couple mechanical stress to acid catalysis within polymeric materials. Mechanocatalysts have the potential to favorably change the chemistry of a polymer by turnover, and may lead to unprecedented mechanoresponsive materials. Major areas of focus are synthetic design of mechanoacids, coupling mechanoacids to synergistic chemistry in polymers, and effective analytical methods to study these unconventional chemical cascades in polymers.
Waste reduction is key to a sustainable materials landscape, and it is achievable through life extension and recycling of polymer materials. We seek to extend the lifetimes of widely used polymer materials by incorporating mechanically triggered damage sensing and healing functionalities. When damage occurs, the mechanophores locally activate to report damage or initiate healing. Major areas of focus to achieve these goals are learning structure-property relationships for efficient force transduction in polymers, chemical synthesis of functional mechanophores, and streamlined analytical tools to study these mechanochemical reactions.
Ultimately we believe our group is well positioned to realize the next major advancement in the field of mechanochemistry as well as further our understanding of matter as it experiences mechanical energy input.
Our mechanochemistry research is done in collaboration with the Autonomous Materials Systems division of the Beckman Institute at the University of Illinois.
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