Multiscale Modeling and Design of Mechanochemically Active Interfaces

Abstract

Mechanochemically active systems experience chemical reactions when subjected to critical levels of mechanical force and have applications in self-healing and early warning systems. One approach towards developing such systems is the use of force responsive chemical groups called mechanophores that can be covalently bonded to polymeric systems. In the past decade a library of mechanophores have been developed - ones that change color, fluoresce, unveil cross-linking sites, and trigger catalysis when activated to force. Though mechanophores in bulk polymers have been extensively studied, the placement of mechanophores at interfaces has attracted little attention. Augmenting interfaces with covalently attached mechanophores has tremendous potential for damage management in polymer composites that fail through interfacial debonding. We refer to a polymer composite augmented with mechanophores as an Interfacial Mechanophore Augmented Composite (IMAC). In this dissertation we investigate the multiscale physics of mechanophore activation in IMACs and capture design principles for IMACs. First, mechanophore activation at an interface is studied using a molecular dynamics (MD) interface model subjected to shear. Our simulations demonstrate that interfacial mechanophores activate starting with the mechanophores aligned along the direction of shear and progress to the the rest of the mechanophores. For interfacial mechanophores to activate the attachments to the substrate need to withstand the force necessary for mechanophores to activate. Given strong attachment, mechanophore activation is primarily governed by interfacial displacements. Next, inspired by our MD study, we connect the macroscopic stress state in an IMAC to mechanophore activation using an extensible link mechanophore model. The extensible link model mechanophore stretches with the attachment points, activating when a desired length change is achieved, and exerts negligible tractions on the underlying system. With this mechanophore model, mechanophore activation can be computed directly from the displacement fields in an IMAC near the interface. We demonstrate this model framework through the classical mechanics systems of (1) a circular filler particle in a planar polymer matrix subjected to remote loading and (2) a three dimensional IMAC with a dispersed, dilute volume fraction of spherical filler particles subjected to uniaxial loading. The interfacial debonding is governed by cohesive zone laws. Our simulations show the interplay between debonding mechanics and mechanophore activation. Mechanophore activation relative to debond depends on two crucial non-dimensional parameters - the length change required to activate relative to critical interface debond length scale and the critical interface debond stress relative to matrix Young's modulus. We compute design maps to showcase the zone where mechanophores can be expected to activate during debond. We also compute the impact of other material parameters and volume fraction of filler particles on mechanophore activation. Lastly, we answer a fundamental question of how the force dependent rate of a mechanophore reaction is affected by the environment in which it is placed. We use a 1D transition state theory approach for the computing reaction rates, with a sinusoidal potential representing the environment superimposed on a mechanophore double-well potential. We find that reaction rates are significantly affected by the addition of the potentials via changes in energy barriers and the creation of metastable states.