Rapid Activation of Non-Oriented Mechanophores via Shock Loading and Spallation

TL;DR

This study uses large-scale molecular dynamics simulations to investigate how shock loading and spallation activate mechanophores in polymers, revealing the importance of many-body effects and different stress conditions.

Contribution

It provides new insights into mechanophore activation mechanisms under dynamic loading, emphasizing many-body intra-molecular distortions in condensed-phase materials.

Findings

Many-body intra-molecular distortions lower activation barriers.

Reactivity under compression is driven by intra-molecular torsions.

Reactivity under tension is governed by tensile stresses.

Abstract

Mechanophores, stimuli-responsive molecules that respond chromatically to mechanochemical reactions, are important for understanding the coupling between mechanics and chemistry as well as in engineering applications. However, the atomic-level understanding of their activation originates from gas phase studies or under simple linear elongation forces directly on molecules or polymer chains containing mechanophores. The effect of many-body distortions, pervasive in condensed-phase applications, is not understood. Therefore, we performed large-scale molecular dynamics simulations of a PMMA-spiropyran co-polymer under dynamic mechanical loading and studied the activation of the mechanophore under various conditions from dynamical compression to tension during unloading. Detailed analysis of the all-atom MD trajectories shows that the mechanophore blocks experience significant many-body intra-molecular distortion that can significantly decrease the activation barrier as compared to when deformation rates are slow relative to molecular relaxation timescales. We find that the reactivity of mechanophores under material compression states is governed by many-body effects of intra-molecular torsions, whereas under tension the reactions are governed by tensile stresses.