In Silico investigations of mechanophore activation in polymers

Abstract

The study of chemical reactions induced by mechanical forces, termed Mechanochemistry, has emerged as a promising field for developing new materials and processes. The advancements in the last two decades in this field have shown that significant deformations in materials can induce mechanochemical reactions which allows for a broad range of applications. Mechanochromism, the response to deformations in terms of changes in color or luminescent behavior, is a key feature that is contributing to numerous applications. In the field of polymer mechanochemistry, this behaviour is governed by small molecular subunits called mechanophores. Specifically, when a bulk material is deformed, its internal stress and strain distribution change, which can create local variations in the forces experienced by specific regions within the material. These local forces lead to the activation of mechanophores and contribute to the overall mechanical behavior of the material. This thesis explores the such complex interplay between mechanical forces and chemical reactions in polymer materials, shedding light on the fundamental mechanisms underlying mechanophore activation processes. Most mechanophores developed to date are activated by stretching forces, that pull the mechanophore from two opposite directions and break a labile bond in it. However, in some cases the bond present in the polymer itself or some other bonds of the mechanophore rupture, which limits the mechanophore activation efficiency. The third Chapter of the thesis focuses on this issue, where the activation of mechanophores as a result of stretching forces is discussed in detail. This chapter is divided into two parts. The first part discusses the activation mechanism of flex-activated mechanophores under tension. A combination of quantum chemical static and dynamic calculations is used to compare three different mechanical deformation modes to identify the most efficient method. The results provide detailed insights into the activation of flex-activated mechanophores in polymers. The second part of the chapter addresses the adjustable threshold parameter by identifying eleven different linkers to fine-tune the activation rates of three different mechanophores. The linkers allow for the adjustment of the threshold barrier, and the results show applicability in the context of the gating mechanochemical process, tuning it from one-step to two-step gating. Another way to apply forces to the mechanophores is by compressing them. Generally, there are mechanophores which activate based on compressive forces, but here we are showing that the activated form of a mechanophore can be reversed using hydrostatic pressure. For this purpose, a spiropyran (SP) mechanophore is activated under stretching forces through linkers. Under hydrostatic pressure the activated form (i.e. merocyanine (MC)) is deactivated back to SP form. By subjecting the SP-MC system to static and dynamic calculations, we established a two-step baro-mechanical cycle for repeated activation and deactivation of the mechanophores, achieved by alternating mechanical stretching forces and hydrostatic pressure. As SP-MC show the mechanochromism behaviour, a time-resolved UV/Vis absorption spectrum is provided to show the interconversion and true mapping of the stress type over the material. The second part of Chapter 4 is dedicated to explore the effect of hydrostatic pressure on a [2,3]-sigmatropic rearrangement. In this study, we demonstrate that under hydrostatic pressure the transition state of a rearrangement can be transformed into a minimum on the potential energy surface. Our simulations, conducted using Born-Oppenheimer molecular dynamics (BOMD) methodology, revealed that a gradual increase in hydrostatic pressure up to 120 GPa results in the formation of the transition state (i.e., the five-membered ring). Conversely, a step-wise reduction in pressure led to the formation of a 70:30 mixture of the product and educt of the sigmatropic rearrangement. To facilitate experimental studies, we have included reference data in the form of simulated IR, Raman, and time-resolved UV/Vis absorption spectra.

Lastly, a multiscale mechanochemical model was developed to simulate the SP mechanophore activation inside linear poly(methyl methacrylate) (PMMA) polymers. To achieve this, we parameterize the force fields of SP and MC molecules in molecular mechanics (MM) to obtain a SP-MC isomerization force-modified potential energy surface (PES) as reference Quantum mechanics (QM) energies. During bulk deformations, such as uniaxial tensile and shear deformation of the system, mechanophores activation is captured at 5.10^7 and 5.10^8 s^-1 strain rates. The molecular rearrangement of the chains as a function of the applied strain provided the insight that the force transduction behaviour from the macroscopic level to the local environment of the mechanophore is a result of direct chain elongation. Interestingly, we also found that there is a strong influence of the stress type on the stress-strain behaviour and resulting mechanochemical activity. Overall, our results provide a detailed understanding of the force transduction behaviour of bulk scale deformation to govern the mechanophore activation.