Journal Club Theme of August 2013: Synthetic Mechanochemistry
Mechanochemistry, as its name implies, is the use of mechanical force to drive chemical changes. One of the most common forms of synthetic mechanochemistry is simply homolytic bond scission. Recently, researchers have been working on designing materials for productive mechanochemical function (e.g. autonomous damage sensing, intrinsic healing). Mechanoresponsive polymers can be created by covalently linking force responsive chemical groups, termed mechanophores, into the polymer backbone or as crosslinks between polymer chains. Mechanophore functionality is based on weakest link concept in which the polymer chains are attached such that a targeted bond will consistently break at lower forces than the adjacent bonds. This local reaction can be conceptualized (and calculated) in terms of a force modified potential energy surface which describes how force reduces the reaction energy barrier and shifts the relative energy levels of the mechanophore states (e.g. 1-4).
Mechanophore-linked polymer response has been experimentally demonstrated in a range of polymer matrices and under various modes of macroscopic deformation (e.g. 5-13). Functions that have been demonstrated include mechanically induced color change, fluorescence, chemiluminscence, chain length extension and contraction, and catalyst release. Mechanophore reactions within bulk polymers take place as a result of the transmission of stress on the macroscale to force on the molecular scale. Therefore, the evolution of the mechanochemical reaction with external loading, depends heavily on the polymer matrix, linking architecture, and force dependent energetics of the mechanophore itself. With the growing body of synthetic capabilities, there is a need for quantitative theoretical understanding of how these polymers will respond to external loading. Such an understanding will facilitate mechanophore-based polymer design.
Our recent paper  describes a modeling framework for mechanochromic glassy polymers. The continuum activation model developed therein relies on two smaller scale simulation methods (molecular dynamics for determining local force-stress relations and ab initio calculations for mechanophore kinetics), and on a continuum mechanical model. The model has been validated against experimental data for the specific case of spiropyran-linked PMMA and found to be surprisingly predictive.
Some questions I’ve been thinking about lately:
What mechanophore enabled functionalities would be industrially relevant?
To what extent do the details of the polymer matrix influence the mechanophore kinetics for a given applied force?
How does local force relate to macroscopic stress? And can/how can the external conditions (e.g. loading history) to activate a given mechanophore be tailored by linking architecture without significantly influencing the polymer mechanical behavior?
How do temporal and spatial inhomogeneities, which are inherent to bulk polymers, play out in terms of the mechanochemical behavior of the material? Or conversely, can we learn something quantitative about this inhomogeneity by monitoring the mechanochemical reaction?
What mechanophore energetic profiles would be useful for creating polymers with a particular functionality? For instance a chain length extending mechanophore that doesn’t activate until a force of 4nN under quasi static rates would not be useful for stress relief because the adjacent polymer chain segments would break at a lower force.