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Journal Club Theme of August 2013: Synthetic Mechanochemistry

Meredith N. Silberstein's picture

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 [14] 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.

 References:

[1] Kauzmann and Eyring, J. Am. Chem. Soc. (1940)   

[2] Bell, Science (1978)

[3] Ong et. al.,  J. Am. Chem. Soc. Comm. (2009)  

[4] Dopieralski et. al.,  Agnew Chem Int Ed. (2011)   

[5] Davis et. al., Nature Letters (2009)

[6] O'Bryan et. al., Applied Materials and Interfaces (2010)  

[7] Black et. al., Journal of Materials Chemistry (2011)

[8] Beiermann et. al., Journal of Materials Chemistry (2011)  

[9] Kingsbury et. al., Journal of Materials Chemistry (2011) 

[10] Y. Chen et. al., Nature Chemistry (2012)  

[11] Deisendruck et. al., J. Am. Chem. Soc. (2012)  

[12] Lee et. al., Macromolecules (2013)

[13] Larsen and Boydston, J. Am. Chem. Soc. (2013)

[14] Silberstein et. al., Journal of Applied Physics (2013)

Comments

Qiming Wang's picture

Hi Dr. Silberstein, Thank you for initiating this very interesting topic. You gave excellent outlooks in this field. Closely related to your third point, recently we are working on using wrinkling instability to control chemistry (activate mechanophores). This is a project in collaboration with Craig Group at Duke Chemistry. Wrinkling (especially post-wrinkling) generated by external physical fields can provide a platform with a patterned stress/strain. As a result, for a mechanoresponsive polymer with a certain threshold of stress for mechanophore activation, a patterned chemical changes are introduced on the polymers, for example patterned color change or fluorescence emission. In this way, we can generate various chemical reaction patterns in the polymer by designing various wrinkling instabilities with the aid of the knowledge of mechanics.  As you mentioned in your outlooks, two of our current concerns are how to molecularly design the mechanoresponsive material to control the threshold of macroscopic stress for mechanophore activation, and how to make patterned chemical reactions into macroscopic functionality.

I guess for different materials the pathways to open the mechanophores are different. So the relations between macroscale stress and local force on the crosslinks may largely depend on the molecular structures. My question is: without knowing the detailed molecular mechanism (maybe without MD simulation), can we have some physical insights on the scaling relationship between the activation intensity and applied shear strain? If possible, could you give a little explanation about this scaling law with mechanoresponsive PMMA in your recent paper (Silberstein, JAP, 2013)? Thank you.

Best Wishes,

Qiming

Meredith N. Silberstein's picture

Wrinkling to pattern chemical changes is a cool idea - I imagine the main challenge will be controlling/generating the right amount of force.

Based on experimental data with glassy polymers so far, there seems to be one consistent physical insight - there needs to be plastic flow for the mechanochemical reaction to take place. Specifically, the yield peak needs to have fully evolved. I interpret this as a mobility limitation (one we do not see in elastomers), but certainly that is up for debate. As far as degree of activation goes, I have not performed enough molecular dynamics simulations or experiments to determine whether the linear scaling between stress and local force is universal across architectures, I suspect that it is not.

With elastomers we can call upon the physical insight from rubber elasticity theory, Kuhn, Flory, etc. For any mechanophore there's an approximate force at which the reaction will take place in a reasonable time frame. On the individual chain scale this force is a function of stretch, Kuhn segment length, and total chain length. With a full polymer network stretch and total chain length are replaced by the deformation gradient and chain length between crosslinks (or physical entanglements). Short version: a more heavily crosslinked elastomer and one with less viscous flow should respond at smaller deformations.

~Meredith

Kejie Zhao's picture

There is a recent special issue of mechanochemistry in Chem. Soc. Rev., that might be complementary to this theme.

http://pubs.rsc.org/en/Content/ArticleLanding/2013/CS/C3CS90058D

Ronald Peters's picture

Very good

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