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Results
The activation strain, activation stress and activation strain energy were higher at higher shear rates and
were linearly dependent on shear rate. In an effort to lower the activation parameters, the major crosslinker
(EGDMA) was replaced with PEG-550. We hypothesized that the longer length of the PEG
chains compared to the spiropyran molecule would enhance load transfer to the spiropyran, thus
leading to mechanical activation at a lower bulk shear stress and strain. The result of changing the
major cross-linker from EGDMA to PEG was a lower activation shear strain, shear stress and strain
energy for all shear rates. A summary of activation parameters is given in Table 1. Control tests were
performed on PMMA with monofunctional spiropyran [2] and PMMA with no spiropyran. As expected,
no color change resulted from the shear testing. (Figure 1) Experiments are in progress to understand
the role of crosslink length, crosslink density, and the incorporation of other mechanophores on the
shear-induced chemical reactions.
Figure 1: Images of active and control samples cross-linked with EGDMA before and after testing at each
shear rate.
Table 1: Strain, stress and strain energy levels at activation for both EGDMA and PEG cross-linked
polymers at each shear rate.
[1] Potisek, S.L., Davis, D.A., Sottos, N.R., White, N.R., White, S.R., Moore, J.S. 2007,
Mechanophore-Linked Addition Polymers. J. Am. Chem. Soc. 129, 13808-13809.
[2] Davis, D.A., Hamilton, A., Yang, Y. Cremar, L.D., Gough, D.V., Potisek, S.L., Ong, M.T. Braun,
P.V., Martínez, T.J., White, S.R., Moore, J.S., Sottos, N.R. 2009, Force-Induced Activation of
Covalent Bonds in Mechanoresponsive Polymers. Nature. 459, 68-72.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 24