Early damage detection of epoxy via poly(vinyl cinnamate) mechanophore using Fourier transform infrared spectroscopy

The employment of mechanophores and mechanochemistry in materials has enabled the development of novel force-responsive materials. Studies exploring the force sensing capabilities of the UV-dimerized cinnamoyl moiety have shown that after severing its cyclobutane bond under an external force, the moiety will revert back to its initial fluorescent state. Current fluorescent detection methods, however, fail to properly detect cyclobutane mechanophore activation in highly opaque samples. In this study, we apply Fourier transform infrared spectroscopy technique to measure a composite's chemical structure and examine activation of the cinnamoyl moiety's cyclobutane bond, regardless of sample transparency. Samples containing 10 wt% poly(vinyl cinnamate) as the active mechanophore, as well as set of samples with an additional 0.5 wt% carbon nanotubes, used to create a completely opaque composite, were developed. Both composites showed an increase in peaks at 1650 cm-1 and 1635 cm-1 after strain, which correspond to the cis and trans isomers of the fluorescent double-bond in the cinnamoyl group. A statistical difference in peak height occurs as early as 4% strain-before the yield point of the composites-indicating that early signal detection is possible. This improved sensing method provides a simpler, faster method for early signal detection over fluorescent imaging.