We report the high strain-rate response of a spiropyran (SP) mechanophore in poly(methylmethacrylate). Previous work on this system has demonstrated a reversible bond scission in the SP under local tensile force, converting it to a fluorescent merocyanine form. A Hopkinson bar was used to apply fast compressive loads at rates from 10 2 to 10 4 s 21 , resulting in significant activation of the SP near fracture surfaces. However, comparison with a similar thermochromic SP reveals that much of the observed activation likely arises from thermal effects during high-rate fracture. These results show the importance of a thermally active control system in distinguishing mechanochromic response during high-rate loading. Microscale fluorescence mapping of the fracture surfaces using a confocal Raman microspectrometer suggests that some distinct mechanical activation may be occurring in craze-like regions during fibril rupture. The thermal response of the SP is useful in its own right for characterizing plastic heating regions during dynamic fracture.

Optical characterization and confocal fluorescence imaging of mechanochromic acrylate polymers. "J. Appl. Phys. 117, 043103 (2015 The development of mechanochromic molecules has opened new pathways for the study of localized stress and failure in polymers. Their application as stress or temperature diagnostics, however, requires suitable measurement techniques capable of detecting the force-and temperature-sensitive chemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial resolution but the energy input during these measurements can itself affect the activation state of the mechanochromic species. Here, we present a systematic study of the effects of laser-based imaging on the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into poly(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman microspectrometer. Localized stress and temperature activation were studied by means of high-rate compressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excitation wavelength-and power-dependence. Suitable correction functions were established and used to account for the observed laser effects. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for localized stress analysis in mechanochromic polymers, offering quantifiable information on the activation state with high spatial resolution. However, laser-mechanophore interactions must be well understood and effects of laser excitation and exposure times must be taken into consideration when interpreting the obtained results.