High strain-rate response of spiropyran mechanophores in PMMA

We report the high strain-rate response of a spiro-pyran (SP) mechanophore in poly(methylmethacrylate). Previ-ous work on this system has demonstrated a reversible bondscission in the SP under local tensile force, converting it to afluorescent merocyanine form. A Hopkinson bar was used toapply fast compressive loads at rates from 102to 104s21,resulting in significant activation of the SP near fracture surfa-ces. However, comparison with a similar thermochromic SPreveals that much of the observed activation likely arises fromthermal effects during high-rate fracture. These results showthe 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 dis- tinct 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.