Optical Characterization and Confocal Imaging of Mechanochromic Polymers

Matthew Van Horn, LT USN

Mechanochromic molecules

Mechanochromic molecules open new pathways for the study of micro-damage in polymers. Their application stress and temperature diagnostics, however, requires measurement techniques capable of detecting the activated force- or temperature-sensitive chemical species with spatial resolution. Confocal imaging techniques offer excellent spatial resolution but, due to the increased energy-input, may affect the activation state of the mechanochromic polymer systems. Here we present a systematic study of the effects of laser-based imaging on the activation and fluorescence of spiropyran (SP) in poly(methyl acrylate) (PMA) and poly(methyl methacrylate) (PMMA) using a confocal Raman microspectrometer. While in the case of SP-functionalized PMA and PMMA no Raman features could unambiguously be linked to the activation state, the fluorescence emitted from the activated SP could be detected by the spectrometer. Illumination of monofuctionalized SP in PMA at different wavelengths and power levels was used to quantify the effects of laser exposure. The characterization revealed a strong wavelength and power dependence. The laser-induced deactivation of the sample was found to follow an exponential power law. The wavelength-specific rate constants varied linearly with the power density. The presented study demonstrates that confocal imaging using conventional Raman spectrometers is a powerful characterization tool for mechanochrmoic polymers, offering quantifiable information on the activation state with high spatial resolution. However, the effects of laser excitation and exposure times must be taken into consideration when interpreting the results. If laser-sample interactions are well understood, the obtained rate coefficients can be used to properly account for these effects

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Dec 21, 2013

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