Optical characterization and confocal fluorescence imaging of mechanochromic acrylate polymers

Abstract

The development of mechanochromic molecules has opened new pathways for the study of localizedstress and failure in polymers. Their application as stress or temperature diagnostics, however,requires suitable measurement techniques capable of detecting the force- and temperature-sensitivechemical species with high spatial resolution. Confocal imaging techniques offer excellent spatial re-solution but the energy input during these measurements can itself affect the activation state of themechanochromic species. Here, we present a systematic study of the effects of laser-based imagingon the activation and fluorescence behavior of mechanochromic spiropyran (SP) integrated into pol-y(methyl acrylate) (PMA) and poly(methyl methacrylate) matrices using a confocal Raman micro-spectrometer. Localized stress and temperature activation were studied by means of high-ratecompressive loading and dynamic fracture. Laser illumination of SP in PMA revealed a strong excita-tion wavelength- and power-dependence. Suitable correction functions were established and used toaccount for the observed laser effects. The presented study demonstrates that confocal imaging usingconventional Raman spectrometers is a powerful characterization tool for localized stress analysis inmechanochromic polymers, offering quantifiable information on the activation state with high spatialresolution. However, laser-mechanophore interactions must be well understood and effects of laserexcitation and exposure times must be taken into consideration when interpreting the obtained results.