Mechanochromic polymer particles crosslinked by a radical-type mechanochromophore†

Abstract

The mechanochromic properties of polymer particles crosslinked by difluorenylsuccinonitrile (DFSN), a radical-type mechanochromophore, are reported in relation to particle size and glass transition temperature. DFSN-crosslinked nanoparticles and microparticles were prepared via emulsion polymerization and suspension polymerization of vinyl monomers, respectively, using a DFSN-containing crosslinker. Mechanical grinding of the prepared white particles induced a pink coloration, originating from cyanofluorenyl (CF) radicals generated by the mechanically induced homolysis of DFSN units within the particles. The mechanochemical response of the particles was evaluated using scanning electron microscopy and electron paramagnetic resonance spectroscopy, revealing that microparticles exhibited a stronger response than nanoparticles due to the differences in pulverization behavior. The mechanochemical response also showed a strong correlation with the glass transition temperatures of the particles, highlighting the importance of polymer chain mobility in the mechanochromic properties of the developed polymer particles.