Observing heterogeneous polymer degradation due to photothermal heating via electron imaging and optical measurements*

We observe the effects of thermally-driven chemical reactions (e.g., polymer degradation) occurring in small volumes within a solid material, where diffusion of reactants and products is limited. Such experiments are achieved by photothermally heating metal nanoparticles incorporated within a matrix. Exposure to light matching the nanoparticle’s localized surface plasmon resonance results in significant heat generation at the particle and an overall inhomogeneous steady state temperature distribution. Within the solid material, regions far from any particle are cooler relative to a particle's immediate vicinity, which can experience temperatures of 100-200 °C at moderate light intensities. Such heterogeneous heating complicates analysis of the degradation as highly degraded regions are intermingled with intact polymer. Electron microscopy imaging of photothermally-treated samples provides a physical map of the strongly-heated areas which contain deteriorated polymer having a different contrast than undamaged material. In addition, homogenous incorporation of fluorescent molecules provides an in-situ internal thermometer, a means to detect polymer degradation due to changes in local stiffness, and the ability to spatially map both quantities.