Steady-state temperature gradients within polymer nanocomposites undergoing continuous-wave photothermal heating*

Metal nanoparticles act as nanoscale heaters via light-induced excitation of a localized surface plasmon resonance which converts the optical energy into local heat. Polymer films doped with a small concentration of nanoparticles can then be probed by applying internal heat at meso-length scales. Steady-state temperatures within a polymer matrix embedded with gold nanorods undergoing photothermal heating using continuous-wave excitation are measured in the immediate spatial vicinity of the nanoparticle (i.e., local temperature) from observing the rate of physical rotation of the asymmetric nanoparticles within the locally created polymer melt. Average temperatures across the entire sample (i.e., global temperature) are simultaneously observed using a fluorescence method from randomly dispersed molecular emitters. Comparing these independent measurements clearly demonstrates the presence of large steady-state spatial temperature gradients. Photothermal heating in this manner has potential utility in creating unique thermal processing conditions for outcomes such as driving chemical reactions, inducing crystallinity changes, or enhancing degradation processes in a manner unachievable by conventional heating methods.