Nanoscale Sensing Vitrification of 3D Confined Glassy Polymers through Refractory Thermoplasmonics

Advances in plasmonics have been fundamentally rooted in minimizing ohmic losses in metallic nanostructures. However, the losses at resonance can play a positive role; for instance, in optical heating, there are two sides to every story. Under laser illumination, plasmonic nanostructures serve not only as near-field enhancers but heat generators. The emerging field of thermoplasmonics opens up unprecedented possibilities to probe temperature-dependent phase transitions locally. In this paper, we develop a new approach behind plasmon-assisted optical heating for spectroscopically recognizing the glass transition temperature (Tg) of spatially confined poly(methyl methacrylate) (PMMA) polymers deposited on a square-shaped titanium nitride (TiN) pad. A local photoheating is controlled through Raman thermometry of a c-Si (100) substrate that functions as a temperature-sensing Raman reporter. The reliability of temperature measurements is corroborated by using both the anti-Stokes/Stokes ratio and the Raman peak shift. We show that optical heating can be adjusted by extruding a c-Si substrate, for example, the temperature increase is achieved by making c-Si pillars beneath the TiN pads longer. This peculiarity gives the possibility to probe the Tg in a broad temperature range for the diversity of glassy polymers. We believe that the developed method will pave the way for 2D mapping structural glass transitions of heterogeneous glassy polymers, polymeric blends, and eventually, 3D confined polymers