Domain-Specific Phase Transitions in a Supramolecular Nanostructure.

Understanding thermal phase behavior within nanomaterials can inform their rational design for medical technologies like drug delivery systems and vaccines, as well as for energy technologies and catalysis. This study resolves thermal phases of discrete domains within a supramolecular aramid amphiphile (AA) nanoribbon. Dynamics are characterized by X-band EPR spectroscopy of spin labels positioned at specific sites through the nanoribbon cross-section. The fitting of the electron paramagnetic resonance (EPR) line shapes reveals distinct conformational dynamics, with fastest dynamics at the surface water layer, intermediate dynamics within the flexible cationic head group domain, and slowest dynamics in the interior aramid domain. Measurement of conformational mobility as a function of temperature reveals first- and second-order phase transitions, with melting transitions observed in the surface and head group domains and a temperature-insensitive crystalline phase in the aramid domain. Arrhenius analysis yields activation energies of diffusion at each site. This work demonstrates that distinct thermal phase behaviors between adjacent nanodomains within a supramolecular nanostructure may be resolved and illustrates the utility of EPR spectroscopy for thermal phase characterization of nanostructures.