Position‐Resolved Fluorescence Polarization Maps Segmental Rigidity in Block Copolymer Micelles in Aqueous and Biological Media

Polymeric micelles are widely used as nanoscale compartments for delivery, sensing, and soft‐matter applications, yet their performance is governed not only by size and morphology but also by the local mobility of polymer segments within the assembly. Here, we describe a position‐resolved fluorescence polarization strategy to map apparent segmental rigidity within block copolymer micelles. By covalently installing the same dansyl fluorophore at defined positions along an amphiphilic block copolymer chain, we selectively label the hydrophobic core, core‐corona interface, and oligo(ethylene glycol) (OEG)‐rich corona of the micelles. The resulting fluorescence polarization profiles reveal a pronounced core‐to‐corona apparent rigidity gradient in water, distinguish block‐segregated micelles from compositionally matched random copolymer assemblies, and report micellar responses to temperature, cosolvent, and salt. This method further enables structural interrogation of polymer micelles in biologically relevant media, revealing the unexpected finding that biological serum exerts opposing effects on apparent chain rigidity in the micellar core and corona. Together, these results establish position‐resolved fluorescence polarization as a practical platform for linking block copolymer structure, self‐assembly, local chain dynamics, and biological stability, opening new opportunities to rationally design micelles with environment‐responsive mechanical properties.