Solvent relaxation studies applied to stimuli-responsive core-shell nanoparticles

Solvent relaxation method (SRM) was used for studying the solvation and behavior of shells of polymeric micelles formed by linear polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) block copolymer (PS-PVP-PEO) in acidic aqueous media. The shell is formed by partially ionized (protonated) PVP and PEO blocks, while the compact spherical core is formed by PS. An amphiphilic fluorescent surfactant, PATMAN was used as a probe. It was shown by independent study that it binds strongly to micelles and that its fluorescent part monitors a thin PEO layer close to the PVP-PEO interface. The behavior of PATMAN in non-viscous low-molar-mass solvents was studied first and then the study of PS-PEO, PS-PVP and PS-PVP-PEO micelles was performed. The comparison of SRM data for all studied systems allows for a plausible explanation of a slightly surprising behavior of PS-PVP-PEO micelles in acidic aqueous buffers where an unexpected aggregation of micelles is observed at low pH.