Local polarity and microviscosity in the hydrophobic cores of amphiphilic star-like and scorpion-like macromolecules

Fluorescence spectroscopy is used to probe the polarity and local friction in aqueous solutions of three amphiphilic polymers. Aggregate core polarity and local friction on the nanoscale are observed by measuring the steady-state and time-resolved fluorescence spectroscopy of coumarin 153 (C153) in three amphiphilic polymers as a model for hydrophobic drug encapsulation. Three solutions of polymers are studied: one amphiphilic scorpion-like macromolecule (AScM), M12P5, and two amphiphilic star-like macromolecules (ASMs), NC12P5 and NC6P5. Both ASMs and AScMs consist of hydrophobic segments of acylated mucic acid and hydrophilic poly(ethylene glycol) chains. In aqueous solution, the scorpion-like M12P5 forms micellar aggregates through hydrophobic interactions, while the star-like polymers are covalently bonded macromolecules that form higher order aggregates. Micelle and aggregate sizes are compared using dynamic light scattering measurements. A shift in the steady-state emission wavelength of C153 indicates distinct differences in polarity between the core environments of the scorpion-like polymer micelles and those of the two star-like polymers. The two reorientation time constants from C153 fluorescence anisotropies show that the hydrophobic cores of M12P5 micelles are relatively flexible. The covalently linked, rigid cores of the two star-like macromolecule nanocarriers, NC6P5 and NC12P5, show a third reorientation time constant for which the local friction of the dye is remarkably higher.