Water content in micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers in aqueous solutions as studied by fluorescence spectroscopy

Fluorescence spectroscopy has been applied in the investigation of water content in micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers at 20 and 50 degreesC. The triblock copolymers examined consist of Pluronic F68(R) (F68) and Pluronic L64(R) (L64), which are known to form micelles with cores of PPO and shells of PEO in aqueous solutions. 2-p-Toluidinylnaphthalene-6-sulfonic acid (TNS) is used as a fluorescence probe. Several fluorescence parameters of TNS are employed as tools to evaluate the water content in the micelles: (1) the wavelength at maximal intensity, which strongly depends on the polarity and viscosity of the surrounding media; and (2) the degree of polarization (P) and lifetime, which give the microviscosity of the media through the Perrin-Weber equation. Aqueous solutions of PEO of various compositions are used as standard solutions. It is found that the water content in F68 and L64 micelles ranges from 10 to 70 wt %, depending on the size of the PEO block, concentration of the polymer, and temperature. The usefulness of fluorescence spectroscopy in the studies of block copolymer micelles is demonstrated along with some comments on the interpretation of P of the TNS probe.