Microviscosity in poly(ethylene oxide)-polypropylene oxide-poly(ethylene oxide) block copolymers probed by fluorescence depolarization kinetics

Triblock copolymers [poly(ethylene oxide) (PEO) and polypropylene oxide (PPO)], Pluronic F127 with 100 PEO blocks on each end, and 65 blocks of PPO in the center were examined in aqueous solution. The "sol" and "gel" phase diagram was determined as a function of concentration and temperature. For further study, the concentration was fixed at 20 wt%, and the temperature dependence of the dynamic viscosity differed from the temperature dependence of fluorescence emission spectra and the microviscosity probed by the fluorescence depolarization kinetics of rhodamine 123 dye, which was dissolved in the continuous hydrophilic phase. The depolarization measurements used single-photon counting after two-photon excitation with a Ti-sapphire femtosecond laser. Although the viscoelastic modulus increased by an order of magnitude when the sol-to-gel transition was crossed, the microviscosity of the hydrophilic continuous medium showed only minor changes. At different temperatures the fluorescence lifetime was the same with a single-exponential time constant, but the fluorescence depolarization of the dye in PPO melt and PEO whose molecular weight and aqueous concentrations were varied, the relative proportions of faster and slower components of the fluorescence depolarization were tentatively attributed to varying ratios of the dye in free solution and associated with micelles.