Local viscosity analysis of triblock copolymer micelle with cyanine dyes as a fluorescent probe

The local viscosity of Pluronic F127 triblock copolymer micelles in water was determined with cyanine dyes as fluorescent probes. These dyes show very weak fluorescence at a low temperature, but show enhanced fluorescence at a temperature higher than the critical micellization temperature (T(cm)). This is because a viscous environment within the micelle suppresses the formation of a nonradiative twisted intramolecular charge transfer (TICT) excited state of the dyes. The good correlation between the fluorescence quantum yields of the dyes and the viscosity and the temperature of the media allows a determination of local viscosity of micelle based on the fluorescence quantum yields. The local viscosity of both core and corona regions of micelles increases at >T(cm) and shows a maximum at a temperature 7-9 °C higher than T(cm), and decreases at higher temperature due to the increased fluidity. The core viscosity is larger than that of the corona, and the corona viscosity increases toward the micelle center. The polymer concentration has different effects on the core and corona viscosity: the corona viscosity increases with a polymer concentration increase at the entire temperature range, whereas the core viscosity increases only at a low temperature. The corona viscosity increase is due to the condensation of a large number of polyethylene oxide (PEO) blocks. In contrast, the dehydration degree of polypropylene oxide (PPO) blocks in the core scarcely changes, and the core has a similar composition regardless of polymer concentration. The larger polymer concentration promotes a micelle formation at lower temperature where the fluidity increase is very weak, resulting in larger core viscosity.