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

Abstract

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 Tisapphire 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 displayed a double-exponential decay. After comparison with 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.

1-Naphthol as an ESPT fluorescent molecular probe for sensing thermotropic microenvironmental changes of pluronic F127 in aqueous media

“…9,34 In the gel phase, the aqueous solubility of the PEO block (corona region) is diminished, which causes the corona region to shrink. 35 The blue shift of the NpO À * fluorescence maximum in the sol state compared to that in water, and its further blue shift on gelation, thus result from the increase in hydrophobicity of the microheterogeneous organisation of aqueous PF127. The trends in the enhancement of NpOH* fluorescence can View Article Online also be understood by a similar model.…”

Fluorescence Spectroscopic Investigation To Identify the Micelle to Gel Transition of Aqueous Triblock Copolymer Solutions

“…4,8,43,72 No sharp change in the anisotropy values around the transition point is in accordance with the anisotropy results of other normal fluorescent probes reported in the literature. [20][21][22]26 With HUF, the r SS values also apparently remain in a similar range (increase only marginally) for the micellar region but indicate a sudden increase for solutions with >25% P123 concentration, indicating the changeover from the micelle to the gel phase. The distinct change in the observed r SS values for HUF around 27-28% P123 is thus attributed to the micelle to gel transition, which is very similar to the reported gelation concentration of P123.…”

“…Recent excitation wavelength dependent rotational dynamic studies of coumarin 480 (C480) in a micelle as well as in the gel phase of P123 also corroborate the above literature findings. 28,32 However, an increase in one of the components of the reorientation time for the cationic probe rhodamine 123 (Rh123) in 20% aqueous F127 ((EO) 100 -(PO) 65 -(EO) 100 ) solution has been observed by Jeon et al, 22 who ascribed it to the micelle-micelle entanglement above the sol-gel transition temperature. Mali et al 26 showed later that the above observation is actually due to the unimer to micelle transition rather than the sol to gel transition, as initially suggested.…”

“…[19][20][21][22]26 If the increase in the rigidity of the probe microenvironment is not sufficient to be reflected in the temperature-dependent anisotropy results, studying the concentration-dependent micelle to gel formation seems to be a viable alternative. This approach has also been attempted but with very limited success by Mali et al, 26 where the interfacial probe Rh110 was employed at different concentrations of P123.…”

Fluorescence spectroscopy of a naturally occurring carbazole alkaloid: murrayanine

“…Micellization studies of such block copolymers provide valuable information regarding the size and morphology of the micelles, as well as the nature of the micellar interior [11]. Although several techniques are available to study the micellization, fluorescence probing is one of the most important methods available, particularly due to its ability to monitor the local environment in close vicinity of the micelle-bound probe molecule [12,13]. Fig.…”