Langmuir 1994,10, 658-661
658
Fluorescence Studies
on
the Properties of Micelle
a
Pluronic F68
Kenichi Nakashima,* Takayuki Anzai, and Yoshinori Fujimoto Laboratory of Chemistry, College of Liberal Arts, University of Saga, Saga 840, Japan
1
Honjo,
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Received May 24, 1993. In Final Form: November 19, 19939
The properties of micelles of Pluronic F68 (F68), a water-soluble triblock copolymer with the average composition (ethylene oxide>7g-(propylene oxide)3Q-(ethylene oxide)78, are studied by fluorescence techniques with octadecyl rhodamine B as a probe. The fluorescence intensity, polarization, and lifetime are observed as a function of F68 concentration or temperature. The results show that the critical micelle concentration (cmc) of F68 is strongly dependent on temperature as suggested by Bahadur et al. from dynamic light scattering (Langmuir 1992,8,1903). The erne’s obtained are 10 g/dL at 20 °C and 0.5 g/dL at 50 °C. From fluorescence anisotropy, the microviscosity of the micelle is estimated to be 1.9 cP at 20 °C and 16 cP at 40 °C. The drastic increase of the microviscosity is attributed to the conformational change of the micelle from a loosely coiled aggregation to a compact structure.
Introduction
soluble block copolymers which have been most extensively studied are a series of Pluronics, commercially available triblock copolymers with a form of polyethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (PEO-PPOPEO). A variety of techniques have been employed, including light scattering, viscometry, nuclear magnetic resonance, and fluorescence spectroscopy.10-14 Pluronic F68 (F68) is a hydrophilic member (HLB > 24) in the series, with the average composition EC^gPOsoE078. In spite of many investigations by various techniques, there are still controversies on the association behavior of this block copolymer in aqueous solutions. Zhou and Chu found the existence of three temperature regions (i.e. unimer, transition, and micelle) from studies by dynamic light scattering (DLS).llc They reported that the critical micelle temperature (CMT) of F68 ranges from 36.3 to 42.3 °C depending on the concentration used (5.171.25 g/dL), and that below CMT, the polymer exists in the form of unimers with hydrodynamic radii of about 2.5 nm. On the other hand, surface tension measurements by Prasad et al.,14a DLS and viscosity studies by Al-Saden et al.,14b and DLS and fluorescence studies by Almgren’s group13c,e provide evidence for association of F68 at lower temperatures and concentrations. The reason for the different interpretations is open to discussion, and data on the properties of F68 micelles have been insufficient. Fluorescence techniques have proved to be powerful tools for studying the microstructure of various colloidal systems, e.g., low-molecular-weight detergent micelles, vesicles, and polyelectrolyte solutions.15 For the case of Pluronics solutions, however, the techniques have been rarely applied,10·13 although they promise to give microscopic, molecular-scale information more than other available techniques. In this study, we investigated micelles of F68 by fluorescence spectroscopy using octadecyl rhodamine B (ORB) as a probe (Figure 1). The fluorescence parameters
Block copolymers have been known to form micelles in solvent selective for one of the blocks.1-3 Many investigations have been performed on the properties of these micelles. Most of the investigations are concerned with the micelles in organic solvents.1-5 Recently, however, block copolymer micelles in aqueous solutions have attracted great attention, because the polymers show interesting micellization and gelation behaviors.6-14 Watera
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Abstract published in Advance ACS Abstracts, February 1,1994.
(1) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976,6, 201. (2) (a) Riess, G.; Rogez, D. Polym. Prepr. 1982,23 (1), 19. (b) Riess, G.; Hurtrez, G.; Bahadur, P. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1985; Vol. 2, pp 324-434. (3) (a) Stacy, C. J.; Kraus, G. Polym. Eng. Sci. 1977,17,627. (b) Leibler, L.; Orland, H.; Wheeler, J.C.
