Langmuir 1990, 6, 514-516
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Fluorescence Probe Techniques Used To Study Micelle Formation in Water-Soluble Block Copolymers Cheng-Le Zhao and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, M5S 1A l , Canada
G6rard Riess Ecole Nationale Superieure de Chimie, Mulhouse, France
Melvin D. Croucher Xerox Research Centre, 2660 Speakman Drive, Mississauga, Ontario, L5K 2L1, Canada Received J u n e 12, 1989. I n Final Form: November 27, 1989 Micelle formation in water by poly(ethy1ene oxide)-polystyrene diblock and PEO-PS-PEO triblock copolymers was studied by the fluorescence probe technique using pyrene as the fluorescent dye. Solutions containing 6 X lo-' M pyrene and varying block copolymer concentrations were prepared, and four separate features of the pyrene fluorescence were examined. In the excitation spectrum, the (0,O)band shifts from 333.5 nm for pyrene in water to 339.5 nm for polymer-bound pyrene. At the cmc, sharp increases were observed in the total fluorescence intensity and mean fluorescence decay time. From the decay time measurements and fluorescence vibrational fine structure (Zl/Z3), the location of the pyrene within the polystyrene phase of the micelle can be determined. For the polymer samples of mean degree we find cmc values in the range of polymerization PE0(240)-PS(37) and PEO(l02)-PS(41)-PEO(102), of 4 X lo-' M (6 X g/L).
Introduction
It has been known for many years that block copolymers form micelles upon dissolution into a solvent selective for one of the blocks.'.' Many experiments have been carried oct to determine the properties of these micelles, such as the critical micelle concentration (cmc), micelle size,lb and the thermodynamics3 of micelle formation. A review of the publications on this topic indicates that most of the studies were performed on block copolymers which form micelles in organic solvents. A wide variety of techniques have been used, including light scattering, electron microscopy, viscometry, osmometry, and ultracentrifugation.'t2 Each technique has its own limitations, and even light scattering, which has been so powerful in this regard, has its difficulties with very dilute solutions of low molecular weight micelle-forming block copolymers. cmc values for block copolymer micelles are normally much lower than those for low molecular weight surfactants and consequently are much more difficult to determine. Nakamura et al.,495 for example, measured the decrease in surface tension when styrene-ethylene oxide block copolymers (PS-PEO) were added to water. Transitions associated with the cmc were not readily distinguished. On the other hand, fluorescent probe tech(1)(a) Tuzar, Z.;Kratochvil, P. Adu. Colloid Interface Sci. 1976,6, 201. (b) Riess, G.; Rogez, D. Polym. Prepr. 1982,23(1),19. (c) Riess, G.; Hurtrez, G.; Bahadur, P. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1985;Vol. 2,pp 324-434. (2)(a) Stacy, C. J.; Kraus, G. Polym. Eng. Sci. 1977,17, 627. (b) Leibler, L.; Orland, H.; Wheeler, J. C. J. Chem. Phys. 1983,79, 3550. (c) Noolandi, J.; Hong, K. M. Macromolecules 1988,16,1443.(d) Munch, M.R.; Gast, A. P. Macromolecules 1988,21,1360. (3) Price, C. Pure Appl. Chem. 1983,55,1563. (4) Nakamura, K.;Endo, R.: Takeda, M. J . Polym. Sci., Polym. Phys. Ed. 1976,14,135. (5)Nakamura, K.;Endo, R.; Takeda, M. J . Polym. Sci., Polym. Phys. Ed. 1976,14,1287.
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niques? which have in many ways revolutionized our knowledge of low molecular weight surfactant micelles,'-'O have only recently been applied to block copolymer micelles."~12 For surfactant micelles, these methods provide information on the cmc, on micelle structure (aggregasalt eftion number,' internal polarity: local vi~cosity,~ fects") and on the dynamics of micellar association.' Part of the ease in interpreting these data comes from the small size and rapid relaxation dynamics of surfactant micelles. In principle, one should be able to obtain equally rich information about aqueous block copolymer micelles, and yet the first experiments reported in the literature have been somewhat discouraging. Ikemi et.al,ll for example, used fluorescent probes in conjunction with lightand X-ray-scattering experiments to study aqueous solutions of a triblock copolymer poly(ethy1ene oxide-co-hydroxyethyl methacrylate), (PEO-PEHMA). These form micelles in which the core (PEHMA) is strongly swollen by water. The first dye they examined,"' 8-anilinonaphthalene-1-sulfonate(ANS), did not show a sharp tran(6) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC.1988, 99,2039. (7)(a) Turro, N. J.; Gritzel, M.; Braun, A. M. Angew. Chem., lnt. Ed. Engl. 1980,19,675.(b) Almgren, M.; LBfroth, J.-E. J. ColloidZnterface Sci. 1980,19,675. (c) Ananthapadmanablian, K. P.; Goddard, E. D.; Turro, N. J., Kuo, P. L. Langmuir 1985,1, 352. (d) Infelta, P. P.; Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974,78, 190. (e) Lianos, P.;Zana, R. Chem. Phys. Lett. 1980,76,62. (0Dederen, J. C.; van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lett. 1979,451.(g) Burkey, T. J.; Griller, D.; Lindsay, D. A., Scaiano, J. C. J . Am. Chem. SOC. 1984,106,1983. (8)Schore, N. E.; Turro, N. J. J . Am. Chem. SOC.1975,97,2488 1974,96,306. (9)(a) Zachariasse, K., Chem. Phys. Lett. 1978,57,429. (b) Turro, N.J.; Aikawa, M.; Yekta, A. J. Am. Chem. SOC.1979,101,772. (10)Lianos, P.;Lang, J.; Strazielle, C.; Zana, R. J . Phys. Chem. 1982, 1019. (11)(a) Ikeni, M.; Odagiri, N.; Tanaka, S.; Shinohara, I.; Chiba, A. Macromolecules 1981, 14,34;(b) 1982,15,281. (12)Turro, N. J.; Chung, C. J. Macromolecules 1984,17, 2123.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 2, 1990 515
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Fi ure 1. Excitation and fluorescence spectra of pyrene (6 X 10-%, M).The excitation spectra (Ae,, = 394 nm) correspond to water (bottom spectrum), a polystyrene film (top spectrum), and to aqueous solutions of the ABA block copolymer at concentration of 4.96 X and 1.20 X lo-' g/L. The fluorescence spectrum is that for the ABA copolymer at 4.96 x g/L.
sition with varying polymer concentration. The more hydrophobic dye A N (1-anilinonaphthalene)gave a much sharper transition at low polymer concentration,'lb in a range where light-scattering signals were undetectably weak. The authors found another transition by light scattering, leading to very large micelles at much higher polymer concentration (c = 1 g/L), and chose to attribute this value to the cmc. More recent experiments by Turro and co-workers" examined a single low molecular weight triblock copolymer, poly(ethy1ene oxide-co-propylene oxide) (PEO-PPO). These authors obtained the curious result that multimolecular micelles form only a t polymer concentrations above 10 wt %. Their fluorescence experiments provide very nice information about the local polarity and fluidity of the system in the site where the dyes are bound. Here we turn our attention to PS-PEO diblock and triblock copolymers, presumably a classic set of molecules whose micellar core should have a low affinity for water, and examine pyrene as a probe for micelle formation. We prepared two polymers of nearly identical molecular weight and composition which differ only in their microstructure. The diblock, which we refer to as AB, has a PEO block of 240 monomer units (mean degree of polymerization) attached to a PS block of 37 monomers. The triblock, ABA, has a central PS block (41 monomers) flanked by PEO blocks each composed to 102 monomer units. The presence of these polymers in aqueous solution affects the fluorescence of a trace of pyrene present in the system. We carry out three different measurements on each solution. From this data, we can evaluate the cmc for these polymers and determine the location of the pyrene probe in the micelle. The cmc for both olymers is located in the region of 4 X M (6 x 10-! g/L).
Experimental Section Styrene-ethylene oxide block copolymers were prepared in THF by anionic p01ymerization.l~We refer to the PEO-PS diblock copolymer as AB and_the triblock copo!ymer as ABA. AB has a molecular weight of Mn(B)= 3700 and Mn(A)= 10 400; ABA has a molecular weight of &,,(B) = 4100 and Mn(A)= 2 X 4500. Both AB and ABA have very low molecular weight dispersity (Mw/Mn< 1.1)as confirmed by gel permeation chro(13)Marti, S.; Nervo, J.; Riess, G. Progr. Colloid. Polym. Sci. 1975, 53, 114.
matography. Pyrene (Aldrich, >99%) was recrystallized twice from freshly distilled ethanol. Sample solutions for analysis were prepared in the following way. To each of several 10-mL volumetric flasks was added a known volume of a pyrene stock solution in acetone, and then the acetone was evaporated. Known amounts of the block copolymer stock solutions were then added and diluted to volume with doubly distilled water. The stoppered flasks were equilibrated at 70 'C for 1 h with magnetic stirring. The pyrene concentration was fixed at 6.0 X M for all samples, whereas the polymer concentration was varied from 6.3 X to 2.0 g/L. Solutions were placed in 12-mm0.d. cylindrical quartz tubes and outgassed by bubbling with oxygen-freenitrogen for 5 min before recording lifetimesor spectra. Fluorescence spectra were recorded on a Spex Fluorolog I1 spectrometer in the front face geometry (22.5'). The slit openings were 0.5 mm. Fluorescence lifetimes were measured by the time-correlated single-photon counting method.14 The fluorescence decay profiies were analyzed by using a nonlinear leastsquares iterative reconvolution method. All samples were examined at 22 "C.
Results and Discussion When PS-PEO block copolymer is added to an aqueous solution of pyrene, several dramatic changes in the fluorescence properties can be observed. There is a small but important shift in the excitation spectrum, a substantial increase in the quantum yield of the fluorescence, and a change in the vibrational fine structure in the fluorescence spectrum. These accompany the transfer of pyrene molecules from a water environment to a microenvironment within the micelles and provide information both on the cmc for micelle formation and the locus of the probe in the system. Figure 1 shows excitation spectra of pyrene in different media. In water, the (0,O) band appears at 333.5 nm, whereas in a polystyrene film it is at 339 nm. When the triblock copolymer ABA is added to the water solution of pyrene, a new band appears at 339.5 nm, corresponding to pyrene in a polystyrene-rich environment. As more triblock copolymer is added, the 339.5-nm peak becomes increasingly prominent and eventually dominates the spectrum. Similar behavior was observed for the AB diblock copolymer. This type of spectral shift for pyrene is unusual.'' It both complicates analysis of the data and opens the possibility for additional experiments that we did not anticipate when we began this work. In Figure 2, we present independent measurements of the intensities (I)of pyrene emission (relative quantum yields) and of the fluorescence decay times as a function of polymer concentration. In water and at low polymer concentration, the pyrene fluorescence decay profiles I ( t ) are exponential. I ( t ) is also exponential at the highest polymer concentrations; the lifetime determined, 350 ns for the diblock copolymer, is substantially longer than that we find (290 ns) for pyrene incorporated into a polystyrene film. The differences found between the diblock and triblock systems are undoubtedly real and suggest that the locus of the pyrene in ABA is a more densely packed polystyrene domain. We present further evidence below to support this point of view. A t intermediate polymer concentrations, I ( t ) is nonexponential. As a consequence, we fit I ( t )to a sum of exponential terms and evaluate the mean decay time ( T ) = S t I ( t ) dt/SI(t) dt. Above a minimum polymer concentration, one sees in Figure 2 a marked increase in ( T). Similar features appear in the plot of I vs c, and we (14) OConnor, D. V.;Phillips, D. Time Correlated Single Photon Counting; Academic Press: London, 1984. (15) Hamai, S . J. Phys. Chem. 1989,93, 2074.
516 Langmuir, Vol. 6, No. 2, 1990
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Figure 2. Plot of the total fluorescence intensity ( 0 , O )of pyrene m) of pyrene emission and mean fluorescence decay time (0, (X ,= 339 nm) as a function of AB diblock copolymer (-) and AAA triblock copolymer (- - -) concentrations.
attribute these changes to the onset of formation of a micellar phase." The transfer of pyrene from an aqueous to a micellar phase leads to a supression of nonradiative decay channels (quenching) and therefore to parallel increases in the lifetimes and quantum yields for emission. This reasoning places the cmc for the triblock M (4 X g/L) and that of copolymer at ca. 1.4 X (6 X g/L). the diblock at ca. 4.5 X One important feature of the fluorescence decay experiments is that they emphasize a distribution of environments for the pyrene molecules. Because the decays are exponential at both extremes of polymer concentration, we believe that there is not a large distribution of pyrene environments within the micelle. Rather, over the polymer concentration range 10-~-10-' g/L, our experiments detect emission from pyrene molecules bound to block copolymers free in solution or to premicellar aggregates, as well as from those incorporated into block copolymer micelles. This view is confirmed by analysis of the vibrational fine structure of the pyrene fluorescence, where the ratio Z1/13 refers to the intensity ratio of the (0,O) band to the (0,2) band in the spectrum. This ratio is very sensitive to the polarity of the medium surrounding a pyrene molecule.15 These values range from 1.9 for water and 1.04 for toluene to 0.6 for aliphatic hydrocarbons. In PS films, we observe a value of 0.95. For polymer concentrations (16) The shift in the excitation spectrum leads to larger increases in T, Figure 2. When these experiments were repeated using a fivefold lower pyrene concentration, intensity vs log c plots identical with those in Figure 2 were obtained. The onset of the intensity increase does not depend upon the pyrene concentration.
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Figure 3. Plot of the intensity ratio Ill&as a function of diblock (-) and triblock (- - -) copolymer concentration for A,, = 339 nm. The dotted line describes the data for both copolymers for A,, = 333 nm.
above 2 g/L, Z1/13 values become constant, 1.05 for the ABA triblock and 1.18for the diblock copolymers. These values confirm the idea that the locus of the pyrene in the triblock micelle is a more compact polystyrene environment than in the diblock and that in the diblock the pyrenes experience some of the polarity associated with the PEO interface. At lower polymer concentrations, 11/13values increase. In this concentration range, the number of pyrene molecules exceeds that of polymer molecules,'6 and those pyrenes free in solution can be excited with some selectivity at 333 nm (upper curve, Figure 3). The fraction of bound pyrenes increases with polymer concentration, and these pyrenes can be excited selectively at 339 nm. Values of Z1/13 decrease from 1.6 to 1.2 and indicate that many of the polymer-bound pyrenes are located in polar environments. This result is consistent with the idea that in this range of polymer concentrations we are observing the consequence of pyrenes associating with single polymer molecules (single molecule micelles) or premicellar aggregates. At higher polymer concentration, formation of micelles permits the partitioning of pyrenes into protected environments where the rates of radiationless decay processes are reduced, resulting in higher quantum yields and longer lived fluorescence. In none of these samples is there any indication of pyrene excimer emission. At higher pyrene concentrations in these systems, excimer formation becomes prominent. Studies are now in progress to see what further information is available from that kind of experiment.
Acknowledgment. We thank NSERC Canada and the Province of Ontario for their financial support of this research.
