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Articles Fluorescence Quenching and Excimer Formation To Probe the Micellization of a Poly(ethylene oxide)-Poly( propylene oxide)-Poly( ethylene oxide) Block Copolymer, As Modulated by Potassium Fluoride in Aqueous Solution Mats Almgren* and Jan Alsins Department of Physical Chemistry, Uppsala University, Box 532, S-75121 Uppsala, Sweden
Pratap Bahadur Department of Chemistry, South Gujarat University, Surat 395007, India Received May 22, 1990. I n Final Form: July 25, 1990 The change in aggregationbehavior of L-64,a poly(ethy1eneoxide)-poly(propy1eneoxide)-poly(ethy1ene oxide) triblock copolymer, with the concentration of added potassium fluoride in aqueous solutions has been studied with fluorescence probes and light scattering and viscosity measurements. An important feature is that the L-64preparation is polydisperse, with some 2% of the material in a more hydrophobic form, probably diblock polymers, which form micellar aggregates at lower temperature and lower salt addition than the bulk of the polymers. This explains nicely the peculiar excimer formation and fluroescence quenching behavior and also a number of other erratic observations previously made.
Introduction Block copolymers with both hydrophobic and hydrophilic blocks display aggregation tendencies similar to those of nonionic surfactants.' EPEs, poly(ethy1ene oxide)-poly(propy1ene oxide)-poly(ethy1ene oxide) triblock copolymers, sometimes called poloxamers, are available in a variety of compositions under trade names such as Pluronics. Their solution properties receive increasing attention, and studies of both macroscopic solution properties (cloud point, surface tension, viscosity)2 and structure and aggregation properties (by scattering method^)^ have been published. More recently various photophysical methods have been applied,4l5methods that have proven their powers in studies of micelles and macromolecules.6 The EPE L-64with a total molecular weight of 2900, 4071 of which is poly(ethy1ene oxide) (PEO), is supposed to form micelles a t temperatures above 25 "C in a 1% (wt) solution, and has a cloud point of 62 "C.In a recent study of several properties of L-64solutions in the temperature range 20-60 "C it was observed that results from different methods were inconsistent, while NMR self-diffusion and static light scattering indicated the formation of small (1) Al-Saden, A. A.; Whatley, T. L.; Florence, A. T. J.Colloid Interface Sci. 1982, 90, 303. (2) Prasad, A. A.; Luong, T. T.; Florence, A. T.; Paris, J.; Vaution, C.; Seiller, M.; Puiseux, F. J. Colloid Interface Sci. 1979, 69, 225. (3) (a) Dwiggins, C. W.; Bolen, R. J.; Dunning, H. N. J . Phys. Che?. 1960, 64, 1175. (b) MacDonald, C.; Wong, C. K. Aust. J. Pharm. SCL 1977.6.85. (c) Zhou, Z.: Chu, B. Macromolecules 1987,20,3091; J.Colloid Interface Sci. 1989, 126, 171. (4) Zhao, C.-L.; Winnik, A. M.; Riess, G.; Croucher, M. D. Langnuir 1990, 6, 514. (5) (a) Turro, N. J.; Chung, C. Macromolecules 1984, 17,2123. (b) Turro, N. J.; Kuo, P.-L. J. Phys. Chem. 1986,90,4205. (c) Turro, N. J.; Kuo, P.-L. Langmuir 1986,2,438. (6) Thomas, J. K. The chemistry of excitation at interfaces; ACS Monograph 181; American Chemical Society: Washington, DC, 1984. Guillet, J. E. Polymer photophysics and photochemistry; Cambridge University Press: Cambridge, 1985.
0743-7463/91/2407-0446$02.50/0
aggregates a t low temperatures, quasi-elastic light scattering revealed the presence of much larger aggregates.' The probable reason for this inconsistency is the polydispersity in the composition of the EPE preparation. Polymers with small PEO and large PPO blocks will start to aggregate prior to the more hydrophilic molecules; if a fraction of diblock material is present, this tendency would be strongly enhanced. Luminescence probes may report on the size and nature of the aggregates. Some studies by Turro et al.s have revealed a complex concentration dependence where a hydrophobic probe molecule was solubilized in a fairly hydrophobic environment above a concentration of about 1%of the EPE, with more pronounced hydrophobicity a t concentrations around 10% (at 20 "C). This was taken to indicate a transition from expanded polymers to unimer micelles a t 1% ,and aggregation into micelles a t lo%, in conformity with a similar process studied by Ikemi et a1.8 for a poly(ethy1eneoxide)-poly(2-hydroxyethyl methacrylate)-poly(ethy1ene oxide) block copolymer. An aggregation of a more hydrophobic part of the material a t the lower concentration would provide an alternative explanation. Turro and K U O also ~ C noticed a peculiar temperature dependence for pyrene excimer formation, which increased with temperature to a maximum around 20 "C and decreased rapidly a t higher temperatures. The growth of small micelles increase the excimer formation since the chance of having more than one pyrene probe per micelle increases. The decrease a t higher temperatures was explained in terms of an increased microviscosity of the micelle interior. Aggregation of more of the EPE into micelles with increasing temperature is a simpler explanation. In order to clarify the aggregation behavior, we have (7) Bahadur, P.; Almgren, M.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A., submitted for publication in J. Colloid Interface Sci. (8) Ikemi, M.; Odagiri, N.; Tanaka, S.; Shinohara, I.; Chiba, A. Macromolecules 1982, 15, 281.
0 1991 American Chemical Society
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undertaken a study of excimer formation of pyrene and quenching with the hydrophobic quencher guaiazulene in solutions of L-64 where the hydrophobicity of the poly(ethylene oxide) blocks was modified, at constant temperature, by the addition of potassium fluoride. The cloud point of PEO is strongly reduced by the addition of salts as studied and discussed comprehensively by Kjellander et al.9 The cloud point of L-64 is reduced from 60 to 34 O C in the prescence of 1.0 M KF. The micelle formation would be expected to be similarily affected and was monitored by measurement of viscosity and static and quasi-elastic light scattering. Experimental Section Materials. Pluronics L-64 from SERVA, Germany, and potassium fluoride (Merck pro Analysi) were used as supplied. Pyrene, Aldrich, was recrystallized twice from ethanol. Guaiafrom Aldrich was used zulene (1,4-dimethyl-7-isopropylazulene) without further purification. An L-64sample from another batch, when analyzed with HPLC on a silicon column with 50:50 watermethanol as eluent, gave a bimodal elution profile, indicating the presence of about 3% of diblock material.10 Static Light Scattering. Measurements were made with an apparatus comprising a Hamamatsu photon counting unit and a 3-mW He-Ne laser for excitation. The optical constant for vertically polarized light is K = 4~n,2(dn/dc)~/(N,X*)
h n
Yavelength
lnml
Figure 1. Fluorescence spectra for pyrene, 5 X M, in L-64, 1% (wt). The excimer emission at 480 nm decreases, and the monomer fluorescence increases monotonously with increasing concentration of added KF. The dashed curve is the solution without KF, the concentrations in the rest are 0.1, 0.2,0.4,and 0.6 M. 1
(1)
where no is the refractive index, dnjdc the refractive index increment, N AAvogadro's number, and X the wavelength (633 nm). The reduced scattering intensity, KCIRw, was obtained by using benzene in calibration measurements. The molecular weight, or aggregationnumber, was obtained from extrapolation to zero concentration. All solutions were filtered through 0.22pm Millipore filters into Hellma cylindrical light-scatteringcells. Dynamic Light Scattering. Measurementswere performed with equipment and methods described earlier." Viscosity. Measurementswere made with an Ubblehodeglass capillary viscometer with flow times for water of about 150 s. The viscometer was thermostated at 40 & 0.05 "C. The intrinsic , evaluated from viscosity,[VI,and the Huggin's constant,k ~were 'I,pjC [VI + [VlZkHC (2) Fluorescence Spectra. Spectra were determined in a Spex Fluorolog Model 212 spectrofluorometer. Fluorescence Decay. Data were collected with the timecorrelated single photon counting technique, using an apparatus described earlier.12 The setupuses a mode-lockedargon-ionlaser (SpectraPhysicsModel171/12)tosynchronouslypumpacavitydumped dye laser (Spectra Physics Models 375, 3448) for excitation at 323nm, using the dye Rhodamine6G, and frequency doubling in a KDP crystal. The pyrene monomer emission was measured at 393 nm and the excimer emission at 530 nm.
Results and Discussion Fluorescence Spectra. Spectra of 5 X M pyrene in 1%) L-64 (3.4 mM) at 30 "C are shown in Figure 1. The strong excimer emission gradually decreases when potassium fluoride is added to the solutions, contrary to the expected behavior from a growth of the micelles. The series of spectra in Figure 2, with 5 X lo+ M of pyrene and a constant concentration, 2 x M, of the hydrophobic quencher quaiazulene, shows still some excimer emission, but mainly monomer emission, the intensity of which (9) Florin, E.; Kjellander, R.; Eriksson, J. C. J. Chem. SOC.,Faraday Trans. 1 1984,80, 2889. (10) We are indebted to Dr. Walther Batsberg, Ria0 National Laboratory, Denmark, for this analysis. ( 1 1 ) Brown, W.; Johnsen, R. M.; Stilbs, P. J. Phys.Chem. 1983,87, 4548. Brown, W.; Johansson, K.; Almgren, M. J.Phys.Chem. 1989,93, 5888. (12) Almgren, M.;Alsins, J.; van Stam, J.; Mukhtar, E. Progr.Colloid Polym. Sci. 1988, 76,68.
navelength lnnl
Figure 2. Fluorescence spectra for pyrene, 0.5 X M, in L-64, 1% (wt), in the presence of 2 X 10-6 M of the quencher quaiazulene. The fluorescence intensity increases with the salt concentration (0, 0.1, 0.2, 0.4, 0.6 M). The intensity scale is roughly the same as in Figure 1. increases with the salt concentration. These observations are compatible with the idea that a portion of the EPEs form miceIlar aggregates without salt present and that the hydrophobic solutes are mainly found in these, at a high local concentration that ensures rapid and efficient excimer formation and quenching. More micelles are formed, and the total volume of the hydrophobic domain increases, when the salt concentration is increased, so that the local concentrations of probe and quencher in the hydrophobic domains decrease, and also their mean number per aggregate. That no direct change of the local environment of the hydrophobic probes occurs when salt is added is apparent from the fact that the monomer emission spectrum remains the same independent of the salt concentration. The intensity ratio between the third and first peaks of the fluorescence spectrum, the so-called III/I ratio, can be used as a measure of the polarity of the environment.'3J4 The value in the present case, 0.89, is close to that reported for 2-propanol14 and also close to values found in various micelles13and block copolymer aggregates! thus indicating solubilization in a rather polar environment, The lifetime of the pyrene fluorescence (at low concentration of pyrene when effects from excimers are absent, see below) which is affected by oxygen quenching, growths by less than 25 % when KF is added. Time-Resolved Measurements. Measurements obtained by using the high pyrene concentration in salt-free (13) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039. (14) Stihlberg, J.; Almgren, M. Anal. Chem. 1986,57,817.
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0
200
400
l
l
600
time / ns Figure 3. Fluorescence decay at 395 nm and excimer growth M, in 1% (wt) L-64 and decay at 480 nm for pyrene, 5 X without added salt. The faster of the two monomer decay curves was obtained with 2 X 10-5 M guaiazulene present and has an exponential tail which merges with that from the decay without quencher.
0
500
1000
time I ns Figure 4. Fluorescence decay curves for pyrene in 1 A (wt) L-64 with 0.6 M KF. The slowest decay, which is close to exponential, was obtained at a low pyrene Concentration, 0.5 X 10-5 M. A somewhat faster and clearly nonexponentialcurve was obtained with 5 X 10-5M of pyrene. The fastest decay was obtained with also 2 X M guaiazulene in the sample. Table I. Apparent Aggregation Numbers, N, of 1 % (wt) L-64 at Various Concentrations of KF Obtained from Fluorescence Decay Curves by Extrapolation of the Exponential Tail to t = 0 KF/M 1061Pyl/M l@iQl/M N (excimer) N (quenching) 0 5 2 240 400 0 5 20 190 40 0.1 5 10 240 80 0.2 5 10 210 100 0.4 5 10 155 110 0.6 5 10 82 I1 0.6 0.5 2 83
solution show a rapid initial decay of the monomer emission at 395 nm, which is matched by the dynamic growth and decay of the excimer at 530 nm, Figure 3. The exponential tail of the monomer emission extends far beyond the excimer region however, showing that a fraction of the pyrene molecules is isolated in small domains. The addition of quencher at a concentration of 2 X 10"' M induces a strong quenching in the initial decay portion but leaves the final tail unaffected; both the decay constant and the fraction of the excited molecules that contributes to final phase are essentially the same as without quencher. These observations strongly support the supposition that in a salt-free solution only a minor part of the EPEs is involved in the formation of large aggregates, which solubilizes the majority of the probes and quenchers. The long-lived tail in the decay stems from pyrene molecules associated with free polymers or polymers forming small aggregates: unimer or oligomer micelles. It is to be expected that some quencher is present also in the small aggregates, but with
the total molecular concentration of L-64 an order of magnitude larger than that of the quencher,the probability of quenching in the small aggregates is small. The fluorescence decay curves at 0.6 M KF with 5 X 10-5 M pyrene show a very different pattern, Figure 4. The excimer emission (shown in Figure 5) extends over the same time range as the monomer decay. There is no indication of a population of pyrene molecules isolated in unimer micelles. The addition of quencher increases the deactivation rate and makes the decays nonexponential. The absence of a well-developedexponential tail with the same decay constant as in a solution without quencher makes the analysis with the Infelta equation15(3) uncertain and results, at best, in apparent aggregation numbers In F(t)/F(O)= - t / + ~ n(exp{-k,t) ~ - 1)
(3)
F ( t ) is the fluorescence intensity at time, t , 70 the decay time without quencher, k, the deactivation rate constant in the aggregates with one quencher, and n the mean number of quenchers per aggregate. With 70 fixed from measurement at a low pyrene concentration (no excimers), we obtain an estimate of the aggregation number, from
by extrapolating the tail of the decay linearily with slope 1 / 7 0 back to t = 0 to give n. The values so obtained, from the decays in Figure 4, and from a decay curve determined at a pyrene concentration of 5 X lo4 M and a quencher concentration of 2 X 10-5 M, were all close to 80. This is larger than the value of about 25, obtained from an interpolation of the values determined in the static light scattering measurements (see below) at 26 "C;the temperature difference could allow for only part of the discrepancy. With the same analysis of the decay curves in the saltfree case and for lower concentrations of added salt, the values presented in Table I were obtained. The apparent aggregation numbers are all larger than those at 0.6 M KF, and there is a large difference between the values from excimer formation and from quenching at each salt concentration: the addition of quenchers to solutions with substantial excimer formation gives little additional quenching in terms of an increase of n; only the rate of the decay increases. As discussed above for the salt-free case, this indicates that the quenching and the excimer formation occur mainly in a small number of large aggregates coexisting with unimers or oligomer micelles, so that the Infelta model is not applicable. Although the agreement between the aggregationnumbers from excimer formation and from quenching gets better with increasing salt concentration and is good at 0.6 M, the disagreement with the light scattering results indicates that the inhomogeneous distribution probably persists also in this case. The growth and decay of the excimer emission report directly on the dynamics in the large aggregates. Figure 5 shows that both the growth and the decay get progressively slower with increasing salt concentration. In a homogeneous solution, this is what happens when the pyrene concentration is reduced. Although the statistics of distribution of the probes among the hydrophobic domains are of importance in the present case, an increase of the total volume of the solubilizing regions is indicated, in agreement with the proposed interpretation. It may also be noted that the growth of the excimer emission shows that the eximers are formed dynamically; they are not due to preformed ground-state dimers or microcrys(15) Infelta, P . P . Chem. Phys. Lett. 1979,61,88.
L.A. Chem. Phys. Lett. 1978,59, 519.
Atik, S. S.;Singer,
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8
j 5 0
500 time / ns
1000
Figure 5. Excimer growth and decay for pyrene, 5 X 10" M, in 1% (wt) L-64at various concentrations (0, 0.1, 0.2, 0.4, 0.6 M) of added KF. Both the growth and the decay become gradually slower when the salt concentration is increased, indicating that the total volume of the hydrophobic domains available for the probes increases.
o r . ' 0
2
"
"
4
"
"
'
6
8
10
12
L-64, wt%
Figure 7. Results from static light scattering measurements on L-64in solutionswith various concentrationsof added KF. Filled symbols refer to measurements at 26 "C,with KF concentrations of (from the top) 0,0.2,0.4,and 1.0 M. The open symbols refer to 40 "C at KF concentrations of 0.2 and 0.4 M.
2
0 k,
,lo.'
/a'
Figure 6. Decay parameters k l and kl from fitting the results in Figure 5 to homogeneous solution excimer kinetics (filled circles). Values calculated using parameters as for ethanol are shown for comparison (diamonds). For further discussion see the text. tals. The fact that the excitation spectra for the monomer and excimer emissions are very similar corroborates this point. Our knowledge about the system does not allow a detailed analysis of the excimer emission in accordance with the treatment by Infelta and Gratzel.16 To quantify the statement about the decrease in the local concentration of pyrene in the large hydrophobic domains, the excimer emission curves in Figure 5 were fitted with a twoexponential expression, as for excimer emission in a homogeneous solution17
where k M and k D are the first-order decay constants for the excited monomer and dimer, respectively, and kDM and k M D are the excimer formation and dissociation rate constants; [PI is the pyrene concentration. Fex(t)is the excimer emission intensity and A a constant. The experimental results in Figure 5 could be represented very well by a general two-exponential function with almost equal amplitudes of opposite sign. The pairs of decay constants estimated for each curve are represented in a k l vs k2 plot in Figure 6 and compared to values calculated from eq 6, using for k M the value measured a t low pyrene concentration, 6 X lo6 s-l, and for k m and kD (16)Infelta, P.P.; Gratzel, M. J. Chem. Phys. 1979,70, 179. (17)Birks, J. B.Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970.
values for pyrene in ethanol, 8.3 and 23 X lo6 s-l, taken from Birks," and in the case of the latter adjusted somewhat upward to account for oxygen quenching at the same rate as for the monomer. The excimer formation rate ~ D M [ P was ] varied from 0 to 30 X 106 s-l, in steps of 2.5 X 106 s-l. The plots are in quite good agreement, in spite of the fact that the calculated values depend sensitively on the values of the parameters. The decay constants obtained a t 0.6 M KF correspond to ~ D M [ P=] 2 X 106 s-l, whereas the value in the salt-free solution is more than 10 times as large. This can be taken as an indication that the local concentration of pyrene in the latter solution is about a factor of 10 larger or, in other words, that less than one-tenth of the material has aggregated in the salt-free case. Stretching the interpretation, we can note that if L-64 had aggregated completely in 0.6 M KF, the local concentration of pyrene in the hydrophobic domain (the PPO core) would be 8.3 mM, which yields kDM = 2.5 X lo8 M-' s-,, as compared to 7 X lo9 M-' s-l in ethan01.l~ The difference is reasonable considering the high microviscosity reported for these micelles,& but as pointed out by a referee, the high microviscosity should have affected kMD as well. Light Scattering and Viscosities. The results from the static light scattering measurements are shown in Figure 7, with the resulting aggregation numbers at 26 and 40 "C shown in Figure 8. Just as in previous reports, the dynamic light scattering data indicate rather large hydrodynamic radii, in the range 85-120 A, the last value referring to a solution with 0.5 M KF at 40 "C. These large values are not consistent with the static light scattering results but are similar to'values found earlier in QELS s t u d i e ~ . l %Large ~ , ~ aggregates formed by a portion of diblock copolymers are the probable cause of deviations at low temperatures. Comparison with the fluorescence quenching results reveals a discrepancy between the aggregation numbers at high concentrations of KF amounting to a factor of 2 or 3. If the micelles are formed by a constant fraction f of the material a t a given temperature and KF concentration, the static light scattering would underestimate the aggregation number by this factor, and the fluorescence quenching would overestimate them by
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450 Langmuir, Vol. 7, No. 3, 1991
l-----l
loo
"
0,O 0,2 0,4 0 , 6 0,8 1,0 [KF] I molar
Figure 8. Aggregation numbers from the static light scattering data for L-64 reported in Figure I .
0' 0,o
0,2
0,4
I 0-6
[KFI, M
Figure 9. Intrinsic viscosity for L-64 a t 40 "C, as a function of added KF.
the same factor. The true number would be the geometrical mean of the two estimates, and the fraction of the material micellized would be given by the square root of the ratio of the light scattering to fluorescence quenching aggregation numbers. The reality is probably more complex, with a concentration dependence of the factor f, which would affect the analysis of the light scattering data. More comprehensive studies of the aggregation behavior of this and related poloxamers will be published elsewhere.18 Intrinsic Viscosity. The intrinsic viscosity for L-64 as a function of the concentration of added KF is shown in Figure 9 for a temperature of 40 "C. The increase in the salt concentration takes the system closer to the cloud point, and the main trend is similar to what has been observed for L-64 and other EPEs on an increase in temperature. The values of the intrinsic viscosity are similar to those found by Al-Saden et a1.l for EPEs in water and by El Eini et al.19 for alkyl poly(ethy1ene oxide) surfactants in water. El Eini et al. deduced from their results that the nonionic micelles must have extended PEO mantles, with water molecules trapped or bound in an amount that increases with the number of EO groups in the monomers, from 5.2 water molecules per EO for micelles to 10.6 for C16E63micelles. For the much smaller micelles that are formed by (&E,,, with m = 4,5,8, Carlstrom and Halle20have recently concluded, from oxygen17 magnetic relaxation studies and earlier neutron scattering results, that the polar headgroup shell is compact, containing less than five and presumably as little as two (18) Brown, W.;Schillhn, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem., in press. Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Submitted for publication in J. Colloid Interface Sci. (19) El Eini, D.I. D.; Barry, B. W.; Rhodes, C. T. J. Colloid Interface Sci. 1976, 54, 348. (20) Carlstrom, G.; Halle, B. J. Chem. Soc., Faraday TraA. 1 1989, 85, 1049.
to three water molecules per EO group. These results are not necessarily in opposition; extrapolation of the hydration numbers from the results of El Eini et al. suggests that about four molecules should be trapped for micelles with four to eight EO groups in the polar head of the surfactant. It seems rather to be expected that the properties of the EO chains in the headgroup shellshould tend toward those of the exended PEO coils when the length increases. The L-64 molecules have on average 26 EO groups, which are divided into two polar blocks. The EO headgroups are thus rather small, and the hydration numbers would be expected to be small, therefore. The effect of the temperature must also be considered. The viscosity measurements were performed at 40 "C where the aggregation was expected to be uniform; earlier determinationsof the intrinsicviscosity and hydrodynamic radius have shown a complex behavior with a broad minimum for both quantities around 40 "C in L-64.' Our interpretation is that diblock aggregation is responsible for the increase at low temperatures, whereasthe attractive interactions that eventually result in phase separation at the cloud point probably are responsible for the increase at high temperatures (and with increasing salt concentration). It has long been claimed that the shell of the headgroups in nonionic C,E, micelles becomes more compact and contains less trapped water when the temperature increases; the neutron scattering data of Zulauf et a1.21gave evidence in that direction. The discussion by Halle et a1.20 showed, however, that the data were not conclusive in this respect, and theoretical model calculations by Bjorling et a1.,22based on K a r l s t r O m ' ~two~~*~~ state model for the poly(ethy1ene oxide) chain, indicate that the temperature may have little effect on the compactness of the mantle of a nonionic micelle-mainly because it is compact already at low temperatures. The recent quasi-elastic light scattering studies by Zhou and Chu3strongly supports that the EPE micelles grew more compact with increasing temperature. Thus, the value of the intrinsic viscosity for L-64 is larger than what corresponds to an aggregate, which behaves as a rigid sphere; the deviation at 40 "C may be due to hydration water moving with the hydrodynamic particle, as proposed by earlier investigators of similar compounds.'Jg The increse with salt concentration is similar to the increase with temperature observed before.' A transition to anisometric particles at temperatures below the cloud point has been suggested for P-85.18 Such a transition is a possible reason for the behavior also in this case.
Conclusion The fluorescence probing results strongly supports the suggestion that the EPEs like L-64 are polydisperse, probably containing a sizable proportion of diblock material which starts to aggregate at lower temperature and salt concentration than the bulk material. This is probably the main reason to many conflicting and erratic observationsconcerningthe aggregationbehavior of EPEs. Acknowledgment. This work was supported by grants from the Swedish Natural Science Research Council. Registry No. L-64,106392-12-5; KF, 7789-23-3;pyrene, 12900-0; guaiazulene, 489-84-9. (21) Zulauf, M.;Weckstrom, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J. Phys. Chem. 1985,89, 3411. (22) Bjorling, M.;Linse, P.; Karlstrom, G. J. Phys. Chem. 1990, 94, 471. (23) Karlstrom, G.J.Phys. Chem. 1985, 89, 4962. (24) Sjoberg, A.; Karlstram, G. Macromolecules 1989, 22, 1325.
