Fluorescence Probe and Pulsed Field Gradient NMR Study of

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Fluorescence Probe and Pulsed Field Gradient NMR Study of Aqueous Solutions of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Block Copolymer F88 Rongbiao Wang,† Helmut Knoll,*,† Frank Rittig,‡,§ and Jo¨rg Ka¨rger‡ Wilhelm-Ostwald-Institut fu¨ r Physikalische und Theoretische Chemie, Fakulta¨ t fu¨ r Chemie und Mineralogie der Universita¨ t Leipzig, Linne´ strasse 2, D-04103 Leipzig, Germany, and Institut fu¨ r Experimentelle Physik I, Fakulta¨ t fu¨ r Physik und Geowissenschaften der Universita¨ t Leipzig, Linne´ strasse 5, D-04103 Leipzig, Germany Received June 26, 2001. In Final Form: September 17, 2001 In aqueous solutions of the triblock copolymer F88 (EO96PO39EO96) micelle formation has been studied by means of dual fluorescence of the probe molecule 1,3-bis(1-pyrene)propane monitoring viscosity changes in its microenvironment. Mobility changes of the polymer molecules on micellization were followed by means of pulsed field gradient (PFG) NMR. These results complement a recent detailed study of cis-trans isomerization of azo dyes as “kinetic probes” for structure formation in aqueous solutions of F88 (Langmuir 2001, 17, 2907). Differences of the critical micellization temperatures determined by various methods, and in H2O and D2O as solvents, are discussed. Our results reflect the particular large content of polye(ethylene oxide) in F88 (80%) compared to other triblock copolymers of this type.

Introduction In water solutions poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers (Pluronics/BASF) behave as nonionic amphiphiles. They show a temperature-dominated aggregation behavior1,2 so that micellization can be followed with one sample at increasing temperature. In recent studies we investigated rate constants of the light-induced thermal cis-trans isomerization of “push-pull” azobenzenes in aqueous solutions of such triblock copolymers. Depending on the polymer block length, concentration, and temperature, a wide range of rate constants, kiso, and S-shaped Arrhenius plots were observed.3,4 The sensitivity of kiso of these azo dyes against changes of their microenvironment are due to changes of micropolarity, microviscosity, and dynamics of hydrogen bonding, caused by changing water content around the probe molecules. This was concluded from the comparison with results in homogeneous poly(ethylene glycol) (PEG)/ water mixtures.4,5 In these mixtures, viscosity and water content could not be varied independently. To study microviscosity changes on micelle formation separately, in this paper 1,3-bis(1-pyrene)propane (P3P) was applied. The viscosity dependence of the dual fluorescence of P3P was recognized some time ago by Zachariasse et al.6 According to a simplified kinetic scheme from the litera* Corresponding author. E-mail: [email protected]. uni-leipzig.de. † Fakulta ¨ t fu¨r Chemie und Mineralogie der Universita¨t Leipzig. ‡ Fakulta ¨ t fu¨r Physik und Geowissenschaften der Universita¨t Leipzig. § Present address: Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195. (1) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (2) Chu, B.; Zhou, Z. In Nonionic Surfactants, Polyoxyalkylene Block Copolymers; Surfactant Science Series 60; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; p 67. (3) Gille, K.; Knoll, H.; Rittig, F.; Fleischer, G.; Ka¨rger, J. Langmuir 1999, 15, 1059. (4) Wang, R.; Knoll, H. Langmuir 2001, 17, 2907. (5) The term poly(ethylene oxide) (PEO) is used for the more hydrophilic chains of the triblock copolymers and poly(ethylene glycol) (PEG) for the model compounds as designated by the manufacturers.

ture, the measurable quantities excimer lifetime τE and fluorescence intensity of the monomer IM and excimer IE, respectively, can be related to the rate constant of excimer formation kexc, eq 1. On the other hand, the dependence of kexc on viscosity, which is effective as microviscosity (ηmicro) here, can be expressed by eq 2.7

τEIM/IE ) (kM/kE)/kexc

(1)

1/kexc ) cηmicro

(2)

kM and kE are the fluorescence rate constants of the monomer and the excimer, respectively. They can be assumed to be temperature independent and can be combined with the temperature-dependent constant c to give C in eq 3.

τEIM/IE ) Cηmicro

(3)

τEIM/IE was used as a measure of microviscosity in recent papers of Zana8,9 and Miyagishi10 et al. at constant temperature. In this study we determined τEIM/IE in PEG/water mixtures and aqueous solutions of F88 (EO96PO39EO96) from the dual fluorescence of P3P at increasing temperatures, to follow microviscosity changes under similar conditions, as kiso data of azo dye probes were determined recently.3,4 According to eq 3, τEIM/IE as a function of temperature T reflects both the temperature dependence of ηmicro and the temperature dependence of c in C. The former can be assumed to be ∝eEa1/RT, where Ea1 has the meaning of an activation energy of microviscosity. For unit viscosity, c ) 1/kexc ∝ eEa2/RT can be assumed. Then Ea2 can be understood as activation energy of 1/kexc at unit viscosity, so that for the homogeneous PEG/water mixtures τEIM/IE ) f(T) ∝ e(Ea1+Ea2)/RT. (6) (a) Zachariasse, K. A.; Ku¨hnle, W. Z. Phys. Chem. 1976, 101, 276. (b) Zachariasse, K. A.; Duveneck, G.; Busse, R. J. Am. Chem. Soc. 1984, 106, 1045. (7) Turley, W. D.; Offen, H. W. J. Phys. Chem. 1985, 89, 2933. (8) Zana, R.; In, M.; Le´vy, H.; Duportail, G. Langmuir 1997, 13, 5552. (9) Zana, R. J. Phys. Chem. B 1999, 103, 9117. (10) Miyagishi, S.; Suzuki, H.; Asakawa, T. Langmuir 1996, 12, 2900.

10.1021/la010975c CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001

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Micelle formation was also followed by a “probe-free” method. PFG NMR measurements were performed in aqueous F88 solutions to determine self-diffusion coefficients D and to calculate hydrodynamic radii RH. These diffusion coefficients complete the data found in the gel phase of F88/water mixtures.11,12 From S-shaped Arrhenius plots of D and kiso values determined in aqueous F88 solutions, critical micellization temperatures (cmt’s) can be derived. Differences between probe molecule methods1,3,4 and PFG NMR and between H2O and D2O are discussed. Experimental Section Materials. 1,3-Bis(1-pyrene)propane (Molecular Probes), poly(ethylene glycol) 4000 (M ≈ 4000 g mol-1, Fluka), and the triblock copolymer F88 (EO96 PO39 EO96, M ≈ 10 700, lot WPDN616B, BASF, Parsippany, NJ) were used as received. For comparison, experimental data of P85 (EO26PO40EO26, M ≈ 4600, lot WPAN628B), determined recently,3 were used. The polymers were dissolved in double distilled water. D2O (Chemotrade Leipzig, 99.8%) was freshly distilled over KMnO4. Concentrations of F88 are given in percent (w/v, 25 °C). PEG/water mixtures were prepared with the molar ratio Z ) 3 of H2O or D2O and ethylene oxide units (according to molecular formula and weight). Flamesealed sample tubes of 7.2 mm o.d. were used for NMR measurements. Fluorescence. Stationary fluorescence spectra were taken on a Fluoromax-2 spectrometer (Spex). P3P was introduced into the air-saturated solutions from a stock solution in acetone to give 1 × 10-6 mol dm-3 for the stationary measurements, and 4 × 10-6 mol dm-3 for the lifetime measurements. Fluorescence intensity at 485 nm versus time first showed an increase at short times due to excimer formation from the monomer. τE was calculated from the tail of the IE,485nm ) f(t) curves,9 omitting the 4-fold of the fluorecence lifetime of the monomer. Time-resolved fluorescence was carried out with a nitrogen laser LN203C (Laser Photonics) as excitation source and a MC photomultiplier (Hamamatsu) as detector. Complete decay curves from 400 flashes were collected by a digital storage oszilloscope 54720A (Hewlett-Packard) and averaged. PFG NMR. Our home-built spectrometer FEGRIS 400, experimental conditions, evaluation precedures, and general behavior of the samples were the same as described for the study of pluronic P85.3 Examples of recent reviews13,14 and details of PFG NMR can be found in the literature.15-19 Viscosimetry. To compare microviscosity probed by P3P in homogeneous solutions of known bulk viscosity, we determined the bulk viscosity of PEG/water mixtures by means of a a LS100 rheometer (Paar-Physica). For the lower viscosity of F88 solutions to be determined for calculations of RH, an Ubbelohde viscosimeter was used.

Results and Discussion Fluorescence Measurements. In some studies dual fluorescence of P3P8-10 or di(pyrenylmethyl ether) (Dipyme)20,21 has been used to compare microviscosities in micelles of classical surfactants8-10,20 and copolymers.21 We measured bulk viscosity and τEIM/IE as a measure of (11) Scheller, H. Thesis, Universita¨t Leipzig, 1997. (12) Scheller, H.; Fleischer, G.; Ka¨rger, J. Colloid Polym. Sci. 1997, 275, 730. (13) So¨dermann O.; Stilbs P. Prog. NMR Spectrosc. 1994, 26, 445. (14) Ka¨rger J.; Fleischer G.; Roland, U. PFG NMR Studies of Anomalous Diffusion. In Diffusion in Condensed Matter; Karger, J., Heitjans, P., Haberlandt, R., Eds.; Friedr. Vieweg & Sohn Verlagsgesellschaft mbH: Braunschweig/Wiesbaden, 1998. (15) Fleischer, G. J. Phys. Chem. 1993, 97, 517. (16) Fleischer, G.; Fujara, F. NMR Basic Princ. Prog. 1994, 30, 159. (17) Ka¨rger, J.; Pfeifer, H.; Heink, W. Adv. Magn. Res. 1988, 12, 1. (18) Ka¨rger, J.; Fleischer, G. TRAC 1994, 13, 145. (19) Heink, W.; Ka¨rger, J.; Seiffert, G.; Fleischer, G.; Rauchfuss, J. J. Magn. Reson., Ser. A 1995, 114, 101. (20) Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991, 95, 2583. (21) Nivaggioli, T.; Alexandridis, P. Hatton, T. A. Langmuir 1995, 11, 730.

Figure 1. PEG4000/water solutions with Z ) 3: Plot of τEIM/ IE vs temperature (left axis). Plot of ηbulk and vioscosity solvent isotope effects (VSIE) for bulk measurements (ηD2O/ηH2O)bulk and for microviscosity (τEIM/IE)D2O/(τEIM/IE)H2O, respectively, vs temperature (right axis).

microviscosity according to eq 3 in PEG4000/water mixtures with Z ) 3, Figure 1. Both τEIM/IE and bulk viscosity decrease similarly with temperature. Assuming that C has the same value for H2O and D2O, (τEIM/IE)D2O/ (τEIM/IE)H2O gives the microviscosity solvent isotope effect VSIEmicro, which decreases from 1.26 to 1.13 between 18 and 60 °C. The ratio of bulk viscosities (ηD2O/ηH2O)bulk decreases from 1.24 to 1.13 in the same temperature range, Figure 1. This coincidence confirms the correlation between bulk viscosity and microviscosity. Moreover, the plots ln(τEIM/IE) vs 1/T between 18 and 60 °C for PEG4000/ water mixtures with Z ) 3 were found to be linear, and activation energies of 42.8 ( 0.1 and 44.4 ( 0.1 kJ mol-1 were obtained for H2O, and D2O solutions, respectively. The corresponding bulk viscosities give 22.7 ( 0.6 and 25.6 ( 0.4 kJ mol-1, respectively. The latter data are reasonable estimates for the activation energy of microviscosityy Ea1. Then the difference between activation energies of bulk viscosity and τEIM/IE of about 20 kJ mol-1 should correspond to Ea2, which can be considered as activation energy for the conformational rearragement in the propyl group in P3P on intramolecular excimer formation. Due to the large molar mass, single molecules of F88 may solubilize P3P molecules, which are not soluble in neat water, not only in micelles but also in or at their unimer coils below the cmt. Therefore the entire temperature range and the aggregation of unimer coils to form micelles in aqueous solutions of F88 could be followed without distribution effects of the probe. Figure 2 shows maxima of τEIM/IE, which are at higher temperatures for 2.5% solutions compared to 10% solutions and also for H2O solutions compared to D2O solutions; see Table 1. These maxima indicate maxima of ηmicro in the microheterogeneous solution. They are due to changes of the microenvironment of the probe molecule P3P on micelle formation, which strongly dominate over the inherent continuously decreasing microviscosity with increasing temperature as dicussed for homogeneous solutions. A comparable maximum of microviscosity near the critical micellar concentration (cmc) was observed by Zana et al.22 at constant temperature and increasing concentrations. At higher temperatures, (τEIM/IE) ) f(T) and therefore microviscosity decreases as in homogeneous solutions with increasing temperature. Above 45 °C, microviscosity in the micelles seems to be independent of polymer concentration, and on H2O and D2O, respectively. From a plot (22) Ka¨stner, U.; Zana, R. J. Colloid Interface Sci. 1999, 218, 468.

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Figure 2. Plot of τEIM/IE vs temperature in aqueous solutions of F88 for different concentrations in H2O and D2O. Lines are guides for the eye; arrows indicate temperatures of maximum microviscosity.

Figure 3. Arrhenius plots of averaged self-diffusion coefficients D of F88 molecules measured by means of PFG NMR for different concentrations in H2O and D2O. Lines are drawn to determine the maxima; arrows indicate the cmt.

Table 1. Critical Micellization Temperatures (cmt in °C) of F88 and P85 Triblock Copolymer Solutions in H2O and D2O Determined from Probe Molecule Methods and PFG NMR Measurements, Respectivelya

maximum, from which D starts to decrease with increasing temperature, indicates decreasing average mobility of the polymer molecules, i.e., beginning mizellization (cmt). The mobility decreases further on increasing temperature due to the increase of the number of micelles and of their size up to a minimum of D, where mobility again increases mainly due to decreasing viscosity of the solvent. The unimers have a residence time in the micelles shorter than our observation times. Therefore, they exchange between the micellar and the unimer state several times during our experiment and we obtain an averaged diffusion coefficient

1.0% 2.3/2.5% 4.6/5% 10% concn concn concn concn (w/v) (w/v) (w/v) (w/v) H2O/D2O H2O/D2O H2O/D2O H2O/D2O Method solubilzed DPH1 P85 29.5 ln kiso ) f(1/T)3 30/29 PFG NMR3 30/31 solubilzed DPH1 F88 34/ ln kiso ) f(1/T)4 32.5/32 PFG NMR 40/37

25.5 24.5/23.5 27/27 26/ 25/24 32.5/27.5

Maxima from Fluorescence Measurements for Comparison max τEIM/IE 40.5/37.5 30/28.5 (ηmicro) max τE 42/40 31/30 a An error of (1 °C is assumed for the determination of the maxima. Therefore differences of >2 °C can be considered as significant differences between the methods and between H2O and D2O.

of ln(τEIM/IE) ) f(1/T) including all data of F88 solutions determined at g45 °C, an activation energy of 47 ( 3 kJ mol-1 can be derived. It corresponds to Ea1 + Ea2 and is similar to those determined by Zana9 with P3P in micelles of a large number of different surfactants. The occurrence of the maxima of microviscosity near to the critical micellization temperatures determined by PFG NMR, see below, indicates the change of the microenvironment of P3P from hydrated PEO/PPO domains of the unimer coils to the almost water free PPO core of the micelles. The large isotope effect of 1.6, and the differences between 2.5 and 10% solutions in the premicellar temperature range, might be due to nonrandom conformation of P3P with respect to the mean distance of the pyrenyl groups during excitation of P3P in or at the unimer coils. Smaller distances would give an increased excimer formation rate and seemingly decreased microviscosity.23 Excimer lifetimes show maxima on increasing temperature which correspond to minima of the radiationless deactivation rate of the excimer; see Table 1. PFG NMR Measurements and Comparison of the cmt Values Derived. The dependence of the self-diffusion coefficients D on temperature is shown in an Arrhenius plot in Figure 3. Coming from low temperatures, the (23) We acknowledge this possible explanation of a referee.

D h ) pmicDmic + puniDuni

(4)

where pmic and puni and Dmic and Duni are the fractions of chains and the self-diffusivities in the micellar and unimer states, respectively.15 cmt data are given in Table 1. The temperatures are higher than those determined from the inflection points of the Arrhenius plots of isomerization rate constants kiso4 and from solubilization studies.1 For P85 (EO26PO40EO26), however, cmt values from all three methods were in better accordance.3 The azo dye is insoluble in pure water and is therefore solubilized by unimer coils in premicellar aqueous solutions. Rate constants kiso of the azo dye3 deviate from the Arrhenius line, when their microenvironment changes on increasing temperature, i.e., when dehydratization of the PPO chains starts. At the same temperature, the solution also becomes capable of solubilizing the even more hydrophobic probe DPH which was used in the solubilization studies.1 In the case of P85 with shorter PEO chains, dehydratization of the PPO blocks and aggregation to form micelles seems to start at about the same temperature. The long PEO blocks of F88, however, stabilize the dehydrating PPO blocks of the unimers in a broader temperature range on rising temperature, before aggregation starts. Therefore, the PFG NMR method gives the real cmt values for F88 at higher temperatures compared to the two other methods mentioned before. A comparable difference of cmt values was also observed in 5% F88 solution,24 comparing I1/I3 data from the hydrophobic polarity probe pyrene (27 °C) with dynamic light scattering (DLS) data (30 °C). The former indicates microenvironmental changes of the probe, where the latter method monitors increasing size of the colloids. (24) Wen, X. G.; Verall, R. E.; Liu, G. J. J. Phys. Chem. B 1999, 103, 2620. The data were taken from Figures 1 and 2, although 35 °C was given as cmt from the DLS in the text of these authors.

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Figure 4. Hydrodynamic radii RH of unimers and micelles of F88, calculated by means of eq 5 from self-diffusion coefficients, D, vs temperature. Lines are guides for the eye and indicate hydrodynamic radii of unimers and micelles.

To interpret the self-diffusion coefficients, hydrodynamic radii RH are calculated using the Stokes-Einstein equation (5), where kB is the Boltzmann constant.

RH ) kBT/6πηD

(5)

The model assumptions of eq 5 do certainly not hold for 10% solutions, as an S-shape Arrhenius plot of bulk viscosity was obtained, whereas for the 2.5% solution an Arrhenius line was found. Therefore, we calculated RH according to eq 5 only for 2.5% solutions, Figure 4. RH ≈ 2.8 nm was obtained for the unimers in good agreement with 2.9 nm determined by DLS.23 This good agreement does not exist anymore for the hydrodynamic radii of the micelles, which in the case of the PFG NMR data analysis result to be 4.25 nm (H2O) and 5.25 nm (D2O), respectively. The DLS experiments yield a value of 12 nm.25 Smallangle neutron scattering (SANS) data give about 8 nm for the hard sphere radius.26 The origin of this difference is still unclear, but one has to have in mind that the different methods are based on different physical phenomena. Moreover, from eq 4 it follows that average diffusion coefficients D h determined by means of PFG NMR are upper limits for diffusion coefficients, Dmic, of polymer molecules aggregated in micelles. Therefore it is reasonable to assume that in the case of F88 with its long water soluble PEO chains, at temperatures where micelles exist, a remarkable fraction puni exists. Using the experimentally determined D h instead of unknown Dmic for the calculation (25) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (26) Mortensen, K.; Brown W. Macromolecules 1993, 26, 4128.

leads to an underestimation of RH of the micelles. While the unimers in H2O and D2O give equal radii RH as approximately found also with P85, the micelle radii are different, and in particular with an inverse order compared to P85. The value of 5.25 nm is about 25% larger for D2O, which is outside the error limits of (10%. For P85 we attempted to explain that the stronger bonding between the EO oxygens and D2O as compared to H2O leads to a stronger coiling of the PEO chain parts and, consequently, increased segment density.3 The more compact PEO corona yields smaller hydrodynamic radii in D2O solutions. If this argument is valid also for F88, a larger aggregation number in D2O compared to H2O has to be assumed to explain the larger radius in D2O.27 Contrary to P85, selfdiffusion coefficients of F88 molecules monitor a clear temperature difference between the cmt in H2O and D2O as do maxima of microviscosity, Table 1. It suggests that the stronger deuterium bonding compared to hydrogen bonding with the oxygens of the ethylene oxide units contributes more to the driving force of aggregation in the case of F88 compared to P85, at equal PPO chain length. Summary and Conclusions Microviscosity, monitored by the dual fluorescence of P3P, shows a maximum on increasing temperature on micelle formation in aqueous solutions of F88. Its temperature coincides with the cmt determined from the maxima of Arrhenius plots of the self-diffusion coefficients, determined by means of PFG NMR. Other hydrophobic probe molecules however experience a change of their microenvironment at lower temperatures in F88 solutions. Hydrodynamic radii derived from self-diffusion coefficients of P85 and F88 differ when exchanging H2O by D2O. This result should be considered on discussion of PFG NMR and SANS measurements, usually performed with D2O. Acknowledgment. The authors thank Dipl.-Phys. B. Kohlstrunk for assistance at the fluorescence lifetime measurements and BASF for providing the samples of Pluronics. R.W. acknowledges a grant and further support by the “Graduiertenkolleg Physikalische Chemie der Grenzfla¨chen” at the University of Leipzig (Deutsche Forschungsgemeinschaft and Sa¨chsisches Staatsministerium fu¨r Wissenschaft und Kunst). H.K. and J.K. thank the Fonds der Chemischen Industrie for financial support. LA010975C (27) We performed preliminary experiments with static pyrene fluorescence quenching by cetylpyridinium chloride, as widely applied, in the in 2.5% solutions of F88 and P85. Comparing only the extent of quenching in H2O, and D2O solutions, respectively, we obtained no difference with P85 but stronger quenching in D2O solutions of F88 compared to H2O solutions. It suggests that in D2O the quencher is distributed among less micelles, i.e., the aggregation number is higher in D2O solutions of F88 compared to H2O solutions.