Langmuir 1995,11, 119-126
119
Microviscosity in Pluronic and Tetronic Poly(ethy1ene oxide)-Poly(propylene oxide) Block Copolymer Micelles Thierry Nivaggioli, Belinda Tsao,?Paschalis Alexandridis, and T. Alan Hatton" Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received September 26, 1994@ The micellar microviscosity afforded by Pluronic and Tetronic poly(ethy1eneoxide)-poly(propy1ene oxide) block copolymer aqueous solutions has been investigated by fluorescence and NMR spectroscopy. Comparison is made with bulk poly(propy1ene oxide) (PPO) samples of different molecular weights. The microviscosity in Pluronic PEO-PPO-PEO copolymer micelles is much larger than that observed in conventional surfactant micelles and depends stronglyon the size ofthe hydrophobicPPO block: the larger this block, the higherthe viscosity. Above the critical micellar temperature (CMT),as temperature increases, the microviscosity decreases. However, this decrease is not as important as that observed in bulk PPO. Hence, the relative microviscosity, defined as the ratio of the two observed phenomena, increases. This suggests structural transformation of the micelles resulting in a core becoming more and more compact as temperature increases. Such results have been confirmed by NMR studies that showed broadening of the PPO peak and relatively constant spin-lattice relaxation time, 2'1, with increasing temperature while the PEO signal remained relatively sharp with an exponential increase in 2'1. In addition, solubilization ofbenzene in Pluronic copolymer micelles as detectedby NMRindicated that benzene partitionspreferentially in the core of the micelle constituted mainly of PPO.
Introduction
Over the last 2 decades, fluorescence probe techniques have played a crucial role in obtaining microstructural Owing to their amphiphilic character, copolymers information on colloidal systems.13-17 The intramolecular containing both hydrophilic and hydrophobic blocks such excimer formation of 1,l'-dipyrenyl methyl ether (dipyme), as poly(ethy1ene oxide) [PEOI and poly(propy1ene oxide) for instance, is an attractive tool for studying hydrophobic [PPOI exhibit many interesting features in aqueous microenvironments. It has been show@ that the extent solutions. The PEO-PPO block copolymers, commercially of excimer emission is dependent on the local friction available from BASF Corp. under the name Pluronics for imposed by the environment. As a result, measurement the linear PEO-PPO-PEO version and Tetronics for the of the monomer to excimer intensity ratio, IMIIE, provides X-shaped polymers, have recently received increased attention owing to their useful mi~ellization,'-~ g e l a t i ~ n , ~ , ~ information about the microviscosity (or microfluidity) experienced by the probe. The term microviscosity is used and solubilization8 characteristics. Micellization and to distinguish the viscosity of the probe environment in solubilization have received particular attention in our the interior of the aggregate from that of the bulk solvent group both from a theoreticalgJOand experimentall1J2 medium.17 In addition, the monomer emission of dipyme point of view. Most studies, using techniques such as resembles that of pyrene which shows vibronic fine light and neutron scattering, surface tension measurestructures, and measurement of the I& ratio provides ments, differential scanning calorimetry, etc., have focused information on the micropolarity. Dipyme has been used on bulk properties and micellar structure but not on the for the study of hydrophobically modified poly(N-isopromicroenvironment affordedby the micelle^.^ Fluorescence pyla~rylamides)'~ and P E O ' S . ~However, ~ ~ ~ ~ it has never and NMR spectroscopy provide a sensitive way of probing been employed to study Pluronic and Tetronic copolymer the latter. micellar solutions.
* Corresponding author. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139. EI Abstract
published in Advance ACS Abstracts, December 15,
1994. (1) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988,126, 171. (2) Wanka, G.;Hofhann, H.;Ulbricht, W. Colloid Polym. Sci. 1990, 266, 101. (3) Malmsten, M.; Lindman, B. Macromolecules 1992,25, 5440. (4) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. ( 5 ) Alexandridis, P.; Hatton, T. A. Colloids Surf., A: Physicochem. Eng. Asp., in press. (6) Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991,95, 1850. (7)Yu, G.-E.;Deng, Y.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J . Chem. SOC.,Faraday Trans. 1992, 88, 2537. (8) Hurter, P. N.; Hatton, T. A. Langmuir 1992, 8, 1291. (9) Hurter, P. N.: Scheutiens, J. M. H. M.: Hatton, T. A. Macromolecules 1993,26, 5030. (10) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Macromolecules 1993,26, 5529. (11) Alexandridis,P.;Holzwarth,J. F.;Hatton, T. A. Macromolecules 1994,27,2414. (12) Alexandridis,P.;Nivaggioli,T.;Hatton, T . k Langmuir, in press.
(13) Zana, R., Ed. Surfactant Solutions: New Methods oflnvestigation; Marcel Dekker: New York, 1986; Chapter 5. (14) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph 181; Caserio, M. C., Ed.; American Chemical Society: Washington, DC, 1984; Chapter 5. (15) Winnik, M. A.; Pekcan, 0.;Croucher, M. D. Scientific Methods for the Study of Polymer Colloids and their Applications; Candau, F., Ottewill, R. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1990, pp 225-245. (16)Photophysical and Photochemical Tools in Polymer Science: Conformation, Dynamics and Morphology; Winnik, M. A., Ed.;D.Reidel Publishing Co.: Dordrecht, 1985; NATO Series, Vol. 182. (17) Kalyanasundaram, K Photochemisty in Microheterogeneous Systems; Academic Press, Inc., Harcourt Brace Jovanovich, Publishers: Orlando, FL, 1987. (18)Georgescauld,D.; Desmasez, J. P.; Lapouyade, R.; Babeau, A.; Richard, H.; Winnik, M. A. Photochem. Photobiol. 1980,31, 539. (19) Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991,95, 2583. (20) Yekta, A.; Duhamel, J.; Brochard, P.;Adiwidjaja, H.; Winnik, M. A. Macromolecules 1993,26, 1829. (21) Yekta. A.: Duhamel, J.: Adiwidjaia, - - H.: Brochard, P.; Winnik, M. A. Langmuir.1993, 9, 881.'
0743-7463/95/2411-0119$09.00/00 1995 American Chemical Society
120 Langmuir, Vol. 11, No.1, 1995
Nivaggioli et al.
Aqueous solutions of Pluronic and Tetronic copolymers (1%(w/ v)) were prepared using Milli-Q water. Dipyme was prepared and purified by a modification ofthe method described by Winnik et al.18 All chemicals used in the synthesis were obtained from Aldrich Chemical Company. Solvents were obtained from Mallinckrodt Specialities Chemical Company and Fisher Chemical Company. Dichloromethane was distilled over CaHz prior to use. Where anhydrous conditions were required, glassware was flame-dried and reactions were run under argon atmosphere. A stock solution of 1 mM dipyme in acetone was prepared, from which 1pL was added to 3 mL of polymer sample, either in bulk or aqueous solution. a Pluronic surfactants are linear triblock copolymers of the form FluorescenceSpectroscopy. All fluorescencespectra were PEO-PPO-PEO. Tetronic surfactants are X-shaped block copolymers of the form (PEO-PPO)~N-CH~CHZ-N(PPO-PEO)Z. recorded on a SPEX FluoroMax spectrofluorometer using a 1.5nm bandpass in the "s" mode (sample counts). The wavelength of excitation was 345 nm in all experiments (sample PM counts NMR spectrometry is also a powerful technique for were always smaller than 2 x 106 countds to ensure linear studying surfactant systems.22 In addition to its versatilresponse of the detector). Step increments and integration times ity, it is a well-established method that provides structural were set at 1nm and 0.5 s, respectively. All samples were aerated, information about systems under investigation. By magnetically stirred, temperature controlled using a thermovariation of the design of NMR experiments, different stated cuvette holder connected to a circulating water bath, and parameters, such as relaxation times and the extent of examined at right-angle geometry. Each spectrum was obtained by averagingthree scans, corrected for scatter using an equivalent electrostatic interactions, can be obtained. In addition, blank solution. The monomer to excimer intensity ratio Z ~ Z E the line widths ofthe proton NMR signalscan give valuable was estimated by measuring zdzE.lstlg In dl cases ZM/ZEvalues information on the motion of the polymer chains. Slowly were averaged over three different experiments. The standard moving chains will give extensively broadened signals. deviations were smaller than 0.05. The dipyme concentration To date, most of the work has been devoted to studying was 0.42 pM in all samples; proper dissolution (absence of small surfactant systems. microcrystals) ofthe probe was controlledby observingexcitation In a recent study23aimed at probing the micropolarity spectra as described in the 1 i t e r a t ~ r e . l ~ afforded by Pluronic copolymer micellar solutions, we Nuclear Magnetic Resonance (NMR)Studies. lH NMR noticed that these hydrophobic microenvironments seem spectra were obtained on Varian UN-300 or Varian VXR-500 to exhibit a relatively high viscosity (microviscosity). spectrometers. All chemical shifts are reported in ppm. Spectra Nakashima and co-~orkers,2~ using fluorescence depotaken in DzO were referenced to residual HDO at 4.63 ppm. Variable temperature measurements required prior temperature larization, also reported an increase of microviscosity for calibration using standard methanol samples and "tempcal" F68 micelles as temperature increases. This was atroutine in the Varian software (VNMRVersion3.1). All samples tributed to conformational change of the micelles from a were stored at 10 "C overnight prior to experiments to avoid loosely coiled aggregation to a compact structure. Only micelle formation. A 20-min equilibration time was allowed in a recent p ~ b l i c a t i o nhas ~ ~the aggregation behavior of between temperatures duringvariable temperature experiments. PEO-PPO-PEO triblock copolymers been investigated One-milliliter samples of either 1%P104 or FlO8 in DzO were using lH NMR, but the focus was on the lineshapes only. used. Observation of solubilized aromatics in copolymers was It was shown that the methylene signal of the PPO performed by adding 2 pL of benzene (HPLC grade) to 1mL of segment broadens upon micellization, indicating that the polymer solutions giving 16 wt % of benzene to polymer. Longitudinal relaxation times (2'1) were measured using the PPO block is located in a more viscous environment. inversion-recovery method as implemented by the Varian We report here an investigation on the microviscosity software. Standard nuclear Overhauser effect (NOE) experiof Pluronic and Tetronic micellar solutions by fluorescence ments were used to help assignments. and NMR spectroscopy, comparing the results with those Viscosity. The viscosity of PPO 725 as a function of obtained in bulk PPO of various molecular weights. A temperature was measured using a Cannon-Fenske capillary calibration curve using the fluorescence data allows viscometer (Standard Test ASTM D 445) immersed in a water quantification of the core viscosity of the micelles as a bath having a temperature stability of 0.1 "C. The time it takes function of temperature. NMR spectroscopy studies of for a defined amount of sample t o go through the capillary was the micellization process at various temperatures show measured to the nearest second and was then converted to that aromatics partition preferentially to the hydrophobic kinematic viscosity using constants supplied by the viscometer environment of the micelle. In addition, NMR relaxation manufacturer (Cannon Instrument Co., State College, PA). The calibration constants derived from viscosity standards were measurements provided information on the motions of available at two temperatures; linear interpolation was employed the chains at different temperatures, confirming fluoresto obtain the constants at the temperatures of our experiments. cence results suggesting that micelles possess a viscous To avoid the need for kinetic energy corrections, flow times were PPO core. kept between 200 and 500 s by choosing an appropriate diameter for the capillary of the viscometer used. The kinematic viscosity Experimental Section of a sample was measured at least 3 times. The conversion from Materials. The linear Pluronic and X-shaped Tetronic PEOkinematic viscosity (expressed in cSt) to absolute viscosity PPO copolymers P65, P85, P105, P103, P123, P104, F108, and (expressed in cP) is straightforward in the case of PPO as its T704 were obtained as a gift from BASF Corp., Parsippany, NJ. density is close to 1.0 g/cm3 over the range of temperatures Their structural descriptions are presented in Table 1. Liquid studied. Therefore, as a good approximation, cSt and CPunits PPO homopolymers of molecular weights 425 (7 = 80 CPat 25 can be interchanged in our graphs. "C), 725 (7% 115 CPat 25 "C), 2000 (7 = 300 CPat 25 "C), and Synthesis. At the suggestion of Professor M. A. Winnik of 3000 (7 600 CPat 25 "C)were obtained from Aldrich Chemical the University of Toronto, we have altered the synthetic Co., Milwaukee, WI. All polymers were used as received. procedures already described.l8 Here, we present the modified reactions and work-up conditions. All products were analyzed (22) Lindman, B. ref 15, Chapter 6, and references herein. by NMR and fluorescence spectroscopy. (23) Nivaggioli,T.; Alexandridis, P.; Hatton,T. A.;Yekta, A.;Winnik, I-(Hydroxymethyl)pyrene. To a solution of 1-pyrene carboxyM. A. Langmuir, in press. aldehyde (3.057 g, 13 mmol) in absolute ethanol (200 mL) was (24) Nakashima, K . ; h z a i ,T.; Fujimoto,Y. Langmuir lBM, 10,658. added NaB& (0.6 g, 16 mmol) under argon atmosphere. The (25) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, mixture was stirred for 45 min. This was then acidified with 1 27,4145. Table 1. Pluronic and Tetronic Copolymers Structural Information" coDolvmer name molwt PPOblockwt POunits EOunits Pluronics P65 3400 1700 29 2 x 18 P85 4600 2300 40 2 x 26 P104 5900 3540 61 2 x 27 P105 6500 3250 56 2 x 37 50 2 x 132 F108 14600 2920 P123 5750 4025 69 2 x 19 Tetronics T704 5500 4 x 825 4 x 14 4 x 12
Langmuir, Vol. 11, No. I, 1995 121
Micellar Microviscosity 120
Dipyme in PPO 2000
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N HCl and diluted with 50 mL of CH2C12. The organic layer was separatedand washed 3 times with saturatedNaHC03 and dried
over anhydrous MgSO4. The solvent was removed in vacuo. This was dried under reduced pressure to give 1.421g (6.1 mmol) of product (yield = 47%). IH NMR (CDCl3)6 8.2 (m, 9H, pyrene), 5.3 (9, 2H, CH2), 1.5 ( 8 , lH, OH). 14Chloromethyl)pyrene. To an ice-cooled solution of 1-(hydroxymethy1)pyrene(0.5 g, 2.15 mmol) in 10 mL of anhydrous CHzClz was added dropwise a solution of thionyl chloride (3 mL, 43 mmol)in 10mL of CH2Clz over a period of 15min under argon atmosphere. During this time the suspension turned pale green. After 15 min, this was allowed to warm to room temperature. Excess thionyl chloride was removed in vacuo and the product was redissolved in 10 mL of anhydrous CHzC12. This was used without further purification. Sodium Salt of l-(hydroxymethyl)pyrene. NaH (0.06 g, 2.58 mmol)was added to a flame-dried flask under argon atmosphere. This was washed twice with cyclohexane to remove the excess oil and 2 mL of DMF was added. To this suspension was added dropwise a solution of 1-(hydroxymethy1)pyrenein 5 mL of DMF under argon atmosphere. This orange mixture was stirred at room temperature for 10 min and warmed to 40 "C for 5 min or until the mixture turned purple. This was then allowed to cool to room temperature and stirred for an additional hour and was used in the next step without further purification. Dipyme. To a solution ofthe sodium salt of 1-(hydromethyl)pyrene in DMF was added dropwise a solutionof 1-(chloromethy1)pyrene in CHzClz at room temperature and under argon atmosphere. This pale red mixture was stirred for 1h and the solvent was removed in uacuo. This was redissolved in CH2C12 to precipitateNaCl and then filteredthrough celite. The solvent was removed in vacuo and the product was obtained by flash chromatographyon silica gel with benzene as eluant. Fractions with Rf = 0.66 were collected and the solvent was removed in vacuo. Dipyme was dried under reduced pressure and stored under vacuum in the dark. lH NMR (CDCl3) 6 8.2 (m, 18H, pyrene), 5.4 (s, 4H, CHd.
Results and Discussion Dipyme in Bulk PPO. The sensitivity of the fluorescence emission spectrum of dipyme to changes in viscosity, as a function oftemperature, is depicted in Figure 1. In this case, the probe was dissolved in bulk PPO (M, = 2000). The fluorescence emission spectra can be roughly divided into two regions. The 360-430-nm region represents the monomer fluorescence emission and shows vibrational fine structures similar to those observed for pyrene in these systems.23 The 1 5 band will be taken as reference for the monomer emission henceforth.l8Jg At longer wavelengths, a structureless broad band, IE, centered at ca. 485 nm represents the intramolecular excimer fluorescence emission. As temperature increases, viscosity decreases, and this band becomes more and more predominant. As a result, the monomer to excimer decreases. This ratio intensity ratio, IM/IE,that is,
46
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Temperature ('C)
600
Wavelength (nm) Figure 1. Normalized (at Is) emission spectra of dipyme in bulk PPO 2000 as a function of temperature.
43
Figure 2. Monomer-to-excimerintensityratio (IM/ZE) of dipyme in bulk PPO 725 (open symbols)and kinematic (bulk)viscosity of the polymer (filled symbols) as a function of temperature. In both cases, an exponential fit ofthe formAexp(-c*T) was found (2' representing the temperature in "C). For ZM/ZE,A = 10.7 and c = 0.043 "C-l. For 7, A = 276 CPand c = 0.041 "C-'. Reexpressing in the Arrheneus form, A exp(-E$RT), gives A = 2.18 x and E, = 35 kJ/mol for ZM/ZEandA = 1.49x CPand E, = 33 kJ/mol for 7. The viscosity scale expressed in CPcan be considered as identical to that expressed in cSt as the density of PPO is close to 1.0 g/cm3. 2.8
1
2.4
-
--.-
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1
PPO 425 PPO 2000 - PPO 3000
-
2 1.8
1.6
1.4 1.2
34
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46
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Temperature ("2) Figure 3. Monomer-to-excimerintensityratio (ZMIZE) of dipyme in bulk PPOs of Merent molecular weights as a function of temperature. is an indicator of the viscosity of the medium surrounding the probe. Note that even at 50 "C (the least viscous case, IM/IE% 1.4) the monomer emission still dominates the spectrum. By comparison, the emission spectrum taken at room temperature in a fluid solvent such as benzene is largely dominated by the excimer band (IM/IE % 0.4).18 The temperature dependence of thd& ratio of dipyme in PPO 725, along with the kinematic (bulk) viscosity of the polymer, is presented in Figure 2. In both cases an exponential decrease, characteristic of an activated process, is observed and the functional behavior (i.e. the temperature dependence) is similar. In fact, there is a direct correlationbetween the fluorescence measurements and the bulk viscosity of the polymer. This type of correlation will be used in the following section to estimate the microviscosity afforded by the copolymer micelles. Figure 3 shows the influence of the molecular weight of PPO on the dipyme IM/IEratio as a function of temperature. Once again, we observe activated processes with similar exponential forms, unlike in fluid solvents such as cyclohexane, where smaller (by approximately 7 times) exponential factors are found. Surprisingly, the monomer to excimer ratios are sensitive to the bulk viscosity (molecular weight) of the polymers. However, it is worth pointingout that the difference inZ& between PPO 425 (77 % 80 CPat 25 " C )and PPO 3000 (77 % 600 CP
26 24
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-
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.-
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Temperature (‘C)
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Temperature (‘C)
Figure 4. Monomer-to-excimerintensity ratio (IM/IE)of dipyme in various Pluronic copolymer micellar solutions as a function of temperature (above the CMTs of the block copolymers). (a) Effect of PPO block with copolymers containing 50% PEO. (b) Effect of PPO block with copolymers having approximately the same PEO blocks. (c) Effect of polymer concentration. (d) Comparison between F108 copolymer micellar solution and bulk PPOs of different molecular weights.
at 25 “C) is only about 20% while the bulk viscosity of PPO 3000 is approximately 7.5 times larger than that of PPO 425. This moderate sensitivity was expected since the probe is sensitive to its immediate surrounding, or microenvironment, and not to the bulk viscosity which depends on more long-range interpolymer entanglements. Nevertheless, for a given temperature, the higher the molecular weight, the higher the IM/IE ratio (Le. the viscosity). These “master”curves are used in the following to compare measurements obtained in the micellar systems. Dipyme in Block Copolymer Micelles. The results for the Pluronic linear triblock copolymer solutions are summarized in Figure 4. All measurements ofIM/IE were made a t temperatures above the critical micellar temperatures (CMTs)ll where dipyme was solubilized by the micelles, as confirmed by the excitation spectra.lg For ease of comparison, all the graphs are of the same scale. Activated processes were observed in all cases with a temperature dependence (i.e. exponential factor) roughly 20% less important than that observed in bulk PPO. The influence of the PPO block is depicted in Figure 4a. For these triblock copolymers containing 50%PEO, the larger the PPO block, the higher the IMIIEratio, suggesting that the microviscosity afforded by these triblock copolymer micelles is strongly affected by the PPO block. It appears that the larger its molecular weight, the more viscous the micelle interior. These observations also imply that dipyme partitions preferentially in the core of the micelle, mainly constituted of hydrophobic PPO, a trend already observed for ~ y r e n e Such . ~ ~ a conclusion is also confirmed
by our NMR studies (see next section). The strong effect of the PPO block on the microviscosity of the micelles is also shown in Figure 4b. In this case, both PEO-PPOPEO polymers have the same molecular weight of PEO block but differ in molecular weights of the PPO block. Once again, the micelles formed from the copolymer with the larger PPO block exhibit higher microviscosity. These results reflect an intrinsic property of the micelles and are not dependent on the total polymer concentration in the solution as shown in Figure 4c. Increasing the polymer concentration by a factor 5 does not affect the IMIIEratio. Comparison of the IM/IE values obtained in bulk PPO and in the copolymer micelles reveals the following trends: at lower temperatures, the IM/IE ratio in the micelles is equivalent to that obtained in bulk low molecular weight PPO. As temperature increases, the IM/IE ratio value approaches that obtained in bulk PPO of higher molecular weight. This typical result is depicted for one copolymer micellar solution in Figure 4d. Although direct comparison of microviscosity and bulk viscosity is not entirely appropriate, the general trend can be described as follows. At 35 “C the microviscosity in F108 micelles, as probed by dipyme, is similar to that of PPO 425, while at 50 “C it becomes similar to that of PPO 2000 which is approximately 5 times more viscous than its lower molecular weight equivalent. Such an effect is also observed when the other Pluronic copolymer micellar solutions are compared with PPO homopolymers of different molecular weights. Figure 5 shows the dipyme scale in pluronic micellar solutions relative to that in bulk PPO 725. Such scale is obtained by dividing the IMIIE
Langmuir, Vol. 11, No. 1, 1995 123
Micellar Microviscosity h
Bn.
1.1
0
1.05
-a
1
.-s
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-
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P65 1Yo
0.9
2 0.85
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PPO 725
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T704 1%
I.L
34
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Temperature ('C) Figure 6. Relative dipyme scale obtained from the ratio of I M I Ein Plurronic copolymer micellar solutionsto that obtained in bulk PPO 725. Note that on the same scale, the values for PPO 2000 would be around 1.05. Therefore, as an approximation, a change of 5% on the relative dipyme scale imply a 2.5fold change in viscosity (see also Figure 2). values obtained in Pluronic solutions by that obtained in bulk PPO 725 as a function of temperature and illustrates more clearly the relative increase in microviscosity with increasing temperature. We also note that the more hydrophilic the copolymer (i.e. P65 in Figure 5), the stronger the temperature effect. These data suggest that the micellar structure becomes more and more compact as the temperature increases, probably as a result of the increased hydrophobicity of the polymer with increasing temperature. This is in accordance with our previous findings showing that the aggregation number ofthis type of copolymer increases with temperature while the micellar hydrodynamic radius remains constant.23 We also note that the molecular weight of the high temperature (50 "C) "viscosity equivalent" PPO is always smaller than that of the PPO block of the copolymer (i.e. for F108 the PPO block molecular weight is 3000 while its "viscosity equivalent" PPO has a molecular weight of 2000). If one compares the&& values (ranging from 1.8to 2.1) obtained in these Pluronic copolymer micelles to those presented in Figures 2 and 3, one can estimate that at 40 "C the microviscosity ranges from the viscosity of PPO 725 to that of PPO 2000. The viscosity ratio between these two homopolymers is approximately 2.6 at 25 "C. Owing to their similar viscosity-temperature dependence, as shown earlier, one can assume that this ratio holds at 40 "C. Therefore, using Figure 2, one can estimate that the microviscosity in Pluronic copolymer micelles ranges approximately from 50 CP(P65) to 130 CP(P123, P105) at 40 "C. This is much larger than in traditional surfactant micelles such as SDS, CTAB, and DTAB, where microviscosities usually range from 10 to 30 CP a t room temperat~re.~' The situation is quite different in Tetronic X-shaped copolymer solutions. As one can see from Figure 6, the IM/IEratio in T704 micellar solution remains identical to that obtained in bulk PPO 725 over the whole temperature range studied. This suggests that the structure of these micelles forbids the same compacting effect observed with the linear PEO-PPO-PEO surfactants. As the PEO chains do not extend as much into the hydrophilic corona, one can imagine that the temperature-induced dehydration effect is not as important in such systems. In addition, the microviscosity afforded by these Tetronic copolymer micelles is smaller than that observed in equivalent molecular weight (and PEO-to-PPO ratio) Pluronic copolymer micellar solutions. This lower microviscosity is probably due t o the presence of the X-shapedjunction at
34
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Temperature ('C) Figure6. Monomer-to-excimerintensity ratio (Ion/lE)of dipyme in Tetronic copolymer micellar solution and in bulk PPO 725 as a function of temperature (above the CMT of the block copolymer).
the center of the polymer chains. Such a junction between the PPO-PEO blocks most likely introduces extra free volume in the micelle and hinders the polymer chains from forming a more compact structure. We determined the hydrodynamic size of Tetronic T704 micelles as previously described.23 We found that the diameter of these micelles is approximately 11.5 nm (in the 40-50 "C temperature range) which is significantly smaller than the diameter of P104 micelles: 16.2 nm.23 We believe that the relatively small size, consequently aggregation number, of the T704 micelles is another indication of the hindrance encountered by the copolymer chains to form a larger, more compact structure. In terms of viscosity, as the dipyme scale for T704 is similar to that observed with PPO 725, one can estimate using Figure 2 that the microviscosity in T704 micelles ranges from 65 CPat 35 "C to 35 CPat 50 "C. Interestingly, smaller microviscosity values were reported by Nakashima and co-workers for Pluronic F68 micellar solutions.24 They estimated a value of 16 CPat 40 "C, a value similar to that observed in conventional surfactant micelles. There are two main reasons that can explain this discrepancy. The first is that F68 is a very hydrophilic copolymelsJl and might form a more compact structure only at higher temperatures (i.e. above 60 "C). The second, as mentioned by Nakashima et al., is that the probe used in their studies, octadecyl rhodamine B (ORB), is cationic and is probably located in the hydrated PEO domains. Such domains, as shown by our NMR data, are less viscous than the hydrophobic PPO core of the micelles where dipyme partitions preferentially. If the second reason is indeed the case, then the study of a given micellar copolymer solution with both dipyme and ORB would allow the determination of the radial microviscosity dependence in the micelles. N M R Studies. lHNMR spectra of P104 and FlO8 were recorded in D2O at various temperatures (Figure 7). Distinct signals for the methyl and methylene protons of the PPO block and the methylene protons of the PEO block are apparent. At temperatures below CMT (as determined in water solutions11which gives an upper limit to the value in D2OZ3), the two doublets correspondingto the methyl and methylene groups of PPO at 0.9 and 3.3 ppm, respectively, and the singlet ofthe menylene protons of PEO at 3.45 ppm are observed. As temperature approaches the CMT of the copolymer, both doublets correspondingt o the PPO segment broaden, with a more significant effect observed for the methylene protons at 3.4 ppm. However, the signal for the methylene moiety of the PEO block remains as a sharp singlet. At
Nivaggioli et al.
124 Langmuir, Vol. 11, No. 1, 1995
A
I 1
"
"
"
l
'
I
1
'
' 1 " " l "
'
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"
"
4.0
3.5
3.0
2.5
2.0
1.5
1.o
PPM
40
35
30
25
20
15
10
PPM
Figure 7. Proton NMR spectra of Pluronic block copolymer solutions as a function of temperature: (a, top) 1%P104 in D2O (CMT = 21.5 "C in HzOll);(b, bottom) 1% F108 in DzO (CMT = 29.5 "C in HzOl').
temperatures above the CMT, both doublets of PPO broaden to a singlet whereas the PEO signal remains unchanged. Such findings are in accordance with observations in a 1w t % F127 solution in D20.25 These observations not only offer a means of determining the CMT of the copolymer but also provide information about the structure of the system at various temperatures. At temperatures below the CMT, the ability to observe distinct signals for all protons indicates that all segments of the solvated polymer can move freely. As temperature approaches the CMT, micellization begins to take place. Broadening of the PPO signals implies that this segment
is in a more confined environment and that the motions of the chains are limited. At temperatures above CMT, no further broadening of these signals indicates that an equilibrium conformation has been reached but movements of the PPO chains are still restricted. These observations confirm results from the fluorescencestudies presented in the preceding section. Upon micellization, the core of the micelles consists mainly of the viscous PPO and the corona is made up of the hydrated PEO. Since PEO is still in contact with the aqueous solvent,the chains can move more freely, and thus the signal remains relatively sharp. Compacting of the PPO chains limits
Micellar Microviscosity 1.4
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Langmuir, Vol. 21, No. 1, 1995 125
1
1
1
PEO methylene group
-c0
PPO methyl group
CMT 0.4
5
10
15
20
25
30
35
40
45
('C) Figure 8. Longitudinal relaxation times, TI, ofPEO methylene and PPO methyl groups for 1%P104 solution in D2O as a function of temperature. The arrow indicates the CMT of a 1% P104 aqueous solution as determined by a dye solubilization method, i.e. CMT = 21.5 O C . l l Temperature
their movement and therefore induces a broadening of the PPO peaks. Similar to the study of lineshape broadenings, analysis of the time constant, TI (known as the longitudinal or spin-lattice relaxation), of the PPO and PEO peaks at various temperatures provides information on the environments of the different groups and their interactions with the rest of the molecule. In the Bloch theory of NMR,26-28magnetization of a diamagnetic sample results in an induced field which builds up over some time interval. Return to thermal equilibrium is treated as an exponential decay with a time constant, TI.In a multiline spectrum, therefore, not all nuclei experience relaxation at the same rate, so a set of TIvalues may exist for the different nuclei environments in a molecule. What are the factors that influence TIrelaxation? There are two pathways of relaxation available for most molecules: spontaneous emission or magnetic (dipolar) interactions.26 The probability of spontaneous emission is low per s) and can be neglected. The major route is dipolar interaction between nuclei modulated by molecular motions. Thus, factors such as temperature, solution viscosity, molecular size, pH, and hydrogen bonding influence relaxation times. TI analysis of lH NMR spectra of 1%P104 solution in D20 at various temperatures is presented in Figure 8. Two contrasting effects are immediately apparent. While the TI values for the methylene peak of the PEO segment increase exponentially with temperature, those for the methyl peak of the PPO segment remained relatively constant, with a slight maximum around the CMT. It is well-known that elevating the temperature speeds up vibrational, rotational, and translational motions of macromolecular chains and decreases the viscosity. This, in turn, increases the interaction with the surrounding solvent or environment and reduces interactions with the rest of the molecule. The result is a decrease in spinlattice relaxation. In other words, 1'2 values increase with increasing t e m p e r a t ~ r e This . ~ ~ is exactly the scenario for the PEO segment, as previously described. For the PPO block, the situation is quite different. In addition to the (26)Derome, A. Modern NMR Techniques for Chemical Research: Pergamon Press: New York, 1986. (27) Slichter, C . P. Princzples of Magnetic Resonance; SpringerVerlag: Berlin, 1980. (28) Abragam, A. Principles ofNuclear Magnetism; OxfordUniversity Press: Oxford, 1983. (29) Dennis, E. A.; Ribeiro, A. A. Magn. Res. Colloid Interface Sci. Symp. 1976,453.
temperature influence, the PPO segment also experiences a viscosity change upon micellization as discussed earlier. Amore viscous environmentslows down chain movements, Increased interactions between the PPO with the rest of the molecule allow faster relaxation, thus smaller T1 values. Such opposing effects, due to temperature and viscosity, result in the values of TIfor the PPO block remaining relatively constant. Notice also that before micellization ( ~ 2 "0, 5 the TI values for both the PEO and the PPO blocks are similar, indicative ofboth existing in a similar environment. After the CMT, dramatic departure of the value of TIfor the PEO from that of PPO illustrates that the two components are in different environments. After the CMT, the exponential increase of 1'2 for PEO with temperature implies significant interactions with surrounding water. This is consistent with the model that the corona of the Pluronics micelles is made up of hydrated PEO. On the other hand, the constant (or slightly decreased) TI values for the PPO signal confirm our fluorescence data indicating a more and more compact structure ofthe PPO core ofthe micelles as temperature increases. Solubilization of Aromatics. Addition of a hydrophobic compound to a solution of Pluronics in D2O allowed direct observation of the partition of this species between the different microdomains formed upon micellization. Variable temperature measurements of a 1%solution of P104 and 2 pL of benzene clearly illustrated the partitioning of this aromatic species to the interior of the micelle. Below the CMT, the benzene signal is apparent at 7.2 ppm as a singlet. After the CMT, two new signals appear at approximately 5 and 1.3 ppm. It is well-known that inclusion of a species in a highly hydrophobic environmentresults in drastic upfield shifts of the affected protons with complementary downfield shift of the hydrogen-bonded protons. Such is the case here. In addition, penetration of the benzene into the micelle is timedependent. After leaving the sample overnight at 35 "C, it was apparent that a significant amount of benzene penetrated into the core of the micelle as shown by the growth of the peak at 5 and 1.3 ppm in Figure 9, notice though that the intensity ofthe benzene peak was reduced probably due to evaporation. The relative amounts of benzene in the two states indicated by the NMR peaks can be used directly to determine a partition coefficient, Kpw= CJC,, where C, is the concentration of benzene in the micelles and C, that in water. A value of 50 is extracted, which is consistent with extrapolation of the data of Hurter and Hattona for the solubilization of polycyclic aromatics in block copolymer micelles. To confirm the relationship of the peaks and to allow unambiguous assignments of these signals, experiments based on the nuclear Overhauser effect (NOE) were of NOE depends on the p e r f ~ r m e d . Observation ~~~~~ change in intensity of one resonance when the transitions of another are perturbed in some way. Such intermolecular cross-relaxation NOE's are dependent on intermolecular distances. These equilibrium NOE difference spectra gave evidence for the relationship between the upfield shiRed benzene peak at 5 ppm and the downfield shifted PPO peak at 1.3 ppm. For example, irradiation of the peak at 1.3 ppm showed enhancement of the upfield shifked benzene peak, and complementary irradiation of the peak at 5 ppm confirmed these interactions. This implies that hydrophobic aromatic molecules, such as benzene, are solubilized in the PPO core of PEO-PPOPEO block copolymer micelles. (30) Neuhaus, D.; Williamson, M. P. The nuclear overhauser effect in structural and conformational analysis; VCH: New York, 1980.
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126 Langmuir, Vol. 11, No. 1, 1995
PPO in contact wlth S O I U ~ I I I Z ~benzene
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Solubllhd Benzene
t
I
I
dLI
Benzene In bulk solvent
2
Methyl of PPO
3 7
6
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3
e
I
.FP
Figure 9. Proton NMR spectra of 1%P104 solution in D20 containing 2 p L of benzene: bottom spectmm, 10 min after reaching 35 "C;top spectrum, after 1day at 35 "C. Notice that the intensity of the benzene peaks are reduced due probably to evaporation.
Conclusion Both fluorescence and NMR spectroscopy have been used to investigate the microviscosity afforded by various PEO-PPO block copolymer micelles as a function of temperature. Such microviscosity is much higher than that observed in conventional surfactant micelles. In linear PEO-PPO-PEO micellar systems, the microviscosity is mainly dependent on the size of the hydrophobic PPO blocks. As temperature increases the fluorescence probe seems to experience a progressively more viscous environment (as compared to that experienced in homopolymers), probably due to a compacting effect of the micelles. At lower temperatures (but above CMT) the microviscosity experienced by the probe is similar to that observed in a low molecular weight PPO, while at higher temperatures it becomes similar to that observed in significantly higher molecular weight PPO. For the X-shaped copolymer micelles such compacting effect is not observed, the microviscosity experienced by the probe varies similarly to that observed in low molecular weight PPO. This low viscosity is probably due to the presence of the X-shaped junctions in the middle of the PEO-PPO blocks that introduce extra free volume in the micelle core. The NMR data also confirm the fact that upon micellization of the linear triblock copolymers, the PPO blocks experience a more viscous environment. Both the line widths and relaxation measurements showed that the PEO segment remained relatively mobile and in contact with aqueous solvent throughout the range of temperature
studied. On the other hand, upon micellization, the line width of the PPO segment increases while the TIvalues remained constant (slightly decreased). These confirm that the PPO segment experiences an environment with increasing viscosity. In addition, solubilization of aromatics in Pluronics micelles was detected by NMR. An upfield-shifted benzene peak indicated that it is encapsulated in a highly hydrophobic environment. Experiments based on the nuclear Overhauser effect (NOE) confirmed these direct interactions between the solubilized aromatic and the PPO segment, supporting previous observations by fluorescence studies.
Acknowledgment. This work was supported by the U.S. Department of Energy under Grant Number DEFG02-92ER14262, and the Emission Reduction Research Center (ERRC), Newark, NJ. We acknowledge Vassiliki Athanassiou, Shinya Fukuda, and Wendy Yeh, who participated in this research program under the M.I.T. Undergraduate Research Opportunities Program (UROP), for their valuable assistance in conducting some of the fluorescence measurements. We are also grateful to Professor M. A. Winnik for his guidance in the synthesis of dipyme and to Dr. A. Yekta for valuable discussion; both are from the University of Toronto. We also thank BASF Corp. (Parsippany, NJ) for providing us with Pluronic and Tetronic surfactant samples free of charge. LA9407626
