Micellization Dynamics and Impurity Solubilization of the Block

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Micellization Dynamics and Impurity Solubilization of the Block-Copolymer L64 in an Aqueous Solution Matthias J. Kositza,† Cornelia Bohne,†,| Paschalis Alexandridis,‡ T. Alan Hatton,§ and Josef F. Holzwarth*,† Abteilung Physikalische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin-Dahlem, Germany, Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received September 14, 1998. In Final Form: December 8, 1998 The dynamics of the micellization of Pluronic® L64 (EO13PO30EO13, PEO/PPO ) 2/3) was studied by employing the iodine laser temperature-jump technique. These studies show a more complex behavior than previously described. Three relaxation processes were observed, depending on the temperature at which experiments were performed. The first two of these relaxation processes are related to the unimer incorporation into micelles and the redistribution of micelles after incorporation of the unimers. The third relaxation process, which had not been previously observed, occurs at high temperatures and is related to the clustering of micelles into larger aggregates. In addition, the presence of impurities and hydrophobic components was shown for the first time to significantly affect the dynamics of L64 micellization.

Introduction Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers (PEO-PPO-PEO, also known by their trademark names Pluronics® (BASF) or Poloxamers (ICI)) are nonionic surfactants used widely in industry.1,2 The aggregation properties of these copolymers in aqueous solutions can be altered by varying the PPO/PEO composition ratios or total molecular weights.3-5 These block copolymers are amphiphilic with the central PPO portion being hydrophobic and the external PEO chains being relatively hydrophilic. Above a certain concentration (critical micellar concentration, CMC) or temperature (critical micellar temperature, CMT) micellization occurs.6 The copolymer micelles have a core of PPO segments surrounded by a PEO corona. A further increase of the temperature above the CMT leads to an enhancement of micellization which is believed to be accompanied by a progressive dehydration of the copolymer chains.7 At sufficiently high temperatures a cloud point (CP) is reached where phase separation into a polymer-rich and a water-rich phase occurs.5,8 The characterization of the structure of Pluronic® micelles has been extensively studied.4,5,7-10 In contrast, * To whom correspondence should be addressed. Phone: +4930-84135516. Fax: +49-30-84135385. E-mail: Holzwarth@ fhi-berlin.mpg.de. † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ State University of New York at Buffalo. § Massachusetts Institute of Technology. | On sabbatical leave from the Department of Chemistry, University of Victoria, British Columbia, Canada V8W 3V6. (1) Schmolka, I. R. Surf. Sci. Ser. 1967, 1, 300. (2) Whitmarsch, R. H. Surf. Sci. Ser. 1996, 60, 1. (3) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490. (4) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (5) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (6) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (7) Goldmints, I.; von Gottberg, F. K.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 3659. (8) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (9) Almgren, M.; Bahadur, P.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A. J. Colloid Interface Sci. 1992, 151, 157.

the study of the kinetics of polymeric micelles has attracted insufficient attention, but is important both from the fundamental and the application point of view. In particular, such studies should improve the understanding of the kinetics of solubilization by such micelles, which are relevant to the use of copolymers in dispersion polymerization, rates of solubilization, and detergency and in the formulation of lubricants.11 There are only a few reports on the micellization dynamics which in part presented conflicting results and interpretations. Indirect methods such as gel permeation chromatography12 and NMR13 lead to unimer residence times in micelles from hours to less than 3 ms, respectively. A variety of timeresolved methods have been employed in which different numbers of relaxation processes were reported.14-18 Ultrasonic relaxation measurements showed a fast relaxation process (µs time domain) that was assigned to the diffusion-controlled association of unimers into micelles.17 In contrast, in Joule-heating and laser temperature-jump experiments a slightly slower relaxation process was assigned to the unimer incorporation into micelles.14,15 In addition, a second relaxation process has been observed, which has a negative amplitude in lightscattering experiments and is slower than the unimer incorporation event.14,17 This process was interpreted to be due to the size redistribution of micelles either by an Aniansson-Wall mechanism14 or by fission/fusion processes.17 A second aspect that has been frequently raised when studying the properties of Pluronic® copolymers is that impurities or hydrophobic species, such as diblock and triblock copolymers, are present in the industrial samples. Many “anomalous” observations of the aggregation be(10) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101. (11) Oh, S. G.; Shah, D. O. J. Am. Oil Chem. Soc. 1993, 70, 673. (12) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5440. (13) Fleischer, G. J. J. Phys. Chem. 1993, 97, 517. (14) Goldmints, I.; Holzwarth, J. F.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 6130. (15) Hecht, E.; Hoffmann, H. Colloids Surf., A 1995, 96, 181. (16) Rassing, J.; Bandyopadhyay, S.; Eyring, E. M. J. Mol. Liq. 1983, 26, 97. (17) Michels, B.; Waton, G.; Zana, R. Langmuir 1997, 13, 3111. (18) Hvidt, S. Colloids Surf., A 1996, 112, 201.

10.1021/la9812368 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/31/1998

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havior of the Pluronic® copolymers have been attributed to the presence of additional components.9,15,19,20 Although there is merit in studying the properties of samples which have practical applications, it is very important that the effect of these impurities on the aggregation behavior be understood, so that the properties of Pluronic® micelles can be rationally designed for the intended applications. In this letter we present dynamic data for the micellization of the Pluronic® L64 copolymer for which a new relaxation process was observed in addition to the two processes previously reported.14,15,17 Furthermore, we show for the first time that the presence of hydrophobic impurities in the industrial samples of L64 can significantly alter the behavior of the micellization dynamics. Experimental Section The Pluronic® L64 (EO13PO30EO13, nominal Mw ) 2900, PEO weight content of 40%, PEO/PPO ) 2/3) and L61 (EO2PO30EO2, nominal Mw ) 1930, PEO weight content of 10%, PEO/PPO ) 1/9) copolymers were a gift from BASF (Parsipanny, NY). L64 was purified by vigorously stirring 1 g in 100 mL of hexane at 20 °C for 30 min, followed by separation of the hexane phase which was discarded. This procedure was repeated three times after which the liquid L64 was stirred for 3 h at 60 °C and dried for 24 h on a water aspirator to extract residual hexane. Purified water (Milli-Q Ultrapure Water System) was employed to prepare aqueous solutions of L64, and these were kept for 6 h at 15 °C before being filtered through 0.22 µm Millipore filters. Light-scattering data were collected at 360 nm and 90° with a RF-5000 Shimadzu fluorimeter or at 0° with a UV-2100 absorption spectrometer. A Haake F3-C bath was employed to control the sample temperature, and the temperature was changed at a rate of 0.2 °C/min by using a Haake PG 20 controller. No hysteresis was observed for the light-scattering intensity when the sample was heated and then subsequently cooled. The iodine laser temperature-jump (ILTJ) setup has been previously described.21 The iodine laser (1315 nm) excites rotational-vibrational states of water leading to heating of the solution by ≈1 °C in 2.4 µs, independent of the solution composition. The relaxation to the new equilibrium after the laser excitation was followed by light scattering (Xe/Hg 200 W arc lamp, 360 nm) measured at 0 ( 15 °C and 90 ( 15 °C. The relaxation decays were averaged for at least four experiments. The upper time limit for measuring the relaxation kinetics is dictated by the cooling of the solution which was detectable after 1.5 s.

Results The light-scattering intensity measured at a constant scattering angle and wavelength is sensitive to the particle size and concentration. The scattering intensity of purified L64 increases monotonically above the CMT (Figure 1). The CMT is obtained from the intercept of the linear regions defined by the baseline at temperatures below the CMT and the region up to 2-3 °C above the CMT (Table 1).6 The CP values were determined in an analogous manner from scattering data collected at 0 °C (Table 1). The CMTs measured are 2-3 °C higher than those determined using an absorption probe molecule,6 but were similar to those determined by ultrasonic relaxation.17 The CP values do not vary in the L64 concentration range employed, in contrast to previous reports.8,22 The effect of impurities on the light-scattering intensities is significant, close to the CMT where additional peaks are observed (Figure 1). The shape of the light-scattering (19) Kausalya, R. N.; Fordham, P. J.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1990, 86, 1569. (20) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548. (21) Holzwarth, J. F.; Schmidt, A.; Wolff, H.; Volk, R. J. Phys. Chem. 1977, 81, 2300. (22) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074.

Figure 1. Light scattering intensities at 360 nm and 90° for aqueous solutions of 2.5% (w/v) purified L64 (0), 2.5% (w/v) industrial L64 (b), and a mixture of purified L64 (2.5% w/v) and L61 (0.25% w/v) (O). Table 1: Dependence of the CMT and CP Values as Obtained from Light Scattering

a

[L64] (% w/v)

CMT (°C)a

CP (°C)b

0.625 1.25 2.5 5

37.0 34.2 32.1 29.7

59.8 59.7 60.1 60.5

Estimated errors of 0.3-0.5 °C. bEstimated errors of 1-1.5 °C.

Figure 2. Response curves on split time basis for the temperature-jump experiment of 2.5% (w/v) purified L64 in water at 37 °C (A) and 47 °C (B). The small spike at short times comes from the iodine laser and corresponds to the time (t ) 2.4 µs) for which the sample was heated.

curve for the industrial sample can be simulated by adding small amounts of the more hydrophobic L61 Pluronic® (open circles, Figure 1) to a solution containing pure L64. It is worth noting that, even for temperatures well above the CMT, the light-scattering intensity is always higher for the impure L64 than for the purified sample. The addition of increasing concentrations of L61 depresses the CP in a linear fashion from 60 °C for the pure sample to 49 °C with the addition of 0.5% w/v L61 to pure L64 (2.5%w/v). ILTJ experiments for pure L64 solutions at temperatures close to the CMT revealed two relaxation processes (τ1, τ2) (Figure 2A). The first and second components have respectively positive and negative amplitudes, suggesting that in the first process the size and/or the number density of micelles increases, whereas the opposite occurs during the second relaxation. As the temperature is raised the relaxation lifetimes and absolute amplitudes for both processes decrease (Figure 3). In addition, the relaxation processes for a 2.5% w/v L64 solution are not detectable

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Figure 4. Concentration dependence of 1/τ1 (A) and 1/τ2 (B) at T-CMT ) 4 °C.

Figure 3. (A) Temperature dependence of the three different relaxation times τ1, τ2, and τ3 for aqueous solutions of 2.5% (w/v) purified L64 (full symbols with a connecting line), 2.5% (w/v) industrial L64 (open symbols with a connecting line), and a mixture of purified L64 (2.5% w/v) and L61 (0.25% w/v) (nonconnected symbols).

above 52 and 42 °C for the first and second process, respectively. The same lifetimes for both processes were obtained when the ILTJ experiments were performed with light-scattering detection at 0 and 90 °C. At high temperatures (>45 °C for 2.5% w/v L64) a new (third) process with a positive amplitude was observed (Figure 2B). The values for τ3 are constant between 45 and 53 °C and then increase (Figure 3). The relaxation kinetics were measured for different L64 concentrations. Since the CMT changes with the Pluronic® concentration, the effect of concentration on τ1 and τ2 was studied at a fixed temperature increment above the CMT (T-CMT ) 4 °C). It was shown for P85 that the micelles have the same aggregation numbers at a constant T-CMT,7 suggesting that these micelles have the same structure. We are assuming that the same behavior holds true for L64 because our preliminary SANS data point in this direction. The reciprocal for τ1 varies linearly with the total L64 concentration as is typical for second-order processes, whereas for 1/τ2 a more complex behavior is observed (Figure 4). It is worth noting that, at a constant temperature, which is at least 4 °C above the CMT of the lowest L64 concentration measured, the 1/τ1 values also vary linearly with the L64 concentration. The presence of impurities when micellization of L64 is complete (i.e., at temperatures above those for which the two impurity peaks are observed in the light-scattering curve (>37 °C, Figure 1)) leads to a significant slowing down of τ2 and τ3 (Figure 3A). However, the values for τ1 are not significantly altered. When increasing amounts of L61 (0-0.375% w/v) are added to pure L64, the τ2 and τ3 values increase in a linear fashion, and the absolute

magnitude of the amplitude for the process related to τ2 (A2) with respect to the amplitude associated with τ1 (A1) increases from A2/A1 ) 0.5 for pure L64 to 0.8 in the presence of 0.375% w/v of L61. Furthermore, in the temperature region around the CMT where the two prominent light-scattering peaks are observed (Figure 1), the dynamics of aggregation are very different for the industrial sample when compared to those observed for pure L64. The first light-scattering peak for the industrial sample occurs at temperatures below the CMT and a very long relaxation process with a positive amplitude (>300 ms) was measured. The dynamics at the falling edge of the second peak show two relaxation processes with opposite amplitudes. The lifetime of the process with the positive amplitude is 376 µs, whereas the process with negative amplitude has a relaxation with a 320 ms lifetime. These values are at least 1 order of magnitude larger than the relaxation processes τ1 and τ2 observed in the region where L64 micellization is complete. The L64/L61 mixtures show trends similar to those observed for the industrial sample, although the relaxation lifetimes for the dynamics in the two peaks are not as long as those for the industrial sample. Discussion The effect of impurities on the properties of micelles in general are well-known. However, the significant effect on the micellization dynamics of block copolymers had not been reported previously. An understanding of how hydrophobic components affect the micellization dynamics can be explored in practical applications. The effect of hydrophobic impurities (or added L61) on the light-scattering curve at temperatures below the CMT can be explained by the formation of mixed aggregates containing mainly the hydrophobic impurities but also some L64. As the temperature is raised, L64 micelles containing the impurities are formed, leading to a higher light-scattering intensity for the industrial sample when compared to that for pure L64 micelles. This fact suggests that the structures of the micelles are different in the two samples. A possible explanation is that the incorporation

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of hydrophobic components into the mixed micelles could lead to an increase in the aggregation numbers. Alternatively, more rigid micelles could be formed when hydrophobic components are present. The impurities or addition of L61 do not affect the value of τ1, which is assigned to the unimer incorporation into micelles (see below). This fact suggests that the incorporation of L64 unimers into micelles is not very dependent on the presence of hydrophobic impurities in the micelle. In contrast, the slowing down of the processes associated with τ2 and τ3 by the hydrophobic components suggests that these processes are dependent on the structural composition of the Pluronic® micelles. It should be emphasized that although hydrophobic components (impurities) lead to significant alterations of the structure and dynamics of Pluronic® micelles, our results with L61/ L64 mixtures show that a controlled combination of different Pluronic® copolymers could lead to dual micellization behavior (i.e., formation of micelles in two distinct temperature regions) and control of the dynamics (i.e., slow down with an increased ratio of the hydrophobic component). Three distinct relaxation processes are observed when the temperature is raised above the CMT. The first relaxation process corresponds to the unimer incorporation into the micelles.14,15 A dependence of the τ1 values on the L64 concentration and a decrease of the lifetimes when the micelle size increases was observed. In addition, no dependence of the lifetime on the scattering angle was observed and the first relaxation process was also measured for salt-jump experiments using the stopped-flow technique.23 These results suggest that the relaxation process is primarily related to the micellization dynamics and not to the time constant characteristic for the fluctuation of particles as previously suggested.17 It is worth noting that the positive intercept observed (Figure 4A) indicates that both the association and dissociation processes contribute to the magnitude of τ1. In this sense, the first relaxation process is equivalent to the first processes in the Aniansson-Wall mechanism24 in which the incorporation of unimers can be described by association and dissociation steps. However, further structural changes, which cannot be explained by the AnianssonWall mechanism, occur after the incorporation of the unimer since the second relaxation process characterized by a decrease of the light-scattering intensity is observed (process 2). The second and third relaxation processes do not correspond to the same dynamic process. The lifetime at the lowest temperature for which the third relaxation process is observed is consistently lower than the lifetime measured for the second relaxation process. The second relaxation process which is associated with a decrease in micellar size and/or density has been previously explained by the Aniansson-Wall formalism14 or the fission-fusion mechanism.17 For the former mechanism, the lifetime of the relaxation process would correspond to the lifetime of the micelles, whereas in the latter mechanism the (23) Kositza, M. J.; Bohne, C.; Hatton, T. A.; Holzwarth, J. F. Progress in Colloid and Polymer Science; Trends in Colloid and Interface Science XIII; Springer: Berlin, in press. (24) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905.

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redistribution in size occurs because of the collision of micelles. Our results are consistent with a bimolecular mechanism, such as the fusion-fission mechanism, since 1/τ2 increases when the L64 concentration is raised. However, the concentration dependence observed is complex, indicating that more than one factor contributes to the redistribution of micelles. One such factor could be the dehydration of the micelles after the incorporation of unimers, since it was observed that a temperature increase for P85 led to micelles with higher aggregation numbers but a smaller water content.7 In addition, the upward curvature of the concentration dependence on 1/τ2 indicates that cooperative effects could also play a role. Although the mechanism is still uncertain, this second process is related to the redistribution of spherical micelles after the incorporation of unimers. The third relaxation process is only observed at temperatures much higher than the CMT, when fairly large micelles are present in solution and the concentration of unimers is relatively low. The steep increase in the lightscattering curve in the region where τ3 is observed suggests that the size of the particles is increasing significantly. Above 52 °C (2.5% w/v) this increase cannot be due to the further incorporation of unimers since all copolymer molecules are incorporated into the micelles, as attested by the disappearance of the first relaxation process and as reported previously6 by the exponential decrease of the CMC with increasing temperature. For this reason, we attribute the third relaxation process to be related to the clustering of micelles which leads to progressive changes in the micelle structure until phase separation occurs at the CP. The relaxation lifetime stays fairly constant until the complete disappearance of the first relaxation process, when the values for τ3 increase as the temperature is raised. This slowing down of the clustering process is related to the fact that the number density of particles is decreasing, since all unimers are micellized and the particles are larger. The behavior of the third relaxation process is clearly very complex and will be the subject of further studies. In conclusion, our systematic temperature studies which cover the whole range between the CMT and CP clearly demonstrate for the first time that the micellization of L64 follows a complex behavior. The deviation of the τ2 from Aniansson-Wall behavior suggests that micellization is different from that observed for simple micelles such as alkyl sulfates. The dynamics of micellization is critically dependent on the temperature which directly affects the size and structure of the micelles. It is this variability of size and structure that leads to the complexity of micellization. We are currently expanding our studies in order to understand the details of the micellization dynamics. Finally, we have shown that the effects of hydrophobic components (impurities) is remarkable and the L64/L61 mixture experiments suggest a way to control the micellization structure and dynamics. Acknowledgment. C.B. thanks the Alexander von Humboldt Foundation for a fellowship. P.A. and T.A.H. thank the Max-Planck-Gesellschaft for special travel grants. LA9812368