Rheological Study of the Shape Transition of Block

Box 124, S-22100 Lund, Sweden, and Grupo de Polı´meros USB, Departamento de Ciencias de los. Materiales, UniVersidad Simo´n Bolı´Var, Caracas 108...
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Langmuir 2007, 23, 11000-11006

Rheological Study of the Shape Transition of Block Copolymer-Nonionic Surfactant Mixed Micelles David Lo¨f,*,† Karin Schille´n,† Miguel F. Torres,‡ and Alejandro J. Mu¨ller*,‡ DiVision of Physical Chemistry, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, S-22100 Lund, Sweden, and Grupo de Polı´meros USB, Departamento de Ciencias de los Materiales, UniVersidad Simo´ n Bolı´Var, Caracas 1080A, Venezuela ReceiVed June 19, 2007. In Final Form: August 7, 2007 A rheological study of mixed micelles formed by PEO-PPO-PEO triblock copolymer P123 and nonionic surfactant C12EO6 in aqueous solutions has been carried out with the purpose of investigating the time dependence of a shape transition of the mixed micelles and characterizing the shape before and after the transition. The rheology results presented in this report give clear evidence that the P123-C12EO6 mixed micelle grows and changes gradually in shape from spherical to elongated (rodlike) geometry with increasing temperature. These results are in accordance with the results found in the parallel dynamic and static light scattering and calorimetrical investigation.1,2 By using steady-state rheology, the time dependence of the sphere-to-rod transition of the mixed micelle system was carefully followed with time and temperature as simultaneously recorded variables in the experiments. This was performed by a designed novel experimental procedure. A temperature ramp was applied at a rate of 2.6 °C/min from a temperature below to a temperature above the shape transition at a constant shear rate while the viscosity of the solution was measured. The investigation was limited to two different compositions, surfactant-to-copolymer molar ratios (MR ) nC12EO6/nP123) of 2.2 and 6.0 with varying total concentration from 1.5 to 21 wt % in comparison with the neat component. At low concentration, a slow transition was observed, which indicated that the mixed micelles are still growing into rods for several minutes after reaching the final temperature. At a total concentration of 4.0 wt % and above, the system reached equilibrium quickly. A concentration-dependent kinetic process is therefore anticipated, which was also found in the time-resolved static light scattering experiments previously performed (Lo¨f, D.; Schille´n, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911). At concentrations above 10 wt %, shear-thinning behavior was observed for the mixed solutions, which strongly suggests the extended shape of the mixed micelles after the shape transition. The obtained zero-shear viscosity at the investigated molar ratios was found to be lower with higher molar ratios, which indicates that the mixed micelles both in the spherical and in the rodlike state becomes smaller with higher content of C12EO6. These results correlate well with the obtained results from the previous dynamic light scattering measurements on the same system (Lo¨f, D.; Schille´n, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911).

Introduction Triblock copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) are nonionic, water-soluble, low-molecular-weight polymers that are abbreviated as PEOPPO-PEO or EOnPOmEOn.3 They are widely used in several industrial and scientific applications4,5 such as detergents, emulsifications, and controlled delivery in pharmaceutical products.6,7 The EOnPOmEOn copolymers exist in a wide range of different compositions8 and display rich phase behavior in water,9,10 depending on their relative block length. In the dilute regime, the copolymers may self-assemble to form micelles, * Corresponding authors. (D.L.) E-mail: [email protected]. Tel: (+46)-046-222 4682. (A.J.M.) E-mail: [email protected]. Tel: (+58)-2129063388. † Lund University. ‡ Universidad Simo ´ n Bolı´var. (1) Lo¨f, D.; Schille´n, K.; Olofsson, G.; Niemiec, A.; Loh, W. J. Phys. Chem. B 2007, 111, 5911. (2) Schille´n, K.; Jansson, J.; Lo¨f, D.; Costa, T. Submitted to J. Phys. Chem. B. (3) Alexandridis, P.; Hatton, A. T. Colloids Surf., A 1995, 96, 1. (4) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490. (5) Mortensen, K. Colloids Surf., A 2001, 183, 277. (6) Kabanov, A. V.; Alakhov, V. Y. Crit. ReV. Ther. Drug Carrier Syst. 2002, 19, 1. (7) Fusco, S.; Borzacchiello, A.; Netti, P. A. J. Bioact. Compat. Polym. 2006, 21, 149. (8) Pluronic and Tetronic Surfactants; Technical Brochure; BASF-Corporation: Parsippany, NJ, 1998. (9) Chu, B.; Zhou, Z. In Nonionic Surfactants; Nace, W. M., Ed.; Surfactant Series; Marcel Dekker: New York, 1996; Vol. 60, p 67.

with a hydrophobic core containing PPO and a hydrophilic waterswollen corona that contains the PEO chains and protects and encloses the PPO core from the surrounding water.11,12 Vesicles are other self-organized structures that also have been found.13,14 The self-assembly arises from the limited solubility of the hydrophobic PPO block and temperature-dependent solubility of the PEO block in water. For those copolymers that form micelles, the micelles are spherical in shape at ambient temperature. Depending on the PEO/PPO ratio, these micelles exhibit a shape transition at elevated temperatures and form rodlike or wormlike micelles.15-21 Recently, we have investigated the micelles of copolymer EO20PO68EO20 (P123) using dynamic and static light scattering as well as differential scanning and isothermal calorimetry in combination.1,2 The micelles of (10) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101. (11) Linse, P. Macromolecules 1993, 26, 4437. (12) Brown, W.; Schille´n, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (13) Schille´n, K.; Bryskhe, K.; Mel’nikova, Y. S. Macromolecules 1999, 32, 6885. (14) Bryskhe, K.; Jansson, J.; Topgaard, D.; Schille´n, K.; Olsson, U. J. Phys. Chem. B 2004, 108, 9710. (15) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (16) Schille´n, K.; Brown, W.; Johnsen, R. M. Macromolecules 1994, 27, 4825. (17) Lehner, D.; Lindner, H.; Glatter, O. Langmuir 2001, 17, 4818. (18) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 4128. (19) Batsberg, W.; Ndoni, S.; Trandum, C.; Hvidt, S. Macromolecules 2004, 37, 2965. (20) Michels, B.; Waton, G.; Zana, R. Colloids Surf., A 2001, 183, 55. (21) Duval, M.; Waton, G.; Schosseler, F. Langmuir 2005, 21, 4904.

10.1021/la701818y CCC: $37.00 © 2007 American Chemical Society Published on Web 09/21/2007

Rheological Study of the Shape Transition

P123 in a 1.0 wt % solution were found to undergo a sphereto-rod shape transition at a temperature of around 61 °C. The rheological behavior of triblock copolymers has been investigated in earlier studies, especially at higher concentrations close to the micellar overlap concentration (e.g., in solution that is approaching a liquid-crystalline phase such as the cubic phase10). A rheological investigation has also been performed on PEO-PPO-PEO copolymers that tend to form elongated micellar rods rather than spherical micelles.20 Brown et al. performed oscillatory shear modulus measurements on the P85 triblock copolymer.12 This copolymer forms both spherical and rodlike micelles in aqueous solution, and at higher concentrations, cubic and hexagonal phases show viscoelastic properties. Glatter et al. reported low-shear viscosity measurements on the cubic and hexagonal phases of the P85 triblock copolymer system at high concentrations.22 Furthermore, Waton et al. studied the zeroshear viscosity of copolymer P84 micelles in the dilute aqueous solution regime and characterized the transition of the growth from spherical to entangled cylindrical aggregates.23 In most of the published studies involving these types of triblock copolymers, the rheological characterization was carried out first before and then after the investigated transition (either phase or shape) of the system at equilibrium. However, Jeong et al.24 employed rheology to study the kinetics and the mechanism of the morphology transition from nonequilibrium to the cylinder microdomain in a polystyrene-block-poly(ethylene-co-but-1-ene)block-polystyrene triblock copolymer system. They measured the storage modulus as function of time during the transition, where the transition appeared to last for a couple of hours. Surfactants are often added to polymer solutions to change or control the structure of the fluid. Polymer and surfactant mixtures display a wide range of applications, and depending on which type of polymer/surfactant pair used, the macroscopic properties of the polymer system changes.1,25-26 In the case of PEO-PPOPEO copolymer-surfactant systems, several studies with both anionic and cationic surfactants have been performed to investigate the different interaction forces and the effect of surfactant addition to the self-organization of the copolymers.27-34 The interaction between PEO-PPO-PEO copolymers and nonionic surfactants has been less investigated.35-37 Wyn-Jones et al. have shown using calorimetry that nonionic surfactant hexa(22) Glatter, O.; Scherf, G.; Schille´n, K.; Brown, W. Macromolecules 1994, 27, 6046. (23) Waton, G.; Michels, B.; Steyer, A.; Schosseler, F. Macromolecules 2004, 37, 2313. (24) Jeong, U.; Lee, H. H.; Yang, H.; Kim, J. K.; Okamoto, S.; Aida, S.; Sakurai, S. Macromolecules 2003, 36, 1685. (25) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, U.K., 1998. (26) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goodard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (27) Jansson, J.; Schille´n, K.; Olofsson, G.; da Silva, R. C.; Loh, W. J. Phys. Chem. B 2004, 108, 82. (28) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866. (29) Jansson, J.; Schille´n, K.; Nilsson, M.; So¨derman, O.; Fritz, G.; Bergmann, A.; Glatter, O. J. Phys. Chem. B 2004, 109, 7073. (30) da Silva, R. C.; Olofsson, G.; Schille´n, K.; Loh, W. J. Phys. Chem. B 2002, 106, 1239. (31) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (32) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5742. (33) Thurn, T.; Couderc, S.; Sidhu, J.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2002, 18, 9267. (34) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Wyn-Jones. E.; Holzwarth, J. F. Langmuir 2001, 17, 183. (35) Hossain, M. K.; Hinata, S.; Lopez-Quintela, A.; Kunieda, H. J. Dispersion Sci. Technol. 2003, 24, 411. (36) Aramaki, K.; Hossain, M. K.; Rodriguez, C.; Uddin, M. H.; Kunieda, H. Macromolecules 2003, 36, 9443.

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(ethylene oxide) monododecyl ether C12EO6 mixed at different molar fractions with F127 shows that the two species interact and that C12EO6 induces the aggregation of F127.38 In recent studies of the aqueous P123 copolymer and the C12EO6 surfactant system, we showed using DLS and SLS that mixed micelles are formed.2 The size of the micelles depends on the amount of C12EO6 as well as the temperature. At ambient temperatures, the mixed micelles decrease with increasing C12EO6 concentration.2 We also investigated the interactions in the P123-C12EO6 mixed system using titration and differential scanning calorimetry (DSC).1 In that investigation, time-dependent isothermal titration calorimetry (ITC) measurements revealed that two processes are present in the narrow molar ratio interval where the rodlike micelles return to a spherical shape. In the work described in our previous study, the kinetics of the shape transition from spherical to rodlike for the P123C12EO6 mixed micelles was also investigated.1 In the dilute regime and at different surfactant-to-copolymer molar ratios (MR ) nC12EO6/nP123), the sphere-to-rod transition was followed both by differential scanning calorimetry and by dynamic and total intensity light scattering measurements. We found that when C12EO6 is mixed with P123 the sphere-to-rod transition temperature decreases to the lowest level for a molar ratio between 2 and 3 as compared to that of the neat P123 micelles. This was attributed to a change in the PEO corona induced by the surfactants. Increasing the molar ratio further decreased the size of the spherical mixed micelles below the transition and therefore increased the transition temperature. This was explained by the extra cost in energy, due to higher curvature, that has to be put into the system to change the shape. The transition also showed concentration-dependent kinetically slow character, which was investigated by monitoring the total light scattering intensity as a function of time at the sphere-to-rod transition temperature. Simultaneously made time-dependent DLS measurements clearly demonstrated micellar growth. Depending on the experimental conditions (e.g., the total concentration), the transition time varied from a few minutes to a couple of hours (a slower process for a lower total concentration). The reason for this behavior is due to a fewer number of collisions per time unit, leading to micellar growth between the mixed micelles in a less concentrated system. The purpose of this work is to investigate further the shape transition of the P123-C12EO6 mixed micelles, especially its time dependence, by employing rheological methods to complement the previous light scattering and calorimetry experiments.1 A sphere-to-rod transition has an important impact on the rheology exhibited by the fluid. In this study, two different C12EO6/P123 molar ratios, 2.2 and 6.0, were investigated at different total concentrations. It is, to our knowledge, the first time that timedependent shear rheology measurements are performed during a temperature scan to study the shape transition of self-assembled aggregates in solution. Materials and Methods Materials. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer P123 was a generously donated by BASF Corporation, Performance Chemicals, Mount Olive, New Jersey. It has an average composition of EO20PO68EO20 and a nominal molar mass of 5750 g mol-1. Nonionic surfactant hexa(ethylene oxide) monododecyl ether (C12EO6) was purchased from Nikkol Chemicals. All chemicals were used without (37) Ivanova, R.; Alexandridis, P.; Lindman, B. Colloids Surf., A 2001, 183, 41. (38) Couderc, C.; Li, Y.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 4818.

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Figure 1. Viscosity as a function of shear rate at different temperatures for a P123-C12EO6 solution of 21 wt % total concentration and at molar ratio 2.2. Region I: 25.0 (O), 30.0 (0), and 34.0 °C (2). Region II: 38.2 (b), 39.4 (×), 40.9 (1), 42.0 (]), and 43.9 °C (9). further purification. It is well known that the PEO-PPO-PEO copolymers are polydisperse both in molar mass and composition.19,29 Distilled, deionized water was used as the solvent. The copolymer and the surfactant stock aqueous solutions were prepared by weight. They were left to equilibrate for at least 1 day at 6 °C before mixing them at different C12EO6/P123 molar ratios (MR ) nC12EO6/nP123). The mixed solutions were left to equilibrate for at least another day before measurements were performed. In this study, the main focus has been placed on MR ) 2.2 and 6.0 along with the neat polymer and surfactant systems. The total concentration used in the P123-C12EO6 mixed solutions varied from 1.5 to 21 wt %. Shear Rheology. In all rheological measurements, a Rheometrics ARES shear rheometer with a double-wall Couette fixture was used. Two types of measurements were performed: measurements of the shear viscosity as a function of shear rate under controlled strain rate and measurements of the shear viscosity as a function of time and temperature with fixed shear rate. A water-circulating bath was employed to regulate the temperature automatically with a PID controller ((0.1 °C). The temperature-scanning procedure used an initial temperature that was approximately 15 °C below the expected shape transitions in our system. The final temperatures were different for the two molar ratios (2.2 and 6.0) and were chosen on the basis of expected shape transitions found in the DSC measurement.1 Additional measurements were also carried out where the final temperatures were increased even further above the expected transition temperature. The temperature-increasing rate was 2.6 °C/ min in all cases. The temperature-scanning range was set to cover the sphere-to-rod transition at each molar ratio, where the DSC curves of ref 1, which are also summarized in Table 1, were used as a guide to estimate this range.

Results and Discussions Molar Ratio ) 2.2. Figure 1 presents the shear viscosity as a function of shear rate obtained from the rheological measurements performed at different temperatures on P123-C12EO6 mixed aqueous solutions at molar ratio 2.2 and with a total concentration of 21 wt %. At this high concentration, the system is still in the isotropic liquid phase. At temperatures below 36 °C, the solutions behave like Newtonian fluids, and the zeroshear viscosities (plateau values) at low shear rates decrease when the temperature increases (solutions denoted as region I in Figure 1). Their behavior is due to the lowering of the viscosity of the solvent (i.e., water) as the temperature increases. In other studies, for instance, the intrinsic viscosity of P94 has been reported to decrease with temperature in the range of 30-50 °C. This reduction has been attributed to a more compact micelle conformation as the temperature increases because of the relative dehydration of the PEO chains.11,39 However, this is not the case (39) Bahadur, P.; Pandya, K. Langmuir 1992, 8, 2666.

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Figure 2. Evolution of the logarithm of the zero-shear viscosity η0 of a 21 wt % P123-C12EO6 solution at molar ratio 2.2 as a function of the reciprocal values of the absolute temperature. Filled squares (9) represent the corresponding evolution for distilled water. Table 1. Transition Temperature Estimated with DSC and DLS Results (data from ref 1) for the Apparent Hydrodynamic Radii before and at the Transition Temperature for Molar Ratios of 2.2, 3.2, 6.0, and 11 molar ratio

Tm/°C (sphere-rod)

RH,app/nm (sphere)

RH,app/nm (rod)

2.2 3.2 6 11

41 43 ∼52 ∼54

8.8 (35 °C) 7.8 (38 °C) 7.7 (45 °C) 5.4 (25 °C)

21 (40 °C) 18 (43 °C) 15 (52 °C) 15 (60 °C)

in our system where, at temperatures below 40 °C, the mixed micelles at high dilution do not significantly change their structure according to ref 2. The dynamic and static light scattering results showed that the P123-C12EO6 mixed micelles experience only minor changes in hydrodynamic size and molar mass with increasing temperature in the range displayed in Figure 1 below 40 °C. Thus, the micellar shape is still spherical, and small changes in the mixed micelles will not affect the viscosity in this temperature range. When the temperature is increased to values close to 40 °C, the shear viscosity increases to values larger than those exhibited at 34 °C (solutions denoted as region II in Figure 1). This implies a growth of the P123-C12EO6 mixed micelles to an extent that greatly affects the viscosity of the solution. Despite the decrease in the water viscosity, a transformation of the structure of the fluid enhances the solution viscosity. At 40 °C, the solution behaves as a shear thinning fluid. The changes observed in the rheology of the solution indicate that rodlike extended mixed micelles have been formed. It is well known that a solution with rodlike particles will align in the direction of flow at a certain shear rate, thereby decreasing its area of resistance. As a result, the viscosity will decrease with increasing shear rate. We may thus conclude that for molar ratio 2.2 a shape transition from spherical to rodlike for the mixed micelles is taking place at temperatures in the vicinity of 40 °C. This coincides perfectly with previous DSC measurements on the same molar ratio where a small DSC peak, positioned at 41 °C, was attributed to the shape transition (Table 1). The DLS measurements performed in ref 1 revealed a change in the apparent hydrodynamic radius RH,app from 8.8 nm at 35 °C to 21 nm at 40 °C for the same molar ratio (Table 1). Figure 2 summarizes the zero-shear viscosity data from Figure 1 in an Arrhernius-type plot. In the low-temperature range (2534 °C or region I), the viscosity of the solution varies according to Arrhenius’s law, which is the classic temperature-dependent behavior of many liquids. In this low-temperature region, the shape of the P123-C12EO6 mixed micelles is spherical. In the temperature range near 40 °C and above (region II), an abrupt viscosity increase in the signals is related to the transformation

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Figure 3. Viscosity at a constant shear rate of 100 s-1 (O) and temperature (0) as a function of time for a 21 wt % P123-C12EO6 solution at molar ratio 2.2.

of the mixed micelles to a rodlike shape. The corresponding viscosity of water is also added to Figure 2, and its slope is parallel to the viscosity in the region representing the spherical shape of the mixed micelles. The activation energy was calculated from the slopes and was determined to be 26 kJ/mol for the micellar solution and 17 kJ/mol for distilled water. The similarity in the slopes (activation energy) confirms that the decrease in viscosity upon increasing the temperature below the transition temperature depends mainly on the solvent (water). To study the course of events during the sphere-to-rod transition for the P123-C12EO6 mixed micelles at a molar ratio of 2.2 and a total concentration of 21 wt %, the viscosity was measured at a constant shear rate (100 s-1) at the same time that the temperature was increased from 25 to 40 °C. The results are displayed in Figure 3 where both the viscosity and the temperature are shown as functions of time. It is observed that as the temperature starts to increase the viscosity decreases because of the decrease in the viscosity of the solvent as previously discussed. At approximately 35 °C, the viscosity reaches a minimum whereupon it starts to increase until the temperature reaches 40 °C. The minimum in the viscosity curve represents the temperature at which this rheological method can detect changes in the system, which most likely corresponds to the sphere-to-rod transition of the mixed micelles defined with rheological means. When comparing the time dependencies of the viscosity with those of the total light scattering intensity (Figures 11 and 12 in ref 1), no slow kinetic effects were observed in the rheology measurements for samples above 4.0 wt % in total concentration. The system reaches equilibrium on a time scale that is too fast to be observed with the rheological instrument used in this study. The light scattering results in ref 1 show that the process of growth is concentrationdependent and that the growth rate increases with increasing total concentration (from 0.25 to 1.5 wt %). We therefore expect a fast equilibrium process at the concentrations investigated in this work (i.e., in the concentration range between 5.0 and 21 wt %). The final size of the rodlike mixed micelles will differ depending on the temperature range within the measurements performed. The data of Figure 4a shows that the maximum viscosity of the P123-C12EO6 solution is a direct function of the final temperature employed in the experiment (at a constant shear rate of 10 s-1). In all cases, a higher final temperature gives a higher final viscosity, which indicates a larger rodlike particle size. When the temperature increases further, the rod length increases even more until phase separation occurs, which is a

Figure 4. (a) Viscosity at a constant shear rate of 10 s-1 as a function of time for a 21 wt % P123-C12EO6 solution at molar ratio 2.2 for temperature scans from 25 °C and with 43.9 (O), 43.2 (2), 42.0 (0), 40.9 (9), 39.4 (]), 38.1 (b), 37.0 (3), and 35.7 °C (×) as final temperatures. (b) Viscosity at a constant shear rate of 10 s-1 as a function of time for a 21 wt % P123-C12EO6 solution at molar ratio 2.2 for temperature scans from 25 °C and with 50.0 (0), 49.1 (×), 47.1 (b), and 45.2 °C (4) as final temperatures.

typical phenomenon for nonionic polymers and surfactants in aqueous solution.1,16,40 Figure 4b presents temperature scan data taken with end temperatures of 45 °C and above. At 47 °C, the viscosity reaches a maximum, after which it decreases with time until a new constant value is obtained. All samples that exhibited this maximum presented a turbid appearance indicative of phase separation. The initial minimum in viscosity exhibited by all of the curves presented in Figure 4b corresponds to a mean temperature value of 34.3 °C. This temperature would thus be the sphere-to-rod transition temperature in the mixed system at a C12EO6/P123 molar ratio of 2.2 and at the high concentration of 21 wt % determined by rheological means. The temperature range encompassing the shape transition is wider in the rheological measurements of Figures 3 and 4 than the range found by light scattering presented in Figure 11 of ref 1, where the transition started less than 1 °C below the expected transition temperature. Furthermore, the sphere-to-rod transition temperature observed with rheology of about 34 °C corresponds to the onset peak position in the DSC curves representing the sphere-to-rod transition (Figure 2 in ref 1). Both the shape transition and the phase separation occur at lower temperatures in the rheology experiments than in the calorimetry and light scattering study. This is probably an effect of the difference in total concentration, which is higher in this study, and is also due to the fact that the applied shear may affect the ability of the micelles to undergo a shape transition.41 Neat P123 and C12EO6 Solutions versus P123-C12EO6 Mixed Solutions. As a reference, the viscosity and temperature (40) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103. (41) Saito, S.; Koizumi, S.; Matsuzaka, K.; Suehiro, S.; Hashimoto, T. Macromolecules 2000, 33, 2153.

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Figure 5. Viscosity at a constant shear rate of 100 s-1 as a function of time for a temperature scan from 25 to 40 °C for a 21 wt % P123-C12EO6 solution at molar ratio 2.2 (b) and its mutual concentrations of pure P123 (17.1 wt %) (×) and C12EO6 (3.1 wt %) (0).

measurements with time were also carried out for the neat P123/ H2O and C12EO6/H2O systems. The concentration of each substance was chosen from the mutual concentration in the mixed solution with a total concentration of 21 wt %, which corresponds to 17.9 wt % of P123 in water and 3.1 wt % of C12EO6 in water. The time-dependent behavior of the viscosity for both solutions is displayed in Figure 5 together with that of the P123-C12EO6 mixed solution at molar ratio 2.2 at 21 wt %. The temperature scans have been omitted. It can be seen in Figure 5 that the P123 solution decreases whereas the viscosity of the C12EO6 solution increases with increasing temperature. The decrease in viscosity of the P123 solution depends mostly on the decrease in viscosity of the water as pointed out above. The increase in viscosity of the C12EO6 has its origin in the growth of the nonspherical micelles with temperature. C12EO6 forms wormlike micelles that grow with temperature at their cmc.2,22,42-47 Upon comparing these scanning curves with that of the mixed micellar solution, it is clearly observed that the initial and final viscosities display values of a higher magnitude for the mixed solution. The total concentration is higher for the mixed micellar system (21 wt %); therefore, it is not unexpected to observe a higher initial value of the viscosity. It is interesting to note that the difference in viscosity before and after the sphere-to-rod transition is larger for the mixed solution than for the pure C12EO6 solution. These results illustrate the synergistic behavior of the mixture compared to that of its components. The trend was the same at lower concentrations as for the solution containing 21 wt % at the same molar ratio (2.2). The only difference was that lower initial and final viscosities were obtained. In Figure 6a, the viscosity of solutions with different total concentrations (5.0, 10, 15, and 21 wt %) as a function of time is displayed. The corresponding temperature-scanning curves (25 to 40 °C) have been excluded from Figure 6 because the same experimental conditions were employed. (The shape of the temperature ramp can be seen in Figure 3.) The minimum in the viscosity-time curve shifts to longer times and higher onset temperatures as the total concentration decreases. At 5.0 wt %, we find an onset temperature of 37.4 °C (Figure 6b). This is in better agreement with the onset (42) Ilgenfritz, G.; Schneider, R.; Grell, E.; Lewitzki, E.; Ruf, H. Langmuir 2004, 20, 1620. (43) Brown, W.; Johnsen, R.; Stillbs, P.; Lindman, B. J. Phys. Chem. 1983, 87, 4548. (44) Brown, W.; Rymde´n, R. J. Phys. Chem. 1987, 91, 3565. (45) Yoshimura, S.; Shirai, S.; Einaga, Y. J. Phys. Chem. B 2004, 108, 15477. (46) Kole, T. M.; Richards, C. J.; Fisch, M. R. J. Phys. Chem. 1994, 98, 4949. (47) Thomas, H. G.; Lomakin, A.; Blankschtein, D.; Benedek, G. B. Langmuir 1997, 13, 209.

Figure 6. (a) Viscosity at a constant shear rate of 100 s-1 as a function of time for a temperature scan from 25 to 40 °C for P123C12EO6 solutions at molar ratio 2.2 but with different total concentrations: 5.0 (×), 10 (4), 15 (9), and 21 wt % (O). (b) Viscosity at a constant shear rate of 10 s-1 as a function of time for a 5 (9) and a 21 wt % (4) P123-C12EO6 solution at molar ratio 2.2. Temperature scans are from 25 to 46 °C and from from 25 to 41 °C.

temperature observed for a dilute solution using light scattering1 than the onset temperature found at 21 wt %. This implies that a higher temperature is needed to give a change in the viscosity at lower concentrations, or at least to observe the changes with a rheometer. No kinetic effects were observed within this concentration range (5.0 to 21 wt %). To see any kinetic effects with rheology, the sphere-to-rod transition has to be slow enough, which means a process on the scale of several minutes. From the earlier time-dependent light scattering measurements presented in ref 1, it was concluded that, for total concentrations of 5.0 wt % and higher, it is not possible to observe any slow kinetic effects of the shape transition at molar ratio 2.2 because the transition process was too fast at these concentrations. It is interesting to note that the final viscosity of the P123C12EO6 solution is a function of the total concentration and temperature. Figure 6b shows that a similar final viscosity is achieved upon changing the temperature of the two different solutions. In the 5.0 wt % case, the temperature needed to be raised to 46 °C, whereas in the 21 wt % case 41 °C was sufficient to reach the same final viscosity. To observe the above-mentioned kinetic effects of the sphereto-rod transition, the viscosity scanning experiment was also measured for 1.5 wt % as the total concentration of the P123C12EO6 mixed solution. The molar ratio was 2.2. The temperature scan was carried out in the same procedure as in previous cases. Several experiments were carried out with eight different final temperatures. The results are displayed in Figure 7. It is observed that after reaching the target temperature (for final temperatures above 42 °C), the viscosity still increased with time even though the temperature was constant, an effect that was not observed at 5.0, 10, 15, and 21 wt % (e.g., see Figure 4a). The increasing-

Rheological Study of the Shape Transition

Figure 7. Viscosity at a constant shear rate of 100 s-1 as a function of time for a 1.5 wt % P123-C12EO6 solution at molar ratio 2.2 for temperature scans from 25 and 47.0 (4), 46.0 (9), 45.1 (]), 44.2 (b), 42.0 (3) 41.1 (×), 40.0 (2), and 39.0 (0) °C as final temperatures.

viscosity process lasted for approximately 1000 s after reaching the final temperature, which is in accordance with the time found in light scattering experiments for the 1.5 wt % mixed solution at molar ratio 2.2.1 The results of Figure 7 also show that the size of the P123-C12EO6 mixed micelle has a temperature dependence above the sphere-to-rod transition temperature. The rods grow in size even further with temperature, most likely to extended rods, and unless the system phase separates, the system will adopt a wormlike micellar structure.2,43-47 Furthermore, the data presented reveals the great sensitivity of the rheometer employed because we can observe changes in the molecular structure (from a spherical to a rodlike shape) of P123-C12EO6 mixed micelles by studying the macroscopic properties at concentrations as low as 1.5 wt % (i.e., in the dilute concentration regime). Furthermore, for P123-C12EO6 solutions with a total concentration of 5.0 wt % or less at molar ratio 2.2, no shear thinning behavior at 40 °C was observed at higher shear rates (data not shown), unlike the 21 wt % solution in Figure 1. For 10 and 15 wt % solutions, there was a small tendency toward shear thinning at very high shear rates (data not shown). However, shear-thinning behavior was observed when the final setting temperature was increased further above 40 °C for samples with lower contents of copolymer and surfactant (1.5 to 5.0 wt %). This can be explained by the further growth of rods into even longer rods of wormlike micellar character at higher temperatures. In fact, there are two ways to obtain shear-thinning behavior: by increasing the concentration or by increasing the temperature. Molar Ratio ) 6. To explore further the mixed micellar growth, a higher molar ratio of 6.0 was investigated using the same procedure as for the previously mentioned molar ratio of 2.2. According to the transition temperature gained in the DSC experiments (presented in Table 1), a sphere-to-rod transition was expected at approximately 51 °C for a mixed solution at molar ratio 6.0. Figure 8 displays the viscosity and temperature as a function of time for the mixed solution at molar ratio 6.0, with a total concentration of 20 wt %. The temperature scan started at 35 °C and ended at different temperatures for each set. Higher plateau viscosity values of the temperature scan were obtained with higher final temperatures. This is due to the temperature dependence of the rod length. At higher temperatures, the rods are longer, and higher viscosity values are obtained. As in the case of MR ) 2.2, the transition is illustrated with a change in viscosity represented with a minimum in the viscosity curve followed by an increase in viscosity. Compared to that for molar ratio 2.2, the plateau value at 51 °C never reaches a perfect plateau value at the final temperature at molar ratio 6.0. One of the reasons may be the

Langmuir, Vol. 23, No. 22, 2007 11005

Figure 8. Viscosity at a constant shear rate of 100 s-1 as a function of time for temperature scans from 35 to 51.3 (9), 49.5 (0), 48.0 (O), 46.0 (4) 43.2 (×), and 40.0 (-) °C for a 20 wt % P123-C12EO6 solution at molar ratio 6.0.

Figure 9. Viscosity at a constant shear rate of 100 s-1 as a function of time for temperature scans for a 21 wt % P123-C12EO6 solution at molar ratio 2.2 (b) and a 20 wt % P123-C12EO6 solution at molar ratio 6.0 (0).

effect of solvent evaporation during the experiment because of the high measurement temperature. The temperature scanning time was also longer than for the molar ratio 2.2 system. Losing solvent to evaporation gives rise to an increase in the concentration of the solution and therefore an increase in the viscosity. It should also be mentioned that a sticky coating was observed on the cylindrical cone between the interface of the solution and the air, which was observed after each of the performed experiments at this molar ratio. As in the previous case (MR ) 2.2), all solutions of molar ratio 6.0 exhibited a concentration dependence with initial and final viscosities that were dependent on the total concentration (results not presented). Higher viscosity values at higher concentrations were obtained. Figure 9 compares the differences in viscosity as a function of time between two solutions of molar ratios 2.2 and 6.0 at 21 and 20 wt % concentration. In each case, the viscosity was measured simultaneously as the temperature was increasing (temperature curve not presented in the Figure). The final temperature in each case was chosen from the peak position in the DSC measurements presented in Table 1, which represents the sphere-to-rod transition of the P123-C12EO6 mixed micelles. The initial temperatures were set to approximately 15 °C below the transition temperatures, which are the same as presented in Figures 3 (molar ratio 2.2, 25 °C) and 8 (molar ratio 6.0, 35 °C). The initial viscosity is lower for molar ratio 6.0 compared to that for molar ratio 2.2. This is due to not only the small difference in the total concentration and the higher initial temperature, which affects the viscosity of the solvent, but also smaller spherical micelles. From dynamic light scattering measurements, it was shown that the hydrodynamic radius of the P123-C12EO6 mixed micelles (spherical and rodlike) decreases with increasing molar

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ratio at both 25 and 40 °C.1,2 This is also easily seen in Figure 9, where the viscosity after the shape transition is higher for molar ratio 2.2, which in turn indicates a larger rodlike micelle. It is important to consider the effect of the solvent. It exhibits lower viscosity at higher temperatures when comparing the two final temperatures, 40 and 51 °C. However, the decrease in water viscosity due to the temperature is less than the viscosity effect due to the difference in micellar size. The shear-thinning behavior was difficult to measure at 51 °C for molar ratio 6.0 because of the effects from solvent evaporation that instead increased the viscosity slightly with time. The shear rate was not high enough to reach a shear thinning state in the sweep rate measurements because the final size of the elongated micelle was smaller at molar ratio 6.0 than at molar ratio 2.2.

Conclusions Steady-state rheology measurements were performed on mixed micelles of PEO-PPO-PEO block copolymer P123 and nonionic surfactant C12EO6. Two different surfactant-to-polymer molar ratios, 2.2 and 6.0, were investigated at different total concentrations, with the purpose of confirming the sphere-to-rod transition found in the previous light scattering and calorimetry study.1 A large increase in viscosity at the transition temperature was observed, which indicates the growth of the mixed micelles. Also, the shear-thinning behavior obtained in sweep-rate measurements for molar ratio 2.2 confirms the morphology change from a spherical to a rodlike shape. No retarded kinetic effect was found in a reasonable time limit in the total concentration range of 5.0-21 wt %. This observation is supported by the time-resolved SLS data in ref 1; the higher the total concentration, the shorter the shape transition rate and the faster the process. However, at 1.5 wt % a kinetically retarded process was observed. The zero-shear viscosity at the equilibrium plateau at the transition temperature showed a larger value for molar ratio 2.2 than for 6.0. That cannot be explained only by a further decrease in the viscosity of the solvent when the temperature is increased. The larger value is also a result of the fact that the mixed micelles

Lo¨f et al.

at molar ratio 2.2 are larger than at molar ratio 6.0, at their transition temperatures. The significantly lower viscosity value at the transition temperature for a solution at molar ratio 6.0 also explains the lack of shear-thinning behavior during the sweep rate measurements. The growth of the rods at molar ratio 6.0 was not large enough to induce this effect properly. However, the data suggest that increasing temperature even further would extend the rods to a wormlike shape, which in turn would imply shearthinning behavior. According to ref 1, mixed micelles of molar ratio 6.0 have a smaller size both before and after the transition than compared to those of molar ratio 2.2. This can be explained by the increasing curvature of the mixed micelles when adding C12EO6 to P123 micelles. Increasing the curvature leads to a decrease in the size of the spherical mixed micelles. A smaller spherical particle, which has a higher curvature, is associated with a larger cost in energy in order to reduce the curvature, which happens during a sphere-to-rod transition. Smaller spherical micelles, therefore, have a higher transition temperature in the investigated concentration regime. Finally, the mutual properties of P123 and C12EO6 as pure components diluted and assembled in water change as they are mixed and interact with each other in aqueous solution. This is why the sphere-to-rod transition temperature decreases by about 20 °C for the mixed micelles at molar ratio 2.2 compared to that of P123 micelles, which occurs at 61 °C.1 Acknowledgment. We thank the ALFA network program “ELAPNET-Polymeric Materials, II-0231-FI” for financial support within the exchange between Sweden and Venezuela. We also acknowledge support from the Swedish Research Council (VR) and the Linne´us Center of Excellens “Organizing Molecular Matter” (K.S.) and the Swedish Fondation for Strategic Research (SSF) (D.L.). Finally, we thank Andrew Bartles, Beloit College, Beloit, Wisconsin, for proof reading the manuscript. LA701818Y