pubs.acs.org/Langmuir © 2010 American Chemical Society
Micellar Crystallization with a Hysteresis in Temperature Marco Walz,*,† Max Wolff,‡ Nicole Voss,† Hartmut Zabel,§ and Andreas Magerl† †
Crystallography and Structural Physics, University of Erlangen-N€ urnberg, Staudtstrasse 3, 91058 Erlangen, Germany, ‡Department of Physics, Uppsala University, Box 530, 75121 Uppsala, Sweden, and § Institute for Experimental Physics/Condensed Matter/EPIV, Ruhr-University Bochum, Universit€ atsstrasse 150, 44780 Bochum, Germany Received June 14, 2010. Revised Manuscript Received July 28, 2010 We have investigated the phase diagram of the triblock copolymer P123 solved in water by viscosity measurements for different concentrations and temperatures. The structures of the different phases were identified by surface sensitive neutron diffraction. We find a pronounced hysteresis between heating and cooling. During heating, a highly viscous crystalline fcc phase is found before melting occurs at 44 °C with a simultaneous drop in viscosity. Upon cooling, first a hexagonal phase with low viscosity develops followed by a highly viscous fcc phase. Phase diagrams for the heating and cooling cycle for different concentrations are provided. The hysteric behavior is discussed in relation to the shape of the micelles.
Introduction Triblock copolymers (registered trade names: Pluronic, Synperonic, Poloxamer) are well-known1,2 as templates for the synthesis of ordered mesoporous materials3-5 as they form micelles in concentrated aqueous solutions. The interaction between polymer micelles can be varied continuously by changing temperature, concentration, or pressure, and aqueous solutions exhibit complex phase diagrams.6 For this reason, micellar solutions are a model system for the study of gelation and crystallization in soft matter.7 Rheometry is an established method to locate the phase boundaries of a solution through the determination of the bulk viscosity. The bulk viscosity, in turn, is strongly correlated with the structure and is, in general, different for different phases. Nevertheless, it lacks direct information on the structure itself. Grazing incidence small angle neutron scattering (GISANS) is known to provide surface-sensitive structural information.8,9 Recently, it has been shown that the structure of triblock copolymer micelles influenced by the surface energy of the solid interface becomes visible in GISANS.10 Further, it was noticed by GISANS that for a 33 wt % (weight percent) solution of P85 in deuterated water the structure formed in the interface region may depend on the thermal history of the sample.11 In the present study on P123, we relate GISANS measurements resolving the crystalline structure formed by the micelles to viscosity measurements and show that both reflect the *To whom correspondence should be addressed. E-mail: walz@ krist.uni-erlangen.de. (1) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1–46. (2) Hamley, I. W. The Physics of Block Copolymers, 1st ed.; Oxford University Press: Oxford, 1998. (3) Wang, Y.; Wang, Y.; Yang, C.-M.; Lu, G.; Sch€uth, F. Langmuir 2006, 22, 5491–5496. (4) Chen, D.; Li, Z.; Wan, Y.; Tu, X.; Shi, Y.; Chen, Z.; Shen, W.; Yu, C.; Tu, B.; Zhao, D. J. Mater. Chem. 2006, 16, 1511–1519. (5) F€orster, S.; Konrad, M. J. Mater. Chem. 2003, 13, 2671–2688. (6) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145– 4159. (7) Chen, W.-R.; Chen, S.-H.; Mallamace, F. Phys. Rev. E 2002, 66, 021403. (8) M€uller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Phys. Chem. Chem. Phys. 1999, 1, 3857–3863. (9) Wolff, M.; Magerl, A.; Zabel, H. Thin Solid Films 2007, 515, 5724–5727. (10) Wolff, M.; Scholz, U.; Hock, R.; Magerl, A.; Leiner, V.; Zabel, H. Phys. Rev. Lett. 2004, 92, 255501. (11) Wolff, M.; Magerl, A.; Zabel, H. J. Phys.: Condens. Matter 2005, 17, S3645–S3650.
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hysteretic temperature behavior. We establish phase diagrams and explain our findings by a changing morphology of the single micelles. Accordingly, a formerly established phase diagram for Pluronic P123 dissolved in water6 has to be extended.
Sample Preparation Pluronic P123 from Sigma-Aldrich was used without further purification. The EO20-PO70-EO20 macromolecules consist of a central part of poly(propylene oxide) terminated by two end groups of poly(ethylene oxide). In aqueous solution, the unimers agglomerate at elevated temperatures or concentrations into micelles with a hydrophobic core and a hydrophilic shell.12-15 For polymer concentrations above 26 wt %, the sample crystallizes into cubic or hexagonal structures depending on the sample conditions resulting in a distinctly higher viscosity. For rheological studies, we prepared solutions of P123 in deionized H2O with polymer concentrations from 25 wt % up to 40 wt %. For the structural investigations with neutrons, the polymer was dissolved in deuterated water to enhance the scattering contrast between the protonated polymer chains and the solvent. With respect to the higher density FD2O of D2O compared to H2O (FH2O), the corresponding concentration of the polymer in D2O (cD2O, wt %) was calculated from the weight fraction cH2O in H2O h i- 1 cD2 O ¼ 1 þ 1=cH2 O - 1 3 FD2 O =FH2 O This relation retains the fraction of the solvent molecules by D2O in reference to light water. In the case of the neutron scattering experiment, a 28 wt % solution of P123 in deuterated water was prepared (corresponding to 30 wt % in H2O) and filled into the sample cell in the liquid state at low temperature to prevent a preceding shear (deformation) of the crystalline phase. A polished silicon block was used as a solid interface. The contact angle between the native silicon oxide, water, and air was about 50°. (12) Loh, W. Encyclopedia of Surface and Colloid Science, 2nd ed.; Taylor & Francis: Boca Raton, 2006. (13) Sasanuma, Y. Macromolecules 1995, 28, 8629–8638. (14) Guo, C.; Liu, H.-Z.; Chen, J.-Y. Colloid Polym. Sci. 1999, 277, 376–381. (15) Guo, C.; Wang, J.; Liu, H.-Z.; Chen, J.-Y. Langmuir 1999, 15, 2703–2708.
Published on Web 08/19/2010
DOI: 10.1021/la102415x
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Figure 1. Viscosity of P123 dissolved by 30 wt % in water (corresponding to 28 wt % in D2O) at a constant shear rate of 5.0 s-1. The crystalline phases are visible from the increased viscosity. The dashed-dotted line shows the background viscosity originating from the solvent trap.
Experimental Details The viscosity measurements were performed with a commercially available rheometer (Bohlin CSR-10) using a constant shear rate of 5.0 s-1. A stainless steel cone-plate with a diameter of 40 mm and an opening angle of 4.0° was used for concentrations up to 30 wt %. For technical limitations, a smaller cone diameter of 30 mm was used for measurements of higher polymer concentrations. Because of the high viscosity, this was without a loss of data quality. The temperature was controlled by a water-based circulator bath with a stability of better than 0.2 °C. The gradient of the heating and cooling cycle was kept constant at 1.0 °C/min. The temperature was determined by a Pt100 sensor mounted inside the rheometer water cycle at the bottom of the sample. To prevent evaporation of the solvent, the measurement system was covered by an encapsulated solvent trap.16 The small angle scattering experiments were carried out in a GISANS setup.17,18 As usual, for reflectometry the Cartesian coordinates x, y, and z are defined with respect to the surface of the sample, with z along the normal to the interface, and x the projection of the transmitted beam onto the interface. The monochromatic and highly collimated neutron beam enters a single crystalline silicon block (50 50 10 mm3) from the side face and becomes scattered at the solidliquid interface. Two double-slit systems for the y- and z-directions define the incident beam collimation, yielding a divergence of 0.3° along the y-axis and 0.06° (fwhm) perpendicular to the scattering plane, i.e., the divergence of incoming beam along the z-axis. Data were taken at the reflectometer ADAM19 at the Institut Laue-Langevin (Grenoble, France), with an incoming wavelength of λ =4.41 A˚ ( 1%. The mean scattering length density of the solution is 4.60 10-6 A˚-2, and the calculated critical angle of total reflection between silicon and sample is 0.227°. Depending on the angle of incidence θi, the penetration depth of the incident beam can be tuned from several nanometers to the micromolar range. In the present study, it was set to 0.3°, which corresponds to a beam penetration depth of 20 μm. Further, a structural ordering may be influenced by the presence of the solid interface leading to reduced dimensionality and anisotropy of the system. In this case, a texture with respect to the interface normal becomes readily visible.9 All presented scattering patterns have been corrected by the refraction that occurs at the interface. During the scattering experiments, the sample was continuously heated or cooled with a constant rate of 20 K/h. (16) Wolff, M.; Steitz, R.; Gutfreund, P.; Voss, N.; Gerth, S.; Walz, M.; Magerl, A.; Zabel, H. Langmuir 2008, 24, 11331–11333. (17) M€uller-Buschbaum, P.; Cubitt, R.; Petry, W. Langmuir 2003, 19, 7778– 7782. (18) Wolff, M.; Magerl, A.; Zabel, H. Eur. Phys. J. E 2005, 16, 141–145. (19) Wolff, M.; Zhernenkov, K.; Zabel, H. Thin Solid Films 2007, 515, 5712– 5715.
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Figure 2. GISANS patterns taken for a 28 wt % solution of P123 in deuterated water. The panels on the left and right side show data taken during heating and cooling, respectively.
Results Figure 1 depicts the viscosity η of a 30 wt % solution of P123 plotted versus temperature between 5 and 67 °C measured at a constant shear rate of 5.0 s-1. The viscosities surrounding the plateau are limited by the exponentially decaying contribution of the solvent trap sealing from 1.1 Pa 3 s (10 °C) to 0.11 Pa 3 s (65 °C). Three different viscosity regions are indicated for the heating cycle in Figure 1 (A,B,C). For low and high temperatures indicated as regions A (T44 °C), respectively, the sample shows low viscosity. At intermediate temperatures (B), the level of viscosity is increased by 2 orders of magnitude. Surprisingly, the viscosity in this region shows a distinctly different temperature dependence during cooling where two regions of increased viscosities can be identified (B1 and B2). First, the viscosity is increased to several pascal-seconds (η ≈ 1-6 Pa 3 s). In the second region, the values of the viscosity in region B1 is comparable with the heating cycle (η ≈ 50 Pa 3 s, 15 < T < 25 °C). The rheological data are well-correlated with the GISANS patterns shown in Figure 2. For low temperatures (T < 15 °C), isotropic small angle scattering is visible, indicating the presence of monomers and micelles in the liquid phase. With increasing temperature, several Bragg peaks become visible. They can be indexed by a cubic dense packing (i.e., ABC layering for cubic face centered structure (fcc)).10,20 This type of structure remains unchanged throughout the entire plateau of high viscosity. At high temperatures (T > 44 °C), the crystalline fcc structure melts, (20) Walz, M.; Magerl, A.; Wolff, M.; Zabel, H. Superlattices Microstruct. 2007, 41, 185–189.
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Figure 3. Phase diagram obtained from rheology at a constant shear rate of 5.0 s-1 for a heating (panel a) and subsequent cooling cycle (panel b).
mirrored in rheometry by a drop in viscosity by 2 orders of magnitude, and in the GISANS pattern by more isotropic forwardscattering. However, investigations during a subsequent cooling cycle produce different results: In the crystalline phase and for decreasing temperatures, first a reduced number of reflections located at different positions as compared to the fcc structure is 6 0, Qz =1/3 Q111, visible. More precisely, the Bragg peaks at Qy ¼ and 2/3 Q111 disappear and are replaced by lattice reflections at Qz = 1/2 Q111. Although the reflections are very broad, the scattering pattern can be explained by a hexagonal structure. Further cooling brings the system back to the fcc phase. With respect to the integration time, this structure is represented by the coexisting fcc reflections as indicated by the yellow brackets (cooling cycle at 16.7 °C). The exact phase boundary is determined by rheometric measurements discussed below. Figure 3 summarizes the phase diagrams of P123 solutions based on rheometric viscosity η(c, T) studies of the continuously sheared sample as a function of the polymer concentration and temperature. The left panel corresponds to heating (a) and the right panel to cooling (b). The dark and light areas represent high and low viscosity, respectively. The contour lines illustrate constant values of the viscosity log10[η(c, T)/Pa 3 s]. The individual phases can be identified on the basis of the structural information extracted from neutron scattering experiments. The absence of the cubic phase during cooling over a large range of temperature and concentration as compared to heating is clearly visible. In relation to this, the phase diagram established earlier6 is valid only when the sample is heated. For cooling, the phase diagram presented in Figure 3b holds. A phase transition from a hexagonal to a cubic structure occurs for the cooling cycle at around 25 °C. At lower temperatures, the crystalline phase melts, as shown by a low viscosity and isotropic scattering. The information on the thermal history is lost in this liquid state, and subsequent heating leads again to the phase diagram reproduced in Figure 3a. We note that by additional GISANS measurements (not shown here) we could confirm phase stability of the hexagonal structure observed during cooling at 37 °C for at least several hours. In rheological studies, this intermediate phase was found to be stable over several days.
Discussion The different orderings observed when entering the crystalline phase from low or high temperatures may be explained by the topology of the single micelle. It is known for similar systems that the micelles change from an initially spherical shape into a more (21) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103–A124.
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elongated shape as the temperature increases.6,21 Along this line of reasoning, it is assumed that the micelles maintain the elongated shape within a certain temperature range when cooling the sample from high temperature. The lower symmetry of rod-like particles results in a hexagonal ordering. The ABC stacking sequence which is required for a fcc structure is unfavored along the c-axis, since long-range forces does not exist over this distances (Figure 4). Further, the elongated micelles may readily align under shear. For a given concentration, elongated micelles entail more free water (packing density of ∼74%) in comparison to spheroids (depending on the c/a ratio, it may reach up to ∼91%). This may be the reason for the low viscosity found in the hexagonal phase, because more water is available within the sliding planes. The small number of observed Bragg peaks makes a determination of the structural ordering difficult. This holds, in particular, for the hexagonal ordering. Two different explanations for the diffraction pattern discussed in the following refer either to a description of the Bragg peaks originating from a hexagonal close packing (hcp) built up of spherical micelles (i) or to an explanation based on elongated micelles arranged in a 2D hexagonal ordering (ii). In both cases, the c-axis is aligned parallel to the interface.20 The different hexagonal arrangements for elongated micelles are illustrated in Figure 4 (panels a, b, and c). Ad (i): Close-packed spherical micelles. This interpretation leads to the question regarding how the next nearest-neighbor correlation (ABA or ABC stacking) is mediated over two micellar layers. The long-range interaction between the micelles is driven by the excluded volume.5 There is no difference in the volume fraction between fcc and hcp. The short-range interaction potential of relevance within a micelle of this nonionic surfactant is the covalent binding of the physiochemical incompatible compounds, which leads to a geometrically determined diameter of the micelles.5 However, its interaction range is smaller than the distance between three layers. A deformation of the spherical shape, or more specifically of the outer shell of a micelle, may mediate between three layers to arrange for the ABC sequencing. Further, the shape deformation is of a higher symmetry for a neighborhood with a face-centered configuration than for a hexagonal coordination. Stacking faults will become important if the next nearestneighbor interaction is negligible, and only direct neighbor correlations with randomly distributed hexagonal and cubic sequencing will appear. As a consequence, most higher-order Bragg reflections will feature only low intensities as observed in GISANS. Ad (ii): Elongated micelles. Three possibilities may be considered: (a) all micelles have identical rod length and they are DOI: 10.1021/la102415x
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Figure 4. The upper row shows different c-axis arrangements of a two-dimensional hexagonal ordering: (a) elongated micelles of well-defined length arrange highly correlated in the c-direction; (b) the correlation of the position along the c-axis is lost; panel (c) illustrates micelles of different lengths, and as a consequence no correlation along the c-axis. The bottom panels show schemes of the scattering patterns. The powder average with respect to the surface normal is depicted. Triple hkl-indexing is used where the l-index corresponds to the direction of elongation (with a c/a ratio of 2). The Bragg reflection originating from the 2D hexagonal ordering are represented in light gray color. The peaks for l 6¼ 0 are colored black. The size of the spots represents the expected intensity. A detailed description of the Bragg spot indexations is given by Walz et al.20
arranged with a well-defined correlation along the direction of the elongation (see Figure 4, panel a); (b) the positional correlation along the elongated direction is lost (Figure 4b). These two types of orderings are comparable to a smectic or nematic liquid crystal phase. Further (c), a randomly distributed rod length may be considered, which implies a lack of positional order along the c-axis (Figure 4c). Schematical diffraction patterns are shown in Figure 4 (panels a0 , b0 , and c0 ). A powder averaging around the normal of the surface is assumed. For case (a), additional Bragg peaks should appear at Q-values as shown in Figure 4a (e.g., 001, 111). In the case of an elongated hexagonal c-axis, the corresponding Bragg reflections appear at smaller Q-values. This is not supported by our experimental findings. As a consequence, the correlation of the cap positions of the micelles in direction of the elongation has to be random (b) or the rod length distribution has to be broad (c) to weaken or even suppress these peaks. In the case of a welldefined lattice parameter along the c-axis (panel a), the powder averaging leads to additional Bragg reflections with index l 6¼ 0 (e.g., 001, 111). The Bragg peaks with mixed indices l ¼ 6 0 and h or k 6¼ 0 vanish as soon as the correlation between the different rows is lost. Furthermore, the intensity of peaks l ¼ 6 0 and h = k = 0 drops, since the intensity from the rods in the c-direction only sums up incoherently. For panel c, all correlations along the elongation are lost and only 2D hexagonal peaks appear (l = 0, panel c0 ). Considering the experimental resolution and background, it is not possible to distinguish between cases (b) and (c). However, there is no strong argument that the distribution of cap-to-cap lengths of the rod-like micelles should be well-defined such that a highly correlated c-axis is formed. The reason for a well-defined micelle diameter is founded in the different block lengths of the polymer, but the c-axis lacks such a correlation. It is well-known that the number of macromolecules per micelle is augmented with increasing temperature.6 At a defined temperature, the extension of a macromolecule forbids continued growth of the micelles. This enforces different morphologies. The isotropic growth becomes suppressed in favor of different morphologies like elongated shapes. We note, that this kind of structure
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fits well to the low viscosities observed, since the fraction of the solvent between the layers in case of elongated micelles is higher as compared to a close-packed structure made from spherical micelles. Considering all reasons mentioned, it appears that the hexagonal structure is formed by elongated micelles with randomly distributed rod lengths (c).
Conclusion The viscosity of aqueous solutions of P123 as measured with a rheometer varies by several orders of magnitude in the accessed temperature range between 5 and 82 °C. These variations reflect different crystallographic structures for the micellar ordering, which can be identified from neutron diffraction in GISANS geometry. We find for the first time a pronounced structural hysteresis that is also reflected in the viscosity between heating and cooling, and which may reflect the stability of the micelles in different morphological forms. In particular, we propose that the formation of a newly found hexagonal ordering during the cooling cycle relates to the transition from spherical to rod-like micelles. This reasoning will be fortified in future research including TEM studies. The hysteretic ordering observed by rheometry is maintained for several days both with and without applied continuous deformation. Acknowledgment. The authors thank R. B. Neder for the support with the scattering simulation package DISCUS.22 We acknowledge financial support from the DFG within the priority program “Micro- and Nanofluidics” (SPP 1164) under contract numbers MA801/12-2 and ZA161/18-2 and the BMBF under contract number 03ZA7BOC. Supporting Information Available: Illustration of the GISANS geometry, the angle of incidence θi, and the projections of the momentum transfer vector. This material is available free of charge via the Internet at http://pubs.acs.org. (22) Proffen, T.; Neder, R. B. J. Appl.: Cryst. 1997, 30, 171–175.
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