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Notes Salt-Induced Microphase Separation and Crystallization in Salt-Polymer Complex Systems Tianbo Liu,† Yi Xie,‡ Li-Zhi Liu,† and Benjamin Chu*,†,§ Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, Anhui, China, and Department of Materials Science & Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275 Received January 14, 2000. In Final Form: June 28, 2000
Introduction Block copolymers of oxyethylene (E) and oxypropylene (P) are nonionic surfactants with numerous applications. Symmetrical triblock copolymers, with structure ExPyEx, where x and y denote the numbers of E and P monomers per block, are commercially available (e.g., from BASF Co., known as Pluronics) with a broad range of block lengths. Self-assembly of EPE-type triblock copolymers in aqueous and nonaqueous solutions has been studied extensively for the past decade.1-10 To minimize the free energy in aqueous solution, the hydrophobic P blocks tend to shrink together to form micellar cores, whereas the hydrophilic E blocks form extended micellar shells. At higher polymer concentrations, the ordered packing of the micelles or the rearrangement of the hydrophilic and hydrophobic regions in solution can lead to the formation of ordered gel structures, for example, cubic, hexagonal, or lamellar structures.11-14 The addition of inorganic salts to aqueous copolymer solutions can either increase or decrease the solubility of polymers in solvents, and lower or enhance the polymer gelation concentration. These phenomena are called “salting in” and “salting out” effects, respectively.15 The * To whom correspondence should be addressed. † Department of Chemistry, State University of New York at Stony Brook. ‡ Department of Chemistry, University of Science & Technology of China. § Department of Materials Science & Engineering, State University of New York at Stony Brook. (1) Nace, V. M., Ed. Marcel Dekker: New York, 1996; especially Chapter 3 by B. Chu and Z. K. Zhou, pp 67-143, and references therein. (2) Chu, B. Langmuir 1995, 11, 414. (3) Wu, G.; Zhou, Z.; Chu, B. Macromolecules 1993, 26, 2117. (4) Wu, G.; Chu, B. Macromolecules 1994, 27, 1766. (5) Brown, W.; Schillen, K.; Hvidt, S. J. Phys. Chem. 1992, 96, 6038. (6) Schillen, K.; Brown, W.; Konak, C. Macromolecules 1993, 26, 3611. (7) Glatter, O.; Scherf, G.; Schillen, K.; Brown, W. Macromolecules 1994, 27, 6046. (8) Mortensen, K.; Pederson, J. Macromolecules 1993, 26, 805. (9) Motensen, K.; Brown, W. Macromolecules 1993, 26, 4128. (10) Motensen, K. J. Phys. Condens. Matter 1996, 8, (25A) A103. (11) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (12) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1994, 27, 7, 5654. (13) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (14) Zhang, K. Ph.D. Dissertation, Lund University, 1994.
micellization and phase diagrams of triblock copolymers in aqueous solution in the presence of inorganic salts have also begun to draw attention.16-19 The development of nanoparticle synthesis and biotechniques20-22 demand a better understanding of such more complicated systems, because the self-assembled surfactants or block copolymers have been considered as suitable synthetic matrixes for making materials with nanoscale modifications.23,24 Recently, we reported a small-angle X-ray scattering (SAXS) study on the Pluronic L64 (E13P30E13)/CdCl2/H2O system.25 At high Cd2+ and L64 concentrations, a new lamellar structure appeared with a domain-domain distance of about 18-19 nm, and the distance varied with solute concentration. This distance was much longer than the length of the block copolymer chains and would not appear without the presence of either Cd2+ or water. We attributed these peaks to the formation of a Cd-polymer complex with Cd2+ ions being the cooperative centers and connecting the oxygen atoms on the L64 chains. The above-mentioned model was a reasonable explanation, based on the available data. However, we realized that it was still not very convincing to study only one copolymer with one technique. A more thorough study should be helpful to further elucidate this phenomenon. In the present work, three additional Pluronic block copolymers, L72, L44, and L35, with nominal compositions of E6P35E6, E10P21E10, and E11P16E11, respectively, were studied in the presence of CdCl2 and water. SAXS and wide-angle X-ray diffraction (WAXD) measurements were performed simultaneously to detect microphase separation and crystallization behaviors in these systems. To determine the binding sites on copolymer chains to Cd2+ ions, a comparison was made between WAXD results on the systems of Pluronic triblock copolymers/CdCl2/H2O and polyoxyethylene (PEO)/CdCl2/H2O. Experimental Section Sample Preparation. Four commercial triblock copolymers L35 (E11P16E11), L44 (E10P21E10), L72 (E6P35E6), and L64 (E13P30E13) were obtained as gifts from the BASF Co., New Jersey, and used without further purification. The CdCl2‚2.5H2O salt (Baker analyzed reagent, product of J. T. Baker Chemical Co.) was first dissolved in water in a centrifuge tube. PEO homopolymer (Mn ) 100 000 Da) was purchased from Sigma. Polymer samples were then mixed with the CdCl2 aqueous solution and (15) Schott, M.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984, 98, 196. (16) Binana-Limbele, W.; Van Os, N. M.; Rupert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 144, 458. (17) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657. (18) Jφrgensen, E. B.; Hvidt, S.; Brown W.; Schille´n K. Macromolecules 1997, 30, 2355. (19) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442. (20) Mann, S. Nature 1993, 365, 499. (21) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (22) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (23) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (24) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (25) Liu, T.; Xie Y.; Liang, D.; Zhou S.; Jassal, C.; McNabb, M.; Hall, C.; Chuang, C.; Chu, B. Langmuir 1998, 14, 7539.
10.1021/la000047y CCC: $19.00 © 2000 American Chemical Society Published on Web 08/19/2000
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Notes Table 1. Parameters of the Pluronics Triblock Copolymers and SAXS Measurements of Pluronics/ CdCl2/H2O Complexes
Figure 1. SAXS profiles of Pluronic triblock copolymer ExPyEx/ CdCl2/H2O tertiary systems with a weight ratio of copolymer: CdCl2:H2O ) 3:1:1. (a) L35 (E11P16E11); (b) L44 (E10P21E10); (c) L72 (E6P35E6). centrifuged at a speed of 8000 rpm (≈7.7 × 103 g) for at least 1 day to make sure that the components were thoroughly mixed. SAXS and WAXD Experiments. SAXS experiments were performed at the X3A2 SUNY Beamline, National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), using a laser-aided, prealigned pinhole collimator.26 The incident beam wavelength (λ) was tuned at 0.128 nm. Fuji imaging plates were used to collect the scattering data with exposure times of 4 min per frame. The sample-to-detector distance was 1162 mm.WAXD experiments were performed at the X27C Beamline, NSLS at BNL. The incident beam wavelength (λ) was tuned at 0.137 nm. Fuji imaging plates were used to collect the diffraction data with exposure times of 1 min per frame. The sample-todetector distance was 91 mm. The distance was corrected by using NIST Standard Materials No. 676, Alumina.
Results and Discussions Formation of Cd2+/Triblock Copolymer Complexes. The four Pluronic PEO copolymers have rather low viscosities and exist as liquids in the bulk state. They can mix with water at any weight fraction and form homogeneous solutions or clear gels. However, by adding a large amount of Cd2+, the sample becomes very viscous and paste-like. Two different copolymer concentrations were studied for each block polymer/CdCl2/H2O system, with the weight ratios of ExPyEx/CdCl2/H2O being 6:2:1 and 3:1:1, respectively. All samples were homogeneous and formed very viscous pastes after a long period of centrifugation. It was difficult to tell whether these materials formed a single phase. It has been reported that a gradual increase in intrinsic viscosity was observed in similar systems at higher salt concentrations (0.5-4.0 g/L),15 which was independent of the nature of the salt (e.g., KBr, KCl, KF). This phenomenon was caused by the influence of the inorganic salts on the micelle formation, as well as the micellar size and shape. SAXS was used to study possible ordered structures of the gel-like systems in the presence of CdCl2. Figure 1 shows the SAXS profiles of three ExPyEx/CdCl2/H2O (weight ratio 3:1:1) systems, with x and y denoting the numbers of E and P units in each block of different triblock copolymers, respectively. At least four scattering peaks with relative peak positions of 1:2:3:4 can be found in each scattering curve, which clearly indicate a lamellar (26) Chu, B.; Harney, P. J.; Li, Y.; Linliu, K.; Yeh, F.; Hsiao, B. S. Rev. Sci. Instrum. 1994, 65, 597.
name
formula
molecular weight
L35 L44 L72 L64
E11P16E11 E10P21E10 E6P35E6 E13P30E13
1900 2100 2600 2900
hydrophobic weight ratio (%)
q1 (nm-1)
lamellar spacing (nm)
50 60 80 60
0.522 0.472 0.394 0.331
12.0 13.3 15.9 19.0
structure. These peaks were certainly due to the addition of CdCl2 salts, because parallel experiments showed that, without salt, the same systems were all amorphous; there was no scattering peak in the SAXS profiles. This conclusion has also been shown previously in our work studying E13P30E13/water systems.25 It is obvious that the addition of Cd2+ induces microphase separation in ExPyEx/ water systems in the presence of only a very small amount of water. The appearance of the high-order scattering peaks suggests that the new structures were highly ordered. The distance between two adjacent layers (d) in the lamellar structures of the ExPyEx/CdCl2/water systems can be calculated from eq 1:
d ) 2p/q1
(1)
with q[≡(4πλ) sin(θ/2), θ being the scattering angle] being the scattering wave factor and q1 being the q value of the primary scattering peak. For ExPyEx/CdCl2/water systems with a weight ratio of 3:1:1, the d values were very large, from 12.0 nm for the L35 (E11P16E11) system, 13.3 nm for the L44 (E10P21E10) system, to 15.9 nm for the L72 (E6P35E6) system, which suggested large-scale ordered structures. The latter could not be due to CdCl2 molecular crystals (in Å), and were even much larger than the lamellar structures formed by Pluronics/water systems (in the scale of several nanometers). For 3:1:1 L64 (E13P30E13)/CdCl2/ H2O system, the q1 was 0.331 nm-1, corresponding to a d spacing of 19.0 nm, as reported previously.25 A plausible explanation is that a new ordered structure caused by salt-block copolymer interaction has formed. The coordination nature of Cd2+ ions enables them to coordinate with electron-rich O atoms on the copolymer chains, especially the O atoms at the ends of each chain, which have the highest electron density. Based on our model, the interlamellar distance should be determined by the lengths of both blocks in the copolymer, as shown schematically in Figure 2. To further prove this model, as shown in Figure 3, the domain-domain distances of the lamellar structures for different ExPyEx/CdCl2/H2O complex systems with a weight ratio of 3:1:1 are plotted versus the total number of segments of each triblock copolymer. A linear relationship was observed. The domain-domain distance increases with the increase of the total segment of block copolymer chains, that is, the distance between Cd2+ ions in two adjacent layers is determined by the lengths of both E and P blocks. For different block copolymers, the weight percentages of P blocks in the whole copolymer chain are different, varying from 20 wt % to 50 wt %. However, the increment of the domain-domain distance is only related to the total segment length, but not sensitive to the chemical composition of the block copolymers. An important feature supporting our model as presented previously is that the block copolymer chains connecting two adjacent Cd2+ ions are quite extended (i.e., there is little difference between the E and P blocks). In aqueous solution, the micellar size is usually determined
Notes
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Figure 2. (a) Schematic diagram of L64 (E13P30E13)/water lamellar structure in the presence of CdCl2. (b) Schematic diagram of proposed lamellar structure of the L64 (E13P30E13)/water/CdCl2 complex block (From ref 25, Figure 5. Used with permission).
of water is available, the conformation difference between E and P blocks seems to be very small, so that E and P segments have similar zigzag lengths and the contributions of each E or P segment to the interlamellar distance are similar. The scattering peaks of the complex systems were much broader than those of block copolymer/water gel systems, suggesting that the size of the complex crystals (or quasicrystals) is much smaller than those of block copolymer gels. The overall size of the complex crystal (t) can be estimated by measuring the half-width of the primary scattering peak at half-height (B) with
t ) λ/(B cos B)
Figure 3. Plot of total number of segments of Pluronic block copolymers, including L35 (E11P16E11), L44 (E10P21E10), L64 (E13P30E13), and L72 (E6P35E6), versus domain-domain distance of the lamellar structure of copolymer ExPyEx/CdCl2/H2O tertiary systems with a weight ratio of Pluronic:CdCl2:H2O ) 3:1:1.
by the length of the hydrophilic block, and the hydrophobic block tends to shrink and form a smaller micellar core.27 However, in the current case, because only a small amount (27) Liu, T.; Zhou, Z.; Wu, C.; Nace, V. M.; Chu, B. J. Phys. Chem. 1998, 102, 2875.
(2)
For example, the crystal size of the E6P35E6/CdCl2/H2O (3:1:1) complex (Figure 3c) is about 2.2 × 102 nm, which is equivalent to about 15 layers of the lamellar packing. Effect of Water Content. The SAXS profiles for the E6P35E6/H2O/CdCl2 system with different water contents (weight ratios being 3:1:1 and 6:2:1, respectively) are shown in Figure 4. Two curves represent quite similar lamellar structures, but the system with the lowest water content shows a shorter interlamellar distance (15.9 nm vs 16.8 nm). This is because the hydrophilic E blocks can be more extended in the presence of a larger amount of water. Therefore, the overall length of the triblock copolymer chains will increase. For different ExPyEx/CdCl2/ H2O systems, the scattering patterns are different when
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Figure 4. SAXS profiles of L72 (E6P35E6)/H2O/CdCl2 tertiary systems with weight ratios of E6P35E6/CdCl2/H2O ) (a) 3:1:1 and (b) 6:2:1.
Figure 5. SAXS profiles of L35 (E11P16E11)/H2O/CdCl2 tertiary systems with weight ratios of E11P16E11/H2O/CdCl2 ) (a) 3:1:1 and (b) 6:2:1.
the water content is low. For example, the SAXS profiles of two E11P16E11/CdCl2/H2O systems are shown in Figure 5 with the component weight ratio of E11P16E11:CdCl2: H2O ) 3:1:1 (Figure 5a) and 6:2:1 (Figure 5b), respectively. The two scattering peaks in curve a show a relative q value of 1:2, which is typical for a lamellar packing. However, for curve b, the two peaks show relative peak positions of 1:1.6. This ratio also appeared in the E13P30E13/ CdCl2/H2O system, as reported previously.25 In this case, several scattering peaks could be distinguished and they could be separated into two groups, with each group suggesting a possible lamellar structure. We attributed one of the lamellar structures (the set of scattering peaks with the q1 peak in Figure 5a, or group A, see ref 25) to be the original lamellar structure, and the other set of peaks (containing the q2 peak in Figure 5b, or group B) should belong to a new ordered lamellar packing. WAXD Measurements on the Salt-Polymer Complexes. WAXD experiments were used to study crystallization in a confined domain. Figure 6 shows WAXD measurements on pure CdCl2 (Figure 6a), and on the complexes formed by CdCl2 and three Pluronic block copolymers (Figures 6b-d), respectively. The appearance
Notes
Figure 6. WAXD profiles of Pluronic triblock copolymer ExPyEx/ CdCl2/H2O tertiary systems with a weight ratio of copolymer: CdCl2:H2O ) 3:1:1 with (a) no copolymer, pure CdCl2; (b) L35 (E11P16E11); (c) L44 (E10P21E10); and (d) L72 (E6P35E6).
of the scattering peaks in the wide-angle region (2θ ) 8-40°) for the complexes indicates crystallization processes in these systems. All the WAXD profiles of the Pluronics/CdCl2/water systems are similar, but they are apparently different from that of the pure CdCl2 salt, which suggests that copolymer chains are involved in the crystallization process. Previous research demonstrated that the PEO homopolymer chains could bind a variety of metal ions strongly.28-30 In a specific case of group IB metal ions, Blumberg and Pollack31 showed that PEO binds HgCl2 and CdCl2 molecularly in crystalline complexes formed by coordination of oxygen atoms along the polymer chains to the metal ions. For both Hg2+ and Cd2+, characteristic diffraction patterns appeared on casting PEO films onto HgCl2 or CdCl2 solutions. The crystal structure of the CdCl2/PEO crystalline complex has been elucidated by fiber X-ray diffraction. The molecular-level “solubility” and the ordered arrangement of CdCl2 and PEO were also observed in the system of triblock copolymer containing E block. For the PEO/CdCl2/H2O system at low CdCl2 salt concentration (Figure 7a), because PEO is also in its crystalline state at room temperature, the two characteristic lines of crystalline PEO at d spacing of 4.76 and 3.90 Å, respectively, could still be detected by WAXD (the two strong scattering peaks in Figure 7a). When the weight percentage of CdCl2 increased (about 40 wt %), the characteristic peaks of crystalline PEO would become much weaker in the WAXD pattern, as shown in Figure 7b. At the same time, a new scattering peak appeared at the diffraction angle (2θ) of 8.2° (Figure 7b). This peak should be attributed to the formation of the PEO-CdCl2 crystalline complex, because it cannot be found in the WAXD profiles of any form of CdCl2 crystals and their hydrated compounds. More accurately, the new peak indicated the formation of cross-linked coordination between adjacent Cd2+ ions, that is, a large amount of Cd2+-Cd2+ bonds have formed, as shown schematically previously. At low CdCl2 concentrations, all the inorganic salts can be dissolved in the hydrophilic PEO region amorphously (in the molecular level), and the average (28) Radhakrishnan, S.; Schultz, J. M. J. Cryst. Growth 1992, 116, 378. (29) Lin, J.; Cates, E.; Bianconi, P. A. J. Am. Chem. Soc. 1994, 116, 4738. (30) Radhakrishnan, S.; Saini, R. J. Cryst. Growth 1993, 129, 191. (31) Blumberg, A. A.; Pollack, S. S. J. Polym. Sci. 1964, 2, 2499.
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Langmuir, Vol. 16, No. 19, 2000 7537
with Cd2+ make the complex crystallize in the local region. The ordered structure at a molecular level should be easy to form if the coordination number and the coordination distance with the oxygen atoms to each CdCl2 are fixed. The PEO/CdCl2 complex becomes insoluble when the amount of effectively cross-linked Cd2+-Cd2+ bonds is high enough.31 A similar cross-linking phenomenon was also observed in the coordination of CdCl2 with Pluronic triblock copolymer systems, with the appearance of the samples becoming thick and translucent, and the viscosity of the samples also increasing rapidly. Although the Clions may have some influence on the viscosity of the blends, we still believe that the coordination cross-linking of E block in ExPyEx with Cd2+ is the most important reason for the enhancement of the viscosity of the systems.
Figure 7. WAXD profiles of PEO/H2O/CdCl2 tertiary systems, with weight ratios of PEO/H2O/CdCl2 ) (a) 3.8:1:1 and (b) 3:1:1.
distance between two adjacent Cd2+ ions should be very great. Then, the cross-linked coordination cannot occur, so that the characteristic peak cannot be detected by WAXD measurements. The three WAXD profiles of ExPyEx/CdCl2/H2O systems in Figure 6 (b, c, and d) are basically identical, and similar scattering peaks can be found at the same q ranges for each curve. The scattering peak positions are independent of the chemical composition of the Pluronic copolymers. This is further evidence that no major difference exists between E block and P block in their contributions to determining the crystal lattice, although Cd2+ ions may tend to choose the electron-rich E block. Also, the profiles of the Pluronics/salt complexes are quite similar to that of the PEO homopolymer/salt complex (Figure 7a), including the characteristic scattering peak at 2θ ) 8.2°, suggesting that the presence of PEO chains in the system is critical for the formation of the salt-polymer complex. In addition, the number of polymer chains coordinated with each Cd2+ ion should be quite similar, no matter what the length of E blocks is or whether they were homopolymer chains or block copolymer chains. A previous study showed that each Cd2+ would coordinate with four oxygen atoms (the other two coordination sites were occupied by the two Cl- ions).28 Therefore, an empirical formula for the complex can be given as CdCl2-(EO)4. We suggest that the coordination binding of oxygen atoms
Conclusions Salt-induced microphase separations and crystallizations of ExPyEx (including E6P35E6, E11P16E11, E13P30E13, and E10P21E10) Pluronic block polymers/Cd2+/H2O complexes were studied by SAXS and WAXD techniques. The coordination binding of CdCl2 to the Pluronics triblock copolymer chains was observed in the presence of a trace amount of water. Lamellar packing was observed in all the systems. We suggested that this lamaller structure was formed by the coordination of the Cd2+ ions to the oxygen atoms on the polymer chains. The domain-domain distance of the lamellar structure was determined by the total segment length of the Pluronic block copolymers, suggesting that the E and P blocks functioned similarly on determining the interlamellar distance of saltcopolymer complexes. The much larger interlamellar spacings in all the complex systems, when compared with the salt-free systems, indicated that the copolymer chains used for the formation of a complex structure had been stretched out. The crystallization of CdCl2/ExPyEx complex was studied by WAXD measurements. Identical X-ray diffraction patterns were obtained in systems of CdCl2 with EPE block copolymers and those with PEO homopolymer chains, suggesting that the coordination should occur mainly between CdCl2 and the oxygen atoms on the E blocks. Acknowledgment. B.C. gratefully acknowledges the support of this work by the U.S. Department of Energy (DEFG0286ER45237.015 and DEFG0299ER45760) and the National Science Foundation (DMR9984102). LA000047Y
