Langmuir 1998, 14, 7539-7542
Formation of a Salt-Polymer Complex in L64/Water/CdCl2 Systems Tianbo Liu,† Yi Xie,† Dehai Liang,† Shuiqin Zhou,† Charanjeet Jassal,†,‡ Michael McNabb,†,§ Christopher Hall,†,| Chun-Lun Chuang,†,⊥ and Benjamin Chu*,†,∇ Department of Chemistry and Department of Materials Sciences and Engineering, State University of New York at Stony Brook, New York 11794-2275 Received April 8, 1998. In Final Form: September 16, 1998
Introduction The micellization and phase separation behaviors of EPE type triblock copolymers (where E and P represent, respectively, polyoxyethylene and polyoxypropylene) in aqueous and nonaqueous solutions, have been widely reported.1,2 To minimize the free energy in solution the solvent-phobic blocks tend to shrink together to form micellar cores while the soluble blocks form extended micellar shells. One of the typical works on the association behavior of EPE triblock copolymers in water and xylene has been reported with a commercial Pluronic L64 (E13P30E13) by Wu, Zhou, and Chu.3,4 At higher polymer concentrations, the ordered packing of hydrophilic and lyophilic regions can form gel-like cubic, hexagonal, or lamellar structures.5 Extensive work has been done to elucidate the phase behavior of polyoxyalkylene triblock copolymers in solution. For L64, Zhang et al.5 and Alexandridis et al.6 have provided very detailed phase diagrams of L64/water and L64/water/p-oxylene systems. The micellization and phase diagrams of triblock copolymers in aqueous solution in the presence of inorganic salts have begun to draw attention. The development of nanoparticle synthesis and biotechniques7,8 demands a better understanding of such more complicated systems. Binana-Limbele et al.9 studied the aggregation behavior and the micellar dynamics of a nonionic surfactant, pentaoxyethyleneglycol monooctyl ether, in water in the presence of different sodium halides. The sodium halides had the effect of decreasing the solubility of polymers in water, resulting in lower cloud-point temperatures and micellar aggregation numbers. Bahadur et al.10 reported that in L64/water and P85 (E25P40E25)/water systems, the addition of KF, KCl, KBr, and KI decreased the cloudpoint temperature (salting-out effect) while the addition of KCNS increased the cloud-point temperature (saltingin effect). By using rheology and light scattering, Jorgensen et al.11 reported the effect of KF on the micellization * To whom correspondence should be addressed. † Department of Chemistry. ‡ Research Education for Undergraduate (REU) Program. § Stony Brook Summer Research Institute. | Research and Engineering Apprenticeship Program. ⊥ Simons Fellowship Program. ∇ Department of Materials Sciences and Engineering. (1) Chu, B.; Zhou, Z. In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; Chapter 3. (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) Zhang, K.; Khan, A. Macromolecules 1995, 28, 3807. (6) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (7) Mann, S. Nature 1993, 365, 499. (8) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (9) Binana-Limbele, W.; Van Os, N. M.; Rupert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 144, 458. (10) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 657.
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and the gelation of P85 triblock copolymer in aqueous solution. A lower gelation concentration was reported in the presence of KF. In this note, we report the effect of CdCl2 on the gelation behavior of L64 in aqueous solution. CdCl2 is an important reagent to synthesize Cd-containing semiconductors8 (e.g., CdS). Therefore, the role of CdCl2 in the phase behavior of L64/water system is very important for further development of the nanosynthesis in block copolymer gels. It is noted here that we have used the terminology “gel” in a broad sense, including very viscous solutions where polymer chains are entangled but without permanent cross-links. Thus, the gel region may not have a sharp boundary from the less viscous solution. At the same time, the transition metal Cd shows a richer behavior than alkali metals because of its d-level electrons and empty electronic orbits. A mixed system of Cd salt and block copolymers could exhibit a variety of more complex phase behavior than the systems reported previously.12 Experimental Section Sample Preparation. The triblock copolymer L64 (E13P30E13) was obtained as a gift from BASF Company and used without further purification. L64 was dissolved in deionized water. To prepare a solution of L64 in water in the presence of CdCl2, the CdCl2 salt was first dissolved in water in a centrifuge tube. L64 was then mixed with the CdCl2 aqueous solution, and the mixture was centrifuged at a speed of 8000 rpm (≈7.0 × 104 g) for at least 1 day to make sure that the components were thoroughly mixed. Cloud-Point Temperature and Gelation Concentration Measurements. The cloud-point temperatures of polymer solutions were measured by detecting a sudden decrease in transmittance of an incident laser beam with increasing temperature. The experimental setup and data interpretation have been described elsewhere.13 The gelation temperatures of the polymer solutions were determined by visual observation. Small-Angle X-ray Scattering (SAXS) Experiments. SAXS experiments were performed at the X3A2 SUNY Beam Line, National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), using a laser-aided prealigned pinhole collimator.14 The incident beam wavelength (λ) was tuned at 0.128 nm. Fuji imaging plates were used as the detection system. The sample-to-detector distance was 1114 mm.
Results and Discussions CdCl2 Effect on Cloud-Point Temperature (Tcl) of L64/Water Systems. The Tcl can be defined as the temperature at which phase separation occurs in the block copolymer solution. The solubility of nonionic surfactants in water usually decreases with increasing temperature. The Tcl of polyoxyalkylene block copolymers tends to show a small concentration dependence in the dilute solution regime. The addition of CdCl2 to the L64/water system lowers the Tcl, as shown in Figure 1. The Tcl of L64 decreases with increasing CdCl2 concentration until the CdCl2 concentration reaches about 2.4 mol/L, which is the largest solubility of CdCl2 in water at about 48 °C. This type of salts, including Na+, K+, Ca2+, and so forth, has been identified as having a “salting-out effect”;15 that is, the (11) Jorgensen, E. B.; Hvidt, S.; Brown, W.; Schillen, K. Macromolecules 1997, 30, 2355. (12) Encyclopedia of Inorganic Chemistry; Wiley: New York, 1994; p 463. (13) Liu, T.; Nace, M.; Chu, B. J. Phys. Chem. 1997, 101, 8074. (14) Chu, B.; Harney, P. J.; Li, Y.; Linliu, K.; Yeh, F.; Hsiao, B. S. Rev. Sci. Instrum. 1994, 65, 597. (15) Schott, M.; Royce, A. E.; Han, S. K. J. Colliod Interface Sci. 1984, 98, 196.
10.1021/la980385n CCC: $15.00 © 1998 American Chemical Society Published on Web 11/20/1998
7540 Langmuir, Vol. 14, No. 26, 1998
Figure 1. Cloud-point temperatures of 4 wt % L64 in aqueous solution at different CdCl2 concentrations.
presence of salt decreases the solubility of surfactants in an aqueous solution. On the contrary, some anions, for example, SCN-, have the “salting in” effect, which increases the solubility of surfactants in water. Phase Diagram of L64/Water/CdCl2 Ternary System. L64, water, and CdCl2 form a ternary system, as shown in Figure 2a. In our study, we fixed the weight ratio of L64 over CdCl2 to be a constant and changed the weight percentage of water in the system. This condition was represented by a dashed line in Figure 2a. A typical phase diagram of L64/water/CdCl2 with a constant CdCl2to-L64 weight ratio is shown in Figure 2b, where the x-axis denotes the weight fraction of L64 versus the total weight of water and L64. The molar ratio of CdCl2 versus L64 was set to be 2.88:1 (or weight ratio 1:5.47). As stated in the Experimental Section, the phase boundaries were
Notes
obtained by visual observation where the “gel state” was defined as the clear, immobile, homogeneous liquid having a very high viscosity which prevented the liquid from flowing when the sample tube was inverted. A liquid solution state with relatively low viscosity could be observed at low temperatures and low L64 concentrations (denoted by L1 in Figure 2b). W+L1 denotes the twophase-coexistence region above the cloud-point temperature curve (denoted by Tcl) where concentrated L64 and CdCl2/water solution could coexist at higher temperatures (I); W denotes essentially the water phase which in our system becomes the CdCl2 solution. We just copied the symbols used in Figure 2c for the corresponding region. By comparing Figure 2b with the phase diagram of the L64/water system (Figure 2c, cited from ref 5), the gelation concentration, which contains different structures (e.g., hexagonal and lamellar) at different temperatures, has shifted to a lower polymer concentration (from an average of about 40 wt % to as low as ≈32 wt %), in good agreement with our previous conclusion that CdCl2 makes water a poorer solvent for L64. The gel-like region has also been compressed to only the low-temperature regime. At very high polymer concentrations (e.g., >75 wt % L64 and at 25 °C), and after a long equilibration time in mixing the very viscous fluid by centrifugation, there appeared another translucent gel-like phase (II), possibly because of the presence of highly concentrated CdCl2 salt. It was difficult to establish whether the multiphase viscous gellike materials had reached equilibrium. To test this point, several samples were equilibrated by temperature cycling and centrifugation over a period of about 6 months. SAXS data were reproduced after the 6-month period. When the system finally reached equilibrium, phase separation could be observed with the upper layer becoming a clear
Figure 2. (a) Variation of water content at constant weight ratio of L64 to CdCl2. The dashed line denotes the condition under which a detailed study has been performed, as shown in Figure 2b. (b) Phase diagram of L64/water/CdCl2 ternary system with a molar ratio of L64 to CdCl2 of 1:2.88 (weight ratio 5.47:1). It is noted that there is no clear boundary between the solution (L1) and the gel-like region, which remains as a homogeneous one-phase solution. (c) Phase diagram of the L64/water system, cited from ref 5, with permission: L1 and L2, liquid; E, hexagonal; D, lamellar; V, bicontinuous cubic; W+L1, macrophase separation.
Notes
Langmuir, Vol. 14, No. 26, 1998 7541
a
Figure 4. SAXS measurement of a L64/water/CdCl2 ternary system with the molar ratio of L64 to CdCl2 ) 1:2.88 and L64/ water ) 71 wt %.
b
Figure 3. SAXS measurements of (a) a L64/water (61 wt %) system at 16 °C and (b) after a certain amount of CdCl2 (L64/ CdCl2 ) 1:1.44 molar ratio) was added to the system in part a at 16 °C.
gel-like solution of very high viscosity and the bottom layer forming a translucent and viscous paste. This two-phase gel-like/paste region appeared at high polymer concentrations. At higher temperatures, the gel-like/paste region became a clear homogeneous solution (L1), indicating that the solubility of CdCl2 in L64/water became better at higher temperatures. With the molar ratio of CdCl2 to L64 being higher, the cloud-point temperatures and the gelation concentrations further decreased. Furthermore, the two-phase “thick gel” region (II) became even larger and appeared at lower polymer concentrations and extended to higher temperatures. SAXS Study of L64/Water/CdCl2 Systems. SAXS was used to study the possible ordered structures in the gel-like region in the presence of CdCl2. The L64/water system has been studied thoroughly by different groups,3-5 with the ordered packing of hydrophobic and hydrophilic parts in aqueous solution leading to hexagonal, lamellar, and bicontinuous cubic structures.5 However, without adding an immiscible organic solvent (e.g., xylene) to the system to enhance the hydrophobicity of micellar cores, the micellar packing was not as ordered as expected, mainly due to the comparatively small difference in the hydrophobicity between oxyethylene and oxypropylene groups. Figure 3a shows a typical SAXS scattering curve for L64 in water (61 wt %) at 16 °C. Only the first two peaks were observable, suggesting a lamellar structure with the relative peak positions 1:2. Similar peak ratios
could also be observed at different polymer concentrations (around >60 and 82 wt %), where the (16) Liu, T.; Zhou, Z.; Wu, C.; Nace, V. M.; Chu, B. Macromolecules 1977, 30, 7624.
L64/water system has again become a liquid state without nanoscaled order. Figure 6 shows SAXS measurements of a ternary system with very low water content and very high polymer and salt concentrations. The scattering peaks can be separated into two groups (labeled as A and B in Figure 6): group A contains the same peaks due to salt/polymer complex in Figures 4 (q ) 0.337 and 0.675 nm-1) and 5b, although the first peak has been merged by the new structures. Group B has two peaks with relative q values of 1:2 (q ) 0.395 and 0.792 nm-1), suggesting a possibly new ordered lamellar structure. The interdomain distance can be calculated by eq 1 to be 16 nm, a little smaller than that of the first lamellar structure. A possible explanation of the so-formed structures has been provided as the following: at comparatively low salt concentrations, the first lamellar structure was formed by block copolymers due to the formation of salt/polymer complex. At higher salt concentrations, more and more Cd2+ ions stayed on one plane, leading to the formation of the second lamellar structure that might cross over the first one. A small amount of water was shown to be necessary for the formation of salt/polymer complex. A water-free mixture of L64 copolymer and CdCl2 provided no scattering peak in SAXS measurements. A possible explanation is given as follows. The lamellar structure formation, as shown in Figures 5b, was mainly due to the salt-oriented packing of L64 blocks. The presence of water was necessary for separation of the two blocks so that the polymer chain packing could become more ordered. Although at this time we just found such a kind of new structure in the L64 system, it is reasonable to consider that it can also exist in other similar block copolymer/ water systems and maybe in the presence of other transition-metal salts. Acknowledgment. B.C. gratefully acknowledges support of this work by the National Science Foundation (Grant DMR9612386). LA980385N
