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Salt-Induced Polymer Gelation and Formation of Nanocrystals in a Polymer-Salt System Li-Zhi Liu,†,| Quan Wan,‡ Tianbo Liu,§ Benjamin S. Hsiao,† and Benjamin Chu*,†,‡ Departments of Chemistry, Materials Science & Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, and Department of Physics, Brookhaven National Laboratory, Upton, New York 11793 Received May 22, 2002. In Final Form: September 16, 2002 Salt-induced polymer gelation and nanocrystal fabrication in a polymer-salt system of Pluronic surfactant L35 (E11P16E11) and saturated CdCl2 aqueous solution were studied by means of small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM). The symbols E11 and P16 denote the ethylene oxide end block with 11 segments and the propylene oxide middle block with 16 segments, respectively. The L35 surfactant started to form a gel-like polymer network with only 2 wt % of the saturated CdCl2 solution (weight ratio of CdCl2:H2O ) 50:50) in the system. When the amount of salt solution was further increased to 5 wt %, nanocrystals composed mainly of CdCl2 were formed in the gel matrix. In this system, the minimized water content was not the driving force for polymer gelation and microphase separation. The poly(ethylene oxide) (PEO) block in the EPE surfactant could not crystallize by itself, but it could be strongly bonded with salt. The formation of uniform nanocrystals with 3-dimensional ordered packing could be achieved in the presence of soft polymer networks. On the basis of the TEM results, the nanocrystals were found to undergo morphological changes from cubic to rodlike structures, depending on the amount of saturated salt solution.
Introduction There have been a number of studies on salt/polymer complexation. Typically, these studies can be classified into two categories: (1) effects of inorganic salts on the aggregation behavior of nonionic polymer surfactants in aqueous solutions;1-4 (2) polymer-induced crystallization of inorganic salts.5-7 The focus in the above two studies is different. In the first case, the systems studied were aqueous solutions, while, in the second case, bulk polymers were used. As the good ionic mobility of poly(ethylene oxide) (PEO) would facilitate diffusion of incoming ions,8 block copolymers with PEO as one of the blocks are widely used in case (1). It is well-known that PEO can bind to a variety of metal ions strongly5,6 and form complexes (such as alkali metal salts).6 The cations are trapped by coordination with the oxygen atoms of the PEO polymer. In the case of polymer-induced crystallization of inorganic salts, the ions are bonded to a polymer matrix and form a complex in the initial stage. The complex network later * Author to whom correspondence should be addressed. † Department of Chemistry, State University of New York at Stony Brook. ‡ Department of Materials Science & Engineering, State University of New York at Stony Brook. § Brookhaven National Laboratory. | Current address: The Dow Chemical Company, 2301 Brazosport Blvd. B-1407 B, Freeport, TX 77541-3257. (1) Binana-Limbele, W.; Van Os, N. M.; Rupert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 144, 458. (2) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs P. Colloid Polym. Sci. 1993, 271, 657. (3) Liu, T.; Xie, Y.; Liang, D.; Zhou, S.; Jassal, C.; McNabb, M.; Hall, C.; Chuang, C.-L.; Chu, B. Langmuir 1998, 14, 7539. (4) Liu, T.; Xie, Y.; Liu, L.-Z.; Chu, B. Langmuir 2000, 16, 7533. (5) Radhakrishnan, S.; Schultz, J. M. J. Crystal Growth 1992, 116, 378. (6) Radhakrishnan, S.; Saini, D. R. J. Cryst. Growth 1993, 129, 191. (7) Lin, J.; Cates, E.; Bianconi, P. A. J. Am. Chem. Soc. 1994, 116, 4738. (8) Bates, J. B.; Farrington, C. G. Fast Ion Transport in Solids; NorthHolland: Amsterdam, 1981.
Figure 1. Composition phase diagram of the CdCl2/H2O/EPE mixtures, where the solid line I(L64) represents the compositions of CdCl2/H2O/L64 mixtures studied before,3,4 the two circles II(L35) represent the compositions of CdCl2/H2O/L35 mixtures studied before,4 and the solid line III(L35) represents the composition region studied in the present work.
acts as the template for subsequent crystallization of the excess salt. In aqueous solutions, the complex between salt and polymer can significantly change the polymer aggregation behavior and the solution properties. One of the interesting systems is the Pluronic surfactant (PEO-PPO-PEO) that contains both PEO and PPO segments capable of forming complexes with many other materials. The phase behavior of this nonionic surfactant in salt solutions is rather complicated, being dependent on the composition of the mixture, as well as the block copolymer composition and the molecular weight. For example, the ExPyEx/water/ CdCl2 tertiary system has been studied in our laboratory.3,4 The phase diagram is shown in Figure 1, and the regions I and II represent the solutions containing L64 (E13P30E13) and L35 (E11P16E11), respectively, which have recently been studied by us.3,4 In these systems, there was enough water, which could lead to aggregation (micellization and/or
10.1021/la0204803 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/27/2002
Gelation and Nanocrystals in a Polymer-Salt System
gelation) by water itself. The aggregation could also occur due to the complexation of the salt and the ether groups in the polymer. In other words, aggregation in the above system could be due to the solvent (water) selectivity (more favorable for the PEO block, less favorable for the PPO block), the complexation of salt and ether groups in the polymer, or a combination of both effects. To understand the complexation behavior in this system and to explore the formation of inorganic nanocrystals in such a gel system, the saturated salt solution (having about 50 wt % of CdCl2) was blended with pure EPE surfactant. In this case, the water content was minimized and the amount of water itself could not make the surfactant aggregate. Thus, the driving force for phase separation came mainly from the interactions between the salt and the polymer. The concentration range studied in this work is shown in region III of the phase diagram in Figure 1. Both behaviors of polymer gelation and formation of nanocrystals in the polymer gel matrix were studied in this region by direct mixing of the EPE surfactant with a saturated aqueous solution of cadmium chloride. The system studied in the present work was somewhat similar to that of the bulk PEO/salt system. The major difference was that the chosen EPE copolymer was in a liquid state at room temperatures. In other words, the present complexation should occur in a gel matrix without the effects of PEO crystallization. We have demonstrated that this unique system would lead to the formation of very uniform nanocrystals with 3-dimensional packing order. Experimental Section Pluronic surfactant L35 (E11P16E11), with a nominal molecular weight of 1900 g/mol, was provided by BASF, New Jersey, and was used without further purification. Cadmium chloride (CdCl2‚ 2.5H2O) was purchased from J. T. Baker Chemical Co., Philipsburg, NJ. Saturated CdCl2 solution in deionized water was first prepared before mixing it with the surfactant. Our measurements showed that the final weight fraction of salt [CdCl2‚ 2.5H2O, including the hydrated crystalline water] in the saturated solution was 62%. The weight fraction of water was 50% if the hydrated water molecules in the salt were excluded. The mixtures were prepared by adding the saturated CdCl2 solution directly into L35. The samples were manually mixed first, and were then placed in centrifuge tubes to force the mixing by centrifugation for at least 1 day in order to make sure that a homogeneous liquid was formed. A series of samples with 1, 2, 5, 10, 25, and 40 wt % of the saturated solution were prepared and studied. For convenience during discussion, the following notations 1%S/L35, 2%S/L35, 5%S/L35, 10%S/L35, 25%S/L35, and 40%S/L35 were used to designate these mixtures. Simultaneous small-angle X-ray scattering (SAXS) and wideangle X-ray diffraction (WAXD) experiments were performed at the X27C beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The X-ray wavelength was tuned at 0.137 nm using a double-multilayered monochromator. Two Fuji imaging plates were used to collect the SAXS/WAXD images. Typical data collection time was 2 min for SAXS and 1 min for WAXD. The WAXD imaging plate had a central opening, allowing the passage of SAXS signals. The SAXS and WAXD signals were calibrated with silver behenate and Al2O3, respectively. TEM measurements were carried out at the University Microscopy Imaging Center, State University of New York at Stony Brook. A JEOL JEM-1200EX electron microscope was used and the applied voltage was 80 kV.
Results and Discussion For the mixtures studied in the present work, the concentration of saturated CdCl2 solution changed from 1 to 40 wt %. The corresponding water content in the series of mixtures was from 0.5 to 20 wt %. In addition,
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Figure 2. WAXD patterns of pure L35, and L35 mixtures with 2 and 5 wt % of the saturated (CdCl2 + H2O) solution.
L35, instead of L64, was chosen for the study (the ethylene oxide content was higher in L35 than that in L64). The higher ethylene oxide (EO) content made the copolymer more soluble in water, and hence microphase separation could occur only in very concentrated solutions of this copolymer. Moreover, the molecular weight of L35 (M ) 1900 g/mol) was about 30% smaller than that of L64 (M ) 2900 g/mol), making L35 more soluble in water than L64 and thereby reducing the chance for phase separation under the same conditions.9 Our SAXS study of the L35/ water mixtures confirmed that microphase separation did not occur in the water/EPE mixture for up to 30 wt % of water. Therefore, it is reasonable to assume that in the chosen compositions of EPE/CdCl2/H2O mixtures, the effect of solvent (water) selectivity for EPE copolymer is not the main driving force for the aggregation behavior in the mixtures. 1. Salt-Induced Gelation. The PEO molecules can complex with CdCl2 (or HgCl2) at the molecular level through coordination of oxygen from the PEO segments and the metal ions.7 In the tertiary mixture of EPE/CdCl2/ H2O, complexes can also be formed between the salt and the PEO blocks.3,4 The PEO homopolymer of similar molecular weight such as L35 is semicrystalline. The crystallization ability of PEO in the L35 block copolymer is suppressed significantly due to the presence of PPO, where the copolymer is non-crystalline and liquidlike. When 1 wt % of saturated CdCl2 solution was added to L35, the mixture remained liquidlike. However, when 2 wt % of saturated CdCl2 solution was added, it became a gel due to the interactions between Cd2+ ions and the oxygen atoms in the polymer chains. WAXD was used to study the segmental packing of amorphous chains. Figure 2 shows that there is a distinct amorphous peak near 2θ ) 18° for pure L35 having a d spacing of 0.44 nm. This peak is due to coherent scattering of neighboring segments with only short-range order, where the corresponding d spacing represents the average interchain distance, i.e., the average distance between adjacent polymer chains is 0.44 nm. This short-range order was substantially reduced by the addition of 2 wt % of saturated CdCl2 solution, as evidenced by the very low scattered intensity in the amorphous peak region (2θ ∼ 18°) from the polymer mixture. The gel state and the absence of the amorphous peak in the 2 wt % S/EPE mixture implies that the strong interactions between the PEO blocks and the Cd2+ ions have severely affected the short-range order of the polymer segments. It is interesting to note that the 2 wt % S/EPE mixture exhibits a small but distinct WAXD peak at 8.7° having a corresponding d spacing of about 0.9 nm. The intensity of this peak increases significantly with further (9) Leibler, L. Macromolecules 1980, 13, 1602.
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Figure 3. SAXS patterns of the pure L35, and L35 mixtures with 2, 5, and 10 wt % of the saturated (CdCl2 + H2O) solution.
addition of salt in the mixture, as shown in the mixture having 5 wt % of salt. A similar peak with a d spacing of 0.96 nm had been reported for the PEO/CdCl2 mixture before,4 which was attributed to the complex formation between PEO and CdCl2. In addition, another report showing a d spacing of 0.7 nm for complex formation in the PEO/CaCl2 mixture has also been made.6 Thus, the observed peak at 8.7° in Figure 2 is not due to pure CdCl2 crystals, as this peak position does not match any CdCl2 crystal reflections. It is also not due to the structure in copolymer, so this peak must be the result of complex formation between the PEO blocks and CdCl2. It is conceivable that the denser complexed domains are distributed homogeneously in the sample, where the d spacing of 0.9 nm represents an average distance between these domains. This argument is also consistent with the observation that the mixture with 2 wt % of the salt solution behaved like a gel. In Figure 3, it is shown that the SAXS profile of the 2 wt % CdCl2 solution with L35 is very different from that of pure L35. The 2%S/L35 mixture shows very strong defused scattering in the low q-range, suggesting that there are many small aggregate domains of heterogeneous sizes formed in the gel system. These aggregates could contain single or multiple CdCl2/PEO complexes and could act as physical cross-linking points (these small aggregates are noncrystalline). The large increase in the scattered intensity could be due to the increasing density difference between the salt-containing domains (the density of pure CdCl2 is 4.05 g/cm3) and the L35 copolymer matrix. The scattering curve of the 2%S/L35 mixture also shows a small scattering peak at q ) 0.78 nm-1 (Figure 3), with a d spacing of about 8 nm. This small scattering peak represented the coherent scattering length from the more homogeneous formation of aggregate domains of similar dimensions that were dispersed in the matrix. The packing of these domains (they are the precursors of nanocrystals, which will be discussed later) became more regular by adding more saturated salt solution. Figure 3 shows the SAXS curve of the mixture with 5 wt % of the CdCl2 solution, exhibiting multiple scattering maxima. When the salt solution was increased to 10 wt % in the mixture, the scattered intensity of the first order peak (at about q ) 0.51 nm-1) became significantly stronger than that in the 5%S/L35 mixture. The intensities of the two higher order peaks also increased slightly. The positions of the multiple-order scattering peak in the mixture with 10 wt % of the saturated solution exhibited the following ratio: 1:(2)1/2:2 (relative to the first scattering peak). The peak position ratio for the 5%S/L35 mixture was also very close to 1:(2)1/2. It is well-known that different microphase structures will exhibit different peak position ratios in
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Figure 4. WAXD patterns of pure CdCl2, and L35 mixtures with 10, 25, and 40 wt % of the saturated (CdCl2 + H2O) solution. (Some curves were shifted in the figure in order to have a better view of the patterns).
the SAXS profile.10 For examples, the body-centered cubic (bcc) structure and the simple cubic structure have the same relative peak position ratios (for the first several peaks) of 1:(2)1/2:(3)1/2:(4)1/2:(5)1/2:(6)1/2; the hexagonally packed cylindrical structure has the peak position ratios of 1:(3)1/2:(4)1/2:(7)1/2; the lamellar structure has the peak position ratio of 1:2:3:4. As our mixtures showed a maximum at the relative peak position of (2)1/2; this could not be present in the cylindrical or lamellar structures. We therefore suggest that the possible domain packing in the studied gel should have a cubic structure. If we assume that the structure has the bcc symmetry, instead of the simple cubic symmetry, then the interdomain distance D can be determined11 from the corresponding Bragg spacings,
D ) (3/2)1/2d110
(1)
where d110 denotes the d spacing of the first peak. As the d spacing for the first peak at q ) 0.51 nm-1 was about 12.3 nm, the interdomain distance D should be about 15 nm. 2. Fabrication of Nanocrystals in the Polymer Gel Matrix. The crystal structures of PEO/CdCl2 and PEO/ HgCl2 complexes have been resolved by the fiber X-ray diffraction method.12 The unit cell structures of these crystal complexes are formed in a way that each Cd2+ (or Hg2+) is coordinated with four oxygen atoms from two PEO segments.12 The crystallization of CdCl2 complex was also observed in the present work. With 2 wt % of the saturated salt solution, the polymer system formed gels and exhibited a strong defuse SAXS scattering profile. By increasing the saturated salt solution to 5 wt %, the SAXS pattern, as shown in Figure 3, exhibited the formation of an ordered domain packing in the mixtures. The corresponding WAXD measurements (Figure 2) showed that besides the first peak (at 8.7°), seen immediately when complexation occurred in the mixture with only 2 wt % of the salt solution, the mixtures with higher concentrations of salt solutions also exhibited many diffraction peaks in the higher angular range, as seen in both Figures 2 and 4. This suggests that these ordered domains consist of nanocrystals of CdCl2/PEO complexes. With further increase in the salt concentration, more nanocrystals were formed in the mixture. As the interdomain distance of the (10) Liu, L.-Z.; Yeh, F.; Chu, B. Macromolecules 1996, 29, 5336. (11) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240. (12) Blumberg, A. A.; Pollack, S. S.; Hoeve, C. A. J. J. Polym. Sci., Part A-2 1964, 2, 2499.
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nanocrystals in the mixture was about 15 nm, the crystal size had to be less than 15 nm. There is direct evidence to support the argument that the salt-containing domains in the mixtures with 5 wt % or more of the salt concentration are due to the formation of nanocrystals consisting of CdCl2 and PEO segments. By comparing the diffraction peaks of the mixtures with those of pure CdCl2 (Figure 4), we found that the majority of the diffraction peaks were located at the same positions. However, several extra peaks around 8.7°, 24°, and 34° were also seen in the mixtures. These peaks were not present in the pure CdCl2 crystals. This observation confirmed that nanocrystals were formed through the complexation of both salt and PEO segments. In the presence of multiple ordered SAXS peaks, these nanocrystals should have relatively uniform size and wellordered packing. The total crystallinity in the 5%S/L35 is nevertheless quite small. The WAXD profiles changed with the concentration of the salt solution in an unusual way. The large amorphous background (Figure 2) from the pure L35 copolymer almost completely disappeared when the copolymer was mixed with 2 wt % of the salt solution. However, this amorphous peak appeared again when the salt concentration was increased to 5 wt % (the presence of multiple diffraction peaks could be attributed to the presence of nanocrystals). In other words, the amorphous peak in the mixture containing 5 wt % of the salt solution was very similar to that of the pure L35 block copolymer. This observation may be explained as follows. By comparing the 5%S/L35 mixture with the 2%S/L35 mixture, the salt in the former case could cocrystallize with the PEO blocks to form the nanocrystals, and thus had less noncrystalline CdCl2/PEO aggregate domains in the mixture. The reduction in the density of the noncrystalline aggregates would allow the free polymer segments, which had not participated in the crystallization process, to possess similar interchain interactions as the segments in pure EPE. Therefore, the amorphous halo, representing the short-range order of the polymer segment, could be seen again in the mixture with 5 wt % of the salt solution. When the salt concentration was increased from 5 to 10 wt %, the WAXD profiles exhibited distinct changes (again see Figures 2 and 4). First, several diffraction peaks disappeared in the mixture with 10 wt % of the salt solution, except for the one at 9.1° with a corresponding d spacing of 0.86 nm, indicating the complexation of salt with PEO blocks persisted. The disappearance of the diffraction peaks suggested that the 10%S/L35 mixture was largely in a noncrystalline state, i.e., the domains no longer contain nanocrystals. Furthermore, the broad X-ray scattering peak (2θ ≈ 18°) from the amorphous segments disappeared again in the 10%S/L35 mixture. The disappearance of this amorphous peak might indicate a significant change in the polymer conformation due to the high density of the physical cross-linking. At present, it is not clear what structural transformation could have happened in the system when the salt concentration changed from 5 to 10 wt %. However, the absence of the amorphous peak clearly suggests that the polymer chain segments no longer possess the short-range order. The SAXS peak positions remained nearly constant when the salt concentration was increased from 5 to 10 wt %. Therefore, the absence of the amorphous peak could suggest that the microdomains could simply become larger, so that the space for the chain segments, which had not participated in the crystallization, became more confined. Further increase in the salt concentration to 25 wt % produced many sharp diffraction peaks. These crystalline
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Figure 5. SAXS pattern of L35 mixtures with 25 and 40 wt % of the saturated (CdCl2 + H2O) solution. (The intensity shown for the sample with 40 wt % of the salt solution is multiplied by 6, to see the high-order peaks clearly; the much lower intensity was due to the larger absorption coefficient by CdCl2.).
Figure 6. Transmitted electron microscopy picture of L35 mixtures with 40 wt % of the saturated (CdCl2 + H2O) solution.
peaks were found to locate on a flat baseline, instead of on an amorphous halo as in the mixture with 5% of the salt solution. The distinct and sharp diffraction peaks with strong intensity indicates that the 25%S/L35 mixture possessed high crystallinity and large crystal size. The peak width in WAXD has been used to evaluate the crystallite size (L) by means of the Scherrer equation13
L ) 0.94λ/(B(2θ) cos θ)
(2)
where λ, 2θ, and B(2θ) are the wavelength, the diffraction angle, and the full width in radians at the half-maximum intensity of the diffraction peak, respectively. For each hkl-reflection, the value of L can be estimated as an average crystal dimension perpendicular to the reflecting planes. The sharp peak at 13.4° with a half-maximum intensity width of 0.185° from the mixture with 25 wt % of the salt solution was used for this purpose. Our calculated result showed that the crystal dimension based on this peak was about 40 nm. It should be pointed out that based on the earlier SAXS data, the maximum domain dimension for the mixtures with 5 and 10 wt % of the saturated salt solution was 12 nm. Therefore, the crystal size (40 nm) in the mixture with 25 wt % of the salt solution was much larger than the estimated domain size of 12 nm in the mixtures with 5 and 10 wt % of the salt solution. This discrepancy may be explained by examining our SAXS data for the mixtures at higher salt concentrations. Figure (13) Guinier, A. X-ray Diffraction; W. H. Freeman & Co.: New York, 1963; p 124.
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Figure 7. Schematic model for L35 mixtures with different amounts of saturated (CdCl2 + H2O) solution, showing the changes in microdomain morphology, domain packing, and WAXD from amorphous L35.
5 exhibits the SAXS pattern of the 25%S/L35 mixture, which is quite different from those of 5%S/L35 and 10%S/ L35 mixtures, as shown in Figure 3. First, the scattered intensity of all higher order peaks were much weaker than that of the 1st order peak, which appeared at a low q value of 0.48 nm-1, corresponding to a d spacing of 13 nm. The relative peak position ratios were 1:(2.5)1/2:2:4:5. The ratio (2.5)1/2 was unusual, suggesting that the microphase separation in this mixture could be in a morphological transition state. By increasing the salt solution further to 40 wt %, the SAXS pattern (Figure 5) showed a relative peak position ratio of 1: (3)1/2:2, which could suggest a cubic domain-packing structure or a structure with hexagonally packed crystal domains. For the mixture with 25 to 40 wt % of the saturated salt solution, the salt content increased by about 5 to 8 times when compared with the mixture at 5 wt % of the salt solution. In contrast, the crystallinity seen from WAXD patterns increased much more than those ratios when compared with the crystallinity of 5%S/L35. However, the interdomain distance of the crystals did not show an appreciable change, suggesting that the crystals might grow along one of the axis. Thus, the WAXD data indicated that the crystal dimension in 25%S/L35 and in 40%S/L35 was about 40 nm along one direction. It is conceivable that the crystal morphology could be rodlike in 25%S/L35 and 40%S/L35 mixtures. As the growth axis was randomly oriented in space, no orientation in the WAXD pattern was observed. Transmitted electron microscopy (TEM) was used to examine the crystal morphology in the 40%S/L35 mixture. For such a measurement, butyl acetate was used to disperse the nanocrystals from the mixture on the TEM grid. It is wellknown that butyl acetate might slightly dissolve or damage the nanocrystals. The dispersion of the sample on the TEM grid could also severely damage the crystal packing. Even
with such a drawback, the TEM image of the sample prepared from the 40%S/L35 mixture (Figure 6) showed some rodlike structures with a length scale of about 40 nm, which is consistent with our analysis. We note that the nanocrystals totally lose packing order in TEM, which is probably due to the use of butyl acetate solvent in sample preparation. Conclusions The salt-induced polymer gelation and subsequent formation of nanocrystals in Pluronic surfactant L35 (E11P16E11) with different amounts of saturated CdCl2 aqueous solutions can be schematically shown in Figure 7. On the basis of our results, the following conclusions can be drawn. The L35 surfactant can form gel with only 2 wt % of the saturated CdCl2 aqueous solution. By increasing the amount of the saturated salt solution (as low as 5 wt %) in the mixture, nanocrystals of CdCl2/PEO complexes were formed with 3-dimensional ordered packing. At higher concentrations of the salt solution (25 and 40 wt %), the nanocrystals can grow along one axis and undergo a morphological change from possible cubic to rodlike structures. Acknowledgment. This work was supported by the U.S. Department of Energy (DEFG0286ER45237.016) and the NSF Materials Research Science and Engineering Center (MRSEC) Program (DMR0080604). T.L. is thankful for the support of the U.S. Department of Energy, Division of Materials Science, under Contract No. DE-AC0298CH10886. The X27C beam line at the National Synchrotron Light Source was supported, in part, by the Department of Energy (DE-FG02-99ER45670). LA0204803
