Effects of Solvent Composition on the Assembly and Relaxation of

Article pubs.acs.org/Macromolecules

Effects of Solvent Composition on the Assembly and Relaxation of Triblock Copolymer-Based Polyelectrolyte Gels Kevin J. Henderson and Kenneth R. Shull* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: The role of solvent selectivity has been explored extensively with regard to its role in the phase behavior of block copolymer assemblies. Traditionally, thermally induced phase separation is employed for generating micelles upon cooling a block copolymer dissolved in a selective solvent. However few amphiphilic, polyelectrolytecontaining block copolymers demonstrate a thermally accessible route of micellization, and solvent exchange routes are frequently employed instead. Here, we describe the use of mixed solvents for obtaining thermoreversible gelation behavior of poly(methyl methacrylate)-poly(methacrylic acid)-poly(methyl methacrylate) (PMMA−PMAA− PMMA) triblock copolymers. One solvent component (dimethyl sulfoxide) is a good solvent for both blocks, and the second solvent component (water) is a selective solvent for the polymer midblock. Rheological frequency sweeps at variable solvent compositions and temperatures demonstrate an adherence to time−temperature−composition superposition, so that changes in the solvent composition are analogous to changes in the Flory−Huggins interaction parameter between end block and solvent. Shift factors used for this superposition are related to the effective activation energy describing the viscosity and stress relaxation response of the triblock copolymer gels. The effectiveness of solvent exchange processes for producing hydrogels with this system is shown to originate from the ability of a small amount of added water to greatly increase the relaxation times of the selfassembled polymer gels that are formed by this process.

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INTRODUCTION The rich phase behavior of block copolymer solutions and melts has been widely reported.1−3 In such assemblies solvent quality plays a fundamental role in the accessible phase space, defining the underlying micellar structure.4 In many organic solvents a strong temperature dependence of the end blocksolvent interaction parameter, χ, permits self-assembly of block copolymer solutions when cooled below an order−disorder transition temperature. When the polymer has an ABA-type triblock copolymer structure, this behavior results in a thermoreversible gelation behavior, as a physically associated network of midblock-bridged micelles forms a percolated structure. Examples include poly(styrene)−poly(isoprene)− poly(styrene) (SIS) block copolymers in squalane5 and tetradecane, 6,7 poly(styrene)−poly(ethylene−butadiene)− poly(styrene) in mineral oil,8 and acrylic polymers with poly(methyl methacrylate) end blocks in higher order alcohols.9,10 Amphiphilic block copolymers containing polyelectrolytes rarely demonstrate thermoreversible gelation behavior in aqueous solution. This results from the strongly disparate differences between favorable solvent conditions for the hydrophilic midblock and hydrophobic end block groups. Most commonly, micellization of block copolymers in aqueous solution is entropically driven, resulting in lower critical solution temperature (LCST) phase behavior.11,12 In some cases solvent exchange is a useful technique for inducing micelle formation,13 and advanced processing techniques are often employed for obtaining sophisticated morphologies for amphiphilic macromolecules in aqueous solution.14 For the triblock copolymer-based gels of interest to us, the solvent © 2012 American Chemical Society

exchange technique relies on composition-driven changes in the χ parameter characterizing the solvent/end block interaction. At the beginning of the solvent exchange process the end blocks are in good solvent conditions (low χ). Micelle formation is observed during solvent exchange when χ is increased to a critical value. The use of mixed solvents to tailor solvent selectivity in block copolymers dates back several decades15−18 and has been the focus of more recent investigations as well.19−22 For example, Fuse et al. have studied the formation of spherical morphologies of a unique amphiphilic copolymer in mixed solvents consisting of water and acetone.19 In work that is particularly relevant to the work that we describe here, Krishnan et al. demonstrated that changes in solvent composition can be equivalent to changes in temperature for triblock copolymer gels formed from a midblock-selective solvent.20 Figure 1 illustrates the phase behavior for a physically associated triblock copolymer gel as a function of χ for the solvent/end block system. In good solvent conditions (χ < 0.5) the polymer chains are fully dissolved in solution. As χ increases dynamic, solvent-swollen end block associations lead to the development of a fully percolated gel network, with successively larger χ values forming more static junctions of lower solvent content. In fully thermoreversible systems, such as acrylic block copolymers in alcohol,10,23 a strong temperature dependence of χ enables a large χ window to be accessed with a single solvent composition, simply by changing the temperature. For the Received: July 13, 2011 Revised: November 11, 2011 Published: January 24, 2012 1631

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similar behavior in PMMA has not been explored. Membranes of PMMA formed by phase inversion in water/DMSO have indicated that PMMA is insoluble for solutions containing 30% water though no lower values are reported.27 For the polyimide system in DMSO-water, a water content of only 0.3% was sufficient for phase separation of a 15% polymer solution. This result can be attributed to the fact that DMSO itself is a marginally good solvent for the polyimide. Because DMSO is a similarly marginal solvent for PMMA at room temperature (Θ temperature = 35 °C),28 a similar trend is expected for the DMSO−water−PMMA system. Here we explore the use of mixed solvents of DMSO−water for forming thermoreversible gels of PMMA−PMAA−PMMA triblock copolymer. Oscillatory shear rheometry is employed to dynamically define the onset of gelation induced by cooling, and frequency sweeps across regular temperature intervals are used to generate a master curve of the frequency-dependent rheological response across temperature and composition. Small angle X-ray scattering is used to characterize micelle size, and the ability to obtain reproducible hydrogels by vapor phase solvent exchange is discussed.

Figure 1. Schematic illustrating the effect of solvent quality and temperature on the phase behavior of physically associated triblock copolymer gel networks. Here, a narrow χ window is accessible across an experimentally accessible range of temperatures, T. Composition A and composition B represent two fixed solvent compositions, with composition B (higher χ) corresponding to a greater concentration of the poor solvent for the associating end blocks of the triblock copolymer.

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mixed solvent systems described in this paper, a smaller χ window is accessible for a given solvent composition, but the location of the accessible χ window can be shifted by changing the solvent composition itself. For example, increasing the fraction of the solvent mixture that acts as a poor solvent for the end block increases the effective value of χ, shifting the χ window from “composition A” to “composition B” in Figure 1. Within each of these windows, χ can be adjusted by changing the temperature, with an increased temperature giving a decrease in χ. The use of mixed solvents to induce micelle formation is quite general, as is the connection between micelle formation and the rheological properties of the block copolymer solution. We are interested in intermediate copolymer concentrations, where the copolymer concentration is large enough so that a one-phase, percolated network is formed,24 but low enough so that a crystalline array of micelles is not formed.4,25 In this regime, corresponding to all of the experiments performed here, the structural features of the micellar gel do not change significantly as χ increases, but the characteristic relaxation time of this gel increases dramatically.23 The process illustrated in Figure 1 can be used to form hydrogels with a well-controlled and reproducible structure, provided that water is the solvent responsible for the increase in χ for the solvent/end block system, and that water and the cosolvent are both good solvents for the midblock of the triblock copolymer. This process has been used previously to form poly(methyl methacrylate)-poly(methacrylic acid)-poly(methyl methacrylate) (PMMA−PMAA−PMMA) triblock copolymer hydrogels, beginning with solutions in dimethyl sulfoxide (DMSO) and inducing gelation by the addition of water by vapor phase transfer.13 With respect to Figure 1, this solvent exchange process is analogous to passing from the bottom of the figure (small χ) to the top (large χ). For a mixed solvent system, this process can be thought of in the context of ternary phase diagram in which the ratio of solvents used determines whether the polymer is present in a homogeneous solution or a micellar network of end block aggregates that are bridged by midblock chains. Such behavior has been studied previously for polyimide systems in water-DMSO,26 though

EXPERIMENTAL SECTION

Sample Information. The triblock copolymer was synthesized via anionic polymerization as described previously,13 and was initially composed of a poly(tert-butyl methacrylate) (PtBMA) midblock and poly(methyl methacrylate) (PMMA) end blocks. The PtBMA midblock length and copolymer polydispersity (PDI = 1.12) were assessed using gel-permeation chromatography (Waters 717plus autosampler connected to a Waters 2410 refractive index detector) in HPLC grade tetrahydrofuran against polystyrene standards. The end block lengths were determined using proton nuclear magnetic resonance spectroscopy (Inova 400 MHz) by comparing integrated peak ratios of the methoxy protons in PMMA (δ = 3.60 ppm) versus the tert-butyl protons in PtBMA (δ = 1.43 ppm) of known molecular weight. The amphiphilic copolymer was obtained by hydrolysis of the tBMA units using a 3 times molar excess of hydrochloric acid (relative to the concentration of tBMA units) in 1,4-dioxane at 80 °C for 6 h. The molecular weights of each block in the final converted polymer are 43k−151k−43k, corresponding to a degree of polymerization of 430−1750−430. For these experiments, 7 wt % solutions of copolymer were dissolved in mixed solvents containing DMSO and water. DMSO(Sigma-Aldrich) was used as received, and solutions were mixed using nanopure water (18.2 MΩ·cm). Rheological Characterization. Rheological measurements were conducted on an Anton Paar Physica Modular Compact Rheometer 300 with a double gap Couette fixture and a Peltier heating/cooling element. Gels were loaded in the liquid state (above 75 °C) into the fixture. Temperature sweep data were collected on cooling at 1 °C/ min with an angular frequency of 10 s−1 and constant strain amplitude of 1%. Master frequency curves were generated through extension of frequency sweeps from 0.1 to 1 s−1 under controlled stress (1−7 Pa) in the linear range at 5 degree intervals from 20 to 75 °C, with shift factors generated by superposition of the loss angle at a reference temperature of 25 °C. Small-Angle X-ray Scattering. SAXS measurements were taken at Sector 5ID-D of the Advanced Photon Source at Argonne National Laboratory at a sample-to-detector distance of 4.0 m and a energy of 17.0 keV with gel samples placed in nylon washers capped with polyimide sheeting. Consequent scattering patterns were fit using a Percus−Yevick hard sphere approximation29,23 to obtain the average micelle size and spacing.

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RESULTS AND DISCUSSION Thermoreversible Gelation. The dynamic gelation behavior within these materials is readily apparent by a sharp

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increase in the modulus upon cooling. Rheologically, the crossover between viscous solution behavior and elastic gel behavior can be defined as the intersection between the storage (G′) and loss (G″) moduli at a specified frequency. This temperature is referred to as the gelation temperature, TGEL, and corresponds to the temperature at which the relaxation time of the material approaches the inverse of the angular frequency of the measurements, in our case 0.1 s. This dynamic signature of gelation has reproducibly been used to define the onset of elastic character in thermoreversible gels in previous work.9,23,30 For the concentration regime of interest to us, which is well above the critical concentration for gelation, this approach is more useful than the commonly used Winter− Chambon approach31 used to detect the onset of a percolating network.10 Figure 2 depicts the temperature sweep rheological data upon cooling at 1 °C/min for 7 wt % polymer mixtures in 4%,

Figure 3. (a) Time−temperature superposition master curves illustrating storage (filled) and loss (unfilled) moduli for 7 wt % copolymer gels in DMSO-water mixtures containing 4% (red circles), 6% (blue squares), and 8% (green triangles) water. Reference temperature: 25 °C. Terminal slopes at low frequencies reflect G′ ∝ ω2 (dashed lines) and G″ ∝ ω (solid line) behavior. (b) Time− temperature−composition superposition of the data from part a, obtained using the composition shift factors, as, denoted by the inset.

constant, changing the solvent quality is analogous to changing the temperature. As a result, each of these master curves generated through time−temperature superposition at a given solvent composition can be shifted horizontally to a reference solvent content to generate a single plot depicting the universal relaxation behavior for the polymer concentration studied. This plot is depicted in Figure 3b, and includes data for 7 wt % gels containing 4%, 5%, 6%, 7%, and 8% water in DMSO. Here, the reference composition where the composition shift factor, as, is equal to one is set at 4% water. These composition shift factors are plotted as a function of the solvent composition in the inset of Figure 3b. Activation Energies. Apparent activation energies, ΔHapp for end block pull-out can be extracted from the temperature dependence of aT.32,33 The apparent activation energy is obtained by assuming that the temperature dependence across the experimentally accessible range follows a simple Arrhenius form:

Figure 2. (a)Temperature dependence of storage (filled) and loss (unfilled) moduli at ω = 10 s−1 for 7 wt % gels in mixed solvents of DMSO-water containing 4% (red circles), 6% (blue squares), and 8% (green triangles) water. (b) Values of TGEL for these same gels as a function of water content.

6%, and 8 wt % water solutions in DMSO and the resulting values of TGEL. For the 4% water solution, the crossover between G′ and G″ occurs near 30 °C, which is just above the freezing point of DMSO (∼18 °C). An increase in water content to 6% results in an increase of TGEL to 50 °C. A further increase to 8% water increases TGEL to 65 °C. Gelation in these systems is driven by changes in the PMMA solubility associated with changes in either temperature or solvent composition. The fact that only 4% water is needed to induce gelation at temperatures above the freezing point of DMSO indicates that PMMA becomes insoluble in DMSO-water mixtures for water contents that are quite small. A similarly small composition window for solubility DMSO-water mixtures has been observed for other polymers as well, including polyimides.26 Master Curves. Master time−temperature superposition curves for these gels are shown in Figure 3a and elucidate the frequency dependence of the rheological response. Similar to acrylic block copolymers in 2-ethyl-1-hexanol,23 these mixed solvent gels demonstrate the response of a viscoelastic liquid at low frequencies (high temperatures), as evidenced by the power law slopes of 1 and 2 for G″ and G′, respectively. At a frequency unique to each sample, the storage and loss moduli cross, and in the limit of high frequencies (low temperature), G′ approaches a plateau value (G0) demonstrating the response of an elastic solid. This plateau modulus is most evident for the 8% water sample, which has a 45° range of accessible temperatures between TGEL and the freezing point of DMSO. The 4% sample, in contrast, only has a 10° window, making the elastic plateau region inaccessible. However, as depicted by Figure 1b, while the concentration of the polymer is kept

aT = AeΔHapp/ RT

(1)

While the behavior in or case is not strictly Arrhenius, we use this analysis as a means to quantify the temperature dependence of the relaxation times within the gels. Figure 4a

Figure 4. (a) Arrhenius plot depicting the temperature dependence of shift factors used for generating master curves in 7 wt % DMSO−water gels containing 4% (red circles), 6% (blue squares), and 8% (green triangles) water. (b) Values of the apparent activation energy, ΔHapp, for these same gels as a function of water content.

shows the temperature dependence of aT for solvents with 4, 6 and 8% water, and Figure 4b shows the value of ΔHapp as a function of water content. For the gels studied here, ΔHapp ranges from 150 to 300 kJ/mol, increasing with increasing water content. These values are larger than the determined 1633

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activation energies for poly(ethylene glycol) chains with alkyl end groups in aqueous solution (ΔHapp∼ 70 kJ/mol),34 and are comparable to SIS copolymers in squalane (ΔHapp ∼ 200 kJ/ mol).5 Triblock copolymers with PMMA end blocks in higher alcohols show larger apparent activation energies, typically between 350 and 500 kJ/mol.10,23 The large value of the apparent activation energy results from a changing value of the glass transition temperature as the solvent quality changes with temperature. Relaxation Behavior. The advantage of employing a mixed solvent in these systems is the ability to tailor both the magnitude and temperature dependence of the relaxation time, while maintaining a similar network structure. The ability to superpose data obtained at different temperatures and different solvent contents is consistent with our expectation that these gels are structurally similar, with a plateaus modulus, G0, that is determined by the gel structure. For our gels this plateau modulus is ∼400 Pa (see Figure 3). The characteristic relaxation time, τ, obtained from the following equation:

τ=

Figure 6. SAXS patterns for 7 wt % gels in mixed solvents of DMSO containing 4% (red), 6% (blue), and 7% (green) water and DMSO solutions after 3 h (dark gray, dashed) and 24 h (black, dotted) of vapor phase exchange with water.

the solvent composition. These results suggest that these samples experience similar thermodynamic conditions during cooling (though at different temperatures) before the structures of these micelles become kinetically frozen below their gelation temperatures. These results provide important insight into the ability of a simple vapor phase solvent exchange process to produce gels with consistent properties.13 The process involves exposing a DMSO solution of PMMA−PMAA−PMMA triblock copolymer to water vapor, with gelation occurring as water diffuses into the DMSO solution. Scattering patterns from samples after 3 and 24 h exposures to water vapor are compared to the mixed solvent samples in Figure 6. The scattering intensity for these samples is substantially higher than the samples with only small amounts of water added. This result is attributed to the deswelling of the PMMA micelle cores, which enhances the contrast with the surrounding solvent.23 The location of the scattering peak is roughly equivalent between all mixed solvent samples and the vapor phase exchange samples, indicating little change in structure. These results suggest that during the vapor phase exchange, a homogeneous micellar structure can be obtained because only a small volume of water is necessary to induce micellization, after which the micelle size and structure are kinetically frozen. During the exchange process, a diffusionlimited water penetration front produces the nanophase structure within the gel before any substantial gel swelling from the added solvent is observed.

η0 aT G0

(2)

where η0 is the zero shear viscosity obtained from the low frequency (high temperature) data, i.e. the limiting value of G″/ ω. The viscosity is proportional to the composition shift factor as and is strongly dependent on solvent composition, as shown in Figure 5a. Over the composition range studied(4−8 wt %

Figure 5. (a) Zero-shear viscosity, η0, as a function of water content for 7 wt % polymer gels at the reference temperature, 25 °C. (b) Relaxation times in DMSO−water mixtures containing 4% (red circles), 6% (blue squares), and 8% (green triangles) water as a function of T − TGEL. Data previously collected for acrylic copolymers in 2-ethyl-1-hexanol are plotted in black.23

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water), η0 at a reference temperature of 25 °C increases by 7 orders of magnitude as the water content is increased from 4% to 8%. Relaxation times obtained from eq 2 are plotted in Figure 5 as a function of the distance from the gel temperature. Published data for PMMA-based triblock gels in 2-ethyl hexanol are included as well. Note that the alcohol-based gels show a 10 order of magnitude difference in relaxation times over a 40 degree temperature interval, whereas the relaxation times for the DMSO-water gels change by ∼5 orders of magnitude over the same range. Small Angle X-ray Scattering. The SAXS patterns for samples of mixed solvent content ranging from 4 to 7% water are shown as the solid lines in Figure 6. Each of these patterns is consistent with the formation of a disordered array of spherical micelles, which is expected for these particular concentrations and relative copolymer block lengths.23,35 A more detailed fit to the Percus−Yevick model23,29 gives an average domain spacing of 65 nm that is nearly independent of

CONCLUSIONS

Here we demonstrate the thermoreversible formation of polyelectrolyte gels from DMSO/water mixtures, using triblock copolymers with poly(methyl methacrylate) endblocks and a poly(methacrylic acid) midblock. Rheological characterization at various temperatures and solvent compositions can be combined into a single master curve of the frequencydependent storage and loss moduli. This result, in conjunction with small-angle X-ray scattering results, indicates that gels with similar structures but dramatically different relaxation times are formed for different solvent compositions. The effect of changing either temperature or solvent composition is to change the effective interaction parameter between the solvent and the end blocks. Gelation in these systems is defined dynamically, in terms of the relaxation time of the polymer gel. Specific results are as follows: 1 A water content of 4 wt % in the solvent mixture is sufficient to induce gelation at room temperature. 1634

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2 The gel temperature increases by about 30 °C when the water content of the solvent is increased from 4 wt % to 8 wt %. 3 The effective activation energy describing the temperature dependence of the relaxation times increases from 100 to 300 kJ/mol as the water content of the solvent is increased from 4 wt % to 8 wt %. These results indicate that the vapor phase exchange technique for micelle assembly and gel formation is successful because only a small amount of water is necessary to generate micelles in these networks. The micelles quickly become kinetically frozen as the viscosity-dependent relaxation behavior jumps by orders of magnitude with only a few percent increase in water content. As water is absorbed by the polymer-DMSO solutions, gel formation occurs at an overall polymer concentration that is very close to the starting polymer concentration in the original DMSO solutions.

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(14) Hayward, R. C.; Pochan, D. J. Macromolecules 2010, 43, 3577− 3584. (15) Tuzar, Z.; Kratochv., P. Makromol. Chem. 1972, 160, 301−311. (16) Mandema, W.; Emeis, C. A.; Zeldenrust, H. Makromol. Chem., Macromol. Chem. Phys. 1979, 180, 2163−2174. (17) Bednar, B.; Devaty, J.; Koupalova, B.; Kralicek, J.; Tuzar, Z. Polymer 1984, 25, 1178−1184. (18) Duval, M.; Picot, C. Polymer 1987, 28, 798−803. (19) Fuse, C.; Okabe, S.; Sugihara, S.; Aoshima, S.; Shibayama, M. Macromolecules 2004, 37, 7791−7798. (20) Krishnan, A. S.; Seifert, S.; Lee, B.; Khan, S. A.; Spontak, R. J. Soft Matter 2010, 6, 4331. (21) Ando, K.; Yamanaka, T.; Okamoto, S.; Inoue, T.; Sakamoto, N.; Yamaguchi, D.; Koizumi, S.; Hasegawa, H.; Koshikawa, N. IOP Conf. Ser.: Mater. Sci. Eng. 2010, 14, 012012. (22) Ando, K.; Yamanaka, T.; Okamoto, S.; Sakamoto, N.; Yamaguchi, D.; Koizumi, S.; Hasegawa, H.; Koshikawa, N. J. Phys.: Conf. Ser. 2010, 247, 012040. (23) Seitz, M.; Burghardt, W.; Faber, K.; Shull, K. Macromolecules 2007, 40, 1218−1226. (24) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1994, 27, 5654−5666. (25) Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J.; Ryu, C. Y.; Lodge, T. P.; Gleeson, A. J.; Pedersen, J. S. Macromolecules 1998, 31, 1188−1196. (26) Kim, J. H.; Min, B. R.; Won, J.; Park, H. C.; Kang, Y. S. J. Membr. Sci. 2001, 187, 47−55. (27) Lin, D. J.; Chang, C. L.; Lee, C. K.; Cheng, L. P. Eur. Polym. J. 2006, 42, 2407−2418. (28) Moyses, S. Int. J. Polym. Anal. Characteriz. 2008, 13, 413−427. (29) Percus, J. K.; Yevick, G. J. Phys. Rev. 1958, 110, 1−13. (30) Erk, K.; Shull, K. Macromolecules 2011, 44, 932−939. (31) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367−382. (32) Tanaka, F.; Edwards, S. F. J. Non-Newtonian Fluid Mech. 1992, 43, 273−288. (33) Tanaka, F.; Edwards, S. F. J. Non-Newtonian Fluid Mech. 1992, 43, 289−309. (34) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37, 695−726. (35) Seitz, M. E.; Burghardt, W. R.; Shull, K. R. Macromolecules 2009, 42, 9133−9140.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

This material is based on work supported under a National Science Foundation Graduate Research Fellowship. Support from the NSF DMR Polymers Program (DMR-0907384) and the NSF MRSEC program (DMR-0520513) is also acknowledged. Portions of this work were performed at the DuPont− Northwestern−Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center located at Sector 5 of the Advanced Photon Source. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the U.S. National Science Foundation through Grant DMR9304725, and the State of Illinois through the Department of Commerce and the Board of Higher Education Grant IBHE HECA NWU 96. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract W-31-10.

(1) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091−1098. (2) Hamley, I. The Physics of Block Copolymers; Oxford University Press: 1998. (3) Matsen, M. W.; Thompson, R. B. J. Chem. Phys. 1999, 111, 7139−7146. (4) Lodge, T. P.; Pudil, B.; Hanley, K. J. Macromolecules 2002, 35, 4707−4717. (5) Vega, D. A.; Sebastian, J. M.; Loo, Y. L.; Register, R. A. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2183−2197. (6) Watanabe, H.; Sato, T.; Osaki, K.; Yao, M. L.; Yamagishi, A. Macromolecules 1997, 30, 5877−5892. (7) Watanabe, H.; Sato, T.; Osaki, K. Macromolecules 2000, 33, 2545−2550. (8) Laurer, J. H.; Mulling, J. F.; Khan, S. A.; Spontak, R. J.; Bukovnik, R. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 2379−2391. (9) Drzal, P.; Shull, K. Macromolecules 2003, 36, 2000−2008. (10) Inomata, K.; Nakanishi, D.; Banno, A.; Nakanishi, E.; Abe, Y.; Kurihara, R.; Fujimoto, K.; Nose, T. Polymer 2003, 44, 5303−5310. (11) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414−2425. (12) Alexandridis, P.; Lindman, B. Amphiphilic block copolymers: selfassembly and applications; Elsevier Science: Amsterdam, 2000. (13) Guvendiren, M.; Shull, K. Soft Matter 2007, 3, 619−626. 1635

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