Imaging of Ionic Channels in Proton Exchange Membranes by the

Jan 13, 2010 - The fidelity of the replica to the channels was confirmed by comparison of the SEM image of the nickel deposit to the transmission elec...
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Imaging of Ionic Channels in Proton Exchange Membranes by the Nickel Replica Method Wataru Kubo,* Kazuhiro Yamauchi, Kyoko Kumagai, Mamiko Kumagai, Kaoru Ojima, and Kenji Yamada Canon Inc., 3-30-2, Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: December 3, 2009

An imaging method for ionic channels in proton exchange membranes in wet conditions is reported. The ionic channels were replicated by the electrodeposition of nickel and then observed using scanning electron microscopy (SEM). The fidelity of the replica to the channels was confirmed by comparison of the SEM image of the nickel deposit to the transmission electron microscopy and atomic force microscopy image of the ionic channels using a sulfonated hydrocarbon block copolymer membrane, which keeps continuous ionic channels even in dry condition. By use of this method, the morphology of the ionic channels in a Nafion membrane in wet conditions was successfully observed. The nickel replica showed a continuous network of 5-10 nm wide channels, which is close to the reported value estimated using small-angle X-ray spectroscopy. The polymer electrolyte fuel cell (PEFC) is extensively developed as an efficient energy converter for automobiles, mobile electronic items and individual houses because of its advantages in compact size, quick response in start/stop operation, and ease of maintenance. A proton exchange membrane (PEM), a polymer membrane, is a key component of PEFC and many studies have reported about the effect of the chemical structure of the polymer on the performance of the PEM for PEFC.1,2 In the PEM, protons carry charges between anode and cathode through ionic channels, which consist of ionic functional groups. Recently several reports pointed out that the microstructure of the PEM, especially the structure of the ionic channels, significantly affects the performance of the PEM for PEFC.3,4 Until now many methods to investigate the microstructure of PEM have been reported such as small-angle X-ray spectroscopy (SAXS),5 transmission electron microscopy (TEM),6 and atomic force microscopy (AFM).7 Strangely enough, however, no useful the method is reported for imaging the cross sections of the ionic channels in the PEM in the wet condition, in which PEFC works. The lack of knowledge about the microstructure, which links the chemical structure with the performance, retards the systematic development of PEMs by complicating feedback from the performance of PEMs to the molecular design of the polymer. Therefore a high throughput imaging method, which allows systematic data collection, is needed to accelerate the development. Recently, Chou et al. reported an excellent idea to observe the ionic channels in a Nafion membrane.8 The idea is summarized as follows. Silver is electrodeposited in the ionic channels in a Nafion membrane on a conducting substrate. After removing Nafion in a solvent, the resulting silver replica of the channels was observed using SEM. However the reported method has two problems that need to be eliminated. First the silver deposit seems to not reflect the fine structure of the ionic channels because the image of the silver deposit in the reported figure showed a rodlike or particlelike structure being just a few micrometers in width, * To whom correspondence should be addressed. E-mail: kubo.wataru@ canon.co.jp. Phone: +81-3-3758-2111. Fax: +81-3-3757-3098.

which is far lager than that of the ionic channels in Nafion membrane estimated using SAXS.9,10 We considered that the large structure is formed by the breaking of the fine wall surrounding the ionic channels by the crystal growth of silver, which is known to form large crystals in elelctrodeposition.11 Second, Nafion is not an appropriate PEM to verify the imaging method of the ionic channels. The ionic channels in Nafion are known not to be connected in dry conditions and become connected with increasing water content of the polymer.9 Therefore the fidelity of the replica prepared using Nafion cannot be examined by comparing the morphology of the replica to that observed using existing imaging method such as TEM, which is usually done in dry conditions. In this study, we report a method to form the close replica of the ionic channels in PEMs. As a solution to the first problem, we chose nickel as a depositing metal. Nickel is known to form fine crystals by the effect of a slight amount of nickel hydroxide, which is formed in situ during the electrodeposition process and suppresses the growth of large crystals.11 Regarding the second problem, we used a sulfonated hydrocarbon block copolymer as a model phase separated system to confirm the fidelity of the replica. Since the ionic channels of the copolymer keep a continuous structure even in dry conditions,12,13 the use of the copolymer allows the comparison of the shape of the replica to the ionic channels observed using existing imaging methods such as TEM and AFM. The consistent shape of the replica in TEM and AFM images proves that this method works well as imaging method for the ionic channels in PEM in wet condition. By use of this method, the ionic channels in the Nafion membrane in wet conditions was observed. The replica shows a fine, three-dimensionally connected structure having a characteristic width close to that estimated using SAXS.9,10 This result proves that this method is applicable even to Nafion, which has very fine ionic channels that change the connectivity with water content. Scheme 1 shows the preparation of the nickel replica. Experimental Section Polystyrene-b-poly(ethylene/propylene)-b-polystyrene (SEPS) was provided by Kuraray. Sulfonated SEPS (S-SEPS, Chart 1) was synthesized from SEPS based on a reported procedure.14

10.1021/jp9082695  2010 American Chemical Society Published on Web 01/13/2010

Ionic Channels in PEMs by the Ni Replica Method

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SCHEME 1: Schematic Description of the Preparation of the Nickel Replica of the Ion-Conducting Channels in PEMs

The ratio of sulfonated polystyrene/total polystyrene was estimated to be 46% by 1H NMR in THF-d8. Nafion solution was purchased from Aldrich (Product No. 527084). All chemicals were used as purchased. Polymer membranes were prepared on a gold-deposited glass substrate by a bar-coating method. S-SEPS was dissolved in THF/methanol (8/2 wt %) mixed solvent and then coated onto the substrate. The thickness of the membranes was about 10 µm. The in-plane proton conductivity of the membrane was measured using a 1255WB system (Solartron) equipped with a four-point-probe attachment under controlled temperature and humidity. Four-point probe electrodes are widely used to measure the conductivity of PEMs. However, it is known that high accuracy cannot be expected because of some problems such as electric field inhomogeneity, nonplanar electrodes on the surface, poorly defined cross sectional areas, and electrode geometry. Here, we used the conductivity measured using the electrodes as an approximate value to confirm that the conductivity of S-SEPS can be used as a model of the microphase separated ionic conducting polymer. To obtain the sectional TEM image of the polymer membrane, an S-SEPS membrane was sliced using an ultramicrotome (80 nm thick) and stained by dipping in 2 wt % 12tungsten(VI) phosphoric acid aqueous solution for 10 min and dried before the observation. TEM images were obtained using a H-800 TEM (Hitachi). AFM images were obtained using a NanoNavi scanning probe microscope (SII) equipped with a SIDF-20 cantilever (SII) operated in tapping mode at 25 °C, about 40% relative humidity. Nickel was electrodeposited using a galvanostat and a three-electrode cell consisting of a nickel wire as a counter electrode, Ag/AgCl (NaCl) as a reference electrode, and 42 mM NiSO4, 7.8 mM NiCl2, 24 µM H3BO3 aqueous solution as an electrolyte. The active electrode area defined by a rubber ring was 0.48 cm2. Typical depositing conditions were 3.3 mA, 20 s for S-SEPS and 10 mA, 10 s for Nafion at 25 °C. Electrodeposited substrates were cut and dipped in THF (SSEPS) or ethanol (Nafion) to expose the section of the nickel deposit from the polymer matrices. Cross-sectional images were obtained using a S5500 SEM (Hitachi).

stability compared to perfluorinated polymers because of their partially aliphatic character.15 Here, we used S-SEPS as a model phase-separated system. The conductivity of the S-SEPS membrane was 2.6 × 10-2 S cm-1 at 50 °C, 80% relative humidity. The value is about 45% of that of a Nafion membrane. The TEM image of a stained S-SEPS section (Figure 1a) shows a micro-phase-separated structure. A dark region, strongly stained by the aqueous staining solution, represents a hydrophilic domain consisting of poly(styrene sulfonate) segments. A bright region, relatively poorly stained, represents a hydrophobic domain mainly consisting of poly(ethylene/propylene) segment. This image indicates that the S-SEPS membrane has continuous ionic channels even in the high vacuum condition, similar to a sulfonated hydrocarbon block copolymer membrane.12,13 The shape of both of the domains seems to indicate that the microphase separated structure is bicontinuous with reference to a previous paper.16 The width of the hydrophilic domain was 10-15 nm and that of the hydrophobic domain was 30-50 nm.

Results and Discussions A kind of polymer with similar chemical structure to S-SEPS is known to exhibit a rich microphase separated morphology and to keep continuous ionic channels even in dry conditions.12,13 However the polymers are also known to have a poorer oxidative CHART 1: Chemical Structure of S-SEPS Figure 1. (a) TEM image of the stained S-SEPS membrane section. (b) AFM phase image of the S-SEPS membrane surface. The phase scale is 0-8°. (c) Sectional SEM image of the nickel electrodeposited in S-SEPS and dipped in THF.

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The AFM phase image of the S-SEPS membrane (Figure 1b) also shows a microphase separated structure. A bright (stiff) region represents the hydrophilic domain and a dark (soft) region represents the hydrophobic domain. The AFM image shows that both domains form a continuous structure. The width of the hydrophilic domain was 10-20 nm and that of the hydrophobic one was 20-40 nm. The sectional image of S-SEPS observed using TEM and the surface image observed using AFM of the S-SEPS membrane indicates that the S-SEPS membrane evidently keeps connected ionic channels even in high vacuum or ambient condition and the microphase separated structure in the S-SEPS membrane is bicontinuous. The width of the hydrophilic domain shown in the AFM image is slightly larger than that in the TEM image, while the width of hydrophobic domain shown in the AFM image is slightly smaller than that in the TEM image. Because the difference is small, it could be insignificant comparing to the effect of AFM tip resolution or slight structural differences between the membrane surface (AFM) and the section (TEM). If it is a significant value, however, the difference can be explained by the water content in the membrane. The water content of the AFM sample observed under room humidity must be higher than that of the TEM sample observed under high vacuum. It is well-known that the PEM expands with increasing water content.17,18 S-SEPS films in ambient humidity showed about a 5% larger thickness than in dry condition. The swelling water is preserved in the hydrophilic domains and expands them. As a result, the hydrophilic domain observed using AFM showed a larger width than that using TEM. On the other hand, the difference of the width of the hydrophobic domains can be explained by the difference of the pressure of the hydrophilic domain. Since water molecules in the PEM should locate mainly in the hydrophilic domain comparing to the hydrophobic domain, the relative pressure of the hydrophilic domain to that of the hydrophobic domain should increase with increasing water content. The hydrophobic domains at the AFM observation should be under pressure and shrunk compared to that at the TEM observation through the increased pressure of the hydrophilic domains. As a result, the hydrophobic domains showed a smaller width using AFM than that using TEM. The micro-phase-separated structure of the block copolymer membrane is strongly affected by solvents used in the membrane preparation.19 In our previous study, a substantial difference of proton conductivity was observed using the PEMs having the same chemical structure but different micro-phase-separated structures. While a PEM having the bicontinuous structure exhibited significant conductivity as a PEM for FC, a PEM having hydrophilic domains isolated in a hydrophobic matrix did not exhibit high conductivity. For example, a PEM prepared using polystyrene-b-poly(3-(sulfonicacid)propylacrylate)-bpolystyrene THF solution forms spherical hydrophilic domains surrounded by a continuous hydrophobic domain and exhibited 1-2 orders of magnitude lower conductivity than that having the bicontinuous structure prepared using the same polymer THF/methanol (8/2 wt %) solution. The S-SEPS membrane exhibits significant conductivity attributed to the bicontinuous structure prepared from the THF/methanol (8/2 wt %) solution. The strong influence of the microphase separated structure on the properties of PEM suggests the importance of imaging the cross sectional structure. To form the replica of the ionic channels, nickel was electrodeposited in the S-SEPS membrane on a substrate. The potential observed during the electrodepositing process was stable at about 1.3 V. The resulting metal-polymer hybrid on

Kubo et al. the substrate had a black-colored homogeneous appearance. Dipping the hybrid in THF overnight dissolved the S-SEPS and exposed the nickel deposit. The cross sectional SEM image of the nickel deposit is shown in Figure 1c. The nickel deposit showed a continuous, branched, twisted, and knotted wirelike structure having 20-60 nm width. The space surrounding the nickel wire showed a continuous structure having 10-20 nm width. The thickness of the nickel deposit was about 8 µm, and the same morphology was observed in the entire membrane from the substrate interface to the top of the nickel deposit. The image of the nickel deposit in Figure 1c shows a similar morphology to the hydrophilic domains shown in TEM and AFM images. Here, we describe three potentially influential factors in the electrodeposition process, electric field, cation exchange, and the deposited nickel metal, perturbing the microphase separated structure of the S-SEPS. It is widely known that the microphase separated structure of the block copolymer can be aligned in an electric field.20,21 However, the alignment process needs high electric field and temperature, for example, 30 kV cm-1 and 250 °C.21 Because the electric field and the temperature used in this study was much lower than those (a few V cm-1 and 25 °C), the electric field applied in the process should be considered not to perturb the microphase separated structure of the S-SEPS. The cation of the S-SEPS is exchanged from proton to nickel ion by the dipping in the electrodepositing solution. We observed the surface of the S-SEPS membrane after dipping in the solution using AFM to investigate the effect of the cation exchange to the micro-phase-separated structure. The AFM image of the S-SEPS dipped in the solution (Figure S1 of Supporting Information) shows a slightly blurred but bicontinuous morphology similar to Figure 1b. This result supports that the exchange of the cation did not perturb the microphase separated structure of the S-SEPS significantly. In the previous paper, McLean et al. described the difference of the microphase separated structure of the polymer having the ionic segment between the proton form and the metal ion form as follows. In the case of the polymer having a carboxylic acid group, the proton form of the polymer did not exhibit a distinct phase separated structure while the metal ion form did. In the case of the polymer having a sulfonic acid group, both forms exhibited a distinct microphase separated structure because of the strong acidity of the sulfonic acid.7 Because S-SEPS showed the distinct microphase separated structure even in proton form and the ionic group is sulfonic acid, the effect of cation exchange on the microphase separated structure of the polymer can be considered insignificant in this study. The nickel crystals formed by the electrodeposition are smaller than those of silver as shown in Figure 1c, which does not show the large crystals destructing the microphase separated structure as in the previous paper.8 However the nickel ions, which did not locate in the ionic domain of the polymer in high concentration before the electrodeposition, concentrated and electrodeposited in the ionic domain as a nickel metal can perturb the structure by their volume. The concentrating process is described as follows. The nickel ion located in the ionic domain of the polymer is electroreduced and deposited as a nickel metal at the Au ionic domain interface. The nickel ion concentration around the interface decreased with the electrodeposition. More ions are supplied by diffusion, caused by the resulting concentration gradient, from the solution thorough the ionic channel and then are additionally electrodeposited. However, the electrodeposition of the nickel metal can be controlled by the depositing condition. For example, Figure S2

Ionic Channels in PEMs by the Ni Replica Method of Supporting Information shows the sectional SEM image of nickel deposit prepared using a 22× higher charge amount than that applied to prepare the sample shown in Figure 1c in the normal Watt’s bath composition. The image indicates that the microphase separated polymer structure was destructed and bulk nickel having 810 nm thickness, corresponding to 8% of S-SEPS membrane thickness, grew by the electrodeposition. (Efficiency of electrolysis is about 40%.) By assumption of the same efficiency and the density of bulk nickel, the total volume of the nickel deposited using the condition of the sample in Figure 1c is 0.37% of that of dry S-SEPS film. When the nickel is homogeneously located in the ionic channel of the polymer as Figure 1c, the volume increase of the ionic domain, which occupies about 22% of the total molecular weight of the polymer, by the electrodeposited nickel is estimated to be less than 2%. The large amount of water preserved in the ionic domain reduces further the value by increasing the basic volume of the ionic channel. This estimation indicates that the electrodeposited nickel prepared in appropriate condition does not perturb the microphase separated structure of the S-SEPS significantly. The morphology of the nickel deposit and the space shown in the SEM image exhibits a structure and dimension close to those of S-SEPS consisting of the hydrophilic and hydrophobic domain shown in the TEM and AFM images except for the width of the deposit or the hydrophilic domain and the space or the hydrophobic domain. The width of the nickel deposit shown in the SEM image is larger than that of the hydrophilic domain shown in the AFM image, and the width of the space is smaller than that of the hydrophobic domain in the AFM image. The difference of the width between the nickel deposit and the hydrophilic domain in the AFM image can be explained by the difference of the water content. Because the S-SEPS membrane was dipped in the aqueous electrolyte solution during the electrodepositing process, the water content of the membrane at the nickel deposition must be higher than that at the AFM observation carried out at 25 °C, about 40% relative humidity. The hydrophilic domain of the S-SEPS membrane at the electrodepositing process must be in a more expanded state than that at the AFM observation. On the other hand, the difference of the width between the space and the hydrophobic domain can be explained by the increased pressure of the hydrophilic domain caused from the increasing of the water content. These results confirm the fidelity of the nickel replica of the ionic channels prepared using this method. We chose nickel as a replicating metal instead of silver as used in the previous paper8 to avoid the destruction of the fine hydrophobic wall, surrounding the ionic channels, by the crystal growth of the metal. It is reported that silver is electrodeposited from a simple acid solution in a coarsely crystalline form like other metallic heavy elements such as copper, zinc and lead.11 In the early stage of this study, we also tried to replicate the ionic channels using silver. However, electrodeposited silver showed a coarse and grainy morphology. The fine nickel replica shown in Figure 1c supports that the change of the ion successfully works to suppress the crystal growth and form the fine structure. Not only the nature of the metal ion but also the electrodepositing condition plays an important role to form a close replica of the ionic channels. In this study, we mainly optimized three parameters, electrolyte concentration, deposition temperature, and current density. To form a close replica of the fine ionic channels surrounded by the very thin hydrophobic wall, our basic concept is suppressing the radial growth of the nickel wire,

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Figure 2. (a) AFM phase image of the Nafion membrane surface. The phase scale is 0-10°. (b) Sectional SEM image of the nickel electrodeposited in Nafion after dipping in ethanol.

which leads to collapse of the thin wall, and emphasizing the longitudinal growth. To meet the requirement, an ion diffusion limited condition is appropriate. This is explained as follows. When nickel ions were supplied in abundance through the ionic channels, the nickel wires grow not only in longitudinal direction but also radially. When the supply of the ions is sufficiently restricted, the wires grow along the ionic channels, through which the ions are supplied. A diluted Watt’s bath having about one-twenty-seventh the concentration to the normal bath was used as an electrolyte solution. The nickel concentration of Watt’s bath is usually around 1.35 M. However this concentration is too high for an ion diffusion limited condition. In the beginning, we tried to form the replica at this concentration. However, the resulting deposit was bulk nickel metal as shown in Figure S2 of Supporting Information. Usually the electrodeposition temperature using the normal Watt’s bath is around 40 °C; however, the high temperature enhances the diffusion of ions. To avoid this problem, the electrodeposition was carried out at 25 °C. Regarding the current density, high current density is suitable to drive the electrodepositing reaction to the ion diffusion limit; however the nickel deposit prepared using high current density, i.e., 10 mA cm-2, showed an inhomogeneous appearance caused by the rapidly electrodeposited nickel, which grew between the S-SEPS membrane and the substrate and separated the membrane from the substrate. Therefore, we used a current density of about one-tenth of that of an usual Watt’s bath (20 - 70 mA cm-2). The method confirmed using S-SEPS was applied to a Nafion membrane. The AFM phase image of Nafion is shown in Figure 2a. Dark (soft) regions represent the hydrophilic domains and bright (stiff) regions represent the hydrophobic domains. In the image, the hydrophilic clusters were embedded in the hydrophobic matrix as is reported for a AFM image.7 The size of the hydrophilic clusters is 3-10 nm, which is slightly larger than that reported in the previous paper.7 Nickel was electrodeposited in the Nafion membrane on the substrate with a 3× higher current than in the case of the S-SEPS

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membrane. The potential shifted gradually from -2.0 to -2.5 V with deposition time. The resulting deposit showed a gray to black, relatively inhomogeneous appearance compared to the deposit in the S-SEEPS membrane. The SEM image of the nickel deposit after removing Nafion is shown in Figure 2b. The nickel deposit showed a three-dimensionally connected wire like structure having 5-10 nm width. Unlike for S-SEPS, the morphology of the nickel deposit is quite different from that of the hydrophilic domains of Nafion in AFM images. The difference reflects the structural change of Nafion with the water content. The hydrophilic domain of Nafion grows with increasing water content from the state embedded in the hydrophobic domain as shown in the AFM image, to a three dimensionally connected state.9 The width of the nickel deposit shown in the SEM image is comparable to the width of the ionic channels in the connected state estimated to be 4-5 nm9 or 1.8-3.5 nm10 by using SAXS. The slightly larger width of the nickel replica than that of the ionic domain estimated using SAXS can be explained by the difference of the water content in the membrane. The deposition current applied to the substrate with Nafion to form the close replica of the ionic channels was higher than that in the case of S-SEPS. To form a close replica of the ionic channels, the electrodepositing reaction should be in the ion transport limited condition as described in the section of S-SEPS. Because the ionic conductivity of the Nafion membrane is higher than that of S-SEPS as reflected in the proton conductivity, the higher current was necessary to drive the electrodeposition in the ion transport limited range. These results confirm that we successfully observed the morphology of the ionic channels of the Nafion membrane in a swollen state, in which PEMFCs work. The replica of the ionic channels in the Nafion membrane shows a densely connected network consisting of narrower channels than in S-SEPS. In addition to the high acidity of the perfluoro-sulfonated polymer, the densely connected network of the ionic channels would contribute to the two times higher conductivity than in S-SEPS. In this paper, we report the imaging method of the ionic channels of PEMs in the condition of high water content, in which PEMFC works. The fidelity of the electrodeposited nickel replica of the ionic channels in a PEM was confirmed by the comparison of the TEM and AFM image of the sulfonated hydrocarbon polymer, retaining the ionic channels even under dry condition, to the sectional SEM image of the nickel deposit. Application of this nickel replica method to Nafion showed the

Kubo et al. image of the nickel replica of the ionic channels in Nafion having the width of 5-10 nm, which is close to the reported value determined using SAXS. This electrodepositing replica method is useful for the design and characterization of PEMs and can contribute to the development of PEMFCs. Acknowledgment. We are grateful to Dr. Hirokatsu Miyata and Dr. Otto Albrecht for reviewing the manuscript. Supporting Information Available: An AFM image of the S-SEPS membrane surface after dipping in the electrodepositing solution and a SEM image of the bulk nickel deposit on the substrate prepared using high charge in a normal Watt’s bath. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mauritz, K. A.; Moore, R. B. Chem. ReV. 2004, 104, 4535. (2) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463. (3) Ding, J.; Chuy, C.; Holdcroft, S. Chem. Mater. 2001, 13, 2231. (4) Bussian, D. A.; O’Dea, J. R.; Metiu, H.; Buratto, S. K. Nano Lett. 2007, 7, 227. (5) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307. (6) Porat, Z.; Fryer, J. R.; Huxham, M.; Rubinstein, I. J. Phys. Chem. 1995, 99, 4667. (7) McLean, R. S.; Doyle, M.; Sauer, B. B. Macromolecules 2000, 33, 6541. (8) Chou, J.; McFarland, E. W.; Metiu, H. J. Phys. Chem. B 2005, 109, 3252. (9) Gebel, G. Polymer 2000, 41, 5829. (10) Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75. (11) O’Sullivan, J. B. Trans. Faraday Soc. 1930, 26, 533. (12) Weiss, R. A.; Sen, A.; Pottck, L. A.; Willis, C. L. Polymer 1991, 32, 2785. (13) Yang, J.-E.; Lee, J.-S. Electrochim. Acta 2004, 50, 617. (14) Xu, K.; Li, K.; Khanchaitit, P.; Wang, Q. Chem. Mater. 2007, 19, 5937. (15) Hodgdon, R. B.; Jr, J. Polym. Sci. 1968, 6, 171. (16) Khandpur, A. K.; Fo¨rster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W.; Almdal, K.; Mortensen, K. Macromolecules 1995, 28, 8796. (17) Weiss, R. A.; Sen, A.; Willis, C. L.; Pottck, L. A. Polymer 1991, 32, 1867. (18) Hinatsu, J. T.; Mizuhara, M.; Takenaka, H. J. Electrochem. Soc. 1994, 141, 1493. (19) Huang, H.; Hu, Z.; Chen, Y.; Zhang, F.; Gong, Y.; He, T.; Wu, C. Macromolecules 2004, 37, 6523. (20) Amundson, K.; Helfand, E.; Quan, X.; Smith, S. D. Macromolecules 1993, 26, 2698. (21) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931.

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