pH-Responsive Aqueous Foams Stabilized by ... - ACS Publications

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pH-Responsive Aqueous Foams Stabilized by Hairy Latex Particles Syuji Fujii,*,† Michiru Mochizuki,† Kodai Aono,† Sho Hamasaki,† Ryo Murakami,§ and Yoshinobu Nakamura†,‡ †

Department of Applied Chemistry, Faculty of Engineering, and ‡Nanomaterials Microdevices Research Center, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan § Department of Chemistry of Functional Molecules, Konan University, 8-9-1 Okamoto, Kobe 658-8501, Japan W Web-Enhanced bS Supporting Information b

ABSTRACT: Polystyrene (PS) latex particles carrying pHresponsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair (PDEA-PS particles) were synthesized by dispersion polymerization and characterized in terms of diameter, diameter distribution, morphology, chemical composition, surface chemistry, and pH-response using scanning electron microscopy (SEM), elemental microanalysis, 1H nuclear magnetic resonance spectroscopy, the laser diffraction method, and zeta potential measurements. The hairy particles can act as pHresponsive stabilizers of aqueous foams by adsorption at the air
water surface. Above pH 8.0, where particles have nonprotonated PDEA hair, which is relatively hydrophobic, particle-stabilized foams are stable for at least 1 month. Optical microscopy and SEM confirmed that flocculated PDEA-PS latex particles were adsorbed at the air
water interface and stabilized the aqueous foams. At pH 6.1 and 7.1, relatively stable foams can be prepared that remain stable for at least 24 h. SEM studies indicated that the PDEA-PS particles were adsorbed at the air
water interface as a monolayer at pH 6.1. At pH 5.1 and 3.1, where the particles have cationic water-soluble PDEA hairs with hydrophilic character, no foam was formed. Rapid defoamation can be induced by lowering the solution pH; the addition of acid caused the in situ protonation of 2-(diethylamino)ethyl methacrylate residues, which impart watersoluble hydrophilic character to the PDEA hair, and the PDEA-PS particles desorbed from the air
water interface. The foaming and defoaming cycles could be repeated at least five times.

’ INTRODUCTION Gas-in-liquid foams occur as intermediates or end-products in diverse industrial sectors, including food manufacturing, cosmetic formulations, and personal care products, as well as in the synthesis of porous materials.1
5 To prevent macroscopic phase separation of the gas and liquid, ionic or nonionic surfactants or polymeric stabilizers (including proteins), which adsorb to gas
liquid interfaces, are usually added. In addition to these molecular-level foam stabilizers, it has been known that finely divided particles can stabilize liquid foams by adsorbing to gas
liquid interfaces.6
15 The adsorption of particles at an air
water interface is critically dependent on the hydrophobicity of the particle, which can be quantified by the contact angle, θ (measured through aqueous phase).1 The angle θ increases with an increase of the hydrophobicity of the particle. Particles have been reported to best stabilize foams when θ is between 43° and 90°, where the particles are partially wetted by both the gas and the liquid phases.13c The energy ΔG required to remove a spherical particle from an air
water interface is at a maximum when θ = 90°. For the submicrometer-diameter particles used in the present study, ΔG is several orders of magnitude greater than the thermal energy. Therefore, the particles are in effect irreversibly bound, in marked contrast to surfactant molecules that adsorb and desorb r 2011 American Chemical Society

reversibly, and foams stabilized by particles of appropriate wettability at the air
water interface exhibit excellent long-term stability.1 On the other hand, the adsorption energy of particles at air
water interface with a contact angle near 0° is so low that the particles cannot form stable foams and aqueous particle dispersions are obtained. The destruction of foams (defoaming) is often required in practical applications such as paper and oil industries.16 However, there is a limited number of examples regarding in situ defoamation. It has been shown that the stability of foams prepared using stimulus-responsive latex particles can be controlled by adjustment of, for example, pH. The contact angle of the particles and consequently the energy required to remove particles from the air
water interface are modified in this way, leading to desorption. Thus, adjustment of the dispersion pH led to the complete defoamation of air-in-water foams when using pH-responsive particulate foam stabilizers. Recently, poly(acrylic acid)-stabilized polystyrene (PS) particles have been reported to act as a pH-responsive particulate foam stabilizer:17 the poly(acrylic acid) stabilizer used in the previous study had polydisperse molecular Received: August 6, 2011 Revised: September 12, 2011 Published: September 12, 2011 12902

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Figure 1. Application of PS particles carrying poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair (PDEA-PS latex particles) as a pH-responsive particulate foam stabilizer.

weight distribution and was adsorbed to the particle surface in looptrain-tail configuration. In this system, particulate foams stabilized under acidic conditions can be destabilized by the addition of aqueous alkaline solution; poly(acrylic acid) on the PS particle surface became deprotonated and hydrophilic upon addition of aqueous alkaline solution and the PS particles desorbed from the air
water interface, which led to defoamation. Herein, we describe the synthesis of PS latex particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair with narrow molecular distribution (Mw/Mn < 1.2) (PDEA-PS particles) and evaluate their ability as a pH-responsive particulate foam stabilizer and acid-induced defoaming agent (Figure 1). In this study, the pHresponsive component is a polybase rather than a polyacid as used in our previous study.17 Detailed characterization of the particlestabilized foams was conducted with respect to their stability, structure, and pH response.

’ EXPERIMENTAL SECTION Materials. Styrene, 2-(diethylamino)ethyl methacrylate (DEA; 99%), isopropanol (IPA; purity 99%), and aluminum oxide (activated, basic, Brockmann 1, standard grade, ∼150 mesh, 58 Å) were purchased from Sigma-Aldrich. Styrene was treated with basic alumina to remove the inhibitor and then stored at
18 °C prior to use.11,18 2,20 -Azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (VA-086) was provided by Wako Chemicals. NaCl (99.5%), NaOH (98%), HCl aqueous solution (37%), and ammonia aqueous solution (28%) were purchased from Sigma-Aldrich. A PDEA-based macroazoinitiator with a degree of polymerization of 60 (the degree of polymerization of the PDEA chain beside the central azo group) was synthesized by atom transfer radical polymerization, as reported previously.18 Deionized water (97% protonated form can be well dispersed in aqueous media. In contrast, above pH 7.1 (near and above the pKa value of 7.6 for PDEA), the PS particles were flocculated, as indicated by a significant increase in the apparent particle diameters (above 4 μm) and diameter distributions. In alkaline media, amine groups were deprotonated and the PDEA stabilizer was precipitated, which led to flocculation of the PDEA-PS particles. The laser diffraction results are in good agreement with those obtained by OM and dynamic light scattering studies (see Figures 3b, S2, and S3). Colloidally stable submicrometer-sized particles at pH 3.1 were in Brownian motion, which was observed by OM, and micrometer-sized flocs could be observed at pH 10.0. Furthermore, the aqueous dispersions at and above pH 7.1 showed signs of sedimentation at the bottom of the vessel and adsorption of the PDEA-PS particles at the air
water interface. These results also confirmed that the PDEA hair is present on the surfaces of the PS latex particles; the hair is protonated and solvated in the acidic aqueous media and acts as an effective colloidal stabilizer, but is deprotonated and precipitated in the alkaline media. Adjusting the pH back to 3 from 10 led to redispersion of the PDEA-PS latex particles, and the Dv was approximately 650 nm after the pH cycle, which is almost the same as that measured before the addition of the base. This pH-modulated dispersion
flocculation cycle was fully reversible at least five times (see Figure S4 and Table S1). Foam stability is dependent on the wettability of the particles at the air
water interface;15 therefore, it was expected that the behavior of foams stabilized with these PDEA-PS particles would change significantly at pH values close to the pKa of the particles in the bulk aqueous dispersion. To evaluate foamability and foam stability, the height of foam layer was measured after shaking glass vessels containing aqueous dispersions of the PDEA-PS latex particles (5.0 wt %) at different pH’s. Figure 5 shows the height of the foam layer, defined as the distance between the 12905

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Langmuir foam/dispersion boundary and the three-phase contact line of air, glass, and foam, as a function of the pH of the aqueous dispersion, immediately after preparation and for some time afterward. Reasonably stable foams, which were stable for at least 1 month, were obtained at high pH (g8.1). In these cases, the foam drains partly, and the aqueous phase separates into a supernatant and sediment phase. The height gradually decreases with time due to water drainage and partial coalescence of air bubbles. It appears that stable foams are only formed under conditions where particles are flocculated in bulk (pH > 7). A small number of bubbles were formed at intermediate pH (6.1, 7.0), but these disappeared within 1 month. No foam could be

Figure 5. pH-dependent behavior of foams prepared using PDEA-PS latex particles (5.0 wt %, 0.1 M NaCl). (a) Photograph of vessels taken 1 week after shaking aqueous dispersions of particles at different pH values. (b) Height of the foam layer as a function of pH of the aqueous dispersion recorded at different times: immediately after preparation ([), after 24 h (9), after 1 week (4), and after 1 month (b).

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prepared at low pH (e.g., 3.1, 4.1, 5.1), where the particles have cationic PDEA hair. Kettlewell et al.11c reported that charge-stabilized cationic PS particles adsorb strongly at a negatively charged air
water interface21 and function as an effective foam stabilizer. Contrary to the results shown by Kettlewell et al., the cationic PDEA-PS hairy particles could not stabilize foams below pH 5.1, where the PS particles have protonated water-soluble PDEA hairs. One possible factor for preventing adsorption of the cationic PDEAPS particles may be an image charge effect at the air
water interface. If charge Q is located in an aqueous medium with a larger dielectric constant than air, then the image charge Q0 is expressed using the following equation:22   ðεa
εw Þ 0 Q ¼
Q ð4Þ ðεa þ εw Þ where εa and εw are dielectric constants of air and water, respectively. Assuming that εa = 1 and εw = 78, then eq 4 gives Q0 as 0.957Q. This means that there are image charges of the cationic PDEA-PS particles that have the same sign and almost the same valency. The PDEA-PS latex particles have higher charge due to the protonated cationic PDEA hair than the charge-stabilized PS particles carrying amidine groups arising from the cationic radical initiator on their surface.11c It is calculated that one PDEA-PS particle has 1.46  107 amine groups, whereas one charge-stabilized cationic PS particle has 2.47  106 amidine groups (see the Supporting Information). This situation results in a stronger electrostatic repulsion from the interface for cationic PDEA-PS particles as compared to that for the charge-stabilized PS particles. This repulsive image force should be superior to the attractive interaction between the anionic air
water interface and cationic PDEA-PS particles; therefore, the PDEA-PS particles cannot adsorb at the air
water interface. The significant influence of pH on the foamability of these aqueous particle dispersions is similar to the effect of pH on foams stabilized by poly(acrylic acid)-coated PS latex particles.17 The microstructures of the particle-stabilized foams were investigated using OM and SEM. An OM image of an aqueous dispersion of the PDEA-PS particles at pH 3.1 after shaking, where approximately 100% of the PDEA hair is protonated, suggests that the PS particles do not stabilize the foams and are dispersed in the media rather than aggregated (Dv, 660 ( 240 nm) (Figure 6a). At pH 6.1, where 97% of the PDEA hair is

Figure 6. OM images of PDEA-PS particle-stabilized foams prepared at pH’s of (a,d) 3.1, (b,e) 6.1, and (c,f) 10.0, where (a)
(c) are immediately after preparation in the wet state and (d)
(f) are in the dried state. 12906

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Langmuir protonated and the PS particles could be stably dispersed in the aqueous media (Dv, 640 ( 200 nm), particle-stabilized polydisperse bubbles and free colloidally stable PDEA-PS latex particles were observed, which indicates that a slightly hydrophobic surface (3% deprotonated PDEA hair) is sufficient for the particles to adsorb at the air
water interface. The foam is composed of almost spherical and nonspherical bubbles with sizes ranging from ca. 40 to 850 μm, as shown in the OM images presented in Figure 6b. At and above pH 7.1, where less than 77% of the PDEA hair is protonated and the particles are flocculated in the aqueous media (Dv > 4 μm), polydisperse bubbles (size range from ca. 70 to 880 μm) with flocculated PDEA-PS particles adsorbed at the bubble surfaces (white arrows in Figure 6c indicate large flocs on the air
water interface) and flocculated particles in the continuous aqueous media were observed. These particle-stabilized foams prepared at pH > 6 were stable and retained their three-dimensional structure even after drying (Figure 7), and OM studies indicated few bubbles were broken during/after evaporation of the aqueous phase overnight at ambient temperature (panels e and f of Figure 6). Very little coalescence occurred, and both visual inspection and OM observations indicated that the bubble size distribution was almost unchanged after drying. Figure 8 shows SEM images obtained for dried foams prepared using the PDEA-PS particles at pH 6.1 (a
c) and 9.0 (d
f). For this SEM study, NH3 aqueous solution was used to adjust the pH rather than NaOH, because NH3 can be removed by evaporation prior to SEM observation. Focusing on the top surface of the dried bubbles confirmed the presence of the latex particles. A relatively smooth surface, where the PDEA-PS particles are near close-packed, was evident for the pH 6.1 system (Figure 8c), whereas a rough surface, due to coagulated PDEA-

Figure 7. Photographs of PDEA-PS particle-stabilized foam (a) before and (b) after drying on a glass substrate. The particulate foam was prepared at pH 10.0.

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PS particles, was observed for the pH 9.0 system (Figure 8f). If the dried foam is deliberately ruptured using a scalpel, then the internal particle microstructure can be readily examined. For the foams prepared at pH 6.1, in most cases, there was strong evidence for the formation of well-defined particle bilayers (Figure 8b). The widespread formation of latex bilayers strongly suggests that most wet air bubbles were stabilized by monolayers of adsorbed latex particles. These monolayers are presumably

Figure 9. (a) Photographs illustrating pH-responsive behavior of foam prepared using PDEA-PS latex particles (5.0 wt %, 0.1 M NaCl): (i) at pH 9.0 after 10 min, (ii) pH adjustment from pH 9.0 to pH 4.0 after 10 min, (iii) pH adjustment from pH 4.0 to pH 9.0 after 10 min, and (iv) pH adjustment from pH 4.0 to pH 9.0 to pH 4.0 after 10 min. (b) Height of the foam layer versus the number of pH cycles.

Figure 8. SEM images of foams stabilized with PDEA-PS latex particles (5.0 wt %, 0.1 M NaCl) at (a
c) pH 6.1 and (d
f) pH 9.0. Parts (b) and (e) are magnified images of (a) and (d), respectively. The inset in (b) shows a magnified image of ruptured foam section, which indicates clear evidence for the existence of bilayer. Parts (c) and (f) depict surface morphology of the foams. NH3 was used to adjust the pH value to pH 9.0. 12907

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Langmuir forced together to form bilayers during water drainage from the drying foams. Similar results were reported by Fujii et al.11a,b and by Dupin et al.23 In some cases, only latex monolayers were observed; the monolayers were formed at the top surface of a dried foam that was directly exposed to bulk air phase and was not overlapped with other bubble surfaces. There is little evidence for the exquisite long-range ordering of latex particles in the latex layers, which was observed by Fujii et al.11a,b The lack of long-range order is presumably related to the somewhat higher polydispersities obtained for the latexes used in the present work; the coefficient of variations are 14.1% for the PDEA-PS particles and ca. 5% for the PS particles used in the previous studies.11a,b For the foams prepared at pH 9.0, latex multilayers (5
12 layers) were observed (Figure 8e), which suggests that air bubbles are stabilized by the flocculated PDEA-PS particles. The multilayer thickness estimated from SEM image analysis (2
5 μm) was smaller than the PDEA-PS floc size determined by the laser diffraction method (ca. 10 μm). Therefore, there is a possibility that water drainage during drying partially breaks the PDEA-PS particle flocs, which leads to thinner foam layer thickness than expected; laser diffraction studies confirmed the decrease of floc size due to partial breakage of the flocs with a stronger shear during measurement (Table S2). There is another possibility that the PDEA-PS flocs on the bubble surface were compressed into a near close-packed arrangement during the evaporation of water. On the basis of these findings for the pH-dependent particlestabilized foams prepared in a batch mode, we investigated the possibility of inducing defoamation of the particle-stabilized foam by subsequent pH adjustment. The particle-stabilized foam prepared at pH 9.0 and allowed to stand for 10 min was rapidly (within 1 min) defoamed (coalesced) by lowering the solution pH of the aqueous phase to pH 4.0 with a small volume of concentrated aqueous HCl, followed by vigorous shaking using the touch mixer at 25 °C (see Figure 9). This is presumably because the PDEA-PS particles acquire a highly hydrophilic surface character in situ as the PDEA hair chains on the particle surface become protonated. Therefore, the PDEA-PS particles are no longer adsorbed at the air
water interface and become detached from it, which leads to breakdown of the foam and phase separation to macroscopic air and water. OM studies recorded during the in situ addition of HCl confirmed that rapid bubble coalescence occurred (see web-enhanced object). This foaming
defoaming cycle is readily reversible at least five times, and essentially the same amount of foam was obtained with shaking (see Figure 9b).

’ CONCLUSIONS Submicrometer-sized PS particles carrying PDEA hair were successfully synthesized by dispersion polymerization, and their size, size distribution, chemical composition, and pHresponse were well characterized. The performance of PDEAPS latex particles as a pH-dependent and pH-responsive particulate foam stabilizer was evaluated. The PDEA-PS particles can stabilize aqueous foams for at least 1 month, at and above pH 8.0, whereas no foam was formed at and below pH 5.1 where the particles are colloidally stable and positively charged. At pH 6.1 and 7.1, relatively stable foams can be prepared that survive for at least 24 h. The critical minimum pH required for stable particle-stabilized foams correlates closely with the pKa value of 7.6 for PDEA chains. The particle-stabilized foams were stable and retained their three-dimensional structure even

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after drying. SEM examination of the dried foams suggested that flocculated PDEA-PS particles were adsorbed at the air
water interface for the foam prepared at pH 9.0. For the foam prepared at pH 6.1, well-defined particle bilayers, similar to those reported previously for sterically/charge stabilized particles,11a,b were observed, which indicates the bubbles were stabilized with PDEA-PS particle monolayers adsorbed at the air
water interface. Defoaming can be simply achieved by decreasing the pH of the system after foamation. Encapsulation of air bubbles in water using such stimulus-responsive latex particles may have potential applications in food manufacturing, cosmetic formulations, and personal care products.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details on calculation of amine group numbers on the PS particle and characterization of dispersion
flocculation transition behavior for the PDEA-PS latex particles using laser diffraction method and optical microscope. This material is available free of charge via the Internet at http://pubs.acs.org.

W b

Web Enhanced Feature. Video of OM studies recorded during the in situ addition of HCl confirmed that rapid bubble coalescence occurred is available in the HTML version of this paper.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Soft-Interface Science” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We thank Wako Chemicals for the kind donation of 2, 20 -azobis[2-methyl-N-(2-hydroxyethyl)propionamide]. ’ REFERENCES (1) Binks, B. P., Horozov, T. S., Eds. Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, 2006. (2) Pugh, R. J. Adv. Colloid Interface Sci. 1996, 64, 67. (3) Sadoc, J. F., Rivier, N., Eds. Foams and Emulsions; NATO ASI Series E; Kluwer Academic Publishers: Dordrecht, 1999; Vol. 354. (4) Studart, A. R.; Gonzenbach, U. T.; Tervoort, E.; Gauckler, L. J. J. Am. Ceram. Soc. 2006, 89, 1771. (5) (a) Carn, F.; Saadaoui, H.; Mass
e, P.; Ravaine, S.; Julian-Lopez, B.; Sanchez, C.; Deleuze, H.; Talham, D. R.; Backov, R. Langmuir 2006, 22, 5469. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (6) Ramsden, W. Proc. R. Soc. London 1903, 72, 156. (7) Sun, Y. Q.; Gao, T. Metall. Mater. Trans. A 2002, 33, 3285. (8) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371. (9) Binks, B. P.; Horozov, T. S. Angew. Chem., Int. Ed. 2005, 44, 3722. (10) (a) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Nat. Mater. 2005, 4, 553. (b) Subramaniam, A. B.; Abkarian, M.; Mahadevan, L.; Stone, H. A. Nature 2005, 438, 930. (c) Subramaniam, A. B.; Mejean, C.; Abkarian, M.; Stone, H. A. Langmuir 2006, 22, 5986. (11) (a) Fujii, S.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2006, 128, 7883. (b) Fujii, S.; Iddon, P. D.; Ryan., A. J.; Armes, S. P. Langmuir 2006, 22, 7512. (c) Kettlewell, S. L.; Schmid, A.; Fujii, S.; Dupin, D.; Armes, S. P. Langmuir 2007, 23, 11381. 12908

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(12) Binks, B. P.; Murakami, R. Nat. Mater. 2006, 5, 865. (13) (a) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Angew. Chem., Int. Ed. 2006, 45, 3526. (b) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2007, 23, 1025. (c) Wong, J. C. H.; Tervoort, E.; Busato, S.; Gauckler, L. J.; Ermanni, P. Langmuir 2011, 27, 3254. (d) Studart, A. R.; Gonzenbach, U. T.; Akartuna, I.; Tervoort, E.; Gauckler, L. J. J. Mater. Chem. 2007, 17, 3283. (14) (a) Pugh, R. J. Langmuir 2007, 23, 7972. (b) Hunter, T. N.; Pugh, R. J.; Franks, G. V.; Jameson, G. J. Adv. Colloid Interface Sci. 2008, 137, 57. (15) Fujii, S.; Murakami, R. KONA 2008, 26, 153. (16) Garrett, P. R., Ed. Defoaming; Theory and Industrial Applications: Surfactant Science Series 45; Marcel Dekker Inc.: New York, 1993. (17) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S.; Schmid, A. Langmuir 2007, 23, 8691. (18) Fujii, S.; Kakigi, Y.; Suzaki, M.; Yusa, S.; Muraoka, M.; Nakamura, Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3431. (19) Fujii, S.; Suzaki, M.; Armes, S. P.; Dupin, D.; Hamasaki, S.; Aono, K.; Nakamura, Y. Langmuir 2011, 27, 8067. (20) Fujii, S.; Suzaki, M.; Nakamura, Y.; Sakai, K.; Ishida, N.; Biggs, S. Polymer 2010, 51, 6240. (21) (a) Ciunel, K.; Arm
elin, M.; Findenegg, G. H.; von Klitzing, R. Langmuir 2005, 21, 4790. (b) Takahashi, M. J. Phys. Chem. B 2005, 109, 21858. (22) (a) Jackson, J. D. Classical Electrodynamics, 3rd ed.; Wiley: New York, 1998; Chapter 4. (b) Yim, H.; Kent, M. S.; Matheson, A.; Stevens, M. J.; Ivkov, R.; Satija, S.; Majewski, J.; Smith, G. S. Macromolecules 2002, 35, 9737. (c) Ahrens, H.; Baltes, H.; Schmitt, J.; M€owald, H.; Helm, C. Macromolecules 2001, 34, 4504. (d) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir 2004, 20, 7412. (23) Dupin, D.; Howse, J. R.; Armes, S. P.; Randall, D. P. J. Mater. Chem. 2008, 18, 545.

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