Relationship between Wall Thickness and Size in Block Copolymer

May 8, 2009 - Dalin Wu , Mariana Spulber , Fabian Itel , Mohamed Chami , Thomas Pfohl , Cornelia G. Palivan , and Wolfgang Meier. Macromolecules 2014 ...
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Relationship between Wall Thickness and Size in Block Copolymer Vesicles† Lie Ma‡,§ and Adi Eisenberg*,§ ‡

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China, and Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada

§

Received April 9, 2009 In this study, we report a new phenomenon dealing with the size-dependent behavior of the wall thickness of block copolymer vesicles, especially the decrease in wall thickness with decreasing vesicle size. Four vesicle-forming copolymers from the polystyrene-b-poly-(acrylic acid) (PS-b-PAA) family (i.e., PS(500)-b-PAA(50), PS(310)-b-PAA (28), PS(240)-b-PAA(15), and PS(412)-b-PAA(46)) were chosen for study. The sizes and wall thicknesses of the vesicles after quenching were determined from the TEM micrographs, and plots were made of the wall thickness versus size for each family of vesicles made from each of the various blocks. First, the effect of the length of the PAA block on the relationship between the wall thickness and the size was examined. In the vesicles prepared from PS(500)-b-PAA(50), the copolymer with the longest PAA block that yields the smallest vesicles, the wall thickness decreases strongly with decreasing size. By contrast, in the case of vesicles made from PS(240)-b-PAA(15), for which a wide size distribution is obtained, only a weak size dependence of the wall thickness is seen. For vesicles made from the copolymer with intermediate PAA block length (i.e., PS(310)-b-PAA(28)), both strong and weak behavior regions are observed depending on the vesicle size range. We suggest that this new phenomenon of the size dependence of the wall thickness can be considered to be another stabilization mechanism for very small vesicles, under conditions where chain segregation is insufficient to stabilize the size. The vesicles can be stabilized by decreasing the wall thickness for very small vesicles, resulting in the increase in area per corona chain, thus decreasing the corona repulsion on the inside. The effects of additives such as NaCl, HCl, or NaOH on the relationship between the wall thickness and the size were also investigated. By shielding the electrostatic repulsion among corona chains in the presence of NaCl, the strong behavior of the vesicles prepared from PS(412)-b-PAA(46) changes to a weak one as the width of the vesicle size distribution increases. In a NaCl concentration region around 10 mM, an opposite effect is seen relative to that observed in small vesicles in that the wall thickness decreases with increasing vesicle size for vesicles larger than ca. 300 nm, an effect ascribed to corona repulsion among the external corona chains. The addition of HCl also drives the relationship to be weaker through the protonation of the carboxylate groups of PAA chains, in an effect similar to that of NaCl. The presence of NaOH is expected to strengthen the relationship via the deprotonation of PAA, which increases the corona repulsion. However, because of the very short length of PAA chains in the system where a weak effect is seen, no significant effect of NaOH addition was observed because the size distribution remained broad.

1. Introduction Vesicles self-assembled from amphiphilic block copolymers in selective solvents have attracted considerable attention recently † Part of the “Langmuir 25th Year: Molecular and macromolecular selfassemblies” special issue. *Corresponding author. E-mail: [email protected].

(1) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728–1731. (2) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967–973. (3) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143–1146. (4) Kita-Tokarczyk, K.; Grumelard, J.; Haefele, T.; Meier, W. Polymer 2005, 46, 3540–3563. (5) Vriezema, D. M.; Hoogboom, J.; Velonia, K.; Takazawa, K.; Christianen, P. C. M.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 2003, 42, 772–776. (6) Jain, S.; Bates, F. S. Science 2003, 300, 460–464. (7) Antonietti, M.; Forster, S. Adv. Mater. 2003, 15, 1323–1333. (8) Du, J. Z.; Tang, Y. P.; Lewis, A. L.; Armes, S. P. J. Am. Chem. Soc. 2005, 127, 17982–17983. (9) Zheng, R. H.; Liu, G. J. Macromolecules 2007, 40, 5116–5121. (10) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501–527. (11) Zhou, Y. F.; Yan, D. Y. Angew. Chem., Int. Ed. 2005, 44, 3223–3226. (12) Peng, H. S.; Chen, D, Y; Jiang, M. Langmuir 2003, 19, 10989–10992. (13) Koide, A.; Kishimura, A.; Osada, K.; Jang, W. D.; Yamasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2006, 128, 5988–5989. (14) Hu, Z. J.; Verheijen, W.; Hofkens, J.; Jonas, A. M.; Gohy, J. F. Langmuir 2007, 23, 116–122. (15) Hamley, I. W. Nanotechnology 2003, 14, R39–R54. (16) Zhang, Y. F.; Luo, S. Z.; Liu, S. Y. Macromolecules 2005, 38, 9813–9820. (17) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481–489.

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and have been studied by a number of groups.1-19 One can anticipate applications of block copolymer vesicles in such fields as drug delivery20-22 and as microreactors23 among others because of the increased stability relative to liposomes. Early studies1 have shown that vesicles in solution are a part of a morphological continuum consisting of spherical micelles, rods, inverse rods, inverse spheres, and vesicles. The free energy of aggregation consists of a sum with numerous contributions. Those of morphological significance involve the interfacial energy between the core and the external solution, the stretching of the core-forming blocks, and the repulsive interactions among the corona chains.24,25 As a result, a range of parameters influencing the above free-energy contributions can be used to control the morphology, such as relative block length, ion content, and solvent composition, all of which (18) Bellomo, E. G.; Wyrsta, M. D.; Pakstis, L.; Pochan, D. J.; Deming, T. J. Nat. Mater. 2004, 3, 244–248. (19) Bang, J.; Jain, S.; Li, Z.; Lodge, T. P.; Pedersen, J. S.; Kesselman, E.; Talmon, Y. Macromolecules 2006, 39, 1199–1208. (20) Tong, R.; Cheng, J. J. Polym. Rev. 2007, 47, 345–381. (21) Harada, A.; Kataoka, K. Prog. Polym. Sci. 2006, 31, 949–982. (22) Rosler, A.; Vandermeulen, G. W. M.; Klok, H. A. Adv. Drug Delivery Rev. 2001, 53, 95–108. (23) Kishimura, A.; Koide, A.; Osada, K.; Yamasaki, Y.; Kataoka, K. Angew. Chem., Int. Ed. 2007, 46, 6085–6088. (24) Zhang, L.; Eisenberg, A. Polym. Adv. Technol. 1998, 9, 677–699. (25) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239–2249.

Published on Web 05/08/2009

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contribute to one or more of the three “morphogenic” free-energy contributions.26 A previous study by Shen et al.27 suggested that many parameters of block copolymer aggregates, such as their morphology and size, can be changed reversibly, indicating that thermodynamic equilibrium is involved. For example, changes in morphology from sphere to rod and rod to vesicle, as well as changes in size, are reversible as a function of water content in mixed water/ organic solvents. Because the organic solvent partitions between the water and the hydrophobic core or the wall of the aggregates, the chain dynamics of block copolymers depend strongly on the solvent composition. At low water contents, the chains are very mobile, whereas at high water contents they may even be glassy. Because vesicles can be thermodynamically stable, the curvature of the vesicles must be under thermodynamic control. Luo et al.28 have shown that a thermodynamic curvature-stabilization mechanism exists that is based on the segregation of the longer corona chain to the exterior interface, whereas the shorter corona chains are segregated to the inside. Because repulsion among the corona chains on the outside is stronger than that inside the vesicles, the curvature is stabilized thermodynamically. It is well known that the size of block copolymer vesicles can be controlled by parameters such as the ratio of the lengths of the hydrophilic block to the hydrophobic block,26f the polydispersity of the hydrophilic block,29 and many solution properties30 that include the water content in the solvent mixture, the polymer concentration, the nature and composition of the solvent, the presence of additives (ions, acids, and bases), and others. The size increase of vesicles is realized through the fusion of vesicles, whereas the size decrease occurs by vesicle fission.27b Control of the wall thickness of block copolymer vesicles, including the scaling relationship between the length of the styrene block and the wall thickness, has been studied by Azzam et al.31 It was shown that, for a series of block copolymers with a constant PAA block length, the wall thickness of block copolymer vesicles is strongly dependent on the PS length. However, for any vesicle system made from one diblock copolymer, the tacit assumption is made that wall thickness is independent of vesicle size. There are more than 600 papers on block copolymer vesicles or polymersomes (the number was obtained from the Web of Science with the keywords “block copolymer vesicles” or “polymersomes”), yet by using standard search techniques, we could not find any suggestion that the wall thickness is size-dependent. In the present study, we report a new phenomenon showing that for small vesicles the wall thickness decreases with decreasing size of vesicles, as will be shown below. Several block copolymers were selected from the polystyrene-bpoly-(acrylic acid) (PS-b-PAA) family. From the micrographs obtained for each group of vesicles prepared from any one block copolymer, sizes and wall thicknesses were measured, and plots were made of the wall thickness versus size. In this way, the graphic relationship between the wall thickness and the size of the vesicles was obtained. In addition, the effects of PAA block length and the presence of additives such as NaCl, HCl, or NaOH were (26) (a) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777–1779. (b) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168–3181. (c) Zhang, L.; Eisenberg, A. Macromolecules 1996, 29, 8805–8815. (d) Yu, Y.; Eisenberg, A. J. Am. Chem. Soc. 1997, 119, 8383–8384. (e) Zhang, L.; Bartels, C.; Yu, Y.; Shen, H.; Eisenberg, A. Phys. Rev. Lett. 1997, 79, 5034–5037. (f) Yu, Y.; Zhang, L.; Eisenberg, A. Macromolecules 1998, 31, 1144–1154. (27) (a) Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9473–9487. (b) Luo, L.; Eisenberg, A. Langmuir 2001, 17, 6804–6811. (28) Luo, L.; Eisenberg, A. J. Am. Chem. Soc. 2001, 123, 1012–1013. (29) Terreau, O.; Luo, L.; Eisenberg, A. Langmuir 2003, 19, 5601–5607. (30) Choucair, A.; Eisenberg, A. Eur. Phys. J. E 2003, 10, 37–44. (31) Azzam, T.; Eisenberg, A. Angew. Chem., Int. Ed. 2006, 45, 7443–7447.

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explored. Finally, the controlling factors and the relationship between the wall thickness and the size of the vesicles are explained in terms of thermodynamic considerations.

2. Materials and Methods 2.1. Materials. Vesicles were prepared from polystyrene-bpoly(acrylic acid) block copolymers (PS-b-PAA) (i.e., PS(500)-bPAA(50), PS(310)-b-PAA(28), PS(240)-b-PAA(15), and PS (412)b-PAA(46)), where the numbers refer to the number-average degree of polymerization. All of these block copolymers were synthesized by sequential anionic polymerization. The details of the procedure were given in a previous publication.26b Theses block copolymers have a polydispersity indexes (PDIs) of ca. 1.10, 1.05, 1.04, and 1.06, respectively, as determined by gel permeation chromatography (GPC). All other chemicals and solvents were of analytical grade and were used as received. 2.2. Preparation of Vesicles. The different PS-b-PAA block copolymers were dissolved in dioxane, a common solvent for both blocks of the copolymer, at a concentration of 1 wt % and then stirred overnight at room temperature. To induce self-assembly, Milli-Q water was added dropwise to the block copolymer solution at a rate of 1 wt % per minute to a final water content of 50 wt %. The colloidal suspension was then quenched with a 10-fold excess of Milli-Q water and dialyzed for a few days against water to remove the dioxane. Vesicles prepared in the presence of additives (such as NaCl, HCl, or NaOH) were obtained by first adding the base, acid, or salt (as an aqueous solution) to the block copolymer solution at a concentration of 1 wt % and then adding the Milli-Q water dropwise to the desired water content, as described above. 2.3. Characterization. The morphology of the vesicles was observed by transmission electron microscopy (TEM), which was performed on a JEOL JEM-2000FX instrument equipped with a CCD camera and operated at an acceleration voltage of 80 kV. Copper TEM grids were precoated with a thin film of Formvar and then coated with carbon. A drop of solution containing 0.05 wt % vesicles was deposited on the resulting grids. Samples were dried in air for 2 days and then in vacuum for another 3 h. 2.4. Measurement of Wall Thickness and Size of Vesicles. The wall thicknesses and sizes of vesicles, which are polydisperse, were measured directly from the TEM images using SigmaScan Pro software. More than 100 vesicles were measured for each group of vesicles prepared from the specific block copolymer. Then plots of the wall thickness versus size were prepared. The statistical analysis of the wall thickness determination is given in the Supporting Information.

3. Results and Discussion 3.1. Effect of Poly(acrylic acid) Block Length. In the first section, we examine the effect of the hydrophilic block length on the relationship between the wall thickness and the size of the vesicles. Three groups of vesicles were prepared under identical conditions by dissolving three different PS-b-PAA copolymers (i.e., PS(500)-b-PAA(50), PS(310)-b-PAA(28), and PS(240)-b-PAA(15)) in dioxane, a common solvent for the block copolymers, and adding water to a concentration of 50 wt %. The wall thickness and the size of the vesicles prepared from each of the above copolymers were determined from TEM micrographs, and then the relationship between the wall thickness and the size were obtained from plots of wall thickness versus size. DOI: 10.1021/la9012729

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Figure 1. TEM image of the vesicles prepared from PS(500)-bPAA(50). The double arrows indicate the wall thicknesses of vesicles with different sizes.

Figure 1 shows a micrograph of the vesicles prepared from PS (500)-b-PAA(50). Wall thicknesses are indicated by lines for the vesicles with different sizes. It is clearly seen that the lines become shorter with decreasing vesicle size. Figure 2 shows the plots of the wall thickness versus the size of vesicles prepared from the three copolymers. For the vesicles with the shortest PAA length, only a weak size dependence is seen (Figure 2c). The vesicle sizes for this family range from ca. 100 nm to almost 700 nm, with the wall thickness ranging from ca. 25 nm to ca. 40 nm. A linear fitted line of these points has a slope (ΔW/ΔD, where W is the wall thickness and D is the size of vesicles) of approximately 0.004. This type of behavior will be referred to as a “weak” relationship. By contrast, for the vesicles made from the diblock with the longest hydrophilic segment, the wall thickness increases strongly with increasing size in that the value of ΔW/ΔD is ca. 0.13 (Figure 2a). Vesicles made from blocks of intermediate hydrophilic block length exhibit both types of behavior in that they give a value of ΔW/ΔD ≈ 0.004 for vesicles larger than ca. 150 nm but a value of ΔW/ΔD ≈ 0.013 for vesicles smaller than ca. 150 nm (Figure 2b). It should be noted that the vesicles made from the block with the shortest PAA chain give a broad distribution of sizes, whereas those made from the copolymer with the longest PAA block yield the narrowest size distribution. Such behavior is consistent with the studies reported previously32 that suggest that block copolymers with shorter PAA blocks result in larger vesicles. For the group of vesicles prepared from PS(240)-b-PAA(15), we see the broadest size distribution (ca. 100 to ca. 650 nm) and the largest average size (ca. 350 nm). The corona repulsion is weakest here; a wide range of sizes are thermodynamically stable, and very large vesicles are seen. By contrast, vesicles made from PS(500)-bPAA(50), the block with the longest PAA chain, showed a narrow size distribution (with an average size of ca. 103 nm). The family of vesicles prepared from PS(310)-b-PAA(28) (i.e., those containing a PAA block of intermediate size) exhibits both a weak effect (ΔW/ΔD ≈ 0.004) and a strong effect (ΔW/ΔD ≈ 0.013), with a crossover at a vesicle size of ∼150 nm. The average vesicle size above the crossover point is ca. 106 nm, whereas below that point it is ca. 260 nm. In this family, we have a sufficient number of short PAA chains to allow the appearance of some large vesicles but also a sufficient number of long chains for (32) Choucair, A.; Lavigueur, C.; Eisenberg, A. Langmuir 2004, 20, 3894–3900.

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small vesicles to appear. We have no evidence that there is a gross segregation of long corona chains to small vesicles and of short corona chains to large vesicles but merely that, at this intermediate block length, a wide range of vesicle sizes are tolerated. The crossover point, which separates weak- and strong-behavior regions, is obviously related to the hydrophilic block length and also the cavity size. A summary of the vesicle and cavity sizes at the crossover point is given in Table 1. As can be seen clearly, the longer the PAA chain, the larger the cavity size and the flatter the interface at which the crossover between weak and strong behavior is observed (i.e., the point at which the value of ΔW/ΔD changes from ca. 0.004 to ca. 0.013). A brief discussion of the crossover point in relation to the hydrophilic block length and the cavity size is in order. For the polymer containing the longest PAA chains (50 units), no crossover point is seen since only relatively small vesicles are obtained. Crossover behavior would have to be located at vesicle sizes >200 nm. The phenomena associated with the vesicles prepared from PS(310)-b-PAA(28) are most instructive. Here we observe a crossover at a vesicle size of ca. 150 nm and a wall thickness of ca. 35 nm, which indicates a cavity size of ∼ 80 nm. These numbers suggest that at this cavity size, the coil dimensions of the hydrophilic chains on the interior interface of the vesicle just fill the space without major repulsive effects. It should be recalled that the chains on the interior interface undoubtedly lie at the lower end of the length distribution in view of the chain segregation phenomenon discussed above. For smaller cavity sizes, stronger repulsion would be observed, and strong ΔW/ΔD behavior would be seen in that region. By contrast, for larger cavity sizes, the corona repulsion in the interior would be weaker, and weak ΔW/ΔD behavior would be seen. Finally, for the shortest PAA chain in PS(240)-b-PAA(15), the crossover vesicle size would have to be located at vesicle sizes smaller than 100 nm (i.e., at a cavity size of 200a >110a ∼103

310 240 28 15 ∼150 200 nm and to drive the behavior from strong to weak. As the vesicle size increases to >300 nm, the surface becomes progressively more flat. With the addition of water, the shielding effect of NaCl on the PAA chains becomes weaker because of the decrease in the NaCl concentration. Therefore, corona repulsion for the longer exterior chains becomes important. As before, the system responds to increasing corona repulsion, this time as a result of the flatting of the vesicles, by again decreasing the wall thickness, thus increasing the area per external corona chain. In Figure 4, we show the effect of HCl on the relationship between the wall thickness and the size of vesicles prepared from PS(412)-b-PAA(46). With the addition of 100 μM HCl, more vesicles in the size range of >200 nm are obtained compared to the HCl-free sample (Figure 4b). Therefore, both strong and weak behavior regions can be observed, showing behavior similar to that seen in vesicles prepared from PS(310)-b-PAA(28) (Figure 2b). However, with still further increases in the HCl concentration, no significant changes in behavior are seen. The addition of HCl acts by protonating the carboxylate group of the PAA chains, thus reducing the repulsive interactions among the corona chains. In addition, the decreased corona repulsion leads to an increase in the aggregation number and thus results in the formation of vesicles with larger sizes; a weak relationship is therefore observed instead of the strong one seen in the absence of HCl. 3.3. Effect of NaOH. In this section, we examine the relationship between the wall thickness and the size of vesicles, which were prepared from PS(240)-b-PAA(15) in the presence of different concentration of NaOH. As shown in Figure 5, the presence of both ca. 12 μM (Figure 5b) and ca. 30 μM NaOH (Figure 5c) results in a broad size distribution and large average sizes (ca. 350 nm), similar to the case of vesicles prepared in the absence of NaOH. In addition, there are no obvious effects of NaOH on the relationship between the wall thickness and the size. A weak effect is observed in both samples. When higher concentrations of NaOH are added (i.e., 60 or 90 μM NaOH), vesicle sizes shift to lower values (i.e., into the region from ca. 100 to ca. 300 nm, Figure 5d,e). However, there are no significant changes in the relationship between the wall thickness and the size in these two cases. It was reported previously that the addition of NaOH can be used to control the morphology of block copolymer aggregates.26c With the addition of NaOH, the morphology changes from vesicles to spherical micelles. NaOH acts on the morphology by DOI: 10.1021/la9012729

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Figure 3. Effect of NaCl on the relationship between wall thickness and size of vesicles prepared from (a) PS(412)-b-PAA(46) and in the presence of NaCl at concentrations of (b) 2, (c) 5, (d) 10, and (e) 20 mM. (f) Statistical analysis of the average wall thickness vs size range from panel d. The asterisk (*) indicates a significant difference (P < 0.05).

Figure 4. Effect of HCl on the relationship between the wall thickness and the size of vesicles (a) prepared from PS(412)-b-PAA(46) and in the presence of HCl at concentrations of (b) 100, (c) 200, (d) 400, and (e) 600 μM. 13734 DOI: 10.1021/la9012729

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Figure 5. Effect of NaOH on the relationship between the wall thickness and size of vesicles (a) prepared from PS(240)-b-PAA(15) and in the presence of NaOH at concentrations of (b) 12, (c) 30, (d) 60, and (e) 90 μM.

increasing the repulsion of PAA chains, which is regarded as one of the three morphogenic contributions to the free energy of aggregations. Because of the addition of NaOH, the PAA chains are ionized, resulting in an increase in electrostatic repulsion among the corona chains. This increase favors a decrease in the radius and the conversion of vesicles to spherical micelles, in which the repulsion in the outer reaches of the corona is lower because of the smaller radius. Not surprisingly, the sizes of the vesicles prepared in the presence of NaOH become smaller than those prepared in the absence of NaOH. With the decrease in vesicle sizes, the repulsive interactions between the corona chains on the inside become stronger. At this point, the new mechanism contributes to the reduction of the wall thickness, which increases the area per corona chain and thus decreases the repulsion of corona chains. NaOH deprotonates acrylic acid, and that would be expected to increase corona repulsion and induce a strong relationship where a weak one was seen before. However, because the chains are already short, the increasing repulsion by the addition of NaOH is insufficient to induce the appearance of a strong relationship. Both in the presence and absence of NaOH, the size distributions are wide, and because a strong relationship should be seen only for small vesicles, no such behavior is observed. There is no reason to expect a NaOH effect on the copolymer with PAA(50) because a strong effect is already seen in the absence of NaOH. Beside, it was shown that NaOH can convert vesicles to micelles.26a

4. Conclusions In summary, we have shown that in vesicles prepared from PS-b-PAA diblock copolymer, the wall thickness decreases with decreasing vesicle size, especially for small vesicles. The relationship between the wall thickness and the size is controlled by the Langmuir 2009, 25(24), 13730–13736

length of the PAA block. A strong relationship is observed in vesicles prepared from PS(500)-b-PAA(50), the copolymer with the longest PAA block that yields the smallest vesicles; by contrast, for vesicles with the shortest PAA block length and largest sizes, only a weak size dependence is seen. Vesicles made from blocks of intermediate PAA block length exhibit both strong- and weak-behavior regions. We suggest that this phenomenon is due to corona repulsion under conditions in which chain segregation is insufficient to stabilize small vesicles. Specifically, if the interior corona repulsion is too large at a particular vesicle size and wall thickness, then the wall thickness decreases to allow the area per corona chain to increase, thus stabilizing small vesicles. This phenomenon can be considered to be another stabilization mechanism for small vesicles that is operative as long as the vesicle wall is capable of reaching dynamic equilibrium. In addition, we examined the effect of additives such as NaCl, HCl, or NaOH on the wall thickness versus size behavior. The addition of NaCl changes the relationship from strong to weak by shielding the electrostatic repulsion among corona chains. However, for vesicles larger than ca. 300 nm, in the presence of 10 mM NaCl an opposite effect is seen in that the wall thickness decreases with increasing vesicle size; for large vesicles, the corona repulsion of the long exterior chains becomes important. The addition of HCl leads to an increase in vesicle size and thus shows an effect of weakening the strong relationship by protonation of the carboxylate groups of the PAA chains and thus reducing repulsion. The addition of NaOH has no obvious effect on the relationship of the wall thickness versus size for the vesicles with short corona chains studied here. Acknowledgment. The stay by L.M. at McGill University was made possible by a New Star fellowship from Zhejiang DOI: 10.1021/la9012729

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University. A.E. is grateful for financial support from NSERC Canada and the Industrial Technology Research Institute of Hsinchu, Taiwan. Supporting Information Available: TEM micrographs of the vesicles prepared from copolymers with different

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length of PAA and vesicles made in the presence of additives. Statistical analysis of the determination of the wall thicknesses. Linear fit of the plots of wall thickness versus the size of vesicles prepared from copolymers with different lengths of PAA. This material is available free of charge via the Internet at http://pubs.acs.org.

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