Control of Morphology and Corona Composition in Aggregates of

Article pubs.acs.org/Langmuir

Control of Morphology and Corona Composition in Aggregates of Mixtures of PS‑b‑PAA and PS‑b‑P4VP Diblock Copolymers: Effects of Solvent, Water Content, and Mixture Composition Renata Vyhnalkova,† Axel H.E. Müller,‡ and Adi Eisenberg*,† †

Department of Chemistry, McGill University, Otto Maass Building, 801 Sherbrooke St. W., Montréal, Québec H3A 2K6, Canada Institut für Organische Chemie, Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: The morphologies and corona compositions in aggregates of mixtures of PS-b-PAA and PS-b-P4VP diblock copolymers are influenced by controllable assembly parameters such as water content, block copolymer molar ratios, and solvent effects as well as the hydrophilic block lengths and block length ratios. All these factors can affect the morphology of the aggregates as well as their corona composition, the latter especially in vesicles, where two interfaces are involved. The morphologies and corona compositions of the aggregates were investigated by transmission electron microscopy and electrophoretic mobility, respectively. They depend, to a large extent, on the solubility of P4VP and PAA in the given organic solvent (e.g., DMF, THF, or dioxane), which influences the coil dimensions of the hydrophilic chains. The water content affects both the size and the shape of the block copolymer aggregates as well as the corona composition. Water acts as a precipitant for the hydrophobic block in the common solvent and, therefore, its progressive addition to the solution changes the interaction parameter with the hydrophobic block. The block copolymer molar ratio has an effect on both the morphology and the corona composition of the aggregates. With increasing PS-b-P4VP content in the mixture, the morphology transforms gradually from large compound micelles (LCMs), through coexistence of LCMs and small spherical micelles (SSMs), and eventually to vesicles. As expected, the corona composition of the aggregates is also affected by the block copolymer molar ratio, and changes progressively from pure PAA to a mixture of PAA and P4VP and to pure P4VP with increasing PS-b-P4VP content. It is clear that the use of mixtures of the soluble chains offers the opportunity of fine-tuning the corona composition in block copolymer aggregates under assembly conditions.

1. INTRODUCTION Amphiphilic block copolymer self-assembly is a very active area of research, in part because one can obtain a wide range of morphologies, such as small spherical micelles (SSMs), rods, large compound micelles (LCMs), and vesicles, as well as bicontinuous and other morphologies.1−30 In aqueous systems, the aggregates consist of a hydrophobic core and a hydrophilic corona. In addition to morphological control, it is also of interest to control the composition of the corona by use of such hydrophilic chains as poly(acrylic) acid (PAA) or poly(4-vinyl) pyridine (P4VP) to obtain acidic, basic, or mixed structures under appropriate pH environments. The control can be achieved by changing the pH of the assembly environment or the block length ratio, as shown in a previous study.31 In the present work, the study is extended to an investigation of the effect of changes in the water content, nature of the solvent, and the molar ratio of the blocks. In previous studies of block copolymer assemblies, a systematic understanding of controllable morphogenetic factors has emerged. These include, among others, polymer block lengths and composition, nature of common solvent, initial polymer concentration, type and concentration of added ions, © 2014 American Chemical Society

method of preparation, and presence of homopolymer and polydispersity.4−6,9,25,27 The factors influencing the properties of small molecule vesicles, that is, liposomes, have received considerable attention.32,33 Vesicles prepared from block copolymers, on the other hand, have been studied much less extensively, especially in regard to their equilibrium status. It was shown previously,4,34 that vesicles are equilibrium structures under a range of conditions since their sizes, and many of their other properties, can be changed reversibly by changing the composition of the solvent mixture or other parameters of the solution in which they are prepared. A thermodynamic curvature stabilization mechanism for block copolymer vesicles has been proposed by Luo and Eisenberg.34 Stabilization consists of preferential segregation of the short hydrophilic corona chains to the interior of the vesicles and of the long chains to the exterior. Segregation increases the corona repulsion on the outside of the vesicle relative to that on the inside and provides thermodynamic Received: July 18, 2014 Revised: August 20, 2014 Published: September 7, 2014 13152

dx.doi.org/10.1021/la5028527 | Langmuir 2014, 30, 13152−13163

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molar ratio of 2:3 (40 mol % of PS-b-PAA chains). The authors concluded,34 from the ζ potential results, that the external corona of the observed vesicles is composed of P4VP in the absence of additives such as HCl or NaOH. The block copolymer combination of PS313-b-PAA11 and PS310-b-P4VP33 in DMF, in the present work, was chosen to reproduce the system of Luo and Eisenberg34 and, initially, to obtain vesicles under the same conditions and in the same molar ratio. However, additional molar ratios were also examined for the same block copolymer combination in order to understand the effect of changing the PAA to P4VP molar ratio. The current work underscores the extreme complexity of the behavior and the complicated nature of the interplay between block length, degree of ionic character, and solubility on the morphology and corona composition, and illustrates the extreme sensitivity of the system to external stimuli. While the present system is extremely complicated in regard to both morphology and corona composition, it is worth exploring because it illustrates very clearly the complex interactions between the polyelectrolyte effect and the effects of block length and solubility (especially in vesicles). The final morphologies and corona compositions depend on the balance of number of factors, only some of which can be controlled independently. The understanding of the phenomena is, at this point, largely qualitative; however, as a result of the study, one can control and fine-tune the morphology, size, and corona composition of the aggregates, and especially vesicles, and, thus, influence their behavior over the wide range of conditions.

stabilization of the curvature. The segregation of a mixture of polystyrene diblock copolymers containing both polystyrene-bpoly(acrylic acid) (PS-b-PAA) and polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was explored.34 One aspect of the evidence of segregation consisted of a comparison of the ζ potential of vesicles of only PS310-b-P4VP33 and of only PS310-bPAA44 with that of vesicles prepared from mixed copolymers of PS310-b-P4VP33 with PS310-b-PAA11. The ζ potential of the mixed systems was identical with that of vesicles prepared from pure PS-b-P4VP, proving that the longer P4VP chains were forming the external corona in the absence of polyelectrolyte effects.34 In the case of mixtures of two block copolymers, such as PSb-PAA and PS-b-P4VP, either the PAA or the P4VP, or a mixture of the two, can be directed to the outside if the vesicles are prepared under appropriate conditions; the interface compositions are dependent on relative coil dimensions during assembly. On the basis of previous studies,34,37 in which the block lengths were approximately the same, at low pH (prep) the corona was expected to consist of P4VP and, as the pH (prep) of the system increases, one would expect the PAA to form the external corona; these effects are expected because of ionization accompanying protonation of P4VP at low pH and deprotonation of PAA at high pH. Surprisingly, as reported in the previous publication,31 it was found that at extreme pH (prep) values, the opposite behavior to that expected on the basis of other studies,34,37 was encountered, in that at high pH (prep) the vesicular corona was composed of P4VP chains while at low pH (prep) the PAA chains were forming the corona. The behavior was, thus, the opposite of what would be expected on the basis of the polyelectrolyte effect.31 Morphological changes are frequently induced by very small changes in the environment.38 The present work extends the previous study by exploring the morphological consequences of changes in factors other than block length, pH, coil dimension, and salt concentration by focusing on the effects of solvent, water content, and molar ratios of hydrophilic blocks of the block copolymers. As was observed before, even a slight change of only the pH, which changes the coil dimensions, can have a major effect on the morphology.31 As an illustrative example of the interplay of effects, one should note that the changes in pH influence coil dimensions; the latter, in turn, modify the polymer concentration in the region close to the interface.31 Therefore, it should not come as a surprise that a systematic variation in the pH changes the relative importance of several morphogenic parameters, so that the system can undergo multiple morphological changes upon changing one variable. Frequently the specific trigger for a morphological transformation is impossible to identify among the many contributing factors. In the present work, only the unambiguous ones will be described. To understand better the self-assembly behavior of block copolymer mixtures in various solvents, the effect of the nature of the solvent on the morphology and corona composition in a mixture of PS310-b-PAA45 and PS310-b-P4VP33 block copolymers was studied at various HCl/P4VP or NaOH/PAA molar ratios (R). The effect of water content on the morphology and corona composition for mixtures of PS-b-PAA and PS-b-P4VP is also described in the present publication. The water content effect was studied specifically for the PS310-b-PAA27 and PS310-bP4VP33 mixture in DMF. Luo and Eisenberg34 studied the morphology and corona composition in a mixture of PS313-bPAA11 and PS310-b-P4VP33 block copolymers in DMF at a

2. EXPERIMENTAL SECTION A solution was prepared in DMF containing 0.5 wt % of each of the diblock copolymers of PS-b-PAA and PS-b-P4VP. DMF is a common solvent for both diblocks. The samples were stirred overnight to ensure thorough mixing. For samples involving addition of acids or bases (HCl or NaOH), a calculated amount needed to reach a given molar ratio, R, of HCl/4VP or NaOH/AA, was added to the copolymer solutions. The solutions containing additives (HCl or NaOH) were again mixed overnight. Deionized water was then added dropwise to the copolymer solutions at a rate of 2.0 wt %/min, until the water content reached 50 wt %. The solution was stirred continuously during water addition. After measuring the pH (prep) (apparent pH, the value as measured for the mixed solvent system in which the aggregates were prepared), the samples were quenched into a six-fold excess of water to freeze the morphology. The samples were then dialyzed for 3 days against mili-Q water of pH 3−4 to keep the colloid solutions from precipitating and to remove all the solvent from the solutions. The aggregates do not undergo any morphological changes at the high water contents present during dialysis. The same conditions as those described above were employed for the studies of the effects of changes in solvent, water content, and mixture composition on the morphology and corona composition, except that the main variable was changed in the study of each effect. The details of each individual study are given in the sections below. Further experimental details concerning evaluation of morphology and corona composition are given in the Supporting Information. It should be noted that in the text below the mixtures of PS-b-PAAx and PS-b-P4VPy will be referred to as AAX/VPY, where x and y are number-average chain lengths; the PS block length is excluded from the notation because in all cases described here it is very similar (∼300 units). For example, the mixture of block copolymers of PS313-b-PAA45 and PS310-b-P4VP33 will be referred to as AA45/VP33. 2.1. Effect of Solvent on the Morphology and Corona Composition in Mixtures of PS310-b-PAA45 and PS310-b-P4VP33. Mixtures of AA45/VP33 were employed for the solvent effect study. The copolymers were dissolved in various solvents, such as DMF, a 13153


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Table 1. Overview of the Solvents Used for the Mixtures of PS310-b-PAA45 and PS310-b-P4VP33 Block Copolymers at Various HCl/P4VP or NaOH/PAA Ratios DMF (HCI)/(PVP) 10.0 7.0 5.0 3.0 1.5 1.0 0.8 0.5 0

DMF/THF (85/15)

(NaOH)/(PAA) 10.0 7.0 5.0 3.0 1.5

(HCI)/(PVP) 10.0 5.0

THF

(NaOH|/(PAA)

(HCI)/(PVP)

PS310-b-PAA45 and PS310-b-P4VP33 10.0 10.0 5.0

5.0

1.5

(HCI)/(PVP)

(NaOH)/(PAA)

10.0

10.0

10.0

5.0

5.0 3.0

5.0

1.5

1.0

1.0

0.8 0

dioxane

(NaOH)/(PAA)

1.5 1.0 0.8

0

0

Figure 1. Schematic representation of the solvent effect on the morphology and corona composition for mixture of PS313-b-PAA45 and PS310-bP4VP33 in DMF.31 mixture of DMF/THF (85/15 w/w), THF and dioxane, for various HCl/P4VP or NaOH/PAA molar ratios, as shown in Table 1. It is important to distinguish the notation “pH” and “pH (prep)” as used here. “pH” corresponds to the pH value at which the electrophoretic mobility was measured in water, while “pH (prep)” refers to the pH values as measured in the mixed solvent system in which the aggregates were actually prepared. 2.2. Water Content Effect on Morphology and Corona Composition in Mixtures of PS313-b-PAA27 and PS310-b-P4VP33 in DMF. Mixtures of AA27/VP33 were employed in the study of the effect of changes in the water content. The molar ratio of block copolymer chains in the mixture was kept constant (1:1), while the water content was varied (20, 35, 50, 65 wt %) at the two given R values, i.e. acidic (HCl/P4VP = 10.0) and basic (NaOH/PAA = 1.7). 2.3. Block Copolymer Molar Ratio Effect on the Morphology and Corona Composition in Mixtures of PS313-b-PAA11 and PS310-b-P4VP33 in DMF. Mixtures of AA11/VP33 were employed in

the study of the effect of changes in the molar ratio. The mixtures did not contain any additives such as HCl or NaOH in the preparative stage. The ratios for this mixture were 1:0 (pure PS313-b-PAA11), 3:1 (75 mol % of the PAA containing block copolymer), 3:2 (60% PAA), 1:1 (50% PAA), 2:3 (40% PAA) (this mixture duplicates that of Luo and Eisenberg,34 1:3 (25% PAA), and 0:1 (pure PS310-b-P4VP33).

3. RESULTS AND DISCUSSION The most commonly obtained morphologies in this study are small spherical micelles (SSMs), vesicles, rods, large compound micelles (LCMs), elongated vesicles and large compound vesicles (LCVs). A schematic representation, symbol, and TEM micrographs of the aggregate morphologies found in the mixtures of PS-b-PAA and PS-b-P4VP are shown in Figure 5 SI in the Supporting Information. Figure 6 SI give a schematic representation of one of the obtained morphologies−the SSMs. 13154


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al.,39 the DMF/THF (85/15 w/w) solvent combination was chosen for this study, despite the fact that some differences exist in block lengths and number of segments in the diblocks (PS313-b-PAA45 and PS310-b-P4VP33) and triblock (PAA26-bPS890-b-P4VP40). The results for the AA45/VP33 mixture in the DMF/THF (85/15 w/w) solution are given in Figure 2. It should be noted

Figure 6 SI shows schematically the formation of an interpolyelectrolyte complex (IPEC)24,35,36 in DMF (Figure 6 SI image B). Self-assembly in water, showing the collapsed PS core, as well as the IPEC, which is collapsed but still swollen (in green), and the excess PAA in the solvent (blue segment outside of the green region) is indicated by Figure 6 SI image C. Finally, on drying, the PS core is surrounded by a shell of the IPEC, which is further surrounded by a shell of the excess PAA (or P4VP if that is present in excess) as shown in Figure 6 SI image D. The most common morphologies found in the mixture of block copolymers include SSMs, LCMs, and vesicles. The corona composition for each of the observed morphologies was examined by ζ potential measurements (for vesicles, only the external interfacial corona could be monitored) as described in the Supporting Information in sections 1.3. and 1.7−1.9. It should be borne in mind that the use of ζ potentials to obtain an estimate of the corona composition yields only approximate values, but it does provide useful qualitative insight into the corona as a function of the overall composition. 3.1. Effect of Solvent on the Morphology and Corona Composition for the Mixture of PS310-b-PAA45 and PS310b-P4VP33. The effect of solvent in the AA45/VP33 (at a 1:1 (50% PAA) molar ratio) was studied at various HCl/P4VP and NaOH/PAA R ratios (as shown in Table 1 above). This particular block copolymer mixture combination was chosen due to relative similarity in the length of the PAA and P4VP blocks. The same mixture was explored extensively in the study of the pH effect.31 The results for mixtures of block copolymers dissolved in various solvents, that is, DMF, DMF/THF (85/15), THF, dioxane, are described in the following sections in terms of the resulting morphologies and corona compositions. The results for the different solvents are given in Figures 1−4, and the overall trends ascribable to solvent effects are summarized in Figure 5. Morphologies and the corona compositions depend on the relative solubility parameter (δ) values of each solvent and polymer pair. The dissolution of the polymer is optimal if the solubility parameters of polymer and solvent are similar. An overview of the values of the solubility parameters of polymers and solvents used in this study is given in Table 1 SI in the Supporting Information. It should be noted that the use of solubility parameters in this context gives only approximations, but is useful in improving our qualitative understanding of this system. 3.1.1. PS310-b-PAA45 and PS310-b-P4VP33 in DMF. The results for the AA45/VP33 mixture in DMF are given in Figure 1. In examining the figure, it is worth recalling that the solubility parameter of DMF (24.8 (MPa)1/2) is comparable to that of acrylic acid (24.6 (MPa)1/2). The value of the solubility parameter of pyridine is 21.8, while that of styrene is 19.0 (MPa)1/2. This specific mixture (AA45/VP33) in DMF was already discussed in detail in the previous publication,31 the results are also summarized in the Supporting Information in section 2.1. To avoid repetition, therefore, a detailed discussion of the morphological features and of the corona composition is omitted from the main text of this publication. However, the figure is reproduced here to facilitate comparison with the results for other solvents. The explanation of the symbols utilized in tables, as well as a brief discussion of the general features of the tables is given in the in the Supporting Information in Figure 5 SI. 3.1.2. PS310-b-PAA45 and PS310-b-P4VP33 in DMF/THF (85/ 15 w/w). Inspired by the study of triblock copolymers by Liu et

Figure 2. Schematic representation of the solvent effect on the morphology and corona composition for the mixture of PS313-b-PAA45 and PS310-b-P4VP33 in DMF/THF (85/15 w/w).

that the value of the solubility parameter of DMF/THF (85/15 w/w) solvent mixture lies between 24.8 and 19.4 (it is approximately 24.0 taking the weight-average for the components), which is closer to the solubility parameter of acrylic acid (24.6) than to that of P4VP (21.8 for pyridine). As shown in Figure 2, as the pH (prep) increases, the morphology changes from a coexistence of LCMs and vesicles at R = 10.0 (A) to LCMs in the R value region between 5.0 (B) and 0 (D). In the basic range, at R = 1.7 (E), large polydisperse vesicles appear of an average size around 600 nm. With increasing pH (prep), the coexistence of LCMs, SSMs, and vesicles is observed at R = 5.0 (F), which, at R = 10.0 (G), changes to a mixture of LCVs, vesicles, and SSMs. The observed morphological changes are, most likely, due to environmentally induced changes in corona dimensions and degree of solubility. At the acidic R values from 10.0 (A) to 1.0 (C), the quaternized P4VP is insoluble in the DMF/THF (85/ 15 w/w) mixture (see solubility for pure DMF as discussed in the Supporting Information in section 1.12.), and forms the core of the inverse micelles, which then coalesce to LCMs, leaving PAA in the corona. Judging from the ζ potential of the sample of R value of 10.0 (A), the corona, at pH = 3.0, is composed of both types of chains, with PAA being dominant. The formation of vesicles, in addition to LCMs, at the acidic R 13155


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values of 10.0 (A) is unexpected. The corona of the LCMs, at the acidic R values of 5.0 (B) and 1.0 (C), is also composed of mostly PAA. At the R value of 0 (D), in the absence of additives, the ζ potential results indicate that the solubility of P4VP is better than that of PAA, since both PAA and P4VP chains are present in the external corona, but with a slight excess of P4VP. The system is probably close to the isoelectric point, which might be the reason for the formation of LCMs. At the basic R value of 1.7 (E), the morphology suddenly changes to vesicles, containing mostly P4VP chains in the corona, possibly due to the improvement of the solubility of P4VP and worsening of the solubility of PAA chains in the solvent mixture. The morphology and corona composition remains essentially unchanged even for the basic R values of 5.0 (F) and 10.0 (G). At the basic R value of 10.0 (G), LCVs are seen, accompanied by vesicles and SSMs. The transition region (corona composed of a mixture of PAA and P4VP) is located at R = 0 (i.e. absence of additives) at which point LCMs are present. Compared to the sample prepared in pure DMF, the transition region here is shifted toward more basic regions. In the acidic region, the corona is composed of mostly PAA chains. By contrast, the corona of the aggregates in the basic regions is composed of mostly P4VP or of pure P4VP, similar to the behavior of mixtures of PS-b-PAA and PS-b-P4VP in DMF (described in the previous publication31). Again, solubility effects are dominant rather than polyelectrolyte behavior. 3.1.3. PS310-b-PAA45 and PS310-b-P4VP33 in THF. The aggregates, obtained from the AA45/VP33 mixture in THF, are mostly combinations of vesicles with aggregates of other morphologies, as shown in Figure 3. The solubility parameter of THF is 19.4 (MPa)1/2, which is comparable with that of P4VP (21.8 for pyridine). The value of the solubility parameter of acrylic acid is 24.6. At the acidic R value of 10.0 (A), a mixture of rods and vesicles containing P4VP chains in the external corona is present. LCMs are not formed at that point; they would be expected at this pH (prep), due to the insolubility of PAA, but good solubility of quaternized P4VP in THF. The fact that the solubility parameter of THF (19.4) is closer to that of P4VP (21.8 for pyridine) than to that of acrylic acid (24.6), might be the reason for vesicle formation under the present experimental conditions, because the dimensions of the shorter P4VP coil might become comparable to those of numerically longer PAA. The initial mixture in the acidic region at R = 10.0 (A) changes, with increasing pH (prep), to large vesicles, accompanied by LCMs at R = 5.0 (B). The morphology changes, possibly because of pH related changes in the solubility of PAA and P4VP chains in THF, and the accompanying changes in coil dimensions. At the acidic R value of 1.0 (C) the morphology remains basically the same, in that a mixture of relatively small vesicles and LCMs is observed. In the absence of additives, at R = 0 (D), LCMs are observed. As the pH (prep) increases to the basic region, LCMs are present, accompanied by vesicles and LCVs at R = 1.7 (E). No explanation can be given at this point for the formation of vesicles and LCVs. The morphology changes again at R = 5.0 (F) to a coexistence of vesicles and LCMs, and then to a coexistence of LCVs and vesicles at R = 10.0 (G). Interestingly, the corona in both acidic and basic regions is composed of P4VP or mostly P4VP. No transition region is seen in this solvent, possibly due to the good solubility

Figure 3. Schematic representation of the solvent effect on the morphology and corona composition for mixture of PS313-b-PAA45 and PS310-b-P4VP33 in THF.

of P4VP in THF. The ζ potential results suggest that THF is not a good solvent for the ionic PAA chains in that pH region. 3.1.4. PS310-b-PAA45 and PS310-b-P4VP33 in Dioxane. The morphologies obtained from the AA45/VP33 mixture of block copolymers in dioxane are shown in Figure 4. It should be noted that the value of the solubility parameter of dioxane is 20.5 (MPa)1/2, which is comparable to that of P4VP (21.8 for pyridine). The value of the solubility parameter of acrylic acid is 24.6. In the AA45/VP33 mixture in dioxane in the acidic region, at R values between 10.0 (A) and 1.0 (D), vesicles are observed with mostly P4VP chains in the external corona. The vesicles are present in those regions possibly due to quaternization of the numerically shorter P4VP chains, as well as to the similarity of the solubility parameters of dioxane (20.5 (MPa)1/2) and pyridine (21.8), which suggests that the solubility of P4VP chains in dioxane should be good. PAA is probably less soluble, which would make its coil dimensions smaller, and therefore the PAA is located in the inner corona. At the acidic R value of 1.0 (D), the large vesicles are accompanied by LCMs. At the R value of 0 (E) (in the absence of additives) as well as in the basic solution of R = 0.75 (F), only LCMs are present, again with mostly P4VP chains in external corona. The reason for LCMs formation is possibly the proximity of the system to the isoelectric point. With an increase in pH (prep), at R values ranging from 1.7 (G) to 10.3 (I), in addition to LCMs, large LCVs are formed. The existence of LCVs may possibly be explained by poor solubility (but not insolubility) of PAANa in dioxane. Similar to the case of THF, at all the ratios the corona consists of only P4VP or mostly P4VP, and no coronal transition region is seen in dioxane. 13156


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Figure 4. Schematic representation of the solvent effect on the morphology and corona composition for a mixture of PS313-b-PAA45 and PS310-bP4VP33 in Dioxane.

Figure 5. Schematic representation of the solvent effect on the morphology (expressed by schematic symbols) and corona composition (expressed by the color codes) in PS313-b-PAA45 and PS310-b-P4VP33 mixtures at pH = 3.0 and 4.2. The corona (organized in the sequence spectral colors) consists of pure PAA (blue), mostly PAA (green), mixture of PAA and P4VP (yellow), mostly P4VP (orange), pure P4VP (pink). The solubility parameter of acrylic acid is 24.6, of vinylpyridine is 21.8, and of styrene is 19.0 (MPa)1/2. 13157


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3.1.5. Solvent Effect on Morphology. Now that the morphologies of the AA45/VP33 mixtures in various solvents, i.e. DMF, DMF/THF (85/15 w/w), THF, and dioxane have been discussed individually, the solvent effect can be summarized in a schematic representation as shown in Figure 5. It is clear that LCMs are distributed all over the table. Vesicles appear in DMF and DMF/THF (85/15 w/w), mostly in the basic regions, in contrast to what is seen in THF and dioxane, where they are present mostly in acidic regions. LCVs are observed in DMF/THF (85/15 w/w), THF and dioxane in acidic regions, but they are not present in DMF. Elongated vesicles are observed only in the acidic region (R = 3.0) of DMF, and rods are found only in THF mixed with vesicles. The reason for formation of elongated vesicles and rods is not clear. In the absence of additives (R = 0) in all solvents, only LCMs are observed. Complex formation via acid−base interactions may be involved here. The morphology depends mainly on the solubility of P4VP and PAA in the given solvent and, thus, on the coil dimensions of PAA and P4VP chains, as described above for the individual solvent systems. 3.1.6. Solvent Effect on Corona Composition. The schematic representations of the solvent effect on the corona composition of PS313-b-PAA45 and PS310-b-P4VP33 mixtures, measured using electrophoresis at pH = 3.0 and 4.2, is given in the two parts of Figure 5 for the two pH values. It is clear from both tables, that in the case of both the DMF and DMF/THF (85/15 w/w), either PAA or a mixture of PAA and P4VP is present in the corona of the aggregates in the acidic region at R = 10.0 (in contrast to the results of Luo and Eisenberg34). As the pH (prep) of the samples increases, the corona composition changes to a mixture of PAA and P4VP, to mostly P4VP, and finally to pure P4VP. The corona composition of the aggregates prepared in THF or dioxane does not show the same trend with increasing pH (prep) as was seen for DMF and DMF/THF (85/15 w/w) solvents. In the case of THF and dioxane, the corona composition of the aggregates does not change with increasing pH (prep), that is, the corona is composed of mostly P4VP or of pure P4VP, possibly due to the similarity of the solubility parameters of vinylpyridine to that of the solvents. As pointed out before, the morphologies and corona compositions depend mainly on the solubilities of P4VP and PAA in the given solvents, and thus on the coil dimensions of the PAA and P4VP chains, for the individual solvent systems. If the solubility parameter of the solvent is similar to that of acrylic acid, which is the case for the solution of P4VP in DMF, the system behaves in such a way that at acidic R values, LCMs are the most commonly encountered structure, containing PAA chains in the corona. This behavior results from the insolubility of P4VPCl in DMF and in the mixture of DMF/THF (85/15 w/w). If the solubility parameter of the solvent is similar to that of P4VP, which is the case for THF and dioxane, vesicles or a mixture of vesicles and LCMs are observed in the acidic regions. The corona of the aggregates is composed of mostly P4VP or only P4VP chains. This behavior possibly results from the similarities of solubility parameter values of the solvent and of P4VP. In the basic regions, primarily vesicles or LCVs containing mostly pure P4VP chains are observed in all solvents. As the solubility parameter value of the solvent approaches that of the solubility parameter of P4VP, more LCMs are observed in basic regions. The reason for the formation of vesicles or LCVs is, most likely, the poor solubility of PAA in the solvents under

basic conditions, that is, if present as poly(sodium acrylate). In the region without additives, only LCMs are observed. Complex formation via acid−base interactions may be the reason for such behavior, with the complexes acting as the cores of the inverse micelles in the LCMs. 3.2. Water Content Effect on Morphology and Corona Composition in the Mixture of PS313-b-PAA27 and PS310b-P4VP33 in DMF. As part of the investigation of the effect of changing various parameters on the morphology and corona composition for mixtures of PS-b-PAA and PS-b-P4VP, the effect of a change in the water content was also explored. The water content effect was studied for the AA27/VP33 mixture in DMF. This block copolymer combination was selected because it is the only one forming vesicles as the sole morphology under both acidic and basic conditions. The mixtures in DMF were evaluated at various water contents and at two pH (prep) values, that is, HCl/P4VP = 10.0 and NaOH/PAA = 1.7. The results are given in Figures 6

Figure 6. Schematic representation of water content effect on the morphology and corona composition for the mixture of PS313-b-PAA27 and PS310-b-P4VP33 in DMF at ratio of HCl/P4VP = 10.0.

and 7, respectively. General morphological and corona composition trends are shown in Figure 8. Not unexpectedly, the water content influences both the size and shape of block copolymers aggregates. Water acts as precipitant for the hydrophobic block in the common solvent.40 As the water content is increased in the polymer solution beyond the cwc, a series of morphological transitions is induced (most commonly from SSMs to rods and eventually to vesicles, with narrow coexistence regions between the pure morphologies).40 3.2.1. PS313-b-PAA27 and PS310-b-P4VP33 at the HCl/P4VP Ratio of 10.0. At the acidic R value of 10.0, coexistence of 13158


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As the water content increases, the morphology remains essentially unchanged at a water content of 35 wt % (B) (see Figure 6), except that rods are no longer present in the system. The corona of the aggregates is composed mostly or only of PAA. At water content of 50 wt % (C) the morphology changes to only vesicles, a trend which is characteristic of the morphological progression with increasing water content.40 The external corona of the vesicles is composed of mostly PAA. The morphology remains basically unchanged at a water content of 65 wt % (D), except that LCVs begin to appear. The corona of the aggregates is, at this point, composed of mixture of PAA and P4VP chains, which means that the system is in the transition region. Most probably, the coil dimensions of the PAA and P4VP chains change at different rates with increasing water content, which makes the P4VP dimensions comparable to those of the PAA at this point. As expected, the size of the vesicles increases with increasing water content. At a water content of 20 wt % (A), the vesicle size is 48 ± 4 nm, but at 65 wt % (D) the size increases by a factor of five to 245 ± 85 nm; the vesicles are highly polydisperse. Luo and Eisenberg explained the increase in vesicle size with water content on the basis of surface energy considerations.41 3.2.2. PS313-b-PAA27 and PS310-b-P4VP33 at the NaOH/PAA Ratio 1.7. The morphologies obtained from the AA27/VP33 mixture in DMF at the basic R = 1.7 are shown in Figure 7. On the basis of the polyelectrolyte effect alone, one would expect the external corona to be composed of PAA; in fact, the opposite is encountered, in that vesicles with an external corona of only P4VP chains are obtained at a water content of 20 wt % (A). The formation of such vesicles under the present experimental conditions occurs, most likely, because the PAANa chains are less soluble in DMF than the P4VP chains (the solubility is reviewed in the Supporting Information in section 1.12.). The morphology remains unchanged with increasing water content up to 65 wt % (D). The size of the vesicles also does not vary significantly, remaining constant at about 80 nm over the whole range. It is surprising that the size of the vesicles does not increase with water content. Most likely, the increased corona repulsion compensates for the increased interfacial energy effect. No transition region is found in the present system; the external corona of the vesicles is composed of only P4VP. 3.2.3. Water Content Effect on Morphology. A schematic representation of the water content effect on the morphology of the AA27/VP33 mixture in DMF at the acidic (R = 10.0) and basic (R = 1.7) ratios is shown in Figure 8. Vesicles are present at both pH values and over the whole range of water contents. In contrast to what is seen at the NaOH/PAA ratio of 1.7, where only similar-sized vesicles are present, the vesicles here are found to coexist with SSMs and rods at the HCl/P4VP ratio of 10.0. 3.2.4. Water Content Effect on Corona Composition. Figure 8 summarizes the results of the water content effect on the corona composition of the aggregates for the AA27/VP33 mixtures in DMF at the acidic (R = 10.0) and basic (R = 1.7) composition ratios. It is clear from the tables of results at both pH 3.0 and 4.2, that the corona composition of the acidic sample of ratio 10.0 is affected by water content; that is, the corona composition changes with water content progressively from solely PAA through mostly PAA to a mixture of PAA and P4VP chains. Possibly, if the water content were to be increased further, the corona composition could change further to mostly P4VP or even only P4VP chains, provided the chain dynamics

Figure 7. Schematic representation of the water content effect on the morphology and corona composition for mixtures of PS313-b-PAA27 and PS310-b-P4VP33 in DMF at a ratio of NaOH/PAA = 1.7.

Figure 8. Schematic representation of the water content effect on the morphology (expressed by schematic symbols) and corona composition (expressed by the color codes) in mixture of PS313-b-PAA27 and PS310-b-P4VP33 block copolymers in DMF at acidic (R = 10.0) and basic (R = 1.7) ratios at pH = 3.0 and 4.2. The corona (organized in the sequence spectral colors) consists of pure PAA (blue), mostly PAA (green), mixture of PAA and P4VP (yellow), mostly P4VP (orange), pure P4VP (pink).

SSMs, rods, and vesicles at a water content of 20 wt % (A) is observed in the DMF solution (see Figure 6). The corona of the aggregates is composed of only PAA chains. The formation of SSMs is possibly induced by the insolubility of the quaternized P4VPCl, which forms the cores. 13159


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extensive study; it was thought advisible, as a first step, to reproduce the results of Luo and Eisenberg34 in the exploration of additional molar ratios in terms of the morphology and corona composition for the same block copolymer combination in order to understand the effect of changing the PAA to P4VP ratio. Mixtures of seven different ratios of PS313-b-PAA11 and PS310-b-P4VP33 blends were prepared in DMF. The morphology and corona composition results are summarized in Figure 9.

remain rapid enough. In the basic sample of ratio 1.7, the corona composition (only P4VP chains) remains unchanged at both pH values (3.0 and 4.2) with increasing water content. This is in marked contrast to the behavior of the acidic sample. Again, the solubility effect is dominant. As mentioned before, in addition to the pH effect, the water content can also influence both the size and shape of block copolymer aggregates. Water acts as a precipitant for the hydrophobic block in the common solvent; therefore, its progressive addition to the solvent changes its interaction parameter with the hydrophobic block. As the water content is increased in the polymer solution, a series of morphological transitions is induced, which, most commonly, progresses from SSMs to rods and eventually to vesicles (with narrow coexistence regions between the pure morphologies).42 The same behavior is seen in the case of the AA27/VP33 block copolymer mixture in DMF at the acidic R value of 10.0; the morphology changes with increasing water content from a mixture of SSMs, rods and vesicles, to a coexistence of spheres and vesicles and finally to only vesicles. These morphological transitions have been ascribed to the changes in the force balance between the three morphogenic factors, that is, core− chain stretching, interfacial energy, and intracoronal chain interactions.2 The free energy of aggregation contains other terms, for example, the entropy change involving the water in the vicinity of the hydrophobic species, or the localization entropy of the junction point at the interface. These latter contributions, however, are not morphogenic. The morphogenic components depend on the structural parameters of the micelles, such as the core dimensions, the aggregation number, and the density of the corona on the surface of the core.42,43 In the case of the AA27/VP33 block copolymer mixture in DMF at a basic R value of 1.7, the water content increase does not influence the morphology above 20%. Vesicles are already formed at that water concentration and remain up to the maximum water content studied. By contrast, the effect on the corona is significant. From the results discussed above, it is clear that in the case of the water content effect, the system behaves similarly to the pH (prep) effect (as described in the previous publication31), in that the corona composition changes with increasing water content gradually from pure PAA, to mostly PAA, to mixture of PAA and P4VP chains, to mostly P4VP and finally to pure P4VP chains. At the acidic R value of 10.0, the corona composition changes gradually, with increasing water content, from only PAA to mostly PAA and to mixture of PAA and P4VP chains. With a further water content increase, the corona is expected to be composed of only P4VP chains because of the high acid content. However, at high water contents the chain dynamics become too slow to be accessible over experimental time scales. In the case of basic R value of 1.7, the corona composition remains unchanged with increasing water content, presumably because of the insolubility of the PAANa in DMF. 3.3. Block Copolymer Molar Ratio Effect on the Morphology and Corona Composition in Mixtures of PS313-b-PAA11 and PS310-b-P4VP33 in DMF. Luo and Eisenberg34 studied the morphology and corona composition in a mixture of PS313-b-PAA11 and PS310-b-P4VP33 block copolymers in DMF at a molar ratio of 2:3 (40% PAA). The authors concluded,34 from the ζ potential results, that the corona of the observed vesicles is composed of P4VP in the absence of additives such as HCl or NaOH. The AA11/VP33 block copolymer combination in DMF was chosen for a more

Figure 9. Schematic representation of the effect of PAA/P4VP molar ratio on the morphology and corona composition in mixture of PS313b-PAA11 and PS310-b-P4VP33 in DMF.

As the content of PS310-b-P4VP33 in the mixture increases, the morphology changes from LCMs in pure PS313-b-PAA11 (A) (molar ratio 1:0) and in the sample with the 3:1 (75% PAA) block copolymer molar ratio (B) to coexistence of LCMs and simple SSMs at a molar ratio of 3:2 (60% PAA) (C) as well as at the equimolar point of PAA/P4VP (D). Only vesicles are observed at molar ratios ranging from 2:3 (40% PAA) (E) to pure PS310-b-P4VP33 (G). The size of the vesicles does not change significantly with increasing PS310-b-P4VP33 content. The transition region (corona composed of a mixture of PAA and P4VP) is located at the 3:1 (75% PAA) block copolymer molar ratio (B). In pure PS313-b-PAA11 (A) the corona is obviously composed of only PAA (R = 1:0) at both pH values, as it must be since no P4VP is present. At ratios above PAA/ P4VP = 3:1 (75% PAA), the corona is composed of mostly P4VP (C) or pure P4VP (D−G), because that is the majority component. LCMs are formed at pure PAA and at molar ratios of 3:1 (75% PAA), 3:2 (60% PAA), and 1:1 (50% PAA) because of the poor solubility of PAA. Vesicles appear beyond 2:3 (40% PAA) ratio because of the presence of the much more soluble P4VP chains. The results obtained are in complete agreement with those of Luo and Eisenberg,34 that is, at the AA11/VP33 block copolymer molar ratio of 2:3 (40% PAA) in 13160


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studied. The results obtained are in a good agreement with those of Luo and Eisenberg.34 With an increase of P4VP content, the morphology changes gradually from LCMs, through the coexistence of LCMs and SSMs, to only vesicles. The corona composition of the aggregates is affected by the molar ratio, and changes progressively from pure PAA to a mixture of PAA and P4VP and finally to pure P4VP with increasing PS310-b-P4VP33 content in the mixture, similar to the changes observed in the study of the pH (prep) effect.31 Under the present experimental conditions and in the absence of additives, the results are in agreement with the size segregation theory. While at this time our understanding of the system is only qualitative, valuable conclusions can be drawn about the importance of relative chain length of the corona forming blocks, the PAA/P4VP ratio, and the solvent effect. Thus, a useful tool has become available for fine-tuning of the morphology and corona composition by appropriate manipulations of PAA/P4VP ratios, which probably can be extrapolated to other coronal pairs and relative block lengths.

DMF, vesicles are the only morphology present, with a corona composed of pure P4VP. With increasing PS310-b-P4VP33 content in the mixture, the morphology transforms gradually from LCMs, through the coexistence of LCMs and SSMs, to only vesicles. As expected, the external corona composition of the aggregates is affected by the block copolymer molar ratio and changes progressively from pure PAA to a mixture of PAA and P4VP and, finally, to pure P4VP with increasing PS310-b-P4VP33 content; this behavior is similar to that seen for the pH (prep) effect.31 Under the present experimental conditions and without additives, the results for vesicles can be understood in terms of the length segregation theory. For pure PS313-bPAA11 (A), the corona, naturally, is composed of PAA. At a low content of PS310-b-P4VP33 (3:1 (75% PAA)) (B) in the mixture, the corona should be composed of mostly PAA or pure PAA; however, because the P4VP chains are longer, the corona is composed, again, of a mixture of PAA and P4VP, as expected from the segregation theory. At the content of PS310b-P4VP33 below equimolar (for example, 3:2 (60% PAA)) (C), but due to the greater length of the P4VP chains, the corona is composed of mostly P4VP, which is consistent with the segregation theory. For equimolar compositions (D), and also at lower block copolymers ratios (E−G), the corona should contain both types of chains in the absence of segregation by length of the PAA and P4VP. Because the P4VP chain is longer than the PAA, the corona is composed of only P4VP.

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ASSOCIATED CONTENT

* Supporting Information S

Description of the experimental aspects of the work, specifically an overview of the block copolymers used, the method of preparing the aggregates, the experimental details of pH measurement, the evaluation of the corona composition, transmission electron microscopy, and electrophoretic mobility; experimental design including the compositions of the mixtures and of the relationship between electrophoretic mobility and corona composition; scheme of data presentation including a table containing the values of solubility parameters of polymers and solvents and an overview of the solubility of PAA and P4VP; an overview of the observed morphologies in the study with schematic representation of IPEC formation along with the results of the pH effect on the morphology for mixture of AA45/VP33 in DMF. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS The goal of the present work is to gain a better understanding of the relative importance of morphogenic factors in block copolymer aggregates, with special emphasis on corona composition in vesicles. The work is an extension of the previous study of pH and block length effects in aggregates composed of mixtures of PS-b-PAA and PS-b-P4VP.31 In the present work, the effects of solubility of the hydrophilic chains, pH, solvent composition, water content, and block length ratios were addressed. The first part of the study is devoted to an exploration of the effect of solvent for the mixture of AA45/VP33. As expected, the degree of similarity of the solubility parameters of solvent and polymers is a major factor. The water content influences both the size and shape of the aggregates. Water acts as a precipitant for the hydrophobic block in the common solvent; therefore, its progressive addition to the solvent changes its interaction parameter with the hydrophobic block. As the water content is increased in the polymer solution, a series of morphological transitions is induced, which, most commonly, progresses from SSMs to rods and eventually to vesicles (with narrow coexistence regions between the pure morphologies).42 The same general behavior is seen in the case of the block copolymer mixture of AA27/ VP33 in DMF. These morphological transitions can be explained by changes in the force balance between the three morphogenic factors. It is clear that in the case of the water content effect on the corona composition, the system behaves similarly to the pH (prep) effect;31 the corona composition changes with increasing water content gradually from pure PAA, to mostly PAA, to a mixture of PAA and P4VP chains, to mostly P4VP and finally to pure P4VP chains in the external corona. Finally, the effect of changes in the block copolymer molar ratio (PAA/P4VP) for a mixture of AA11/VP33 in DMF was

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 514-398-6934. Fax: 514-398-3797. E-mail: adi. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank to Dr. Hongwei Shen for synthesizing PS-b-P4VP diblock copolymers, Dr. Futian Liu for synthesizing PS-b-PAA diblock, and Dr. Tony Azzam for synthesizing the homopolymer of poly(4-vinylpyridine) used to determine the solubility and to measure absorbance curves. We would also like to thank NSERC for providing financial support in this study. Axel Müller acknowledges a visiting professorship sponsored by NSERC.

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ABBREVIATIONS Polystyrene-block-poly(acrylic acid), PS-b-PAA; polystyreneblock-poly(4-vinylpyridine), PS-b-P4VP; poly(acrylic acid), PAA; poly(4-vinylpyridine), P4VP; mixture of PS300-b-PAAx and PS300-b-P4VPy, AAX/VPY; HCl/4VP or NaOH/AA, R; small spherical micelles, SSMs; large compound micelles, 13161


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LCMs; large compound vesicles, LCVs; tetrahydrofuran, THF; dimethyl formamid, DMF; solubility parameter, δ; zeta potential, ζ; transmission electron microscopy, TEM; Supporting Information, SI; IPEC, interpolyelectrolyte complex

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