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Langmuir 2007, 23, 4618-4622
Reorganization of Hydrogen-Bonded Block Copolymer Complexes Nathalie Lefe`vre, Charles-Andre´ Fustin, and Jean-Franc¸ ois Gohy* Unite´ de Chimie des Mate´ riaux Inorganiques et Organiques (CMAT) and Research Center in Micro- and Nano-Materials and Electronic DeVices (CeRMiN), UniVersite´ catholique de LouVain, Place L. Pasteur 1, 1348 LouVain-la-NeuVe, Belgium ReceiVed NoVember 30, 2006. In Final Form: January 15, 2007 Poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymers and poly(acrylic acid) (PAA) have been mixed in organic solvents. Complexation via hydrogen bonding occurs between the P4VP and PAA blocks. Those insoluble complexes aggregate to form the core of micelles surrounded by a corona of PS chains. Reorganization of these structures occurs upon addition of acidic or basic water, which results in the breaking of the hydrogen bonds between the P4VP and PAA blocks. After transfer of the initial complexes in acidic water, micelles consisting of a PS core and a protonated P4VP corona are observed. In basic water, well-defined nanoparticles formed by the PS-b-P4VP copolymers are obtained. It is demonstrated that these nanoparticles are stabilized by the negatively charged PAA chains. Finally, thermally induced disintegration of the micelles is investigated in organic solvents.
Introduction The micellization of diblock copolymers in a selective solvent for one of the blocks is a very easy method to produce welldefined nanoobjects that may be useful for a variety of applications.1 Micelles from diblock copolymers are characterized by a rather simple structure consisting of a core formed by the insoluble blocks surrounded by a corona of the soluble blocks.2 Although spherical micelles are the most widely reported examples, other morphologies including rods, vesicles, and other intricate morphologies have been described, depending on the chemical nature and the composition of the diblock copolymers.3 Moreover, more complex structures than core-corona micelles have been obtained whenever ABC triblock copolymers are considered for micellization. In that respect, various multicompartment micelles have been recently obtained. Information on these micelles can be found in a recent review.4 To broaden the range of micellar structures and functionalities, specific noncovalent interactions have been recently considered as driving forces for micellization. In this respect, noncovalent interactions between mutually interacting polymer blocks or between a polymer block and low molecular weight functional molecules such as surfactants have been used. The complexes resulting from these noncovalent interactions may be insoluble and further aggregate into a micellar core. These micellar cores are surrounded by a corona formed by the polymer blocks which were not involved in the complexation process.5,6 Some early examples of these strategies can be found in, e.g., complexes formed by mixing poly(ethylene oxide)-block-poly(sodium methylacrylate) diblock copolymers either to cationic surfactants7,8 or to quaternized poly(4-vinylpyridine) homopolymers,9 and in mixtures of poly(ethylene oxide)-block-poly(L-lysine) and (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, UK, 1998. (2) Riess, G. Prog. Polym. Sci. 2003, 28, 1107. (3) Gohy, J.-F. AdV. Polym. Sci. 2005, 190, 65. (4) Fustin, C. A.; Abetz, V.; Gohy, J.-F. Eur. Phys. J. E 2005, 16, 291. (5) Zhou, S.; Chu, B. AdV. Mater. 2000, 12, 545. (6) Jiang, M.; Li, M.; Xiang, M.; Zhou, H. AdV. Polym. Sci. 1999, 146, 121. (7) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 3519. (8) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (9) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6797.
poly(ethyleneoxide)-block-poly(asparticacid)blockcopolymers10-12 More examples of similar complexes can be found in ref 3. All these examples based on aqueous systems rely on electrostatic interactions between the mutually interacting blocks to create water-insoluble complexes that further aggregate into well-defined micelles. Besides ionic interactions, hydrogen bond interactions have also been used to trigger complexation between polymer blocks and further aggregation.13 To ensure the success of the complexation process, competitive hydrogen bonding between polymer blocks and solvent molecules has to be minimized. Therefore, complexation has to be generally performed in apolar and/or aprotic organic solvents. Such a strategy has been exemplified recently in the works of Jiang’s group on hydrogenbonded complexes formed by poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers and low-MW molecules bearing a carboxylic acid headgroup in chloroform. The resulting complexes may aggregate into micelles or not, depending on the nature of the low-MW molecule. For example, aggregating (insoluble) complexes were observed in chloroform after complexation of PS-b-P4VP with formic acid, while nonaggregating (soluble) ones were formed with molecules with a longer aliphatic tail such as acetic, stearic, or decanoic acid.14 The detrimental effect of alkyl chains on the aggregation of the complexes could be overcome by using molecules with perfluorinated tails. In that case, aggregating complexes were always observed.15-17 In a very recent paper, we have studied the formation of micelles in an organic solvent by mixing PS-b-P4VP copolymers with PAA homopolymers.18 Strong hydrogen bonding between the P4VP and PAA blocks induced the formation of complexes which further aggregate into a micellar core surrounded by PS chains. (10) Harada, A.; Kataoka, K. Science 1999, 283, 65. (11) Harada, A.; Kataoka, K. Macromolecules 1995, 28, 5294. (12) Harada, A.; Kataoka, K. J. Controlled Release 2001, 2, 85. (13) Chen, D.; Jiang, M. Acc. Chem. Res. 2005, 38, 494. (14) Chen, D.; Peng, H. S.; Jiang, M. Macromolecules 2003, 36, 2576. (15) Yao, X.; Chen, D.; Jiang, M. J. Phys. Chem. B 2004, 108, 5225. (16) Hu, Z.; Varshney, S.; Jonas, A. M.; Gohy, J.-F. J. Am. Chem. Soc. 2005, 127, 6526. (17) Hu, Z.; Verheijen, W.; Hofkens, J.; Jonas, A. M.; Gohy, J.-F. Langmuir 2007, 23, 116. (18) Lefe`vre, N.; Fustin, C. A.; Varshney, S.; Gohy, J.-F. Submitted for publication to Polymer.
10.1021/la063477i CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
Reorganization of Block Copolymer Complexes Scheme 1. Reorganization of the Micellar Structure Due to the Addition of Acidic or Basic Water
Langmuir, Vol. 23, No. 8, 2007 4619 Addition of Water. Water at different pH values, obtained with HCl or NaOH, was added gradually to the initial solutions. After each water addition, the solutions were stirred for 20 min for equilibration. Once the volume fraction of water reached 90%, the solutions were dialyzed against pure water (at the appropriate pH) for 24 h to remove DMF. Dynamic Light Scattering Measurement. The hydrodynamic radius, Rh, of the micellar complexes was measured by dynamic light scattering (DLS). The experimental autocorrelation function, g(t), is commonly expressed in the form of a cumulant expansion:
[
g(t) ) exp -Γ1t +
() ()
Γ2 2 Γ3 3 t t + ... 2! 3!
]
(1)
where Γi is the ith cumulant and Γ1 ) Dq2, where D is the translational diffusion coefficient and q is the absolute value of the scattering vector. The diffusion coefficient is related to the hydrodynamic radius Rh by the Stokes-Einstein equation:
The influence of the relative lengths of the different blocks and the quality of the solvent (THF or DMF) toward the complexes on the size of the micelles was investigated. In DMF, which is a better solvent for the complexes, nonaggregating, soluble complexes were observed when the interacting blocks were sufficiently small. In all other cases, micelles whose size mainly depends on the length of the P4VP blocks were obtained.18 In the present investigation, we will study the reorganization of these micelles upon modulation of the hydrogen bonds in the complexes. To achieve this goal, different strategies will be used. The first one consists in the modification of the functional groups responsible for hydrogen bonding in the complexes. Addition of acidic water will protonate the P4VP blocks, while basic water will lead to the deprotonation of the carboxylic acid groups. In both cases, the initial hydrogen-bonded complexes will be disrupted. Because an aqueous medium will affect the solubility of the other blocks (PS or neutral P4VP), a reorganization of the micellar structure is also expected during the addition of acidic or basic water. A second strategy will be to modulate the assembly/ disassembly of the micelles containing hydrogen-bonded cores in organic solvents by a variation of the temperature. Experimental Section Materials. The polymers were purchased from Polymer Source Inc. apart from the PS standard, which was purchased from TOSOH Corp. Two poly(styrene)-block-poly(4-vinylpyridine) copolymers with the abbreviations PS-b-P4VP (34000-3000) and PS-b-P4VP (20000-19000) (where the numbers in parentheses represent the number-average molecular weight, Mn (g/mol), of each block) were used. Their polydispersity is 1.1 in each case. Three homopolymers were also used: a poly(acrylic acid) homopolymer with an Mn of 20000 g/mol and a polydispersity of 1.1, a poly(styrene) homopolymer with an Mn of 38000 g/mol, and a poly(4-vinylpyridine) with an Mn of 18000 g/mol and a polydispersity of 1.1. N,N-Dimethylformamide (DMF) was of analytical grade. Preparation of Micelles. Micelles have been prepared by mixing known amounts of PS-b-P4VP copolymers with PAA homopolymers. The amount of each polymer and the volume of solvent have been adjusted to obtain a concentration of 10 g/L after mixing. Moreover, an acrylic acid/4-vinylpyridine 1/1 (mol/mol) stoichiometry was used for all samples. The micellar solutions were obtained by first dissolving one partner into DMF, followed by the addition of the second partner as a powder. The solutions were stirred for 1 h before being diluted 10 times to obtain a concentration of 1 g/L. After dilution, the solutions were left to equilibrate for 1 h. Identical micelles were obtained whether PS-b-P4VP is first dissolved and PAA is added as a powder, or the reverse. Solutions containing only the copolymer PS-b-P4VP or the homopolymer P4VP or PS were also prepared at the same concentration as the micellar systems.
Rh ) kbT/6πηD
(2)
where kb is the Boltzmann constant and η the viscosity of the solvent. DLS measurements were performed on a Malvern CGS-3 apparatus equipped with a He-Ne laser with a wavelength λ of 632.8 nm. The temperature was controlled at 25 °C, and the measurements were done at an angle of 90°. The polydispersity index (PDI) of the micelles was estimated from the Γ2/Γ12 ratio in which Γ1 and Γ2 represent the first and second cumulants, respectively. The experimental data have also been analyzed by the CONTIN method, which is based on an inverse-Laplace transformation of the data and which gives access to a size distribution histogram for the analyzed micellar solutions. All the micellar solutions have been filtered before DLS measurements. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was performed on a Leo 922 microscope operating at 200 kV accelerating voltage in bright field mode. Samples for TEM measurements were prepared by spin-coating the different solutions on a carbon-coated grid, followed by dipping the grid in pure THF to remove nonadsorbed materials, and finally by a drying of the grid in vacuum. Two staining agents have been used: RuO4, which contrasts both P4VP and PS blocks, and iodine, which contrasts only the P4VP block. The staining time was 1 h for RuO4 and 1.5 h for iodine.
Results and Discussion In a previous study, we have prepared well-defined micelles from hydrogen-bonded complexes obtained by mixing PS-bP4VP diblock copolymers and PAA homopolymers in organic solvents.18 In the present study, two types of experiments have been carried out on these systems. First, the influence of water addition at different pH values has been studied. According to the pH, the addition of water can either deprotonate the carboxylic acid moieties of the PAA or protonate the pyridine groups of the P4VP, breaking the hydrogen bonds formed between the P4VP and PAA blocks in the starting solution. Moreover, the presence of water will change the solubility of the different blocks. Second, the influence of the temperature on the stability of the hydrogen bonds between P4VP and PAA blocks has been studied. First, we focus on the micellar system formed between PSb-P4VP (20000-19000) and PAA (20000) in DMF. Three different situations have been studied (see Scheme 1). The first one consists in adding acidic water at a pH low enough to protonate the 4VP units (pH 2). The second case is the addition of basic water with a pH much higher than the pKa of the PAA block (pH 9) to deprotonate the carboxylic acid groups. The last situation is intermediate between the first two and consists in adding water
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Lefe` Vre et al.
Figure 1. Evolution of the Rh with the volume fraction of acidic water at pH 2.
with a pH slightly lower than the pKa of the P4VP in its protonated form and slightly higher than that of the PAA (i.e., pH 4.8). The P4VP and the PAA blocks are thus both partially ionized, but hydrogen bonds can, in principle, still be formed between them. The ionization of the P4VP or PAA block, accompanied by the insolubilization of the PS block due to the presence of water, should induce a reorganization of the initial structures to yield new objects. Figure 1 presents the results obtained in DLS (cumulant analysis) for the addition of acidic water (pH 2) to the initial DMF solution. Upon addition of 10% (v/v) acidic water, a strong increase (above 200 nm) of Rh is observed. Rh then decreases gradually with increasing water content to stabilize around 130 nm. After dialysis to remove the DMF, the size of the objects does not change much, increasing only slightly. Those objects are in fact micelles with a core formed by the insoluble PS blocks and a corona constituted by the protonated P4VP blocks, the PAA homopolymer probably remaining in the form of unimers in the solution since it is soluble at this concentration in acidic water (see Scheme 1). As the chains forming the corona are charged, electrostatic repulsions between pyridine units appear, inducing a stretching of the chains. These repulsions explain the larger size compared to that of the initial micelles. For the first two points (10% and 20% water), sizes larger than those at higher water content are observed. This is attributed to a transition regime between the initial structure and the reorganized micelles, as was observed previously for micelles formed by interpolyelectrolyte complexes.19 Micelles transferred in pure acidic water by dialysis have been observed by TEM (see Figure 2). RuO4 has been used to contrast the P4VP and PS blocks and iodine to contrast selectively the P4VP block. Objects with a radius of about 20 nm have been observed in both cases. The size measured by DLS (130 nm) is much larger than that measured by TEM because, in solution, the charged P4VP chains forming the corona are highly stretched. Upon closer inspection of the TEM image with iodine staining, one can see that the edges of the micelles are darker than the centers, which is not the case for RuO4 staining. This observation is another indication that, after addition of acidic water, micelles with a PS core and an extended P4VP corona are formed. Since the TEM imaging has been performed in the dry state, the P4VP corona is collapsed and the “thickness” of the corona is thus larger around the core than on top of it, inducing a darker contrast of the edges. Since RuO4 stains both PS and P4VP, a homogeneous contrast is observed in this case. (19) Gohy, J.-F., Varshney, S.; Antoun, S.; Je´roˆme, R. Macromolecules 2000, 33, 9298.
Figure 2. TEM images of micelles formed between PS-b-P4VP (20000-19000) and PAA (20000) in pure acidic water (after dialysis).
One last experiment has been performed to confirm the structure obtained after addition of acidic water. PS-b-P4VP (2000019000) has been dissolved in DMF at the same concentration as for the complex formation, and acidic water (pH 2) has then been added as described previously. Micelles, with a PS core and a protonated P4VP corona, exhibiting a size very similar to that obtained starting from the corresponding PS-b-P4VP/PAA complex have been observed. This experiment also evidences that the PAA does not play a role in the final structure since the same objects are obtained in both cases. The DLS results obtained in the case of the addition of basic water (pH 9) are reported in Figure 3. In this case, the Rh stays more or less constant when the volume fraction of basic water increases. These objects have been transferred in pure basic water by dialysis and imaged by TEM (see Figure 4). They have a similar size, 15 nm radius, whatever the staining agent. Since both PS and P4VP blocks are hydrophobic in basic pH, one would expect the precipitation of the PS-b-P4VP (2000019000) after transfer in pure basic water, but this was not observed. To better understand those rather surprising results, we have
Reorganization of Block Copolymer Complexes
Figure 3. Evolution of the Rh with the volume fraction of basic water at pH 9.
Figure 4. TEM images of micelles formed between PS-b-P4VP (20000-19000) and PAA (20000) in pure basic water (after dialysis).
performed several complementary experiments. First, we have checked the behavior of the PS-b-P4VP (20000-19000) without PAA. The copolymer was dissolved in DMF at the same concentration, and basic water was then added gradually. Well-
Langmuir, Vol. 23, No. 8, 2007 4621
defined objects with an Rh of 28 nm were obtained up to a volume fraction of water of 50%. They were imaged by TEM, and a radius around 16 nm whatever the staining agent was measured. For higher water content the formation of a precipitate was observed. This first experiment shows clearly that the PAA plays an important role in the stabilization of the objects since when it is present in the starting system, stable objects can be obtained in pure water, as opposed to only a DMF/water (50/50) mixture when it is absent. The second experiment consisted in checking the solubility of the different components of the systems, i.e., PS and P4VP, in mixtures of DMF and basic water (pH 9). A PS homopolymer (Mn ) 38000) and a P4VP homopolymer (Mn ) 18000) were thus dissolved in DMF before addition of basic water step by step. Unsurprisingly, the PS precipitated immediately when water was added. On the other hand, the P4VP homopolymer formed a stable solution (as confirmed by DLS) up to a volume fraction of water around 70%. P4VP is thus soluble in mixtures of DMF and water up to a 30/70 ratio, and this explains the stability of the objects formed by the PS-bP4VP copolymer. Upon addition of basic water, the PS blocks precipitate and form micellar cores stabilized by the still soluble P4VP chains. When the water content becomes too high, the P4VP chains are no longer soluble and precipitation is observed. The stability limit is lower for the PS-b-P4VP copolymer (50% (v/v) water) compared to the P4VP homopolymer (70% (v/v) water) because of the PS block. To further prove that micelles with a PS core and a P4VP corona are indeed obtained, we have dissolved the PS-b-P4VP (20000-19000) copolymer in DMF and have then added basic water step by step up to a volume fraction of water of 50% as described previously. The pH was then lowered to 2 by adding HCl. The decrease of pH induced an increase of the Rh from 28 to 53 nm. This confirms that the objects are indeed micelles with a PS core and a P4VP corona, the addition of HCl inducing a protonation of the pyridine moiety and hence a stretching of the coronal chains because of the electrostatic repulsions. For the mixture of PS-b-P4VP with PAA, the addition of basic water breaks the hydrogen bonds between the P4VP and PAA chains and induces the precipitation of the PS chains, yielding micelles with a PS core and a P4VP corona. When the volume fraction of water increases, the P4VP chains also collapse, but the nanoparticles thus formed are stabilized by the charged PAA chains (see Scheme 1). However, the exact role of the PAA in the stabilization of these objects is not yet known, but it probably interacts with the P4VP chains through charge-dipole interactions, creating a corona to stabilize the nanoparticles and allowing stable objects to be obtained even in pure basic water. Such behavior is similar to the block copolymer free strategy developed by M. Jiang’s group, where the core and corona blocks of the micelles are linked not by covalent bonds but by hydrogen bonds.13,20,21 To further evidence the role of negatively charged PAA chains in the stabilization of hydrophobic P4VP-containing nanoparticles, we have added basic water to complexes made in DMF from P4VP (18000) and PAA (20000). Stable nanoparticles with an Rh of 64 nm have been obtained in pure basic water. We have also prepared in DMF a mixture of a PS-b-P4VP copolymer, with a much smaller P4VP (3000) block compared to the PS block (34000), with PAA (20000), and basic water was then added step by step. A precipitate was observed immediately upon addition of a small amount of basic water, evidencing that when the P4VP/PS fraction is too small, the interactions between (20) Wang, M.; Jiang, M.; Ning, F.; Chen, D.; Liu, S.; Duan, H. Macromolecules 2002, 35, 5980. (21) Yuan, X.; Jiang, M.; Zhao, H.; Wang, M.; Zhao, Y.; Wu, C. Langmuir 2001, 17, 6122.
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4622 Langmuir, Vol. 23, No. 8, 2007
These different results show that the disintegration of the micelles with a complexed core can indeed be controlled by the temperature according to the solvent used. In DMF, which is a stronger competitor for the formation of hydrogen bonds and a better solvent for the complexes,18 an increase in temperature can induce a crossing of the solubility limit of the complexes, yielding soluble complexes, and/or a breaking of the hydrogen bonds, yielding noncomplexed PS-b-P4VP and PAA chains. In THF, which is a rather weak competitor for the hydrogen bonds and a poorer solvent for the complexes,18 an increase of temperature up to 60 °C is not sufficient to disintegrate the micelles.
Conclusions Figure 5. Evolution of the Rh with the volume fraction of acidic water at pH 4.8.
P4VP and PAA chains are not sufficient to stabilize the nanoparticles. These two experiments confirm the hypothesis on the interaction between P4VP and PAA chains in the stabilization of the nanoparticles. The last case that has been studied is the addition of water at pH 4.8. The DLS results are reported in Figure 5. The Rh first increases slightly and then decreases to stabilize around 20 nm. These results are similar to those obtained for strongly acidic water (pH 2; see above), except that the increase in size upon addition of 10% (v/v) water is much less pronounced. The Rh values measured in DLS are smaller than in the case of strongly acidic water because the P4VP is only partially protonated, so the chains are less stretched. Contrary to the experiment at pH 2 and 9, we were unable to transfer the objects in pure water, a precipitate being formed after dialysis. This is probably due to the fact that the P4VP and PAA chains are only partially ionized, and the objects are thus not sufficiently stabilized to remain stable in pure water. To induce a breaking of the micelles formed by P4VP/PAA complexes, a stimulus other than the addition of water at different pH values can be used: an increase of temperature. Temperature was increased by steps of 10 °C, starting from 25 °C, and the evolution of the Rh was followed by DLS. Different systems have been studied. The first one consists in a mixture of PSb-P4VP (20000-19000) and PAA (20000) in DMF. The starting micelles have an Rh of 29 nm, and it stays constant up to 55 °C. At 55 °C, the Rh decreases to 5 nm, a size characteristic of free PS-b-P4VP chains or soluble complexes,18 signaling that the micelles have been disintegrated. The temperature was then decreased back to 25 °C. After 3 h at 25 °C, micelles were again observed, in agreement with the reaggregation of the hydrogenbonded complexes. The second studied system is the same mixture of PS-b-P4VP (20000-19000) and PAA (20000), but in THF instead of DMF. Since the boiling point of THF is 67 °C, the micellar solution could only be heated up to 60 °C. The initial micelles have an Rh of 58 nm, and in this case no change was observed in the DLS data during the heating to 60 °C.
In this paper, the reorganization of micelles containing hydrogen-bonded interpolymer complexes in the core has been studied. We have demonstrated that the addition of water at a controlled pH led to the disintegration of the initial complexes and to the formation of other types of aggregates. Whenever acidic water was added, the 4-vinylpyrine units became protonated and were no longer able to form hydrogen bonds with acrylic acid units. Moreover, an inversion of the initial micellar structure was immediately observed, even upon addition of a very small amount of acidic water. In these inverted micelles, a PS core is surrounded by a protonated P4VP corona (see Scheme 1). This behavior could be compared to some extent to the “schizophrenic” micellization behavior reported by Armes and co-workers.22 These micelles could be transferred in pure acidic water by a dialysis process. A similar behavior was noted at pH 4.8, although the transfer in pure water was not possible. Addition of basic water also led to the disruption of the initial hydrogen-bonded complexes through the ionization of the carboxylic acid moieties of PAA into negatively charged carboxylates. This process was also accompanied by a reorganization of the initial micelles. In this case, the formation of well-defined nanoparticles with a core-shell structure was observed. These nanoparticles contain a PS core surrounded by a P4VP shell, stabilized by the charged PAA chains (see Scheme 1). These nanoparticles could also be transferred into pure basic water by dialysis. Finally, the initial micelles could be disintegrated by increasing the temperature if the starting solvent is DMF but not in the case of THF, highlighting the role of the nature of the solvent in the stability of the formed aggregates. Acknowledgment. N.L. thanks the FRIA for financial support. C.-A.F. is Charge´ de Recherches FNRS. J.-F.G. is grateful to the STIPOMAT Program from the European Science Foundation. LA063477I (22) Liu, S.; Billingham, N. C.; Armes, S. Angew. Chem., Int. Ed. 2001, 40, 2328.
