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Langmuir 2007, 23, 10732-10740
Dendrimer-Influenced Supramolecular Structure Formation of Block Copolymers Anja Kroeger, Xingfu Li, and Adi Eisenberg* Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada ReceiVed May 8, 2007. In Final Form: July 11, 2007 The water content-dependent supramolecular structure formation of polystyrene-block-poly(acrylic acid) (PS-bPAA) copolymer in the presence of a fourth-generation amine-terminated poly(amido amine) dendrimer (PAMAM) is investigated by dynamic light scattering, turbidity measurements, and transmission electron microscopy. The solvent system for this study is a mixture of dioxane/THF and water. A very complex turbidity profile is observed with increasing water content in the system and is explained by the presence of various aggregated structures based on strong interactions between the amine-containing dendrimers and the poly(acrylic acid) blocks of the polymer. The onset of the self-assembly of single chains of PS-b-PAA (primary structure) into single and multiple dendrimer core inverse micelles (secondary structure) is detected as very low water contents of cw < 2% wt (cwc). These micelles consist of dendrimers coated with PAA blocks, which are connected to the corresponding PS chains that form the corona. Further addition of water leads to an association of these micelles into compound multiple dendrimer core inverse micelles (tertiary structure) in the range of cw ) ∼6 to ∼10% wt. At still higher water content, some of the acrylic acid chains of the block copolymer move from the vicinity of the dendrimer to the outside of the aggregates, resulting in a decrease in the size of the formed structures and the acquisition of progressively increasing hydrophilic character of the aggregates. Multiple dendrimer core inverse onion micelles are formed, which agglomerate into compound multiple dendrimer core inverse onion micelles at cw ) ∼12 to ∼18% wt. Above this water content, vesicular structures are formed. The complexity is unusual for block copolymer systems and illustrates the importance of strong interactions in structure formation.
Introduction The self-organization of amphiphilic block copolymers in selective solvents or solvent mixtures, resulting in the formation of colloidal-sized aggregates of various morphologies, has been studied intensively.1-16 The morphologies, which include spheres, rods, bicontinuous structures, lamellae, tubules, micelles, and polymeric vesicles,3,5-7 among others, can be controlled by different parameters. The effects of polymer properties such as the molecular weight,3 the relative block length,7-9 the chemical nature of the repeat unit, and the polydispersity8,10 as well as the * Corresponding author. E-mail:
[email protected]. (1) van Hest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; Van Genderen, M. H. P.; Meijer, E. W. Science 1995, 268, 1592. (2) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (3) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (4) Lee, J. C.-M.; Santore, M.; Bates, F. S.; Discher, D. E. Macromolecules 2002, 35, 323. (5) Antonietti, M.; Foerster, S. AdV. Mater. 2003, 15, 1323. (6) (a) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (b) Zhang, L.; Bartels, C.; Yu, Y.; Shen, H.; Eisenberg, A. Phys. ReV. Lett. 1997, 79, 5034. (c) Yu, Y.; Eisenberg, A. J. Am. Chem. Soc. 1997, 119, 8383. (d) Yu, G.-E.; Eisenberg, A. Macromolecules 1998, 31, 5546. (e) Kita-Tokarczyk, K.; Grumelard, J.; Haefele, Th.; Meier, W. Polymer 2005, 46, 3540. (7) Shen, H.; Eisenberg, A. Angew. Chem. 2000, 112, 3448. Shen, H.; Eisenberg, A. Angew. Chem., Int. Ed. 2000, 39, 3310. (8) Azzam, T.; Eisenberg, A. Angew. Chem., Int. Ed. 2006, 45, 7443. (9) Nardin, C.; Hirt, Th.; Leukel, J.; Meier, W. Langmuir 2000, 16, 1035. (10) Ding, J.; Liu, G. Macromolecules 1997, 30, 655. (11) (a) Santore, M. M.; Discher, D. E.; Won, Y.-Y.; Bates, F. S.; Hammer, D. A. Langmuir 2002, 18, 7299. (b) Nukova, A. T.; Gordon, V. D.; Cristobal, G.; Talingting, M. R.; Bell, D. C.; Evans, C.; Joanicot, M.; Zasadzinski, J. A.; Weitz, D. A. Macromolecules 2004, 37, 2215. (12) Lim-Soo, P.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 923. (13) Solomatin S. V.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2004, 20, 2066. (14) Choucair, A.; Eisenberg, A. Eur. Phys. J. E 2003, 10, 37. (15) Yu, K.; Bartels, C.; Eisenberg, A. Langmuir 1999, 15, 7157. (16) 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.
presence of additives (ions, surfactants, homopolymers),11-13 the solvent nature or the composition of solvent mixtures (including water content) and solution conditions,3,14,15 including polymer concentration, pH, and temperature12,13,16 on the selforganization of block copolymers have been explored in detail. As a result of these studies, the properties of such supramolecular structures can be varied extensively, which results in a wide range of potential applications, e.g., in technical, medical, and pharmaceutical fields. In particular, in connection with the last-mentioned applications, extensive studies have been performed on the use of micelles as carrier systems of hydrophobic drugs.17 Block copolymer vesicles have been considered to be alternatives to liposomes in their ability to incorporate in their interior cavities water-soluble molecules or even larger structures.12,23 Hydrophilic drugs, such as doxorubicin, were loaded into copolymer vesicles via active loading using a pH gradient.18 Large hydrophilic biomolecules (e.g., DNA or proteins) can also be encapsulated into block copolymer aggregates, including (17) Kataoka, K., Kabanov, A., Ed. Colloids Surf., B: Biointerfaces 1999, 16. (18) Choucair, A.; Lim-Soo, P.; Eisenberg, A. Langmuir 2005, 21, 9308. (19) (a) Cornelissen, J. J. L. M.; Fischer, M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 1998, 280, 1427. (b) Brown, M. D.; Schatzlein, A.; Brownlie, A.; Jack, V.; Wang, L.; Tetley, L.; Gray, A. I.; Uchegbu, I. F. Bioconjugate Chem. 2000, 11, 880. (c) Lee, J. C.-M.; Bermudez, H.; Discher, B. M.; Sheehan, M. A.; Won, Y.-Y.; Bates, F. S.; Discher, D. E. Biotechnol. Bioeng. 2001, 73, 135. (d) Nardin, C.; Meier, W. ReV. Mol. Biotechnnol. 2002, 90, 17. (e) Napoli, A.; Boerakker, M. J.; Tirelli, N.; Nolte, R. J. M.; Sommerdijk, N. A. J. M.; Hubbell, J. A. Langmuir 2004, 20, 3487. (f) Brannan, A. K.; Bates, F. S. Macromolecules, 2004, 37, 8816. (g) Mecke, A.; Dittrich, Ch,; Meier, W. Soft Matter 2006, 2, 751. (h) Wittemann, A.; Azzam, T.; Eisenberg, A. Langmuir 2007, 23, 2224. (20) Korobko, A. V.; Jesse, W.; Maarel, J. R. C. Langmuir 2005, 21, 34. (21) (a) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (b) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (22) (a) Tomalia, D. A.; Huang, B.; Swanson, D. R.; Brothers, H. M., II; Klimash, J. W. Tetrahedron 2003, 59, 3799. (b) Chen, H.; Banaszak Holl, M.; Orr, B. G.; Majoros, I.; Clarkson, B. H. J. Dent. Res. 2003, 82, 443. (23) Ding, J.; Liu, G. J. Phys. Chem. B 1998, 102, 6107.
10.1021/la701334r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2007
Structure Formation of Block Copolymers
vesicles.19,20 The encapsulation of DNA, for example, was achieved by emulsifying an aqueous DNA solution in a solution of the amphiphilic diblock copolymer in toluene; a subsequent change in the composition of the organic solvent resulted in a collapse of the hydrophobic block brush and the formation of a capsule, which could be dispersed in an aqueous medium to form the copolymer vesicles.20 Compared with the extensive research on encapsulation of biomolecules into liposomes, studies on the encapsulation of large biological units into copolymer aggregates are very limited, in part because large hydrophilic biomolecules are sensitive to organic solvents and the pH of the solution. The paucity of studies dealing with the encapsulation of biological species into block copolymer aggregates underlines the inherent difficulties of the process and suggests the need for further research. In the present study, we explore the encapsulation of a fourth-generation amine-terminated (G4-NH2) poly(amido amine) (PAMAM) dendrimer21 into block copolymer assemblies, including micelles and vesicles, with the aim of improving our understanding of the encapsulation process. The polystyreneblock-poly(acrylic acid) copolymer (PS-b-PAA) was chosen because it is a well-known and well-understood system and the self-organization has been studied extensively. Dendrimers were chosen because of their inherent chemical simplicity and their stability with respect to the conditions under which colloidalsized aggregates are prepared. They are thus very useful model compounds for complex structures such as proteins, which have similar sizes and/or similar terminal groups,22 in studies involving incorporation into vesicles. The size of the G4-NH2 PAMAM dendrimer in solution is approximately 5 nm, i.e., close to that of the globular protein hemoglobin. Furthermore, many proteins, such as serum albumin, also have amine residues and/or carboxylic acid residues on their surfaces. Although the chemical stability of the dendrimers is very different from that of the proteins, one can nonetheless obtain useful information about incorporation patterns and driving forces if one studies the incorporation of dendrimers into block copolymer aggregates as a preliminary step to the study of more sensitive or more complicated structures. Also, the structures and sizes of dendrimers can be varied systematically, which is not the case with proteins. There are many factors to consider when one prepares copolymer aggregates with the intention of incorporating dendrimers, as is done here. In dendrimer-free solutions of PSb-PAA in organic solvents at room temperature, the content of added water is a decisive factor in the formation of different aggregate morphologies.5,23-25 With increasing water content, micelles first appear at a water content called the critical water concentration (cwc), and subsequent structure formation is influenced by the water content. Furthermore, in the case of the presence of the G4-NH2 PAMAM dendrimer, strong interactions between the amine groups of the dendrimer and the carboxylic acid residues of PAA in the copolymer are present, and the effect of such strong interactions on structure formation is also of interest. It should be noted that the effect of polyelectrolyte complex formation on block copolymer self-assembly has been explored extensively.26 In those cases, however, the interactions occur between flexible polyelectrolyte chains and are thus very different (24) Choucair, A.: Laviguer, C.; Eisenberg, A. Langmuir 2004, 20, 3894. (25) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (26) (a) Kataoka, K.; Togawa, H.; Harada, A.; Yasugi, K.; Matsumoto, T.; Katayose, S. Macromolecules 1996, 29, 8556. (b) Harada, A.; Kataoka, K. Macromolecules 1998, 31, 288. (c) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 1998, 14, 6101. (d) Cohen Stuart, M. A.; Besseling, N. A.; Fokkink, R. G. Langmuir 1998, 14, 6846. (e) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg,
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in their effect from those in the present system, which involves interactions between a rigid sphere and a flexible polymer. Therefore, the aim of the present article is to elucidate the effect of interactions between the G4-NH2 PAMAM dendrimer with the PAA groups of the PS-b-PAA diblock copolymer on supramolecular structure formation by using dynamic light scattering (DLS) and turbidity measurements as well as transmission electron microscopy (TEM). The turbidity method is a very useful screening technique that detects the presence of regions of rapid changes, for example, if the investigated system is studied as a function of water content and it allows the selection of interesting regions for more detailed investigations by DLS and TEM. Although light scattering data help one to assess the structure and aggregation behavior of produced assemblies, the data interpretation needs a model of the structure of the individual objects and their interaction. Here, direct imaging methods (e.g., TEM) come into play, which help to corroborate the assumptions that enter the interpretation of scattering data. A combined and critical discussion of the pertinent results of structure elucidation by direct imaging as well as by scattering techniques allows the optimum identification of the supramolecular structures. Experimental Section Materials. Polystyrene-block-poly(acrylic acid) copolymer PS310b-PAA36, where the numbers refer to the number-average degree of polymerization, was synthesized by sequential anionic polymerization and has a polydispersity index PDI of 1.03, as determined by sizeexclusion chromatography (SEC) using polystyrene standards. The details of the synthesis and the characterization procedures were given in a previous publication.27 Sample solutions of a polymer concentration (cp ) 0.5% wt) were prepared by dissolving the polymer in 75/25% wt dioxane/THF under continuous stirring for several hours at room temperature. 1,4-Dioxane (99.0%) and tetrahydrofuran (THF, 99.9%) were purchased from Fisher Scientific. The fourth-generation amine-terminated (G4-NH2) poly(amido amine) (PAMAM) dendrimer consists of a tetra-functional core of ethylenediamine (>NCH2CH2N 20% wt. As seen in Figure 7.5, almost all the aggregates change into vesicles with a size of rz ) ∼40 ( 11 nm (Table 2). In comparison to these results, in dynamic light scattering experiments, more aggregates of larger dimensions are detected because of the association of vesicles. Consequently, aggregate sizes of Rh > ∼800 ( 80 nm result, which do not correspond
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Figure 7. Water content dependence of the hydrodynamic radii as well as the turbidity of aggregates formed in dendrimer-containing solutions (RAm/Ac ) 0.06) of PS310-b-PAA36 in dioxane/THF at T ) 20 °C. Arrows are directed to points at which the TEM micrographs were obtained. The z-average radii of individual aggregates obtained by several micrographs were based on statistics for ca. 200 items.
to the dimension of freely diffusing vesicles but rather to that of associated vesicles.
Conclusions In this article, we present a detailed analysis of the water content-dependent structure formation of a strongly interacting system of a PS310-b-PAA36 copolymer and a fourth-generation poly(amido amine) dendrimer in dioxane/THF. Normal waterinduced micellation is observed in the absence of dendrimer, yielding micelles with a hydrophobic polystyrene core and a hydrophilic poly(acrylic acid) corona (Figure 4A), which, at high water concentration, convert into vesicles. In the presence of dendrimer, at a constant amine to acid group ratio of 0.06, the condensation of PAA blocks around the dendrimer is observed, yielding hydrophobic single or multiple dendrimer core inverse micelles (Figure 4B,C). These micelles further aggregate into compound multiple dendrimer core inverse micelles (Figure 4D) with increasing water content. At still higher water concentrations, some of the PAA blocks move from the vicinity of the dendrimer to the outside of the micelles. This reorganization process results in a hydrophilic character and a decreasing dimension of the formed multiple dendrimer core inverse onion micelles (Figure
4E). With increasing water content, these hydrophilic structures further aggregate into compound multiple dendrimer core inverse onion micelles (see Figure 4F) to reduce the size of the interface, and finally, vesicles are formed. The level of complexity encountered here points out the importance of strong interactions in determining the morphology in block copolymer self-assembly and suggests that this field is worthy of extensive further studies. The effect of the amine to acid ratio in the present system is now under investigation. Acknowledgment. We gratefully thank Professor Dr. G. Wegner as well as Ch. Rosenauer and B. Mueller (Polymer Analytic Group) of the Max-Planck-Institute for Polymer Research in Mainz for their kind support and assistance with the dynamic light scattering experiments. Financial support by the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. Supporting Information Available: Details of photon correlation spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org. LA701334R
