Formation of Vesicles in Block Copolymer-Fluorinated Surfactant

CeRMiN, UniVersite´ Catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium, and. Molecular and Nanomaterials, Department of ...
0 downloads 0 Views 1MB Size
Download PDF
116

Langmuir 2007, 23, 116-122

Formation of Vesicles in Block Copolymer-Fluorinated Surfactant Complexes† Zhijun Hu,‡,§ Wendy Verheijen,| Johan Hofkens,| Alain M. Jonas,§ and Jean-Franc¸ ois Gohy*,‡ Unite´ de Chimie des Mate´ riaux Inorganiques et Organiques (CMAT) and Research Center in Micro- and Nanoscopic Materials and Electronic DeVices (CeRMiN), UniVersite´ Catholique de LouVain, Place Pasteur 1, 1348 LouVain-la-NeuVe, Belgium, Unite´ de Chimie et de Physique des Hauts Polyme` res (POLY) and CeRMiN, UniVersite´ Catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium, and Molecular and Nanomaterials, Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, 3001 HeVerlee, Belgium ReceiVed May 30, 2006. In Final Form: July 5, 2006 Spherical micellar aggregates have been obtained in chloroform by mixing poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers with perfluorinated surfactants (FS) bearing a carboxylic acid head. These micellar aggregates are resulting from the self-assembly of the insoluble P4VP/fluorinated complexes into a core surrounded by the soluble PS coronal chains. Their characteristic features have been studied as a function of various parameters including the composition of the PS-b-P4VP copolymer, the tail length of the fluorinated surfactant, the 4VP/FS molar ratio, the number of carboxylic acid group (1 or 2) on the surfactant, the presence of the PS block and of the fluorine atoms on the surfactant. Dilution of these initial micellar aggregates triggers a morphological reorganization resulting in the formation of more stable vesicles. The extent of this morphological transition is related to the solubility of the P4VP/fluorinated complexes during the dilution process. This transition is complete for short P4VP/FS complexes, incomplete for long P4VP/FS complexes, and not observed whenever an R,ω-difunctional FS is used in P4VP/FS complexes, leading to a cross-linked core. Finally, the spheres-to-vesicles transition has been advantageously used in order to encapsulate molecules, as demonstrated by confocal fluorescence microscopy.

Introduction Micellar aggregates made from block copolymer-surfactant complexes are currently a topic of intense research, stemming from both the academic and application point of view. Indeed, such systems have potential importance in quite different fields such as drug carriers,1 coatings, cosmetics, food, and enhanced oil recovery.2 Block copolymer-surfactant complexes are able to selforganize into hierarchical nanostructures, as illustrated by the works of Ikkala and co-workers.3 These complex structures result from the combination of the morphological characteristic features of both block copolymers and polymer-surfactant complexes and have been widely investigated in bulk.4 In these materials, the so-called “hairy-rod” or “comblike” polymer-surfactant structures, which result from noncovalent interactions such as electrostatic interactions or hydrogen-bonding between a polymer and mutually interacting surfactants, and are organized at a typical size of 1-6 nm,5 are combined with the block copolymers † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom correspondence should be addressed. E-mail: [email protected]. ‡ CMAT and CeRMiN, Universite ´ Catholique de Louvain. § POLY and CeRMiN, Universite ´ Catholique de Louvain. | Katholieke Universiteit Leuven.

(1) Thu¨nemann, A. F.; General, S. Macromolecules 2001, 34, 6978. (2) Zhou, S.; Chu, B. AdV. Mater. 2000, 12, 545. (3) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407. (4) See, for example: (a) Valkama, S.; Kosonen, H.; Ruokolainen, J.; Haatainen, T.; Torkelli, M.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Nat. Mater. 2004, 3, 872. (b) Ruotsalainen, T.; Turku, J.; Heikkila¨, P.; Ruokolainen, J.; Nyka¨nen, A.; Laitinen, T.; Torkelli, M.; Serimaa, R.; ten Brinke, G.; Harlin, A.; Ikkala, O. AdV. Mater. 2005, 17, 1048. (5) Ruokolainen, J.; Ma¨kinen, R.; Torkelli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557.

microphase-separated structures which are characterized by a typical length scale of 10-200 nm.6 Beside the bulk nanostructures, self-organization of block copolymer-surfactant has been considered in a selective solvent for the uncomplexed blocks. This should lead to micellar aggregates in which the core is formed by the insoluble polymer block-surfactant complexes whereas the corona contains the soluble uncomplexed blocks. Such an approach has been considered in water-based systems obtained by mixing a diblock copolymer with a low molar mass surfactant known for its specific interaction with one of the blocks of the copolymer.7-14 Vesicles have been generally observed in these block copolymersurfactant complexes, in relation with the layered structures found in the core-forming polymer block-surfactant complexes. Micellar aggregates from block copolymer-surfactant complexes have been rarely studied in apolar solvents. The direct formation of vesicles has been however reported in mixtures of a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer with perfluorooctanoic acid (PFOA) in chloroform.15 Although both starting components were molecularly dissolved (6) Bates, F. S.; Fredrickson, G. H. Annu. ReV. Phys. Chem. 1990, 41, 525. (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) Gohy, J.-F.; Mores, S.; Varshney, S. K.; Je´roˆme, R. Macromolecules 2003, 36, 2579. (10) Solomatin, S. V.; Bronich, T. K.; Bargar, T. W.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2003, 19, 8069. (11) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481. (12) Bekturov, E. A.; Kudaibergenov, S. E.; Frolova, V. A.; Khamzamulina, R. E.; Schulz, Rolf C.; Zoeller, J. Makromol. Rapid Commun. 1991, 12, 37. (13) Pispas, S. J. Phys. Chem. B 2006, 110, 2649. (14) Khanal, A.; Li, Y.; Takisawa, N.; Kawasaki, N.; Oishi, Y.; Nakashima, K. Langmuir 2004, 20, 4809. (15) Peng, H.; Chen, D.; Jiang, M. J. Phys. Chem. B 2003, 107, 12461.

10.1021/la061532h CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2006

Vesicle Formation in Block Copolymer Complexes

in chloroform, vesicles were observed after their mixing under ultrasounds. These vesicles were thought to consist of an insoluble layer formed by PS-b-P4VP/PFOA complexes surrounded by soluble PS blocks. The incorporation of flurorinated moeities in polymer-surfactant mixtures is an interesting route to create coating imparting low friction properties to surfaces.16 In a preliminary report, we have used a similar approach to the one described in ref 15 in order to create vesicles from PSb-P4VP/perfluorinated surfactants (FS) complexes.17 Surprisingly enough, spherical micellar aggregates were observed, which however rearrange into vesicles upon dilution. The discrepancy between our results and the previous ones could be accounted for different molecular characteristics of the used PS-b-P4VP copolymers or for a different preparation method for the complexes. In this respect, a high molecular weight, nearly symmetric PS-b-P4VP copolymer was used in ref 15, whereas a lower molecular weight, highly asymmetric one was considered in ref 17. Moreover, the methods used for the preparation of block copolymer-surfactant complexes mixtures were different in the two studies: we prepared the complexes under stirring whereas slow addition of the surfactant to the copolymer and ultrasonic treatment were used in ref 15. In this paper, we investigate into more details the reorganization process of the initial spherical micellar aggregates formed in PS-b-P4VP/FS complexes toward vesicles. In this respect, various parameters including the structure of the FS, the P4VP/FS molar ratio in the complexes, and the composition of the PS-b-P4VP copolymer have been varied. Finally, we show that this transition can be successfully used for the encapsulation of molecules. Experimental Section Materials. All materials were used without further purification. Chloroform, perylene bisimide and decanoic acid were purchased from Sigma-Aldrich (Bornem, Belgium), perfluorodecanoic acid (PFDA, CF3-(CF2)8-COOH), perfluoropentanoic acid (PFPA, CF3-(CF2)3-COOH) and perfluorosebacic acid (PFSA, HOOC(CF2)8-COOH) were purchased from ABCR Inc. (Karlsruhe, Germany). Two poly(styrene)-block-poly(4-vinylpyridine) (PS327b-P4VP27 and PS192-b-P4VP179 , the number in subscript being the average degree of polymerization of the corresponding block) copolymers both with a polydispersity index of 1.1 and P4VP179 and P4VP27 homopolymers both with a polydispersity index of 1.1 were obtained from Polymer Source Inc. (Dorval, Que´bec). Dialysis was performed with Spectra/Por membranes (cutoff of 6000-8000 Da) from Spectrum Laboratories Inc. (Rancho Dominguez, CA). Preparation of the Complexes. Stock solutions of the PS-bP4VP copolymers and of the FS were prepared in chloroform at a concentration of 1 g/L. No aggregated species were detected by dynamic light scattering for both the PS-b-P4VP and FS solutions, confirming that chloroform is a nonselective solvent in the two cases. Calculated amounts of these solutions were then mixed under stirring at 20 °C to obtain PS-b-P4VP/FS mixtures in which the 4VP/FS molar ratio was varied from 10/1 to 1/1 mol/mol. These solutions were equilibrated for one night before characterization. These initial solutions were then diluted by addition of known amounts of chloroform. For encapsulation experiments, a fluorescent dye, perylene bisimide, was dissolved in chloroform (4 × 10-6 g/L), and this solution was used to dilute the initial micellar solutions. The diluted solution was then dialized against pure chloroform for 1h in order to remove nonencapsulated dye. Instrumentation. Atomic force microscopy (AFM) measurements were obtained in tapping mode with a Veeco Nanoscope IV Multimode microscope operated in air. Both height and phase images were recorded simultaneously. Etched Si tips with a resonance (16) Antonietti, M.; Henke, S.; Thu¨nemann, A. F. AdV. Mater. 1996, 8, 41. (17) Hu, Z.; Jonas, A. M.; Varshney, S. K.; Gohy, J.-F. J. Am. Chem. Soc. 2005, 127, 6526.

Langmuir, Vol. 23, No. 1, 2007 117 Table 1. Average Diameter from AFM Pictures (DAFM, nm) and Apparent Hydrodynamic Diameter as Measured by DLS (Dh,app, Nm) of the Different PS-b-P4VP/Fluorinated Surfactant Micellar Aggregates Investigated in This Studya 4VP/FS molar ratio sample

1/1

PS192-b-P4VP179/PFDA PS327-b-P4VP27/PFDA PS192-b-P4VP179/PFPA PS327-b-P4VP27/PFPA PS192-b-P4VP179/PFSA PS327-b-P4VP27/PFSA a

4/3

2/1

4/1

10/1

25-27 33-33 39-42 43-78 86-134 14-15 11-13 12-14 12-15 23-20 34-41 11-14 10-10 53-62 19-21

Results are present as DAFM - Dh,app pairs.

frequency of approximately 170 kHz and a spring constant of 15 Nm-1 were used, and the scan rate was in the range of 1-2 Hz. The diameter of the micellar aggregates (listed in Table 1) was measured in the lateral dimension without considering any tip-convolution correction and was obtained by averaging measurements from at least 50 different objects. Transmission electron microscopy (TEM) was performed on a Leo 922 microscope, operating at 200 kV accelerating voltage in bright field mode. Samples for AFM and TEM were prepared by spin coating solutions of copolymer/surfactant complexes onto carbon-coated mica. For TEM observations, the samples were floated from the mica substrate in water, collected on a copper TEM grid, and stained by RuO4 vapor before observation. Dynamic light scattering (DLS) measurements were performed on a Brookhaven Instruments Corp. BI-200 apparatus equipped with a BI-2030 digital correlator with 136 channels and a Spectra Physics He-Ne laser with a wavelength λ of 633 nm. A refractive index matching bath of filtered Decaline surrounded the scattering cell, and the temperature was controlled at 25 °C. The DLS data were analyzed by the CONTIN routine, a method based on a constraint inverse Laplace transformation of the data and which gives access to a size distribution histogram for the aggregates.18 Confocal laser scanning microscopy images of the vesicles were acquired with an Olympus Fluoview FV500 scanning unit and software coupled to an IX70 Olympus microscope. Continuous wave 488 nm excitation was performed with an argon-ion laser (163-C1210, Spectra Physics). The excitation light was directed on the sample through an oil immersion objective lens (Olympus, 100×, 1.4 NA). The excitation light is filtered from the fluorescence light by a 488 nm dichroic mirror and the appropriate band-pass filters.

Results and Discussion Various PS-b-P4VP/FS mixtures have been prepared in chloroform at an initial concentration of 1 g/L and analyzed by DLS, AFM, and TEM. The effect of the following parameters on the characteristic features of the initial spherical micellar objects has been systematically studied: the composition of the PS-b-P4VP copolymer, the 4VP/FS molar ratio, the length of the perfluorinated tail of the surfactants, the number of functional groups on the surfactants (1 or 2), the effect of the PS block, and the effect of the perfluorinated versus hydrogenated tails. We have concentrated on the effect of dilution on these initial micellar aggregates and the observed spheres-to-vesicles morphological transition. The characteristic features of the vesicles obtained after dilution is discussed as a function of some of the parameters described here above. Finally, an application of this transition for the encapsulation of a fluorescent dye is presented. Effect of the Composition of the PS-b-P4VP Copolymer. Two different copolymers have been investigated, i.e., PS327b-P4VP27 and PS192-b-P4VP179. The total degrees of polymerization of these two copolymers are comparable (354 and 371), but their compositions are quite different. These copolymers (18) Berne, B. J., Pecora, R. J., Eds.; Dynamic Light Scattering; Wiley: Toronto, 1976.

118 Langmuir, Vol. 23, No. 1, 2007

Hu et al.

Figure 2. Influence of the 4VP/PFDA molar ratio on the morphology of the micellar aggregates formed by PS327-b-P4VP27/PFDA) complexes at 1 g/L. 4VP/PFDA molar ratio of 4/1 (a), 2/1 (b), 4/3 (c), and 1/1 (d) (AFM height contrast pictures, scale is shown near d).

Figure 1. Influence of the 4VP/PFDA molar ratio on the morphology of the micellar aggregates formed by PS192-b-P4VP179/PFDA complexes at 1 g/L. 4VP/PFDA molar ratio of 10/1 (a), 4/1 (b), 4/1 (c), 2/1 (d), 4/3 (e), and 1/1 (f), (a, b, d, e, and f: AFM height contrast pictures, scale is shown near d; c: example of TEM picture).

showed the formation of spherical micellar objects once they have been mixed with FS. These spherical micellar objects were systematically smaller for the PS327-b-P4VP27/FS mixtures whatever the 4VP/fluorinated surfactant molar ratio, the length of the perfluorinated tail of the surfactants, and the number of functional groups on the surfactants. Typical data related to the size of the initial spherical aggregates can be found in Table 1. This observation is not surprising since the spherical micellar aggregates formed in PS-b-P4VP/FS mixtures are thought to consist of a P4VP/FS core surrounded by PS coronal chains. Since the size of the core, the total size, and the number of aggregation of block copolymer micelles are known to mostly depend on the degree of polymerization of the insoluble block,19 larger micelles should be observed for the micellar objects derived from the PS192-b-P4VP179 copolymer (Table 1 and Figure 1). According to the volume fraction of the P4VP/FS insoluble blocks as compared to the soluble PS ones and to the layered hierachical structures which is expected to be formed inside the P4VP/FS cores, vesicles are expected to be formed especially in case of the PS192-b-P4VP179/FS complexes. Obviously, spherical micellar aggregates have been only observed (Figure 1). This observation is not in agreement with previous observations on similar systems for which vesicles were observed17 and certainly indicates that nonequilibrium micellar aggregates have been formed after mixing the copolymer and surfactant solutions. These aggregates should be kinetically frozen since no evolution of the mixed solutions has been detected on a time scale of several months. Finally, one should mention the good agreement between the lateral diameter of the micellar aggregates as measured by AFM (19) Gohy, J.-F. AdV. Polym. Sci. 2005, 190, 65.

and the apparent hydrodynamic diameter (Dh,app) as determined by DLS (see Table 1). Since the structure of the micellar aggregates is dependent on the concentration (see later), only apparent Dh (not extrapolated to zero concentration) have been considered in the following. This indicates that the morphological characteristic features of the dried micellar aggregates are not so different from the situation observed in solution. Effect of the 4VP/FS Molar Ratio. Various amounts of FS have been mixed with the PS-b-P4VP copolymers and the micellar characteristic features have been monitored as a function of the 4VP/FS molar ratio. Since 4VP units are soluble in CHCl3, larger micelles with swollen core are expected whenever the 4VP/FS molar ratio is larger than 1. This effect is indeed clearly observed in case of the PS192-b-P4VP179 /FS mixtures (Figure 1 and Table 1): the larger the 4VP/FS molar ratio, the larger the micellar aggregates. In the case of PS327-b-P4VP27/FS mixtures, a nearly constant size for the micellar aggregates has been noted (Figure 2 and Table 1). This can be accounted for by the very short P4VP block of this copolymer for which a limited swelling is expected even for high 4VP/FS molar ratio. Finally, it should be noted that the morphology of the micellar aggregates is always spherical whatever the 4VP/FS molar ratio (Figures 1 and 2). Effect of the Length of the Perfluorinated Tail of the Surfactants. The tail length of the FS has been varied in order to check the influence of this parameter on the micellar characteristic features. In this respect, two surfactants have been investigated, namely perfluorodecanoic acid (PFDA) and perfluoropentanoic acid (PFPA). The micellar aggregates formed by both types of FS have been visualized by TEM and AFM and characterized by DLS. Slightly smaller spherical objects have been systematically observed for PS-b-P4VP/PFPA complexes, all of the other parameters being kept constant (compare Figure 3a and Figure 1b and Figure 3b and Figure 2a, and see also data listed in Table 1). It thus appears that the length of the fluorinated surfactant tail has only a limited effect on the micellar characteristic features and that the dimension of the core slightly depends on this parameter. Effect of the Number of Functional Groups on the Surfactants. In all of the previously described experiments, FS with one carboxylic acid headgroup have been considered. In a

Vesicle Formation in Block Copolymer Complexes

Langmuir, Vol. 23, No. 1, 2007 119

Figure 5. Micellar aggregates formed by P4VP179/PFDA (a) and P4VP27/PFSA (b) complexes at an initial concentration of 0.1 g/L (4VP/PFDA molar ratio of 4/1 in both cases; AFM height contrast pictures).

Figure 3. Micellar aggregates initially formed at a concentration of 1 g/L by (a) PS192-b-P4VP179/PFPA and (b) PS327-b-P4VP27/ PFPA and vesicles observed after dilution at 0.1 g/L by (c) PS192b-P4VP179/PFPA and (d) PS327-b-P4VP27/PFPA (4VP/PFPA molar ratio of 4/1 in all cases; a and b: AFM height contrast pictures; c and d: TEM pictures).

Figure 6. AFM images (a: heigth contrast; b: phase contrast) of a thin film formed by PS192-b-P4VP179/decanoic acid complexes (4VP/decanoic acid molar ratio of 1/1).

Figure 4. Micellar aggregates formed by PS192-b-P4VP179/PFSA (a,b) and PS327-b-P4VP27/PFSA (c,d) complexes at an initial concentration of 1 g/L (a,c) and after dilution to a concentration of 0.1 g/L (b,d) (AFM height contrast pictures).

further experiment, we have also investigated complexes between a bifunctional FS bearing carboxylic acid groups at both ends, namely perfluorosebacic acid (PFSA). PFSA has been complexed with both PS192-b-P4VP179 and PS327-b-P4VP27 copolymers (Figure 4). In this case, one molecule of surfactant is expected to interact with two molecules of 4VP. Spherical micellar aggregates are again observed as visualized by AFM (Figure 4), whose characteristic features are similar to the ones previously reported for the PFDA-based analogues (Table 1 and Figure 4). In conclusion, the number of functional groups on the FS has no effect on the morphology of the obtained micellar aggregates for a constant 4VP/carboxylic acid group content. However, crosslinked micellar cores are expected in the case of PFSA-based complexed since one surfactant molecule may bind 4VP units from two different PS-b-P4VP chains. Since this cross-linking process is expected to occur randomly among P4VP chains, the packing of the FS fluorinated tails in the micellar core could be perturbed, resulting in larger and less dense cores than in the case of the monofunctional FS (see data in Table 1).

Effect of the PS Block. P4VP homopolymers (P4VP179 and P4VP27) with degrees of polymerization identical to the P4VP blocks of the PS-b-P4VP copolymers have been considered as precursors for micellar aggregates. Various 4VP/FS molar ratios have been considered. Precipitation of the complexes has been observed whenever the 4VP/FS molar ratio was lower than 4/1. Soluble aggregates were however observed for a 4VP/FS molar ratio of 4/1 (Figure 5). These aggregates are ill-defined and bigger than the ones observed in case of PS-b-P4VP/FS complexes, highlighting the role of the soluble PS chains in controlling the self-assembly process of the complexes. These ill-defined aggregates are thought to result from the aggregation of insoluble P4VP/FS segments surrounded by soluble uncomplexed P4VP segments. This also indicates that swollen micellar cores are likely formed in the case of micelles from PS-b-P4VP/FS complexes with a 4VP/FS higher than 1. Effect of the Perfluorinated Tail versus an Alkyl Tail. The hydrogenated analogue of PFDA (i.e., decanoic acid) has been complexed with the PS-b-P4VP copolymers. No micellar aggregates have been detected in these mixtures, whatever the 4VP/decanoic acid molar ratio. The resulting hydrogen-bonded complexes are always soluble in chloroform and do not further aggregate. Thin films showing a lamellar morphology (Figure 6) are indeed formed whenever the PS-b-P4VP/decanoic acid complexes are spin-coated onto a flat substrate. This experiment also shows that the perfluorinated tail is a prerequisite for micellar aggregates to be observed. The driving forces for aggregation are thus suspected to be a combination of the hydrogen bonding interaction between the nitrogen atom of the 4VP units and the carboxylic acid group of the surfactant and van der Waals interactions among perfluorinated tails. Effect of Dilution on the Micellar Characteristic Features. In a previous communication, we reported that dilution of spherical micellar aggregates formed by PS327-b-P4VP27/FS complexes with various 4VP/FS molar ratio quantitatively yielded large vesicles. This observation was accounted for by a reorganization of the initial “nonequilibrium” micellar aggregates

120 Langmuir, Vol. 23, No. 1, 2007

into “equilibrium” vesicular structures. A key issue in this process seems to be the preparation method of the complexes. The simple mixing process used in this study could actually lead to nonequilibrium spherical nanoparticles in which the FS molecules are randomly organized in the core. Because these complexes are of limited solubility, the time needed to reach equilibrium structures could be extremely long. Even for short surfactants tails (see results on complexes with PFPA), nonequilibrium structures are observed. A way to trigger the reorganization of the ill-defined P4VP/FS cores could be to increase the solubility of the complexes. This could be achieved by diluting the initial “nonequilibrium” nanoparticles. Following dilution of the system, partial resolubilization or further swelling of the initially formed P4VP/PFDA complexes could occur. Recomplexation with fluorinated surfactant molecules remaining in the solution, or loosely trapped in the micellar cores, could then be observed, leading again to less soluble complexes and reaggregation. The reorganization of fluorinated tails in these complexes and the subsequent formation of the layered structures characteristic of polymer/surfactants complexes3-5 would nicely explain the spheres-to-vesicles transition. To give credit to this possible mechanism of reorganization, different experiments have been carried out. First of all, the reversibility of the spheres-to-vesicles transition has been checked. It was found than at least a 5-fold dilution factor was required to effectively observe the transition. Lower dilution factors do not increase sufficiently the solubility of the nonequilibrium P4VP/FS complexes. Moreover, the transition was shown to be irreversible since spherical micellar aggregates could not be obtained by concentrating solutions of vesicles. Furthermore, the initial spherical micellar aggregates, which have been usually prepared at a concentration of 1 g/L, are also observed after mixing PS-b-P4VP and FS solutions of lower concentrations (e.g., 0.1 g/L). This clearly confirms that kinetically trapped nonequilibrium spherical micellar aggregates are initially formed that further rearrange into equilibrium vesicles. The effect of dilution seems thus to be more complicated than a simple shift from an insoluble region to a soluble one in the phase diagram of the P4VP/FS complexes. Indeed, a real “stimulus”, which is in this case the addition of solvent, needs to be applied to trigger the transition. Second, the extent of the transition should be linked to the solubility of the P4VP/FS complexes. In this respect, the morphological transition should be favored whenever shorter P4VP/FS complexes are considered. It was indeed found that the transition is complete in the case of PS327-b-P4VP27/FS complexes. Whenever the same dilution process is applied for the initial spherical micellar aggregates formed by the PS192-bP4VP179/FS complexes, the spheres-to-vesicles transition is incomplete or even not observed, depending on the 4VP/PFDA molar ratio (Figure 7). This observation can be accounted for by the lower solubility of the longer P4VP179/FS complexes as compared to the short P4VP27/FS complexes. Interestingly enough, the remaining spherical aggregates were observed to be essentially located inside the formed vesicles, whatever the PS192b-P4VP179/FS molar ratio and the length of the surfactant tail. Third, PS-b-P4VP/PFSA complexes do not show the spheresto-vesicles morphological transition. Since these complexes likely contain cross-linked cores due to the two carboxylic groups of PFSA, no reorganization could therefore occur in a further step. Effect of the Used PS-b-P4VP/FS Complexes on the Characteristic Features of the Vesicles. The spheres-to-vesicles morphological transition was followed by several techniques including DLS, TEM, and AFM. The CONTIN size distribution

Hu et al.

Figure 7. TEM images of PS192-b-P4VP179/PFDA complexes after dilution to 0.1 g/L [4VP/PFDA molar ratio of 4/1 (a), 2/1 (b), and 1/1 (c)].

histograms of the PS-b-P4VP/PFDA complexes with a 4VP/ PFDA molar ratio of 4/1 before and after dilution with chloroform are shown in Figure 8. These results clearly show that the morphological transition is incomplete in the case of PS192-bP4VP179/PFDA since two populations are seen after dilution: one associated to the initial micellar aggregates and one to the vesicles. In the case of PS327-b-P4VP27/PFDA complexes, only the population associated to vesicles was visualized in the CONTIN histogram. The vesicles have been observed by TEM and AFM. Since these measurements have been performed in

Vesicle Formation in Block Copolymer Complexes

Langmuir, Vol. 23, No. 1, 2007 121

Figure 8. CONTIN size distribution histograms of PS192-b-P4VP179/ PFDA (a,b) and PS327-b-P4VP179/PFDA (c,d) complexes at an initial concentration of 1 g/L (a,c) and after dilution to 0.1 g/L (b,d) (4VP/ PFDA molar ratio of 4/1 in each case).

the dried state, the structures visualized in Figures 7 and 9 may not correspond to the real situation in solution. In this respect, the dried vesicles as observed by TEM and AFM look like rings or donuts, due to the deformation of the vesicles during the drying process. Furthermore, no direct correlation could be established between the size of the vesicles as measured by TEM and AFM and their Dh,app as measured by DLS. The effect of the 4VP/PFDA molar ratio on the spheres-to-vesicles transition was investigated. No effect was detected for PS327-b-P4VP27/PFDA complexes. In the case of PS192-b-P4VP179/PFDA complexes, the transition was not observed for 4VP/PFDA molar ratio lower than 2/1 as discussed before (Figure 7). It has been also demonstrated that the length of the fluorinated surfactant tail has no effect on the spheres-to-vesicles transition (compare Figure 3, panels c and d, to Figures 7 and 9) and that the presence of the PS block is a prerequisite for the transition to be observed. Indeed, no transition has been observed for the soluble P4VP179/ PFDA and P4VP27/PFDA complexes. Finally, the effect of the number of carboxylic acid groups on the FS was previously discussed in this paper. Encapsulation of a Fluorescent Dye inside PS-b-P4VP/ Fluorinated Surfactant Vesicles. The spheres-to-vesicles morphological transition observed in this paper may be advantageously used in order to trap molecules of interest. If the initial spherical micellar aggregates are diluted with chloroform containing the molecules to be encapsulated, the subsequent morphological transition will lead to vesicles filled with molecules dissolved in chloroform dispersed in the same mixture of chloroform and molecules. Because these vesicles contain perfluorinated alkyl tails in their walls, permeation of nonfluorinated molecules through them is not expected. Therefore, a further dialysis of the diluted solutions against pure chloroform should finally lead to a system in which vesicles filled with chloroform and molecules are diffusing in pure chloroform. To further evidence this hypothesis, we have selected a fluorescent dye, perylene bisimide, as a candidate for encapsulation in vesicles formed by the PS327-b-P4VP27/PFDA complexes. Because micrometer-sized vesicles are formed by these complexes, optical microscopy can be used to visualize the encapsulation process. Confocal laser scanning microscopy images were indeed directly recorded on the dialyzed solutions containing perylene bisimideloaded vesicles (Figure 10). The success of our approach is demonstrated by the fact that fluorescence is essentially observed from the vesicles that are also present in the transmission image.

Figure 9. TEM images of PS327-b-P4VP27/PFDA complexes after dilution to 0.1 g/L. 4VP/PFDA molar ratio of (a) 4/1, (b) 2/1, and (c) 1/1.

The fluorescence emission spectrum recorded from this image is similar to the one of perylene bisimide molecules dissolved in chloroform, demonstrating that the dye molecules are freely dispersed inside the vesicles.

Conclusions In this contribution, we have highlighted the importance of the sample preparation method on the morphological characteristic features of aggregates formed by block copolymer-fluorinated

122 Langmuir, Vol. 23, No. 1, 2007

Hu et al.

starting components, kinetically frozen nonequilibrium spherical micellar aggregates are formed. The characteristic features of these spherical micellar aggregates are however controlled by a variety of parameters including the composition of the PSb-P4VP copolymer, the 4VP/FS molar ratio, the length of the perfluorinated tail of the surfactants, the number of functional groups on the surfactants (1 or 2), and the presence of the PS block. We have also demonstrated that aggregation is driven by two factors: the hydrogen bonding between the carboxylic acid group of the surfactant and the nitrogen atom of 4VP, and additional interactions among perfluorinated chains of the surfactants. Upon dilution with chloroform, these spherical micellar objects reorganize irreversibly into vesicles, which are believed to represent the equilibrium state of the system. This reorganization process is essentially directed by the increased solubility of the P4VP/FS complexes as solvent is added. Finally, the accordingly obtained vesicles with perfluorinated alkyl chains in the walls can be used as containers for molecules of interest. These molecules can be encapsulated by a very selective and simple process which requires dilution of the initial micellar aggregates by a solution of the molecules of interest, followed by a dialysis step to eliminate nonencapsulated molecules. These vesicles containing encapsulated molecules are stable for several weeks. Figure 10. Optical microscopy picture (left) and corresponding confocal laser scanning microscopy image (right) of PS327-b-P4VP27/ PFDA vesicles (C ) 0.01 g/L; 4VP/PFDA 4/1 molar ratio) loaded with perylene bisimide.

Acknowledgment. Z.H., A.M.J., and J.F.G. thank the “Fondation Louvain” for financial support (me´ce´nat Solvay). J.F.G. thanks the STIPOMAT ESF Program.

surfactant complexes in chloroform. Whenever the aggregates are prepared by simple mixing of chloroform solutions of the

LA061532H