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Micellar Aggregation in Blends of Linear and Cyclic Poly(styrene-b-isoprene) Diblock Copolymers Nadia Ouarti,*,† Pascal Viville,† Roberto Lazzaroni,† Edson Minatti,‡ Michel Schappacher,§ Alain Deffieux,§ Jean-Luc Putaux,| and Redouane Borsali*,§ Service de Chimie des Mate´ riaux Nouveaux, Universite´ de Mons-Hainaut/Materia Nova, 20 Place du Parc, 7000 Mons, Belgium, Departamento de Quı´mica da Universidade Federal de Santa Catarina, Floriano´ polis, SC, Brazil, Laboratoire de Chimie des Polyme` res Organiques, CNRS, ENSCPB and Bordeaux-1 University, 16 Avenue Pey Berland, 33607 Pessac Cedex, France, and Centre de Recherches sur les Macromole´ cules Ve´ ge´ tales, ICMG-CNRS (affiliated with the Joseph Fourier University of Grenoble), BP 53, 38041 Grenoble Cedex 9, France Received April 8, 2005. In Final Form: June 14, 2005 The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS290-b-PI110 copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 °C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.
1. Introduction In the context of the development of nanotechnologies, diblock copolymer systems, which consist of connected blocks formed by two different monomer species, represent a fascinating class of polymer materials, due to their specific properties. In particular, they spontaneously selfassemble into micelles in the presence of a selective solvent for one of the blocks. This process originates from the minimization of the interactions between the solvent and the insoluble segment. The formed micelles are characterized by a core-shell structure in which the nonsoluble block forms the core and the soluble chains form the diffuse corona. These micellar systems are used in many industrial and pharmaceutical preparations as dispersants, stabilizers, wetting agents, etc.1,2 They are also of great interest as biomaterials, as control agents for drug delivery, and more recently as vectors for gene therapy.3-7 The colloidal behavior of diblock copolymers in solution has been intensively studied both theoretically and experimentally.1,2,8-18 Depending on factors such as the * To whom correspondence should be addressed. E-mail: borsali@ enscpb.fr (R.B.),
[email protected] (N.O.). † Universite ´ de Mons-Hainaut/Materia Nova. ‡ Universidade Federal de Santa Catarina. § ENSCPB and Bordeaux-1 University. | ICMG-CNRS. (1) Riess, G.; Hurtrez, G.; Bahadur, P. Block Copolymers, 2nd ed.; Wiley: New York, 1985; Vol. 2, p 324. (2) Alexandridris, P.; Lindman, B. Amphiphilic Block Copolymers; Elsevier: Amsterdam, 2000; p 1. (3) Torchilin, V. P. J. Controlled Release 2001, 73, 137. (4) Kataoka, K.; Harada, A.; Nagasaki, Y. Adv. Drug Delivery Rev. 2001, 47, 113. (5) Kabanov, A. V.; Batrakova, E. V.; Alakhov, V. Y. J. Controlled Release 2002, 82, 189. (6) Kakizawa, Y.; Kataoka, K. Adv. Drug Delivery Rev. 2002, 54, 203. (7) Katayose, S.; Kataoka, K. Bioconjugate Chem. 1997, 8, 102.
sample preparation, the copolymer concentration and its molecular architecture, and the nature of the solvent, the self-assembly process of diblock copolymers leads to the formation of a variety of micellar morphologies: spheres,19-21 cylinders,22 wormlike structures,23 sunflowerlike structures,24,25 lyotropic liquid crystalline structures,26 or vesicles.27-29 As a further step, blending block copolymers offers the attractive opportunity of combining properties not found (8) Price, C. In Developments in Block Copolymers; Goodman, I., Ed.; Applied Science: London, 1982; p 39. (9) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998; p 131. (10) Antonietti, M.; Heinz, S.; Schmidt, M.; Rosenauer, C. Macromolecules 1994, 27, 3276. (11) Forster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (12) Calderara, F.; Riess, G. Macromol. Chem. Phys. 1996, 197, 2115. (13) Noolandi, J.; Hong, K. M. Macromolecules 1983, 16, 1443. (14) Nagarajan, R.; Ganesh, K. J. Chem. Phys. 1989, 90, 5843. (15) Whitmoore, M. D.; Noolandi, J. Macromolecules 1985, 18, 657. (16) Halperin, A. Macromolecules 1987, 20, 2943. (17) Svaneborg, C.; Pedersen, J. S. Macromolecules 2002, 35, 1028. (18) Pedersen, J. S.; Svaneborg, C.; Almdal, K.; Hamley, I. W.; Young, R. N. Macromolecules 2003, 36, 416. (19) Henselwood, F.; Liu, G. Macromolecules 1997, 30, 488. (20) Harada, A.; Kataoka, K. Science 1999, 283, 65. (21) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (22) Pedersen, J. S.; Hamley, I. W.; Ryu, C. Y.; Lodge, T. P. Macromolecules 2000, 33, 542. (23) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960. (24) Minatti, E.; Viville, P.; Borsali, R.; Schappacher, M.; Deffieux, A.; Lazzaroni, R. Macromolecules 2003, 36, 4125. (25) Borsali, R.; Minatti, E.; Putaux, J.-L.; Schappacher, M.; Deffieux, A.; Viville, P.; Lazzaroni, R.; Narayanan, T. Langmuir 2003, 19, 6. (26) Svensson, M.; Alexandridris, P.; Linse, P. Macromolecules 1999, 32, 637. (27) 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. (28) Won, Y. Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B. 2002, 106, 3354. (29) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967.
10.1021/la050935z CCC: $30.25 © 2005 American Chemical Society Published on Web 08/26/2005
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in the pure copolymers, properties that can be potentially useful for novel applications. Another interesting feature of copolymer blends is the possibility to control the morphology by adjusting the relative amount of each component. Most previous studies dealt with blends of a diblock copolymer (AB) with a homopolymer30-39 (A or B); particular attention has also been devoted to mixtures of diblock copolymers (AB) with other diblock copolymers40-45 (AB or BC). Those investigations have mostly focused on chemically identical diblocks (AB/AB blends) in the bulk. In particular, poly(styrene-b-isoprene), PS-b-PI, binary blends have been studied46-51 in terms of the influence of the molecular weight ratio and the volume fraction of the copolymers. For instance, Hashimoto et al.48 have investigated the self-assembly of binary blends of PS-b-PI with different compositions and molecular weights by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). They found that, in most cases, the mixtures exhibit a lamellar morphology. They also studied the miscibility criteria for the binary mixtures of nearly symmetric PS-b-PI copolymers organizing into lamellae. They show that when the molecular weight ratio is smaller than 5, the two copolymer components are molecularly miscible and form a single ordered microdomain morphology. In contrast, when the ratio is higher than 10, the two copolymers phase-separate and form their own microdomain morphology. It is important to stress here that, for those binary blends of PS-b-PI, the most studied architecture is the linear one, which consists of a long sequence of PS monomer units covalently bonded to a chain of isoprene monomer units. Recently, we have shown24,25 that chain cyclization is also a parameter of great importance, which strongly affects the micellar aggregation of PS290-b-PI110 diblock copolymers in heptane (a selective solvent for the PI segments). Our results indicate that, whatever the copolymer concentration (up to 20 mg/mL, i.e., the highest concentration value we have considered), the linear diblock chains form spherical micelles with a PS core and a PI shell, with a diameter of 40 nm. The corresponding cyclic (30) Zin, W. C.; Roe, R. J. Macromolecules 1984, 17, 183. (31) Roe, R. J.; Zin, W. C. Macromolecules 1984, 17, 189. (32) Whitmore, M. D.; Noolandi, J. Macromolecules 1985, 18, 2486. (33) Shull, K. R.; Winey, K. I. Macromolecules 1992, 25, 2637. (34) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378. (35) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240. (36) Han, C. D.; Baek, D. M.; Kim, J. K.; Kimishima, K.; Hashimoto, T. Macromolecules 1992, 25, 3052. (37) Baek, D. M.; Han, C. D.; Kim, J. K. Polymer 1992, 33, 4821. (38) Matsen, M. W. Macromolecules 1995, 28, 5765. (39) Maurer, W. W.; Bates, F. S.; Lodge, T. P.; Almdal, K.; Mortensen, K.; Fredrickson, G. H. J. Chem. Phys. 1998, 108, 2989. (40) Shi, A.-C.; Noolandi, J. Macromolecules 1995, 28, 3103. (41) Cifra, P.; Karasz, F. E.; MacKnight, W. J. Macromolecules 1989, 22, 3649. (42) Shi, A. C.; Noolandi, J. Macromolecules 1994, 27, 2936. (43) Shi, A.-C.; Noolandi, J.; Hoffman, H. Macromolecules 1994, 27, 661. (44) Kim, J. K. Polymer 1995, 36, 1243. (45) Jiang, M.; Huang, T.; Xie, J. Macromol. Chem. Phys. 1995, 196, 803. (46) Sakurai, S.; Umeda, H.; Yoshida, A.; Nomura, S. Macromolecules 1997, 30, 7614. (47) Sakurai, S.; Irie, H.; Umeda, H.; Nomura, S. Macromolecules 1998, 31, 336. (48) Hashimoto, T.; Yamasaki, K.; Koizumi, S.; Hasegawa, H. Macromolecules 1993, 26, 2895. (49) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 4371. (50) Hashimoto, T.; Tanaka, H.; Hasegawa, H. In Molecular Conformation and Dynamics of Macromolecules in Condensed Systems; Nagasawa, M., Ed.; Elsevier: Amsterdam, 1998; p 257. (51) Hashimoto, T.; Koizumi, S.; Hasegawa, H. Macromolecules 1994, 27, 1562.
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diblock copolymer yields either smaller so-called “sunflower” micelles (33 nm in diameter) at very low concentrations, or giant wormlike micelles (33 nm wide up to 1 µm in length) at relatively high concentrations, resulting from the unidirectional self-assembly of the sunflower micelles. This phenomenon occurs because the micelles lower their free energies by changing their shape from small sunflowers to cylindrical.52 Following that work, in the present paper we report on the morphology of blends of linear and cyclic PS290-PI110 diblock copolymer micelles formed in heptane. Two methods were used to prepare these mixed diblock copolymer micelles: the first one consisted in blending the solid copolymers as powders before their solubilization in heptane, in which they form micellar morphologies. The second one consisted in mixing the copolymers already solubilized in heptane. In that case, micelles are already formed in each solution before blending. Dynamic light scattering (DLS) was used to identify the ongoing dynamics of different blends in heptane, while atomic force microscopy (AFM) measurements were performed on thin deposits formed on a substrate after heptane evaporation. In addition, cryo transmission electron microscopy (cryoTEM) images were recorded on dilute systems after in situ freeze-drying of the embedding heptane. In parallel to the studies on blends, we have also investigated the effect of annealing on the solid-state organization of the micelles formed by the linear and cyclic PS-b-PI diblock copolymers separately or in blends, to probe their thermal stability. The main reason is that temperature is likely to induce morphological transitions.53-56 It is worth noting that annealing experiments on deposits of diblock copolymers are generally carried out ex situ.18,57 Here, the effect of the annealing treatment on the film micellar morphology was followed in situ with an atomic force microscope equipped with a heating stage. 2. Experimental Section The polymerization used here has already been described in detail in previous papers.24,25,59 The cyclic form of the poly(styreneb-isoprene) copolymer is obtained by direct end coupling of the R- and ω-ends of the linear poly(styrene-b-isoprene) copolymer. The polymerization yields exactly the same DP for each block in the linear and cyclic copolymer systems, since the cyclic compound is made by closure of the linear diblock. The volume fraction of PS, ΦPS, in both diblock copolymers is equal to 0.78, corresponding to DPPS block ) 290 (as determined by SEC analysis) and DPPI block ) 110 (estimated from the NMR isoprene/styrene signal integration and DPPS block) and to a total molar mass of 37 × 103 g/mol. The polydispersity index for the two copolymers is lower than 1.1. The different samples analyzed in this work were prepared either by mixing either the copolymer powders before their solubilization in heptane or by directly mixing the copolymer solutions in heptane. In both cases, the concentration of the final solution was 3 mg/mL. The DLS experiments have been described in detail in our previous paper.59 The autocorrelation functions were measured (52) Larue, I.; Adam, M.; Da Silva, M.; Sheiko, S. S.; Rubinstein, M.; Macromolecules 2004, 37, 5002. (53) Bott, R.; Wolff, T.; Zierold, K. Langmuir 2002, 18, 2004. (54) Gorski, N.; Kalus, J. Langmuir 2001, 17, 4211. (55) Kumar, S.; Sharma, D.; Khan, Z. A.; Din, K. U. Langmuir 2001, 17, 5813. (56) Yin, H. Q.; Mao, M.; Huang, J. B.; Fu, H. L. Langmuir 2002, 18, 9198. (57) Han, C. D.; Vaidya, N. Y.; Kim, D.; Shin, G.; Yamagushi, D.; Hashimoto, T. Macromolecules 2000, 33, 3767. (58) Iatrou, H.; Hadjichristidis, N.; Meier, G.; Frielinghaus, H.; Monkenbusch, M. Macromolecules 2002, 35, 5426. (59) Ouarti, N.; Viville, P.; Lazzaroni, R.; Minatti, E.; Schappacher, M.; Deffieux, A.; Borsali, R. Langmuir 2005, 21, 1180.
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using an ALV laser goniometer, which consists of a 22 mW HeNe linear polarized laser with a 632.8 nm wavelength, and the scattering angle ranges from 20° to 150°. The relaxation times (or frequencies) were obtained through the cumulant method or/and Contin analysis to yield the effective diffusion coefficient and the mean apparent hydrodynamic radius Rh (StokesEinstein) relation as a function of the scattering angle (1).
Rh )
kT 2 q 6πηΓ h
(1)
Thin films of these blends for AFM characterization were prepared at ambient temperature by solvent casting. Typically, 15 µL of a 3 mg/mL solution in heptane was deposited on a freshly cleaved mica substrate. After complete evaporation of heptane at room temperature, tapping mode (TM) AFM was used for imaging thin deposits at 25 °C. AFM images were recorded with a Nanoscope IIIa microscope from Digital Instruments (Veeco, Santa Barbara, CA) operating in ambient atmosphere at room temperature. For thermal experiments, the same solvent casting procedure was employed to prepare the samples. Annealing experiments were performed in situ. For that purpose, the microscope was equipped with a sample holder underneath which a heating element allowed uniform annealing of the sample. The temperature was progressively increased from 25 to 100 °C by steps of 5 °C. The sample and the tip were kept at the same temperature during the whole experiment. After each temperature step, the system was allowed to reach thermal equilibrium before imaging started. In situ freeze-drying cryo-TEM specimens were prepared using the method described elsewhere.61 As heptane is known to dissolve into liquid ethane, thin films of 2 mg/mL micelle suspensions, formed on “lacey” carbon films (NetMesh, Pelco), were rapidly frozen into liquid nitrogen. The specimens were mounted in a Gatan 626 specimen holder cooled by liquid nitrogen. Once transferred into a Philips CM200 “cryo” electron microscope, operated at 80 kV, the specimen was slowly warmed to a temperature of approximately -115 °C, at which heptane sublimes and the heating was stopped. After the return to thermal stability at -180 °C, the samples were observed under low illumination and images were recorded on Kodak SO163 films.
3. Results and Discussion 3.1. Micellar Morphology in the Blends. We have studied various copolymer mixtures in heptane, corresponding to the addition of an increasing amount of linear PS290-b-PI110 diblock copolymer to the cyclic PS290-b-PI110 diblock copolymer. One notes that the correlation functions were described by a single relaxation time for all blends in heptane, and the resulting diameter of the newly formed morphology, as measured by DLS, is shown in Figure 1. In the solution containing only the cyclic copolymer (0% linear), the large average size (>100 nm) is consistent with the presence of wormlike micelles resulting from the unidirectional aggregation of individual sunflower micelles of the cyclic copolymer. This result has already been extensively discussed.24,25 Upon addition of a small amount of the linear copolymer (a few percent), the size of the micelles dramatically decreases. At about 10% linear copolymer, the average micelle size drops to a value close to 40 nm. Then, upon further enrichment in linear copolymer, the size of the micelles slightly, but constantly, increases to reach a value close to 45 nm. These DLS data have been supplemented by TM-AFM analyses. The TM-AFM height images in Figure 2 show the morphology of deposits obtained from solutions containing 0%, 5%, 50%, and 100% linear diblock copolymer, the total copolymer concentration being kept con(60) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (61) Putaux, J.-L.; Minatti, E.; Lefe`bvre, C.; Borsali, R.; Schappacher, M.; Deffieux, A. Faraday Discuss. 2005, 128, 163.
Figure 1. Size of the micelles formed by blending linear and cyclic PS290-PI110 block copolymer powders before solubilization in heptane, as measured by DLS. The total copolymer concentration is 3 mg/mL.
stant at 3 mg/mL. As illustrated in Figure 2a, the pure cyclic PS290-b-PI110 diblock copolymer forms cylindrical micelles with an average diameter of 33 ( 0.5 nm. These cylinders, constituted by the unidirectional self-assembly of 33 nm wide sunflower micelles,24,25 are several hundred nanometers long, consistent with the large size revealed by the DLS measurements. Upon addition of 5% linear copolymer, the morphology drastically changes from long cylinders to much smaller objects (Figure 2b). This is consistent with the strong decrease in particle size observed by DLS upon addition of small amounts of the linear copolymer. In contrast to what could have been intuitively expected (i.e., a mixture of spherical and cylindrical micelles), these deposits only contain spherical micelles with a diameter of 33 ( 0.6 nm. Interestingly, this value is similar to that of individual sunflower micelles formed from the pure cyclic copolymer.24,25 Figure 2c shows the height image obtained for an equimolar blend of linear and cyclic PS290-b-PI110 copolymers. This mixture also leads to the formation of spherical micelles densely packed on the surface and having a narrow size distribution: 35 ( 0.7 nm. Finally, the pure linear copolymer assembles into 40 nm wide micelles (Figure 2d). The AFM data therefore fully confirm the evolution observed in the DLS measurements. To understand this morphological evolution, let us recall the most likely cause of the difference in morphology between the micelles made of the pure cyclic system (cylinders) and those of the pure linear system (spheres). When the linear copolymer is solubilized in heptane, the PI blocks can fully extend into the solvent to stabilize the PS core. This yields 40 nm wide spherical micelles. In contrast, for the cyclic counterpart, the PI segments have to fold back into the PS core, which most probably reduces their stabilizing capacity by a certain extent. As a result, self-assembly has to proceed to a further stage and larger cylindrical micelles are formed instead of spheres. The incorporation of linear copolymer molecules into the micelles made of the cyclic molecules is expected to improve the global stabilizing capacity of the PI shell, so that individual elementary micelles become again the most stable form of micellar self-assemblies and that wormlike micelles no longer tend to form. Figure 3 schematically shows the three types of micelles: those made of the pure linear copolymer (top), those made of the pure cyclic copolymer (middle), and those with the mixed composition (bottom).
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Figure 2. TM-AFM height images (1 × 1 µm2) of micelles of PS290-PI110 block copolymers formed from a 3 mg/mL solution in heptane by blending linear and cyclic copolymer powders: (a) with no linear copolymer added in the solution; (b) mixture of 5% linear copolymer with 95% cyclic copolymer; (c) mixture of 50% linear copolymer with 50% cyclic copolymer; (d) with 100% linear copolymer. The vertical gray scale is 30 nm for all the images except for the last image (40 nm).
Figure 4. Cryo-TEM images of micelles formed by blending linear and cyclic PS290-PI110 diblock copolymers in a 10/90 ratio, after in situ freeze-drying of the thin crystalline film of embedding heptane. In (a), the micelles are seen clinging to the edges of the lacey carbon network while, in (b), they are seen lying on an extended surface of carbon of the same specimen.
Figure 3. Schematic representation of the three types of micelles: those made of the pure linear copolymer (top), those made of the pure cyclic copolymer (middle), and those with the mixed composition (bottom).
Cryo-TEM observations were also performed to check whether the slow drying of the solvent in air, which takes
place during sample preparation for AFM, could affect the morphology of the micelles. After the in situ freezedrying procedure to remove the embedding crystalline heptane, micelles appear, clinging around the holes in the lacey carbon film (Figure 4a) or, in some cases, evenly spread on extended flat parts of the carbon substrate (Figure 4b). These cryo-TEM images are snapshots reflecting the micellar organization in the solution. They clearly show that the large majority of micelles are small spherical micelles and are rather monodisperse, with a diameter close to 30 nm, consistent with the size measured from AFM images. However, in contrast to the AFM observations, a few short cylindrical objects, with a width equal to that of sunflower micelles, are also observed. When examined at higher magnification, these cylinders
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Figure 5. TM-AFM height images (1 × 1 µm2) of micelles of PS290-PI110 diblock copolymers formed by blending linear and cyclic copolymer molecules from separate 5 mg/mL solutions in heptane: (a) 50/50 linear/cyclic blend; (b) 5/95 linear/cyclic blend. The vertical gray scale is 30 nm for both images.
seem to be formed by the association/fusion of individual sunflower micelles. This clearly confirms that the isolated micelles observed here are mainly formed by molecules of the cyclic copolymer. The absence of 40 nm spherical micelles typical of the pure linear copolymer indicates that all the linear chains are incorporated in the micelles with the cyclic copolymer. In other terms, the isolated micelles also contain a certain number of linear molecules (Figure 3). The incorporation of both linear and cyclic molecules within common micelles also implies that the linear and the cyclic copolymers are fully miscible, suggesting that the composition of each individual micelle observed here is equivalent to that of the starting solution. It is interesting to mention that Hashimoto et al.48 have shown that the miscibility of nearly symmetric PS-b-PI diblock copolymers depends on their relative molecular weight. The present results indicate that PS-b-PI diblock copolymers having the same molecular weight but different molecular architectures are also fully miscible. To further confirm that the morphological evolution we observed did not depend on the sample preparation procedure, we have investigated the same mixtures of linear and cyclic PS290-b-PI110 diblock copolymers prepared using two different methods. We first prepared separate solutions of the two copolymers in heptane, and we then mixed the solutions in various ratios at a temperature of 50 °C: in other words, micelles were already present in solution before blending of the two copolymers. For the sake of conciseness, we only present here AFM data for the 50/50 and the 5/95 linear/cyclic blends (Figure 5). A typical AFM height image for the 50/50 blend is presented in Figure 5a. Again, one may have expected to observe the coexistence of spherical and cylindrical objects. Instead, the deposit exclusively contains spherical micelles similar to those formed by mixing the two powders prior to solubilization (Figure 2b,c). The morphological change of the wormlike micelles originally present in the solution made of the cyclic copolymer to elementary spherical micelles upon mixing with the solution of the linear copolymer highlights the dynamic nature of micellar objects. The interchange of molecules between micelles most probably occurs here because the blends are prepared at 50 °C, allowing sufficient mobility for all the chains. This eventually leads to a uniform 50/50 composition in all micelles and thus to the presence of elementary spherical objects only, both in solution and in the deposits. Even when the amount of added linear copolymer is as
low as 5%, the deposit only shows spherical micelles (Figure 5b). These AFM data demonstrate that the morphology obtained for the blends is independent of the preparation procedure: indeed, when linear and cyclic copolymer molecules are mixed, they always self-assemble to form elementary spherical micelles. These results, corroborating the DLS data, are important since it is known that the control of the self-assembly of diblock copolymers at the mesoscale is a key parameter for several potential applications in nanotechnology. For instance, the size and compactness of micellar aggregates can have a significant impact on their efficiency to encapsulate and release substances.62 3.2. Thermal Stability of the Formed Micelles. The main aim of the annealing experiments is to determine the thermal stability of the micelles vs self-assembly and to determine whether the thermal behavior of the micelles depends on the molecular architecture of the copolymer chains. Note that the glass transition temperature (Tg) of polystyrene is close to 100 °C, while that of polyisoprene is around -70 °C. Figure 6 presents the AFM results obtained by increasing the temperature from 25 to 100 °C for the equimolar blend of linear and cyclic PS290-b-PI110 diblock copolymers. Starting from the usual spherical morphology at room temperature, we observe that this type of organization is maintained as long as the temperature is lower than 70 °C. Note however that the contours of the individual objects gradually fade away starting from 60 °C. Spheres then begin to coalesce, eventually forming a homogeneous film at 100 °C. This behavior can be explained by the fact that, as the temperature increases, the interactions between the coronae formed by the PI segments increase so that the PI chains of different coronae coalesce; this process leads to the formation of a PI film which entirely covers the surface, because the surface energy of PI is lower to that of PS.63 We have observed the same behavior for the blends with different compositions, as well as for the deposits formed from the pure linear and the pure cyclic diblock copolymers (data not shown here). Indeed, micelles can be observed in AFM images up to 70 °C; as the temperature exceeds this value, the coalescence of the micelles makes the individual objects hardly visible. (62) Fo¨rster, S.; Planterberg, Th. Angew. Chem., Int. Ed. 2002, 41, 688-714. (63) Brown, R. A.; Masters, A. J.; Price, C.; Yuan, X. F. Comprehensive Polymer Science; Pergamon Press: New York, 1989; Vol. 2.
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Figure 6. TM-AFM height images (1 × 1 µm2) showing the effect of the gradual increase of temperature on the micellar morphology for a deposit of the 50/50 linear/cyclic blend. The mixture was prepared by blending the copolymer powders before their solubilization in heptane. The vertical gray scale is 30 nm for all the images.
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hmn is the measured height at the pixel mn, and 〈h〉 is the mean height of the imaged area. Here, the RMS surface roughness is used to follow the coalescence of the micelles: as the coronae of neighboring micelles interpenetrate, the valleys between the objects get filled up and the surface topography gets smoother. The roughness remains almost constant for temperatures below 50 °C, and then it decreases significantly between 50 and 70 °C. This behavior indicates that in that temperature range the micelle coronae begin to interpenetrate. Between 70 and 90 °C, the roughness of the surface remains quasiconstant, and then it sharply decreases. This phenomenon can be explained as follows: the PI corona has a very small thickness compared to the diameter of the hard PS core (1.6 nm vs 18 nm, as estimated previously59). Therefore, even when the PI coronae have fully coalesced (i.e., above 70 °C), the presence of the rigid PS cores slightly below the surface still imparts a significant roughness. Another possible explanation for this behavior could be the formation of a fuzzier interface between the core and corona. This interface region would be somewhat softer that the PS core itself, and its width is not expected to change much with temperature. When the temperature finally exceeds 90 °C, i.e., for temperatures very close to the PS glass transition temperature, the PS cores are likely to begin softening, thereby leading to full relaxation to a smooth, PI-covered surface. A similar evolution of the roughness vs temperature is observed for all the compositions investigated, including the micelles made of pure cyclic and pure linear copolymer molecules. 4. Conclusions
Figure 7. Evolution of the surface RMS roughness versus the increase of temperature for the equimolar blend. The line through the experimental data is a guide to the eye.
As expected, on cooling back to room temperature, the final homogeneous surface morphology is maintained. We do not observe the classical phase-separated morphology of thin films of block copolymers, probably because of the thin layer of the lower surface energy component, i.e., PI, covering the whole surface. The evolution of the surface root-mean square (RMS) roughness, as measured on 1 × 1 µm2 AFM images, with an increase of the temperature is shown in Figure 7 for the equimolar blend. The RMS surface roughness corresponds to the standard deviation (σ) of the symmetrical Gaussian distribution of height values over a given area; it is expressed as follows:
σRMS )
1 N
[hmn - 〈h〉]2]1/2 ∑ mn
[
where N is the number of sampling points over the image,
The starting point of this study was the observation that PS-b-PI block copolymers with the same composition but with different chain architectures (linear or cyclic) form micelles with different morphologies in a selective solvent for PI: micelles of the pure cyclic PS290-b-PI110 copolymers are wormlike cylindrical objects built by unidirectional self-assembly of 33 nm wide sunflower micelles, while the linear block copolymer having the same composition forms 40 nm wide spherical micelles. When blends are examined, the formation of the cylindrical aggregates appears to be inhibited by the presence of the linear copolymer (even when it represents only 5% of the total polymer mass) because the incorporation of linear copolymer chains stabilizes elementary micelles against unidirectional self-assembly leading to cylinders. In contrast to the morphological properties, the thermal stability of the micelles is quite similar for all systems (the pure copolymers and the blends), as far as the observation of the coalescence with AFM is concerned. The interpenetration of the PI coronae is complete around 70 °C, while the surface becomes fully smooth only when the glass transition temperature of the PS hard cores is reached. Acknowledgment. The Bordeaux-Mons collaboration is supported by the Tournesol program of the Ministe`re de la Communaute´ Franc¸ aise de Belgique and the Ministe`re Franc¸ ais des Affaires Etrange`res. Research in Mons is supported by the Belgian Science Policy Program “Poˆle d’Attraction Interuniversitaire en Chimie Supramole´culaire et Catalyse Supramole´culaire-PAI 5/3”, FNRS-FRFC, the European Commission, and the Re´gion Wallonne (Phasing Out Program-Hainaut). The LCPO members acknowledge the CNRS, the re´gion Aquitaine, and FEDER for financial support. LA050935Z
