Langmuir 2008, 24, 3699-3708
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Articles Micellization of PEO/PS Block Copolymers at the Air/Water Interface: A Simple Model for Predicting the Size and Aggregation Number of Circular Surface Micelles Louise Descheˆnes,*,† Mosto Bousmina,‡ and Anna M. Ritcey§ Food Research and DeVelopment Centre, Agriculture and Agri-Food Canada, St-Hyacinthe, Que´ bec, Canada J2S 8E3, Canada Research Chair on Polymer Physics and Nanomaterials, Chemical Engineering Department, LaVal UniVersity, Que´ bec, Canada G1K 7P4 (CREPEC) and Hassan II Academy of Science and Technology, Rabat, Morocco, and Chemistry Department, CERSIM, LaVal UniVersity, Quebec, Canada G1K 7P4 ReceiVed July 17, 2007. In Final Form: January 10, 2008 Isotherms of monolayers of poly(ethylene oxide) (PEO) and polystyrene (PS) triblock copolymers spread at the air/water interface were obtained by film balance technique. In a low concentration regime, the PEO segments surrounding the PS cores behave the same way as in monolayers of PEO homopolymers. Langmuir-Blodgett (LB) films prepared by transferring the monolayers onto mica at various surface pressures were analyzed by atomic force microscopy (AFM). The results reveal that these block copolymers form micelles at the air/water interface. Within the micelles, the PS blocks act as anchoring structures at the interface. In several cases, aggregation patterns were modified by the dewetting processes that occur in Langmuir-Blodgett films transferred to solid substrates. High transfer surface pressures and mestastable states favored these changes in morphology. A flowerlike surface micelle model is proposed to explain the organization of the surface circular micelles. The model can be generalized and applied to diblock copolymers as well. The model permits prediction of the aggregation number and the size of circular surface micelles formed by PEO/PS block copolymers at the air/water interface.
Introduction Amphiphilic block copolymers are of great scientific interest for several reasons including their crucial role in the control of interfacial phenomena. Applications for these materials are found in such diverse fields as microelectronics, medicine, and personal care products. Block copolymers containing PEO as the hydrophilic block also offer the advantage of being biocompatible. Triblock copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) blocks, known as poloxamers, have been extensively studied.1-4 Related systems composed of PEO and polystyrene (PS), that is, PEO-PS-PEO triblocks, have received much less attention. However, the potential for using the resulting self-assembled structures to develop nanopatterns and templates has recently attracted important interest.5-7 Amphiphilic block copolymers are known for their ability to form monolayers at the air/water interface. Typically, the hydrophobic block anchors the molecule at the interface and the * To whom correspondence should be addressed. E-mail: deschenesl@ agr.gc.ca. Telephone: 450-768-3243. † Agriculture and Agri-Food Canada. ‡ Chemical Engineering Department, Laval University and Hassan II Academy of Science. § Chemistry Department, CERSIM, Laval University. (1) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103-A124. (2) Alexandridis, P.; Hatton, T. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 743-754. (3) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604-2612. (4) Amiji, M.; Park, K. Biomaterials 1992, 13, 682-692. (5) Li, M.; Ober, C. K. Mater. Today 2006, 9, 30-39. (6) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725-6760. (7) Hamley, I. W. Nanotechnology 2003, 14, R39-R54.
Table 1. Characteristics of the Block Copolymers commercial name
block copolymer n-m-n
MW (g/mol)
polydispersity
P3706 P3711 P764 P763 P1952
6-69-6 95-67-95 61-177-61 70-27-70 170-39-170
7700 15 400 23 800 9000 19 100
1.10 1.03 1.07 1.11 1.06
hydrophilic block is soluble in the water subphase. Compared to PEO-PPO block copolymers, the interfacial behavior of block copolymers containing polystyrene is quite remarkable. In 1991, Zhu et al.8,9 suggested that they tend to form surface aggregates at the air/water interface. Such a conclusion about twodimensional micelles at the air/water interface was obtained from experiments carried out on polystyrene/poly(vinylpyridinium) block polyelectrolytes (PS-P4VP+). This phenomenon has since been observed for various PS-X diblock copolymers, with X being a neutral or charged hydrophilic block. Within the amphiphilic neutral PS-X systems, surface aggregation has been reported for polystyrene-poly(2-vinylpyridine) (PS-P2VP),10-12 polystyrene-poly(methyl methacrylate) (PS-PMMA),13 and PS(8) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 55835588. (9) Zhu, J.; Lennox, R. B.; Eisenberg, A. Langmuir 1991, 7, 1579-1584. (10) Meli, M.-V.; Badia, A.; Gru¨tter, P.; Lennox, R. B. Nano Lett. 2002, 2, 131-135. (11) Choi, M.; Chung, B.; Chun, B.; Chang, T. Macromol. Res. 2004, 12, 127-133. (12) Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. Macromolecules 2006, 39, 684-689. (13) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H. Macromolecules 2001, 34, 48424851.
10.1021/la702141h CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008
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Figure 1. Comparison of experimental Π-A isotherms for spread monolayers of selected PEO-PS-PEO triblock copolymers at 20.0 ( 0.5 °C. An enlargement of the high concentration regime is shown in the upper right of the graph.
PEO.14-18 It has been suggested that when PS-containing diblock copolymers are spread at the water surface, they aggregate spontaneously upon solvent evaporation16 to form what have been called starfish and jellyfish micelles.19 Most of the studies reporting surface micellization of block copolymers at the air/water interface involve the analysis of the resulting Langmuir-Blodgett (LB) films by either transmission electron microscopy (TEM) or atomic force microscopy (AFM). Confirmation that the observed surface aggregates of block copolymers do not result from the monolayer transfer process but actually exist at the air/water interface has been obtained through several different approaches. Neutron reflectivity measurements revealed that the PS14-PEO160 diblock copolymer forms aggregates from solution at the air/water interface, yielding surface clusters composed of an anchoring PS lens surrounded by a PEO corona.14 The same authors reported that, for adsorbed PS14-PEO160 layers, surface micellization occurs at a solution concentration of 2.5 × 10-6 g/mL, a concentration lower than the observed bulk critical micelle concentration (cmc) (3.5 × 10-5 g/mL). Matsuoka et al. have developed a direct approach based on X-ray reflectometry to monitor surface micelles.20 More recently, Kim et al. demonstrated evidence of block copolymer surface aggregation by inserting a cross-linkable unit into the PS-PEO structures.21 Finally, recent work by Wen et al. shows that the nature of the spreading solvent influences the morphology of LB films of PS-b-P2VP.22 Most of the investigations carried out on the self-assembly of block copolymers at the air/water interface have concerned diblock copolymers. Only a few studies have examined the aggregation of other types of PS-PEO block copolymers at the air/water interface: surface aggregates have been obtained from (PS-PEO)3 (14) Dewhurst, P. F.; Lovell, M. R.; Jones, J. L.; Richards, R. W.; Webster, J. R. P. Macromolecules 1998, 31, 7851-7864. (15) Gonc¸ alves da Silva, A. M.; Simo˜es Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327-5330. (16) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, B. Langmuir 1999, 15, 7714-7718. (17) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432-5436. (18) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921-1927. (19) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 47274730. (20) Matsuoka, H.; Mouri, E.; Matsumoto, K. Rigaku J. 2001, 18, 54-68. (21) Kim, Y.; Pyun, J.; Fre´chet, J. M. J.; Hawker, C. J.; Frank, C. W. Langmuir 2005, 21, 10444-10458. (22) Wen, G.; Chung, B.; Chang, T. Polymer 2006, 47, 8575-8582.
star block copolymers.23 To the best of our knowledge, the only published study on the behavior of PEO-PS-PEO triblock copolymers at the air/water interface is that of Rivillon et al.24 While block copolymer micellization in the bulk has been the subject of numerous scientific publications and resulted in the development of various models,25-28 models that properly describe the surface micelles formed by these systems are definitely missing. This is particularly true in the case of triblocks. Monte Carlo simulations of the formation of surface micelles from short block copolymers at an adsorbing smooth surface concluded that these micelles have many similarities with those formed in bulk solution.29 One specific characteristic of the surface micelles that is highlighted in this study is that they are frozen-in at the random positions where they form. This model does not, therefore, yield the highly regular patterns that have been observed experimentally. Cox et al.30 compared three qualitative models of the organization of diblock PS-PEO copolymers at the air/ water interface. Their conclusions agreed with a model in which the PS cores sit on a PEO film, but they excluded the formation of a PEO brush, which is usually observed upon compression in the case of PEO-PPO-PEO monolayers.31 Despite the various available qualitative studies, the quantitative treatment to adequately predict the aggregation number and to describe how the size of these surface micelles scales with the number of statistical segments composing the hydrophobic and hydrophilic blocks is still missing. A better understanding of the factors involved in the formation of block copolymer surface micelles would be of great interest for the development of block copolymer-based nanoscale templates for nanolithography and biosensing devices and for microporous and mesoporous structure development. The surface film balance method for the study of surface monolayers is a technique that is particularly useful for (23) Logan, J. L.; Masse, P.; Dorvel, B.; Skolnik, A. M.; Sheiko, S. S.; Francis, R.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005, 21, 3424-3431. (24) Rivillon, S.; Muno˜z, M. G.; Monroy, F.; Ortega, F.; Rubio, R. G. Macromolecules 2003, 36, 4068-4077. (25) Mortensen, K.; Brown, W.; Almdal, K.; Alami, E.; Jada, A. Langmuir 1997, 13, 3635-3645. (26) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998; Chapter 3. (27) Pedersen, J. S. J. Appl. Crystallogr. 2000, 33, 637-640. (28) Riess, G. Prog. Polym. Sci. 2003, 28, 1107-1170. (29) Milchev, A.; Binder, K. Langmuir 1999, 15, 3232-3241. (30) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417-4421. (31) Sedev, R.; Steiz, R.; Findenegg, G. H. Physica B 2002, 315, 267-272.
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Figure 2. Typical Π-A isotherm indicating regions discussed in the text. The critical molecular areas AL and A0 correspond to the limiting area of the high compression regime and the limiting molecular area at zero pressure, respectively.
the determination of molecular organization.32,33 In the present study, triblock copolymers with different PS/PEO ratios were studied by film balance technique and atomic force microscopy (AFM). Characteristics of the self-assembled structures are discussed, and a simple geometrical model is proposed to describe the scaling of the surface micelle dimensions formed by PEOPS-PEO diblock and triblock copolymers with molecular parameters. Materials and Experimental Techniques Chemicals. Commercial and custom-made triblock copolymers were obtained from Polymer Source Inc. in Montreal, Canada and used as received without further purification. Polydispersity data were obtained from the supplier. For the samples considered here, the hydrophobic block consists of polystyrene (PS) chains containing 27-177 monomers, whereas the hydrophilic poly(ethylene oxide) (PEO) blocks are composed of 6-170 monomers. In this paper, the PEOn-PSm-PEOn copolymer chains will be denoted as n-m-n. The used specific structural copolymer properties are provided in Table 1. A homopolymer of poly(ethylene glycol), having a molecular weight of 4600 g/mol and identified as PEO105, was purchased from Sigma-Aldrich Canada Ltd. A homopolymer of polystyrene having a molecular weight of 10 509 g/mol, identified as PS100, was obtained from Dow. High-performance liquid chromatography (HPLC) grade chloroform was purchased from Fisher. Water was purified using a NANOpure II purification system (Barnstead/ Thermolyne); it had a surface tension and resistivity of 72 mN/m and 18.2 MΩ cm, respectively. Film Balance Experiments. Surface-pressure isotherms were recorded for monolayers spread from chloroform solutions (1 mg/ mL) onto a nanopure-quality water subphase in a Langmuir-Blodgett (LB) Teflon trough (KSV 3000 LB-system). Solutions were spread using a microliter syringe typically delivering 5 uL drops. To ensure complete evaporation of the solvent, a time lag of 15 min was applied between the deposition of the polymer solution and the beginning of compression of the spread molecules. Pressure-area isotherms were measured with symmetric compression at a rate of 5 mm/min at 20 °C. The surface pressure was measured using a platinum Wilhelmy plate located midway between the two barriers and oriented perpendicular to the barriers, that is, parallel to the direction of barrier movement. The complete system was operated on an antivibration table. Film Deposition. Langmuir films were transferred onto freshly cleaved ruby mica from S&J Trading Inc., Glen Oaks, NY. After spreading from chloroform solutions, at least 15 min was allowed (32) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (33) Suresh, K. A.; Bhattacharyya, A. Pramana 1999, 53, 93-106.
Figure 3. Compression isotherms plotted with respect to (a) the average area per PEO repeat unit and (b) the average area per PS repeat unit. for solvent evaporation. The barriers were then compressed to the target surface pressure at a speed of 10 mm/min. The transfers were carried out with a substrate speed of 5 mm/min while maintaining constant surface pressure. Film transfer ratios of 0.95-1.0 were obtained. The resulting films were individually stored at room temperature in clean closed vials. Film Imaging. LB films were imaged using tapping mode AFM with a Multimode III atomic force microscope (Digital Instruments, Santa Barbara, CA). OTESPA7 cantilevers with a frequency of 150300 Hz were obtained from Digital Instruments (DI). Surface scans were run with a DI type E scanner and an applied frequency of 1 Hz. Typically, for each type of surface, five different samples were scanned to ensure reproducibility and the reported results in this paper are averages.
Experimental Results Isotherms. Experimental Π-A isotherms of the PEO-PSPEO block copolymers investigated in the present study are shown in Figure 1. For comparison purposes, the isotherm of a PEO homopolymer with a degree of polymerization of 105 (PEO105) is also plotted in the same figure. A generalized isotherm is shown in Figure 2, indicating the critical zones of surface pressure and molecular areas which will be discussed throughout the paper. Previous studies have demonstrated that the PEO homopolymer adsorbs to the air/water interface despite its solubility in water, allowing for the existence of a 2D monolayer in a low surface concentration regime.34-36 A “pseudoplateau” is observed for the homopolymer isotherm at a medium surface concentration. In this case, no steep rise is observed after the plateau, since the polymer does not contain a hydrophobic block to anchor it to the interface. Upon compression beyond the plateau, the PEO (34) Descheˆnes, L.; Bousmina, M.; Ritcey, A. M. Submitted for publication. (35) Henderson, J. A.; Richards, R. W. Macromolecules 1993, 26, 45914600. (36) Kim, M. W.; Cao, B. H. Europhys. Lett. 1993, 24, 229-234.
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Figure 4. Typical AFM images of PEO-PS-PEO LB films.
homopolymer is believed to detach from the surface and dissolve into the subphase. A number of the block copolymers exhibit a similar plateau in their compression isotherms. This is the case for most of the PEO-PS-PEO copolymers studied here. When present, the pseudoplateau of the PEO-PS-PEO triblock copolymers appears at higher surface pressures than that found for the PEO homopolymer (N ) 105). The surface pressure-area (Π-A) isotherms of the triblock copolymers in Figure 1 are very similar to those obtained by other authors for spread monolayers of PS-PEO diblock copolymers,15,17,37 as well as for similar triblocks (70-27-70 and 61-177-6124). As expected, the length of the constituent blocks has a major effect on the profile of block copolymer isotherms. The effect of the polymer structure can be easily visualized by plotting the surface pressure as a function of the average surface area per monomer. Isotherms plotted with the molecular area normalized with respect to the number of EO and PS segments are shown in parts a and b, respectively, of Figure 3. Figure 3 illustrates the concentration regime in which the isotherms are influenced by the size of each of the constituent blocks. Figure 3a demonstrates that, except for 6-69-6, all of the isotherms fall on a single curve for surface areas corresponding to the semidilute regime. This indicates that the isotherm in this regime is mainly dependent on PEO block length for all polymers except 6-69-6. Beyond the pseudoplateau, all polymers appear to be affected by the PS/PEO ratio. In the semidilute regime, the Flory coefficient of the system can be obtained from the des Cloizeaux equation,38 which describes the relationship between the surface pressure (Π) and the surface concentration (Γ) of a polymer by
∏ ) CA-y ) KΓy
with
y)
dν dν - 1
(1)
where Π is the surface pressure, C and K are proportionality constants, A is the molecular area, y is the scaling exponent, Γ (37) Faure´, M. C.; Bassereau, P.; Carignano, M. A.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3, 365-375. (38) des Cloizeaux, J. J. Phys. (Paris) 1975, 36, 281-291.
is the surface concentration, and d is the geometrical dimension. In two dimensions, y ) 2ν/(2ν - 1), where ν is the Flory coefficient that the values are 0.77 and 0.57 for good and theta solvent conditions, respectively.39,40 The molecular area can be conveniently related to the surface concentration by
A)
MW 6.023Γ
(2)
where A is the molecular area in Å2, MW is the molecular weight, and Γ is the surface concentration expressed in mg/m2. According to eq 1, the exponent y, and thus the Flory coefficient, can be evaluated from the slope of a plot of log Π versus log Γ. For all of the triblock copolymers investigated in this work, a coefficient of ν ) 0.77 was obtained. This result confirms that, in the semidilute regime, the PEO blocks determine the shape of the isotherm and that their conformation is purely 2D, as observed for the PEO homopolymers.34 These results are similar to those obtained for PS-PEO diblock copolymers.24,41 AFM. Typical AFM images of LB films on mica from the different PEO-PS-PEO triblock copolymers are shown in Figure 4. Aggregation was observed for all of the copolymers investigated. Different types of domains were observed, including dots, stripes, islands, and more complex three-dimensional lattices. The evolution from dots to 3D complex lattices appears to be pressure-dependent. This is particularly obvious in the case of highly asymmetric block copolymers. Additional AFM images of the two extreme cases investigated (6-69-6 and 170-39-170) are given in Figure 5 for different surface pressures. Both the size and the shape of the domains seem to depend strongly on the PS/PEO ratio. For two polymers (6-69-6 and 170-39-170), important superaggregation (circular micelle assemblies) and breaks in the monolayer structure are observed in LB films (Figure 5). The surface micelle assembly can take place (i) during the solvent evaporation step or (ii) during the LB transfer. For 6-69(39) Le Guillou, J. C.; Zinn-Justin, J. Phys. ReV. Lett. 1977, 39, 95-98. (40) Le Guillou, J. C.; Zinn-Justin, J. Phys. ReV. B 1980, 21, 3976-3998. (41) Faure´, M. C.; Basserau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32, 8538-8550.
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Figure 5. AFM images of 6-69-6 and 170-39-170 at different surface pressures.
Figure 6. Evolution of 95-67-95 pattern at medium and high surface pressure.
6, LB films show very large empty spaces between the domains, even at the lowest surface pressure (2 mN/m) of monolayer transfer (Figure 4a). For this copolymer, circular micelle aggregation is thought to occur before the LB transfer because of the high block copolymer asymmetry. This hypothesis was verified by preliminary Brewster angle microscopy (BAM) experiments (image not shown). In BAM, 6-67-6 was the only copolymer showing a broken monolayer. This behavior explains the discrepancy of 6-69-6 in Figure 3a. In the case of 170-39170, irregular bicontinuous rod lattices and dendritic superstructures are observed for films transferred at 15 and 20 mN/m, but not at 10 mN/m or lower transfer surface pressures (Figure 4c and d). Similar branched morphologies were recently reported for LB films of a PS-PEO multiarm star copolymer with high block asymmetry ((PS30)10-(PEO568)10).42 Between these extremes of structural asymmetry in the PS and PEO relative dimensions, intermediate superaggregation morphologies were observed. For 70-27-70 at high pressure, dots are replaced by regular zigzag patterns. In comparison, 9567-95 has an intermediate behavior. At 8 mN/m, dot patterns present at lower surface pressures evolve to form cylinders (stripes) and a large-scale aggregation pattern is observed, suggesting a dewetting process (Figure 4f). At higher transfer surface pressures, the LB films of 95-67-95 appear as a uniform layer (Figure 6c). Interestingly, disruptions were occasionally (42) Gunawidjaja, R.; Peleshanko, S.; Genson, K. L.; Tsitsilianis, C.; Tsukruk, V. V. Langmuir 2006, 22, 6168-6176.
observed between 8 and 15 mN/m (Figure 6a and b). Such disrupted patterns have also been reported for PS-b-PMMA diblock copolymers.43 Such transition raises the question of whether this pattern results from nucleation sites (dust particles) or spinodal dewetting. A summary of observed domains is presented in Table 2. In the case of samples exhibiting dot morphologies, the aggregation number (Nagg) was evaluated from the AFM images in conjunction with the Π-A isotherms. Nagg was determined by dividing the number of dots (circular domains) per unit area by the molecular area from the Π-A isotherms at the corresponding transfer pressure. The height profile of the dots was generally found to be hemispheric with the condition used to scan the samples. A typical height profile of 61-177-61 is presented in Figure 7. The data of Table 2 indicate that the aggregation number, Nagg, is largely influenced by the PS/PEO balance. The exact relationship, however, seems far from straightforward. Copolymers having a large dissymmetry in block size are likely to adopt morphologies other than circular micelles leading to dots. This trend is favored by higher transfer surface pressures. It is also an indication that circular surface micelles under dot patterns are not in an equilibrium state for these polymers. For very high hydrophobic content (6-69-6), dots aggregate to generate large flat lamellae-like structures even at very low surface pressure. (43) Orso, K. A.; Green, P. F. Macromolecules 1999, 32, 1087-1092.
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Figure 7. Height profile of a LB film of 61-177-61 transferred to mica at a surface pressure of 4 mN/m. Table 2. Characteristics of Surface Aggregates
polymer
surface pressure (mN/m)
pattern
6-69-6
2
dot islands
61-177-61
4 5
dot dot
95-67-95
2 4 5 8
dot dot dot hemicylinders wormlike disrupted layer (Figure 5b)
10 70-27-70
170-39-170
2 4 5 10 20
dot dot dot irregular wormlike zigzag
4 5
dot dot
8 15
dot cylinders honeycomb-like
monolayer height (Å)
domain width (Å)a
54 30.7 ( 0.8
540 ( 23 235 ( 35
13.6 ( 0.4
domain height (Å)
Naggb 382c
375 ( 15 455 ( 10
27.4 ( 1.6
60 76
365 ( 22 330 350 ( 10 332 ( 16
7.9 ( 1.5 9.77 7.3 ( 0.3 4.5 ( 0.4
22 23 27
276 ( 10 247 ( 11 202 ( 6 300 ( 28
2.9 ( 0.3 3.8 ( 0.4 8(1 5.8 ( 0.2
15 15
12 ( 2
6.3 ( 2
6.9
282 ( 20 270 ( 22 371 ( 15 312 ( 13 603 ( 30 194 ( 12
3.8 ( 0.5 74.7 ( 1.4 43
7 7 14 14
a For circular patterns (dots), it is the average dot-to-dot distance. b Nagg ) aggregation number. c From individual dots composing the edge fringe, Figure 4b.
cylinders, and wormlike and lamellae-like morphologies observed for the investigated triblock copolymers were also reported for LB films of PS-PEO18 and PS-PVP11,19 diblock copolymers. However, bicontinuous lattices (honeycomb-like) and dendritic structures are unusual for LB films of amphiphilic block copolymers. Recently, dendritic patterns were reported for the first time for LB films of PEOnPSn multiarm star copolymers.42
Discussion Figure 8. Scheme of block copolymer aggregation upon solvent evaporation at the air/water interface.
For very low hydrophobic content (170-39-170), the observed LB films pass from dots to 3D lattices at transfer surface pressures higher than that of the pseudoplateau. In these two extreme cases, the LB films show exposed substrate surfaces (Figures 4 and 5). Intermediate PS/PEO ratios and low transfer surface pressure seems to favor circular micelle patterns (dots). The circular surface micelles showed a tendency to organize into hexagonal lattices in all cases, when this type of aggregate was observed. Dots,
For the PEO-PS-PEO block copolymers investigated in the present work, increasing the hydrophilic coronal chain length decreases the aggregation number of the spherical micelles. The same trend was observed by Eisenberg et al. for polystyrenepoly(acrylic acid) (PS-PAA) diblock copolymers.44,45 The scenario of aggregation appears to be quite simple. Upon spreading of the solution at the interface, the solvent evaporates. The PS segments concentrate in the chloroform medium, while the PEO (44) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J. Chem. 1999, 77, 1311-1326. (45) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168-3181.
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Figure 9. PEO-PS-PEO flower-micelle model.
segments probably play their surfactant role at the chloroform/ water interface. The size and the nature of the blocks that first come into contact with each other at the interface seem to determine the aggregation number. The copolymers with large PS segments and small PEO ones should aggregate faster, resulting in a high aggregation number. In the reverse situation (low PS and high PEO content), as illustrated in Figure 8, the spread PEO blocks reduce the probability of PS segments encountering each other, and the aggregation number is consequently much smaller. The individual units sketched at the beginning of the process correspond to individual molecules. The number of molecules illustrated is not proportional to the number expected in the micelles for the corresponding block sizes. Only a fraction of molecules is represented. The important point to keep in mind is the relative proportions of PS and PEO segments and the deformation of the PS core of the micelle in the resulting aggregates. The two-dimensional surface micelles considered are formed from spreading a solution in a nonselective solvent onto the air/water interface. The solution spreading induces a 2D positioning of the molecules at the interface, whereas the solution-surface interaction (chloroform/water) induces polymer orientation. The PEO block adsorbs at the chloroform/water interface to decrease the energy of the system. Upon solvent evaporation, the PS block dewets the adsorbed PEO layer, forming isolated clusters. It is assumed that low solution concentrations and less volatile solvents favor thermodynamically stable micelles. It can be expected that a combination of high concentrations, highly volatile solvents, and relatively high molecular weight polymers (with entanglements) can lead to kinetically frozen micelles. Micellization of the system upon solvent evaporation results in domains with a dense core of PS surrounded by an outer corona of swollen PEO coils. The PS blocks are confined to the interface and are assumed to collapse in packing constraints of φPS ≈ 1, as predicted for block copolymer micelles in solution,46 with φPS being the volumic fraction of PS in the space occupied by this block. A 2D PEO layer acting as a surfactant carpet isolates the PS core from the water subphase, which decreases the total Gibbs energy of the system. Based on an analysis of (46) Halperin, A. Macromolecules 1987, 20, 2943-2946.
the experimental isotherms and AFM imaging, a flowerlike micellization model is proposed to explain the organization of PEO-PS-PEO block copolymers spread at an air/water interface. Flowerlike Micelle Model for Block Copolymer Aggregates at the Air/Water Interface. The following model is developed based on the assumption that each individual block copolymer molecule is placed in the 2D plane of the air/water interface. During solvent evaporation, several molecules aggregate as shown in Figure 8. Once solvent evaporation is complete, the 3D PS segments collapse and scale according to RF ) aKNK1/3, where RF is the radius of the polymer coil, aK is the effective segment size (the Kuhn length), and NK is the number of Kuhn segments in the coil. NK ) N/mK, with mK being the number of monomers per Kuhn segment and N being the total number of monomers in the polymer chain. The Kuhn length and mK are important parameters in scaling approaches, as they take into account the tacticity, the intramolecular interactions, and the rigidity of the chains. The circular micelles are essentially constituted of a PS core, forming a disk lying on 2D PEO segments, and surrounded by other PEO segments, which form the micelle corona. Since the PEO segments are in good solvent condition at the air/water interface,34 it can be assumed that the chains will organize in a pattern maximizing the number of PEO segments in the surrounding corona. This means that the two PEO blocks attached to each PS block situated at the external limit of the micelle core (identified as PS in Figure 9) will contribute solely to the corona, as opposed to being under the PS disk. The carpet of PEO segments between the PS core and the aqueous phase is assumed to be formed by the PEO blocks attached to the PS blocks of the molecules forming the center of the core of the micelle (identified as PSχ in Figure 9). Figure 9 gives a simple representation of micelle organization at the end of solvent evaporation. The area of the core of the 2D micelle (Ac) is equivalent to the total areas of the PS blocks involved in the aggregate. In the case of a PEO-PS-PEO triblock copolymer,
Ac ) NaggAPS ) NaggkπRPS2
(3)
where APS is the surface area of the PS block of each molecule included in the micelle and k is a factor expressing deformation.
3706 Langmuir, Vol. 24, No. 8, 2008
Descheˆ nes et al.
The deformation of the PS segments will be neglected in a first step but will be discussed later. The core radius is then defined by
Rcore ) (Ac/π)1/2 ) RPSNagg1/2
(4)
The micelle area, Am, includes the area of the corona, formed by PEO blocks attached to the PS blocks (APEO), and is thus given by
Am ) Ac + Acorona ) NaggAPS + Acorona
(5)
The number of PEO blocks surrounding the micelle, NPEO, is twice that of the PS blocks located at the core/corona interface (NPS), and the area of the corona is therefore
Acorona ) NPEOAPEO ) 2NPSAPEO
(6)
where APEO is the area of a single PEO block at the air/water interface. NPS depends on the aggregation number. It is given by
NPS )
APS APS
(7)
πNaggRPS2 - π(RPSNagg1/2 - 2RPS)2 (8) For simplification purposes, it is first assumed that RPS is the same for PS segments in the center of the core and at the core/ corona interface. In other words, an average PS radius is used. Substituting APS from eq 8 into eq 7, we obtain
πNaggR2 - π(RPSNagg1/2 - 2RPS)2 πRPS2
Nagg )
8AB 4AB - (A0 - AA) - [16AB2 - 8AB(A0 - AA)]1/2
) 4(Nagg1/2 - 1) (9)
Substituting eq 9 into eq 6 yields to the relationship between the number of PEO segments in the corona and the aggregation number (eq 10):
Acorona ) 8(Nagg1/2 - 1)APEO
(10)
Combining eqs 5 and 10 results in a simple quadratic relationship:
Am ) APSNagg + 8APEO(Nagg1/2 - 1)
(11)
The micelle area is related to the average molecular area at zero pressure (A0), as expressed in eq 12. As indicated in Figure 2, A0 can be obtained from the Π-A isotherms:
Am ) NaggA0
(12)
Substituting Am as given by eq 12 into eq 11 gives the following:
Nagg(A0 - APS) - 8APEONagg1/2 + 8APEO ) 0
(13)
From the above equation, the aggregation number is given by
Nagg ) 8APEO 4APEO - (A0 - APS) - [16APEO2 - 8APEO(A0 - APS)]1/2 (14)
(15)
The flower-micelle model predicts circular micelle formation within a specific range of block lengths. The limiting cases will be encountered when (a) NA . NB and (b) NA , NB.
(a) NA . NB
(A0 f AA)
Nagg ≈
8AB f∞ 4AB - 4AB
(16)
This situation (NA . NB) will thus favor the formation of lamellaelike morphologies. This is supported by our experimental results in the case of the copolymer 6-69-6. When the hydrophobic anchoring block is very small compared to the hydrophilic blocks (NA , NB), the flower-micelle model predicts a limiting aggregation number of 47 units for triblock copolymers (eq 17).
(b) NA , NB
(A0 - AA f AB) Nagg ≈
APS ) Ac - π(Rc - 2RPS)2 )
NPS )
Generalization of the Model and Limiting Cases. The model could be generalized for Bm-An-Bm triblock copolymers made up of hydrophobic anchoring blocks A and hydrophilic blocks B:
8AB 3AB - (8AB2)1/2
≈ 47 (17)
For highly asymmetric molecules with very short hydrophilic blocks (NA . NB), the circular micelles are expected to be unstable. This instability would drive the circular micelles to form superaggregates (micelle aggregation) to decrease the free energy of the system. Our results are in very good agreement with this prediction. For 6-69-6, very few isolated dots are observed. The primary pattern obtained corresponds to a lamellar morphology, even at low transfer surface pressures (Figure 4a). For the triblock copolymers 70-27-70 and 170-39-170, aggregation numbers of 7 and 14, respectively, were obtained for LB films transferred at low surface pressures. However, above 5 mN/m, the circular domains start to transform into larger superstructures, with wormlike and zigzag patterns for 70-27-70 and dendritic ones for 170-39-170. This change in morphology can be interpreted as an indication that the structures are not equilibrium ones (NA , NB) and the circular micelles probably correspond to metastable states for these copolymers. In the case of 95-67-95 (Nagg ) 25), a higher surface pressure is required to induce a morphology change in the LB films (8 mN/m) as compared with 70-27-70 and 170-39-170. The most symmetric copolymer treated in this study, 61-177-61, exhibits an aggregation number of 75 and forms circular domains that are stable at surface pressures up to 15 mN/m. (No LB films were examined for higher transfer pressure for this molecule). The model thus appears to yield valid predictions for the PEO-PS-PEO triblock copolymers. Furthermore, the results indicate that the range of relative block sizes within which circular micelles are stable is relatively restricted. Table 3 summarizes the predicted and experimental values of Nagg obtained for LB films transferred at a surface pressure of 2 mN/m. The calculated values of Nagg were obtained from eq 14 combined with the experimental values of A0 and AL. The use of the experimental values of AL instead of the calculated values of APS compensates for neglecting the PS deformation factor k. The experimental values can be used to extract information about PS chain stretching. At A0, the PEO segments correspond to 2D collapsed coils in a helix 11/2 configuration with the following scaling parameters: aK ) 7 Å and C∞ ) 4.81.34 The aggregation number obtained from eq 14 therefore reflects the organization
PEO/PS Block Copolymers at the Air/Water Interface
Langmuir, Vol. 24, No. 8, 2008 3707
Table 3. Aggregation Number and PS Stretching Ratio copolymer (PEO-PS-PEO) 6-69-6 61-177-61 95-67-95 70-27-70 170-39-170 b
Nagg (AFM)a
Nagg (eq 14)
382 and +b 76 22 15 7, 14
4142 85 27 15 8
PS stretching ratio (APS - AL)/APS 0.0 -0.1 -0.1 -0.7 -1.5
a Nagg of LB transfer obtained at a surface pressure of 2 mN/m. Domains of various sizes were observed.
Table 4. Aggregation Characteristics of Calculated and Experimental Domainsa copolymer PS-PEO 140-80 215-113 459-82
Nagg (AFM)
references Nagg PS stretching ratio experimental values (eq 18) (APS - AL)/APS
181 62 341 57 527 and +b 512
0.2 0.2 0.4
16, 30 16 18
a Transfer surface pressure of LB films ) 2 mN/m. b Domains of various sizes were observed.
of the aggregates at the air/water interface (with the values used for the calculations being obtained from the Π-A isotherms). The aggregation number obtained by AFM corresponds instead to the size of the aggregates after water evaporation and dewetting on the substrate. Although quite simple, the model gives excellent correlations between the experimental and calculated Nagg values. Furthermore, the stretching ratio provides a measure of PS chain deformation in the nanodomains. The most stable circular micelles have a low PS stretching ratio (61-177-61, 95-67-95), whereas the unstable ones have high absolute stretching ratios (70-27-70 and 170-39-170). A negative value for the stretching ratio indicates that the deformation is experienced in an axis parallel to the interface (AL > APS). In the case of 6-69-6, since the pattern at 2 mN/m is not dots but lamellae, the aggregation number is an average of the size of the different aggregates present at the interface (Figure 4a). The surface contains less than 0.1% dots. The experimental Nagg value of 4142 gives the average number of molecules per superaggregate. The stretching ratio is equal to 0 because the system is already relaxed by superaggregation and the PS chains are not deformed anymore. Extension of the Flower-Micelle Model to the Diblock Copolymer Case. Based on the same model, corresponding equations for diblock copolymers can be derived. The resulting expression for the aggregation number of diblocks is given by the following:
Nagg )
4AB 2AB - (A0 - AA) - 2[AB2 - AB(A0 - AA)]1/2
(18)
The flowerlike model also predicts lamellae morphologies for the diblock copolymers in the limiting case of NA . NB. For long hydrophilic chains linked to a very short anchoring block, NA , NB, Nagg ) 4. Thus, the model predicts that diblock copolymer aggregates could exist under a circular micelle pattern for longer hydrophilic chains than the triblock copolymers (for a similar size of PS block). Equation 18 was verified for diblock copolymers using data available in the literature (Table 4). For the reported diblock copolymers, NPS > NPEO in all cases. The PS stretching ratio indicates that, at the air/water interface, circular micelles are deformed in a direction normal to the interface
(AL < APS). For lower stretching ratios (0.2), regular arrays of circular micelles are observed for LB films of 140-80 and 215113. The difference between Nagg calculated from the model and Nagg obtained from AFM indicates that the dewetting process induces circular micelle aggregation. This superaggregation allows the relaxation of PS chains. For 459-82, the AFM images of LB films transferred at 2 mN/m show a distribution of dots of various sizes.16 In the latter case, instability is greater, resulting in a distribution of domain size and the loss of hexagonal arrays. The experimental results support the flowerlike model for the diblock copolymers. For relatively large PS blocks, the circular micelles tend to aggregate in bigger spherical structures. Replacement of the air/water interface by an air/mica interface during LB transfer is believed to induce shrinkage of the PEO segments and initiate the superaggregation phenomenon. Deformation of Aggregates upon LB Transfer. When surface micelles are transferred to hydrophilic substrates (e.g., mica, SiO2), shrinking of PEO chains during drying reduces the proportion of area occupied by PEO chains relative to that occupied by PS blocks. The reduction of the PEO area paves the way for the formation of superaggregates and morphology changes. In the case of transfers to hydrophobic substrates, the wetting of PS segments drives the morphology changes. The type of substrate and the need for the system to relax the deformation of the PS segments are determining factors. Domains with high PS stretching in the direction perpendicular to the interface (NPS . NPEO) will relax in a direction parallel to the interface, favoring the development of lamellae-like structures (6-69-6). Domains with high PS stretching in the direction parallel to the interface will relax by extending the PS melt in an axis normal to the interface, favoring wormlike and cylindrical structures (70-27-70, 95-67-95). In the extreme cases of PS stretched in 2D, the relaxation results in 3D dimensional lattices such as gyroidlike and dendritic morphologies (e.g., 170-39170). These processes are more likely to take place when the transfer is carried out at high surface pressures, because the surface concentration is high and most of the PEO segments are located in the subphase, leaving fewer of them to surround the PS cores.
Conclusions The present study shows that amphiphilic triblock copolymers of PEO-PS-PEO self-assemble into highly monodisperse, stable flowerlike surface micelles, ordered in hexagonal arrays for the block copolymers without important structural asymmetry regarding the relative number of PS and PEO monomers. At low surface pressure, the core of the aggregates consists of a monolayer of PS blocks surrounded by the 2D swollen PEO blocks. Analysis of the results indicates that the size (radius and aggregation number) of these micelles is controlled by the ratio of the PS block area to PEO block area prevailing at the solvent deposition/ evaporation step. Circular flower micelles formed by block copolymers with high PS/PEO ratios superaggregate into lamellae-like morphologies. Highly asymmetric block copolymers with low PS/PEO ratios superaggregate into cylinders or wormlike or dendritic structures. The Langmuir-Blodgett deposition step on mica is believed to induce morphology modification by dewetting processes driven by PS chain relaxation. The uniform size distribution of the aggregates formed by PEO-PS-PEO block copolymers indicates an equilibrium state, or a long-lived metastable state. The circular disklike micelles of 6-69-6 and 170-39-170 definitely belong to the metastable category. For the other block copolymers studied, it is not clear whether the morphological evolution observed with increasing pressure is
3708 Langmuir, Vol. 24, No. 8, 2008
due to a change from a metastable state to an equilibrium one or whether the increase in surface pressure forces the PEO chains into the subphase and thus induces contact and aggregation between neighboring circular micelles. Additional experimental work is required to distinguish between these two possibilities. The proposed flowerlike micelle model developed here correctly describes the surface micellization of PS/PEO di- and triblock copolymers spread from chloroform solutions (1 mg/mL) at the air/water interface. To the best of our knowledge, no studies have yet been published that examine the stability of PS/PEO block copolymer self-assemblies at the air/water interface. Such a study would
Descheˆ nes et al.
help to improve understanding of surface micellization of these molecules at interfaces and would assist in identifying the role of entropy versus enthalpy in the formation of these aggregates. The results of the present study suggest that it may be possible to pattern surfaces with molecular aggregates of nanometric size which can be controlled by substrate type, relative solvent quality for each block, and transfer surface pressure. Acknowledgment. This work was supported by funding from Agriculture and Agri-Food Canada. LA702141H
