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Effect of Temperature on the Interfacial Behavior of a Polystyrene-b-poly(methyl methacrylate) Diblock Copolymer at the Air/Water Interface Yongsok Seo,*,† Chung Yeon Cho,† Minyoung Hwangbo,† Hyoung Jin Choi,*,‡ and Soon Man Hong§ Intellectual Textile System Research Center (ITRC) and School of Materials Science and Engineering, College of Engineering, Seoul National UniVersity, Shillim9dong 56-1, Kwanakgu, Seoul, Korea 151-744, Department of Polymer Science and Technology, Inha UniVersity, Yonghyun4dong, Namku, Inchon, Korea, 402-751, and Hybrid Materials Research Center, Korea Institute of Science and Technology, Hawolgokdong 391-1, Sungbukku, Seoul, Korea 136-701 ReceiVed September 5, 2007. In Final Form: October 30, 2007 Monolayers of a polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymer at the air-water interface were studied by measuring the surface pressure-area isotherms at several temperatures. Langmuir film balance experiments and atomic force microscopy showed that the diblock copolymer molecules formed surface micelles. In the plot of the surface pressure versus surface area per repeating unit, the monolayer changed from the gas phase to the liquid expanded phase at lower surface pressure for systems at low temperature compared to those at high temperature. In addition, a plateau, corresponding to the transition from the liquid expanded to liquid condensed phase, appeared in that plot at lower surface pressure for systems with a higher subphase (water) temperature. Hysteresis was observed in the compression-expansion cycle process. Increasing the subphase temperature alleviated this hyteresis gap, especially at low surface pressures. The minimum in the plot of the surface pressure versus surface area per repeating unit in the expansion process (which arises from the transition) and the transition plateau appeared more vividly at higher water temperature. These dynamic experimental results show that PS-PMMA diblock copolymers, in which both blocks are insoluble in water, do not form complicated entanglements in two-dimensional space. Although higher water temperature provided more entropy to the chains, and thus more conformational freedom, it did not change the surface morphology of the condensed film because both blocks of PS-PMMA are insoluble in water.
Introduction Amphiphilic block copolymers are known to form surface micelles when spread at the air/water interface.1,2 Self-assembly of diblock copolymers into more compact structures has been observed in both two- and three-dimensional space.3 Eisenberg and co-workers have studied a number of ionic diblock copolymers that form surface micelles.4,5 They found that ionic diblock copolymers in which one block is soluble in water form surface * To whom correspondence should be addressed. E-mail: ysseo@ snu.ac.kr (Y.S.);
[email protected] (H.J.C.). † Seoul National University. ‡ Inha University. § Korea Institute of Science and Technology. (1) (a) Zhang, Y. X.; Da, H. A.; Hogen-Esch, T. E.; Bulter, G. B. In Water Soluble Polymers: Synthesis, Solution Properties, and Application; Shalaby, S. W., McCormik, C. L., Buller, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; p 159. (b) Bock, J.; Varadaraj, R.; Schultz, D. N.; Maurer, J. J. In Macromolecular Complexes in Chemistry and Biology. Solution Properties of Hydrophobically Associating Water-Soluble Polymers; Dubin, P., Bock, J., Davis, R., Schulz, D. N., Thies, C., Eds.; SpringerVerlag: Berlin, 1994; p 33. (c) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers; Elsevier: Amsterdam, 2000. (d) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, Chapter 1, pp 1-83. (2) Deveraux, C. A.; Baker, S. M. Macromolecules 2000, 33, 1734. (3) (a) Schuch, H.; Klingler, J.; Rossmanith, P.; Frechen, T.; Gerst, M.; Feldthusen, J.; Mu¨ller, A. H. E. Macromolecules 2000, 33, 1734. (b) Mortensen, K.; Brown, W.; Almdal, K.; Alami, E.; Jada, A. Langmuir 1997, 13, 3635. (c) Poppe, A.; Willner, L.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, 7462. (d) Jada, A.; Hurtrez, G.; Siffert, B.; Riess, G. Macromol. Chem. Phys. 1996, 197, 3697. (e) Hickl, P.; Jada, A. Macromolecules 1996, 29, 4006. (4) (a) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727. (b) Zhu, J.; Eisenberg, A.; Lennox, B. J. Am. Chem. Soc. 1991, 113, 5584. (c) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (d) Zhang, L.; Barlow, R. J.; Eisenberg, A. Macromolecules 1995, 28, 6055. (e) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (f) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923.
micelles with diverse morphologies. This micellar polymorphism arises from variations in the balance between block sizes within the copolymer as well as the solubility of the hydrophilic part.2,4,5 As the soluble block in the copolymer was made progressively shorter, the morphology of the aggregates changed from spherical to rodlike, lamellar, or vesicular and finally to large compound vesicles.5 When both blocks of a diblock copolymer are insoluble to water, different interactions between the blocks induce surface aggregation under compression.6,7 Such block copolymers have been observed also to form various micellar morphologies depending on the relative block length ratio (spherical, rodlike, large compound disk like)5,8 Previously, we investigated the surface aggregation behavior of polystyrene-b-poly(methyl (5) (a) Li, S.;Clarke, C. J.; Eisenberg, A.; Lennox, R. B. Thin Solid Films 1999, 354, 136. (b) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417. (c) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714. (6) (a) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432. (b) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921. (c) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004, 37, 6511. (d) 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. (e) Cheyne, R. B.; Moffit, M. G. Langmuir 2005, 21, 5453. (f) Kim, Y.; Pyun, J.; Fre’chet, J. M. J.; Hawker, C. J.; Frank, C. W. Langmuir 2005, 21, 10444. (g) Gunawidjaja, R.; Peleshanko, S.; Genson, K. L.; Tsitsilianis, C.; Tsukruk, V. V. Langmuir 2006, 22, 6168. (7) (a) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; J.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. (b) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52. (c) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (8) (a) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6556. (b) Li, Z.; Zhao, W.; Quinn, J.; Rafailovich, M. H.; Sokolov, J.; Lennox, R. B.; Eisenberg, A.; Wu, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (c) Shin, K.; Rafailovich, M. H.; Sokolov, J.; Chang, D. M.; Cox, J. K.; Lennox, R. B.; Eisenberg, A.; Gibaud, A.; Huang, J.; Hsu, S. L.; Satija, S. K. Langmuir 2001, 17, 4955.
10.1021/la702745w CCC: $40.75 © 2008 American Chemical Society Published on Web 02/01/2008
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methacrylate) (PS-PMMA) diblock copolymers with different molecular weights.9∼11 From a molecule’s structural viewpoint, these block copolymers are intriguing because the polystyrene block is insoluble in water and surface inactive whereas the poly(methyl methacrylate) block is also insoluble, but surface active. As a result, PS-PMMA forms two-dimensional surface micelles when compressed.10,11 Our finding indicated that the aggregated morphology of these copolymers was completely determined by the interaction between the two blocks. On the basis of atomic force microscopy (AFM) findings, we described a self-consistent picture of surface micelle morphology and aggregation phenomena.9,10 Specifically, we observed that PS-PMMA copolymers having high molar masses in both blocks aggregate to form surface micelles in which the PS segments form the core while the PMMA blocks form the coronas.10,12 Once formed, the surface micelles did not disperse as separate chains even after the surface pressure returned to zero. These findings were subsequently confirmed by others.13 Since both blocks of PS-PMMA molecules are insoluble in water, they were not affected by the ions in the water nor by the conformation of the copolymers in the water. However, the structural evolution for block copolymers is a thermodynamic process that is definitely affected by the thermodynamic properties of the aqueous subphase. In this study, we investigated the effect of temperature on the interfacial behavior of a PS-PMMA diblock copolymer at the air/water interface with a view to developing a better understanding of the structural evolution of the PS-PMMA molecules in two-dimensional space.
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Figure 1. π-A isotherms obtained by compressing monolayers of PS-PMMA diblock copolymer in a compression and expansion cycle at different temperatures. deposited onto freshly cleaved mica by pulling it out of the water at a speed of 5 mm/min while compressing the film at a constant pressure. The morphology of each LB film was observed by an atomic force microscope (PSDA III) equipped with a microfabricated V-shaped silicon nitride cantilever on a 5 µm scanner in the noncontacting mode.
Results and Discussion Experimental Section Materials. We used a PS-PMMA diblock copolymer with a number average molar mass (Mn) of 140 000 g/mol for the PS block and 656 000 g/mol for the PMMA block. This copolymer, which is the same as the diblock copolymer used in our previous study,9,10 was purchased from Polymer Source Inc. (Canada). The polydispersity index (PDI) of the PS-PMMA was 1.32. The PMMA block was composed of 45% syndiotactic and 55% atactic moieties. Neat polystyrene and PMMA were purchased from Pressure Chemicals Co. The Mn of PS was 200 000 g/mol with a PDI of less than 1.05. The Mn of PMMA was 255 000 g/mol with a PDI of less than 1.15. Spectrograde chloroform was used without further purification to make dilute solutions (0.1 mg/mL). Surface Pressure Measurement. Using a stock solution of PS-PMMA dissolved in chloroform at a concentration of 0.1 mg/mL, a monolayer was prepared on the water surface. The water used had been purified and deionized after passing through membrane filters (resistivity 18.1 MΩ). The surface pressure isotherms were obtained using a Langmuir film balance KSV minitrough (Helsinki, Finland). The surface temperature was controlled using a Lauda circulating bath. The water temperature was set to a predetermined temperature prior to solution spreading. Since the solution concentration is in a dilute solution regime, we can safely assume that the solvent evaporation rate does not affect the monolayer morphology.9 To ensure the cleanness of the film balance before each measurement, the water surface was swept several times. After the polymer solution was spread on the water surface, the system was left for 20 min to completely evaporate the solvent. The layer floating on the subphase was then symmetrically compressed by moving two mobile barriers at a predetermined speed. The compression speed was set slow enough not to affect the final film morphology. The resulting LB film was (9) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H. Macromolecules 2001, 34, 4842. (10) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 8735. (11) Seo, Y.; Esker, A. R.; Sohn, D.; Kim, H.-J.; Park, S.; Yu, H. Langmuir 2003, 19, 3313. (12) (a) Kumaki, J.; Nishikawa, Y.; Hashimoto, T. J. Am. Chem. Soc. 1996, 118, 3321. (b) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 1998, 121, 423. (c) Kumaki, J.; Kawauchi, T.; Yashima, E. J. Am. Chem. Soc. 2005, 127, 5788. (13) Chung, B.; Park, S.; Chang, T. Macromolecules 2005, 38, 6122.
In our previous study on the interfacial behavior of PS-PMMA diblock copolymers, we found that the diblock copolymer molecules stay on the water surface at high compression.9 We also found that the surface pressure isotherms of PS-PMMA diblock copolymer exhibited two phase transitions before reaching the solid state, one due to surface micelle formation and the other to the transition from the liquid expansion state to the liquid condensation state (the LE-LC transition).9,10 Figure 1 shows the effect of temperature on the dynamic behavior of the PSPMMA monolayer in the experimentally accessible temperature range (10-40 °C). Overall behaviors are as follows: The compression part of the isotherms can be divided into four different regions. For convenience, we designate these regions as L1 (010 mN/m), T1 (10-20 mN/m), L2 (20-55 mN/m), and T2 (>55 mN/m). In L1, the molecules are in the separate (gaseous) state initially and then subsequently form surface micelles as they are compressed. As shown in Figure 2, the conformation change in the monolayer can be schematically represented as follows: As soon as the compression starts, many surface micelles are formed. Micelle formation reduces the enhanced entropy they received from the water by extending the PMMA moiety outward.12 With higher compression, the skirts of the surface micelles begin to touch each other, with a large free area (macrovoids) between them (Figure 2). Uniform micellar films were formed under conditions of low surface pressure with higher temperature (Figure 1). Further compression changes the film state from the liquid expanded state to a liquid condensed state by removing the free area of macrovoids (T1 region). This manifests as the plateau observed in the plot of the surface pressure versus surface area per repeating unit. This transition is a first-order transition since the provided energy does not enhance the surface pressure.14 Further compression after the plateau drives the PMMA segments into a more compact conformation by removing the free area of (14) Li, S.; Clarke, C. J.; Lennox, R. B.; Eisenberg, A. Colloids Surf., A 1998, 133, 191.
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Figure 2. A schematic representation of the PS-PMMA diblock copolymer micelles at different surface pressures. The temperature was 40 °C (from Seo et al.9). (A) When the solution is spread on the water surface, few aggregates are formed because of the long PMMA block. As soon as the compression starts, self-assembly of diblock copolymers in two dimensions occurs to form “surface micelles”. PMMA segments are radially extended to accept the space restriction. (B) In the compression process, a detectable rise in pressure above the baseline occurs when the branches from different aggregates begin to sterically interact with each other. We termed the free space between micelles as “free area” (“macrovoids”). (C) The free space between aggregates is compressed to remove the free area of macrovoids and, hence, to form a more compact and uniform film (transition). (D) Further compression above the transition surface pressure forces the PMMA segment in a more compact conformation by removing the free area of microvoids in the PMMA corona.
microvoids in the PMMA segments (L2 region in Figure 1).9 However, it should be emphasized again that PS-PMMA molecules stay on the water surface because the atactic and syndiotactic PMMA segments are in thermodynamic environments that are worse than the “Θ” condition.11,12 Some isotherms at low subphase temperatures do not fall between those of PMMA and PS due to incompatible interaction between PS and PMMA segments. Several features are of interest in the isotherms of Figure 1. First, the onset area per molecule for surface micelle formation, which was determined by the extrapolation of the slope in the L1 region to 0 mN/m, decreases with increasing temperature. This agrees with our previous finding that PS-PMMA molecules move more freely on the water surface as the temperature increases and hence more easily form surface micelles.9 Second, the plateau in the T1 region corresponding to the LE-LC transition is more clearly defined at higher temperature. The transition process becomes more evident if we plot the static elastic modulus [s ) -dπ/d(ln A)] versus the surface area per repeating unit (Figure 3).14,15 When the subphase temperature was 40 °C, two clear maxima appeared along with two local minima. At other subphase temperatures, the minimum at larger surface area is less evident. This minimum is characteristic of a phase transition.15 When the PS-PMMA chains form the surface micelles, the surface-inactive PS blocks of several molecules form the aggregate cores whereas the surface-active PMMA segments form the outer corona of the surface micelle. The monolayer includes less free area between micelles at low temperature compared to high temperature (Figure 1).9 Rearrangement of the micelle skirts (PMMA segments) occurs (15) Joncheray, T. J.; Bernard, S. A.; Matmour, R.; Lepoittevin, B.; El-Khouri, R. J.; Taton, D.; Gnanou, Y.; Duran, R. Langmuir 2007, 23, 2531.
Figure 3. Static elastic modulus versus the surface area per repeating unit at 40 °C.
more easily at high temperature because of the lower chain modulus at high temperature and the existence of a larger free area. As a result, the plateau appearance corresponding to the LE-LC transition becomes more pronounced at high temperature and the transition proceeds at lower surface pressure. The minimum of s at high surface area (Figure 3) corresponds to this transition. This behavior differs from that observed for polystyreneb-poly(ethylene oxide) (PS-PEO) diblock copolymer.14,16 Specifically, the plateau surface pressure increased with temperature for PS-PEO diblock copolymers because high temperature induced greater adsorption of PEO segments at the interface and better solubility.16
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Figure 4. π-A isotherms obtained by expanding monolayers of PS-PMMA diblock copolymer in a compression and expansion cycle at different temperatures. For clarity, only a compression isotherm at 40 °C is displayed.
Another point worthy of note is that, when the monolayer is compressed and then expanded through the region T2, the isotherm shows a hysteresis (Figure 4). When the PMMA coronas are compressed above the T2 transition surface pressure, they are in the fully compressed solid film state. For the PS-PMMA diblock copolymer, the slope (dπ/dA) decreases with temperature in region L1 but increases in L2. As the temperature increases, the slope after the plateau region increases (L2 region) because a more compact film is formed in the plateau region than at lower temperature. The monolayer then shows a more condensed film like behavior.10 Hence, the hysteresis decreases with increasing temperature. Once the pressure is relieved, they disassemble into separate micelles (as verified below by AFM, further coalescence between micelles seldom occurs for this block copolymer).10,11 Micelle overlapping was not observed, consistent with our previous findings for this copolymer.9 The expansion isotherm follows the same track until the minimum point is reached, irrespective of the subphase temperature. When the external constraints are relieved, the monolayer of micelles rapidly expands. By the same token, the minimum (dip) in the expansion process due to the overexpansion16 (relaxation of the long chain conformation), thus, appears more clearly at higher temperatures. Overexpansion allows the plateau in the expansion process to proceed at a lower surface pressure than that in the compression process. In the plateau region, the transition surface pressure increases slowly with increasing temperature up to 30 °C. Then at 40 °C, it shows a large increase comparable to that in the compression process. Although the applied temperature is not close to the glass transition temperature of PMMA thin films, higher temperature provides more thermal energy to the chain molecules and softens them. PMMA thin films have a glass transition temperature of 60 °C for isotactic PMMA and 110 °C for syndiotactic PMMA and have a local segment transition of 70 °C for syndiotactic PMMA17 (the PMMA segment of the diblock copolymer is composed of almost half syndiotactic and half atactic portions). Figure 5 shows a plot of (16) (a) Goncu¨alves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G Langmuir 1996, 12, 6547. (b) Goncu¨alves da Silva, A. M.; Guerreiro, J. G.; Rodrigues, N. G.; Rodrigues, T. O. Langmuir 1996, 12, 4442. (c) Goncu¨alves da Silva, A. M.; Simoes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327. (d) Bertelho do Rego, A. M.; Pellegrino, O.; Martinho, J. M. G.; Lopes da Silva, J. Langmuir 2000, 16, 2385. (17) Shin, H.; Lee, H.; Jun, C.; Jung, Y. M.; Kim, S. B. Vib. Spectrosc. 2005, 37, 69.
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Figure 5. Transition pressure versus subphase temperature. Table 1. LE-LC Transition Parameters of PS-PMMA Diblock Copolymer20 ∆H ∆S (cal mol-1 of (cal K-1 mol-1 of T πt ∆At (°C) (mN/m) (Å2/segment) PMMA segment) PMMA segment) 10 15 23 30 40
18.2 16.9 15.9 13.8 12.3
-0.66 -0.82 -0.97 -1.66 -2.67
0.04 0.05 0.06 0.09 0.15
10.7 13.4 16.6 20.1 47.6
the transition surface pressure versus subphase temperature; the two variables show an almost linear relationship. From the plot of the transition pressure versus temperature as shown in Figure 5, we can predict that the surface pressure goes to 0 at a temperature of 93 °C, which is coincidentally close to the glass transition temperature of a syndiotactic PMMA thin film.17 If we take into consideration the fact that the glass transition temperature in the ultrathin film should be lower than that of the bulk polymer because of the sorbed water molecules in the thin film, this is quite a remarkable coincidence.18 Another notable feature is that, after the first expansion, the surface pressure at 40 °C does not fully return to zero even after a very long time. This implies that, at high temperature, the micelle structure requires an extremely long time to return to the fully relaxed state. Lennox et al.19 calculated the enthalpy (∆H) and entropy (∆S) changes at the LE-LC transition using the corrected form of the two-dimensional Clausius-Clapeyron equation, ∆H ) T∆At(∂πt/∂T - ∂γa/∂T) ) T∆S, where ∂γa/∂T is the rate of change of the pure water surface tension with temperature (-0.14 mN m-1 K-1), ∂πt/∂T is the transition surface pressure change with temperature, and ∆At is the area change during the transition. ∂πt/∂T is obtained from the plot of the transition surface pressure versus subphase temperature as shown in Figure 5. The calculated ∆H and ∆S values are summarized in Table 1. Since ∂πt/∂T < 0 and ∆At < 0, ∆S > 0. ∆At increases with increasing temperature, and so does ∆S, which implies that, as the temperature is increased, a condensed monolayer of surface micelles with a more disrupted structure is formed. As the water temperature increases, the interaction between the water molecule and the PMMA segments (18) (a) Morita, H.; Tanaka, K.; Kajiyama, T.; Nishi, T.; Doi, M. Macromolecules 2006, 39, 6233. (b) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000, 33, 7588. (c) Kajiyama, T.; Tanaka, K.; Takahara, A. Polymer 1998, 39, 4665. (d)Kajiyama, T.; Tanaka, K.; Satomi, N.; Takahara, A. Sci. Technol. AdV. Mater. 2000, 1, 31. (19) Zhu, J.; Lennox, R. B. J.; Eisenberg, A. Langmuir 1991, 7, 11579. (20) Lee, J. S. M.S. Thesis, Ajou University, Suwon, Korea, 2001.
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Figure 6. AFM images of the PS-PMMA monolayers at different subphase temperatures (π ) 0 mN/m).
Figure 7. AFM images of the PS-PMMA monolayers at different surface pressures (T ) 40 °C).
becomes weak. Moreover, at higher temperatures, the block copolymer molecules on the water move more freely due to their higher thermal energy and the chain modulus is also reduced. Thus, chain self-aggregation and micelle formation at higher temperature occur at a lower surface pressure, and condensation at LE-LC transition also occurs at a lower surface area (Figure 1). This process continues until a uniform micelle film is formed. Because of the presence of a large free area at high temperature, the surface area change becomes larger. This results in a large entropy change during the transition process at higher temperatures. However, the absolute values of ∆S in transition are small and similar (∆S ) 0.04-0.15 kcal K-1 mol-1 of PMMA segment) to transition ∆S values of PS-b-PtBMA [polystyrene-b-poly-
(tert-butyl methacrylate)].4,16 This implies that both monolayers show transitions structurally similar to each other. Lennox et al.19 pointed out the small ∆S value for the nonionic system implicates that the final state has a degree of order similar to that of the as-cast state and that a substantial chain reorientation process (a transition from a two-dimensional to a threedimensional state) is not taking place during the micelle formation. The transition would require a substantial increase in entropy. For example, for the PS-b-PVP C10I (poly(styrene-b-decylated vinylpyridine iodide)) system, the ∆S value was 7.6 (kcal K-1 mol-1)/mol of VP residue at 25 °C).4 These understandings can be further corroborated by the morphological observation. When a diblock copolymer contains
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a block that is soluble in the subphase (water) or interacts with the subphase, various morphologies can develop during the transition process.4,5 However, when both blocks are insoluble in water, the block copolymer molecules remain on the water surface and the morphology development during the transition is just a dynamic process whose characteristics are determined by the molecular weight and relative block length ratio of the copolymer.10 When the solution was spread on the water surface at low temperature, few aggregates were formed because of the long PMMA block (Figure 6). After surface pressure is applied, spherical shapes corresponding to aggregates of PS moieties appear. Although there is a subtle change in the development of the surface film morphology as the subphase temperature is increased, this temperature effect is insufficient to completely change the morphology. This is obvious in the AFM photos in Figure 7. Depending on the subphase temperature, more aggregates were formed at low surface pressure. As the temperature is increased, however, conformation changes in the PMMA segments become easier, allowing aggregation of the block copolymer molecules with higher entropy even at zero compression. We did not observe further aggregation of the surface micelles into other surface morphologies such as rodlike or large planar morphologies.9,13 In two-dimensional space, PMMA blocks in different molecules (or micelles) do not penetrate each other.12 PMMA coronas prevent the core PS moieties from merging into larger agglomerates with different morphologies. Even at high surface pressure and at high temperature, the size of the PS core does not grow noticeably. Thus, the aggregates are quite stable to further aggregation, indicating that the entropy change is not affected by the nature of the hydrophobic block. AFM micrographs can be used to determine the aggregation number of surface micelles. Using the total area method,7,10 the aggregation number after the plateau transition surface pressure was determined to be about 7 molecules/micelle, which is in agreement with our previous results.9 This number does not change with the temperature, indicating that the temperature of the subphase affects
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the kinetics of micelle formation but has little effect on the thermodynamics of micelle formation. Therefore, we can conclude that, for diblock copolymers in which both blocks are insoluble in water, micelle morphology is decided by the inherent parameters of the molecules (absolute molar mass and the relative block length ratio of the block copolymer).
Summary We investigated the effect of temperature on the twodimensional self-assembly of the nonionic diblock copolymer PS-b-PMMA (surface micelles) at the air/water interface using the LB trough technique and AFM. Easy formation of surface micelles and an early transition into micellar aggregates were observed at high temperature, and the π-A curves showed more condensed film like behavior. Thermodynamic factors in a twodimensional film show that the entropy increases with increasing temperature. A positive entropy implies that the transition process receives the energy from the subphase, and surface micelles with a more disrupted structure are formed. A high subphase temperature facilitates the transition of micelle aggregates. Our rough estimate of the film transition temperature was close to the transition temperature reported by others for PMMA thin films. Finally, we conclude that thermal energy provided by the water subphase affects the formation of surface micelles and the monolayer dynamic behavior of the monolayer, but does not change the micelle morphology unless the block copolymer contains a block that strongly interacts with water (or is soluble in water) or the molar mass is quite low. Acknowledgment. We thank Professor Jaeho Kim and Mr. Jong Suk Lee of Ajou University for their help with the experiments. This work was supported by the Korea Research Foundation (Grant No. 0417-20060134, Y.S.), the SRC/ERC program of MOST/KOSEF (Grant R11-2006-065, Y.S.), KOSEF (Grant R01-2006-000-10062-0, H.J.C.), and the 21C Frontier Recycling Program of MOCIE (Grant 0417-20050058, S.M.H.). LA702745W
