Langmuir 2006, 22, 6587-6592
6587
Molecular Reorganization of Rod-Coil Diblock Copolymers at the Air-Water Interface Jie Zhang, Hongqing Cao, Xinhua Wan,* and Qifeng Zhou Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed March 29, 2006. In Final Form: May 9, 2006 Amphiphilic rod-coil diblock copolymers consisting of flexible poly(ethylene oxide) (PEO) and rodlike poly{(+)2,5-bis[4′-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS) with predominant hydrophobic contents formed ordered monolayers at the air-water interface. The structures of monolayers transferred to a mica substrate at different surface pressures by the Langmuir-Blodgett method were investigated by atom force microscopy (AFM). For PEO104-bPMBPS17 copolymer (the subscripts denote the number-averaged polymerization degree of each block), a complete spectrum of molecular reorganization at variable surface pressures was observed. Spherical surface aggregates of LB monolayers were spontaneously formed during the solvent evaporation after the deposition of polymer solution. Continuous compression led to the pancake-to-brush conformation transition of the copolymer that PEO chains desorbed from the air-water interface and went into the water subphase. The effective content of the rod block changed continuously from 61% at zero pressure to 93% at the start of the monolayer collapse, igniting the molecular reorganization. As a result, coalescence of individual spherical aggregates into long cylindrical aggregates with an increase of the surface pressure was observed. For a series of block copolymers PEO104-b-PMBPSm (m ) 17, 30, 45, 53), as the rod contents increased from 61% to 83%, the morphological transition from spherical aggregates to long cylindrical aggregates in orientational order developed at zero pressure, which showed a similar dependence on the effective contents of the rod block to PEO104-b-PMBPS17 at different pressures. In comparison to coil-coil block copolymers PEO-b-PS, the rod-coil block copolymers PEO-b-PMBPS exhibited distinct structure reorganization behavior, in which the orientation of rod block might play an important role.
Introduction One of the most fascinating properties of block copolymers is their ability to self-assemble into ordered nanostructures not only in selective solvent and melts1-4 but also at the interface and surface.5-9 Self-assembly of amphiphilic or surface-adsorbing block copolymers at the air-water interface can form twodimensional monolayers on the nanometer scale order. The monolayer films at the interface can be transferred to a solid substrate using the Langmuir-Blodgett (LB) transfer technique, which provides prospective application in electrooptical materials,10 nanopatterned substrates for microlithography,11 and separation membranes.12 A variety of studies on LB monolayer of amphiphilic block copolymers give rise to two-dimensional surface aggregates that range from small circular objects to cylindrical structures and * Corresponding author. Tel: 86-10-62754187. Fax: 86-10-62754187. E-mail:
[email protected]. (1) Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; John Wiley & Sons: Hoboken, NJ, 2003. (2) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J. Chem. 1999, 77, 1311. (3) 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. Liu, G. J.; Qiao, L. J.; Guo, A. Macromolecules 1996, 29, 5508. (4) Ruokolainen, J.; Makinen, R.; Torkkeli, M.; MaKela, T.; Serimaa, R.; Brinke, G. T.; Ikkala, O. Science 1998, 280, 557. Li, C. Y.; Tenneti, K. K.; Zhang, D.; Zhang, H. L.; Wan, X. H.; Chen, E. Q.; Zhou, Q. F.; Carlos, A. O.; Igos, S.; Hsiao, B. S. Macromolecules 2004, 37, 2854. (5) Seo, Y.; Im, J. H.; Lee, J. S.; Kim, J. H. Macromolecules 2001, 34, 4842. (6) Zhu, J. Y.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (7) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 1998, 120, 423. (8) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 5464. (9) Li, H. B.; Liu, Q. T.; Qin, L. D.; Xu, M.; Lin, X. K.; Yin, S. Y.; Wu, L. X.; Su, Z. M.; Shen, J. C. J. Colloid Interface Sci. 2005, 289, 488.
to large planar aggregates.13,14 The polymorphism depends not solely on the absolute size of each block but rather relies on a more subtle balance between the relative sizes of the two blocks in a fashion more complicated than that observed for block copolymers in solution and in bulk. The ratio of block sizes of copolymers at the air-water interface will change continuously if the water-soluble flexible blocks such as poly(ethylene oxide) (PEO) transform from a pancakelike to a brushlike conformation and submerge into water during compression.15-19 Therefore, variable effective compositions may lead to the surface aggregation or the morphological transition of aggregates even though the chemical compositions remain unchanged.20,21 Significant advances in the field of polymer chemistry enable the synthesis of nontraditional block copolymers with complex architectures, such as rod-coil block copolymers,22-24 star block (10) Li, F. Y.; Huang, C. H.; Jin, L. P.; Wu, D. G.; Zhao, X. S. J. Mater. Chem. 2001, 11, 3002. (11) Remmers, M.; Neher, D.; Wegner, G. Macromol. Chem. Phys. 1997, 198, 2551. (12) Maximychev, A. V.; Matyukhin, V. D.; Stepina, N. D.; Yanusova, L. G. Thin Solid Films 1996, 285, 866. (13) Zhu, J. Y.; Eisenberg, A.; Lennox, R. B. J. Phys. Chem. 1992, 96, 4727. (14) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. (15) Zhu, J. Y.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (16) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 8735. (17) Kim, Y.; Pyun, J.; Fre´chet, J. M. J.; Hawker, C. J.; Frank, C. W. Langmuir 2005, 21, 10444. (18) Goncu¨alves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547. (19) Xu, Z.; Holland, N. B.; Marchant, R. E. Langmuir 2001, 17, 377. (20) Tsukruk, V. V.; Genson, K. L.; Peleshanko, S.; Markutsya, S.; Lee, M.; Yoo, Y. S. Langmuir 2003, 19, 495. (21) Kampf, J. P.; Frank, C. W.; Malmstrom, E. E.; Hawker, C. J. Langmuir 1999, 15, 227. (22) Holzmueller, J.; Genson, K. L.; Park, Y.; Yoo, Y. S.; Park, M. H.; Lee, M.; Tsukruk, V. V. Langmuir 2005, 21, 6392.
10.1021/la060844h CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006
6588 Langmuir, Vol. 22, No. 15, 2006 Scheme 1. Chemical Structure of Block Copolymers PEOn-b-PMBPSm
copolymers,25,26 dendritic polymers,27,28 and Janus micelles.29 This allows access to study the role of architectures systematically and explore the underlying relationship between the threedimensional structure and the self-assembly process. Among the most studied polymers, a lot of attention has been paid to rodcoil block copolymers consisting of both flexible and rigid components, which exhibit distinct self-assembling abilities beyond their coil-coil counterparts.30 The orientation order of the rod block induced by two-dimensional geometrical restrictions plays an important role in supramolecular structures at the airwater interface.23-24,31 Herein, this work focuses on the investigation of amphiphilic rod-coil block copolymers containing a flexible PEO segment and rigid poly{(+)-2,5-bis[4′-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS) segment with different molecular weights (Scheme 1). For PMBPS homopolymers, the bulky terphenyl group laterally attached to the backbones imparts significant steric hindrance and enhances the stiffness of polymer backbones to take an extended conformation.32 We report the interfacial behavior of PEO-b-PMBPS at the air-water interface and their surface morphologies of the LB monolayer film. Block copolymers used in this study have varied contents of predominant hydrophobic PMBPS block from 61% to 83% with the same length of PEO block, which plays the role of a hydrophilic anchor. It is conceivable that crowding of PMBPS rods at the air-water interface will lead to the orientation of rods, which will affect the microstructure of the LB monolayer film. Moreover, it is expected that the variable effective compositions induced by the conformation change of PEO will tend toward structure reorganization and lead to interesting surface aggregates. Experimental Section Synthesis of Diblock Copolymers. The synthesis of diblock copolymers PEO-b-PMBPS by atom transfer radical polymerization (ATRP) has been described in a recent communication.33 The macroinitiator, ω-methoxypoly(ethylene glycol) 2-bromoisobutyrate (PEO-Br), was obtained by esterification of the terminal hydroxy group of commercially available poly(ethylene glycol) monomethyl ether with 2-bromoisobutyryl bromide. The monomer, (+)-2,5-bis[4′-((S)-2-methylbutoxy)phenyl]styrene (MBPS), was prepared as (23) Park, Y.; Choi, Y. W.; Park, S.; Cho, C. S.; Fasolka, M. J.; Fasolka, D. S. J. Colloid Interface Sci. 2005, 283, 322. (24) Sommerdijk, N. A. J. M.; Holder, S. J.; Hiorns, R. C.; Jones, R. G.; Nolte, R. J. M. Macromolecules 2000, 33, 8289. (25) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004, 37, 6511. (26) Peleshanko, S.; Gunawidjaja, R.; Jeong, J.; Shevchenko, V. V.; Tsukruk, V. V. Langmuir 2004, 20, 9423. (27) Cardullo, F.; Diederich, F.; Echegoyen, L.; Habicher, T.; Jayaraman, N.; Leblanc, R. M.; Stoddart, J. F.; Wang, S. P. Langmuir 1998, 14, 1955. (28) Karthaus, O.; Ijiro, K.; Shimomura, M. Langmuir 1996, 12, 6714. (29) Xu, H.; Erhardt, R.; Abetz, V.; Mu¨ller, A. H. E.; Goedel, W. A. Langmuir 2001, 17, 6787. (30) Lee, M.; Cho, B. K.; Zin, W. C. Chem. ReV. 2001, 101, 3869. (31) Cho, C. S.; Kobayashi, A.; Goto, M.; Akaike, T. Thin Solid Films 1995, 264, 82. (32) Yu, Z. N.; Wan, X. H.; Zhang, H. L.; Chen, X. F.; Zhou, Q. F. Chem. Commun. 2003, 8, 974. (33) Zhang, J.; Yu, Z. N.; Wan, X. H.; Chen, X. F.; Zhou, Q. F. Macromol. Rapid Commun. 2005, 26, 1241.
Zhang et al. Table 1. Characteristics of Block Copolymers PEOn-b-PMBPSm Mna PMBPS (g mol-1) Mw/Mnb content (wt %)
abbreviatn
copolymersa
P-17 P-30 P-45 P-53
PEO104-b-PMBPS17 PEO104-b-PMBPS30 PEO104-b-PMBPS45 PEO104-b-PMBPS53
11 900 17 400 23 800 27 300
1.09 1.11 1.13 1.12
61 73 81 83
a The degrees of polymerization of PEO and PMBPS segments and the molecular weights are obtained by 1H NMR spectra. b Determined by GPC in THF with polystyrene as standards.
previously reported.32 The copolymerization was carried out by the initiating system of the macroinitiator PEO-Br/CuBr/N,N,N′,N′,N′′pentamethyldiethylenetriamine (PMDETA) ([PEO-Br]/[CuBr]/ [PMDETA] ) 1:1:1) at 90 °C. The reaction solution was then filtered through alumina for the removal of copper(II). The crude products were further purified by repeatedly dissolving in THF and precipitating in methanol several times. Finally, the white products were dried under vacuum. Their molecular weight distributions were characterized by gel permeation chromatography (GPC) performed in tetrahydrofuran (THF) on a Waters 515 chromatograph equipped with a set of Waters Styragel columns HT2+HT3+HT4 and Waters 2410 refractive index detector. Linear polystyrene was used as the calibration standards. Each of diblock copolymers has a relatively narrow polydispersity lower than 1.2. The compositions of the copolymers were estimated from 1H nuclear magnetic resonance spectra obtained on a Bruker ARX 400 spectrometer with tetramethylsilane as the internal standard and CDCl3 as the solvent at room temperature. The diblock copolymers have the same flexible chains of 104 ethylene oxide units and the variable length of the rigid block of 17, 30, 45, and 53 MBPS units (abbreviated as P-17, P-30, P-45, and P-53). Characteristic data for the copolymers are listed in Table 1. Film Balance Measurements. Solutions of the copolymers were prepared in HPLC grade chloroform with a concentration of 0.20.4 mg/mL. A 30 cm × 10 cm Nima 610 Langmuir-Blodgett trough (Nima, Coventry, U.K.) was used for the isotherm measurements and LB film transfers. The surface pressures were measured using the Wilhelmy plate method with a filter paper plate. The subphase was deionized water purified with a Milli-Q system to 18.2 MΩ/cm resistivity. The water temperature was maintained at 30.0 °C. In a typical experiment, 50-100 µL of the polymer solution was spread in small drops on the water surface using a Hamilton microsyringe. After a 15-20 min waiting period of solvent evaporation, the barrier was compressed at a constant rate of 15 cm2/min, and the pressurearea (π-A) isotherm was recorded. Langmuir-Blodgett Film Transfer. For LB film transfer, a 1 cm × 1.5 cm freshly cleaved mica was first immersed in the subphase before the polymer solution was spread. After the desired surface pressure was reached, followed by a 15 min wait at such pressure, the surface monolayer was transferred onto the substrate by the vertical dipping method at a constant speed of 3 mm/min. The transfer ratios of monolayer deposition were calculated by the ratio of areas of monolayer removed from subphase at constant pressure to the areas of substrate immersed in water. The transfer ratio of each sample in our experiments was about 1.0 ( 0.1. The LB films were dried overnight and the atom force microscopy (AFM) images were acquired with a NanoScope IIIa multimode AFM in tapping mode (Digital Instruments) with a microfabricated silicon cantilever (thickness ) 1.7-2.3 µm, width ) 60 µm, length ) 90 µm, tip height ) 15-20 µm, resonance frequency ) 260-420 kHz, and spring constant ) 23-91 N/m, MikroMasch). The alignment factor (F) of cylinders, based on the analysis of AFM images with the size of 1 × 1 µm2, was evaluated by the equation of F ) (3〈cos2θ〉 1)/2. (θ represents the angle between the axis direction of cylinders and the alignment direction parallel to the barrier.)
Results Interfacial Behavior at the Air-Water Interface. Typical pressure-area isotherms of PEO and PMBPS homopolymers at
Rod-Coil Diblock Copolymers
Figure 1. π-A curves of PEO and PMBPS homopolymers. The dotted line shows the limiting area of the MBPS monomer.
the air-water interface are represented in Figure 1. For the PEO homopolymer with number-averaged molecular weight (Mn) of 5000 g/mol, a plateau at a surface pressure of nearly 7 mN/m could be observed in the isotherm, which is associated with the dissolution of the PEO chain into the water subphase. There is a conformational transition, through which the PEO segments adopting a flattened conformation in contact with water surfaces a pancake structuresleave the surface and go into the water subphase, forming a brush conformation.34 The result coincides with the earlier observations which show molecular weight effect of PEO on transition pressure. Under similar conditions, Marchant et al. found a plateau at 5.1 mN/m for PEO with Mn of 2000 g/mol and at 10.7 mN/m for PEO with Mn of 400 000 g/mol.19 Homopolymer PMBPS does not spread to be a monolayer on the water surface due to the lack of the hydrophilic group. The increase in surface pressure measured by the film balance is due to the crowding of PMBPS rods upon compression, alike to hydrophobic polystyrene.35 Therefore, the π-A curve of PMBPS is an apparent isotherm. Only the condensed phase is observed for PMBPS. The limiting surface area/MBPS monomer, calculated by the extrapolation of the steep rise in the surface pressure to a zero level, is 0.36 nm2. For diblock copolymer P-17, the isotherm has a very slow increase in surface pressures at large molecular areas which extend from about 37 nm2/molecule to 10 nm2/molecule, followed by a steep increase in surface pressures in the small molecular area regime (Figure 2). Since PMBPS shows no surface active features, the slow rise in the π-A curves of P-17 is attributed to the pancake-to-brush conformation transition of PEO chains, which is similar to the pseudoplateau previously observed for block copolymer PS-b-PEO with relatively short PEO chains.18 The continuous variation of surface pressures with the molecular areas at the phase transition in the present work is considered to be a diffuse first-order transition. The isotherm of P-30 shows the same feature as that of P-17 except shorter slow increase extension, because the area occupied by the pancake at the onset of the transition is dependent on not only the length of PEO chains but also their contents in block copolymers. As the lengths of the PMBPS block increase further, no feature of isotherms can been seen due to decreasing contents of hydrophilic PEO block less than 20 wt %, and the curves are alike to the homo-PMBPS. All monolayer films are unstable and collapse at a relatively low pressure of near 25 mN/m. All pressure-area isotherms for PEO-b-PMBPS are consistently shifted to higher surface areas with increasing length (34) Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 786. (35) Kumaki, J. Macromolecules 1988, 21, 749.
Langmuir, Vol. 22, No. 15, 2006 6589
Figure 2. π-A curves of block copolymers PEO-b-PMBPS. The inset shows the relation of the limiting areas of block copolymer to NMBPS.
of the PMBPS block. The limiting surface area/molecule increases with the molecular weight of the PMBPS block. The plot of the limiting areas of block copolymers against the number-average polymerization degree of PMBPS (NMBPS) exhibits a linear relation, by which the area/monomeric unit is estimated to be 0.35 nm2. It is slightly smaller than the limiting area 0.36 nm2 calculated from the homo-PMBPS. Surface Aggregates in the LB Monolayer. To gain additional insight into the surface behavior of PEO-b-PMBPS, we analyzed surface structures of LB monolayer films transferred on the mica substrate. As shown in Figure 3, AFM images of the LB films for block copolymer P-17 exhibit distinct interfacial morphologies at different pressures. At zero surface pressure, i.e., at the onset of the deposition of polymer solution, small spherical aggregates of about 26 ( 5 nm with relatively narrow size distribution were observed. The height of aggregates is about 1.6 ( 0.4 nm. As the monolayer was compressed to low pressure of nearly 1 mN/ m, the distance between neighboring spherical aggregates decreased and the average diameter and the height of the spherical domains stays relatively constant. They started to pack more closely and cohered into cylindrical aggregates at a higher pressure of nearly 2 mN/m. P-17 exhibited a cylindrical structure coexisting with spherical aggregates at pressures more than 3 mN/m. The cylinders have the diameters of 22 ( 4 nm and the length less than 200 nm. The height of cylinders is similar to that of spheres. This confirms a flat arrangement of PMBPS rod under these conditions. The diameters of cylinders are slightly smaller than that of spheres. The length of cylindrical aggregates increased with the surface pressure. At the same time, the amount of spherical aggregates decreased and the cylindrical structure dominated the LB monolayer film. The cylinders showed a random orientation below the short transition region, but signs of ordering were observed at higher surface pressures about 7 mN/m, where the cylinders aligned themselves parallel to the barrier and perpendicular to the compression and transfer directions. The alignment factor of cylinders was calculated to be about 0.4. When the pressure was as high as 10 mN/m, the space between cylinders almost vanished, and also the monolayer appeared to start to collapse. The block polymer P-30 monolayer deposited on the substrates at various surface pressures also possessed a variety of molecular reorganizations similar to those of P-17. While short cylinders formed as well as spherical aggregates at the onset of the solution spreading, spheres developed to long cylinders as the pressure increased to 3 mN/m. P-45 and P-53 initially formed long cylinders in an orientational order and finally collapsed at the very low pressure of nearly 2 mN/m. For all rod-coil block copolymers
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Figure 3. Structures of the surface aggregates obtained from P-17 in different deposition pressures: (A) 0 mN/m; (B) 1 mN/m; (C) 2 mN/m; (D) 3 mN/m; (E) 4 mN/m; (F) 5 mN/m; (G) 6 mN/m; (H) 7 mN/m; (I) 10 mN/m. The sizes of all images are 1 × 1 µm2. All height scales are 10 nm.
under different conditions, the height of aggregates keeps constant to 1.6 ( 0.4 nm, implying the face-on conformation of the PMBPS rod at the interface. Rich polymorphic transitions from block copolymers with an increase of the PMBPS block length were also observed. Figure 4 shows AFM images of aggregates from block copolymers at zero pressure. While P-17 predominantly formed spheres, P-30 containing relatively a higher PMBPS block length showed the mixture of spheres and cylinders, and P-53 and P-45 exhibited long cylinders with the in-plane orientation locally. Characteristics of surface aggregates are tabulated in Table 2. The diameters of cylinders from P-17 and P-30 are smaller than those of spheres, corresponding to a decrease of PEO corona around the PMBPS core on water surface during compression. The diameters of the cylindrical aggregates also grow with the lengths of the PMBPS block.
Discussion π-A Isotherm. The interpretation of π-A isotherms of rodcoil block copolymers PEO-b-PMBPS in the former section emphasized the individual molecules. In fact aggregates were formed in the monolayers at different conditions during compression, so interpretation of the isotherms must include the surface aggregates. First of all, the correct mechanism of the surface aggregation must be known. Three different mechanisms
of formation of block copolymer surface aggregates at the airwater interface have been proposed: deposition of micelles in solution; compression-induced surface aggregation; spontaneous surface aggregation at the onset of deposition of polymer solution.36 Since the spreading solution of block copolymers in chloroform at the concentration used shows no aggregates confirmed by dynamic light scattering measurements, we can preclude the possibility of deposition of micelles in solution. Surface aggregates are observed at zero surface pressure. Considering that our block copolymers are not at their thermodynamic equilibrium because of the high Tg of the hydrophobic PMBPS block, surface aggregation is most likely to be a spontaneous process, which occurs upon deposition of the block copolymer solution during the evaporation of the solvent. So spontaneous surface aggregation is the probable source of surface aggregation of PEO-b-PMBPS systems. It follows that upon increasing surface pressures the morphological transition for block copolymers occurs. Therefore, the compression-induced aggregation is another mechanism of surface aggregation. The surface aggregation of PEO-b-PMBPS at the air-water interface can be described as the following: Upon deposition of the polymer solution at the air-water interface, the block copolymers spontaneously self-assemble into surface aggregates (36) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714.
Rod-Coil Diblock Copolymers
Langmuir, Vol. 22, No. 15, 2006 6591 Table 3. Effective Composition Dependence of Morphologies of Surface Aggregates Formed from P-17 at Variable Surface Pressures surf compressn press Aπa APEO,πb ratio of WPMBPS’% (mN/m) (nm2) (nm2) PEO (%)c (wt %)d 0 1 2 3 4 5 6 7
35 28 24 21 19 16 14 10
28 21 17 14 12 9 7 3
100 75 61 50 43 32 25 11
61 68 72 76 78 83 86 93
morphology sphere sphere sphere + cylinder sphere + cylinder cylinder + sphere cylinder cylinder cylinder
a Aπ represents instantaneous area/P-17 molecule at different surface pressures. b APEO,π represents area of each PEO chain at different pressures, APEO,π ) Aπ - APMBPS (APMBPS is assumed to keep constant during compression and be equal to the limiting area/P-17 molecule, APMBPS ) 7 nm2). c The compression ratio of PEO chains shows the ratio of area of each PEO chain at a certain pressure (APEO,π) to that at zero pressure (APEO,0). d Effective PMBPS content WPMBPS’% ) 1/(APEO,π/ APEO,0 × WPEO%/WPMBPS% + 1).
Figure 4. Monolayers deposited at zero pressure from block copolymers with different PMBPS block lengths: (A) P-17 (1 × 1 µm2); (B) P-30 (1 × 1 µm2); (C) P-45 (2 × 2 µm2); (D) P-53 (2 × 2 µm2). All height scales are 10 nm. Table 2. Characteristics of Structures of LB Monolayer Films of PEO-b-PMBPS as a Function of Surface Pressures copolymers
π (mN/m)
P-17
0 1 2 4 6 0 2 4 0 2 0
P-30 P-45 P-53
morphology
area/ molecule (nm2)
av diameter of aggregates (nm)
sphere sphere sphere + cylinder cylinder + sphere cylinder cylinder + sphere cylinder cylinder cylinder cylinder cylinder
37 29 24 19 14 36 24 18 28 21 29
26 ( 5 26 ( 5 26 ( 5, 22 ( 4 22 ( 4, 26 ( 5 22 ( 4 27 ( 4, 34 ( 4 27 ( 4 27 ( 4 46 ( 6 46 ( 6 58 ( 7
during the solvent evaporation. For block copolymers P-17 and P-30, the isotherms of which show the pseudoplateau region, spherical aggregates adopt the “starfish” conformation with the PEO corona absorbed at the interface.36 Upon compression, the stretched PEO corona chains of surface aggregates begin to interact with each other in their pancake conformation, which gives rise to an increase of surface pressures. The pseudoplateau region in the isotherms relates to the dissolution of the PEO corona into water subphase. The aggregates transform from “starfish” to “jellyfish” conformation, and at the same time the morphology changes from spheres to cylinders.36 After the pseudoplateau, the surface pressures increase fast when the hydrophobic PMBPS cores start to experience the repulsive interaction with the neighbors. For block copolymers P-45 and P-53 which form cylindrical aggregates spontaneously at zero pressure, the isotherms exhibit no pseudoplateau. It is assumed that when the hydrophobic PMBPS block dominates at the airwater interface, the PEO chains go underneath the hydrophobic core rather than radially extended around the core. Therefore, it could be concluded that the pseudoplateau region in the π-A isotherm originates from the conformation change of PEO corona chains of spherical aggregates for our block copolymer system. LB Monolayer Film. Rod-coil block copolymers PEO-bPMBPS form ordered monolayers at the air-water interface.
Although the aggregates we observe with the LB transferred films of PEO-b-PMBPS block copolymer are two-dimensional surface aggregates rather than the three-dimensional aggregates structures in bulk or in aqueous solution, the driving force behind the aggregation behavior should be the same. In this work, the repulsion of the PMBPS block against both water and the PEO block and the tendency of the orientation of PMBPS induced by two-dimensional geometrical restrictions drives the PMBPS blocks to aggregate. The morphologies of surface aggregates are dependent on the size of two blocks. As the content of PMBPS increases from 61% to 83%, the structure of aggregates formed at zero pressure changes continuously from spheres to mixture of sphere and short cylinders and to long cylinders (Figure 4). The similar morphological transition could also be realized from block copolymer P-17 upon variation of surface pressures. This kind of transition is assumed to be associated with the molecular reorganization. During the solvent evaporation period when the block copolymers are allowed to rearrange themselves at the air-water interface, spherical micelles form with the orientationally packed PMBPS rod in the core and the extended PEO corona around the core. Continuous compression leads to dissolution of flexible PEO chains into water. As a result, the effective composition of the rod blocks at the interface is changing continuously. It is supposed that the area decrease during compression arose only from the PEO area change and in the wake of monolayer collapse all PEO chains completely submerged into water. That is, the area of the PMBPS block (APMBPS) approximates the limiting area of P-17 and keeps constant during compression. Therefore, the area of PEO at a certain pressure (APEO,π) can be calculated according to the pressure-area isotherm. The compression ratio of PEO chains is considered as the ratio of APEO,π to the area at the start of pressure elevation (APEO,0). Finally, the effective contents of PMBPS (WPMBPS’%) could be estimated as WPMBPS’% ) 1/(APEO,π/APEO,0 × WPEO%/ WPMBPS% + 1). The calculated data are shown in Table 3. As the effective content of PMBPS increases to about 72%, spheres tend to transfer to cylinders, which allow more rods packing into the core with the in-plane orientation. The enthalpy gain achieved by this reorganization balances the loss of entropy caused by compressed and folded PEO tails. At the effective content of PMBPS about 83%, cylinders are the dominant structure. This result coincides well with the dependence of morphologies on the chemical compositions of block copolymers at zero pressure.
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So the effective volume ratio of two blocks might be the key factor which determines the morphologies. Comparison with Coil-Coil Block Copolymer PS-b-PEO. One goal of this work is to elucidate the effect of architectures on the surface aggregation of block copolymers. It is interesting to compare with the coil-coil block copolymer PS-b-PEO to understand our rod-coil block copolymer system well. PMBPS and PS have the same flexible backbones. However, the bulky terphenyl group laterally attached enhances the stiffness of PMBPS backbones due to steric hindrance. The conformation difference between PS and PMBPS leads to a quite different surface aggregation behavior. The interfacial behavior and microstructures of PEO-b-PS block copolymers at the air-water interface have been studied systematically during the past decade by several research groups.17-18,36-44 Aggregates are spontaneously formed at the onset of copolymer solution spreading. The structure of aggregates depends greatly on the amphiphilic balance between the ethylene oxide and the styrene units. Spheres are formed predominantly at the content of PS
