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Macromolecules 2004, 37, 3804-3808
Effect of Composition Distribution on Microphase-Separated Structure from BAB Triblock Copolymers A. Noro, M. Iinuma,† J. Suzuki,‡ A. Takano, and Y. Matsushita* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received November 27, 2003; Revised Manuscript Received March 2, 2004
ABSTRACT: The effects of composition distribution on microphase-separated structures formed by monodisperse BAB triblock copolymers were investigated. Monodisperse nine parent BAB triblock copolymers consisting of polystyrene for A and poly(2-vinylpyridine) for B were prepared by anionic living polymerization. These nine copolymers were designed such that polystyrene volume fraction, φs, ranged from 0.1 to 0.9, and they were blended to produce samples with various composition distributions but with constant average composition at φs of 0.5. Periodic simple alternating lamellar structures were observed for solvent cast and well-annealed blend films as long as composition distribution is relatively low. It has been found that microdomain spacing increases with increasing composition distribution of copolymer samples up to 1.25 in terms of Mw(S)/Mn(S), where Mw(S) and Mn(S) are weight-average and number-average molecular weights of polystyrene blocks, respectively, as was the case of the AB diblock copolymer system, though the increment is larger for triblock than diblock. Furthermore, it was clarified that triblock copolymer tends to macrophase separate into several regular microphase-separated structures more easily than diblock copolymer.
Introduction It is well-known that block copolymers composed of incompatible components form microphase-separated structures. So far, morphology change of block copolymers has been investigated focusing on molecular parameters such as molecular weight,1-6 composition,7-11 and manner of chain connectivity12-17 as variable indicators. Furthermore, somewhat complex systems such as block copolymer/homopolymer blends18-21 and block copolymer/block copolymer blends22-24 having different molecular parameters were also studied in recent years. Throughout these works, monodisperse block copolymers were used as samples because morphology changes very sensitively depend on these molecular parameters. On the contrary, the polymers in application field in general possess relatively wide molecular weight and composition distributions; however, these distributions have not been quantitatively discussed since usual block copolymers have both wide molecular weight and composition distributions together, and it is hard to separate the individual effect from these two distributions. Recently, we have succeeded in evaluating the composition distribution dependence on microphase-separated structures from polystyrene-block-poly(2-vinylpyridine) (SP) diblock copolymers of the AB type25 quantitatively. For quantitative discussion, we defined a new composition distribution index, Mw(S)/Mn(S), where Mw(S) and Mn(S) denote weight-average and number-average molecular weights of polystyrene block. In the AB diblock system we have found that the blends with wide composition distributions show homogeneous † Present address: Tokai Rubber Co. Ltd., 3-1 Komaki-higashi, Komaki, Aichi 485-8550, Japan. ‡ Present address: The Computing Research Center, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba 3050801, Japan. * To whom correspondence should be addressed. E-mail: yushu@ apchem.nagoya-u.ac.jp.
single structures up to 1.7 in terms of Mw(S)/Mn(S) and that the microdomain spacing monotonically increases with Mw(S)/Mn(S). Furthermore, the blend system has been phase-separated macroscopically into at least two microphase-separated structures if Mw(S)/Mn(S) exceeds 1.8. Including our previous studies, many of the studies on block copolymers are for AB diblock copolymers since they are easier to synthesize than the other block copolymers, and hence the former possess the simplest and the most basic structures.4,6 On the other hand, triblock copolymers of the BAB type have also been dealt with considerably.26-29 This type of copolymer has the distinct feature that their mid-blocks have no chain ends and accordingly adopt two conformations in bulk, i.e., loop and bridge conformations; this is apparently different from AB diblock copolymers. Thus, it is expected that BAB triblock copolymers show different morphological and mechanical features from AB diblock; therefore, it is very important to study the structural difference between AB and BAB. In this work, paying attention to the difference in conformations between AB diblock copolymers and BAB triblock copolymers, the effect of composition distribution on microphase-separated structures of BAB triblock copolymers was investigated quantitatively by using nearly monodisperse samples with various composition distributions. Experimental Section Parent poly(2-vinylpyridine)-block-polystyrene-block-poly(2-vinylpyridine) (PSP) triblock copolymers were synthesized by sequential two-step anionic polymerization in tetrahydrofuran (THF) at -78 °C with naphthalene-potassium as a bifunctional initiator.28 To characterize polystyrene blocks, small amounts of polystyrene solutions for all copolymers were separated as precursors in the course of polymerization. Molecular weight distributions of parent polymers and the corresponding polystyrene precursors were estimated by a gel permeation chromatography (GPC) system, HLC-8020 of Tosoh
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Table 1. Molecular Characteristics of Parent Polymers sample PSP-19 PSP-28 PSP-37 PSP-46 PSP-55 PSP-64 PSP-73 PSP-82 PSP-91
Mwa/105 (Mw/Mn)b 1.61 1.58 1.44 1.39 1.56 1.64 1.65 1.35 1.48
1.08 1.07 1.07 1.03 1.05 1.04 1.05 1.05 1.06
φsc 0.112 0.224 0.328 0.422 0.496 0.578 0.687 0.820 0.922
M(S)d/104 (Mw/Mn(pre))e 1.68 3.32 4.47 5.59 7.42 9.15 11.0 10.9 13.6
Table 2. Composition Distributions and Domain Spacings of All Blend Samples blend no.
1.04 1.03 1.04 1.03 1.05 1.04 1.04 1.04 1.05
I
II
a Weight-average molecular weights measured by multiangle laser light scattering. b Apparent molecular weight distributions determined by gel permeation chromatography. c Volume fractions of polystyrene blocks measured by pyrolysis-gas chromatography. d Molecular weights of polystyrene blocks calculated by using M , w φS, and bulk densities of two component polymers, i.e., 1.05 for polystyrene and 1.14 for poly(2-vinylpyridine). e Molecular weight distributions of polystyrene precursors determined by gel permeation chromatography.
III
code
Ma/105
Mw(S)/Mn(S)
D/nm
D/D0b
E3 E5 E7 E9 G3(0.2) G5(1.13) G7(2.5) G9(4.5) G9(7.5) G9(20) Q-2 Q-4 MG(0.02) MG(0.06) MG(0.09)
1.52 1.53 1.51 1.51 1.55 1.53 1.52 1.51 1.51 1.51 1.51 1.52 1.51 1.51 1.51
1.04 1.10 1.15 1.25 1.01 1.06 1.09 1.15 1.18 1.22 1.39 1.44 1.34 1.30 1.27
40.7 41.8 43.7 45.1 41.1 41.5 42.1 43.1 44.8 45.1 macro macro macro macro macro
0.99 1.02 1.06 1.10 1.00 1.01 1.02 1.05 1.09 1.10
a The calculated average molecular weight using M values in w Table 1. b D0, 41.1 nm, is lamellar domain spacing for PSP-55.
Corp., with two GMHXL columns of Tosoh while weight-average molecular weights of copolymers were determined by a multiangle laser light scattering (MALLS), DAWN EOS enhanced optical system of Wyatt Technology. Compositions of all copolymers were measured by pyrolysis-gas chromatography (pyrolysis-GC), GC-2010 of Shimadzu equipped with PY2020s pyrolyzer of Frontier Laboratory having an ultra alloy column and a FID detector. Details of the measurements were described in ref 25. Molecular characteristics of parent polymers are listed in Table 1. φs of these polymers is covering the range 0.1 < φs < 0.9 with its step of about 0.1, and all copolymers have the weight-average molecular weights between 135K and 165K. By using these parent copolymers, three series of blend samples were prepared. Series I is equal mass amount blends, and series II is the blends from several parent polymers with Gaussian distribution function as expressed in eq 1, while series III includes blend manners according to quadratic (Q2) and quartic (Q-4) functions as shown in eq 2, where m equals 2 or 4, and also includes the modified Gaussian distribution function defined by eq 3, and w(φs) denotes weight fraction of parent polymers having corresponding φs.
w(φs) ) (2π)-1/2σ-1 exp{-(φs - 0.5)2/2σ2}
(1)
w(φs) ) (φs - 0.5)m + A
(2)
w(φs) ) B - (2π)-1/2σ-1 exp{-(φs - 0.5)2/2σ2}
(3)
Series I consists of E3, E5, E7, and E9, which include 3, 5, 7, and 9 parent polymers. Samples of series II are G3(0.2), G5(1.13), G7(2.5), G9(4.5), G9(7.5), and G9(20), in which the numbers in parentheses mean squares of the standard deviation in eq 2. MG(0.02), MG(0.06), and MG(0.09) in series III have common σ2 value of 2.55 and with B of 0.02, 0.06, and 0.09, respectively, in eq 3. The average composition of all blends is designed to be approximately 0.5 for φs. Characteristics of all blends are listed in Table 2. In evaluating Mw(S)/ Mn(S), it was assumed that Mw/Mn of nine parent polymers and their precursors are all unity since the largest value in Table 1 is 1.08, and so forcing this assumption could be considered to offer no essential effect on the results.25 Recognizing this assumption, Mw(S)/Mn(S) values in Table 2 must be minimum, and the actual value should be somewhat larger. Mixing manners from nine parent polymers for eight blends are schematically shown in Figure 1. Sample films for morphological observations were obtained by solvent-casting from dilute solutions of tetrahydrofuran and well-annealed at 150 °C for 5 days. Microphase-separated structures of the films were observed by a transmission electron microscope (TEM), H-800, of Hitachi under acceralation voltage of 100 kV for ultrathin sections of samples
Figure 1. Relative amounts of parent polymers used for preparing various blends. Vertical axes express weight fractions, w(φs), of the nine parent polymers included in blends, while horizontal axes are polystyrene volume fractions of the parent polymers in bulk. stained with osmium tetroxide. Small-angle X-ray scattering (SAXS) was also performed to investigate structures precisely using two SAXS apparatuses. One is the machine installed in the beamline 15A at Photon Factory in Tsukuba equipped with a X-ray imaging plate placed at 2405 mm from sample position. The wavelength of the incident X-ray beam was 0.1508 nm. The other one is the new Rigaku SAXS system, Nano Viewer,
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Figure 2. A typical image of a transmission electron micrograph for blend E9. The brighter phase is polystyrene while the darker phase expresses poly(2-vinylpyridine).
Figure 4. Composition distribution dependence of the reduced domain spacing, D/D0, of all blends with single microphaseseparated structure. D0, 41.1 nm, is the domain spacing of one of the parent copolymers, PSP-55. The hatched portion denotes the macrophase-separated region.
Figure 3. Comparison of SAXS diffraction patterns for the blends whose composition distribution is smaller than 1.27. Scattering profile for a parent polymer, PSP-55, is added as a reference. The curves are displayed in the order of the magnitude of composition distribution, that is, PSP-55, G3(0.2), E3, G5(1.13), G7(2.5), E5, G9(4.5), E7, G9(7.5), G9(20), E9 from top to bottom. with a confocal max flux multilayer mirror operated at 45 kV and 60 mA. The wavelength of the source X-ray was 0.1542 nm. An imaging plate was used as a detector, and the camera length adopted was 738 mm.
Results
Figure 5. Transmission electron micrographs showing different three kinds of microphase-separated grains for MG(0.09) and Q-2 whose Mw(S)/Mn(S) are 1.27 and 1.39, respectively.
Figure 2 shows a bright field transmission electron micrograph for blend E9. It is apparent from Figure 2 that E9, whose Mw(S)/Mn(S) equals 1.25, possesses very periodic lamellar structure with flat interfaces. All the other samples with composition distribution narrower than that of E9 were confirmed to be qualitatively similar to the micrograph in Figure 2, though they were not shown to avoid duplicated information. Figure 3 compares SAXS diffraction patterns of a parent block copolymer, PSP-55, whose φs is approximately 0.5, and 10 blends, of which composition distributions are smaller than 1.27, as a function of the magnitude of scattering vector q ()4π sin θ/λ), where 2θ and λ are the scattering angle and the wavelength of X-rays. All the curves include integer number order peaks composed of strong odd number peaks and weak even number ones reflecting the feature of periodic lamellar structures whose average compositions of two component blocks are close to each other. These results indicate that any molecules with different compositions do not localize in lamellar microdomains if the composition distribution is not so extremely wide. The same behavior was previously observed for an AB diblock system.25 In Figure 3, SAXS profiles are displayed in the order of composition distribution. As composition distribution increases from top to bottom, the locations of diffraction peaks shift to lower q values. These results indicate that
domain spacing increases with increasing composition distribution, as was the case of AB diblock system. Applying Bragg’s conditions to the magnitude of scattering vectors corresponding to the locations of the diffracted peaks, microdomain spacings, D, were evaluated, and they are listed in Table 2. Nondimensional normalized numbers, D/D0, where D0 denotes the domain spacing for the pure parent block copolymer, PSP55, were also listed. In Figure 4, D/D0 are plotted against Mw(S)/Mn(S) together with that for a pure parent block copolymer PSP-55, that is, D/D0 equals unity. From this figure it is definite that lamellar domain spacing increases with an increase of composition distribution. Figure 5 shows typical each three locations of bright field transmission electron micrographs for MG(0.09) and Q-2 whose Mw(S)/Mn(S) are 1.27 and 1.39, respectively. It is apparent that these blend samples show at least three kinds of microphase-separated grains. They are periodic lamellar structures and two kinds of cylindrical structures with cylinders from polystyrene and poly(2-vinylpyridine) embedded in matrix of counterparts, though the average compositions of the blends are around 0.5. MG(0.06) and MG(0.02) whose Mw(S)/ Mn(S) are 1.30 and 1.34 were also confirmed to give essentially the same micrographs as in Figure 5.
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BAB Triblock Copolymers 3807
Figure 6. Transmission electron micrographs showing different five kinds of microphase-separated grains observed for Q-4 with Mw(S)/Mn(S) of 1.44.
Figure 7. Comparison of SAXS diffraction patterns for the blends whose composition distribution is larger than 1.25. The curves are displayed in the order of the magnitude of composition distribution, that is, MG(0.09), MG(0.06), MG(0.02), Q-2, and Q-4, from top to bottom.
Alternatively, Figure 6 shows five typical locations of bright field micrographs for Q-4 whose Mw(S)/Mn(S) is 1.44. This blend shows two kinds of cylindrical structures and another two kinds of spherical structures besides of periodic lamellar structure. Thus, macrophase separation has preferentially occurred on these blends. Figure 7 shows SAXS diffraction profiles of these blends; they are MG(0.09), MG(0.06), MG(0.02), Q-2, and Q-4 from top to bottom. As the composition distribution value increases, diffraction peaks deriving from lamellar structure are getting harder to recognize. At the same time it is getting difficult to assign the diffracted intensity patterns to a certain particular microphaseseparated structure. This result reflects the fact that the samples mostly contain polycrystals composed of grains with several different microphase-separated structures as is recognized from the bottom curve in Figure 7. Discussion Figure 8 compares the composition distribution dependence of D/D0 between triblock and diblock copolymers against Mw(S)/Mn(S).25 From this figure one notices that (a) the increment of D/D0 for PSP triblock is larger than that for SP diblock and (b) the PSP triblock copolymer system more easily macrophase-separates than the SP diblock copolymer system. At first, we discuss the reason for (a). Figure 9 schematically compares chain conformations in lamellar microdomains between BAB triblock and AB diblock copolymers, both of which consist of monodisperse but compositionally nonuniform block copolymer molecules. As was discussed in ref 25, the average interfacial area for a junction point of each block copolymer in blends with wide composition distribution is smaller than that for the regular monodisperse copolymers. This is be-
Figure 8. Composition distribution dependence of the reduced domain spacing, D/D0, of all blends with single lamellar microphase-separated structure for both the SP diblock copolymer system and the PSP triblock copolymer one. Open circles are for SP diblock copolymers, while filled circles are for PSP triblock copolymers.
Figure 9. Schematic comparison of chain conformation in microdomains formed by BAB triblock copolymer molecules (left) and AB diblock ones (right).
cause the segments on the chain end of the longer block is localized at the center of lamellar microdomain, since the chain segments from the shorter block have to be concentrated in the vicinity of lamellar interface. For triblock copolymer of the BAB type whose mid-blocks have no chain ends, on the other hand, a certain fraction of the chain adopts a loop conformation28 where the localization and resulting concentration of the chain segments on a shorter mid-block must be more strict than diblock copolymer system since chain elongation toward normal to lamellar interface is suppressed for the loop conformation.30 This can cause the enhancement of chain localization for the longer chain at the center of microdomain, and therefore, the increment of D/D0 for BAB triblock could be larger than that for AB diblock. This phenomenon is similar to the expansion of domain size due to segregation of longer homopolymer chains at the center of microdomains for block copolymer/ homopolymer blend system.21,31 Neutron reflectivity measurements are now undergoing, and the difference in degree of localization of segments on shorter and also
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on longer block between AB diblock and BAB triblock will be clarified. The second feature can be understood more easily. Apparently more or less some fraction of triblock copolymer possesses bridge conformation in bulk. If the chain with short polystyrene block adopts bridge conformation in lamellar structure, it has to be quite naturally elongated toward the direction normal to lamellar interface. The shorter chain could reach its elongation limit easily when Mw(S)/Mn(S) goes up, and eventually the system tends to separate macroscopically among several microphase-separated structures by selecting the state with lower total free energy than that for the uniform microphase-separated state, in which the extreme entropy loss for short chain has to occur. In summary from these experimental results on monodisperse poly(2-vinylpyridine)-block-polystyreneblock-poly(2-vinylpyridine) block copolymers having molecular weights of around 150K with various composition distributions, we conclude that (1) they show periodic and uniform lamellar structures if composition distribution value is smaller than 1.25, (2) their lamellar domain spacing increases with an increase in composition distribution value, (3) the increment of domain spacing is considerably larger than that of diblock copolymer system, and (4) macrophase separation is favorable if composition distribution value exceeds 1.27; this value is much smaller than that of the diblock system. Acknowledgment. The authors thank Professor T. Tanji at Department of Electrical Engineering in Graduate School of Engineering, Nagoya University, and Mr. S. Arai and Mr. Y. Yoshida at the Center for Integrated Research in Science and Engineering in Nagoya University for their help in taking transmission electron micrographs. We also thank Miss K. Aoki for her help in measuring molecular weight of parent copolymers and Mr. T. Hikage at High Intensity X-ray diffraction Laboratory in Nagoya University for his help in measuring small-angle X-ray scattering. This work was partially supported by the Ministry of Education, Science, Sports, and Culture, Glant-in-Aid program #12450383 and #13031040, and also supported by the 21st century COE Program entitled “The Creation of Nature-Guided Materials Processing”. This research was also supported in part by a grant from Daiko Foundation. The authors are thankful for their financial assistance. References and Notes (1) Helfand, E.; Wasserman, Z. R. Macromolecules 1976, 9, 879. (2) Leibler, L. Macromolecules 1980, 13, 1602.
Macromolecules, Vol. 37, No. 10, 2004 (3) Ohta, T.; Kawasaki, K. Macromolecules 1986, 19, 2621. (4) Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules 1980, 13, 1237. (5) Hadziioannou, G.; Skoulios, A. Macromolecules 1982, 15, 258. (6) Matsushita, Y.; Mori, K.; Saguchi, R.; Nakao, Y.; Noda, I.; Nagasawa, M. Macromolecules 1990, 23, 4313. (7) Inoue, T.; Soen, T.; Hashimoto, T.; Kawai, H. J. Polym. Sci., Part A-2 1969, 7, 1283. (8) Richards, R. W.; Thomason, J. L. Polymer 1981, 22, 581. (9) Hasegawa, H.; Tnaka, H.; Ymasaki, K.; Hashimoto, T. Macromolecules 1987, 20, 1651. (10) Mogi, Y.; Kotsuji, H.; Kaneko, Y.; Mori, K.; Matsushita, Y.; Noda, I. Macromolecules 1992, 25, 5408. (11) Matsushita, Y. J. Polym. Sci., Part B: Polym. Phys. Ed. 2000, 38, 1645. (12) Hadjichristidis, N.; Iatrou, H.; Behal, S. K.; Chledzinski, J. J.; Disco, M. M.; Garner, R. T.; Liang, K. S.; Lohse, D. J.; Milner, S. T. Macromolecules 1993, 26, 5812. (13) Matsushita, Y.; Watanabe, J.; Katano, F.; Yoshida, Y.; Noda, I. Polymer 1996, 37, 321. (14) Matsushita, Y.; Momose, H.; Yoshida, Y.; Noda, I. Polymer 1997, 38, 149. (15) Gido, S. P.; Lee, C.; Pochan, D. J.; Pispas, S.; Mays, J. W.; Hadjichristidis, N. Macromolecules 1996, 29, 7022. (16) Takano, A.; Kadoi, O.; Hirahara, K.; Kawahara, S.; Isono, Y.; Suzuki, J.; Matsushita, Y. Macromolecules 2003, 36, 3045. (17) Takano, A.; Kondo, K.; Ueno, M.; Ito, K.; Kawahawa, S.; Isono, Y.; Suzuki, J.; Matsushita, Y. Polym. J. 2001, 33, 732. (18) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240. (19) Mayes, A. M.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. Macromolecules 1992, 25, 6523. (20) Matsushita, Y.; Torikai, N.; Mogi, Y.; Noda, I.; Han, C. C. Macromolecules 1993, 26, 6346. (21) Torikai, N.; Takabayashi, N.; Noda, I.; Koizumi, S.; Mori, Y.; Matsushita, Y. Macromolecules 1997, 30, 5698. (22) Hashimoto, T.; Yamasaki, K.; Koizumi, S.; Hasegawa, H. Macromolecules 1993, 26, 2895. (23) Court, F.; Hashimoto, T. Macromolecules 2001, 34, 2536. (24) Yamaguchi, D.; Hashimoto, T. Macromolecules 2001, 34, 6495. (25) Matsushita, Y.; Noro, A.; Iinuma, M.; Suzuki, J.; Ohtani, H.; Takano, A. Macromolecules 2003, 36, 8074. (26) Uchida, T.; Soen, T.; Inoue, T.; Kawai, H. J. Polym. Sci., Part A-2 1972, 10, 101. (27) Richard, R. W.; Thomason, J. L. Polymer 1981, 22, 581. (28) Matsushita, Y.; Nomura, M.; Watanabe, J.; Mogi, Y.; Noda, I.; Imai, M. Macromolecules 1995, 28, 6007. (29) Karatasos, K.; Anastasiadis, S. H.; Pakula, T.; Watanabe, H. Macromolecules 2000, 33, 523. (30) Chain elongation suppression for cyclic copolymer has been observed by Matsushita et al., and the manuscript is in review. According to this report, domain spacing D of cyclic SI diblock copolymer, where only loop conformation exists, increases in proportion to 0.59 power of molecular weight M (D ∝ M0.59), while that for SIS triblock copolymer is 0.68 (D ∝ M0.68). Moreover, D of linear SI diblock copolymer has been confirmed to increase in proportion to 2/3 power of molecular weight M (D ∝ M2/3) by Hashimoto et al.4 (31) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378.
MA035784Q
