Article pubs.acs.org/Macromolecules
Fddd Phase Boundary of Polystyrene-block-polyisoprene Diblock Copolymer Melts in the Polystyrene-Rich Region Yi-Chin Wang,† Kuniaki Matsuda,† Myung Im Kim,† Ayaka Miyoshi,† Satoshi Akasaka,† Shotaro Nishitsuji,† Kenji Saijo,† Hirokazu Hasegawa,† Kazuki Ito,‡ Takaaki Hikima,‡ and Mikihito Takenaka*,†,‡ †
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, 615-8510, Japan Structural Materials Science Laboratory SPring-8 Center, RIKEN Harima Institute Research, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
‡
ABSTRACT: We investigated the Fddd phase in the polystyrene (PS) rich region or f PI < 0.5 of the phase diagram for polystyrene-block-polyisoprene (SI) diblock copolymers by using small-angle X-ray scattering and transmission electron microscope, where f PI is a volume fraction of polyisoprene (PI) in a SI diblock copolymer. Five SI diblock copolymers which have different f PI values were prepared, in the range of 0.369 < f PI < 0.392 by living anionic polymerization. We found the Fddd phase as an equilibrium phase in the range of 0.370 ≤ f PI ≤ 0.373 and 19.5 < χN < 21.1 where χ and N are, respectively, the Flory−Huggins interaction parameter between PS and PI, and polymerization index of the diblock copolymer. The Fddd phase boundary in PS-rich region is much narrower than that in PI-rich region or f PI > 0.5 in terms of f PI and χN. The asymmetry of the Fddd region between PS-rich and PI-rich region can be explained by the conformational asymmetry between PS and PI chains.
I. INTRODUCTION
In addition to the morphologies mentioned above, it has been found that the Fddd structure exists as an equilibrium phase in diblock copolymer melts. Tyler and Morse reexamined the phase diagram of diblock copolymers by using SCFT and predicted that the Fddd phase is stable in diblock copolymer within a narrow region between L and G phases.8 Yamada et al.,9 Ranjan and Morse10 also found the Fddd structure is stable in diblock copolymer theoretically. Experimentally we have found the Fddd structure exists between L and G phases in SI diblock copolymer at f PI > 0.5, where f PI is the volume fraction of polyisoprene (PI) in SI diblock copolymers.11 We checked the stability of the Fddd structure after long time annealing and the thermoreversibility, and we have confirmed that the Fddd structure exists as an equilibrium structure in SI diblock copolymers.12 We also found that the stable region of the Fddd phase exists at 0.629 ≤ f PI ≤ 0.649 and 25.6 < χN < 29.8.13 Jung et al. also found the Fddd phase in SI (f PI = 0.645) diblock
Block copolymers are composed of two or more kinds of chemically different polymers connected by a covalent bond. Diblock copolymer A-b-B is the simplest block copolymer and its phase behaviors1−6 can be represented in terms of χN and f, where N is the polymerization, f is the volume fraction of A component in A-b-B, and χ is the Flory−Huggins interaction parameter per monomer between A and B, which is usually proportional to inverse of temperature. The theoretical and experimental studies on the phase diagrams of diblock copolymers have done extensively. Matsen and Schick have calculated the phase boundaries of lamellae (L), gyroid (G), hexagonally packed cylinder (C), and sphere on the bodycentered lattice (S) for diblock copolymers by using selfconsistent-field theory (SCFT).5 Experimentally, the phase diagram for polystyrene-block-polyisoprene (SI) diblock copolymer near order−disorder transition has been constructed by Khandpur.6 They have determined the phase boundaries of L, G, C, and S, and found qualitative agreement between experimental and theoretical phase diagram.6,7 © XXXX American Chemical Society
Received: January 2, 2015 Revised: February 18, 2015
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DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules copolymer.14 It is anticipated that the Fddd phase should exist on the other side of phase diagram, or polystyrene (PS)-rich region or f PI < 0.5. Ahn et al. have already found that the Fddd structure coexists with G in PS-rich region of SI diblock copolymer.15 However, the detail of the Fddd phase in PS-rich region has not examined yet. In this study, we thus synthesized five SI diblock copolymers with various compositions in PS-rich region and investigated their phase behavior by small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM). Then the stable region of the Fddd structure and the phase boundary were determined in PS-rich region of SI diblock copolymers.
sample reached its equilibrium state and then quenched it into liquid nitrogen to freeze morphology formed at 160 °C. The frozen sample was microtomed into 70 nm thickness at −10 °C and stained with OsO4. In TEM image, thus, the dark part corresponds to polyisoprene. We used JEM-2000FX (JEOL Ltd., Japan) with the acceleration voltage being 200 kV.
III. RESULTS AND DISCUSSION Figure 1a shows the temperature dependence of SAXS profiles of I3 ( f PI = 0.373). The scattered intensity I(q) is plotted as a function of wavenumber q (q = (4π/λ) sin(θ/2); θ is the scattering angle and λ is the wavelength of incident X-ray). Below 165 °C, several distinct peaks appear, indicating that I3 is in the ordered state. At 145 and 150 °C, the peaks at q/qm = 1, 2, and 3 are found in the SAXS profiles with qm= 0.342 and 0.344 nm−1, respectively, where qm is q at the first order peak. These peak positions indicate that the lamellar structure was formed in I3 at 145 and 150 °C. On the other hand, at 155 and 160 °C, the ratios of q/qm become different from those of the lamellar structure. SAXS peaks are located at q/qm= 1, 1.21, 1.54, 1.68, 1.71, 1.81, 2.00, 2.50, 2.65, 2.76, and 2.93 with qm= 0.344 nm−1. Enlarged SAXS profile at 155 °C is shown in Figure 1(b). This series agrees with the peak ratio of the Fddd structure as observed in PI-rich SI diblock copolymer previously.11 These peaks can be indexed as 111, 004 (q/qm = 1), 113 (1.21), 115 and 131 (1.54), 040 (1.68) 133 (1.71), 202 (1.81), 222 (2.00), 242 (2.47), 311 (2.67), 313 (2.75), and 315 (2.92). We estimated (a:b:c) = (1:2.08:3.62) with a = 20.7 nm by using
II. EXPERIMENTAL SECTION II-1. Materials. Five SI diblock copolymer samples were synthesized via living anionic polymerization at 50 °C in benzene under an argon environment using sec-BuLi as an initiator. Numberaverage molecular weight (Mn) and polydispersity index (PDI) of SI diblock copolymers were determined by size exclusion chromatography (SEC). For SEC measurement, THF was used as the eluent at 40 °C and the chromatograms were recorded with a refractive index detector. Volume fraction of polyisoprene, f PI, was determined by using 1H nuclear magnetic resonance (NMR) spectroscopy. We also identified that polyisoprene has a high degree of 1,4-addition (more than 95%) and a small degree of 3,4-addition (less than 5%) by 1H NMR. The characterizations of the samples are listed in Table 1. Prior
Table 1. Molecular Characteristics of SI Diblock Copolymers Used in This Study code
Mn (×104) (g·mol−1)
f PI
Mw/Mn
I1 I2 I3 I4 I5
3.06 3.71 2.16 1.94 2.54
0.367 0.370 0.373 0.376 0.385
1.01 1.03 1.01 1.01 1.03
qhkl = 2π[h2 /a 2 + k 2/b2 + l 2/c 2]1/2
(1)
where a, b, and c are unit cell parameters and h, k, and l are Miller indices for a, b, and c, respectively. At 165 °C a single broad peak is observed, indicating that I3 is in disordered state at 165 °C. We also found the Fddd structure in I2 (f PI = 0.370) at 252−268 °C. Figure 2 shows the TEM image of I3 at 160 °C. The black ovals are connected with trivalent junctions in the TEM image, which are representative patterns of Fddd structure,11,16 agreeing with the results of SAXS profile. The results of SAXS and TEM for I3 indicate that I3 exhibits L−FdddDisorder transition. This transition does not contain Fddd-G transition, which is different from the phase behavior of Fddd in PI-rich region.12 In order to check whether the Fddd phase exists as an equilibrium phase at f PI < 0.5, we examined the stability of the Fddd structure. We checked the stability of Fddd structure after long time annealing, and the thermoreversibility between L and Fddd structure. Figure 3 shows the time change in SAXS profiles during long time annealing at 155 °C. Even after 31 h, we still found q/qm = 1.22, characteristic peak of Fddd, indicating that Fddd survives after long time annealing. Figure 4 shows the result on the check of thermoreversibility between L and Fddd phases. First we annealed I3 sample at 125 °C or L region for 24 h. Figure 4a corresponds to SAXS profile after annealing at 125 °C and the peaks at q/qm = 1 and 2 indicate L is formed at 125 °C. Subsequently, I3 was heated up to 155 °C and annealed for 20 h. SAXS profiles after the annealing at 155 °C is shown in Figure 4b. The peak appears at q/qm = 1.22, suggesting that L is transformed into Fddd by annealing at 155 °C. Finally, we cooled the sample to the L region or to 115 °C and annealed the sample at 115 °C for 10 h. In SAXS profiles after annealing at 115 °C shown in Figure
to each experiment, we made 5 wt % polymer solution in toluene with 0.2 wt % 2,6-di-tert-butyl-4-methylphenol as an antioxidant agent and then cast them at 30 °C for 1 week. The cast films were further dried at room temperature in vacuum for 1 day. Thickness of the as-cast films was about 0.1 mm. II-2. Small-Angle X-ray Scattering (SAXS). Synchrotron SAXS experiments were performed at BL-15A in KEK and BL45XU in SPring-8 to examine the phase behavior of the samples. At BL-15A, the X-ray wavelength and the sample-to-detector distance were, respectively, 0.15 nm and 2000 mm. Imaging plates were used as a detector. At BL45XU, the wavelength and the sample-to-detector distance were, respectively, 0.12 nm and 3300 mm. A CCD with Image Intensifier was used as a detector. We also performed in-house SAXS experiment with conventional small-angle X-ray scattering apparatus (NANO-Viewer IP system, Rigaku, Co, Ltd., Japan). The wavelength and the sample-to-detector distance were, respectively, 0.154 nm and 2000 mm. RAXIS-IV was used as a detector. We installed cast film samples sandwiched between Kapton films into the SAXS sample holder with their thickness being 2.5 mm. Prior to SAXS experiment, the samples were annealed at 230 °C corresponding to disorder state for 30 min and at 120 °C for 1 day in vacuum, and then quenched to room temperature. SAXS profiles were measured after attaining the equilibrium states. The obtained data was corrected for air scattering (BL-15A, BL45XU, and in house) and electrical background and the distortion due to the CCD camera (BL45XU). Then we obtained the 1D SAXS profiles by circularly averaging the 2D data. II-3. Transmission Electron Microscope (TEM). We performed TEM observation to identify the Fddd structure in real space. Prior to TEM observation, we annealed the sample I3 at 160 °C until the B
DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Temperature dependence of SAXS profiles for I3 (f PI = 0.373). The profiles are shifted vertically to avoid overlapping. (b) SAXS profile at 155 °C. The inset is the enlarged part of the high q-regime.
Figure 2. TEM image of I3 ( f PI = 0.373) annealed at 160 °C.
4c, the peak at q/qm = 1.22 disappears and the peaks are found at q/qm = 1 and 2, indicating that the L structure is recovered from the Fddd structure after annealing at 115 °C. These results have confirmed that the thermoreversilibity exsits between L and Fddd and that the Fddd region exists as an equilibrium phase in PS-rich region. While Fddd structure exhibited in I2 and I3, the Fddd phase was not observed in the other samples. Figure 5 shows the temperature dependence of SAXS profiles of I5 which has the largest PI volume fraction in the SI samples, or f PI =0.385. The peaks appear at q/qm = 1, 2, and 3 below 180 °C, suggesting that I6 forms the L structure in its ordered state. A broad peak for the disordered state is observed in the SAXS profile at 190 °C and I5 exhibits an L−disorder transition. Similar to the I5, the L−disorder transition is found in I4.
Figure 3. SAXS profile of I3 ( f PI = 0.373) annealed at 155 °C for 31 h.
Temperature dependence of the SAXS profile for I1 with the smallest PI volume fraction in the SI samples, or f PI = 0.367, is shown in Figure 6. Distinct peaks at q/qm = 1, 2, 3, and 4 indicate the L structure was formed at 120 °C. On the other hand, SAXS profiles above 120 °C in the ordered state have the peaks located at q/qm = 1, 1.15, 1.52, 1.9, 2.58, and 2.9, which agree with those calculated for the G structure. This change in the peak positions indicates L structure transformed into G structure. At 230 °C, the SAXS profile shows broad peak and the L−G−disorder transition has been confirmed in I1. C
DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Temperature dependence of SAXS profiles for I1 (f PI = 0.367).
Figure 4. SAXS profiles of I3 ( f PI = 0.373) after each step during thermal protocol: (a) after annealing at 125 °C for 24 h, (b) after annealing at 155 °C for 20 h, and (c) after annealing at 115 °C for 3 h.
We determined the phase boundary of the Fddd phase of the SI diblock copolymer in the PS-rich region in terms of f PI and temperature. To eliminate the effects of molecular weight on the phase boundary, we plotted our identified phases in parameter space of χN on f PI, instead of temperature on f PI. To convert temperature to χ, we employed the following relation by Khandpur et al.:6 χ = 71.4/T − 0.0857
(2)
Figure 7 shows the phase boundary of the Fddd phase in the PS-rich region. The Fddd phase in PS-rich region locates at 0.370 ≤ f PI≤ 0.373 and 19.5 < χN < 21.1. The Fddd phase exists near the phase boundary between G and L. Figure 8 shows the Fddd region in PS and PI-rich regions. The Fddd phase in PI-rich region ranges over 0.629 ≤ f PI≤ 0.649 and 25.6 < χN < 29.8 and the Fddd region in the PS-rich region is much narrower than that in the PI-rich region. Besides, the location of the Fddd phase in PS-rich region is not symmetric to that in the PI-rich region. The Fddd phase in the PS-rich region is shifted to smaller χN values and is located closer to f PI = 0.5 than that in the PI-rich region. The asymmetry in the phase diagram of diblock copolymers has been found in the SI diblock copolymer and Bates et al. suggested that the conformational asymmetry can have an effect on the phase diagram and phase boundaries.17 Recently, Matsen has developed a new scheme of selfconsistent-field theory to calculate the phase diagram for block copolymers precisely.18 He calculated the phase diagram of AB diblock copolymers with the scheme including the effects of conformational asymmetry and found that conformational asymmetry strongly affects the location of each phase including the Fddd phase in the parameter space of χN and f PI. According
Figure 5. Temperature dependence of SAXS profiles for I5 ( f PI = 0.385).
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DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
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effects were considered as a possible reason for the direct transition from Fddd to disordered state. Observation of the Fddd phase as an equilibrium phase both in PS-rich and PI-rich regions of SI diblock copolymers contradicts the prediction by Miao and Wickham,20 where they calculated the stability of the Fddd phase in diblock copolymers under the effects of thermal fluctuations and found that the Fddd phase is metastable with respect to L and disorder phases. The contradiction may arise from the fact that the calculations of the effects involve a considerable number of approximations.
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CONCLUSIONS We investigated phase behavior of five SI diblock copolymers in PS-rich region, with different volume fraction of PI ranging from 0.367 ≤ f PI ≤ 0.385 by the technique of SAXS and TEM. The Fddd structure was found in I2 and I3 samples with 0.37 ≤ f PI ≤ 0.373 and exhibited L−Fddd−disorder transition. The stability of the Fddd structure after long-time annealing and the confirmation of the thermoreversibility between L and the Fddd structures clarified that the Fddd structure exists as an equilibrium phase. According to the phase diagram of SI diblock copolymers, the Fddd phase in PS-rich region is much narrower than that in PI-rich region, and the χN range of the Fddd phase in PS-rich region is shifted to lower χN region than that in PI-rich region. The asymmetry of the Fddd phase in PSrich rich regions agrees with the phase diagram calculated with self-consistent field theory including the effects of conformational asymmetry by Matsen, indicating that the conformational asymmetry causes the asymmetry of the Fddd phase in the phase diagram of SI diblock copolymers. Fddd phase was firstly found in triblock coplymer melts by Baily et al.,19 and Matsen et al. predicted the existence of Fddd phase in a wide range of other block copolymer architectures.20 These facts including our results suggest that the Fddd phase could end up being a common phase among block copolymer systems.
Figure 7. Phase boundary of the Fddd phase in the PS-rich region. Solid line corresponds to the phase boundary determined by Khandpur et al.6
to the phase diagram calculated for aA/aB = 1.5, the Fddd region in the A-rich region extends to the high χN region and is biased to fA = 0.5 while the Fddd region in the B-rich region shifts to fA = 0.5 and stays in a lower χN region than that in the A-rich region, where ai = bi/vi, with bi and vi being statistical segment length and associated statistical segment length of i-th component. This tendency agrees with our results qualitatively. Theoretical calculations demonstrated that conformational asymmetry have a profound influence on order−ordertransitions (OOTs). The conformational asymmetry of SI diblock copolymer has been reported to be aPI/aPS = 1.22; thus, we can conclude that the conformational asymmetry causes the Fddd region in the SI diblock copolymer to be asymmetric. In addition to conformational asymmetry, thermal concentration fluctuations also affect the phase boundary of Fddd structure.19 Unlike the transition sequences found in PI-rich side (L−Fddd−G−disorder),13 the SI diblock copolymers in the PS-rich region showed L−Fddd−disorder transitions with increasing temperature and G is not found between Fddd and disorder with increasing temperature. Thermal fluctuation
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AUTHOR INFORMATION
Corresponding Author
*(M.T.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
Figure 8. Phase boundary of the Fddd phase in PS-rich and PI-rich regions. E
DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of RIKEN (Proposal No. 20100082, and 20130015) and at BL15A at KEK (2009G056, and 2011G066). This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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REFERENCES
(1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (3) Leibler, L. Macromolecules 1980, 13 (6), 1602−1617. (4) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29 (4), 1091− 1098. (5) Matsen, M. W.; Schick, M. Phys. Rev. Lett. 1994, 72 (16), 2660− 2663. (6) Khandpur, A. K.; Forster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W.; Almdal, K.; Mortensen, K. Macromolecules 1995, 28 (26), 8796−8806. (7) Hajduk, D. A.; Takenouchi, H.; Hillmyer, M. A.; Bates, F. S.; Vigild, M. E.; Almdal, K. Macromolecules 1997, 30 (13), 3788−3795. (8) Tyler, C. A.; Morse, D. C. Phys. Rev. Lett. 2005, 94 (20), 208302. (9) Yamada, K.; Nonomura, M.; Ohta, T. J. Phys.: Condens. Matter 2006, 18 (32), L421−L427. (10) Ranjan, A.; Morse, D. C. Phys. Rev. E 2006, 74 (1), 011803. (11) Takenaka, M.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Shimizu, H.; Kim, M. I.; Hasegawa, H. Macromolecules 2007, 40 (13), 4399−4402. (12) Kim, M. I.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Hasegawa, H.; Ito, K.; Takenaka, M. Macromolecules 2008, 41 (20), 7667−7670. (13) Kim, M. I.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Hasegawa, H.; Ito, K.; Takenaka, M. Macromolecules 2009, 42 (14), 5266−5271. (14) Jung, J.; Park, H. W.; Lee, J.; Huang, H.; Chang, T.; Rho, Y.; Ree, M.; Sugimori, H.; Jinnai, H. Soft Matter 2011, 7 (21), 10424− 10428. (15) Ahn, H.; Shin, C.; Lee, B.; Ryu, D. Y. Macromolecules 2010, 43 (4), 1958−1963. (16) Bailey, T. S.; Hardy, C. M.; Epps, T. H.; Bates, F. S. Macromolecules 2002, 35 (18), 7007−7017. (17) Bates, F. S.; Shultz, M. F.; Khandpur, A. K.; Förster, S.; Rosedale, J. H. Faraday Discuss. 1994, 98, 7−18. (18) Matsen, M. W. Eur. Phys. J. E 2009, 30 (4), 361−369. (19) Bates, F. S.; Rosedale, J. H.; Fredrickson, G. H. J. Chem. Phys. 1990, 92 (10), 6255−6270. (20) Miao, B.; Wickham, R. A. J. Chem. Phys. 2008, 128 (5), 054902/ 1−054902/5. (21) Matsen, M. W. Macromolecules 2012, 45 (4), 2161−2165.
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DOI: 10.1021/acs.macromol.5b00001 Macromolecules XXXX, XXX, XXX−XXX
