Article pubs.acs.org/Macromolecules
Fddd Structure in Polystyrene-block-polyisoprene Diblock Copolymer/Polystyrene Homopolymer Blends Yi-Chin Wang,† Myung Im Kim,† Satoshi Akasaka,† Kenji Saijo,† Hirokazu Hasegawa,† Takaaki Hikima,‡ and Mikihito Takenaka*,†,‡ †
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, 615-8510, Japan RIKEN Spring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
‡
S Supporting Information *
ABSTRACT: We have induced Fddd structure in polystyreneblock-polyisoprene diblock copolymer (SI) which does not have Fddd phase by adding polystyrene (hPS) homopolymer. We prepared the SI/hPS blend samples in the range of 0.627 < ϕPI < 0.646 and investigated the phase boundary of Fddd phase by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), where ϕPI is volume fraction of polyisoprene (PI) in SI/hPS. We found the Fddd phase as an equilibrium phase in the range of 0.631 ≤ ϕPI ≤ 0.641 and 26.6 < χN < 30.7, where χ and N are respectively the Flory− Huggins interaction parameter between PS and PI and polymerization index of the SI. The Fddd phase boundary in the SI/hPS is similar to that in the neat SI block copolymer since the effect of the addition of the hPS with the low molecular weight on the phase diagram corresponds to that of the same change of PS volume fraction in the neat block copolymer.
I. INTRODUCTION Block copolymers that are composed of chemically different polymers connected by a covalent bond can self-assemble into various microstructures via microphase separation. Diblock copolymer A-b-B is the simplest diblock copolymer, and its microstructures depend on several parameters such as the volume fraction (f) of A component in A-b-B, the Flory− Huggins interaction parameter (χ) between A and B, and the polymerization index (N) of A-b-B. The phase behavior in A-bB has also been studied extensively, both theoretically and experimentally.1−7 Among these studies, a bicontinuous microdomain morphology with the symmetry of Fddd space group was first found by Bailey et al.8 and confirmed by Epps et al.9 in the ISO triblock terpolymer. Tyler and Morse suggested that the stability region of Fddd phase should extend to the diblock limit by using SCFT10 and Ginzburg−Landau thoery.11 Masten suggested that the Fddd phase is expected to occur in a wide range of block copolymer systems.12 Recently, we reported Fddd structure in a polystyrene-blockpolyisoprene diblock copolymer (SI) with number-average molecular weight Mn = 2.64 × 104 g/mol, polydispersity index Mw/Mn = 1.02, and volume fraction of polyisoprene f PI = 0.629.13 We then confirmed the stability of Fddd structure and found Fddd structure exists as an equilibrium structure in the SI diblock copolymer.14,15 We also found that the stable region of Fddd phase exists at 0.37 ≤ f PI ≤ 0.373 and 19.53 < χN < 21.12.16 Previous studies17−22 reported that blending homopolymer with diblock copolymer causes order−order transition. We can © XXXX American Chemical Society
anticipate that diblock copolymer/homopolymer blend exhibits Fddd structure even though the neat block copolymer does not have Fddd region. In this study, we, hence, investigated the phase behaviors of SI/PS homopolymer (hPS) blends by smallangle X-ray scattering (SAXS) and transmission electron microscopy (TEM) and explored how blending homopolymer affects the Fddd region.
II. EXPERIMENTAL SECTION II-1. Materials. We synthesized a SI diblock copolymer sample via living anionic polymerization in benzene at 50 °C under an argon environment. sec-BuLi was used as an initiator. We determined number-average molecular weight (Mn) and polydispersity index (PDI) of SI were determined by using size exclusion chromatography (SEC) using polystyrene standards. 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 NMR. hPS was purchased from TOSOH Corporation. The characterizations of the samples are listed in Table 1. We prepared
Table 1. Molecular Characteristics of SI and HPS code
Mn (×103) (g mol−1)
f PI
Mw/Mn
SI hPS
26.1 5.73
0.651
1.01 1.02
Received: January 3, 2016 Revised: February 16, 2016
A
DOI: 10.1021/acs.macromol.6b00007 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules the films of neat SI and five blend samples (B1 to B5) by casting 5 wt % polymer solution in toluene with 0.2 wt % 2,6-di-tert-butyl-4methylphenol as an antioxidant agent at 30 °C for 1 week. The blend ratios of the five samples are listed in Table 2. The cast films were further dried at room temperature in vacuum for 1 day.
Table 2. Characteristics of Blend Samples code
SI/hPS (w/w)
ϕPI
B1 B2 B3 B4 B5
99.2/0.8 98.4/1.6 97.6/2.4 96.7/3.3 95.9/4.1
0.646 0.641 0.636 0.631 0.627
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 behaviors of the samples. At BL-15A, the X-ray wavelength and the sample-to-detector distance were respectively 1.5 Å and 2000 mm. Imaging plates were used as a detector. At BL45XU, the wavelength and the sample-to-detector distance are respectively 1.2 Å and 3300 mm. A CCD with image intensifier was used as a detector. Prior to SAXS experiment, the cast films were annealed in the SAXS holder at 230 °C corresponding to disorder state for 30 min and then at 120 °C for 1 day under vacuum and quenched to room temperature. SAXS profiles were measured after attaining the equilibrium states during heating processes. The obtained data were corrected for air scattering (BL-15A, BL45XU) 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. We also used a conventional SAXS apparatus (NANO-Viewer IP system, Rigaku, Co, Ltd., Japan) that is the interoperable system of Kyoto University. It consists of 1.2 kW (40 kV, 30 mA) rotating-anode X-ray generator (RA-Micro7HF) with multilayer optics (Conforcal Max-Flux optics) for focusing and monochromatizing at 0.154 nm, a 2300 mm camera (1000 mm from the source to the sample and 1300 mm from the sample to detector) including three pinhole slits between the source and the sample, and a two-dimensional (2D) imaging plate (IP) detector (RAXIS IV++). Exposure time for taking a SAXS pattern is 3600 s. The obtained 2D data were corrected for the absorption of the sample, subtracted air and background scattering, and converted to 1D SAXS data by circularly averaging. II-3. Transmission Electron Microscopy (TEM). Prior to TEM observation, we annealed the samples for 8 h and then quenched the samples into liquid nitrogen to freeze morphologies formed at annealed temperature. Ultrathin sections of ca. 50 nm thickness were obtained by cryo-ultramicrotoming and stained with OsO4. In TEM image, thus, the dark part corresponds to polyisoprene. The TEM observation has been performed using JEM-2000FX with the acceleration voltage being 200 kV.
Figure 1. Temperature dependence of SAXS profiles for neat SI ( f PI = 0.651). The profiles are shifted vertically to avoid overlapping.
Figure 2. TEM images of neat SI at (a) 120 °C and (b) 140 °C. Dark region corresponds to PI. White scale bar corresponds to 50 nm.
lamellar structure. The SAXS profiles have peaks located at q/ qm = 1, 1.15, 1.82, 1.91, 2, 2.08, 2.52, and 2.90, agreeing with those of the gyroid structure (G). The TEM image shown in Figure 2b exhibits a 4-fold gridlike pattern, which is typical of the projection of a double-gyroid structure onto the (001) plane, in agreement with the SAXS result. Thus, the neat SI shows L−G disorder with increasing temperature and does not have Fddd region. The temperature dependence of the SAXS profiles for the blend sample B1 (ϕPI = 0.646) is shown in Figure 3. At lower temperature or 110 to 125 °C, the peak ratios become 1, 2, and 3, indicating L is stable. At 130 °C, the scattering profile does not show multiple integer peak ratios. The SAXS peaks are located at q/qm = 1, 1.22, 1.55, 1.72, 1.81, 1.95, and 2.00 with qm = 0.315 nm−1, agreeing with the peak ratio of the Fddd structure as identified in SI diblock copolymer previously.13 This fact suggests that the blend of hPS with SI can induce the order−order transition from L to Fddd at 130 °C. The peaks can be indexed as 111, 022, 004 (q/qm = 1), 113 (1.22), 115
III. RESULTS AND DISCUSSION Figure 1 shows the temperature dependence of SAXS profiles of the neat SI ( f PI = ϕPI = 0.651). The scattered intensity I(q) is plotted as a function of wavenumber q (q = (4π/λ) sin(θ/2); θ is scattering angle and λ is wavelength). At 165 °C, the SAXS profiles exhibits a single broad peak indicating that the neat SI is in the disordered state. Below 165 °C, several peaks are found in the SAXS profiles, and the neat SI is in its ordered state. At 120 and 125 °C, the peaks appear at q/qm= 1, 2, and 3 in the SAXS profiles, where qm is q at first-order peak. These integer multiples of peak position indicate that the lamellar structure (L) was formed at 120 and 125 °C. As shown in Figure 2a, the striped pattern in TEM image at 120 °C also suggests the lamellar structure. On the other hand, at 130 and 140 °C, the ratios of q/qm become different from those of the B
DOI: 10.1021/acs.macromol.6b00007 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 3. Temperature dependence of SAXS profiles for B1 (ϕPI = 0.646). The profiles are shifted vertically to avoid overlapping. The peaks are smeared at 145−165 °C since we measured with a conventional SAXS apparatus.
Figure 4. Temperature dependence of SAXS profiles for B2 (ϕPI = 0.641). The profiles are shifted vertically to avoid overlapping. The peaks are smeared at 150−165 °C since we measured with a conventional SAXS apparatus.
and 131 (1.54), 040 (1.68), 133 (1.71), 202 (1.81), and 222 (2.00). We estimated (a:b:c) = (1:2.00:3.46) with a = 23.1 nm by using 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 135 °C, the SAXS profile has the peaks at q/qm = 1, 1.15, 1.53, 1.83, 2, and 2.52 with qm = 0.313 nm−1. These ratios agree with that of a doublegyroid structure. Thus, we found L-Fddd-G order−order transition with increasing temperature in B1 blend. Similar to B1, the blend samples B2 and B3 also have Fddd phase and exhibit L-Fddd-G order−order transition with increasing temperature. Figures 4 and 5 show the temperature dependence of SAXS profiles of B2 (ϕPI = 0.641) and B3 (ϕPI = 0.636), respectively. Fddd phase are found at 135−140 °C in B2 between L and G phases. B3 is also found to have Fddd phase at 145 °C between L and G phases. The Fddd regions shift toward higher temperature with decreasing ϕPI, which agrees with the tendency of the Fddd phase of the neat SI. A reflection at q = 0.302, slightly less than the principal peak position (q/qm= 1), is observed at 145 °C in Figure 5. By using eq 1, we estimated the unit cell parameters ratio of the B3 blend sample is (a:b:c) = (1:1.99:3.66). The q022/q111 and q004/q111 for the SI/hPS blend are 0.96 and 1.00, respectively, so that the 022 position of the B3 blend is splitted and locates at lower q. It should be noticed that unit cell parameters ratio of (1:2:2√3) causes the near coincidence of 004, 111, and 022 positions in SCFT calculation by Tyler et al.10 The temperature dependence of SAXS profiles of B4 (ϕPI = 0.631) is shown in Figure 6. Differing from B1 to B3, we found
Figure 5. Temperature dependence of SAXS profiles for B3 (ϕPI = 0.636). The profiles are shifted vertically to avoid overlapping.
the Fddd phase at 155−160 °C in B4, and the Fddd phase directly becomes disordered state without G phase between the C
DOI: 10.1021/acs.macromol.6b00007 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 6. Temperature dependence of SAXS profiles for B4 (ϕPI = 0.631). The profiles are shifted vertically to avoid overlapping. The peaks are smeared at 110 °C since we measured with a conventional SAXS apparatus.
Figure 7. Temperature dependence of SAXS profiles for B5 (ϕPI = 0.627). The profiles are shifted vertically to avoid overlapping. The peaks are smeared at 145 °C since we measured with a conventional SAXS apparatus.
Fddd and disordered state. While the Fddd structure exhibited in B1 to B4, the Fddd phase was not observed in B5 (ϕPI = 0.627). Figure 7 shows the temperature dependence of SAXS profiles of B5. For all measurement temperatures except 165 °C, the peaks in the SAXS profiles appeared at peak ratios of 1, 2, and 3; thus, the sample forms lamellar structure in its ordered state. At 165 °C, a single broad peak appeared, which means the disordered state. Even though there is a very small difference in the volume fraction of polyisoprene between B4 and B5, B5 did not exhibit Fddd and gyroid structure. We determined the phase boundary of the Fddd phase of SI/ hPS blends in terms of ϕPI and temperature. To compare the phase boundary of the blends with that of the neat SI, we plotted our identified phases in parameter space of χN on ϕPI, instead of temperature on ϕPI, where N is the polymerization index of SI. To convert temperature to χ, we employed the following relation by Khandpur et al.:6 χ = 71.4/T − 0.0857
Figure 8. Phase boundary of the Fddd phase of SI/hPS blends. Broken line corresponds to the phase boundary of the neat SI.15
Information on the details of the calculation). (χN)S,neat and (χN)S,blend are 13.15 and 13.20, respectively. Thus, we expect that the upper and lower limits of the Fddd region are not strongly affected by the addition of hPS. The addition of homopolymer under wet brush condition affects the curvature of the interface and causes the horizontal shift of the phase boundary. The addition of hPS shifts the Fddd phase boundary in PI-rich region toward smaller ϕPI. As calculated by Matsen,22 the amount of the horizontal shift increases with decreasing molecular weight of homopolymer. In our case, the horizontal shift by the addition of hPS corresponds to that by the change of the same amount of volume fraction of PS in the neat block copolymer. Thus, the Fddd phase region of the blends is well superimposed onto that of the neat SI.
(2)
Figure 8 shows the phase boundary of the Fddd phase of SI/ hPS blends. The Fddd region of the blends extends to 0. 0.631 ≤ ϕPI ≤ 0.641 and 26.6 < χN < 30.7. Compared to the Fddd region of the blends with the Fddd region of the neat SI, both regions are almost identical. Hashimoto et al. investigated the effects of the addition of homopolymer on the order−disorder transition point of block copolymer/homopolymer.19−21 They calculated the mean-field spinodal point (χN)S of block copolymer/homopolymer by using random phase approximation (RPA).19 We calculated (χN)S of the neat blockcopolymer (χN)S,neat and the blend system (χN)S,blend with volume fraction of homopolymer ϕH = 0.04 (corresponding to B4) with RPA (see Supporting D
DOI: 10.1021/acs.macromol.6b00007 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
ethylene oxide) Triblock Copolymers. Macromolecules 2002, 35 (18), 7007−7017. (9) Epps, T. H.; Cochran, E. W.; Hardy, C. M.; Bailey, T. S.; Waletzko, R. S.; Bates, F. S. Network Phases in ABC Triblock Copolymers. Macromolecules 2004, 37 (19), 7085−7088. (10) Tyler, C. A.; Morse, D. C. Orthorhombic Fddd Network in Triblock and Diblock Copolymer Melts. Phys. Rev. Lett. 2005, 94 (20), 208302/1−208302/4. (11) Ranjan, A.; Morse, D. C. Landau theory of the orthorhombic Fddd phase. Phys. Rev. E 2006, 74 (1), 011803. (12) Matsen, M. W. Effect of Architecture on the Phase Behavior of AB-Type Block Copolymer Melts. Macromolecules 2012, 45 (4), 2161−2165. (13) Takenaka, M.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Shimizu, H.; Kim, M. I.; Hasegawa, H. Orthorhombic Fddd Network in Diblock Copolymer Melts. Macromolecules 2007, 40 (13), 4399− 4402. (14) Kim, M. I.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Hasegawa, H.; Ito, K.; Takenaka, M. Stability of the Fddd Phase in Diblock Copolymer Melts. Macromolecules 2008, 41 (20), 7667−7670. (15) Kim, M. I.; Wakada, T.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Hasegawa, H.; Ito, K.; Takenaka, M. Determination of the Fddd Phase Boundary in Polystyrene-block-polyisoprene Diblock Copolymer Melts. Macromolecules 2009, 42 (14), 5266−5271. (16) Wang, Y. C.; Matsuda, K.; Kim, M. I.; Miyoshi, A.; Akasaka, S.; Nishitsuji, S.; Saijo, K.; Hasegawa, H.; Ito, K.; Hikima, T.; Takenaka, M. Fddd Phase Boundary of Polystyrene-block-polyisoprene Diblock Copolymer Melts in the Polystyrene-Rich Region. Macromolecules 2015, 48 (7), 2211−2216. (17) Hong, K. M.; Noolandi, J. Theory of phase equilibriums in systems containing block copolymers. Macromolecules 1983, 16 (7), 1083−1093. (18) Whitmore, M. D.; Noolandi, J. Theory of phase equilibria in block copolymer-homopolymer blends. Macromolecules 1985, 18 (12), 2486−2497. (19) Tanaka, H.; Hashimoto, T. Stability limits for macro- and microphase transitions and compatibilizing effects in mixtures of A-B block polymers with corresponding homopolymers. Polym. Commun. 1988, 29 (7), 212−216. (20) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Ordered structure in mixtures of a block copolymer and homopolymers. 2. Effects of molecular weights of homopolymers. Macromolecules 1990, 23 (20), 4378−4386. (21) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Ordered structure in mixtures of a block copolymer and homopolymers. 1. Solubilization of low molecular weight homopolymers. Macromolecules 1991, 24 (1), 240−251. (22) Matsen, M. W. Phase Behavior of Block Copolymer/ Homopolymer Blends. Macromolecules 1995, 28 (17), 5765−5773.
IV. CONCLUSIONS We have induced the Fddd structure in SI diblock copolymer which does not have the Fddd phase by adding hPS. We prepared the SI/hPS blend samples in the range of 0.627 < ϕPI < 0.646 and investigated the phase boundary of Fddd phase by using SAXS and TEM. We found the Fddd phase as an equilibrium phase in the range of 0.631 ≤ ϕPI ≤ 0.641 and 26.6 < χN < 30.7, where χ and N are respectively the Flory−Huggins interaction parameter between PS and PI and polymerization index of the SI. The Fddd phase boundary in SI/hPS is similar to that in the neat SI block copolymer, indicating that the effect of the addition of the low molecular weight hPS on the Fddd phase region corresponds to that of the PS volume fraction change in the neat SI block copolymer.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00007. Calculation of (χN)S of the neat block copolymer (χN)S,neat and the blend system (χN)S,blend by using random phase approximation (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (M.T.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of RIKEN (Proposal No. 20140072 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.
■
REFERENCES
(1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics: Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41 (1), 525−557. (3) Leibler, L. Theory of Microphase Separation in Block Copolymers. Macromolecules 1980, 13 (6), 1602−1617. (4) Matsen, M. W.; Bates, F. S. Unifying Weak- and StrongSegregation Block Copolymer Theories. Macromolecules 1996, 29 (4), 1091−1098. (5) Matsen, M. W.; Schick, M. Stable and unstable phases of a diblock copolymer melt. 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. Polyisoprene-Polystyrene Diblock Copolymer Phase Diagram near the Order-Disorder Transition. Macromolecules 1995, 28 (26), 8796−8806. (7) Hajduk, D. A.; Takenouchi, H.; Hillmyer, M. A.; Bates, F. S.; Vigild, M. E.; Almdal, K. Stability of the Perforated Layer (PL) Phase in Diblock Copolymer Melts. Macromolecules 1997, 30 (13), 3788− 3795. (8) Bailey, T. S.; Hardy, C. M.; Epps, T. H.; Bates, F. S. A Noncubic Triply Periodic Network Morphology in Poly(isoprene-b-styrene-bE
DOI: 10.1021/acs.macromol.6b00007 Macromolecules XXXX, XXX, XXX−XXX
