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Interplay between Cubic and Hexagonal Phases in Block Copolymer Solutions Moon Jeong Park and Kookheon Char* School of Chemical Engineering & Nanosystems InstitutesNational Core Research Center (NCRC), Seoul National University, Seoul 151-744, Korea
Joona Bang and Timothy P. Lodge* Department of Chemical Engineering & Materials Science, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received August 2, 2004. In Final Form: November 17, 2004
The phase behavior of a symmetric styrene-isoprene (SI) diblock copolymer in a styrene-selective solvent, diethylphthalate, was investigated by in situ small-angle X-ray scattering on isotropic and shear-oriented solutions and by rheology and birefringence. A remarkable new feature in this phase diagram is the coexistence of both body-centered cubic (bcc) and hexagonally close-packed (hcp) sphere phases, in a region between close-packed spheres (cps) and hexagonally packed cylinders (hex) over the concentration range φ ≈ 0.33-0.45. By focusing on the transitions among these various ordered phases during heating and cooling cycles, we observed a strong hysteresis: supercooled cylinders persisted upon cooling. The stability of these supercooled cylinders is quite dependent on concentration, and for φ g 0.40, the supercooled cylinders do not revert to spheres even after quiescent annealing for 1 month. The spontaneous formation of spheres due to the dissociation of cylinders is kinetically hindered in this case, and the system is apparently not amenable to any pretransitional fluctuations of cylinders prior to the cylinder-to-sphere transition. This contrasts with the case of cylinders transforming to spheres upon heating in the melt. The application of large amplitude shear to the supercooled cylinders is effective in restoring the equilibrium sphere phases.
Introduction Block copolymers are known to exhibit various microdomain structures. The principal factors that control the morphological behavior of diblock copolymer melts are well understood.1,2 Systematic experimental investigations have delineated a common set of equilibrium phases for a variety of chemically different diblock copolymers,3 and the identification and location of these phases are in remarkable agreement with theoretical predictions.4-8 The transition from one type of ordered microdomain structure to another is known as an order-order transition (OOT). Since χ, N, and f are the factors that determine the equilibrium phases (χ, N, and f are the interaction parameter, the degree of polymerization, and the composition, respectively) and χ depends on temperature,9-12 * Authors to whom correspondence should be addressed. E-mail:
[email protected] (T.P.L.);
[email protected] (K.C.). (1) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (2) Fredrickson, G. H.; Bates, F. S. Annu. Rev. Mater. Sci. 1996, 26, 501. (3) Bates, F. S.; Schulz, M. F.; Khandpur, A. K.; Fo¨rster, S.; Rosedale, J. H. J. Chem. Soc., Faraday Discuss. 1994, 98, 7. (4) Leibler, L. Macromolecules 1980, 13, 1602. (5) Fredrickson, G. H.; Helfand, E. J. Chem. Phys. 1987, 87, 697. (6) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. (7) Matsen, M. W.; Schick, M. Phys. Rev. Lett. 1994, 72, 2660. (8) Matsen, M. W.; Bates, F. S. J. Chem. Phys. 1997, 106, 2436. (9) Helfand, E.; Wasserman, Z. R. In Developments in Block Copolymers; Goodman, I., Ed.; Applied Science: London, 1982; Vol. 1. (10) Hashimoto, T.; Shibayama, M.; Fujimura, M.; Kawai, H. In Block Copolymers, Science and Technology; Meier, D. J., Ed.; Harwood Academic Publishers: London, 1983. (11) Vavasour, J. D.; Whitmore, M. D. Macromolecules 1992, 25, 5477. (12) Laradji, M.; Shi, A.-C.; Noolandi, J.; Desai, R. C. Macromolecules 1997, 30, 3242.
an OOT can occur by changing the temperature.13-34 Reported OOTs include those between lamellae and (13) Sakurai, S.; Momii, T.; Taie, K.; Shibayama, M.; Nomura, S. Macromolecules 1993, 26, 485. (14) Sakurai, S.; Umeda, H.; Taie, K.; Nomura, S. J. Chem. Phys. 1996, 105, 8902. (15) Hajduk, D. A.; Gruner, S. M.; Rangarajan, P.; Register, R. A.; Fetters, L. J.; Honeker, C.; Albalak, R. J.; Thomas, E. L. Macromolecules 1994, 27, 490. (16) Floudas, G.; Ulrich, R.; Wiesner, U. J. Chem. Phys. 1999, 110, 652. (17) Sakurai, S.; Kawada, H.; Hashimoto, T.; Fetters, L. J. Macromolecules 1993, 26, 5796. (18) Koppi, K. A.; Tirrell, M.; Bates, F. S.; Almdal, K.; Mortensen, K. J. Rheol. 1994, 38, 999. (19) Ryu, C. Y.; Lee, M. S.; Hajduk, D. A.; Lodge, T. P. J. Polym. Sci., Polym. Phys. Ed. 1997, 35, 2811. (20) Ryu, C. Y.; Vigild, M. E.; Lodge, T. P. Phys. Rev. Lett. 1998, 81, 5354. (21) Ryu C. Y.; Lodge, T. P. Macromolecules 1999, 32, 7190. (22) Kimishima, K.; Koga, T.; Hashimoto, T. Macromolecules 2000, 33, 968. (23) Lee, H. H.; Jeong, W.-Y.; Kim, J. K.; Ihn, K. J.; Kornfield, J. A.; Wang, Z.-G.; Qi, S. Macromolecules 2002, 35, 785. (24) Kim, J. K.; Lee, H. H.; Gu, Q.-J.; Chang, T.; Jeong, Y. H. Macromolecules 1998, 31, 4045. (25) Sota, N.; Sakamoto, N.; Saijo, K.; Hashimoto, T. Macromolecules 2003, 36, 4534. (26) Modi, M. A.; Krishnamoorti, R.; Tse, M. F.; Wang, H.-C. Macromolecules 1999, 32, 4088. (27) Hajduk, D. A.; Takenouchi, H.; Hillmyer, M. A.; Bates, F. S.; Vigild, M. E.; Almdal, K. Macromolecules 1997, 30, 3788. (28) Hajduk, D. A.; Ho, R. M.; Hillmyer, M. A.; Bates, F. S.; Almdal, K. J. Phys. Chem. B 1998, 102, 1356. (29) Sakurai, S.; Umeda, H.; Furukawa, C.; Irie, H.; Nomura, S.; Lee, H. H.; Kim, J. K. J. Chem. Phys. 1998, 108, 4333. (30) Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J.; Mai, S.-M.; Booth, C. Phys. Chem. Chem. Phys. 1999, 1, 2097. (31) Schulz, M. F.; Bates, F. S.; Almdal, K.; Mortensen, K. Phys. Rev. Lett. 1994, 73, 86. (32) Fo¨rster, S.; Khandpur, A. K.; Zhao, J.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W. Macromolecules 1994, 27, 6922.
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hexagonal cylinders (hex),13-16 between hex and spheres on a body-centered cubic lattice (bcc),17-26 between lamellae and gyroid,27-30 and between gyroid and hex.16,31-34 The kinetics and pathways of such OOTs have been considered by a number of theoretical models.19-21,35-37 The hex f bcc transition of asymmetric diblock copolymers has been studied experimentally for poly(ethylene propylene)-poly(ethyl ethylene) (PEP-PEE),18 di- and triblock copolymers of styrene and isoprene (SI and SIS),17,19-25 and di- and triblock copolymers of styrene and ethylene-butene-1 (SEB and SEBS)26 using smallangle X-ray scattering (SAXS) and/or transmission electron microscopy (TEM). At higher temperatures, the bcc phase was observed, while the hex phase emerged when the temperature was lowered, and this transition was consistently found to be thermally reversible. Previous experimental studies on the hex f bcc transition have also been performed on shear-oriented samples. Recent experiments by Ryu et al.19-21 and Kim et al.23,38 indicate that the hex-to-bcc transition occurs epitaxially and quite rapidly for the case of shear-aligned cylinders, which was also reported in earlier studies by Sakurai et al.17 and Koppi et al.18 The epitaxial nature was confirmed by scattering, and the rapid kinetics was inferred from isochronal viscoelastic measurements. The recent studies have also suggested that the transition is mediated by the presence of undulating cylinders, which lead to the epitaxial growth of bcc spheres. All the systems examined showed the same epitaxial relationship: the cylinder axis coincides with the [111] direction of the bcc lattice, and d100,hex ≈ d110,bcc. If a small molecule additive is employed, the resulting phase behavior is destined to be more complex, but potentially more interesting and valuable. For instance, the dramatic changes in block copolymer phase behavior relative to the melt state have been recently documented for six SI diblock copolymers in three polystyrene (PS)selective solvents (the dialkyl phthalates dimethyl phthalate (DMP), diethyl phthalate (DEP), and dibutyl phthalate (DBP)), one neutral solvent (dioctyl phthalate (DOP)), and one polyisoprene (PI)-selective solvent (tetradecane).39-41 The dialkyl phthalates are common plasticizers that allow the block selectivity to be tuned by modest changes in the chemical structure of the solvent. Several of the OOTs from cylinders to bcc spheres in block copolymer solutions have been reported using SAXS, small-angle neutron scattering (SANS), rheology, and/or birefringence measurements.39-44 Sakurai et al.42 reported the thermoreversible OOT between the bcc phase and hex for PS-PI/DOP solutions with different polymer concentrations. Alexandridis et al.43,44 established the binary (33) Khandpur, A. K.; Fo¨rster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W.; Almdal, K.; Mortensen, K. Macromolecules 1995, 28, 8796. (34) Vigild, M. E.; Almdal, K.; Mortensen, K.; Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J. Macromolecules 1998, 31, 5702. (35) Qi, S.; Wang, Z.-G. Phys. Rev. Lett. 1996, 76, 1679. (36) Qi, S.; Wang, Z.-G. Phys. Rev. E 1997, 55, 1682. (37) Laradji, M.; Shi, A. C.; Desai, R. C.; Noolandi, J. Phys. Rev. Lett. 1997, 78, 2577. (38) Kim, J. K.; Lee, H. H.; Ree, M.; Lee, K.-B.; Park, Y. Macromol. Chem. Phys. 1998, 199, 641. (39) Hanley, K. J.; Lodge, T. P.; Huang, C.-I. Macromolecules 2000, 33, 5918. (40) Lodge, T. P.; Pudil, B.; Hanley, K. J. Macromolecules 2002, 35, 4707. (41) Lodge, T. P.; Hanley, K. J.; Pudil, B.; Alahapperuma, V. Macromolecules 2003, 36, 816. (42) Sakurai, S.; Hashimoto, T.; Fetters, L. J. Macromolecules 1996, 29, 740. (43) Alexandridis, P. Macromolecules 1998, 31, 6935. (44) Svensson, M.; Alexandridis, P.; Linse, P. Macromolecules 1999, 32, 637.
Park et al.
concentration-temperature phase diagram and the microstructure of a poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) block copolymer in formamide. A thermoreversible transition from a micellar cubic phase to hex was observed, and it is that predicted by the calculated binary concentration-temperature phase diagram. One clear distinction of the cylinder-to-sphere transition in block copolymer solutions from that of block copolymer melts is that the cylinder f sphere transition can occur in solution by cooling the sample. In contrast, for asymmetric block copolymer melts, the cylinder f sphere transition is observed upon heating the sample. In this case, the domain spacing of the cylinders decreases, the degree of block segregation decreases, and the thermal fluctuations increase as the temperature is increased, causing the cylinders to split into spheres. However, for block copolymer solutions showing the cylinder f sphere transition upon cooling, the solvent fraction in the core phase decreases appreciably with decreasing temperature, which is accompanied by an increase in the cylinder radius and the aggregation number per unit length.45 As a result, the transformation of the cylinder microphase to spheres may be more difficult. However, the thermoreversiblilty of the transition between cylinders and spheres for block copolymer solutions has not been thoroughly documented. We report here a detailed study of the long-lived transient or metastable structures during the OOT from cylinders to spheres in block copolymer solutions. Most recently, we investigated the epitaxial transitions associated with the sphere f cylinder transition in a particular SI/DEP block copolymer solution using in situ SAXS.46 These measurements suggested that the pathway from shear-aligned spherically ordered close-packed (facecentered cubic (fcc) or hexagonally close-packed (hcp)) microstructures to hex involved an intermediate coexistence of bcc and hcp. In the reverse case, we thus expect the transition from hex f bcc/hcp if the transitions are truly thermoreversible. In this study, we focus on the same SI block copolymer in DEP, but over a range of concentrations, which exhibit both cylindrical and spherical morphologies over a narrow range of temperatures. Rheological experiments were performed to map out TOOT. These measurements provide a direct method to probe the in situ temporal evolution of structural features related to the cylinder f sphere transition. SAXS data were also collected in a modified shear cell with the beam directed along the vorticity direction, and the epitaxial relationships in such transitions are described. Experimental Section Materials. A polystyrene-block-polyisoprene diblock copolymer was synthesized by standard anionic polymerization procedures.39 The block molecular weights, MPS ) 15 200 g/mol and MPI ) 15 400 g/mol, were determined by a combination of sizeexclusion chromatography and NMR, and the sample is designated SI(15-15). The polydispersity index is 1.02. The slightly PS-selective solvent diethyl phthalate (DEP) was obtained from Aldrich, and the block copolymer solutions (in units of vol %) were prepared using CH2Cl2 as a cosolvent; the CH2Cl2 was removed under a gentle flow of nitrogen until the solution reached constant weight. Rheology. An Advanced Rheometrics Expansion System (ARES) was used to measure the dynamic storage modulus (G′) and loss modulus (G′′) of block copolymer solutions in a parallel plate fixture (25 mm diameter and a 0.5-1 mm gap). All (45) Lodge, T. P.; Bang, J.; Park, M. J.; Char, K. Phys. Rev. Lett. 2004, 92, 145501. (46) Park, M. J.; Bang, J.; Harada, T.; Char, K.; Lodge, T. P. Macromolecules 2004, 37, 9064.
Interplay between Phases in Block Copolymer Solutions measurements from 30 to 130 °C were performed in the linear viscoelastic regime with a small strain (1.0%). The temperature control was accurate to within (1 °C, and all the measurements were taken under a nitrogen atmosphere. Temperature scans were carried out at heating and cooling rates of 0.5 °C/min and at a fixed low frequency of 0.5 rad/s. This protocol is well established as an effective means to locate the OOT as well as the order-disorder transition (ODT). The 36, 38, and 40% block copolymer solutions are specifically chosen to show the representative features of the cylinder f sphere transition upon cooling. Static Birefringence. Static birefringence was also used to locate the OOT and the ODT in these block copolymer solutions. Vertically polarized light from a He-Ne laser was directed through the sample and a horizontally polarized analyzer placed in front of a photodiode detector. Samples in isotropic states (disordered or cubic) do not depolarize the light with no signal detected, while anisotropic lamellar or hexagonal phases are birefringent. The solutions were confined between two glass disks and sealed with a high-temperature adhesive, and they were subject to a slow temperature increase or decrease (
