Morphology and Rheology of SIS and SEPS Triblock Copolymers in

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Morphology and Rheology of SIS and SEPS Triblock Copolymers in the Presence of a Midblock-Selective Solvent J. H. Laurer,†,§ S. A. Khan,‡ and R. J. Spontak*,†,‡ Departments of Materials Science & Engineering and Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695

M. M. Satkowski, J. T. Grothaus, and S. D. Smith* Corporate Research Division, The Procter & Gamble Company, Cincinnati, Ohio 45239

J. S. Lin Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received October 14, 1998. In Final Form: July 30, 1999 While numerous fundamental studies have sought to elucidate the effect of a parent homopolymer on the morphological characteristics and mechanical properties of microphase-ordered block copolymer blends, few comparable efforts have extended such studies to concentrated copolymer solutions in the presence of a low-molar-mass block-selective solvent. In this work, we investigate the microstructures that form in blends of a poly(styrene-block-isoprene-block-styrene) (SIS) triblock copolymer with a midblock-selective aliphatic mineral oil. To discern the influence of midblock/oil compatibility on blend morphology and properties, identical blends with a poly[styrene-block-(ethylene-alt-propylene)-block-styrene] (SEPS) copolymer, the hydrogenated variant of the SIS copolymer, have likewise been examined. The saturated midblock of the SEPS copolymer is responsible for the observed shifts in morphology stability limits and higher dynamic elastic shear moduli relative to the SIS analogue. These results reveal that the morphologies and properties of such triblock copolymer/oil blends are sensitive to the chemical/statistical nature of the copolymer midblock and may be judiciously tailored to satisfy application-specific requirements.

Introduction Significant research effort continues to be directed at predicting1-5 and characterizing6-11 the nanoscale microstructures that result upon spontaneous self-organization of block copolymers. Such broad interest in this class of materials stems from the recognized interdependence of block copolymer morphology and properties, and * To whom correspondence should be addressed. † Department of Materials Science & Engineering. ‡ Department of Chemical Engineering. § Present address: Department of Materials Science & Engineering, University of Pennsylvania, Philadelphia, PA 19104. (1) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 7641. Matsen, M. W. J. Chem. Phys. 1998, 108, 785. (2) Sens, P.; Marques, C. M.; Joanny, J.-F. Macromolecules 1996, 29, 4880. (3) Laradji, M.; Shi, A.-C.; Noolandi, J.; Desai, R. Macromolecules 1997, 30, 3242. (4) Benedicto, A. D.; O’Brien D. F. Macromolecules 1997, 30, 3395. (5) Dobrynin, A. V.; Leibler, L. Macromolecules 1997, 30, 4756. (6) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (7) Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994, 27, 4063. (8) 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. Bates, F. S.; Maurer, W. W.; Lipic, P. M.; Hillmyer, M. A.; Almdal, K.; Mortensen, K.; Fredrickson, G. H.; Lodge, T. P. Phys. Rev. Lett. 1997, 79, 849. (9) Chen, Z.-R.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Science 1997, 277, 1248. (10) Mortensen, K. Curr. Opin. Coll. Interface Sci. 1998, 3, 12. (11) Laurer, J. H.; Fung, J. C.; Sedat, J. W.; Agard, D. A.; Smith, S. D.; Samseth, J.; Mortensen, K.; Spontak, R. J Langmuir 1997, 13, 2177. Laurer, J. H.; Smith, S. D.; Samseth, J.; Mortensen, K.; Spontak, R. J. Macromolecules 1998, 31, 4975.

expedites the development of microstructured polymeric materials with tailored properties for new applications.12-15 It is well-established16 that a homogeneous block copolymer microstructure of desired interfacial curvature and dimensions can be reliably obtained by blending a block copolymer with a parent homopolymer. In similar fashion, the ability of a block copolymer to order into a periodic microstructure17-19 can be controllably modified through the addition of a relatively low-molar-mass solvent. Since a block-selective solvent is expected to be distributed more uniformly within its host microdomain and wet the resident blocks more effectively than a similarly compatible homopolymer,20 the morphologies and properties of block copolymer blends containing such a solvent may not necessarily parallel those of copolymer/homopolymer (12) Mao, G. P.; Wang, J. G.; Ober, C. K.; Brehmer, M.; O’Rourke, M. J.; Thomas, E. L. Chem. Mater. 1998, 10, 1538. (13) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (14) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557. (15) Schneider, A.; Geppert, S.; Spontak, R. J.; Gronski, W.; Finkelmann, H. Mater. Res. Soc. Symp. Proc., in press. (16) Winey, K. I.; Thomas, E. L.; Fetters, L. J. Macromolecules 1992, 25, 2645; J. Chem. Phys. 1991, 95, 9367. (17) Lodge, T. P.; Pan, C.; Jin, X.; Liu, Z.; Zhao, J.; Maurer, W. W.; Bates, F. S. J. Polym Sci. B: Polym. Phys. 1995, 33, 2289 and references therein. (18) Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J.; Ryu, C. Y.; Lodge, T. P.; Gleeson, A. J.; Pedersen, J. S. Macromolecules 1998, 31, 1188. Hanley, K. J.; Lodge, T. P. J. Polym. Sci. B: Polym. Phys. 1998, 36, 3101. (19) Alexandridis, P.; Olsson, P.; Lindman, B. Langmuir 1998, 14, 2627. Alexandridis, P.; Spontak, R. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 130. (20) Siqueira, D. F.; Nunes, S. P.; Wolf, B. A. Macromolecules 1994, 27, 234; 1994, 27, 4561.

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blends of equal composition. To ascertain if copolymer/ solvent blends behave in an analogous fashion as copolymer/homopolymer blends in terms of morphology and property development, we have examined two series of chemically similar triblock copolymers blended with a lowmolar-mass midblock-selective solvent that serves as an extender oil. The retention of periodic morphologies at relatively low oil content and the formation of a physically cross-linked micellar solution at high oil content allow this study the unique opportunity to bridge the existing gap between studies of morphological transitions in triblock copolymer/homopolymer blends21 and those of the microstructure in thermoreversible block copolymer gels.22-26 The thermodynamic incompatibility between the blocks of a block copolymer constitutes the principal driving force responsible for a copolymer to microphase-order into a wide variety of periodic microstructures differing in interfacial curvature.1,6,8,27-29 This incompatibility can be expressed in terms of χN, where χ denotes the FloryHuggins interaction parameter and N is the number of statistical units along the copolymer backbone. In the case of blends containing a block copolymer and a blockselective solvent, (in)compatibility interactions likewise exist between each block of the copolymer and the added solvent. A nonselective solvent, for instance, promotes a reduction in the value of χN for the copolymer by a factor of (1 - φ)1.6, where φ is the solvent volume fraction.17 Addition of a block-selective solvent will not only reduce the copolymer χN but may also influence the interfacial curvature of an existing microstructure.18 Thus, controlled modification of block copolymer microstructure and properties may be achieved through systematic variation of (i) the concentration of solvent molecules and (ii) the magnitude of solvent-block interactions. These variations are investigated in the present study through the complementary use of a poly(styrene-block-isoprene-blockstyrene) (SIS) triblock copolymer and its hydrogenated analog, a poly[styrene-block-(ethylene-alt-propylene)block-styrene] (SEPS) copolymer. Since the styreneincompatible solvent employed throughout this study is an aliphatic mineral oil, the EP midblock is expected a priori to exhibit greater affinity for this oil than the I midblock, suggesting that, at identical φ, a SEPS/oil blend may exhibit a different microstructure and measurably different properties relative to the comparable SIS/oil blend. If such distinguishing characteristics are observed, (21) Lee, S.-H.; Koberstein, J. T.; Quan, X.; Gancarz, I.; Wignall, G. D.; Wilson, F. C. Macromolecules 1994, 27, 3199. Kane, L.; Norman, D. A.; White, S. A.; Spontak, R. J. Mater. Res. Soc. Symp. Proc. 1997, 461, 75. Norman, D. A.; Kane, L.; White, S. A.; Smith, S. D.; Spontak, R. J. J. Mater. Sci. Lett. 1998, 17, 545. (22) Mischenko, N.; Reynders, K.; Mortensen, K.; Scherrenberg, R.; Fontaine, F.; Graulus, R.; Reynaers, H. Macromolecules 1994, 27, 2345. Reynders, K.; Mischenko, N.; Mortensen, K.; Overbergh, N.; Reynaers, H. Macromolecules 1995, 28, 8699. (23) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J.-P. Macromolecules 1996, 29, 1269. (24) Laurer, J. H.; Bukovnik, R.; Spontak, R. J. Macromolecules 1996, 29, 5760. Laurer, J. H.; Mulling, J. F.; Khan, S. A.; Spontak, R. J.; Bukovnik, R. J. Polym. Sci. B: Polym. Phys. 1998, 36, 2379. Laurer, J. H.; Mulling, J. F.; Khan, S. A.; Spontak, R. J.; Lin, J. S.; Bukovnik, R. J. Polym. Sci. B: Polym. Phys. 1998, 36, 2513. (25) Kleppinger, R.; Reynders, K.; Mischenko, N.; Overbergh, N.; Koch, M. H. J.; Mortensen, K.; Reynaers, H. Macromolecules 1997, 30, 7008. Kleppinger, R.; Mischenko, N.; Theunissen, E.; Reynaers, H.; Koch, M. H. J.; Almdal, K.; Mortensen, K. Macromolecules 1997, 30, 7012. (26) Quintana, J. R.; Diaz, E.; Katime, I. Langmuir 1998, 14, 1586. (27) Leibler, L. Macromolecules 1980, 13, 1602. (28) Fredrickson, G. H.; Helfand, E. J. Chem. Phys. 1987, 87, 697. (29) Entropic considerations may also promote microphase ordering. See, for example: Russell, T. P.; Karis, T. E.; Gallot, Y.; Mayes, A. M. Nature 1994, 368, 729.

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then tunable solvent-midblock compatibility provides yet another design parameter that can be used to tailor the copolymer microstructure and properties of these blends. Experimental Section Materials. The SIS and SEPS copolymers were synthesized via living anionic polymerization initiated by sec-butyllithium in cyclohexane at 60 °C. According to 1H NMR, the styrene mass fraction was 0.55 in each copolymer. From GPC analysis, the number-average molecular weight and polydispersity of the SIS copolymer were 260 000 and 1.05, respectively. Hydrogenation of the SIS copolymer to the SEPS copolymer, determined to be 93% complete, resulted in slight reductions in the measured molecular weight (223 000) and polydispersity (1.03) due to differences in the hydrodynamic volume of EP relative to I. The hydrogenation process converted the isoprene midblock into an EP midblock without adversely affecting the phenyl ring of the S endblocks.30 Two series of copolymer/oil blends were prepared by diluting the SIS and SEPS triblock copolymers with an aliphatic white mineral oil (Witco 380PO). According to the ASTM-D2502 testing protocol, its molecular weight was 468. Methods. Blends of each copolymer were prepared over the range 0.55 g wS g 0.05 in increments of 0.05 (wS denotes the mass fraction of styrene in the system) by solvent-casting from 5% (w/v) toluene solutions. [In close proximity to expected or identified morphological transitions, additional compositions were prepared to refine the transition boundaries.] The solutions composed of oil and copolymer in toluene were subsequently cast into Teflon molds for quiescent solvent removal under a toluenerich blanket at 25 °C over the course of 72 h and then heated to remove residual solvent.24 Electron-transparent sections for transmission electron microscopy (TEM) were obtained by microtomy in a Reichert-Jung Ultracut-S cryoultramicrotome maintained at -110 °C. Resultant sections, estimated to be ca. 70-100 nm thick, were selectively stained to enhance phase contrast according to the following protocols: (i) SEPS/oil blends were exposed for 5 min to the vapor of a 0.5% RuO4(aq) solution, and (ii) SIS/oil blends were exposed for 90 min to the vapor of a 2% OsO4(aq) solution. Note that RuO4 selectively stains the styrene-rich microdomains in the SEPS blends, while OsO4 stains the isoprene copolymer midblock in the SIS blends. Images of the sections were acquired immediately (within 24 h) on a Zeiss EM902 electron spectroscopic microscope, operated at 80 kV and electron energy loss (∆E) settings ranging from 25 to 125 eV. Specimens imaged several months after sectioning/ staining exhibited no gross evidence of microstructural transformation. Small millimeter-size pieces of several films were also subjected to rapid vitrification, by hand-plunging into liquid ethane (cooled by liquid nitrogen), for cryofracture replication. Vitrified samples were transferred under liquid nitrogen to a JEOL JFD-9000C cryofracture-replication unit, which was maintained at -170 °C and ca. 10-6 Torr. The sample was fractured with a fresh razor blade, and the resultant fracture surface was coated with Pt/C at 45° (for shadowing) and C at 90° (for stabilization) prior to removal from the chamber and subsequent dissolution of the substrate in toluene. Images were collected on the same microscope described above. Small-angle X-ray scattering (SAXS) of the SEPS blends was performed on the 10 m instrument31 at Oak Ridge National Laboratory using Cu KR radiation (λ ) 0.154 nm at 40 kV and 120 mA) and a sample-to-detector distance of 5.119 m. Twodimensional scattering patterns were collected on a 20 × 20 cm2 position-sensitive detector. The scattering intensity was stored in a 64 × 64 data array and was subsequently corrected (on a cell-wise basis) for instrument background, dark current, and detector efficiency. Higher resolution SAXS data were also acquired at Procter & Gamble using Cu KR radiation from a Rigaku RU-300 rotating anode operated at 40 kV and 40 mA with a 0.2 × 0.2 mm focal size. Two-dimensional patterns were collected using the Siemens HI-STAR wire detector and the Anton Paar HR-PHK collimation system (Graz-Strassgang, Austria). (30) Adams, J. L.; Quiram, D. J.; Graessley, W. W.; Register, R. A.; Marchand, G. R. Macromolecules 1998, 31, 201. (31) Wignall, G. D.; Lin, J. S.; Spooner, S. J. Appl. Crystallogr. 1990, 23, 241.

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Figure 1. Series of transmission electron micrographs of the neat (unmodified) SIS (a) and SEPS (c) copolymers and their aliphatic oil blends in which the styrene fraction (wS) is 0.50 (b and d, respectively). In the systems composed of the SIS copolymer, the isoprene microdomains appear electron-opaque (dark) due to selective OsO4 staining. The styrene microdomains, on the other hand, appear dark in the SEPS systems due to RuO4 staining. Instrumental smearing was minimal due to the small beam size and large sample-to-detector distance.32 In both cases, the twodimensional data were collapsed into a one-dimensional (intensity versus wave vector, q) format by integrating azimuthally over the range 0.05 e q (nm-1) e 1.00, where q ) (4π/λ) sin(θ/2) and θ is the scattering angle. The averaged data from Oak Ridge National Laboratory were further converted to absolute differential scattering cross sections through the use of precalibrated secondary standards,33 thereby yielding absolute intensities (expressed in cm-1). The dynamic mechanical properties of the two blend series were measured at 25 °C with a Rheometrics Mechanical Spectrometer RMS800, using 25 mm (for the SIS/oil blends) or 10 mm (for the SEPS/oil blends) parallel plates separated by a mean gap size of 0.80 mm, depending on the specific specimen tested. Dynamic strain sweeps were performed to identify the linear viscoelastic (LVE) regime at an oscillatory frequency (ω) of 10 rad/s over a strain (γ0) range of 0.5-150%. Blends exhibiting broad LVE regimes were tested up to 500% strain. The dynamic storage (elastic) and loss (viscous) shear moduli (G′ and G′′, respectively) were deduced from the measured shear stress (τ) according to the relationship τ ) γ0G′ sin ωt + γ0G′′ cos ωt, where t denotes time. Dynamic frequency sweeps measured moduli at ω ranging from 10-1 to 102 rad/s at 1.0% strain. In both series, blends with wS ) 0.10 were examined at ω as low as 10-2 rad/s to ensure that the frequency range was sufficiently low to discern important rheological characteristics. (32) Kane, L.; Satkowski, M. M.; Smith, S. D.; Spontak, R. J. Macromolecules 1996, 29, 8862. (33) Russell, T. P.; Lin, J. S.; Spooner, S.; Wignall, G. D. J. Appl. Crystallogr. 1988, 21, 629.

Results and Discussion Morphological Characteristics. The neat SIS and SEPS copolymers and their blends with about 10 wt % oil (wS ) 0.50) exhibit the lamellar morphology, as evidenced by the TEM images in Figure 1. [Copolymer mass fractions may be readily determined from wS/0.55. Compositions are not expressed in volume fractions due to the uncertainty in the mass densities of the constituent materials.] It is important to remember that OsO4 staining of the SIS blend series causes the isoprene blocks to appear electron opaque (dark) in TEM micrographs, whereas the styrene blocks appear dark in the SEPS blend series due to RuO4 staining. In parts b and d of Figure 1, the absence of macrophase-separated oil (unstained in both blends) confirms that the additive has been completely solubilized within the midblock microdomains. Upon comparing the solubility parameter of a typical mineral oil, ca. 14.1 MPa1/2, with those of polyisoprene (16.6 MPa1/2), poly(ethylene-alt-propylene) (16.2 MPa1/2) and polystyrene (18.6 MPa1/2), the mineral oil is clearly midblock-selective.34 Moreover, on the basis of these data, the mineral oil is anticipated a priori to be a better solvent for the EP midblock than for the I midblock. Since the oil contains no aromatic moieties, it is presumed to be excluded, for the most part, from styrene-rich microdomains at equilibrium near ambient temperature, an assertion supported (34) Mangaraj, D.; Bhatnagar, S. K.; Rath, S. B. Makromol. Chem. 1963, 67, 75.

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Figure 2. Small-angle X-ray scattering patterns in which scattering intensity is presented as a function of scattering vector (q) for lamellar SEPS/oil blends of differing compositions, identified by the value of wS in the figure. Each successive data set beyond that of the neat copolymer (wS ) 0.55) has been shifted vertically by an order of magnitude to facilitate discrimination.

by several previous copolymer/oil studies.35-37 Limited solubility of the oil within the styrene microdomains is, however, expected on the basis of prior experimental observations.24 The lamellar morphologies of Figure 1 consist of alternating endblock and midblock microdomains of comparable thickness and period. By exercising precautions to minimize specimen beam damage, the relative sizes of the styrene domains in Figure 1 are estimated to be nearly equal (ca. 20 nm) in both blends. The lamellar morphology is observed in the SEPS/oil blends over a relatively broad composition range, 0.55 e wS e 0.35. In marked contrast, only the neat SIS copolymer and the SIS/oil blend with wS ) 0.50 exhibit lamellae in the SIS/oil blend series. To obtain quantitative information on the lamellar periodicity as a function of oil content, SAXS has been performed on these SEPS/oil blends. Scattering curves obtained from blends possessing the lamellar morphology are shown in Figure 2 and are seen to exhibit reasonably strong interdomain interference, as evidenced by the existence of two reasonably well-defined intensity maxima at each composition. Shifts in peak position correspond to changes in the lamellar period (D) according to Bragg’s law, D ) 2π/q*, where q* denotes the position of the firstorder maximum. Figure 3 reveals that D initially decreases and then increases with decreasing wS. The initial reduction in D (which is beyond experimental uncertainty) is curious, since the addition of oil to the EP microdomains is anticipated to induce lamellar swelling. An explanation for this anomaly is that a small quantity of oil serves to relax the EP blocks, which are stretched along the lamellar normal, and shrink the EP-rich lamellae. If this occurs, the cross-sectional area per EP block must increase and, as a consequence, promote a comparable increase in the cross-sectional area per S block (to maintain uniform density) and a corresponding reduction in the thickness of the S lamellae. Electron micrographs of the lamellar SEPS/oil blends reveal, however, that the thickness of the S lamellae remains nearly constant (within uncertainty) irrespective of composition. These seemingly contradictory observations are resolved if (i) the S lamellae in the blend with 10 wt % oil are (35) Polizzi, S.; Stribeck, N.; Zachmann, H. G.; Bordeianu, R. Colloid Polym. Sci. 1989, 267, 281. (36) Flosenzier, L. S.; Torkelson, J. M. Macromolecules 1992, 25, 735. (37) Mischenko, N.; Reynders, K.; Koch, M. H. J.; Mortensen, K.; Pedersen, J. S.; Fontaine, F.; Graulus, R.; Reynaers, H. Macromolecules 1995, 28, 2054.

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Figure 3. Dependence of the lamellar period (D) on blend composition (wS) for the SEPS/oil blends, as discerned from SAXS. The dashed line represents a linear fit to the data beyond the initial reduction in D. Note that the abscissa is reversed (decreasing in magnitude from left to right) since wS decreases upon increasing the oil fraction in these blends.

slightly swollen due to the uptake of oil and (ii) the equilibrium solubility of oil within the S lamellae is attained in blends with at most 10 wt % oil. In SEPS/oil blends with more than 10 wt % oil, the presence of oil should therefore have no further effect on the S microdomains (assuming that all blends are prepared in identical fashion), and any measurable change in D can be attributed solely to oil-induced swelling of the EP microdomains along the lamellar normal.38 As seen by the least-squares regression displayed in Figure 3, D within this composition regime is found to increase almost linearly with decreasing wS (increasing oil content). In marked contrast, limited data acquired from the SIS/oil series (not shown) reveal that D is surprisingly insensitive to oil content in these blends. Swelling of the I-rich microdomains ultimately induces a transition in blend morphology to dispersed S-rich cylinders at different compositions in the SIS/oil and SEPS/ oil blends. As seen in Figure 4a, styrene cylinders are observed in the SIS/oil blend with wS ) 0.40, whereas swollen lamellae persist in the SEPS/oil blends down to wS ) 0.35. Another difference between these blend series, evident in Figure 4b, is that the cylindrical microdomains formed in the SEPS/oil blend at wS ) 0.30 are not nearly as well oriented within grains as those observed in the SIS/oil blend with wS ) 0.40 (Figure 4a). This difference, not expected to be related to the difference in composition between the two blends in Figure 4, becomes more pronounced in the SEPS/oil blends as wS is reduced further to wS ) 0.20 (data not shown). Endblock micellization occurs in blends containing more than 60 wt % oil (wS < 0.20) in SIS/oil blends, but more than 70 wt % oil (wS < 0.15) in SEPS/oil blends. The formation of glassy (S-rich) micelles promotes the development of a physically cross-linked midblock network, resulting in the formation of thermoplastic elastomer gels (TPEGs).22-26,36,37,39,40 An illuminating image of a TPEG clearly revealing all of its pertinent features is shown in Figure 5 for the SIS/oil blend with wS ) 0.20. Stained isoprene blocks are visible as looped midblock coronae measuring ca. 3-7 nm thick surrounding styrenic micellar cores.41 To put this thickness in perspective, the gyration diameter for an unperturbed looped I midblock is estimated to be ca. 16 nm, in which case the looped I blocks (38) Torikai, N.; Takabayashi, N.; Noda, I.; Koizumi, S.; Morii, Y.; Matsushita, Y. Macromolecules 1997, 30, 5698. (39) Ohlsson, B.; To¨rnell, B. Polym. Eng. Sci. 1996, 36, 1547. Ohlsson, B.; Hassander, H.; To¨rnell, B. Polym. Eng. Sci. 1996, 36, 501. (40) Flosenzier, L. S.; Rohlfing, J. H.; Schwark, A. M.; Torkelson, J. M. Polym. Eng. Sci. 1990, 30, 49; 1990, 30, 1180. (41) TEM images showing the coronal region of diblock copolymer micelles in a selective solvent have been previously reported. See, for example, refs 12 and 20.


Figure 4. Transmission electron micrographs of SIS/oil and SEPS/oil blends exhibiting the cylindrical morphology. Styrene cylinders appear light in the blend with wS ) 0.40 (a), whereas they appear dark (and less ordered) in the SEPS/oil blend with wS ) 0.30 (b).

comprising the coronae are contracted. One explanation for such collapsed coronae lies in the inherent incompatibility between the I midblocks and the mineral oil. This possibility is, however, doubtful since the oil is completely incorporated within the SIS copolymer (i.e., no evidence of copolymer/oil macrophase separation exists). Another consideration is that the OsO4 stain, which selectively reacts with double bonds of I to form polar osmate esters, further increases the degree of midblock/oil incompatibility, thereby promoting more extensive coronal contraction. A second interpretation in this vein is that OsO4induced cross-linking of adjacent I blocks promotes contraction of the coronal loops. In either of these cases, the OsO4 stain permits direct visualization of individual coronae at the expense of collapsing the looped I midblocks. The stained coronal shell surrounding each micelle is responsible for the micelles appearing more electronopaque than the surrounding matrix in Figure 5. The mean diameter of the core, measured directly from images such as the one in Figure 5a, is about 34 nm, which is nearly equal to twice the unperturbed gyration diameter (≈15 nm) of an S endblock. [The slight difference may, in part, reflect solubilized oil.24] Recall that the unstained matrix consists principally of mineral oil but also includes bridged midblocks. While it is certainly not possible to image individual bridged midblocks, the enlargements shown in the inset of Figure 5a and in Figure 5b suggest that some bridged midblocks have associated to form bundles that connect neighboring micelles. The bundles, after OsO4

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Figure 5. TEM images of the OsO4-stained SIS/oil blend with wS ) 0.20, revealing dispersed styrene micelles. Stained isoprene-rich regions appear dark, allowing direct visualization of a midblock-rich corona around each micelle and bundles of associated bridged midblocks in the oil-rich matrix. The inset in a is a 2× enlargement of the center of the image, and the image provided in b is a further enlargement of the same field of view to facilitate visualization of the fine features present in the micrograph.

staining, are sufficiently large and numerous to be visualized. In many instances, these bundles exhibit lateral dimensions that are comparable to the coronal width. The existence of heterogeneously bundled, or associated, midblock bridges has not been previously considered in triblock copolymer blends, let alone observed by TEM, and is consistent with other observations reported here indicating that the aliphatic oil is not as good a solvent for I as it is for EP. As in the case of the coronal loops, however, OsO4 staining may likewise be responsible for the observed degree of association among the midblock bridges. Thus, the bundles visible in Figure 5 provide direct evidence for midblock bridging in SIS/oil blends, but their morphological characteristics may not be representative of the bridged midblocks in unstained blends. Corresponding styrene micelles of the SEPS/oil blends are shown for two blend compositions in Figure 6, in which the endblocks have been stained, thereby eliminating visualization of coronae or associated bridges. These micellar cores measure about 30 nm in diameter, in good agreement with the measured core diameters in Figure 5. Significant alignment of the micelles is observed in these images, but since the imaged section is substantially

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Figure 6. Series of TEM images of micellar SEPS/oil blends. In the direct images (acquired from cryosectioned films), RuO4-stained styrene micelles appear dark at two different blend compositions (in wS): (a) 0.15 and (b) 0.10. Cryofracture-replication images of the same blends are presented in c and d, respectively. Micelles appear as small bumps in the replicas, while large elongated features are attributed to fracture at grain boundaries. The grain boundaries evident in a and b appear qualitatively similar to the branchlike features in c and d.

thicker than the micelle radius, the nearest-neighbor distances on a single plane are indiscernible. Cryofracture replicas of these blends are also included for comparison in Figure 6. Parts c and d of Figure 6 do not suffer from the same section thickness complications (i.e., superimposed features in projection) as parts a and b, since the only microstructural details that are visible in TEM images of the replicas are those that lie along the fracture plane. When a solid composed of dispersed nanoscale features is fractured, the fracture plane typically follows along the periphery (or a part thereof) of the features, rather than cross-fracturing through them. In Figure 6, images of the replicas show small spherical indentations that measure ca. 20-25 nm across. These indentations are attributed to the micelle cores, since the fracture plane is most likely unaffected by fully wetted EP coronae.20 According to the replicas displayed in this figure, the micellar core size is smaller (by 5-10 nm) than the corresponding TEM measurements from sectioned/stained specimens. Since the TEM images are obtained via projection through the entire micelle, this size discrepancy indicates that the fracture plane in each image shown in parts c and d of Figure 6 intersects most of the micelles either above or below their equators (as evidenced by several pits in each image appearing to measure closer to 30 nm in diameter). This explanation could be conclusively tested in blends consisting of a highly volatile solvent, in which case the solvent could be sublimed (etched) prior to replication.20 With the present relatively low-volatility

solvent, however, this is not possible. Also evident in these replica images are elongated, branch-like structural elements of comparable width as the micelles. These elements possess the same sizes and shapes expected for grain boundaries in these systems. Local ordering of micelles, for instance, is apparent in the direct images displayed in parts a and b of Figure 6. The boundaries separating adjacent misaligned micellar regions are expected to influence the fracture response of the blend, especially at cryogenic temperatures, and appear qualitatively similar to the anomalous features evident in the replicas. Since this is one of the first reports24 of cryofracture replicas of micellar triblock copolymer/oil blends, insufficient experimental evidence remains at the present to provide an unequivocal explanation for these curious features. It is nonetheless clear from images such as those in Figures 5 and 6, however, that the copolymer/oil blends with wS e 0.15 exhibit nanoscale micelles and that this microstructure has a dramatic effect on the local fracture behavior (replicas of the pure oil are completely structureless24). Corresponding SAXS patterns for the micellar SEPS/ oil blends are shown in Figure 7 and provide information with respect to both interdomain and intradomain interference. Interdomain scattering is responsible for the two scattering maxima at q < 0.3 nm-1 and results from interference between neighboring scattering centers (i.e., the styrene micelles). A distinct minimum separates these initial scattering peaks from a broad maximum at higher


Figure 7. SAXS profiles for three micellar SEPS/oil blends in which wS is varied from 0.15 to 0.05 (see labels). These profiles have been shifted along the intensity axis by successive orders of magnitude, relative to that of the blend with wS ) 0.15, to facilitate scrutinization.

Figure 8. Experimental morphology diagram for SIS/oil and SEPS/oil blends showing the composition ranges over which the lamellar (circles), cylindrical (triangles), and micellar (diamonds) morphologies have been observed in this study. For the SEPS/oil blends (open symbols), the variation in D (from SAXS) with respect to wS is also indicated. Morphology (not D) data from the SIS/oil blends (filled symbols) are included at the top of the figure for comparison. Broadening of the lamellar regime is evident upon midblock hydrogenation, revealing that the hydrogenated (EP) midblock of the SEPS copolymer is more incompatible with the S endblocks of the copolymer than the unsaturated (I) midblock of the SIS copolymer.

q, which arises from interference between the associated endblocks within each micelle. These scattering results are similar to those reported by Mischenko et al.37 for TPEGs composed of a poly[styrene-block-(ethylene-cobutylene)-block-styrene] (SEBS) triblock copolymer and oil. Since the first maximum is the result of interdomain interference, Bragg’s law can also be applied, as in the lamellar blends, to extract information regarding the interdomain distances. It is important to recognize that the values of D obtained in this manner correspond directly to interdomain spacings only for the lamellar (planar) morphology. These D values are displayed alongside those measured from the SEPS/oil blends exhibiting cylindrical and lamellar morphologies in Figure 8. The relative positions of the scattering peaks in Figure 7 (≈1:x2) allow assignment of either a simple cubic (sc) or a body-centered cubic (bcc) lattice to the micellar microstructure.42 The absence of higher order scattering peaks in these SAXS profiles (and clear lattice orientation in the TEM images) precludes determination of the specific cubic lattice and, consequently, the intermicellar distances from the data in Figure 7. Since Bragg plane spacings and intermicellar distances are related by a constant (in the case of hexagonal packing, this is strictly true only for the basal plane),43 (42) Bates, F. S.; Cohen, R. E.; Berney, C. V. Macromolecules 1982, 15, 589.

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apparent trends in the composition-dependent variation of D within nonlamellar regimes may be nevertheless discussed. The morphological behavior of SIS/oil and SEPS/oil blends is summarized in the experimental morphology diagram presented in Figure 8. The Bragg spacings (D) derived from SAXS for each SEPS/oil blend composition examined are also included in this figure. The dependence of D on wS within the lamellar regime has already been discussed. For cylinder-forming blends, an increase in the oil content (or, alternatively, a decrease in wS) promotes a marginal reduction in D. Since hexagonal packing is expected for block copolymer cylinders,44-46 this decrease in D does not necessarily translate to a decrease in interdomain distance. For hexagonally packed cylinders of finite length, the interdomain distance depends on cylinder length, as the basal-plane separation (equal to the lattice parameter) does not include neighboring domains above and below it. It may be concluded, however, that closer packing in the basal plane can be achieved at wS ) 0.20 relative to wS ) 0.33. Within the micellar range, reduction in wS yields an increase in D. Block copolymers typically micellize on a bcc lattice, suggesting that the observed increase corresponds to a proportional increase in micellar spacing. [It is important to note at this juncture that some solvated diblock copolymers order on a facecentered cubic (fcc) lattice.47] This trend is consistent with earlier reports23-25 of triblock copolymers in the presence of a midblock-selective solvent. Morphological differences induced by hydrogenation of the copolymer midblock are also summarized in Figure 8. Substantial broadening of the composition threshold over which lamellae are observed is the most striking result of midblock hydrogenation. This result gives rise to a dilemma, since midblock/oil compatibility is greater for the EP midblock than for the I midblock, which should allow the oil to be distributed more uniformly within the EP microdomains of the SEPS blends. Recall that interfacial curvature in a block copolymer blend reflects the efficacy of chain packing and solvent (or homopolymer) wetting along the interface. For a given blend composition, changes in interfacial curvature are expected to occur more readily in the SEPS blends due to more extensive midblock wetting than in the SIS blends. In comparable SIS blends, poor midblock/oil compatibility results in a greater population of contracted midblock loops near the microdomain interphase, in which case the midblocks are insufficiently wetted to favor a morphological transition.21,48,49 The data shown in Figure 8 therefore suggest that a consideration other than midblock/oil compatibility is responsible for the extended lamellar regime in the SEPS/oil blends. As shown50 for copolymer/homopolymer blends, widening of the lamellar regime in blends occurs upon increasing the incompatibility between the blocks of a copolymer. This can be achieved by either increasing the block lengths, (43) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: London, 1978; p 56. (44) Matsen, M. W.; Schick, M. Macromolecules 1994, 27, 7157. (45) Sakurai, S.; Hashimoto, T.; Fetters, L. J. Macromolecules 1996, 29, 740. (46) Bates, F. S.; Schultz, M. F.; Khandpur, A. K.; Fo¨rster, S.; Rosedale, J. H.; Almdal, K.; Mortensen, K. Faraday Discuss. 1994, 98, 7. (47) Hamley, I. W.; Pople, J. A.; Fairclough, J. P. A.; Terrill, N. J.; Ryan, A. J.; Booth, C.; Yu, G. E.; Diat, O.; Almdal, K.; Mortensen, K.; Vigild, M. J. Chem. Phys. 1998, 108, 6929. (48) Ni, S.; Sakamoto, N.; Hashimoto, T.; Winnik, M. A. Macromolecules 1995, 28, 8686. (49) Almdal, K.; Rosedale, J. H.; Bates, F. S.; Wignall, G. D.; Fredrickson, G. H. Phys. Rev. Lett. 1990, 65, 1112. (50) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378.

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Laurer et al.

Figure 9. Frequency spectra of the dynamic elastic and viscous shear moduli (G′, open symbols, and G′′, filled symbols, respectively) at γ0 ) 1.0% for SIS/oil blends exhibiting lamellar (wS ) 0.50, circles), cylindrical (wS ) 0.30, triangles), and micellar (wS ) 0.10, diamonds) morphologies. All three blends exhibit rheological behavior that is consistent with the presence of a three-dimensional network.

Figure 10. Dynamic elastic shear modulus (G′) as a function of strain amplitude (γ0) at ω ) 10 rad/s for SIS/oil blends exhibiting lamellar (wS ) 0.50, O), cylindrical (wS ) 0.40, 4), and micellar (wS ) 0.20, )) morphologies. The dynamic viscous (loss) modulus (G′′) is included for the micellar blend ([) to emphasize the extent of the linear viscoelastic regime that persists to relatively high γ0 for this morphology.

and hence the incompatibility χN, in copolymers of constant chemical composition (as in ref 50) or by changing the chemical characteristics of the blocks, and consequently χ, at constant N (as is done here). Thus, the broader lamellar stability regime in the SEPS/oil blends relative to that in the SIS/oil blends is deemed a consequence of greater incompatibility between S and EP, which outweighs the competing compatibility between the EP and mineral oil. To ascertain the extent to which midblock hydrogenation affects copolymer incompatibility, the solubility parameters provided earlier can be used to compute χ from χ ≈ vref∆δ2/RT, where vref is a reference molar volume, ∆δ is the difference in solubility parameters between two polymers, R is the gas constant, and T denotes absolute temperature. Under isothermal conditions, vref/ RT does not change appreciably between the S/EP and S/I pairs, in which case S/EP incompatibility is calculated to be about 44% greater than S/I incompatibility at constant N. Since interfacial tension scales as χ1/2 (in the strongsegregation limit), the interfacial tension between S and EP is estimated to be approximately 20% higher than between S and I. On the basis of midblock/oil compatibility considerations (although midblock flexibility must also be borne in mind, since EP has a lower critical entanglement molecular weight than I), the micellar coronae visualized in Figure 5 for the SIS/oil blend with wS ) 0.20 are likely to be narrower than those in the SEPS/oil blends. Moreover, the associated bridged midblock bundles evident in the OsO4-stained SIS/oil blend, if representative of the blend and not a specimen-preparation artifact, are not expected to be present in the identical SEPS/oil blend due to greater midblock/oil compatibility. On the basis of chemical considerations, the oil is expected to be more tightly bound in the midblock network of the SEPS/oil blends, thereby resulting in TPEGs with improved mechanical properties.51,52 This expectation is tested in the next section by comparing the results of dynamic rheological tests performed on both series of blends. Rheological Behavior. Composition-induced differences in blend morphology are found to have pronounced consequences on the dynamic rheology of the copolymer/ oil blends examined here. The variation of the dynamic elastic modulus (G′) with frequency (ω) at ambient temperature is presented in Figure 9 for SIS/oil blends possessing lamellar (wS ) 0.50), cylindrical (wS ) 0.40),

and micellar (wS ) 0.20) morphologies. The blends with lamellar and cylindrical morphologies clearly exhibit significantly larger G′ values relative to that of the blend possessing the micellar morphology. Between the lamellar and cylindrical blends, the former displays a marginally larger G′. In the blends represented in Figure 9, G′ is virtually independent of ω by over 3 orders of magnitude, regardless of the morphology (similar frequency responses are observed in the SEPS/oil blends, which are not included here for that reason). Moreover, G′ remains consistently larger (by a factor of 3-4) than G′′ over the entire frequency range examined. Both characteristicssG′ independent of ω and G′ > G′′ over a broad ω rangesindicate that these materials exist as 3-D networks.53-55 While the frequency spectra of G′ for the lamellar and cylindrical blends show relatively insignificant differences between the two systems, a comparison of their strain dependence in Figure 10 reveals a startling distinction: the shear modulus of the lamellar blend is very strainsensitive, decreasing more rapidly with increasing strain (γ0) than that of the cylindrical blend. Note, however, that both blends exhibit a surprisingly small linear viscoelastic regime (ca. 1% strain) in marked contrast to the micellar blend, which appears strain-independent over the experimental range of γ0 up to 100%.56 Another noteworthy feature of the data provided in Figure 10 is that G′′ for the micellar blend is smaller than G′ by about an order of magnitude, which is consistent with gel-like behavior. Within the micellar regime, G′ measured for both SIS/oil and SEPS/oil blends with wS ) 0.10 is likewise independent of ω (over 4 orders of magnitude), as evidenced by the frequency spectra presented in Figure 11. Thus, the micellar SIS/oil and SEPS/oil blends investigated here, just as their SEBS/oil analogs,24 behave as physical gels. According to Figure 11, the magnitude of G′ is consistently higher (by at least 100%, at ω ) 10-2 rad/s) for a SEPS/oil blend with wS ) 0.10 relative to the SIS/oil blend of identical composition. This observation, which cannot be explained solely on the grounds of chain statistics, supports the conclusion drawn earlier, namely, that the hydrogenated midblock in the SEPS copolymer exhibits greater compatibility with the aliphatic oil than the

(51) Yu, J. M.; Blacher, S.; Brouers, F.; L’Homme, G.; Je´roˆme, R. Macromolecules 1997, 30, 4619. (52) Watanabe, H.; Sato, T.; Osaki, K.; Yao, M.-L.; Yamagishi, A. Macromolecules 1997, 30, 5877. Sato, T.; Watanabe, H.; Osaki, K. Macromolecules 1996, 29, 6231.

(53) Prud’homme, R. K. In Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symp. Ser. 462; American Chemical Society: Washington, DC, 1991; p 18. (54) Khan, S. A.; Zoeller, N. J. J. Rheol. 1993, 37, 1225. (55) Raghavan, S. R.; Khan, S. A. J. Rheol. 1995, 39, 1311. (56) Specimen geometry restricts the minimum achievable strain. For some specimens, strains as low as 0.50% were tested. Since γ0 g 1.0% were tested for all blend compositions, results for lower γ0 are not included in Figure 10. Data not shown were found to follow the same trends apparent in this figure.


Figure 11. Dependence of the dynamic shear moduli G′ (circles) and G′′ (triangles) on ω for SEPS/oil (open symbols) and SIS/oil (filled symbols) micellar blends with wS ) 0.10. The observed invariance of G′ with respect to ω is representative of all the micellar blends investigated here and is consistent with the formation of physical gels. As in Figure 9, these data have been acquired at γ0 ) 1.0% and 25 °C.

Langmuir, Vol. 15, No. 23, 1999 7955

solvent at equilibrium. If this is the case, then G′ evaluated at Cmin corresponds to a Rouse modulus in the semidilute regime.23 Within this concentration regime, the plateau h n denotes the modulus (G) is given by CRT/Mn, where M number-average molecular weight. Values of G in this regime at 25 °C are calculated to be (0.95 ( 0.10) × 105C dyn/cm2 for the SIS/oil blends and (1.11 ( 0.11) × 105C dyn/cm2 for the SEPS/oil blends (the range in G reflects the (10% uncertainty in Mn from GPC measurements). Lines possessing these slopes and passing through the data at Cmin are included for comparison in Figure 12. Note that these lines do not differ significantly from each other and intersect three data points (from both blend series), suggesting that a concentration regime exists in this series in which midblock/solvent compatibility has little impact on G′. Conclusions

Figure 12. G′ presented as a function of copolymer concentration (C) for SEPS/oil (open symbols) and SIS/oil (filled symbols) blends exhibiting micellar (circles) and cylindrical (triangles) morphologies. The solid lines represent power-law fits to the data, whereas the dashed and dotted lines correspond to the linear relationship between G′ and C for each blend series in the Rouse semidilute regime (see the text).

unsaturated midblock in the SIS copolymer, thus facilitating the formation of entangled loops and midblock bridges.57 As seen in Figure 12, the difference in G′ between the two blend series decreases as the copolymer concentration (C, expressed in g copolymer/cm3 solution) is reduced, indicating that midblock/oil compatibility becomes less significant at high copolymer dilution. The solid lines in this figure are power-law functions of the form G′ ∼ Cβ that are fitted to the data. Regressed values of β are 2.68 (SEPS/oil) and 1.77 (SIS/oil). Previous results24 obtained from a series of TPEGs composed of a SEBS copolymer and the same mineral oil, but compressionmolded at 180 °C, exhibit similar scaling behavior. Over this relatively broad concentration range, the nonlinear dependence of G′ on C (Cβ, where β > 1) appears to be consistent with the “flower” micelle model proposed by Semenov et al.58 According to this picture, entanglements between adjacent micellar coronae, as well as the existence of bridged midblocks, contribute to the measured elastic modulus. The value of G′ at the lowest copolymer concentration in each blend series (Cmin, equal to 0.08 for the SEPS/oil series and 0.16 for the SIS/oil series) clearly deviates from the highly correlated scaling relationships discussed above. A possible explanation for such deviation (which is beyond experimental uncertainty) at these low concentrations is that the contribution of entangled midblock loops to gel elasticity becomes negligible so that G′ depends only on the population of bridged midblocks in a good (57) Yu, J.; Je´roˆme, R.; Teyssie´, P. Polymer 1997, 38, 347. (58) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Macromolecules 1995, 28, 1066.

The effect of midblock hydrogenation on morphology and mechanical properties in blends of triblock copolymers with an aliphatic mineral oil has been examined using a combination of TEM, SAXS and dynamic rheology. Both SIS and SEPS systems exhibit the principal block copolymer morphologies of lamellae, cylinders, and micelles as the concentration of copolymer in the blends is reduced. Evidence of EP midblock relaxation, followed by substantial swelling, in EP-rich lamellae is observed in the SEPS/oil blends upon incorporation of oil. The composition regime over which the lamellar morphology is stable depends on midblock hydrogenation: the SEPS/ oil blends possess a relatively broad lamellar regime relative to that of comparable SIS/oil blends due to greater S/EP incompatibility. In the SIS/oil blends, endblock-rich micelles are observed, by direct TEM, to possess a midblock-rich corona consisting of looped midblocks, as well as bundled bridges consisting of associated midblocks that connect neighboring micelles. Dynamic rheological tests confirm that these blends exhibit strain and frequency responses that are consistent with shear-thinning systems and, at high copolymer dilution, the presence of gel networks. Blends possessing the micellar morphology exhibit higher dynamic elastic moduli at large strains compared to blends with either lamellar or cylindrical morphologies, and this trend is more pronounced in the SEPS/oil blends, suggesting that the hydrogenated and more flexible EP midblocks form a more effective network than their parent isoprene midblocks in the presence of an aliphatic oil. In very dilute SEPS/oil blends, the dynamic elastic modulus is comparable in magnitude to the Rouse modulus and, in light of previous analysis,23,58 is presumed to reflect contributions solely from bridged midblocks. These results reveal that microdomain ordering and property evolution in triblock copolymer/oil blends extending over a very broad composition range depend sensitively on the competition between midblock/oil and midblock/endblock compatibility. Acknowledgment. This work was supported by the Shell Development Co., the Southeastern Universities Research Association, and, in part, the Division of Materials Science, U.S. Department of Energy under Contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation. We also thank M. Rubenstein for valuable discussions and one of the reviewers for insightful comments. LA981441N

Morphology and Rheology of SIS and SEPS Triblock Copolymers in

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