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Mesogel Networks via Selective Midblock Swelling of Lamellar Triblock Copolymers Megan R. King,† Scott A. White,‡ Steven D. Smith,§ and Richard J. Spontak*,†,| Departments of Materials Science & Engineering and Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695, Polymer Science & Technology Department, Becton Dickinson Technologies, Research Triangle Park, North Carolina 27709, Corporate Research Division, The Procter & Gamble Company, Cincinnati, Ohio 45239, and Institut fu¨ r Makromolekulare Chemie, Albert Ludwigs Universita¨ t Freiburg, D-79104 Freiburg, Germany Received November 24, 1998. In Final Form: September 7, 1999 A lamellar ABA triblock copolymer brought to equilibrium in the presence of a B-compatible solvent generally swells or transforms into an A-dispersed (cylindrical or micellar) morphology, depending on solvent content. If the A blocks of the copolymer are glassy, they serve as physical cross-links and stabilize a gel network in which A microdomains are linked through a combination of connected B bridges and entangled B loops. An alternate route by which to introduce solvent into a triblock copolymer, as well as retain the local molecular anisotropy (and bridge population) of the initial lamellar morphology, is to selectively swell the B block and, at sufficiently high solvent concentrations, produce a mesogel. In this work, we describe the formation of mesogels from two chemically related triblock copolymers and employ dynamic mechanical analysis and transmission electron microscopy to examine the features of the resultant nonequilibrium materials.
* To whom correspondence should be addressed at North Carolina State University (
[email protected]). † North Carolina State University. ‡ Becton Dickinson Technologies. § The Procter & Gamble Co. | Albert Ludwigs Universita ¨ t Freiburg.
vature of the blend morphology, and increases as the homopolymer molecular weight is reduced. In a selective solvent, block copolymers likewise selforganize into a wealth of nanoscale morphologies to minimize the system free energy.11-14 Resultant microdomains formed by AB diblock copolymer molecules are coupled through entanglement of the solvent-compatible blocks, whereas those produced by ABA triblock copolymer molecules can be linked through both midblock bridges and entangled loops if the solvent is midblock compatible.15 We now focus our attention on this specific case. Consider a lamellar ABA copolymer to which a B-selective solvent is added. The fraction of bridged midblocks is less than 0.50 in the neat (unmodified) material (predicted16 to be about 0.44 for highly segregated styrene-diene copolymers), but decreases as the solvent concentration is increased.17 Experimental studies have shown that, upon equilibration in the presence of a midblock-selective solvent, the lamellae of ABA copolymers transform into cylinders and eventually micelles as the solvent content is increased. Over a broad solvent concentration range, the network formed by the copolymer molecules remains sufficiently intact for such blends to behave as physical (thermoreversible) gels, as revealed by rheological measurements.14,15,18-20 Since, at high solvent concentrations, these gels typically consist of A-rich micelles and
(1) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (2) Binder, K. Adv. Polym. Sci. 1994, 112, 181. (3) Templin, M.; Franck, A.; Du Chesne, A.; Leist, H.; Zhang, Y.; Ulrich, R.; Scha¨dler, V.; Wiesner, U. Science 1997, 278, 1795. (4) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557. (5) Hashimoto, T.; Koizumi, S.; Hasegawa, H. Macromolecules 1994, 27, 1562. (6) Koneripalli, N.; Levicky, R.; Bates, F. S.; Matsen, M. W.; Satija, S. K.; Ankner, J.; Kaiser, H. Macromolecules 1998, 31, 3498. (7) Spontak, R. J.; Fung, J. C.; Braunfeld, M. B.; Sedat, J. W.; Agard, D. A.; Kane, L.; Smith, S. D.; Satkowski, M. M.; Ashraf, A.; Hajduk, D. A.; Gruner, S. M. Macromolecules 1996, 29, 4494. (8) Sakurai, S.; Irie, H.; Umeda, H.; Nomura, S.; Lee, H. H.; Kim, J. K. Macromolecules 1998, 31, 336. (9) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378. Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 7893. (10) Lee, S.-H.; Koberstein, J. T.; Quan, X.; Gancarz, I.; Wignall, G. D.; Wilson, F. C. Macromolecules 1994, 27, 3199.
(11) 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. (12) Hecht, E.; Mortensen, K.; Hoffmann, H. Macromolecules 1995, 28, 5465. Mortensen, K.; Talmon, Y.; Gao, B.; Kops, J. Macromolecules 1997, 30, 6764. (13) Alexandridis, P.; Olsson, P.; Lindman, B. Langmuir 1998, 14, 2627. (14) Laurer, J. H.; Khan, S. A.; Spontak, R. J.; Satkowski, M. M.; Grothaus, J. T.; Smith, S. D.; Lin, J. S. Langmuir 1999, 15, 7947. (15) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J.-P. Macromolecules 1996, 29, 1269. (16) Matsen, M. W.; Schick, M. Macromolecules 1994, 27, 187. See also Li, B. Q.; Ruckenstein, E. Macromol. Theory Simul. 1998, 7, 333. (17) Nguyenmisra, M.; Mattice, W. L. Macromolecules 1995, 28, 1444. (18) Yu, J. M.; Blacher, S.; Brouers, F.; L′Homme, G.; Je´roˆme, R. Macromolecules 1997, 30, 4619. Yu, J. M.; Je´roˆme, R.; Overbergh, N.; Hammond, P. Macromol. Chem. Phys. 1997, 198, 3719. (19) Mortensen, K.; Almdal, K.; Kleppinger, R.; Mischenko, N.; Reynaers, H. Physica B 1997, 241, 1025.
Block copolymers remain a fascinating class of polymeric materials due to their ability to spontaneously selforganize into periodic microstructures in the same fashion as low-molar-mass surfactants.1,2 For this reason, these materials constitute ideal candidates for a variety of emerging nanoscale technologies.3,4 The predominant morphological features (e.g., interfacial curvature and characteristic dimensions) of block copolymers are tunable through tailored chemical synthesis or, alternatively, through the addition of block-selective additives, such as a parent homopolymer, solvent, or cosurfactant. While a miscible cosurfactant (i.e., a second block copolymer of dissimilar molecular weight and/or composition) is constrained such that its junction locates at the interphase shared by the two copolymer species,5-8 incorporation of a homopolymer into an ordered block copolymer is complicated by the degree to which the homopolymer molecules wet the host blocks of the copolymer.9,10 The extent to which homopolymer-induced block wetting occurs dictates the interfacial chain packing, and, hence, cur-
10.1021/la9816407 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/09/1999
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Figure 1. Schematic diagram of ABA triblock copolymer isogels (a) and mesogels (b) in a B-selective solvent (b). At sufficiently high solvent concentration, copolymer molecules in (a) micellize as a result of equilibration. In this case, the B blocks form bridges and coronal loops, which can, depending on concentration, connect neighboring micelles. Solvent is introduced into a lamellar mesophase in (b) and swells the B lamellae to an equilibrium capacity. If the A microdomains are solid and insoluble, the copolymer molecules retain their initial degree of midblock bridging, thereby resulting in more highly connected (and stiffer) gel networks, relative to their isogel analogs, at high solvent content.
exhibit isotropic properties (unless shear-oriented), we hereafter refer to such equilibrated gels as isogels. A crucial consideration in conventional investigations of block copolymer blends (with a homopolymer, solvent or second copolymer) is the extent to which equilibrium is attained. Equilibration in block copolymer systems can be confounded if any of the copolymer blocks becomes glassy or semicrystalline at a temperature above ambient, thereby favoring potentially long-lived metastable morphologies. This complication can, however, be exploited to produce ABA triblock copolymer/solvent networks that retain the molecular anisotropy and equilibrium bridge population associated with the lamellar morphology of the neat copolymer. As depicted in Figure 1, blend preparation in this case relies on selectively swelling the B microdomains of the copolymer in the lamellar mesophase without adversely affecting the solid, insoluble A microdomains, which serve as physical cross-link sites. At high solvent concentrations, such networked blends are expected to exhibit rheological properties similar to those of isogels and, according to Zhulina and Halperin,21 can be considered mesogels. While midblock-selective swelling of ABA copolymers is well-established, prior efforts22-24 to produce highly swollen mesogels with lamellar A microdomains have been only partially successful or have not sought to elucidate microstructureproperty relationships. In the present work, we examine (20) 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. (21) Zhulina, E. B.; Halperin, A. Macromolecules 1992, 25, 5730. (22) Skoulios, A.; Tsouladze, G.; Franta, E. J. Polym. Sci. 1963, 64, 507. Franta, E.; Skoulios, A.; Rempp, P.; Benoit, H. Makromol. Chem. 1965, 87, 271. (23) Folkes, M. J.; Keller, A.; Odell, J. A. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 833, 847. (24) Chen, C. M.; MacKintosh, F. C.; Williams, D. R. M. Langmuir 1995, 11, 2471.
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the morphological and property development of highly swollen mesogels and compare these results to those of corresponding isogels. A poly(styrene-b-isoprene-b-styrene) (SIS) copolymer and its hydrogenated derivative, a poly[styrene-b-(ethylene-alt-propylene)-b-styrene] (SEPS) copolymer, were synthesized via living anionic polymerization initiated by sec-butyllithium in cyclohexane at 60 °C. The S mass fraction (from 1H NMR) was 0.55 in each copolymer. According to gel permeation chromatography (GPC) using polystyrene standards, the number-average molecular weight (M h n) and polydispersity of the SIS copolymer were 260 000 and 1.05, respectively. Upon 93% hydrogenation of the SIS copolymer to the SEPS copolymer, slight reductions in both M h n (223 000) and polydispersity (1.03) were measured due to differences in the hydrodynamic volume of EP relative to I. The midblock-selective solvent employed here was an aliphatic white mineral oil (Witco 380PO), with a molecular weight of 468. Mesogels were prepared by first dissolving the neat copolymers in toluene (SIS) or cyclohexane (SEPS) at 5% (w/v) and then casting the solutions into Teflon molds. Upon slow solvent evaporation over the course of 3 weeks, the resultant films were dried under vacuum for 6 h from 25 to 90 °C and 30 min at 102 °C to remove residual solvent and refine the lamellar microstructure. Small pieces of each film were subsequently exposed to oil for predetermined time intervals, and the resultant mass uptake at each time was measured by gravimetric analysis. To ensure statistical meaningfulness, these tests were repeated for eight different specimens at each time interval. Isogel films were prepared in similar fashion with two exceptions: (i) the midblock-selective oil was added to the initial solutions prior to casting, and (ii) the solutions were dried for 3 days prior to heat treatment under vacuum. The properties of the neat and solvated films were investigated by small-amplitude oscillatory tensile tests performed at 25 °C on a Rheometrics RSA-II or a Rheometrics DMTA-IV. Measurements were conducted at strain levels ranging from 0.05 to 1.0% (confirmed to lie within the linear viscoelastic regime), depending on oil content and the type of copolymer under examination. Swollen films were sectioned at -110 °C in a ReichertJung Ultracut S cryoultramicrotome and immediately subjected to the vapor of either 0.5% RuO4(aq) (which stains the phenyl rings of the S blocks in SEPS/oil mesogels) for 7 min or 2.0% OsO4(aq) (which stains the unsaturated bonds of the I blocks in SIS/oil mesogels) for 90 min. Transmission electron microscopy (TEM) images of stained sections were acquired on a Zeiss EM902 electron spectroscopic microscope, operated at 80 kV and energy loss settings of 25-75 eV. Figure 2 shows the time-dependent change in oil content upon swelling the SIS and SEPS copolymers. In both cases, an equilibrium oil concentration is achieved: after about 8 h in the SIS/oil series and 150 h in the SEPS/oil series. Due to the chemical similarity between the EP midblock of the SEPS copolymer and the saturated aliphatic oil, the maximum oil uptake in the SEPS/oil system (154% mass increase, or 60 wt % oil) is much greater than that in the SIS/oil system (34% mass increase, or 25 wt % oil). For this reason, only the SEPS/oil blends at high solvent concentrations (>50 wt % oil) can be considered true mesogels. An interesting feature of Figure 2 is that the equilibrium solubility in the SIS/oil blend corresponds closely to the oil content at which the cylindrical morphology first appears14 in the analogous isogels. In marked contrast, the maximum oil solubility in the SEPS/oil blend
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Figure 2. Time dependence of solvent content in SEPS (O) and SIS (b) copolymers exposed to a midblock-compatible oil at 25 °C. The dashed horizontal line denotes the oil content at which cylinders are first observed in SIS/oil isogels, whereas the solid horizontal line identifies the concentration at which solvent-induced lamellar contraction occurs14 in SEPS/oil isogels. The arrow signifies a reproducible kink in the data, and the crosshatched regions display the ranges over which SEPS/oil isogels undergo lamellar f cylinder (L f C) and cylinder f micelle (C f M) transitions.
series is similar in magnitude to that signifying the cylinder f micelle transition in the isogel series. Another feature of Figure 2 that warrants mention is the kink evident (upon repeated testing) in the oil content curve for the SEPS/oil system after about 5 h of swelling. Existence of such a kink suggests that the mechanism governing oil dissolution within the copolymer matrix changes. It is intriguing that the oil concentration at which this kink occurs (21 wt % oil) is almost identical to that at which the microdomain period decreases14 by 8% (at 18 wt % oil) in the isogel series. Consistent with a change in swelling response, Chen et al.24 report a crossover in the time (t) dependence of relaxation associated with the marching solvent penetration front from t1/6 at short times to t1/2 at long times. The frequency (ω) dependence of the tensile storage modulus (E′) is presented in Figure 3a for the neat SEPS copolymer, as well as for four copolymer/oil blends: two (one produced by swelling and the other by solutioncasting) with 28 wt % oil and two with 60 wt % oil. Complementary data from the SIS copolymer are not included, since this copolymer is incapable of forming a highly swollen mesogel in the presence of the oil employed here. For the neat copolymer, E′ is nearly independent of ω, as is expected for a solidlike (or networked) material.25 As seen in Figure 3a, similar frequency spectra are likewise characteristic of the SEPS/oil blends, irrespective of the method by which the blends are prepared, confirming that all the equilibrium (isogel) and nonequilibrium (mesogel) blends examined here exhibit physical gel behavior. An interesting feature of Figure 3a is that, at constant composition, E′ of the mesogels is higher (in some cases, by more than an order of magnitude) relative to that of the isogels. For comparison, values of E′ evaluated at the lowest frequency measured (ω ) 0.1 rad/s) are displayed as a function of gel composition for several isogels and mesogels in Figure 3b. In both gel series, an increase in solvent fraction promotes a substantial reduction in E′, which decreases by more than an order of magnitude for (25) Prud’homme, R. K. ACS Symp. Ser. 1991, No. 462, 18.
Figure 3. Variation of the dynamic tensile storage modulus (E′) with (a) frequency and (b) gel composition at 25 °C. Displayed in (a) are the frequency spectra of the neat SEPS copolymer (O) and SEPS/oil mesogels (open symbols) and isogels (filled symbols) with 28 (triangles) and 60 (diamonds) wt % oil. The solid lines in (a) are power-law fits to the data. In (b), E′ measured at ω ) 0.1 rad/s for SEPS/oil mesogels (0) and isogels (b) is presented as a function of oil content. The solid lines in (b) connect the data.
the mesogels and 2 orders of magnitude for the isogels, at oil concentrations up to 60 wt %. It is evident from this figure that (i) the mesogels consistently possess a higher E′ than do the isogels and (ii) this disparity becomes increasingly more pronounced as the oil content increases. Recall that an increase in oil concentration in the isogel series is accompanied by a decrease in midblock bridging and the eventual transformation of the S lamellae to cylinders over the composition range examined. Independent of solvent concentration, mesogels are presumed to possess comparable midblock bridging as the parent SEPS copolymer (in the absence of solvent), as well as retain the lamellar morphology to higher solvent dilution. The reduction in E′ with increasing oil content evident in Figure 3b is therefore attributed to the addition of solvent and the accompanying decrease in physical cross-link density. While the decrease in E′ for the isogels likewise reflects increasing solvent content, it is more pronounced than that for the corresponding mesogels due to fewer bridged copolymer midblocks and, consequently, a lower cross-link density at a given solvent concentration. A comparable, albeit more noticeable, reduction in E′ with increasing oil content is also observed in the SIS/oil series (data not shown). The transmission electron micrographs displayed in Figure 4 are acquired from the SEPS/oil mesogel with 60 wt % oil. These images reveal that the morphology consists
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Figure 5. TEM image of a swollen SIS/oil blend containing 25 wt % oil and exhibiting the lamellar morphology. In this micrograph, the I blocks of the copolymer are selectively stained with OsO4 and appear dark.
Figure 4. Transmission electron micrographs of the SEPS/oil mesogels containing 60 wt % oil. Both micrographs reveal the presence of RuO4-stained styrenic microdomains, some appearing lamellar and others cylindrical. The images in (a) and (b) differ slightly in section thickness. Note in (b) that some lamellae possess undulating surfaces.
of S-rich lamellae, with undulating surfaces, and cylinders. The width of the large lamellae in these micrographs is about 54 ( 4 nm, which exceeds the lamellar thickness measured18 from the neat SEPS copolymer (27 nm from small-angle X-ray scattering, SAXS, and 23 nm from TEM). The smaller lamellae measure ca. 30 nm across, in closer agreement to the thickness of the neat S lamellae and suggesting that many of the lamellae in Figure 4 are tilted relative to the electron beam. While regions of cylinders and tubules (bilayers that have wrapped around to form a hollow cylinder filled with solvent) are observed in images of this blend, the representative morphology appears to be lamellar in nature. Shown for comparison in Figure 5 is a TEM image of a swollen SIS/oil blend with 25 wt % oil. In this image, the copolymer orders into alternating lamellae in which the I blocks appear electron opaque (dark). Electron micrographs of SEPS/oil or SIS/ oil isogels14 at high solvent concentrations possess only S-rich cylinders or micelles. Similar morphological results
have been reported20,26,27 for copolymer/oil isogels composed of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) copolymers. Over limited concentration ranges, the copolymer micelles in such blends may order on a body-centered cubic27,28 (or, in some cases, a face-centered cubic29) lattice. While bilayered morphologies have been observed30-32 in near-equilibrated block copolymer/ homopolymer blends at high copolymer dilution (at which micelles are expected), it must be remembered that the microstructural features evident in Figure 4 are highly nonequilibrium in nature. Selective midblock swelling of a lamellar triblock (or higher order multiblock) copolymer can therefore yield a gel with a relatively high physical cross-link density and unique morphological anisotropy that can, in principle, be extended to large length scales upon shear-induced microstructural alignment33 prior to swelling. Acknowledgment. R.J.S. is indebted to the Alexander von Humboldt Foundation for financial support. We thank Dr. A. Halperin for encouraging this study and for sharing his insight. LA9816407 (26) Laurer, J. H.; Bukovnik, R.; Spontak, R. J. Macromolecules 1996, 29, 5760. (27) Prasman, E.; Thomas, E. L. J. Polym. Sci. B: Polym. Phys. 1998, 36, 1625. (28) 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. Kleppinger, R.; Mischenko, N.; Theunissen, E.; Reynaers, H.; Koch, M. H. J.; Almdal, K.; Mortensen, K. Macromolecules 1997, 30, 7012. (29) 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. (30) Gido, S. P.; Lee, C.; Pochan, D. J.; Pispas, S.; Mays, J. W.; Hadjichristidis, N. Macromolecules 1996, 29, 7022. (31) Laurer, J. H.; Smith, S. D.; Samseth, J.; Mortensen, K.; Spontak, R. J. Macromolecules 1998, 31, 4975. (32) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Makromol. Chem., Macromol. Symp. 1992, 62, 75. (33) Chen, Z.-R.; Kornfield, J. A. Polymer 1998, 39, 4679.
