Microphase separation in block copolymers

171 Microphase separation in block copolymers David J Lohse* and Nikos Hadjichristidis'l Block copolymers have been the focus of intense scientific and commercial development because of their ability to organize into precise structures on the scale of ten to one hundred nanometers. The wide variety of morphologies exhibited by linear block copolymers made from two chemically distinct monomers has been extensively studied over many years and is now fairly well understood. Three extensions of these studies have uncovered new classes of structures and new relations between copolymer composition and morphology. These include block copolymers with a nonlinear chain architecture, linear terpolymers with three chemically different blocks, and mixtures of linear diblocks with molecular weight or compositional differences. These results have greatly expanded the range of domain sizes and types of morphology that can be found in block copolymer materials. Addresses "Exxon Research & Engineering Co, Route 22 East, Annandale, NJ 08801, USA; e-mail: djlohse@erenj.com tUniversity of Athens, Department of Chemistry, Panepistimiopolis, Athens, 157 71, Greece; e-mail: nhadjich@atlas.uoa.gr Current Opinion in Colloid & Interface Science 1997, 2:171-176 Electronic identifier: 1359-0294,002·00171 © Current Chemistry Ltd ISSN 1359·0294 Abbreviations G gyroid OBOO ordered bicontinuous double diamond 0100 ordered tricontinuous double diamond P2VP poly(2'vinyl pyridine) PB polybutadiene PEB poly(ethylene·co-butene) PI polyisoprene PMMA poly(methyl methacrylate) PS polystyrene SAXS small angle X'ray scattering lEM transmission electron microscopy Introduction The key feature that has sparked both scientific and technological interest in block and graft copolymers is the fact that they spontaneously form phases on the scale of 1O-100nm, often with a high degree of order. This is a result of the covalent linkages between polymeric chain sections which are chemically distinct, plus the fact that most polymers are not miscible in each other. To reduce the enthalpy due to the unfavorable interactions between the unlike monomers, there is a driving force to phase separate just as in a blend of homopolymers. This phase separation can only proceed so far, however, as the covalent bonds between the blocks or graft sections must be maintained, and so the scale of this microphase separation must be similar to the size of the molecules themselves. A great deal of effort has been expended into discovering the origins of these structures, as is well explained in a recent review [1]. In the temperature-composition region, where the blocks are strongly separated, spherical, cylindrical, and lamellar morphologies were first discovered; later, other forms of bicontinuous domains were also found. Although there is still some uncertainty as to exactly how morphology maps onto a temperature-composition diagram at weaker segregation, in general such a morphology map is now quite well understood for linear block copolymers made from two chemically distinct types of blocks. The new profusion of morphologies and the breaking of the morphology/composition relations of linear diblocks has come from looking at block polymers that either have more than two types of blocks or are not linear in architecture. This review describes newly discovered classes of structure and the new relations found to exist between copolymer composition and morphology. Nonlinear block copolymers In contrast to the case for linear block polymers, very little was known, until recently, about the morphologies of nonlinear block polymers, simply because the availability of such molecules with well controlled structure was lacking [2]. Over the past few years, the synthesis of many forms of nonlinear block polymers has been demonstrated [3-7]. Miktoarm block copolymers ('mikto' comes from the Greek word ｾｴｋｌＶ＼［Ｌ meaning mixed) are star-shaped molecules with chemically different arms that belong to the simplest family of nonlinear block copolymers. These novel polymers have provided the means to initiate studies concerning the influence of chain architecture on microphase structure, the order-e disorder transition temperature, ordering kinetics, and the compatibilization capability of such nonlinear block copolymers. Their synthesis is based on anionic polymerization techniques and controlled chlorosilane coupling chemistry [Soo). In this way, one can produce a molecule with, say, two arms of polystyrene (PS) and one of polyisoprene (PI) radiating from a central silicon core. Inverse starblock copolymers can be made by a similar scheme [9). Careful characterization by size exclusion chromatography, light scattering, spectroscopy, and other techniques is necessary all along the synthetic pathway to insure that polymers of high molecular and compositional homogeneity are obtained. These new materials can exhibit interesting morphological features either by moving the borders of the classical morphology map or by forming new morphological structures. In Figure I, the micrographs of an AzB and an A3B block copolymer having approximately the same volume fraction of A (-SO volume %) are shown 172 Materials aspects [soo]. The 3-miktoarm has a bicontinuous structure, whereas the 4-miktoarm copolymer forms hexagonally packed cylinders. At the same volume fraction, a linear diblock (AB) would exhibit a lamellar morphology. These transmission electron microscopy (TE1\1) results were confirmed by small angle X-ray scattering (SAXS). From these and other results it is quite clear that architecture significantly influences the type of morphology that will occur at a given volume fraction. The structure of a (Pl)zPS miktoarrn polymer with Sl volume % of PS [10°] is shown in Figure 2. This is a randomly oriented 'worm micelle' morphology that is not found in linear block polymers. This has been shown to be the equilibrium state of the system by casting the films from selective solvents and prolonged annealing [11°]. Figure 2 Figure 1 TEM micrograph of a (PlbPS miktoarm polymer with 81 volume % PS. Also shown is a schematic of the three-d imensional structure of the domains. Reproduced with permission from [10°1. type [12]. His treatment involves calculations of the total free energy per chain from a balance of the interfacial tension forces (which tend to decrease interfacial area) with the energy of stretching the chains away from the interface (which tends to increase interfacial area). In the strong segregation limit, the free energies for the various morphologies are found to depend only on volume fraction ,..,y and E, where E = ｾ｛ｂｩ｝ＱＯＲ ne PB ' n:I is the number of arms of type i (= A, B) and Pi is the packing length of polymer i: p TEM micrographs of (PJ)2PS (a) and (PlbPS (b) miktoarm polymers w ith approximately 50 volume % PS. Reproduced with permission from [8"]. Milner has developed a theoretical approach to predict the morphologies of miktoarrn star copolymers of the AnB = 1\1 p < R2 >0 NA where p is density, 1\1 molecular weight, N A Avogadro's number, and < R2> 0 the mean-square end-co-end distance of the polymer. As < R2>0 is proportional co 1\1, p is an intrinsic parameter that is dependent only on polymer type. The morphology of the miktoarm at given values of ¢ and E can be predicted by finding which morphological type (spheres, cylinders, etc) has the lowest free energy Microphase separation in block copolymers Lohse and Hadjichristidis at that point. These predictions are compared with data obtained for PS/PI miktoarm polymers in Figure 3 [S··]. In most cases the predictions hold up, and where they do not the data are close to the predicted boundaries. This discrepancy may be due to multiple domain effects. The ability of block and graft copolymers to compatibilize polymer blends is well known [16]. As these polymers act at the interface of the blend, it is apparent that their architecture will have a strong influence on their effectiveness as compatibilizers [17·). Dielectric spectroscopy has been used to show that the kinetics of ordering of the miktoarrn is much slower than for linear chains [IS). This slowing down is due to the constrained mobility of the branched molecules. Moreover, the relaxation times of blocks within the microphase-separated domains are also related to the architecture of the polymers. It is clear that there are many new and interesting features of these nonlinear block polymers, and there are probably many more to be discovered. Figure 3 6 5.5 5 4.5 4 E 3.5 3 2.5 Linear block terpolymers 2 1.5 1 173 0 0.1 Comparison of observed morphologies of various AnB miktoarm copolymers with the theory of Milner [121. Letters indicate the observed morphologies: Cs=cylinders of B, B=bicontinuous, L=lamellar, CA=ylinders of A. Predicted morphologies are written out in full. Reproduced with permission from [S"l. In the case of inverse starblock copolymers, all of the samples had a composition of 50 volume % but differed in the asymmetry ratio, which is the ratio of the outer block to the inner block molecular weight [9]. The samples examined had asymmetry ratios of one, two, and four. At this composition, linear diblocks showed a lamellar morphology. For the inverse starblocks, a transition from lamellar to a bicontinuous cubic morphology was seen by TEi\1 and SAXS as the ratio went from two to four. In order to distinguish between the ordered bicontinuous double diamond (OBOO) and gyroid (G) morphologies, both of which are bicontinuous cubic, projections of their structures along various symmetry axes were simulated by computer and then compared with the TEM images. This showed that the morphology was more consistent with the p2mm symmetry of OBOO than with the c2mm of G. The nonlinear block polymers have been shown to be more stable to microphase separation than linear ones. By SAXS and rheology it was shown [13] that the AzB polymers remain disordered at molecular weights where linear AB polymers will microphase separate (or, equivalently, it has been shown that at the same molecular weight, an AzB has a lower order-edisorder transition temperature than the AB) [13]. This is due to the greater mixing of the A and B monomers at the interface between the microphases for the AzB and generally agrees with mean field theories for such polymers [14,15). Compared to the large literature on the morphology of block polymers with two chemically distinct kinds of blocks, very little has been published on the microphase separation of those having three distinct blocks. Not only are there synthetic challenges in preparing such polymers, but the need to image three types of domains also presents difficulties. If two of the blocks are miscible with each other, then one again has the case of two microphases and morphologies in line with those of diblocks [19,20]. Evidence of three microphases was first seen in 19S0 [21], but only recently has this rich area begun to be well explored. One system that has been well studied is that in which the three blocks are made from PI, PS, and poly(2-vinyl pyridine) (P2VP). One set of studies looked at polymers with PS midblocks and equal volume fractions of the PI and P2VP endblocks [22,23). From 33 to 50 volume % of PS, three-phase, four-layer lamellar structures were seen. It was also shown that the size of the lamellae varied as the molecular weight of the blocks to the 2/3 power, just as for diblocks [24). At 50 to 66 volume % PS, the two end blocks formed two separate but mutually interpenetrating networks in the PS matrix; this was designated as the 'ordered tricontinuous double diamond' or OTOO morphology [23). Between 66 and SO volume % PS, the endblocks formed two types of tetrahedrally arranged cylinders in the PS matrix, and above SO% of the midblock they were found in spheres [22). When the order of connecting the blocks in the system was changed so that PI became the midblock, similar results were seen by Matsushita el al. [25). Evidence of both the lamellar morphology (at near equal block volumes) and the OTOO (when the PI midblock was 66 %) was obtained. All of these results were consistent with several theoretical approaches [26,27). A PS/PI/P2VP triblock with essentially equal block lengths was also examined by Gido et al., however, and did not show a lamellar morphology [2S). This sample showed a cylindrical core-shell structure, with 174 Materials aspects a central P2VP core and a PI shelI in the PS matrix. One possible reason for the discrepancy between these results and those of Matsushita et al. is that the PI microstructure was different in the two samples, which would affect the interactions between them. It was clearly shown, however, that structures not found in diblocks can exist in block polymers with three distinct types of blocks. Stadler and coworkers have examined two sets of triblocks that show three microphases. The first set had blocks in the folIowing order: PS, polybutadiene (PB), and poly(methyl methacrylate) ＨｐｍｾｉａＩ Ｎ In the second set the midblock was converted to poly(ethylene-co-butene) (PEB) by saturating the PB block [29]. As in the Matsushita work, the endblock volumes were kept equal, however, the Stadler group examined the cases where the midblock was equal to or shorter th an the endblocks; moreover the incompatibility between dissimilar endblocks was relatively weaker than that between the midblock with its two endblock neighbors [30,31]. At 38 volume % of midblock (PB or PEB), a three-phase four-layer lamellar structure was seen. At 17 volume % midblock, the morphology was one of PS and P?-.Il\IA lamelIae with PB (or PEB) cylinders at the interface between them ('cylinder at walI' morphology). At 6 volume % PB midblock, spheres of PB were found at the interface .between the PS and PMl\IA lamellae ('balI at walI'). By tapping mode scanning force microscopy, the surface morphology of this structure has been seen [32]. When the midblock was 6 volume % PEB, however, the structure was one of PEB rings around PS cylinders in the P;"Il\lA matrix (see Fig. 4). The differences between the last two cases can be explained by the change in interactions between the blocks upon the switch from PB to PEB [33°]. Even more strikingly, this group has recently examined cases where the endblocks do not have equal volume [34°°]. At PS/PB/PMMA ratios of 24/7/69 and 26/12/62 they saw PS cylinders surrounded by PB helices In the PMl\IA matrix (Fig. 5). This novel structure IS especialIy striking as it shows a case where there is no chirality at a molecular level, but a chiral structure develops in an assembly of the molecules. Finally, they have also shown that PS/pEB/PM?-.IA can be used to cornpatibilize blends of poly(2,6-dimethyl-l,4-phenylene ether) and polyfsryrene-co-acrylonirrile) [35]. This can also be done by PS/pl\Il\1A diblocks [16], but the presence of the elastorneric midblock has a profound effect on the morphology and mechanical properties of the blend. Figure 4 ES-Cyl:nder (a) SEB 7 100 36 PS EB p (b) ps : 5% p SEa .6 80 10 PS EB :8: 6% 1A: ｾＴ Schematic representation of the structures of PS/PES/PMMA triblocks (SESM) with 17 (a) and 6 volume % (b) of the PES midblock (PES: white, PMMA : gray. PS: dark). Published with permission from [30] . Blends of linear diblock copolymers much potential to break the relation of composition and structure. The ability to control morphology by blending together several linear dib locks has been less stud ied than the first two topics in this review, but this technique also seems to be vcry powerful. To .datc, no new morphologies have been seen from these systems, but there seems to be Mixtures of very short diblocks with longer ones have been examined both theoretically [36] and experimentally [37°]. Although some of the short ch ains were dissolved in the matrix of the longer ones, a significant fraction Microphase separation in block copolymers Lohse and Hadjichristidis Figure 5 175 IS 000 Da molecular weight alternating with similar blocks of PI [440 ] . This incommensurability of block length was sufficient to induce phase separation, even when every other feature of the molecules was identical. Conclusions Overall, it seems clear that there is much potential to manipulate the form of microphase separation and to induce macrophase separation by the methods outlined herein: making nonlinear block polymers, blending linear diblock polymers, and making linears with three distinct block types. Due to the great deal of work needed to synthesize and examine all of the possible systems, the theoretical development should stay ahead of the experimental results for some time. This great enhancement in the ability to manipulate morphology over that using just linear diblocks not only will be of interest scientifically but also has the potential for new commercial applications, such as in membranes. Schematic representation of the helical structure of a 26/12/62 PS/PB/PMMA triblock copolymer. A=PS, B=PB, C=PMMA. Published with permission from [34··]. References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: went to the interface of the longer diblock. There they reduced the interfacial tension, leading to greater folding of the lamellar domains. The domain sizes also shrank as the short diblock was added, because its presence at the interface relieved the need for stretching of the long blocks. The type of structure was not changed in these cases, but the sizes and shapes of the domains were. The ability of a pair of diblocks to remain miscible has been tested by several groups. A pair of diblocks that are similar in composition will still show macrophase separation if their molecular weights are too different; the incommensurability of the blocks means that one or the other block would have to be too stretched or too compressed in the mixed state. This has been seen by SAXS, TEt\1 [38,39], and neutron reflectivity [40]. Self-consistent mean field theories for blends of diblocks have been developed that agree with these data, and they give results that are remarkably similar to each other [41",42 0 ] . These theories also predict that diblocks that have the same molecular weight but differ significantly in composition will also show macrophase separation due to the interactions between the segments. This has been verified by simulations [43]. Very rich phase diagrams for such diblock blends, as a function of molecular weight and composition, are predicted. Finally, macrophase separation has also been seen when the compositions and molecular weights of the two polymers are identical. In this case the only difference was the way in which the monomers were grouped into blocks. One component was a PSjPI diblock, with each block having a molecular weight of 60000 Da; the other was an octablock polymer, with four PS blocks of 00 of special interest of outstanding interest 1. Bates FS, Fredrickson GH: Block copolymer thermodynamics: theory and experiment Annu Rev Phys Chem 1990, 41 :525-557. 2. Bates FS: Polymer-polymer phase behavior. Science 1991, 251:898-905. 3. latrou H, Hadjichristidis N: Synthesis of a model 3-miktoarm star terpolymer. Macromolecules 1992, 25:4649-4651. 4. latrou H, Hadjichristidis N: Synthesis and characterization of model 4-miktoarm star co- and quaterpolymers. Macromolecules 1993,26:2479-2484. 5. latrou H, Avgeropoulos A, Hadjichristidis N: Synthesis of model super H-shaped block copolymers. Macromolecules 1994, 27:6232-6233. 6. latrou H, Siakali-Kioulafa E, Hadjichristidis N, Roovers J, Mays JW: Hydrodynamic properties of model 3-miktoarm star copolymers. ) Polym Sci Phys 1995, 33:1925-1932. 7. Avgeropoulos A, Poulos Y, Hadjichristidis N, Roovers J: Synthesis of model 16-miktoarm (vergina) star copolymers of the AeBe type. Macromolecules 1996, 29:6076-6078. 8. Tselikas Y, latrou H, Hadjichristidis N, Liang KS, Mohanty K, Lohse OJ: Morphology of miktoarm star block copolymers of styrene and isoprene. ) Chem Phys 1996, 105:2456-2462. This paper shows how the type of morphology maps onto miktoarm structure and composition. The remarkable breaking of the composition-morphology rules of diblocks is clearly demonstrated. 9. Tselikas Y, Hadjichristidis N, Lescanec RL, Honeker CC, Wohlgemuth M, Thomas EL: Architecturally-induced tricontinuous cubic morphology in compositionally symmetric miktoarm starblock copolymers. Macromolecules 1996, 29:3390-3396. Pochan OJ, Gido SP, Pispas S, Mays JW, Ryan AJ, Fairclough JPA, Hamley IW, Terrill NJ: Morphologies of microphase-separated A2B simple graft copolymers. Macromolecules 1996, 29:5091-5098. See annotation to [11·. 10. 11. Pocham OJ, Gido SP, Pispas S, Mays JW: Morphological transitions in an 125 simple graft block copolymer: from folded sheets to folded lace to randomly oriented worms at equilibrium. Macromolecules 1996,29:5099-5105. Herein the existence of a new 'worm micelle' structure is shown. 176 Materials aspects 12 Milner ST: Chain architecture and asymmetry in copolymer microphases. Macromolecules 1994 , 27:2333-2335. 13. Aoudas G, Hadjichristidis N, latrou H, Pakula T, Fischer 8N: Microphase separation in model 3-miktoarm star copolymers (simple graft) and terpolymers. 1. Statics and kinetics. Macromolecules 1994, 27:7735-7746. 14. De la Cruz MO, Sanchez IC: Theory of microphase separation In graft and slar copolymers. Macromolecules 1986, 19:2501-2508. 15. Dobrynin AV, Erukhimovich IV: Computer-aided comparative investigation of architecture influence on block copolymer phase diagrams. Macromolecules 1993 ,26:276-281- 16. Dalla S, Lohse DJ: Polymeric Compatibilizers: Uses and Benelits in Polymers Blends. Munich: Hanser Publishers; 1996. Lyatsl<aya Y, Gersappe D, Balazs AC: Effect of copolymer architecture on the efficiency of compatibilizers. Macromolecules 1995, 28:6278-6283. The eHect of copolymer structure on compatibilizing efficiency is predicted in this paper. 17. 18. Aoudas G, Hadjichristidis N, latrou H, Pakula T: Microphase separation in model 3-miktoarm star co- and terpolymers. Macromolecules 1996,29:3139-3146. 19. Fielding-Russell GS, Pillai PS: Phase structure of solution cast films of a-methyl styrene/butadiene/styrene block copolymers. Polymer 1974,15:97-100. 20. Fellers U, Firer EM, Defauti M: Synthesis and properties of block copolymers. 4. Poly(p-tert-butylstyrene-diene -p-tertbutyl styrene) and poly(p-tert-butylstyrene-isoprene-styrene). Macromolecules 1977, 10:1200-1207. 21. Riess G, Schli enger M, Marti S: New morphologies in rubbermod ified polymers. J Macromol Sci 1980, B17:355-374. 22. Mogi V, Kotsuji H, Kaneko V, Mori K, Matsushita Y, Noda I: Preparation and morphology of triblock copolymers of the ABC type. Macromolecules 1992, 25:5408-5411. 23. Mogi V, Matsushita Y, Mori K, Noda I: Trlcontinuous morphology oftriblock copolymers of the ABC type. Macromolecules 1992, 25:5412-5415. 24. Mogi Y, Kotsuji H, Mori K, Matsushita Y, Noda I, Han CC: Molecular weight dependence of the lamellar doma in spacing of ABC triblock copolymers and the ir chain conformations in lamellar domains. Macromolecules 1993 , 26:5169-5173. 25. Matsushita Y, Tamura M, Noda I: Tricontinuous double-diamond structure formed by a styrene-isoprene-2 vinylpyridine triblock copolymer. Macromolecules 1994, 27:3680-3682. 26. Kane L, Spontak RJ: Microstructural characteristics of strongly segregated AXB triblock terpolymers possessing the lamellar morphology. Macromolecules 1994, 27:1267-1273. 27. Nagazawa H, Ohta T: Microphase separation of ABC-type triblock copolymers. Macromolecules 1993, 26:5503-5511. 28. Gido SP, Schwark DW, Thomas EL, Goncal ves MdC: Observation of a non-constant mean curvature Interface in an ABC triblock ccpclymer, Macromolecules 1993, 26:2636-2640. 29. Auschra C, Stadler R: Synthesis of block copolymers with poly(methyl methacrylate): P(B-b-MMA), P(EB-b-MMA>, P(S-b-B-b -MMA), and P(S·b·EB-b·MMA). Polym Bull 1993, 30:257-264. 30. Auschra C, Stadler R: New ordered morphologies in ABC triblock copolymers. Macromolecules 1993, 26:2171-2174. 31. Beckmann J, Auschra C, Stadler R: 'Ball at the wall' • a new lamellar multiphase morphology in a poly(styrene-block- butadiene-block-methyl methacrylate) triblock copolymer. Macromol Rapid Commun 1994, 15:67-72. 32. Stocker W, Beckmann J, Stadler R, Rabe JP: Surface reconstruction of the lamellar morphology in a symmetric poly(styrene-block-buladiene·block·methyl methacrylate) triblock copolymer: a lapping mode scanning force microscopy study. Macromolecules 1996, 29 :7502-7507. Stadler R, Auschra C, Beckmann J, Krappe U, Voigt-Martin I, Leibler L: Morphology and thermodynamics of symmetric poly(A-block-B-block-C) triblock copolymers. Macromolecules 1995, 28:3080-3097. The sensitivity of the morphology of ABC triblocks to the level 01interactions between the polymer segments is demonstrated in this paper. A switch in morphology occurs simply by saturating the midblock to turn it from polybu· tadiene into polylethylene-co-but ene) . 33. Krappe U, Stadler R, Voigt-Martin I: Chiral assembly in amorphous ABC triblock copolymers. Formation of a helical morphology in polystyrene'block-polybutadiene-blockpoly(methyl methacrylate) block copolymers. Macromolecules 1995, 28:4558-4561. This paper shows how a chiral morphology can arise from the self-assembly of nonchiral molecules. 34. •• 35. Auschra C, Stadler R: Polymer alloys based on poly(2,6dimethyl-1,4-phenylene ether) and poly(styrene-coacrylonitrile) using poly(styrene-b-(ethylene-co-butylene)-bmethyl methacrylate) triblock copolymers as compatibilizers. Macromolecules 1993, 26:6364-6377. 36. Shi AC, Noolandi J: Effects of short diblocks at interfaces of strongly segregated long diblocks. Macromolecules 1994, 27:2936-2944. 37. Lin EK, Gast Ap, Shi AC, Noolandi J, Smith SD: Effect of short diblock copolymers at internal interfaces of large diblock copolymer mesophases. Macromolecules 1996, 29:5920-5925. This paper shows how short diblock cosurfactants can modify the interface of a microphase-separated long diblock to control domain size and shape. 38. Hashimoto T, Yamasaki K, Koizumi S, Hasegawa H: Ordered structure in blends of block copolymers. 1. Miscibility criterion for lamellar block copolymers. Macromolecules 1993, 26:2895-2904. 39. Hashimoto T, Koizumi S, Hasegawa H: Ordered structure in blends of block copolymers. 2. Self-assembly for lamellaforming copolymers. Macromolecules 1994,27:1562-1570. 40. Mayes AM, Russell TP, Deline VR, Satija SK, Majkrzak CF: Block copolymer mixtures as revealed by neutron reflectivity. Macromolecules 1994, 27:7447-7453. Shi AC, Noolandi J: Binary mixtures of diblock copolymers: phase diagrams with a new twisl Macromolecules 1995, 28:3103-3109. The rich phase diagrams 01 binary blends 01 diblocks are predicted in this paper. It sets clear targets for synthetic eHorts in this area 41. 42. Matsen MW, Bates FS: One-component approximations for binary diblock copolymer blends. Macromolecules 1995, 28:7298-7300. This paper shows the phase behavior of diblock blends in a number of cases. 43. Huh J, Angerman H, ten Brinke G: Symmetric blends of complementary diblock copolymers: multiorder parameter approach and Monte Carlo simulations. Macromolecules 1996, 29:6328-6337. Spontak RJ, Fung JC, Braunfield MB, Sedat JW, Agard DA, Ashraf A, Smith SD: Architecture-induced phase immiscibility in a diblock/multiblock copolymer blend. Macromolecules 1996, 29:2850-2856. In this paper, immiscibility arises between polymers of the same molecular weight and composition solely from the organization of the blocks. 44.