Complex Phase Behavior of a Disordered “Random” Diblock

2250

Langmuir 1997, 13, 2250-2258

Complex Phase Behavior of a Disordered “Random” Diblock Copolymer in the Presence of a Parent Homopolymer Jonathan H. Laurer,† Arman Ashraf,‡ Steven D. Smith,‡ and Richard J. Spontak*,† Department of Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907; and Corporate Research Division, The Procter & Gamble Company, Cincinnati, Ohio 45239-8707 Received August 13, 1996. In Final Form: February 5, 1997X Previous efforts addressing binary blends of a block copolymer and a parent homopolymer have principally employed ordered copolymers in either the intermediate- or strong-segregation regimes. In this work, blends composed of a disordered (75/25)-b-(50/50) poly[(styrene-r-isoprene)′-b-(styrene-r-isoprene)′′] (S/ I)′-b-(S/I)′′ random diblock copolymer (RBC) and homopolystyrene (hS) have been investigated. Blend morphologies, characterized by transmission electron microscopy, are correlated with hS concentration and molecular weight, as well as with changes in the hS Tg, as measured by thermal calorimetry. At low hS fractions (up to 20 wt % hS), the S/I block sequences in the RBC induce competition between attractive and repulsive interactions with hS molecules, resulting in the formation of thin hS channel structures in a continuous RBC matrix. An increase in hS concentration or molecular weight serves to broaden the channels until the morphology resembles macrophase-separated hS domains containing micelle-like RBC dispersions. In blends with relatively high hS fractions (greater than 80 wt % hS), repulsive interactions between RBC and hS molecules are responsible for the formation of macroscopic RBC domains in a continuous hS matrix. These blend morphologies demonstrate that localized interactions between homopolymer molecules and each block of a copolymer exist, and can be probed, in the disordered state.

I. Introduction Morphological development in microphase-ordered block copolymers continues to be a subject of considerable research interest, since (i) these materials provide valuable insight into macromolecular self-assembly and (ii) their thermal, mechanical, and transport properties can be tailored for specific commercial applications.1-3 Numerous efforts addressing neat (unmixed) AB diblock copolymer systems have demonstrated that block copolymer morphologies, as well as their bulk properties, depend on molecular composition, statistical segment length asymmetry, and the extent of interblock thermodynamic incompatibility, the magnitude of which can be expressed by χN (where χ denotes the Flory-Huggins interaction parameter and N corresponds to the number of statistical segments along the chain).4-7 Microphase ordering occurs when χN > (χN)ODT, where the subscripted ODT refers to the order-disorder transition, which occurs at a χN of about 10.5 in the mean-field (N f ∞) limit.4 While, for a given copolymer composition, χN is routinely controlled through judicious choice of monomer species, temperature, and molecular weight, it can also be altered through chemical modification, e.g., by varying the monomer ratio * To whom correspondence should be addressed. † North Carolina State University. ‡ The Procter & Gamble Company. X Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Sperling, L. H. Introduction to Physical Polymer Science, 2nd ed.; Wiley: New York, 1992. (2) Legge, N. R., Holden, G., Schroeder, H. E., Eds. Thermoplastic Elastomers: A Comprehensive Review; Hanser: New York, 1987. (3) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387. (4) Leibler, L. Macromolecules 1980, 13, 1602. (5) Fredrickson, G. H.; Helfand, E. J. Chem. Phys. 1987, 87, 697. (6) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. Bates, F. S. Science 1991, 251, 898. (7) Hamley, I. W.; Gehlsen, M. D.; Khandpur, A. K.; Koppi, K. A.; Rosedale, J. H.; Schulz, M. F.; Bates, F. S.; Almdal, K.; Mortensen, K. J. Phys. II 1994, 4, 2161.

S0743-7463(96)00807-4 CCC: $14.00

(composition) of one or more of the constituent blocks.8-11 Control over morphology (and χN) in applications wherein the operating temperature and monomer species are fixed is, however, more readily achieved by blending an AB diblock copolymer with a parent homopolymer (hA). The morphologies of ordered AB/hA blends depend principally on three considerations: (i) the volume-fraction composition of the AB copolymer, (ii) the concentration of added hA, and (iii) the ratio of the molecular weight of hA to that of the host A block (i.e., MhA/MA). Upon solubilization, the added homopolymer can swell its resident microdomain12,13 or perturb the interfacial curvature, thereby inducing a transition to a stable new morphology.14-18 Most studies of AB/hA blends in which A and B correspond to polystyrene and a polydiene, respectively, have focused on highly ordered copolymers, since weakly segregated or disordered diblock copolymers composed of these monomer species possess relatively short block lengths19 (in which case systematic variation of MhA/MA may be impractical). This shortcoming can be (8) Annigho¨fer, F.; Gronski, W. Colloid Polym. Sci. 1983, 261, 15. (9) Hashimoto, T.; Tsukahara, Y.; Tachi, K.; Kawai, H. Macromolecules 1983, 16, 648. (10) Bu¨hler, F.; Gronski, W. Makromol. Chem. 1986, 187, 2019; 1988, 188, 2995. (11) Zielinski, J. M.; Spontak, R. J. Macromolecules 1992, 25, 5957. Kane, L.; Spontak, R. J. Macromolecules 1994, 27, 1267. (12) Winey, K. I.; Thomas, E. L.; Fetters, L. J. Macromolecules 1991, 24, 6182. (13) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378. Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240. (14) Winey, K. I.; Thomas, E. L.; Fetters, L. J. J. Chem. Phys. 1991, 95, 9367. Winey, K. I.; Thomas, E. L.; Fetters, L. J. Macromolecules 1992, 25, 422, 2645. (15) Spontak, R. J.; Smith, S. D.; Ashraf, A. Macromolecules 1993, 26, 956. (16) Disko, M. M.; Liang, K. S.; Behal, S. K.; Roe, R.-J.; Jeon, K. J. Macromolecules 1993, 26, 2983. (17) Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994, 27, 4063. (18) Matsen, M. W. Phys. Rev. Lett. 1995, 74, 4225. Matsen, M. W. Macromolecules 1995, 28, 5765. (19) Tanaka, H.; Hashimoto, T. Macromolecules 1991, 24, 5398.

© 1997 American Chemical Society

Disorder “Random” Diblock Copolymer

Langmuir, Vol. 13, No. 8, 1997 2251 Table 1. Molecular Characteristics of the Materials Used in This Studya designation

Figure 1. Schematic representation of the random diblock copolymer employed in this study. Each block is a random S/I copolymer, with the block compositions (75/25 and 50/50) illustrated by the relative ratios of light (S) to dark (I) circles. The vertical line denotes the block junction.

alleviated by altering the monomer ratio within one or both blocks of the copolymer so that the chemical properties of the copolymer, including χ, can be controllably adjusted for a given N.9,20-22 Note that, while the blocks are not pure A (or B), the copolymer still consists of discrete, chemically dissimilar, contiguous sequences and therefore, from a molecular architecture standpoint, can be considered a block copolymer rather than a random copolymer (RC). In this work, the morphological characteristics of binary blends composed of an (A/B)′-b-(A/B)′′ random block copolymer (RBC), which consists of two random contiguous sequences differing in A/B monomer ratio, and hA are examined by transmission electron microscopy (TEM). Since, from previous efforts,21 the RBC employed here is microstructurally disordered, exhibiting no evidence of microphase separation, the blend morphologies reported here reflect the effect of delocalized attractive A/hA and repulsive B/hA interactions on copolymer/homopolymer phase miscibility in the disordered state. II. Experimental Section A. Materials. A poly[(styrene-r-isoprene)′-b-(styrene-r-isoprene)′′] (S/I)′-b-(S/I)′′ RBC was synthesized by living anionic polymerization in cyclohexane, using a cocatalyst comprised of sec-butyllithium and a potassium alkoxide. As described in detail elsewhere,20-22 the potassium alkoxide salt was necessary to obtain random S/I copolymers of invariant (nontapered) composition during the course of polymerization. The RBC employed in this work, depicted schematically in Figure 1, was produced with two RC blocks of nearly equal molecular weight, one possessing 75/25 (wt %) S/I and the other 50/50 S/I. The overall styrene content of the molecule was 68 wt %, as discerned by 1H NMR, and M h n ≈ 85 000 (on an S equivalent basis) and M h w/M hn )1.03 from GPC analysis. Upon correcting for the difference in hydrodynamic volume between S and I monomers, the RBC can be considered nearly symmetric, with block masses of approximately 40 000 each. In addition, three polystyrene homopolymers, with M h n values of 15 000, 30 000, and 120 000 and polydispersity indices less than 1.05 were also synthesized via living anionic polymerization to examine RBC/hS blends with corresponding MhS/MS/I ratios of 0.38, 0.75, and 3.0 (MS/I is the molecular weight of the S-rich 75/25 S/I block in the RBC). Material Designation. Each blend is designated as m/whS, where m ) MhS/1000 and whS denotes the wt % of hS in the blend. To distinguish among the three polystyrene homopolymers employed in this work, they are hereafter referred to as hSm (see Table 1). B. Methods. Predetermined masses of RBC and hS were dissolved in toluene to produce 4% (wt/v) solutions, which were subsequently cast into Teflon trays at ambient temperature. Upon slow solvent evaporation over the course of 3 weeks, films measuring about 2 mm thick were postannealed at 130 °C for (20) Smith, S. D.; Ashraf, A.; Clarson, S. J. ACS Polym. Prepr. 1993, 34, 672. Smith, S. D.; Ashraf, A.; Clarson, S. J. ACS Polym. Prepr. 1994, 35, 467. Ashraf, A. M. S. Thesis, University of Cincinnati, 1994. (21) Ashraf, A.; Smith, S. D.; Satkowski, M. M.; Spontak, R. J.; Clarson, S. J.; Lipscomb, G. G. ACS Polym. Prepr. 1994, 35, 581. Smith, S. D.; Ashraf, A.; Satkowski, M. M.; Spontak, R. J. ACS Polym. Prepr. 1994, 35, 651. Spontak, R. J.; Smith, S. D. Ashraf, A. Polym. Mater. Sci. Eng. 1994, 70, 149. (22) Ashraf, A.; Smith, S. D.; Clarson, S. J. Macromolecules, submitted.

RBC hS15 hS30 hS120

classification

M h nb

random block copolymer 80 000 homopolystyrene 15 000 homopolystyrene 30 000 homopolystyrene 120 000

WSc

wS ′

wS′′

0.68 0.75 0.50 1.00 1.00 1.00

a ′and ′′ denote the blocks of the RBC. b Number-average molecular weight (from GPC). c Overall styrene mass fraction (from 1H NMR).

several hours under vacuum. Electron-transparent specimens for electron microscopy were produced by sectioning the annealed films in a Reichert-Jung Ultracut-S cryoultramicrotome maintained at -100 °C. Electron contrast between microphases for TEM was achieved by staining the isoprene units with the vapor of a 2% aqueous OsO4 solution for 90 min. The stained sections were imaged with a Zeiss EM902 electron spectroscopic microscope, operated at 80 kV and ∆E ) 50 eV. Pieces of unstained bulk films were encapsulated in aluminum pans for differential scanning calorimetry (DSC) using a Perkin-Elmer DSC-7 calorimeter. Thermograms were obtained from ambient temperature to 120 °C at a rate of 20 °C/min under a dry nitrogen atmosphere to minimize thermo-oxidative degradation.

III. Results and Discussion The morphologies produced here reflect a progressive shift from influential attractive S/hS interactions to exclusively repulsive RBC/hS interactions as hS content and MhS are increased. At high hS content, the blends exhibit morphologies which will be shown to demonstrate principally repulsive interactions with the RBC, even for relatively low MhS, despite the presence of S in each block. This is not surprising, since, according to both theoretical considerations23 and experimental studies,24,25 random copolymers exhibit composition-dependent miscibility with their parent homopolymers. Conversely, at low hS fractions, sufficient RBC is present so that the S monomers comprising the RBC molecules undergo different extents of repulsive interactions with added hS: less repulsive (75/25 S/I block) and more repulsive (50/50 S/I block). Because the blend compositions investigated here probe these two extreme cases, they are discussed below in separate sections addressing hS-lean blends (exhibiting attractive and repulsive interactions) and hS-rich blends (exhibiting predominantly repulsive interactions). As previously reported,21 examination of the neat RBC by TEM reveals a granular texture reminiscent of weakly segregated, low-M poly(styrene-b-isoprene) (SI) diblock copolymers and indicative of composition fluctuations.6,15 Hence, this (75/25)-b-(50/50) RBC can be considered structurally disorderedsi.e., χN < (χN)ODTsat ambient temperature, even though its molecular weight is about 80 000. In contrast, an SI diblock copolymer of comparable molecular weight (80 000) exhibits the lamellar morphology.26 From small-angle X-ray scattering (SAXS) analysis of a symmetric SI copolymer with M h n ≈ 13 600,27 the (23) Coleman, M. M.; Graf, J. F.; Painter, P. C. Specific Interactions and the Miscibility of Polymer Blends; Technomic: Lancaster, PA, 1991. See also, Wohlfarth, C. Makromol. Chem., Theory Simul. 1993, 2, 605. (24) Sakurai, S.; Izumitani, T.; Hasegawa, H.; Hashimoto, T.; Han, C. C. Macromolecules 1991, 24, 4844. Braun, D.; Yu, D.; Kohl, P. R.; Gao, X.; Andradi, L. N.; Manger, E.; Hellmann, G. P. J. Polym. Sci., Polym. Phys. Ed. 1992, 30, 577. Maier, T. R.; Jamieson, A. M.; Simha, R. J. Appl. Polym. Sci. 1994, 51, 1053. (25) Chai, Z. K.; Sun, R. N.; Karasz, F. E. Macromolecules 1992, 25, 6113. Janarthanan, V.; Kressler, J.; Karasz, F. E.; MacKnight, W. J. J. Polym. Sci., Polym. Phys. Ed. 1993, 31, 1013. (26) Spontak, R. J.; Fung, J. C.; Braunfeld, M. B.; Agard, D. A.; Kane, L.; Smith, S. D.; Satkowski, M. M.; Ashraf, A.; Hajduk, D. A.; Gruner, S. M. Macromolecules 1996, 29, 4494. (27) Hong, S.-U.; Laurer, J. H.; Hajduk, D. A.; Smith, S. D.; Spontak, R. J.; Duda, J. L. Macromolecules, to be submitted.

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Figure 2. Random poly(styrene-r-isoprene) copolymer composition presented as a function of copolymerization in the presence of 30:1 sec-butyllithium/potassium alkoxide. The invariance of composition with conversion seen here signifies nearly equal styrene and isoprene reactivity ratios.

temperature-dependent S-I thermodynamic incompatibility (χN) can be expressed as (-4.72 + 5420/T)N, where T denotes absolute temperature. From this correlation, the SI copolymer with M h n ) 80 000 is estimated to possess a χN of about 70, clearly above (χN)ODT as derived from mean-field4 and fluctuation5 theories. Variation of the S/I monomer ratio in each block can therefore be used to lower χN at constant T and N,28 as well as probe localized interactions in RBC/hS blends, as demonstrated in the following sections. This work focuses on the variation of RBC/hS blend morphology with hS molecular weight and concentration. While an in-depth analysis of the monomer sequence distribution of the blocks comprising the RBC is beyond the scope of the present study, this issue is briefly addressed below. Anionic copolymerization of styrene and isoprene with lithium initiators yields blocky copolymers due to the different reactivity ratios (rk) of the constituent (k) monomers. As we have demonstrated earlier20,21 and present here (Figure 2) for a S/I RC with 60 wt % S, copolymerizations modified with potassium alkoxide at a ratio of 30:1 sec-butyllithium/potassium alkoxide yield relatively constant composition with conversion profiles. From such data, it can be concluded that the monomer reactivity ratios of styrene (rS) and isoprene (rI) are nearly equal under the conditions of the copolymerization. Note that, in the presence of sec-butyllithium without potassium alkoxide, isoprene is consumed substantially faster than styrene, resulting in a large rI, but a small rS. To ascertain the relative magnitude of these ratios, ozonolysis degradation of the isoprene segments, followed by analysis of the styrene oligomer mass distributions, has also been performed on a series of poly(styrene-risoprene) and poly(styrene-r-butadiene) RCs, as described in detail elsewhere.22 The data displayed in Figure 2, coupled with the results from the ozonolysis, reveal that the reactivity ratios of styrene and isoprene, when copolymerized in the presence of 30:1 sec-butyllithium/ potassium alkoxide at 60 °C, are both nearly equal to unity. This crucial outcome confirms that the constituent blocks of the RBC investigated here each possesses random monomer sequencing. An added benefit of using the potassium alkoxide cocatalyst to synthesize random styrene-isoprene copolymers (or blocks, as in the present RBC) is the minimal effect on the microstructure of the isoprene segments, as compared to other common copolymerization methods (e.g., addition of polar additives, such as ethers or amines). Under the conditions employed (28) Dobrynin, A. V.; Erukhimovich, I. Ya. J. Phys. I 1995, 5, 365. Angerman, H.; ten Brinke, G.; Erukhimovich, I. Macromolecules 1996, 29, 3255.

Laurer et al.

here, homopolyisoprenes produced with sec-butyllithium alone are typically 94% 1,4-cis (as measured by 1H NMR), while the isoprene segments in RCs synthesized with secbutyllithium and potassium alkoxide are 92-93% 1,4-cis. A. hS-Lean Blends. Electron micrographs of blends consisting of relatively low concentrations (5-20 wt %) of hS varying in MhS illustrate the unique morphological features of these systems. It is well-established that, in typical immiscible hA/hB (or hA/RC) blends, the minor component (e.g., A) minimizes the number of energetically unfavorable contacts with the matrix component by forming globular or spheroidal dispersions that reduce the surface-to-volume ratio of A. This characteristic behavior is not, however, observed with some of the RBC/ hS15 blends seen in Figure 3, which reveal that hS15 (bright) is excluded from the RBC (dark) matrix in narrow channel structures that do not represent minimization of S-I contacts. Note that these morphologies differ from similarly appearing ones reported29-32 for hA-rich AB/hA blends (e.g., high-impact polystyrene, HIPS) in which the AB copolymer self-assembles into bilayered (membrane) structures. In this work, hS, not the RBC, constitutes the minor component and phase-separates to form the observed channel structure. The electron micrographs of the 15/5, 15/10 and 15/20 blends presented in Figure 3 confirm that attractive RBC/ hS interactions are significant in blends with low-M hS. The hS morphology can be best described as channel-like at the lowest whS (Figure 3a), with each hS channel measuring ca. 50 nm wide. Since block copolymers in the disordered state are known33 to adopt a Gaussian conformation, it is conceivable that the hS chains comprising these channels do nearly the same, in which case the radius of gyration (Rg) can be estimated from the valence bond model1 (which accounts for a fixed 109.5° bond angle). For hS15, Rg≈ 4.7 nm, indicating that the structures observed in Figure 3a consist of very few (about 5) unperturbed hS chains laterally packed. The tendency to form narrow hS layers is a direct consequence of attractive interactions between the hS chains and the 75/25 S/I block of the RBC. Repulsive interactions in the same blend are responsible for the general shape of the hS channel aggregates, which appear as loose ellipsoidal domains measuring up to about 4.5 µm along the major axis. As the amount of hS15 is increased (Figure 3b and c), many of the hS channels widen, with the narrowest averaging about 150 nm wide in the 15/10 blend (Figure 3b). This increase signifies a threefold increase in the number of associated hS layers upon a twofold increase in whS. Since the hS in the 15/10 blend is confined to loosely organized domains of nearly equal size and population as those seen in Figure 3a, the layers of hS molecules must become more densely packed (as compared to those in the 15/5 blend) to reflect the increase in hS concentration. Upon doubling whS again, the morphology of the 15/20 blend (Figure 3c) is more accurately described as ellipsoidal hS globules containing RBC inclusions. While these hS globules are noticeably larger than the hS-rich domains observed in the 15/10 blend, significant variation in domain size is observed, due most likely to sampling (29) Gebizlioglu, O.; Argon, A.; Cohen, R. Polymer 1985, 26, 529. (30) Lo¨wenhaupt, B.; Hellmann, G. P. Polymer 1991, 32, 1065. Fischer, M.; Hellmann, G. P. Macromolecules 1996, 29, 2498. (31) Adedeji, A.; Jamieson, A. M. Polymer 1993, 34, 5038. Adedeji, A.; Jamieson, A. M.; Hudson, S. D. Polymer 1995, 36, 2753. Adedeji, A.; Jamieson, A. M.; Hudson, S. D. Macromolecules 1995, 28, 5255. (32) 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. (33) Almdal, K.; Rosedale, J. H.; Bates, F. S.; Wignall, G. D.; Fredrickson, G. H. Phys. Rev. Lett. 1990, 65, 1112.

Disorder “Random” Diblock Copolymer

Figure 3. Series of TEM micrographs of the hS-lean RBC/ hS15 blends with varying whS (in wt %): (a) 5; (b) 10; (c) 20. Note that the 15/5 blend exhibits narrow (ca. 50 nm wide) hS (light) channel aggregates in loosely connected hS domains. In the 15/10 and 15/20 blends, these channels widen, resulting in RBC (dark) inclusions within the hS domains.

considerations.34 The RBC inclusions, which might be envisioned as residual matrix material separating exces(34) Since the hS domains are larger than the thickness of the TEM sections (ca. 100 nm), the size of each domain in projection will depend on where the domain is intersected by the section.

Langmuir, Vol. 13, No. 8, 1997 2253

sively wide hS channels, measure approximately 50 nm in diameter and are surprisingly uniform in size. In these low-MhS blends, the existence of attractive interactions between the RBC and hS at low whS effectively prevents formation of pure hS globules, since such interactions appear to balance the repulsion required to form hS globules without RBC inclusions. By increasing the content of hS in the blend, S/hS and I/hS contacts become more probable, and the general domain shape is formed by a systematic widening of the hS channel structure. Attractive RBC/hS interactions also explain the presence of stable RBC nanoscale inclusions within the hS domains. In this regard, the inclusions can be considered micelles, since one block of the RBC undergoes attractive (less repulsive) interaction with the hS comprising the domains, while the other self-assembles to reduce unfavorable hS contact. Unlike previous studies of block copolymer micelles,35,36 however, it must be remembered that both blocks of this RBC consist of S and I. Moreover, the RBC employed here is thermodynamically disordered, whereas conventional micelles tend to consist of copolymer molecules that, in the absence of homopolymer, reside in the intermediate- or strongsegregation regimes, i.e., χN > (χN)ODT. As will be shown later, further addition of hS15 is required to exclude RBC chains more completely from the hS domains. The effect of doubling MhS on RBC/hS blend morphology is illustrated for the 30/5, 30/10, and 30/20 blends in Figure 4. This increase in molecular weight is accompanied by an increase in repulsive interactions that effectively preclude development of the narrow channel structure, even at 5 wt % hS. As Figure 4a reveals, though, the hS domains do not appear globular (circular or elliptical in projection). Rather, they exhibit surface protrusions that, due to the increase in interfacial area, are suggestive of attractive interactions between the RBC and hS30. In addition, the domains consist of interconnected RBC inclusions, appearing as dark RBC spheres surrounded by a hS-rich corona. Increasing the concentration of hS30 in the blend affects the distribution of RBC dispersions within the hS domains and yields smoother hS domain boundaries. Micrographs of the 30/10 and 30/20 blends are provided in Figure 4b and c and reveal that the blend morphologies do not change appreciably upon doubling the concentration of hS30 from 10 to 20 wt %. One noticeable difference in these blends is that the RBC inclusions in the 30/10 blend (Figure 4b) are marginally more disperse (i.e., less interinclusion connectivity) than those in the 30/5 blend (Figure 4a). This is attributed to enlargement of the hS domains due to the change in blend composition, which eliminates the protrusions seen in the 30/5 blend. Upon further addition of hS, in the 30/20 blend (Figure 4c), the inclusions are observed to aggregate, becoming larger and less uniformly dispersed. The inclusion diameters are approximately 50 and 100 nm in the 30/10 and 30/20 blends, respectively. Such aggregation results in a reduction in interfacial area between RBC and hS molecules within the hS domains, which is consistent with the expected MhS-induced increase in repulsive RBC/hS interactions. The hS domains, on the other hand, vary significantly in size for each blend composition (due again to sampling considerations34), but they appear to enlarge upon further addition of hS. Upon comparing the morphologies in Figures 3 and 4, it is interesting to note that the effect of increasing MhS (at constant hS concentration) on blend morphology is comparable to that of increasing whS at fixed MhS. (35) Kinning, D. J.; Thomas, E. L. Macromolecules 1984, 17, 1712. (36) Nojima, S.; Roe, R.-J.; Rigby, D.; Han, C. C. Macromolecules 1990, 23, 4305.

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Figure 4. TEM micrographs of the hS-lean RBC/hS30 blends with varying whS (in wt %): (a) 5; (b) 10; (c) 20. Concurrent attractive and repulsive RBC/hS interactions are responsible for the irregularly shaped hS domains in part a, whereas an increase in repulsive interactions yields macroscopic hS domains containing nanoscale RBC inclusions in parts b and c.

Morphologies of blends consisting of hS120, the highest MhS examined in this study, are shown in Figure 5 and, as anticipated, exhibit the least evidence of attractive interactions, even at the lowest whS, due to the relative size of the hS molecules (recall that MhS/MS/I is about 3.0).

Laurer et al.

Figure 5. TEM micrographs of the hS-lean RBC/hS120 blends with varying whS (in wt %): (a) 5; (b) 10; (c) 20. As seen in previous figures, RBC inclusions and aggregates reside within the macrophase-separated hS domains.

As is seen in the micrographs in Figure 5, RBC inclusions reside within the large hS domains at all three hS concentrations examined. These inclusions vary considerably in size and spatial placement, suggesting that these high-MhS blends are far removed from the hS channel structures seen in Figure 3a. It may be possible, however,

Disorder “Random” Diblock Copolymer

Figure 6. Image of the 120/20 blend showing the existence of RBC fine structure (spherical and wormlike micelles) within a single hS120 domain. Small spherical micelles measure about 80 nm across, whereas the wormlike (cylindrical) micelles are approximately 45 nm wide.

for the narrow hS channels to form if the hS120 concentration is lowered below 5 wt %. In the 120/5 blend (Figure 5a), the hS domains appear globular, consisting of relatively uniform RBC inclusions measuring about 50 nm in diameter. As observed in Figures 3 and 4, the hS domain boundary becomes smoother upon increasing whS (see Figure 5b and c). Also observed in the RBC/hS120 blends is a more pronounced dependence of hS domain size on whS. The median area of hS domains in the 120/20 blend is, for instance, nearly double that of the domains in the 120/5 blend. Although comparable trends are observed in blends possessing lower MhS, it appears to be more pronounced in the hS120 blends, presumably due to the significantly larger M, and correspondingly greater repulsion, of hS. In addition to the large hS120 domains, a few dispersed hS channel structures reminiscent of those seen in the 15/5 blend (Figure 3a) are also observed in the 120/20 blend (data not shown). Recall that, while membranelike structures (e.g., vesicles) are often observed in macrophase-separated copolymer/homopolymer blends,29-32 they are commonly the result of copolymer, not homopolymer, self-organization. A micrograph showing the interior of a lone hS120 domain is provided in Figure 6 and demonstrates that spherical RBC inclusions ranging from 80 to 450 nm in diameter, as well as cylindrical or wormlike RBC micelles measuring about 45 nm wide, reside within the hS120 domains. Loss of contiguous S and I interactions between the RBC and hS presumably prevents the formation of microscopically homogeneous hS globules when hS constitutes the minority component of the blend. As shown schematically in Figure 7, short hS molecules are more likely to experience attractive interactions with the S-rich block of the RBC than are long hS molecules, since the short chains are more sensitive to the local concentration of S in the RBC matrix. High-magnification images of the 15/5, 30/5, and 120/5 blends are presented in Figure 8 to provide a more detailed assessment of RBC/hS interactions. Since the neat RBC is disordered, exhibiting weak composition fluctuations, the solubility and morphology of relatively small quantities of added hS are expected to reflect the subtle structure of the RBC melt. Close comparison of the micrographs in Figure 8 with those provided in Figure 3 for hS15 blends supports the

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Figure 7. Conceptual illustration of the different interactions between RBC molecules (see Figure 1) and (a) low- and (b) high-M hS molecules in RBC/hS blends. Attractive interactions are shown as thin arrows, whereas repulsive interactions are displayed as thick arrows. The number of arrows indicates the relative magnitude of each type of interaction.

observation made earlier, namely, an increase in whS at fixed MhS yields a blend morphology similar to that obtained by increasing MhS at fixed whS. Once a dispersed hS morphology is attained, further addition of hS induces RBC aggregation and ultimately results in phase inversion (discussed later). Before presenting the morphologies of hS-rich blends, it is useful to compare the results obtained thus far with SAXS data reported by Tanaka and Hashimoto19 for a series of SI/hS blends in which (i) the SI copolymer (48 wt % S and M h n)31 600) is disordered and (ii) the concentration and molecular weight of added hS is systematically varied. Their results clearly demonstrate that χ(T) is dependent on both whS and MhS. As either of these two factors is increased, separately or concurrently, and macrophase separation becomes more complete, χ(T) exhibits a limiting functionality of the form A + B/T that is representative of the SI copolymer in the absence of added hS. Since χ is a measure of S-I monomer interactions, it constitutes a very sensitive probe of local mixing. The variation of χ(T) at low whS and MhS indicates that hS molecules are capable of preferentially interacting with the S blocks of the copolymer, even though the copolymer is disordered. In their work, however, they did not attempt to correlate hS morphology with χ. Similarly, Schubert et al.37 have used both small-angle neutron scattering and neutron reflectivity to show that, in a series of blends composed of deuterated hS and a statistical poly[(cyclohexyl acrylate)1-x-stat-(n-butyl methacrylate)x] copolymer, the enthalpic and entropic contributions to χ(T) (A and B, respectively) are dependent on copolymer composition (x). B. hS-Rich Blends. Blends in which the RBC constitutes the minor component exhibit, for the most part, morphologies resembling conventional macrophase-separated blends. The micrographs shown in Figure 9 are obtained from the high-whS blends (i.e., 15/95, 30/95, and 120/95). As before, the dark regions correspond to stained I units, which exist only in the RBC. Therefore, all of the dark regions in the micrographs discussed below are rich in RBC, while the light matrix is hS. In blends with hS15, macrophase-separated ellipsoidal RBC domains measure up to 2.3 µm along their major axis, which is significantly smaller than the hS domains in the hS15-lean blends (see (37) Schubert, D. W.; Abetz, V.; Stamm, M.; Hack, T.; Siol, W. Macromolecules 1995, 28, 2519.

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Figure 8. Series of high-magnification TEM micrographs of RBC/hS blends with the lowest whS examined in this study (5 wt %): (a) 15/5; (b) 30/5; (c) 120/5. These images show the MhSinduced progression of hS aggregation upon RBC/hS macrophase separation. The hS channels in part a give way to widened hS channels and dispersed RBC inclusions in parts b and c.

Figure 3). Nevertheless, the micrographs in Figure 9a and b exhibit morphological characteristics indicative of attractive RBC/hS interactions in these two blends. Nanoscale RBC dispersions measuring about 125 nm in

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Figure 9. TEM micrographs of the hS-rich RBC/hS15 blends with the highest whS examined in this study (95 wt %): (a) 15/95; (b) 30/95; (c) 120/95. Attractive RBC/hS interactions are responsible for the nanoscale RBC dispersions (arrows).

diameter are evident in the 15/95 blend (Figure 9a). Due to their size, the interfacial area separating RBC and hS15 molecules is relatively large (2000 nm2/dispersion). Similar, but fewer, dispersions are likewise seen in the 30/95 (Figure 9b) and 120/95 (Figure 9c) blends. As the molecular weight of added hS is increased, no visible evidence of mixing is apparent. In fact, an invariant macrophase-separated RBC domain morphology is found

Disorder “Random” Diblock Copolymer

Figure 10. Representative first-scan DSC thermograms of the 30/80, 30/90, and 30/95 blends, as well as the neat hS30 homopolymer, showing the variation of hS Tg with blend composition. Second scans reveal comparable Tgs. Arrows indicate the Tg values used to compare blend miscibility.

in the hS30 and hS120 blends at each of the different whS values examined here. While not presented here, micrographs of these blends also show that, for a given blend composition, the RBC domains increase in size with MhS, which is consistent with enhanced RBC/hS demixing. Concurrent near-edge X-ray absorption fine structure (NEXAFS) spectroscopic analysis38 of the 120/20 and 120/ 80 blends reveals, however, that the RBC domains in the 120/80 blend are not fully demixed, since they contain a measurable fraction of hS. It is of interest to note that the hS-rich blend morphologies obtained in the present study are similar in appearance to those observed in blends composed of an alternating poly(styrene-co-methyl methacrylate) SMMA copolymer and poly(methyl methacrylate) hMMA homopolymer.39 In 50 and 65 wt % hMMA blends, however, dispersed SMMA globules in the hMMA matrix are observed only upon perdeuteration of the hMMA. The hS-rich blends examined here therefore exhibit overwhelmingly repulsive interactions between the RBC and homopolystyrenes, despite the presence of attractive S monomer sequences along the entire copolymer chain. Micrographs obtained for the entire hS-rich blend series confirm the trend observed in the previous section addressing hS-lean blends, namely, that increasing the hS concentration causes RBC molecules to coalesce to minimize RBC-hS contacts. This is consistent with thermodynamic considerations, since a S/I RC of comparable molecular weight to the RBC employed here must be almost pure S to be miscible with hS at ambient temperature (recall that about half of the RBC is only 50/50 S/I). At some undetermined whS between 20 and 80 wt % hS, RBC molecules must be excluded from the hS domains. On the basis of the results presented here, the hS concentration at which phase separation occurs clearly depends on MhS. C. Blend Miscibility. The degree of mixing in a microphase-ordered block copolymer can be confirmed through thermal analysis of the glass transition temperature (Tg) of each block. This type of analysis has been conducted here to ascertain if the Tg of hS is depressed, indicating enhanced RBC/hS miscibility. Note that only hS Tg data are presented, since the single RBC Tg is difficult to detect by thermal calorimetry. Several representative thermograms for the hS-rich blends with hS30 are provided in Figure 10 and clearly show the hS Tg at every blend composition. The inflection point of each Tg (indicated by the arrows in Figure 10) is utilized in the

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Figure 11. Dependence of hS Tg on whS for three MhS: (4) 15 000; (O) 30 000; (]) 120 000. The decrease in Tg from high to low whS reflects an increase in RBC/hS miscibility. The dashed lines are linear interpolations over the composition range not examined in this work, and the error bar (lower right) provides an estimate of the uncertainty in the DSC data.

comparison presented in Figure 11. The dashed lines included in Figure 11 correspond to a linear interpolation across the intermediate blend compositions not investigated in this study. As expected from free-volume considerations, the neat hS Tg decreases from 99 to 93 °C as MhS decreases from 120 000 to 15 000. In addition, the hS Tg is found to decrease monotonically with decreasing whS and MhS. In the hS15 blend series, for instance, the hS Tg decreases from 93 °C at 95 wt % hS to 83 °C at 5 wt % hS. This observation indicates that RBC/hS mixing is more prevalent at low hS concentration and molecular weight due to more attractive (less repulsive) interactions between RBC and hS molecules. As seen in Figure 11, the Tg dependence on whS is least significant in the hS120 blend series, supporting the conclusion reached earlier that long hS chains undergo less attractive interactions with the S sequences along the RBC backbone (see Figure 7). This behavior is also consistent with the morphologies observed in the hS120 blends. While the dimensions of the hS domains increase with hS content in the hS-lean blends (Figure 5), the morphology and size of the RBC inclusions remain nearly constant. According to the data provided in Figure 11, the hS120 Tg is relatively constant over the range 5-20 wt % hS. In the hS-rich blends with 80 to 95 wt % hS (Figure 11), the blend morphologies reflect predominantly repulsive interactions between the RBC and hS120, and the measured increase in Tg over this range (Figure 11) corresponds to enhanced RBC/hS120 demixing. A similar, but more pronounced, trend is observed in Figure 11 for the hS15 and hS30 blend Tgs, which are found to overlap (within experimental error) in both the hS-rich and hSlean blends. Micrographs of these blend series reveal an increase in the size of macroscopic hS domains upon increasing the hS content from 5 to 20 wt %. In Figure 11, the corresponding hS Tgs increase steadily over this concentration range. As the RBC domains in the hS-rich blends decrease in size and number with increasing whS, the hS Tgs continue to increase steadily. The dependence of hS Tg on blend composition in Figure 11 is most noticeable for blends composed of either hS15 or hS30 and, with the morphologies seen in Figures 3 and 4, confirms that the hS15 and hS30 molecules interact, and mix, more with the RBC molecules than do the hS120 molecules. IV. Conclusions

(38) Smith, A. P.; Laurer, J. H.; Ade, H. W.; Smith, S. D.; Ashraf, A.; Spontak, R. J. Macromolecules 1997, 30, 663. (39) Galvin, M. E.; Heffner, S.; Winey, K. I. Macromolecules 1994, 27, 3520.

The phase behavior and morphology of binary blends of hS and a random diblock copolymer have been investigated here as functions of hS molecular weight and

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concentration using electron microscopy and thermal calorimetry. In blends possessing low-M hS at low hS concentrations, randomly positioned S monomer sequences in the RBC molecules undergo attractive interactions with the hS molecules, resulting in narrow hS channel-like, structures in a continuous RBC matrix. From these and related results,19 addition of a parent homopolymer or a (macro)molecule capable of interacting preferentially with one block of a disordered block copolymer can be utilized to probe concentration fluctuations in the melt. As either whS or MhS is increased, the hS channel structures widen until the morphology resembles globular hS domains containing micelle-like RBC inclusions. When hS is the majority phase in hS-rich blends, phase inversion occurs and macroscopic RBC-rich domains form, suggesting that the RBC/hS interactions are predominantly repulsive (although an unknown extent of intradomain mixing does occur38). Thermal analysis of these blends reveals that, for a given blend composition, the extent of RBC/hS mixing is dependent on homopolystyrene molecular weight. Generally speaking, an increase in hS concentration or molecular weight results in an increase in repulsive RBC/ hS interactions, as discerned from both morphological and thermal analyses. Identification of these trends has been greatly facilitated through the use of a disordered RBC, rather than a neat SI copolymer. Recall that for a symmetric SI copolymer, microstructural disorder could

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only be achieved if the molecular weight of the copolymer is relatively low, in which case almost oligomeric hS would be required to probe the same interactions as those reported in this work. Since the monomer ratios in the blocks comprising the RBC, as well as the effective segmental interaction parameter, can be systematically varied through tailored synthesis, it is apparent that this novel family of block copolymers affords tremendous promise as a viable means by which to examine the phase behavior of block copolymers or copolymer/homopolymer blends at ambient temperature and at a relatively constant, and (from a processing standpoint) manageable, N. These materials also provide a unique opportunity to probe the effect of monomer sequence distribution on selforganization and mesophase formation.28,40 Acknowledgment. This work was supported by grants from the National Science Foundation (CMS-9412361), Sigma Xi, and the NCSU Faculty Research and Professional Development fund. We are grateful to the NCSU College of Engineering for start-up support (R.J.S.) and to Prof. B. D. Freeman (NCSU) for the generous use of his calorimeter. LA960807Y (40) Fredrickson, G. H.; Milner, S. T. Phys. Rev. Lett. 1991, 67, 835. Fredrickson, G. H.; Milner, S. T.; Leibler, L. Macromolecules 1992, 25, 6341.