Article pubs.acs.org/Macromolecules
Ternary Phase Behavior of a Triblock Copolymer in the Presence of an Endblock-Selective Homopolymer and a Midblock-Selective Oil Arjun S. Krishnan,†,⊥ Steven D. Smith,§ and Richard J. Spontak*,†,‡ †
Departments of Chemical & Biomolecular Engineering and ‡Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States § Global Chemical Technologies, Miami Valley Innovation Center, The Procter & Gamble Company, Cincinnati, Ohio 45252, United States S Supporting Information *
ABSTRACT: Bicomponent block copolymers are known to exhibit rich phase behavior in systems containing one or two block-selective homopolymers or solvents. In this study, we combine these efforts by investigating ternary blends composed of an ABA triblock copolymer, an A-selective homopolymer and a Bselective oil. A styrenic thermoplastic elastomer is selected here because of its ability to form a physical network upon microphase separation and thus impart significant elasticity and toughness to such blends. Synchrotron small-angle X-ray scattering is employed to classify the nanostructures of blends varying in composition, homopolymer molecular weight, and oil type, and the results are used to construct ternary morphology diagrams that reveal the phases present at the glass transition temperature of the styrenic endblocks. Of all the classical and complex morphologies commonly observed in binary copolymer blends and solutions, only the bicontinuous gyroid consisting of styrenic channels in a mixed midblock/oil matrix is consistently absent. Variations in nanostructural dimensions with blend composition are provided for selected morphologies.
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can suffer from kinetic limitations due to high melt viscosities,25 solvated block copolymers possess greater molecular mobility and an improved propensity to achieve nearly equilibrated nanostructures. Several noteworthy studies have endeavored to elucidate the phase behavior of ternary block copolymer systems, which can be a daunting task due to the large parameter space involved. The classic ternary phase diagram reported by Alexandridis and co-workers26 demonstrates that nine different copolymer morphologies exist in blends varying in the concentrations of a relatively low-molecular-weight amphiphilic triblock copolymer, water, and oil. Investigations of diblock copolymers blended with their two parent homopolymers have been critical to the development27−33 of a molecular-level understanding of bicontinuous microemulsions. A topic of recent and growing interest involves ABC triblock copolymers wherein ternary phase diagrams display the morphologies of copolymer molecules varying in block length. Li et al.34 have provided one such diagram illustrating the wealth of micellar morphologies accessible in aqueous solutions of an ABC triblock copolymer, whereas Abetz35 and Epps and coworkers36 have focused on solvent-free copolymers. Corresponding theoretical treatments,37−39 following or driving such
INTRODUCTION Miscible block copolymer/homopolymer blends have attracted significant experimental and theoretical interest over the past 20 years because they serve several key purposes.1,2 From a technological standpoint, they provide a facile and attractive route by which to tailor the end-use properties of block copolymers without having to resort to custom chemical synthesis. This becomes a particularly important consideration for commercial copolymers such as thermoplastic elastomers,3,4 which are routinely used in large quantities and in an increasingly broad range of (nano)technologies. Of equal significance, though, is the fundamental insight that can be gleaned with regard to the factors responsible for mixing within the nanostructure of self-assembled block copolymers. The pioneering studies of Winey and Thomas5,6 and Hashimoto and co-workers,7−9 as well as others,10−17 have established the design rules that not only govern controlled swelling of targeted microdomains in block copolymers, but also desired morphological transitions induced by systematically altering interfacial curvature. Incorporation of a parent or block-selective homopolymer into a microphase-ordered block copolymer can have a profound effect on copolymer phase behavior, as elucidated by the theoretical predictions of Matsen.18,19 Of equal interest are the seminal works of Lodge and coworkers,20−24 who examined the influence of solvents varying in block selectivity on the phase behavior of diblock copolymers. Unlike copolymer/homopolymer blends, which © 2012 American Chemical Society
Received: February 28, 2012 Revised: May 6, 2012 Published: July 25, 2012 6056
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Figure 1. Ternary phase diagram for the system composed of a styrenic triblock copolymer (SEPS), an endblock-selective homopolystyrene (hPS6), and midblock-selective mineral oil (MO). Morphologies have been assigned on the basis of SAXS profiles acquired at ambient temperature and are color-coded with the labels provided. The two-phase region is likewise designated.
at relatively low electric fields compared to chemically crosslinked homopolymers. In addition, this networked material design has been extended49,51 to the fabrication of ionic polymer−metal composites. Prior studies have explored ABA/ O systems with a third component, such as a parent52,53 or block-selective54 homopolymer that preferentially swells the micelles formed by the copolymer endblocks. Alternatively, the ternary blend can contain a second midblock-selective species. Analysis of the mechanical properties of copolymer/cosolvent systems reveals55,56 that, insofar as the two solvents are completely miscible but differ substantially in viscosity, the systems can exhibit time−composition rheological equivalence. In the present work, we employ synchrotron small-angle X-ray scattering (SAXS) to probe the morphological development of a triblock copolymer in the presence of an endblock-selective homopolymer and a midblock-selective oil.
experimental studies, provide complementary information regarding the spatial distribution of the constituent species present. In this study, we, too, are interested in ternary phase diagrams that employ triblock copolymers (specifically, ABA thermoplastic elastomers) and combine the concepts associated with block copolymer/homopolymer blends with those of selectively solvated block copolymers. The ternary blends examined here consist of an ABA triblock copolymer, an endblock-selective homopolymer (hA) and a midblockselective oil (O). Our interest in these ternary systems stems from the unique mechanical properties afforded by binary ABA/O blends, especially at high oil concentrations. Such systems, previously referred40,41 to as thermoplastic elastomer gels (TPEGs) due to the nature of the copolymer network that develops largely as a result of bridged midblocks stabilized by glassy endblock-rich micelles, are able to achieve giant strains prior to failure, undergo strain cycling with negligible hysteresis, and endow integrated systems with considerable vibration dampening.42 They are also capable of substantial shear-induced alignment at elevated temperatures43,44 and serve as excellent model systems by which to study the structural characteristics of immobilized micelles at rest45,46 or under deformation.47 More recently, we have shown42,48−50 that such materials function as dielectric elastomers, attaining electroactuation strains as high as ∼300%
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EXPERIMENTAL SECTION
Materials. A poly[styrene-b-(ethylene-co-propylene)-b-styrene] (SEPS) triblock copolymer with 65 wt % S and a number-average molecular weight (M̅ n) and polydispersity index (PDI) of 61 kDa and 1.01, respectively, was provided by Kuraray America Inc. (Houston, TX) and used as-received. Two polystyrene homopolymers were synthesized via living anionic polymerization in cyclohexane at 60 °C with sec-butyllithium as the initiator. Molecular weight characteristics 6057
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were measured by size-exclusion chromatography, and the PDI values were less than 1.09. These homopolymers are designated as hPSm, where m denotes M̅ n in kDa. Two oils were used here: an aliphatic:alicyclic (70:30) mineral oil (MO, Hydrobrite 380; Sonneborn, Inc., Tarrytown, NY) with a molecular weight of ∼500 Da and purely aliphatic squalane (SQ, Mayumi; Japan Health Products, Inc., Granada Hills, CA) with a molecular weight of 422 Da. Reagent-grade toluene was purchased from VWR (West Chester, PA) and used without further purification. Methods. Ternary SEPS/hPS/O blends were prepared by first dissolving predetermined quantities of each component in toluene and then allowing the toluene to evaporate over the course of 5 days. The resultant films were annealed at 120 °C under nitrogen for 16 h to remove residual solvent and promote morphological development. At this temperature the MO and SQ were unaffected, since their normal boiling points are 230 and 350 °C, respectively. Portions of the films were cut into specimens for SAXS analysis performed at Argonne National Laboratory (ANL) and Brookhaven National Laboratory (BNL). Scattering experiments at ANL were conducted on undulator beamline 12-ID for which the sample-to-detector distance and beam size were 3 m and 0.5 mm × 1 mm, respectively. Specimens were exposed to the X-ray beam at ambient temperature for 0.05−0.5 s, and 2D scattering patterns were collected on a MAR CCD detector. Experiments at BNL were performed on bending magnet beamline X10A with a sample-to-detector distance of 1.7−2.1 m and a beam size of 0.8 mm × 0.6 mm. In this case, 2D scattering patterns were collected on a Bruker CCD detector. Correcting the 2D patterns for specimen transmittance, followed by azimuthal integration, yielded intensity as a function of momentum transfer (q), where q = (4π/λ) sin(θ/2), λ is the X-ray wavelength (0.133 nm at ANL or 0.154 nm at BNL), and θ is the scattering angle. Profile analysis was performed on a Wavemetrics Igor-based platform.
unmarked region extending from pure hPS6 to pure MO at low SEPS concentrations is opaque to the unaided eye (with large domains visible by optical microscopy) and is classified as macrophase-separated. No attempt is made to discern the partitioning of the copolymer in these systems. Generally speaking, as the SEPS concentration is increased, six different block copolymer morphologies can be distinguished: SPH1, CYL1, LAM, BIC2, CYL2, and SPH2. While details are provided in following sections, a brief description of each is given here to establish an overview of the system under investigation. In the first case, spherical micelles composed of S cores are (i) surrounded by an EP corona that is swollen by MO and (ii) spatially positioned on a cubic lattice, the details of which are discussed later. A representative SAXS profile corresponding to spherical micelles (SPH1) on a face-centered cubic (fcc) lattice is presented in Figure 2a. In addition to
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RESULTS AND DISCUSSION General Design Considerations. On the basis of the SEPS copolymer molecular weight and architecture, in conjunction with the composition provided by the manufacturer, we estimate the size of each endblock to be 19.8 kDa. Since the polystyrene homopolymers employed in this study should remain largely miscible within the S-rich microdomains that form upon copolymer self-organization, they should not be much larger than the endblocks, as prescribed by independent experimental and theoretical studies.1 Conversely, if the homopolymers are too short, they will behave as plasticizing agents for the S-rich microdomains and consequently reduce the upper glass transition temperature (Tg), which, in turn, will compromise the unique and attractive mechanical properties of this class of solvated macromolecules. For these reasons, the homopolymer molecular weights were selected as 6 kDa (hPS6) and 25 kDa (hPS25). Furthermore, although MO is 30% alicyclic and can slightly plasticize the S microdomains, it has been used extensively in prior studies reported40,41,46,52,57,58 for this class of materials, in which case it is retained in this work. We likewise recognize that SQ has also been employed in investigations59,60 of selectively solvated triblock copolymers and include it for comparison. Since (i) the objective of this work is to elucidate the phase behavior of block copolymer/ homopolymer/oil blends, (ii) previous studies have focused specifically on binary systems composed of triblock copolymers with either added homopolymer61 or midblock-selective oil,62 and (iii) the homopolymers and aliphatic oils are, for the most part, immiscible at ambient temperature, we concentrate almost exclusively on ternary blends in this study. Ternary Phase Behavior. The ternary morphological diagram mapped out for the SEPS/hPS6/MO system at ambient temperature is presented in Figure 1. Much of the
Figure 2. Representative SAXS profiles acquired from SEPS/hPS6/ MO blends varying in composition (in wt %) and morphology: (a) 10.0/0.5/89.5, SPH1; (b) 30.0/3.0/67.0, CYL1; (c) 40.0/20.0/40.0, LAM; (d) 80.0/4.0/16.0, BIC2; (e) 90.0/5.0/5.0, CYL2; and (f) 20.0/ 80.0/0.0, DIS. The principal peak (located at q0) is designated by an arrowhead, whereas each higher-order peak is identified by an arrow and its peak ratio relative to q0. This annotation scheme will be utilized in subsequent figures. 6058
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Only ternary blends with copolymer fractions equal to or greater than 10 wt % are considered further here. Spherical Morphology (SPH1). A relatively small cubic phase (SPH1) is visible up to ∼30 wt % SEPS in MO. As alluded to earlier, the copolymer in this composition window forms an isotropic, physically cross-linked network consisting of dispersed micelles with glassy S-rich cores that connect highly swollen midblock bridges. An example of a SAXS profile acquired from a 10.0/0.2/89.8 (w/w/w) SEPS/hPS6/MO blend is displayed in Figure 3a. This system lies very close to the SEPS/MO axis and is selected for scrutinization because it is representative of the systems that likewise exhibit SPH1 morphologies at higher copolymer concentrations in Figure 1.
spherical micelles, cylindrical micelles with S cores (CYL1) are observed to reside on a hexagonal lattice within a mixed matrix of EP and MO, as evidenced by the SAXS profile shown in Figure 2b. Alternating lamellae (LAM) and inverted bicontinuous morphologies (BIC2), which can be envisaged as bilayered sheets and triply-coordinated channels, respectively, are apparent from the SAXS profiles displayed in Figures 2c and 2d, respectively. In most cases the BIC2 morphologies exist as soft EP/MO channels in a glassy matrix and can be confidently indexed as gyroid with Ia3̅d symmetry.63 In the CYL2 morphology, hexagonally-packed cylinders consisting of mixed EP/MO cores in a polystyrene matrix, as signified by the SAXS profile in Figure 2e, develop and eventually give way to an inverted spherical morphology (cf. Figure 2f) at low MO concentrations. Unlike the ordered micelles in SPH1, those in DIS possess a soft EP/MO core and remain largely disordered. Since the SEPS copolymer effectively serves as a macromolecular surfactant for hPS6 and MO, we first consider the case in which the concentration of SEPS is systematically increased. It should be remembered, however, that an increase in SEPS concentration also affects the propensity for the EP midblock to form bridges64 and, by connecting glassy microdomains, the molecular network capable of withstanding large strains and exhibiting a highly elastic (electro)mechanical response. Such networks can only develop when the EP midblocks (with or without MO) constitute a continuous matrix (as in the SPH1 and CYL1 morphologies) or the continuous channels or alternating sheets expected in the BIC2 or LAM morphologies, respectively. At nearly equal fractions of hPS6 and MO, an increase in the concentration of the SEPS copolymer induces a LAM → CYL2 transition, which, in some instances, involves the formation of an intermediate BIC2 morphology. It is interesting that, while an inverted bicontinuous morphology is observed, the noninverted analogue composed of triply-periodic styrenic channels in a matrix of MO-swollen EP has not been detected in this phase diagram. We return to address this curiosity later. At the highest copolymer concentrations (>90 wt %) interrogated, the SEPS copolymer and MO are only partially miscible, in which case some of the MO undergoes macrophase separation. In general, structural variability decreases with increasing SEPS loading so that more morphologies form as the SEPS concentration is lowered. This observation indicates that the copolymer monolayer separating hPS6 and MO becomes progressively susceptible to subtle changes in curvature. As the critical gel concentration (cgc) is approached at very low SEPS fractions, the ability of the midblocks to form a load-bearing network decreases because the EP midblocks comprising the corona of copolymer micelles tend to form loops instead of a mixture of loops and bridges due to the enlarged intermicellar distance. Blends in this composition regime are expected to behave more like viscoelastic liquids than elastic solids. The possibility that, at concentrations below the cgc, discrete populations of copolymer molecules remain physically connected through the formation of flocs21,61,62 must also be considered. At still lower concentrations of SEPS in hPS- and/ or MO-rich systems, the critical micelle concentration (cmc) establishes the condition at which the copolymer molecules are first able to self-organize into micelles. Identification of the conditions corresponding to the cmc in hPS, MO or a combination thereof is of general and significant interest. While they have not been investigated in the present study, they have been reported7,9,65−67 elsewhere in the case of binary systems.
Figure 3. (a) SAXS profile collected from the 10.0/0.2/89.8 SEPS/ hPS6/MO blend exhibiting the fcc spherical morphology. (b) SAXS profiles acquired from 30.0/0.6/69.4 SEPS/hPS6/MO and SEBS/ hPS6/MO blends (curves 1 and 2, respectively). In both cases, the hPS6 concentration is 2 wt % of the copolymer. The blend containing SEPS (curve 1) possesses peaks indicating coexistence of the bcc (filled arrowhead/arrows) and hcp (open arrowhead/arrows) morphologies, whereas the blend with SEBS (curve 2) exhibits the bcc morphology. 6059
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(curve 2). Relative to the other SAXS profile, the peaks are more sharply defined, indicating that the spherical micelles are more highly ordered on their lattice. From the peak ratios located at 1.00:1.44:1.73:2.00 relative to q0 at 0.164 nm−1, this profile is consistent with the bcc morphology.5,69,70 Although the interaction energy between the EB midblock and MO is expected to differ slightly from that between the EP midblock and MO, we contend that this difference is not the primary cause for this morphological difference. Rather, we attribute this difference to the compositions of the two copolymers. The endblocks are of comparable length (19.8 vs 21.6 kDa in the SEPS and SEBS copolymers, respectively), but the midblock in the SEBS copolymer (101 kDa) is significantly longer than that in the SEPS copolymer (21.3 kDa). At 10 wt % copolymer, the SEPS molecules self-organize into micelles possessing relatively thin coronas. For this reason, they are referred71 to as “crewcut” micelles. The SEBS molecules, on the other hand, form more commonly encountered “hairy” micelles with thicker coronas. Such variation in copolymer morphology due to a difference in coronal thickness is consistent with the McConnell−Gast criterion,72 which states that the bcc lattice is preferred over the fcc lattice when the micellar corona is larger and intermicellar interactions are softer due to interpenetration. The effect of adding hPS6 to the SEPS/hPS6/MO blend in Figure 3b at the expense of reducing the concentration of MO at constant SEPS concentration is considered in Figure 4. Comparison of the SAXS profile in Figure 4b with the one displayed in Figure 3b reveals that the 30.0/1.5/68.5 system in Figure 4 possesses modestly larger (by ∼10%) micellar cores than the complementary 30.0/0.6/69.4 blend shown in Figure
The electron contrast responsible for such scattering derives from the difference between the aromatic-rich micelle cores and the aliphatic-rich matrix (including the micelle coronas). Broad form factor, P(q), peaks at high q provide information regarding the size and shape of the micelle cores and can be accurately fitted to a model for polydisperse spheres, from which the corresponding core diameter determined by regression analysis is 11.4 ± 0.3 nm. [Examples of fitted data are provided as Supporting Information for illustrative purposes.] Structure factor, S(q), peaks at lower q can be used to assign the scattering elements to a periodic lattice, if one exists. By selecting the peak at 0.185 nm−1 as the principal peak (q0), the Bragg peak ratios relative to q 0 are located at 1.00:1.62:1.91:2.52, which again verifies existence of the fcc lattice. Absence of the [200] reflection, corresponding to a Bragg peak ratio of 1.15, can be attributed to the orientation of the specimen during loading.68 This morphology is retained upon increasing the hPS6 concentration to 1.5 wt %, beyond which samples become visibly hazy and thus signal the onset of macrophase separation. Precise determination of the phase boundary demarcating microphase- and macrophase-separated regions requires additional analysis to complement SAXS and is beyond the scope of this work. Displayed in Figure 3b is the SAXS profile for the 30.0/0.6/ 69.4 SEPS/hPS6/MO system (curve 1). Assigning q0 to the broad principal peak visible at 0.211 nm−1 yields Bragg ratios of 1.00:1.41:1.60:2.00:2.40, which do not correspond to any known morphology. If we presume that two different morphologies coexist, however, then the broad peak with a slight shoulder is a consequence of two overlapping peaks (one at 0.211 nm−1 and the other at 0.195 nm−1). The solid arrows assigned to curve 1 in Figure 3b correspond to the commonly encountered body-centered cubic (bcc) morphology with peak ratios of 1.00:1.41:2.00, whereas the open arrows identify peak ratios of 1.00:1.73:2.65, which are typically associated with a hexagonally close-packed (hcp) spherical morphology. Broad P(q) peaks indicative of dispersed spheres are present at 0.751 and 1.186 nm−1, respectively. In the absence of a more complex form factor, we propose that the existing morphology is a mixture of hcp and bcc spheres, for which precedence exists. Lodge and co-workers61 have, for instance, reported on such coexistence after decoupling the Bragg peaks of two morphologies in a solvated diblock copolymer system subjected to shear alignment. The observation of coexisting bcc and hcp peaks in Figure 3b is therefore fortuitous in our system. Comparison of this SAXS profile with the one in Figure 3a, which represents a SEPS/hPS6 ratio of 50:1, reveals that the increase in SEPS concentration achieved mainly by reducing the MO fraction increases the mean micellar core diameter (15.5 from 11.4 nm) due to an increase in association number (i.e., the number of copolymer molecules/micelle). The transformation from micelles positioned on an fcc (with a possible, but indiscernible, coexisting hcp) lattice at low SEPS concentrations to micelles residing on a bcc lattice at higher copolymer concentrations can be explained, at least in part, in terms of coronal interactions. When coronal overlap among micelles is considerable, bcc packing is favored due to its smaller entropic penalty relative to fcc packing.69 To investigate this relationship further, we include a SAXS profile acquired from a comparable ternary blend containing a poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) triblock copolymer (30 wt % styrene and M̅ n = 144 kDa from Kraton Polymers, Houston, TX) for comparison in Figure 3b
Figure 4. A series of SAXS profiles obtained from ternary SEPS/ hPS6/MO blends with 30 wt % SEPS: (a) 30.0/0.0/70.0, (b) 30.0/ 1.5/68.5, and (c) 30.0/3.0/67.0. The intermediate blend (b) exhibits peaks indicating the coexistence of bcc (filled arrowhead/arrows) and hcp (open arrowhead/arrows) morphologies, whereas the peak positions in (c) reveal the presence of a hexagonally-packed cylindrical morphology. 6060
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3b, thereby confirming that the added hPS is incorporated within the cores. Similar homopolymer solubilization has been previously reported54 for SEBS-based TPEGs possessing a distorted (possibly bcc) lattice. The present result provides evidence that, at a different hPS6 concentration, the mixed bcc + hcp morphology remains thermodynamically favored. (Although the principal peak for the hcp spheres is not as discernible as it is in Figure 3b, we estimate its position at 0.192 nm−1 from the corresponding higher-order peaks in Figure 4b.) Further increasing the hPS6 content, however, promotes a pronounced morphological change. Doubling the fraction of hPS6 to 3.0 wt % and concurrently reducing the MO concentration to 57 wt % yields the SAXS profile included in Figure 4c. In this case, the distinct P(q) peaks indicative of spheres are absent, and the Bragg peak ratios relative to q0 at 0.187 nm−1 are 1.00:1.73:2.00:2.65, which corresponds to the scattering signature of cylinders positioned on a hexagonal lattice. Addition of hPS6 at constant copolymer concentration thus induces a change in interfacial curvature of the dispersed Srich microdomains, which leads to a morphological transition from spherical micelles on a mixed bcc + hcp lattice to hexagonally-packed cylinders. The overall concentration of polystyrene (PS) from both the copolymer endblocks and added homopolymer in the blend is ∼22.5 wt % at this transition. Cylindrical Morphology (CYL1). A morphology consisting of PS-rich cylinders dispersed in a highly MO-swollen EP matrix is observed for blends with SEPS concentrations as low as 20 wt % and as high as 60 wt %. Specimens located in region CYL1 of Figure 1 exhibit the characteristic peak ratios of hexagonallypacked cylinders, as detailed above. Figure 5 displays SAXS profiles for two systems with different blend compositions: 30.0/12.0/58.0 and 20.0/18.0/62.0 SEPS/hPS6/MO. As in Figure 4, the peak ratios extracted from the former are
consistent with those expected for a hexagonally-packed cylindrical lattice. In the latter case, however, the second reflection (at √3) is missing. Absence of this signature peak is attributed to the presence of an overlapping minimum in P(q), which is responsible for a marked reduction in Bragg peak intensity.73,74 These two compositions are selected for discussion here because the overall PS fraction in each blend is similar (∼31 wt %). Since the densities of the EP midblock and MO are comparable (∼0.84 g/cm3), the overall volume fraction of PS in both systems is nearly identical (∼27 vol %). Despite this composition (and morphological) equivalence, the position of q0 is significantly different in each system: 0.155 and 0.127 nm−1 for the 30.0/12.0/58.0 and 20.0/18.0/62.0 SEPS/ hPS6/MO blends, respectively. These values yield cylindrical periodicities (D), calculated from 4π/(q0√3), of 46.9 and 57.3 nm, respectively.75 Similar composition-induced variation in D is evident in Figures 6a and 6b for ternary systems with an overall PS concentration of 35 wt %. In Figure 6a, the cylindrical radius (R) is observed to decrease modestly from 17.5 to 11.1 nm as the concentration of SEPS is increased, and the extent of swelling in the cylinders and surrounding matrix is decreased due to accompanying reductions in the concentrations of hPS6 and MO, respectively. Included for comparison in Figure 6b are values of D measured for ternary SEPS/hPS6/MO systems also with an overall PS concentration of 35 wt %. As expected, Dconsistently increases as the concentration of SEPS decreases (and MO increases) due to selective swelling of the EP midblocks. In Figures 6a and 6b, the concentration of SEPS is systematically varied at a constant PS level. Next, we consider a case in which the concentration of SEPS is held constant at 40 wt % and the concentration of added hPS6 is increased. Figure 6c confirms that, under these conditions, an increase in hPS6 content promotes a nonlinear increase in R (due to cylindrical swelling), which likewise serves to reduce D (due to less matrix swelling at lower MO fractions). Lamellar Morphology (LAM). The LAM morphology is predominant in Figure 1 when the overall PS content is greater than 40 wt %. A representative SAXS profile is displayed for the 40.0/20.0/40.0 SEPS/hPS6/MO system in Figure 2c and confirms the existence of integral Bragg peak ratios of 1.00:2.00:3.00:4.00 relative to the principal peak at q0. Values of D extracted from such profiles measured from blends with an overall PS content of 51 wt % are provided in Figure 7 and reveal similar trends previously identified with respect to the CYL1 morphology in Figure 6b. An increase in SEPS concentration at the expense of simultaneously lowering the concentrations of hPS6 and MO, for example, is responsible for promoting a noticeable decrease in D due to reduced swelling. In Figure 8, we first consider the dependence of D on SEPS concentration in ternary systems exhibiting the LAM morphology at a constant hPS6 concentration of 30 wt %. As expected from the results shown in Figure 7, a reduction in the concentration of SEPS is accompanied by an increase in the fraction of MO present and yields a systematic increase in D due to enhanced EP swelling. Conversely, the results presented in the inset of Figure 8 display the fractional change in D upon a 10 wt % change in hPS6 concentration at different SEPS concentrations and reveal a surprising result: D increases (the most at low SEPS concentrations) as the MO fraction decreases. For this to occur, the thickness of the S lamellae must swell to a greater extent than the EP lamellae contract. If the mass densities of hPS6 and MO are comparable or if the
Figure 5. SAXS profiles acquired from ternary SEPS/hPS6/MO blends with equal overall PS compositions of 31 wt %: (a) 20.0/18.0/ 62.0 and (b) 30.0/12.0/58.0. 6061
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Figure 7. Lamellar periodicity (D) as a function of SEPS concentration in ternary SEPS/hPS6/MO blends possessing an overall PS fraction of 51 wt %. The solid line serves as a guide for the eye.
Figure 8. Dependence of D on SEPS concentration in ternary SEPS/ hPS6/MO blends possessing a constant hPS6 concentration of 30 wt % and exhibiting the lamellar morphology. The inset shows the relative change in D that occurs upon increasing the concentration of hPS by 10 wt % at different SEPS concentrations. The solid lines serve as guides for the eye.
increase in hPS6 loading at constant SEPS concentration induces more localization and greater swelling of the S lamellae. In contrast, the MO molecules should more fully wet the EP blocks (which form bridges and loops within the EP lamellae). Besides these composition-dependent changes in nanostructural dimensions, the mechanistic route by which the LAM morphology is accessed warrants discussion at this juncture. At 50 wt % SEPS in MO, the observed morphology in Figure 1 is CYL1. Addition of hPS6 to the blend (yielding 50.0/10.0/40.0 SEPS/hPS6/MO) results in a transformation to the LAM morphology, suggesting that an intermediate morphology is not thermodynamically favored. The same is true (in the absence of hPS6) as the copolymer loading in MO is increased from 57 wt % (CYL1) to 60 wt % (LAM). It seems therefore unlikely that this transition proceeds through an
Figure 6. Variation of nanostructural dimensions with composition for ternary SEPS/hPS6/MO blends possessing the CYL1 morphology. The radius (R) and periodicity (D) of cylinders in ternary SEPS/ hPS6/MO blends possessing an overall PS composition of 35 wt % are shown as functions of SEPS concentration in (a) and (b), respectively. In (c), the dependence of R on hPS6 content is displayed at a constant SEPS concentration of 40 wt %. The solid lines serve as guides for the eye.
density of MO is lower than that of hPS6, this scenario implies that the hPS6 molecules only partially wet the brushes formed by the S blocks comprising the S lamellae and are localized, to an expectedly small extent, along the lamellar midplane. An 6062
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intermediate morphology,57 which would consist of networked (possibly bicontinuous) PS channels in an EP/MO matrix. This is not, however, the case on the opposite side of the LAM regime in Figure 1. Inverted Bicontinuous Morphology (BIC2). As the composition of the ternary system is varied so that the overall PS concentration lies between 52 and 56 wt % and the concentration of hPS6 is relatively low (
