Article pubs.acs.org/Macromolecules
Effects of Polymer and Salt Concentration on the Structure and Properties of Triblock Copolymer Coacervate Hydrogels Daniel V. Krogstad,†,‡ Nathaniel A. Lynd,‡ Soo-Hyung Choi,‡,% Jason M. Spruell,‡ Craig J. Hawker,†,‡,§ Edward J. Kramer,†,‡,∥ and Matthew V. Tirrell*,⊥,# †
Department of Materials, ‡Materials Research Laboratory, §Department of Chemistry and Biochemistry, and ∥Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States ⊥ Institute for Molecular Engineering, University of Chicago, 5747 South Ellis Avenue, Chicago, Illinois 60637, United States # Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Structure−property relationships were established for complex coacervate hydrogels formed from binary aqueous solutions of oppositely charged ABA triblock copolymers. The charged triblock copolymers were synthesized by functionalizing poly[(allyl glycidyl ether)-b-(ethylene oxide)-b-(allyl glycidyl ether)] with either guanidinium or sulfonate functional groups. When aqueous solutions (ca. 5−40 wt %) of these oppositely charged polymers were mixed, the electrostatic interactions of the functionalized blocks led to the association of the oppositely charged end-blocks into phase-separated complex coacervate domains bridged by the uncharged, hydrophilic PEO midblock. The resulting structures were studied by small-angle X-ray scattering (SAXS) and dynamic mechanical spectroscopy. The organization of the coacervate domains was shown to affect substantially the viscoelastic properties of the hydrogels, with the storage modulus increasing significantly as the mixtures transformed from a disordered array of domains to an ordered BCC structure with increasing block copolymer concentration. As the polymer concentration was further increased to 30 wt %, a hexagonal structure appeared, which coincided with a 25% drop in the modulus. Further structural changes, resulting in variations in the viscoelastic response, were also induced through changes in salt concentration. The viscoelastic properties and the physical nature of the cross-links have important implications for the applicability of these gels as injectable drug delivery systems.
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INTRODUCTION Hydrogels have many potential uses in next-generation biomaterial systems including protein,1 cell,2,3 DNA3,4 and drug delivery systems,3−8 cell and tissue growth scaffolds,3,8,9 bioadhesives,3 and biological sensors.3,7,8 They consist of a majority of water and can be synthesized to exhibit a wide range of viscoelastic properties,3,7,9 including those comparable to many different body tissues.10 Additionally, they can be designed to be injectable, biodegradable, or responsive to small changes in environmental conditions including changes in temperature, pH, or salt concentration.3,11,12 Localized delivery of drugs, cells, or DNA through a minimally invasive procedure such as injection is ideal for many applications.2,3,9 In such a configuration, the materials must be injected through a syringe with a minimum amount of pressure and then rapidly solidify once inside the body to prevent the loss of the hydrogel components or the therapeutic cargo.2,3,12 Many types of hydrogels have been synthesized utilizing both chemical and physical cross-linking; however, not all of them are appropriate for injectable delivery systems. Frequently, hydrogels consist of polymers that are covalently cross-linked to form a three-dimensional network, which provides structural rigidity to the material. These chemically © 2013 American Chemical Society
cross-linked hydrogels have very good mechanical properties but are not typically injectable, biodegradable, or environmentally responsive. To overcome these limitations, injectable chemically cross-linked gels have been developed in which the gel components, a photoinitiator, and the cargo are mixed before injection and then cross-linked upon irradiation with UV-light postinjection.1,3,10 A potential problem with this strategy is the burst release of the cargo in vivo before crosslinking as well as the difficulty associated with performing crosslinking reactions within the body. Biodegradability in chemically cross-linked hydrogels is also an issue; therefore, a number of systems have been designed with biodegradable units. Aimetti et al. developed a photoinitiated PEG-based system that incorporated a peptide unit that could be easily cleaved by human neutrophil elastase (HNE), an enzyme common at sites of inflammation.1 Given these challenges, there are still few systems that meet all of the desired requirements for good bioperformance. Received: November 6, 2012 Revised: January 18, 2013 Published: February 6, 2013 1512
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The opportunities to improve upon chemically cross-linked hydrogels provide an impetus for further study of hydrogels based on physical associations. These hydrogels consist of cross-linking domains that are held together by hydrophobic, electrostatic, or hydrogen-bonding interactions.3,7 The relatively weak and reversible associations in these systems allow the gels to be biodegradable, injectable, and responsive to external stimuli. The most common physically associated hydrogels rely on hydrophobic interactions. These hydrophobic systems include a number of well-characterized temperature-responsive systems and have a wide range of properties and applications.2,3,5−9,11,13−16 These polymers either have an upper (UCST)5,7,11 or lower (LCST)8,11,14 critical solution temperature such that it is a polymer solution in a syringe (either at a temperature lower or higher than body temperature, respectively), but when injected into the body at 37 °C, the temperature change results in the formation of a hydrogel. The sol−gel transition temperature is easily manipulated via the relative ratios of the polymer blocks.5,11,14 Other injectable hydrophobically driven hydrogels have been developed in which they can be made injectable through a shear-thinning mechanism.2,3,9,15 This mechanism allows for potentially faster gel formation after injection with a minimal amount of cargo loss. While this strategy is often effective, synthesis and purification of amphiphilic polymers can be difficult owing to differences in solubility among the different pieces of the molecules. Electrostatic hydrogels not only have the benefit of being completely hydrophilic, but the nature of the interactions facilitates tunability of structure and properties with salt concentration and pH. It is well-known that oppositely charged polyelectrolytes can be mixed together and form liquid−liquid macrophase-separated domains through a process known as complex coacervation.4,17−20 In order to make use of this process, several groups have synthesized block copolymers with a neutral, water-soluble block and a charged block. When oppositely charged block copolymers are mixed together, micelles were formed in which the polyelectrolytes microphaseseparated into complex coacervate domains with the neutral block forming the corona.21−31 Additionally, Lemmers et al. have used complex coacervation to form hydrogels by mixing triblock copolymers with negatively charged end-blocks and a neutral mid-block mixed with positively charged homopolymers.32,33 These coacervate domains have higher water content in the cores than hydrophobic systems, allowing easy encapsulation of charged drugs, proteins, and DNA. Furthermore, manipulation of the pH of the polymer solution enables injection of a polyelectrolyte solution followed by rapid coacervation at the pH levels of the body.7 Hunt et al. previously reported a poly[(allyl glycidyl ether)-b(ethylene oxide)-b-(allyl glycidyl ether)], P(AGE-b-EO-bAGE), ABA triblock copolymer system in which the AGE groups were functionalized with various charged groups. It was found that the hydrogels exhibiting the highest shear storage moduli were a mixture of triblock copolymers that were functionalized with guanidinium and sulfonate charged moieties, and as shown in Figure 1, these triblock copolymers self-assemble when mixed in aqueous solution to form coacervate domains with the PEO mid-blocks serving as bridges in the networks.34 In contrast to mixtures of triblock copolymers with homopolymers, hydrogels formed from only triblock copolymers will have a higher number of bridges and
Figure 1. Chemical structure of the (a) guanidinium-functionalized and (b) sulfonate-functionalized P(AGE-b-EO-b-AGE) triblock copolymers and the (c) schematic of coacervate formation. When the two polymer solutions were mixed, electrostatic interactions between the oppositely charged end-blocks caused the formation of complex coacervate domains that act as physical cross-links in the hydrogels.
have the potential to form stronger gels. In this study, we report the effect of variations in polymer concentration, salt concentration, pH, and stoichiometry of the charged moieties on the structure and viscoelastic properties of these coacervate hydrogels derived from binary solutions of guanidiniumfunctionalized and sulfonate-functionalized triblock copolymers.
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EXPERIMENTAL PROCEDURES
Polymer Synthesis and Functionalization. The triblock copolymers were synthesized by anionic ring-opening polymerization of allyl glycidyl ether from a 20 000 Da PEO-diol macroinitiator and functionalized using thiol−ene click chemistry as described previously by Hunt et al.34 The P(AGE31-EO455-AGE31) triblock copolymer precursor had a number-average molecular weight of 27 000 Da (1H NMR spectroscopy) and a polydispersity of 1.14 (size exclusion chromatography). The guanidinium-functionalized polymer had a molecular weight of 36 700 Da, and the sulfonate-functionalized polymer had a molecular weight of 38 100 Da. Hydrogel Preparation. The guanidinium-functionalized and the sulfonate-functionalized polymers were dissolved separately in water or aqueous solutions of NaCl at the desired concentration and stoichiometry. The sulfonate-functionalized polymer solution was then added to the guanidinium-functionalized polymer solution and mixed with a vortex at room temperature for 30 s. Dynamic Mechanical Spectroscopy. Dynamic mechanical spectroscopy experiments were performed on a Rheometrics Scientific Ares II rheometer using the parallel plate geometry (25 mm diameter). The samples were allowed to equilibrate for 3 h prior to rheometry. The hydrogel samples were scooped onto the plate, and a time sweep at 1 Hz and 1% strain was performed for 75 min to observe gel reformation within the sample stage. All frequency sweep measurements were carried out in the linear viscoelastic regime at 1% strain. A temperature of 25 °C was actively maintained for all experiments. These experimental conditions were determined to minimize the 1513
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amount of drying of the hydrogels while on the plate (a 15 wt % sample was measured to be 15.4 wt % after the rheology test using these conditions). Longer wait times or higher temperatures, however, increased the water loss from the hydrogels. Small-Angle X-ray Scattering. Samples for small-angle X-ray scattering (SAXS) experiments were prepared in the same manner as for the rheometry experiments. After 3 h, the samples were packed with a centrifuge into 1.5 mm diameter quartz capillaries. The capillaries were flame-sealed to prevent drying of the hydrogels. SAXS experiments were performed at beamline 8-ID-E at the Advanced Photon Source, Argonne National Laboratory using 7.35 keV X-rays and a detector distance of 2.18 m.
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RESULTS AND DISCUSSION Polymer Concentration and Stoichiometry. The dynamic mechanical response of P(AGE31-EO455-AGE31) functionalized triblock copolymer hydrogels with various polymer concentrations was investigated and is shown in Figure 2. The shear loss modulus, G″, is not shown for the 20,
Figure 3. SAXS patterns of samples at varying polymer concentration. The structure of the samples range from a disordered array of spherical domains (D: 5, 10, and 15 wt %), spherical domains on a BCC lattice (B: 20 and 25 wt %), and hexagonally packed cylinders (H: 30, 35, and 40 wt %) as the polymer concentration was varied. The SAXS patterns were shifted vertically for clarity.
intermediate (15−25 wt %), and concentrated (more than 25 wt %). Dynamic mechanical spectroscopy (Figure 2) reveals that at concentrations of less than 15 wt % the mixture was a complex fluid as indicated by the presence of a crossover frequency (ωx), below which G″ exceeds G′, within the experimental frequency range (0.005 Hz < ω < 10 Hz). Interestingly, for all of the samples, G′ did not scale as ω2 at the lowest ω, indicating that even at the lowest concentrations, the samples do not behave as simple liquids. At 5 wt %, ωx was 1.7 Hz and decreased as the polymer concentration was increased to 10 and 15 wt %. The SAXS pattern (Figure 3) of the 5 and 10 wt % mixtures exhibited a broad intensity peak near q* = 0.03 Å−1, consistent with a diffuse, disordered array of coacervate domains. As the polymer concentration was increased from 5 to 15 wt %, q* increased, indicating a decreased spacing between disordered domains. At 15 wt %, weak reflections can be observed that are attributed to a largely disordered structure with very weak ordering on a cubic lattice. As the polymer concentration is increased above 15 wt % into the intermediate regime, a shift of ωx to below the experimental range (Figure 2) and a large increase in G′ were observed. In this regime, G′ depends only weakly on frequency, indicating a solidlike material with G′ increasing by a factor of 15 at 0.005 Hz and a factor of 2 at 1 Hz from the 15 wt % sample to the 20 wt % sample. Overall, as the polymer concentration was increased from the dilute regime at 5 wt % to the intermediate regime at 25 wt %, G′ at 1 Hz increased by a factor of almost 25 to a maximum of ∼8.5 kPa at 25 wt %. Investigation of the intermediate regime using SAXS (Figure 3) shows that the coacervate domains ordered on a well-defined, body-centered cubic (BCC) lattice for the 20 and 25 wt % samples. The 2D SAXS pattern for the 20 wt % BCC sample is shown in Figure 4. Surprisingly, the 2D scattering pattern consisted of
Figure 2. Dynamic mechanical spectra of hydrogels at varying polymer concentration. The G′ data are shown as solid symbols whereas the G″ data have open symbols. Both G′ and G″ are shown for the 5, 10, and 15 wt % samples in order to clearly see the crossover frequency. The G″ for the 20, 25, and 30 wt % samples was not shown for clarity since these profiles did not show a crossover frequency in the range displayed and G′/G″ was greater than 2 over the entire tested frequency range.
25, and 30 wt % samples for clarity since G′/G″ > 2 over the entire frequency range for each of these samples (full spectra are presented in Figure S1). Additionally, SAXS was performed in order to correlate the concentration-dependent changes in viscoelastic response (Figure 2) to possible morphological transitions. Radially integrated SAXS patterns for polymer concentrations ranging from 5 to 40 wt % are shown in Figure 3. Comparing the results presented in these two figures, it becomes obvious that the structural organization of the coacervate domains has a major, determining effect on the viscoelastic properties. Specifically, three distinct concentration regimes can be identified based on viscoelastic behavior and morphological characteristics: dilute (less than 15 wt %), 1514
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polymers can therefore slide along the cylinder in order to remove the strain placed on the system, resulting in a decreased modulus. Ryu et al. were able to align cylinders formed by PS− PI−PS block copolymers in three directions and showed a significant decrease in the modulus of a material with cylinders that were aligned parallel to the shear direction versus cylinders that were aligned perpendicular to the shear direction.36 In the case of the 30 wt % coacervate-based hydrogels, the cylinders had not been macroscopically aligned and showed only a small decrease in mechanical response due to averaging over domains with different orientations. The effect of varying the stoichiometric ratio of the two oppositely charged triblock copolymers was also investigated. Significantly, the dynamic mechanical data were nearly identical for samples that had a molar ratio of 45:55 to 60:40 guanidinium-functionalized block copolymer:sulfonate-functionalized block copolymer (G:S) (Figure S2). At ratios of 40:60 and 70:30 G:S, the frequency dependence of the viscoelastic response was only slightly affected by the changes in stoichiometry; however, the storage modulus decreased in magnitude by a factor of 5 as compared to the stoichiometrically matched 50:50 G:S sample. To better understand the structure of these samples (Figure S3), SAXS was performed and clearly showed that the 45:55, 50:50, and 55:45 G:S samples all had well-defined BCC structures. At 40:60 and 60:40 G:S, the samples became very weakly ordered; the more off-stoichiometric samples of 30:70 and 70:30 G:S consist of disordered domains. The minimum q*, i.e., the largest domain spacing, occurs at the stoichiometrically matched 50:50 G:S hydrogel. This small range of stoichiometric ratios in which coacervation occurs is in agreement with previous studies of complex coacervates in both homopolymers and diblock copolymers.20,26,28,32 At stoichiometric ratios further from unity, the majority of the polymers were fully dissolved and did not contribute to network formation. Salt Concentration and pH. Salt concentration was found to have a significant effect on the hydrogels as seen in the dynamic mechanical spectra (Figure 5) and SAXS data (Figure 6) for the 20 wt % samples. The coacervate domains consist of ionic polymers, so the weakening of the gels with increased salt was expected with both the dynamic mechanical data and SAXS showing little change from 0.0 to 0.25 M NaCl. However, q* increased for these samples, indicating a decreased domain spacing. The most profound changes occurred as the salt concentration was increased above 0.25 M. In these cases, the dynamic mechanical data show that G′ decreased, and there was a significant change in the frequency dependence of the dynamic mechanical spectrum. At 0.5 M NaCl, G′ and G″ were approximately equal at high frequencies (>0.3 Hz) and scaled as ω0.54 until a critical frequency at which the storage modulus became frequency independent, while the loss modulus continued to decrease. The overlapping of the moduli at the high frequencies would typically indicate that this was a critical salt gelation concentration, but the divergence of the moduli at the lower frequencies is atypical and the physical reason is currently unknown. Above the critical salt gelation concentration, 0.75 and 1.0 M, the electrostatic interactions were weakened, and the number of elastically active PEO bridges decreased so that the samples no longer formed an interconnected network and the materials behaved as polyelectrolyte solutions with G″ greater than G′ over the entire measured frequency range. It is again worth mentioning
Figure 4. Two-dimensional SAXS pattern for the 20 wt % sample showing reflections from the deformation texture caused by the operation of the common slip systems of BCC materials: {110}⟨1̅11⟩, {211}⟨1̅11⟩, and {321}⟨1̅11⟩.
specific reflections that are associated with a deformation texture arising from the operation of the common slip systems of BCC structures: {110}⟨1̅11⟩, {211}⟨1̅11⟩, and {321}⟨1̅11⟩. While typically seen in BCC metals, this pattern was first reported and indexed in block copolymer systems by Torija et al.35 In that report, the domains of the poly(styrene-b-ethylenealt-propylene) diblock copolymers in squalene were aligned using large-amplitude oscillatory shear in order to observe the reflections from a texture produced by operation of the four possible slip systems of mobile dislocations in BCC whose Burgers vector is the BCC close-packed ⟨1̅11⟩ direction. These results provide insight into how block copolymer systems deform, indicating that the materials plastically deform through the same close packed slip systems as in metals. The deformation texture observed in the coacervate hydrogels is likely due to the shear applied to the hydrogel as it was packed into the capillary. A second transition within the polymer concentration range, from the intermediate to the concentrated regime, occurred between 25 and 30 wt %. In Figure 2, it can be seen that while all three samples tested at 20 wt % and above show a gel-like response, there was a 25% decrease in G′ as the concentration increased from 25 to 30 wt %. In all of the other samples, the magnitude of G′ increased with increasing polymer concentration. The SAXS data (Figure 3) again provided insight into this behavior. The data clearly show a BCC structure at 20 and 25 wt % and a hexagonally packed cylinders structure at 30 wt %, suggesting that gels with cylindrical coacervate domains had a reduced modulus. This phenomenon has been observed for a number of block copolymer systems,36−38 and the reasons behind the decrease in modulus are well understood. While the BCC spheres were tethered on a three-dimensional lattice, the hexagonally packed cylinders, alternatively, were only tethered by the PEO bridging mid-blocks in two dimensions and retain a liquidlike state of order in the third dimension. The BCC spheres, when exposed to the oscillating strain of the rheometer, behaved elastically in three dimensions, whereas the cylinders responded elastically in two, but in a viscous manner along the cylindrical axis. The end-blocks of the 1515
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1 M NaCl and above, the driving force for coacervate formation was diminished, and only weak features in the SAXS patterns were present, implying limited coacervation with most of the polymers dissolved in solution, forming a block copolyelectrolyte solution. These SAXS and dynamic mechanical data correlate well with each other with the exception of the 0.75 M sample. The dynamic mechanical data indicated that the sample was a block copolyelectrolyte solution, but the SAXS data indicated that coacervation still occurred. This discrepancy may imply that while some domains were formed, they were likely to be weakly associated so they appeared to be fluidlike in the dynamic mechanical spectrum. The effect of pH was also investigated using dynamic mechanical spectrometry (Figure S4) and SAXS (Figure S5). The viscoelastic response was not significantly affected by changes in pH from pH 1 to 10. However, at pH 13, the sample formed a complex fluid without structural order over the entire frequency range tested. SAXS experiments performed on samples at a variety of pH values indicated that a BCC structure was seen for pH values from 1 to 10, whereas at pH 13, the sample consisted of disordered coacervate domains (Figure S5) with the domain spacing being approximately equal for all samples from pH 1 to 10. The effect of pH on the structure of the gels is not surprising given the pKa of the guanidinium group (∼12), and at pH values near the pKa of guanidinium, many of these functional groups would be in their base form. Conversely, the pKa of the sulfonate group is less than −2, so it can be considered to be primarily in an ionic state at all pH values tested. Comparisons of Salt and Polymer Concentration. Increasing salt concentration and increasing polymer concentration had opposite effects on the morphology of the hydrogels. A distinct difference in how salt and polymer concentration affect the mechanical properties could also be observed. The 10 wt % sample (Figure 3) and the 20 wt % sample with 0.5 M salt (Figure 6) had very similar SAXS profiles with a broad peak indicating the existence of a disordered arrangement of coacervate domains; however, the dynamic mechanical spectra were distinctly different. For the 10 wt % sample (Figure 2), the frequency sweep shows a response that was indicative of a complex fluid as evidenced by the crossover frequency. In contrast, the frequency sweep of the 20 wt %, 0.5 M salt sample (Figure 5) shows an overlap of G′ and G″ at the highest frequencies and a divergence with G′ > G″ at the lower frequencies. These two cases can be explained by the unique structure of these triblock copolymer systems. In the first case, coacervate domains were formed, but the polymer concentration was insufficient to form the necessary PEO midblock bridges to create a network throughout the sample, and instead, coacervate micelles were formed. As shear was applied to the material, the micelles interact and behave as a complex fluid. However, in the sample with increased salt concentration, salt ions partially screened the polyelectrolytes and caused many of the chains to be dissolved into the solution, resulting in weaker associations in the coacervate domains. The domains in this case had lower polymer concentrations but were still able to form a continuous network of PEO bridges throughout the sample. The well-defined nature of the triblock copolymers allows a phase diagram (Figure 7) to be constructed that compiles the SAXS and dynamic mechanical data and presents the region of stability for each phase as a function of the polymer and NaCl concentration. As polymer concentration was increased or salt
Figure 5. Dynamic mechanical spectra of hydrogels at 20 wt % with varying NaCl concentration. The G′ data are the solid symbols, and the G″ data are the corresponding open symbols. Note that the 0.5 M NaCl sample has a slope approximately proportional to ω0.5, which is typically the relationship for systems at the critical point of gelation. Additionally, the G′ and G″ of the 1 M NaCl samples never show the frequency dependence (ω2 and ω, respectively) of simple liquids.
Figure 6. SAXS patterns of 20 wt % samples at varying salt concentration. As the salt concentration was increased, the structure transitions from a BCC structure (0, 0.05, 0.1, and 0.25 M) to disordered domains (0.5 and 0.75 M) to a polyelectrolyte solution (1, 1.5, and 2 M). The SAXS patterns were shifted vertically for clarity.
that G′ does not scale as ω2 for either of these samples, and thus they are not simple liquids. The SAXS data in Figure 6 at high NaCl concentrations show that the 0.50 and 0.75 M samples were disordered with no higher order lattice reflections; however, there was a welldefined first-order peak which indicated that coacervate domains were still formed and had a preferential spacing. At 1516
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investigated. It was determined that the materials form structures ranging from disordered spheres to BCC spheres to hexagonally packed cylinders as the polymer concentration was increased. Mechanical properties were found to be sensitive to polymer concentration with a significant increase in the modulus upon increasing the concentration with a concomitant change in the structure from disordered domains to an ordered BCC gel. However, there was a decrease in the modulus when hexagonally packed cylinders became the dominant structure. While these coacervate gels proved to be robust to salt concentrations up to 0.25 M, mechanical properties were weakened at high concentrations, and above 1.0 M NaCl, the gels were not formed at all. The key finding in this study is that by varying the polymer and salt concentration, a range of tunable structures and mechanical properties could be obtained, allowing for the selection of required properties for potential applications as membranes, in injectable drug delivery or as tissue growth scaffolds. Various lines of further work are being pursued, including investigating kinetic effects on structure formation and rheological evolution, examination of other types of copolymer structures, such as diblock copolymers, influencing structure development by templating and application of fields, and moving to still higher polymer concentrations by dehydration.
Figure 7. Phase diagram showing the structure of the system when varying both polymer and salt concentration. The phase diagram is a compilation of the results from the SAXS and dynamic mechanical data. The lines have been drawn to guide the eye.
concentration was decreased, the samples became more ordered. Close examination of the phase diagram revealed that for the 30 wt % sample, as salt concentration was increased from 0 M NaCl to 0.25 M NaCl, the sample had a hexagonal structure, but as the salt concentration was increased to 0.375 M, there was no indication of the hexagonal structure and, instead, a weakly ordered BCC structure resulted. This is potentially caused by a greater percentage of chains being fully dissolved in solution, so the effective polymer concentration that was available to form coacervate domains, was closer to 25 wt %, and thus, a BCC structure was formed. Additionally, for all of the polymer concentrations tested, there was some high salt concentration in which the material behaved as a polyelectrolyte solution. As expected, the salt concentration necessary to induce this behavior increased with polymer concentration. Lastly, it is important to note that there is a large region in the phase diagram in which both disordered and BCC structures were observed. This phenomenon was predicted for other copolymer systems in which multiple components are present.39 Whether this is true, two-phase coexistence or lack of full equilibration remains to be explored more fully. Overall, this phase diagram bears some resemblance to that developed by the triblock and homopolymer system developed by Lemmers et al. with the exception that no ordered phases were observed in their system.33 Additionally, even at the highest polymer concentrations tested (20 wt %), they only formed complex fluids similar to the dilute regime in our binary system (
