Block polymer self-assembly provides a versatile platform for creating useful materials endowed with three-dimensional periodic network morphologies that support orthogonal physical properties such as high ionic conductivity and a high elastic modulus. However, coil configurations limit conventional linear block polymers to finite ordered network dimensions, which are further restricted by slow self-assembly kinetics at high molecular weights. A bottlebrush architecture can circumvent both shortcomings owing to extended backbone configurations due to side chain crowding and molecular dynamics substantially free of chain entanglements. However, until now, network morphologies have not been reported in AB bottlebrush block copolymers, notwithstanding favorable mean-field predictions. We explored the phase behavior by small-angle X-ray scattering of 133 poly(ethylene-alt-propylene)-b-polystyrene (PEP-PS) diblock and PEP-PS-PEO triblock bottlebrush copolymers prepared by ring-opening metathesis polymerization (ROMP) of norbornene-functionalized poly(ethylene-alt-propylene) (PEP), poly(styrene) (PS), and poly(ethylene oxide) (PEO) macromonomers with total backbone degrees of polymerization N bb between 20 and 40. The PEP-PS diblocks exhibited only cylindrical and lamellar morphologies over the composition range of ca. 30−70%. However, addition of variable-length bottlebrush PEO blocks to diblocks containing 30−50% PS led to the formation of a substantial core−shell double gyroid (GYR) phase window containing 20 bottlebrush triblock specimens, which is the focus of this report. Encouragingly, the GYR unit cell dimensions increased as d ∼ N bb 0.92 , portending the ability to access larger network dimensions than previously obtained with linear AB or ABC block polymers. This work highlights extraordinary opportunities associated with applying facile ROMP chemistry to multiblock bottlebrush polymers.
A series of symmetric poly [(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-block-polystyrene (PsOEGMA-PS) diblock copolymers were synthesized as a model system to probe the effect of charge fraction on the phase behavior of charged-neutral single-ion conducting diblock copolymers. Small-angle X-ray scattering (SAXS) experiments showed that increasing the charge fraction does not alter the ordered phase morphology (lamellar) but increases the order− disorder transition temperature (T ODT ) significantly. Additionally, the effective Flory−Huggins interaction parameter (χ eff ) was found to increase linearly with the charge fraction, similar to the case of conventional salt-doped diblock copolymers. This indicates that the effect of counterion solvation, attributed to the significant mismatch between the dielectric constant of each block, provides the dominant effect in tuning the phase behavior of this charged diblock copolymer. We therefore infer that electrostatic cohesion (local charge ordering induced by Coulombic interactions), which is predicted to suppress microphase separation and lead to asymmetric phase diagrams, only plays a minor role in this model system.
The phase behavior of ternary polymer blends comprising poly(cyclohexylethylene) (C) and polyethylene (E) homopolymers and a compositionally asymmetric CE diblock copolymer with f C = 0.67 was investigated, where f C is the volume fraction of C. The morphology was established in the phase prism (volume fractions of C, E, and CE vs temperature) by optical transmission, small-angle X-ray scattering, and small-angle neutron scattering measurements. The locations of lamellar (LAM), hexagonally packed cylinders and gyroid ordered phases are shifted significantly toward lower fractions of the C homopolymer compared to previous results obtained from ternary polymer blends with a symmetric diblock copolymer (f C = 0.5). Conversely, the Scott line of critical points, which delineates the boundary between single-phase disorder and macroscopic phase separation, remains virtually unchanged, coincident with the fraction of the C homopolymer associated with the binary homopolymer blend critical composition. A central finding of this study is that the line of nearly congruent order−disorder transitions, where the LAM phase melts virtually directly into the disordered state, is decoupled in composition from that of the Scott line of critical points. A wide range of phase space between the compositions associated with the congruent transition and Scott line was identified as containing a microemulsion morphology. This study demonstrates that diblock copolymer compositional asymmetry significantly impacts the ordered phase regime but has a marginal effect on the region displaying macroscopic phase separation. It also provides useful guidance for tuning the interfacial curvature, a crucial factor in the formation of bicontinuous microemulsions.
Salt-doped A/B/AB ternary polymer blends demonstrate a plethora of nanostructured morphologies tunable by the composition of homopolymers (A and B) and the corresponding diblock copolymer (AB). Here, we report a complete phase diagram of lithium bis(trifluoromethane) sulfonimide (LiTFSI)-doped low-molar-mass polystyrene (PS)/poly(ethylene oxide) (PEO)/symmetric PS-b-PEO block copolymer (SO) blends and evaluate the spatial distribution of homopolymers in the resulting microstructures. In the isothermal phase triangle at 120 °C and r = [Li+]/[EO] = 0.06, a wide region of lamellae (LAM) is bracketed by small zones of double gyroid (GYR) and wide regions of hexagonally packed cylinders (HEX); adjacent to HEX is a significant region of the C15 Laves phase. At a high total homopolymer composition ϕH = ϕPS,homo + ϕPEO,homo+LiTFSI, the copolymer brush becomes saturated and begins to exclude homopolymers, resulting in a rapid domain size increase and inducing the formation of higher curvature phases, as suggested by small-angle X-ray scattering (SAXS). This phenomenon is distinct from charge-neutral ternary blends. Moreover, small-angle neutron scattering (SANS) profiles of selectively deuterated lamellar and bicontinuous ternary blends with contrast variation confirm the existence of a pure PS homopolymer layer in the middle of the PS microdomain.
Ternary blends comprising a diblock copolymer and the corresponding homopolymers provide ready access to a variety of morphologies, but the impact of charge on their phase behavior has not been studied extensively. Here, three partially charged ternary blends consisting of symmetric poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)] (POEGMA#), polystyrene (PS), and POEGMA#-PS with different charge fractions were prepared, where # denotes the mole fraction of charged monomers in the POEGMA chain. The phase behavior was systematically studied using small-angle X-ray scattering, and isothermal phase diagrams at 120 °C were constructed. The resulting isothermal planes contain a wide lamellar phase window with a slight deviation from symmetry for ion-containing ternary blends due to the homopolymer size difference. Given the absence of any significant change in the phase diagrams as the charge fraction increases, the role of charge is primarily to increase the segregation strength in the ternary system, as in the case of partially charged diblock polymers. In addition, examination of the increase in domain size along the volumetrically symmetric isopleth reveals a "dry-brush"like swelling behavior of homopolymers in all three blends, presumably due to the space-filling nature of the oligomeric ethylene glycol side chains on POEGMA.
The effect of long-wavelength ultraviolet photo-cross-linking on microphase-separated coumarin-containing block polymers was studied by photorheometry and small-angle X-ray scattering. This model system consisted of three photo-cross-linkable diblock polymers of poly(methoxyethyl acrylate)-b-poly(hexyl methacrylate-co-coumarin methacrylate) with different volume fractions of the cross-linkable coumarin-containing block, which microphase separated into lamellar and cylindrical morphologies. All polymers stiffened upon exposure to 365 nm light, with much greater relative increases in moduli recorded for lamellae-forming polymers (ca. 3200% increase) compared to the cylinder-forming polymer (ca. 550% increase). Disordering transitions that were evident in un-cross-linked samples were no longer observed after cross-linking in the ordered state, and domain sizes were found to remain stable to heating. The photo-cross-linking reaction only proceeded under active irradiation (i.e., cross-linking does not persist when the UV radiation is turned off), indicating a high degree of spatiotemporal control over curing in this system. Finally, at constant concentration of couamarin within the cross-linkable block, the cure rate was largely independent of polymer composition, suggesting a constant local concentration of coumarin moieties within the segregated cross-linkable domains. These findings establish a set of specific structure–property relationships governing the phase-selective photo-cross-linking of diblock polymers that can guide the design of robust nanostructured materials.
In this work, we systematically investigated the structure and phase behavior of ternary mixtures containing an AB bottlebrush diblock copolymer and A and B linear homopolymers, where A is polystyrene and B is poly(ethylene oxide), using a combination of small-angle X-ray scattering and cloud point measurements. We found that the overall phase behavior of such mixtures closely resembles the linear counterparts, with lamellae occurring in the copolymer-rich region, macrophase separation in the homopolymer-rich region, and a bicontinuous microemulsion in between. However, the increase in lamellar and microemulsion domain spacing with increasing homopolymer content is significantly weaker than that found with ternary mixtures containing linear diblock copolymers and depends on the relative size of the homopolymers and bottlebrush side chains. We attribute this behavior to an unconventional spatial distribution of the homopolymer within the bottlebrush architecture of the diblock copolymer. These ternary mixtures offer a promising platform for the design of cocontinuous materials with an expanded parameter space made possible by the application of bottlebrush polymers.
Charged block copolymers that can self-assemble into a host of nanostructures offer a great opportunity as nextgeneration battery electrolytes with outstanding ionic conductivity and mechanical robustness. The impact of ions on the selfassembly of charged block copolymers, however, remains to be fully understood. In this article, we report the phase behavior of charged-neutral block copolymers where a relatively nonpolar matrix was employed in the charged block to boost the strength of electrostatic interaction. The phase behavior was established using small-angle X-ray scattering (SAXS). We found that the overall shape of the phase boundary between ordered and disordered states is asymmetric, tilting toward the charged block-lean side of the phase portrait. However, the composition windows of the ordered phases, such as lamellae and hexagonally packed cylinders observed in this study, are comparable to those of neutral diblock copolymers and are not obviously affected by the incorporation of charges. The results obtained in this work provide insight into the impact of ions on the self-assembly of charged block copolymers as well as the design of nanostructured polymer electrolytes.
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