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.
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