We investigated the temperature-dependent phase behavior and interaction parameter of polyethylenebased multiblock copolymers with pendant ionic groups. These step-growth polymers contain short polyester blocks with a single Li + SO 3 − group strictly alternating with polyethylene blocks of x-carbons (PESxLi, x = 12, 18, 23). At room temperature, these polymers exhibit layered morphologies with semicrystalline polyethylene blocks. Upon heating above the melting point (∼130 °C), PES18Li shows two order-to-order transitions involving Ia3̅ d gyroid and hexagonal morphologies. For PES12Li, an order-to-disorder transition accompanies the melting of the polyethylene blocks. Notably, a Flory−Huggins interaction parameter was determined from the disordered morphologies of PES12Li using mean-field theory: χ(T) = 77.4/T + 2.95 (T in Kelvin) and χ(25 °C) ≈ 3.21. This ultrahigh χ indicates that the polar ionic and nonpolar polyethylene segments are highly incompatible and affords well-ordered morphologies even when the combined length of the alternating blocks is just 18−29 backbone atoms. This combination of ultrahigh χ and short multiblocks produces sub-3-nm domain spacings that facilitate the control of block copolymer self-assembly for various fields of study, including nanopatterning.
We investigated the nanoscale morphologies and ionic conductivities of polyethylene-based multiblock copolymers as single-ion conducting polymer electrolytes. These polymers contain short polar blocks with a single sodium sulfonate group separated by polyethylene blocks of fixed length (PESxNa, x = 10, 12, and 18). At room temperature, these multiblock copolymers exhibit layered ionic aggregates with semicrystalline polyethylene backbones. For PES12Na and PES18Na, the layered ionic aggregate morphologies transition into Ia3̅d gyroid morphologies upon melting the polyethylene blocks and further transition into hexagonal morphologies at higher temperatures. With a shorter polyethylene block, PES10Na exhibits a layered to hexagonal transition at the melting temperature, without an intermediate gyroid morphology. The phase diagram of these PESxNa polymers is reminiscent of conventional diblock copolymers and identifies the presence of gyroid morphologies at a polar block volume fraction of ∼0.27 to 0.41, which is broad compared to typical diblock copolymers. Temperature-dependent ionic conductivities reveal faster ion transport through bicontinuous gyroids than hexagonal ionic aggregate morphologies and a relationship between conductivity and the characteristic distance between ionic aggregates. This study presents material design strategies for single-ion conducting polymers with a bicontinuous ionic aggregate and toward efficient ion transport.
We use single-particle tracking (SPT) to explore the role of nanoparticles/polymer interactions and polymer molecular weight on nanoparticle (NP) diffusion in unentangled polymer melts. The very dilute NP concentrations (∼10 −7 wt %) in SPT measurements enable tuning NP/polymer interactions so that the systems with unfavorable or neutral NP/polymer interactions in polymer melts can be studied without nanoparticle aggregation. Here, the diffusion coefficients of weakly interacting (methyl-capped, CH 3 QDs) and strongly interacting (carboxylic acid-capped, COOH QDs) nanoparticles (radius = 6.6 nm) in poly(propylene glycol) (PPG) melts were measured via SPT. Mean-squared displacements and van Hove distributions of nanoparticle motion are consistent with Brownian motion of single nanoparticles in the long-time diffusion regime. The effective COOH QD size increases with PPG molecular weight as M w 0.5 , indicating a long-lived bound layer. However, for weakly interacting CH 3 QDs, the effective nanoparticle radius is independent of PPG M w due to the absence of a bound layer. In contrast to ensemble average methods (i.e., X-ray photon correlation spectroscopy), SPT methods directly detect spatial and temporal diffusion behavior of individual nanoparticles and provide previously inaccessible information about nanoparticle diffusion in polymer melts.
We demonstrate enhanced Li + transport through the selectively solvated ionic layers of a single-ion conducting polymer. The polymer is a precisely segmented ion-containing multiblock copolymers with well-defined Li + SO 3 − ionic layers between crystallized linear aliphatic 18-carbon blocks. X-ray scattering reveals that the dimethyl sulfoxide (DMSO) molecules selectively solvate the ionic layers without disrupting the crystallization of the polymer backbone. The amount of DMSO (∼21 wt %) calculated from the increased layer spacing is consistent with thermogravimetric analysis. The ionic conductivity through DMSO-solvated ionic layers is >10 4 times higher than in the dried state, indicating a significant enhancement of ion transport in the presence of this solvent. Dielectric relaxation spectroscopy (DRS) further elucidates the role of the structural relaxation time (τ) and the number of free Li + (n) on the ionic conductivity (σ). Specifically, DRS reveals that the solvation of ionic domains with DMSO contributes to both accelerating the structural relaxation and the dissociation of ion pairs. This study is the initial demonstration that selective solvation is a viable design strategy to improve ionic conductivity in nanophase separated, single-ion conducting multiblock copolymers.
We demonstrate that ionic functionality in a multiblock architecture produces highly ordered and sub-3 nm nanostructures in thin films, including bicontinuous double gyroids. At 40 °C, precise ion-containing multiblock copolymers of poly(ethylene- b -lithium sulfosuccinate ester) n (PES x Li, x = 12 or 18) exhibit layered ionic assemblies parallel to the substrate. These ionic layers are separated by crystalline polyethylene blocks with the polymer backbones perpendicular to the substrate. Notably, above the melting temperature ( T m ) of the polyethylene blocks, layered PES18Li thin films transform into a highly oriented double-gyroid morphology with the (211) plane ( d 211 = 2.5 nm) aligned parallel to the substrate. The cubic lattice parameter ( a gyr ) of the double gyroid is 6.1 nm. Upon heating further above T m , the double-gyroid morphology in PES18Li transitions into hexagonally packed cylinders with cylinders parallel to the substrate. These layered, double-gyroid, and cylinder nanostructures form epitaxially and spontaneously without secondary treatment, such as interfacial layers and solvent vapor annealing. When the film thickness is less than ∼3 a gyr , double gyroids and cylinders coexist due to the increased confinement. For PES12Li above T m , the layered ionic assemblies simply transform into disordered morphology. Given the chemical tunability of ion-functionalized multiblock copolymers, this study reveals a versatile pathway to fabricating ordered nanostructures in thin films.
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