Rate of Molecular Exchange through the Membranes of Ionic Liquid Filled Polymersomes Dispersed in Water

The permeation of 1-ethyl-3-methylimidazolium ([EMIM]), 1-butyl-3-methylimidazolium ([BMIM]), and 1-butylimidazole through the bilayer membranes of nanoemulsion-like polymersomes was investigated by nuclear magnetic resonance spectroscopy (NMR) techniques. 1,2-Polybutadiene-<i>b</i>-poly­(ethylene oxide) (PB–PEO) polymersomes in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[TFSI]) were prepared by a cosolvent method and then migrated to the aqueous phase, which is not miscible with the IL, at room temperature. In this way stable, nanoscopic domains of the IL (average diameter ca. 200 nm) were dispersed in water. Two similarly sized molecules, charged [EMIM] and neutral 1-butylimidazole, were employed as tracer molecules, and proton NMR (¹H NMR) and pulsed-field-gradient NMR (PFG-NMR) experiments were conducted. Furthermore, transient ¹H NMR was used with [BMIM] to estimate how rapidly the charged molecules can go through the hydrophobic membrane into the polymersome interior. The molecules in the nanoemulsion solution showed two distinct sets of peaks due to the magnetic susceptibility difference across the membrane. This difference in ¹H NMR gave direct evidence of permeation of the molecules and the relative populations within the polymersomes versus in the aqueous exterior. The escape and entry rates were evaluated by fitting the PEG-NMR echo decay curves with a two-site exchange model. The molecules could permeate through the hydrophobic PB membranes on a time scale of seconds, but the entry and escape rates for the charged molecule ([EMIM]) were approximately 10 times slower than the neutral molecule (1-butylimidazole). These results confirm that this system has the potential to serve as a nanoreactor, facilitating reactions with various kinds of molecules including both charged and neutral molecules. It combines the facile transport and mixing of a majority aqueous phase with the multiple advantages of IL as a reaction medium. The ability to shuttle the polymersomes reversibly between aqueous and ionic liquid phases offers a convenient route to product separation and catalyst recovery

Tunable Upper Critical Solution Temperature of Poly(<i>N</i>‑isopropylacrylamide) in Ionic Liquids for Sequential and Reversible Self-Folding

We demonstrate sequential folding of micropatterned polymer actuators by tuning the upper critical solution temperature (UCST) of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) copolymers in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis­(trifluoro-methylsulfonyl) imide. Incorporation of comonomers having different hydrogen-bonding capacities, acrylic acid and methyl acrylate, is shown to shift the UCST of PNIPAM to higher and lower temperatures, respectively. Relying on the ability to tune the transition temperature through copolymerization along with the wide thermal range afforded by the IL as a solvent, we fabricated a photopatterned self-folding device which shows reversible and sequential bending of three sets of hinges. Such sequential and reversible bending of microactuators offers potential for the design of complex self-folding origami and soft robots

Interfacial Tension-Hindered Phase Transfer of Polystyrene‑<i>b</i>‑poly(ethylene oxide) Polymersomes from a Hydrophobic Ionic Liquid to Water

We examine the phase transfer of polystyrene-<i>b</i>-poly­(ethylene oxide) (PS–PEO) polymersomes from a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[TFSI]), into water. The dependence of the phase transfer on the molecular weight and PEO volume fraction (<i>f</i>_PEO) of the PS–PEO polymersomes was systematically studied by varying the molecular weight of PS (10 000–27 000 g/mol) as well as by varying the volume fraction of PEO (<i>f</i>_PEO) between 0.1 and 0.3. We demonstrate a general boundary for the phase transfer in terms of a reduced tethering density for PEO (σ_PEO), which is independent of the molecular weight of the hydrophobic PS. The reduced PEO tethering density was controlled by changing the polymersome size (i.e., increased polymersome sizes increase σ_PEO), confirming that it is the driving force in the transfer of PS–PEO polymersomes at room temperature. The phase transfer dependence on σ_PEO was also analyzed in terms of the free energy of polymersomes in the biphasic system. The quality of the aqueous phase, which affects the interfacial tension of the PS membrane, influenced the phase transfer. We systematically reduced the interfacial tension by adding a water-selective solvent, THF, which has a similar effect to increasing σ_PEO. The results indicate that the interfacial tension between the membrane and water plays an important role in the phase transfer with the corona and that the phase transfer can be controlled either by the dimensions of the polymersomes or by the suitability of the solvent for the membrane. The interfacial tension-hindered phase transfer of polymersomes in the biphasic water–[EMIM]­[TFSI] system will inform the design of temperature-sensitive and reversible nanoreactors and the separation of polydisperse particles according to size by tuning the quality of the solvent

Size Control and Fractionation of Ionic Liquid Filled Polymersomes with Glassy and Rubbery Bilayer Membranes

We demonstrate control over the size of ionic liquid (IL) filled polymeric vesicles (polymersomes) by three distinct methods: mechanical extrusion, cosolvent-based processing in an IL, and fractionation of polymersomes in a biphasic system of IL and water. For the representative ionic liquid (1-ethyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl) imide ([EMIM]­[TFSI])), the size and dispersity of polymersomes formed from 1,2-polybutadiene-<i>b</i>-poly­(ethylene oxide) (PB–PEO) and polystyrene-<i>b</i>-poly­(ethylene oxide) (PS–PEO) diblock copolymers were shown to be sensitive to assembly conditions. During mechanical extrusion through a polycarbonate membrane, the relatively larger polymersomes were broken up and reorganized into vesicles with mean size comparable to the membrane pore (100 nm radius); the distribution width also decreased significantly after only a few passes. Other routes were studied using the solvent-switch or cosolvent (CS) method, whereby the initial content of the cosolvent and the PEO block length of PS–PEO were systemically changed. The nonvolatility of the ionic liquid directly led to the desired concentration of polymersomes in the ionic liquid using a single step, without the dialysis conventionally used in aqueous systems, and the mean vesicle size depended on the amount of cosolvent employed. Finally, selective phase transfer of PS–PEO polymersomes based on size was used to extract larger polymersomes from the IL to the aqueous phase via interfacial tension controlled phase transfer. The interfacial tension between the PS membrane and the aqueous phase was varied with the concentration of sodium chloride (NaCl) in the aqueous phase; then the larger polymersomes were selectively separated to the aqueous phase due to differences in shielding of the hydrophobic core (PS) coverage by the hydrophilic corona brush (PEO). This novel fractionation is a simple separation process without any special apparatus and can help to prepare monodisperse polymersomes and also separate unwanted morphologies (in this case, worm-like micelles)

Enhancing Water Absorption in Sulfonated Poly(arylene ether sulfone) Polymer Electrolyte Membranes by Reducing Chain Entanglement through Constrained Deswelling

The water uptake of a polymer electrolyte membrane is a critical parameter that determines the dimensional stability and transport behavior in various energy conversion devices. In this study, the water uptake of a sulfonated poly(arylene ether sulfone) (SPAES) membrane was controlled solely by the number of chain entanglements without employing any water absorbents. Through the constrained deswelling process, the SPAES membrane achieved a significant enhancement in water uptake, increasing by up to 210% at room temperature. This notable improvement in water uptake originates from the reduction in elastic friction, represented by the number of chain entanglements, against the volume expansion resulting from the absorption of water by the sulfuric acid groups. Evidently, the controlled deswelling procedure led to biaxial stretching of the SPAES membrane, causing an increase in its surface area and a decrease in thickness. At the microscopic level, this controlled deswelling process might prompt the alignment of hydrophilic channels along the plane directions. These changes brought about by the controlled deswelling process resulted in changes to the membrane’s tensile characteristics and its transport behavior for protons and hydrogen gas

Oil-in-Oil Emulsions Stabilized by Asymmetric Polymersomes Formed by AC + BC Block Polymer Co-Assembly

We demonstrate a facile route to asymmetric polymersomes by blending AC and BC block copolymers in oil-in-oil emulsions containing polystyrene (PS) and polybutadiene (PB) in chloroform (CHCl₃). Polymersomes were prepared by mixing polystyrene-<i>b</i>-poly­(ethylene oxide) (SO) and polybutadiene-<i>b</i>-poly­(ethylene oxide) (BO) in the oil-in-oil emulsion, where the droplets and continuous phase are PS- and PB-rich, respectively. The polymersome structure was directly visualized using dye-labeled SO and BO with confocal fluorescence microscopy; SO and BO with a high O block fraction co-assemble to produce asymmetric polymersomes. As the O block is insoluble in both PS and PB, we infer that the detailed structure of the polymersomes is a bilayer in which the S and B blocks face the PS-inner and PB-outer phases, respectively, while the common O blocks form the core membrane. This structure is only observed for sufficiently long O blocks. It is remarkable that although all the polymers are soluble in CHCl₃, such elaborate structures are created by straightforward co-assembly. These asymmetric polymersomes should provide robust bilayer membranes around emulsion droplets, leading to stable nanoscopic dispersions of two fluids

Permeability of Rubbery and Glassy Membranes of Ionic Liquid Filled Polymersome Nanoreactors in Water

Nanoemulsion-like polymer vesicles (polymersomes) having ionic liquid interiors dispersed in water are attractive for nanoreactor applications. In a previous study, we demonstrated that small molecules could pass through rubbery polybutadiene membranes on a time scale of seconds, which is practical for chemical transformations. It is of interest to determine how sensitive the rate of transport is to temperature, particularly for membranes in the vicinity of the glass transition (<i>T</i>_g). In this work, the molecular exchange rate of 1-butylimidazole through glassy polystyrene (PS) bilayer membranes is investigated via pulsed field gradient nuclear magnetic resonance (PFG-NMR) over the temperature range from 25 to 70 °C. The vesicles were prepared by the cosolvent method in the ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl) imide ([EMIM]­[TFSI]), and four different polystyrene-<i>b</i>-poly­(ethylene oxide) (PS-PEO) diblock polymers with varying PS molecular weights were examined. The vesicles were transferred from the ionic liquid to water at room temperature to form nanoemulsion solutions of polymer vesicles in water. The exchange rate of 1-butylimidazole added to the aqueous solutions was observed under equilibrium conditions at each temperature. The exchange rate decreased as the membrane thickness increased, and the exchange rate through the glassy membranes was three to four times slower than through the rubbery polybutadiene membranes under the same experimental conditions. These results demonstrate that the permeability through nanosized membranes depends on both the dimension and chemistry of membrane-forming blocks. Furthermore, the exchange rate was investigated as a function of temperature in the vicinity of the <i>T</i>_g of PS-PEO membranes. The exchange rate, however, is not a strong function of the temperature in the vicinity of the membrane <i>T</i>_g, due to a combination of the nanoscopic dimension of the membrane, and some degree of solvent plasticization

Anhydrous Proton Conducting Polymer Electrolyte Membranes via Polymerization-Induced Microphase Separation

Solid-state polymer electrolyte membranes (PEMs) exhibiting high ionic conductivity coupled with mechanical robustness and high thermal stability are vital for the design of next-generation lithium-ion batteries and high-temperature fuel cells. We present the in situ preparation of nanostructured PEMs incorporating a protic ionic liquid (IL) into one of the domains of a microphase-separated block copolymer created via polymerization-induced microphase separation. This facile, one-pot synthetic strategy transforms a homogeneous liquid precursor consisting of a poly­(ethylene oxide) (PEO) macro-chain-transfer agent, styrene and divinylbenzene monomers, and protic IL into a robust and transparent monolith. The resulting PEMs exhibit a bicontinuous morphology comprising PEO/protic IL conducting pathways and highly cross-linked polystyrene (PS) domains. The cross-linked PS mechanical scaffold imparts thermal and mechanical stability to the PEMs, with an elastic modulus approaching 10 MPa at 180 °C, without sacrificing the ionic conductivity of the system. Crucially, the long-range continuity of the PEO/protic IL conducting nanochannels results in an outstanding ionic conductivity of 14 mS/cm at 180 °C. We posit that proton conduction in the protic IL occurs via the vehicular mechanism and the PEMs exhibit an average proton transference number of 0.7. This approach is very promising for the development of high-temperature, robust PEMs with excellent proton conductivities

Highly selective porous separator with thin skin layer for alkaline water electrolysis

© 2022Advanced porous separators with thin selective skin layers to reduce the hydrogen permeation are developed for applications in alkaline water electrolysis. A thin skin layer based on crosslinked polyvinyl alcohol (cPVA) is fabricated on a porous substrate by a facile and scalable ultrasonic spray coating process. As the number of ultrasonic spraying cycles increases, the resulting separator demonstrates a decrease in the large-diameter pore fraction, an increase in the bubble-point pressure, and a reduction in the hydrogen permeability without a significant increase in the areal resistance. As a result, the optimized separator with a cPVA skin layer combines a low ionic resistance of 0.267 Ω cm2, a high bubble point pressure of 2.71 bar, and a low hydrogen permeability of 1.12 × 10−11 mol cm−2 s−1 bar−1. The electrolytic cell assembled with cPVAZ-30 achieves current densities of 861 mA cm−2 and 1890 mA cm−2 at 2.0 V and 2.6 V, respectively, in a 30 wt% KOH electrolyte solution at 80 °C.Y