10 research outputs found
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>-polyÂ(ethylene
oxide) (PBâPEO) polymersomes in the ionic liquid (IL) 1-ethyl-3-methylimidazolium
bisÂ(trifluoromethylsulfonyl)Âimide ([EMIM]Â[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
(<sup>1</sup>H NMR) and pulsed-field-gradient NMR (PFG-NMR) experiments
were conducted. Furthermore, transient <sup>1</sup>H 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 <sup>1</sup>H 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
Interfacial Tension-Hindered Phase Transfer of Polystyreneâ<i>b</i>âpoly(ethylene oxide) Polymersomes from a Hydrophobic Ionic Liquid to Water
We
examine the phase transfer of polystyrene-<i>b</i>-polyÂ(ethylene
oxide) (PSâPEO) polymersomes from a hydrophobic
ionic liquid, 1-ethyl-3-methylimidazolium bisÂ(trifluoromethylsulfonyl)Âimide
([EMIM]Â[TFSI]), into water. The dependence of the phase transfer on
the molecular weight and PEO volume fraction (<i>f</i><sub>PEO</sub>) of the PSâPEO polymersomes was systematically studied
by varying the molecular weight of PS (10âŻ000â27âŻ000
g/mol) as well as by varying the volume fraction of PEO (<i>f</i><sub>PEO</sub>) between 0.1 and 0.3. We demonstrate a general boundary
for the phase transfer in terms of a reduced tethering density for
PEO (Ï<sub>PEO</sub>), 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 Ï<sub>PEO</sub>), confirming that it is the driving
force in the transfer of PSâPEO polymersomes at room temperature.
The phase transfer dependence on Ï<sub>PEO</sub> 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 Ï<sub>PEO</sub>. 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 waterâ[EMIM]Â[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-methylÂimidazolium bisÂ(trifluoroÂmethylÂsulfonyl)
imide ([EMIM]Â[TFSI])), the size and dispersity of polymersomes formed
from 1,2-polybutadiene-<i>b</i>-polyÂ(ethylene oxide) (PBâPEO)
and polystyrene-<i>b</i>-polyÂ(ethylene oxide) (PSâPEO)
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 PSâPEO 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
PSâPEO 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 membraneâs 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 (CHCl<sub>3</sub>). Polymersomes
were prepared by mixing polystyrene-<i>b</i>-polyÂ(ethylene
oxide) (SO) and polybutadiene-<i>b</i>-polyÂ(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 CHCl<sub>3</sub>, 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><sub>g</sub>). 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 bisÂ(trifluoromethylsulfonyl)
imide ([EMIM]Â[TFSI]), and four different polystyrene-<i>b</i>-polyÂ(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><sub>g</sub> 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><sub>g</sub>, 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 polyÂ(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
StructureâConductivity Relationships in Ordered and Disordered Salt-Doped Diblock Copolymer/Homopolymer Blends
We examine the relationship
between structure and ionic conductivity
in salt-containing ternary polymer blends that exhibit various microstructured
morphologies, including lamellae, a hexagonal phase, and a bicontinuous
microemulsion, as well as the disordered phase. These blends consist
of polystyrene (PS, <i>M</i><sub>n</sub> â 600 g/mol)
and polyÂ(ethylene oxide) (PEO, <i>M</i><sub>n</sub> â
400 g/mol) homopolymers, a nearly symmetric PSâPEO block copolymer
(<i>M</i><sub>n</sub> â 4700 g/mol), and lithium
bisÂ(trifluoroÂmethane)Âsulfonamide (LiTFSI). These pseudoternary
blends exhibit phase behavior that parallels that of well-studied
ternary polymer blends consisting of A and B homopolymers compatibilized
by an AB diblock copolymer. The utility of this framework is that
all blends have nominally the same number of ethylene oxide, styrene,
Li<sup>+</sup>, and TFSI<sup>â</sup> units, yet can exhibit
a variety of microstructures depending on the relative ratio of the
homopolymers to the block copolymer. For the systems studied, the
ratio <i>r</i> = [Li<sup>+</sup>]/[EO] is maintained at
0.06, and the volume fraction of PS homopolymer is kept equal to that
of PEO homopolymer plus salt. The total volume fraction of homopolymer
is varied from 0 to 0.70. When heated through the orderâdisorder
transition, all blends exhibit an abrupt increase in conductivity.
However, analysis of small-angle X-ray scattering data indicates significant
structure even in the disordered state for several blend compositions.
By comparing the nature and structure of the disordered states with
their corresponding ordered states, we find that this increase in
conductivity through the orderâdisorder transition is most
likely due to the elimination of grain boundaries. In either disordered
or ordered states, the conductivity decreases as the total amount
of homopolymer is increased, an unanticipated observation. This trend
with increasing homopolymer loading is hypothesized to result from
an increased density of âdead endsâ in the conducting
channel due to poor continuity across grain boundaries in the ordered
state and the formation of concave interfaces in the disordered state.
The results demonstrate that disordered, microphase-separated morphologies
provide better transport properties than compositionally equivalent
polycrystalline systems with long-range order, an important criterion
when optimizing the design of polymer electrolytes
Hydrophilic Channel Alignment of Perfluoronated Sulfonic-Acid Ionomers for Vanadium Redox Flow Batteries
It is known that uniaxially drawn
perfluoronated sulfonic-acid ionomers (PFSAs) show diffusion anisotropy
because of the aligned water channels along the deformation direction.
We apply the uniaxially stretched membranes to vanadium redox flow
batteries (VRFBs) to suppress the permeation of active species, vanadium
ions through the transverse directions. The aligned water channels
render much lower vanadium permeability, resulting in higher Coulombic
efficiency (>98%) and longer self-discharge time (>250 h). Similar
to vanadium ions, proton conduction through the membranes also decreases
as the stretching ratio increases, but the thinned membranes show
the enhanced voltage and energy efficiencies over the range of current
density, 50â100 mA/cm<sup>2</sup>. Hydrophilic channel alignment
of PFSAs is also beneficial for long-term cycling of VRFBs in terms
of capacity retention and cell performances. This simple pretreatment
of membranes offers an effective and facile way to overcome high vanadium
permeability of PFSAs for VRFBs