6 research outputs found
Nanoconfinement and Chemical Structure Effects on Permeation Selectivity of Self-Assembling Graft Copolymers
Permeation of small molecule solutes
through thin films is typically
described by the solution-diffusion model, but this model cannot predict
the effects of nanostructure due to self-assembly or additives. Other
models focusing on diffusion through isolated nanopores indicate that
confining permeation to channels slightly larger than the size of
the solute can lead to an increased influence of solute–pore
wall interactions on permeation rate. In this study, we analyze how
differences in polymer nanostructure affect the relative contributions
of solute size and polymer–solute interactions on transport
rate. We compared the diffusion rates of several small molecules through
two polymer thin films: A cross-linked, homogeneous film of polyÂ(ethylene
glycol phenyl ether acrylate) (PEGPEA) and a graft copolymer with
a polyÂ(vinylidene fluoride-<i>co</i>-chlorotrifluoroethylene)
(PÂ(VDF-<i>co</i>-CTFE)) backbone and PEGPEA side chains
that self-assemble into continuous ∼1–3 nm PEGPEA domains
through which transport occurs. We correlated these rates with the
size of each solute and its chemical affinity to PEGPEA, as measured
by the difference between their solubility parameters. Diffusion rate
through the homogeneous polymer film was controlled by solute size,
whereas diffusion rate through the copolymer was strongly controlled
by the difference between the solubility parameters. Furthermore,
permeation selectivity between two selected molecules was 2.5×
higher for the nanostructured copolymer, likely enhanced by the nanoconfinement
effects. These initial results indicate that polymer self-assembly
is a promising tool for designing polymeric membranes that can differentiate
between solutes of similar size but differing chemical structures
Selective Transport through Membranes with Charged Nanochannels Formed by Scalable Self-Assembly of Random Copolymer Micelles
Membranes that can
separate compounds based on molecular properties
can revolutionize the chemical and pharmaceutical industries. This
study reports membranes capable of separating organic molecules of
similar size based on their electrostatic charge. These membranes
feature a network of carboxylate-functionalized 1–3 nm nanochannels,
manufactured by a simple, scalable coating process: a porous support
is coated with a packed array of polymer micelles in alcohol, formed
by the self-assembly of a water-insoluble random copolymer with fluorinated
and carboxyl functional repeat units. The interstices between these
micelles serve as charged nanochannels through which water and solutes
can pass. The negatively charged carboxylate groups lead to high separation
selectivities between organic solutes of similar size but different
charge. In single-solute diffusion experiments, neutral solutes permeate
up to 263 times faster than negatively charged compounds of similar
size. This selectivity is further enhanced in experiments with mixtures
of these solutes. No permeation of the anionic compound was observed
for over 24 h. In filtration experiments, these membranes separate
anionic and neutral organic compounds while exhibiting water fluxes
comparable to that of commercial membranes. Furthermore, carboxylate
groups can be functionalized, creating membranes with nanopores with
customizable functionality to enable a broad range of selective separations
Self-Assembling Zwitterionic Copolymers as Membrane Selective Layers with Excellent Fouling Resistance: Effect of Zwitterion Chemistry
Membranes with high
flux, ∼1 nm pore size, and unprecedented protein fouling resistance
were prepared by forming selective layers of self-assembling zwitterionic
amphiphilic random copolymers on porous supports by a simple coating
method. Random copolymers were prepared from the hydrophobic monomer
2,2,2-trifluoroethyl methacrylate (TFEMA) and four zwitterionic monomers
(sulfobetaine methacrylate, sulfobetaine 2-vinylpyridine, sulfobutylbetaine
2-vinylpyridine, and 2-methacryloyloxyethyl phosphorylcholine) by
free radical polymerization. All copolymers microphase separated to
form bicontinuous ∼1.2 nm nanodomains with the zwitterionic
domains acting as nanochannels for the permeation of water and solutes.
The resultant membranes all had a ∼1 nm size cutoff independent
of zwitterion chemistry. There were, however, significant differences
in the hydrophilicity, water uptake, water flux, and fouling resistance
among membranes prepared with different zwitterionic monomers. Membranes
prepared from the copolymer with 2-methacryloyloxyethyl phosphorylcholine
were the most hydrophilic and had the highest water permeance, higher
than that of commercial membranes of similar pore size. Furthermore,
these membranes showed unprecedented fouling resistance, exhibiting
no measurable flux decline throughout a 24 h protein fouling experiment.
The structure–property relationships gleaned from this survey
of different zwitterion structures serves as a guideline to develop
new zwitterionic materials for various applications such as membranes,
drug delivery, and sensors
Self-Cleaning Membranes from Comb-Shaped Copolymers with Photoresponsive Side Groups
In this study, we present a novel
self-cleaning, photoresponsive
membrane that is capable of removing predeposited foulant layers upon
changes in surface morphology in response to UV or visible light irradiation
while maintaining stable pore size and water permeance. These membranes
were prepared by creating thin film composite (TFC) membranes by coating
a porous support membrane with a thin layer of novel comb-shaped graft
copolymers at two side-chain lengths featuring polyacrylonitrile (PAN)
backbones and photoreactive side chains, synthesized by atom transfer
radical polymerization (ATRP). Photoregulated control over membrane
properties is attained through a light-induced transition, where the
side chains switch between a hydrophobic spiropyran (SP) state and
a zwitterionic, hydrophilic merocyanine (MC) state. The light-induced
switch between the SP and MC forms changes surface hydrophilicity
and causes morphological changes on the membrane surface as evidenced
by atomic force microscopy (AFM). Before any phototreatment, the as-coated
membrane surface comprises mostly hydrophobic SP groups that allow
the adsorption of organic solutes such as proteins the membrane surface,
reducing flow rate. Once exposed to UV light, conversion of the SP
groups to hydrophilic MC groups leads to the release of adsorbed molecules
and the full recovery of the initial water flux. A fouled membrane
in the more hydrophilic MC form is also capable of self-cleaning upon
conversion to the less hydrophilic SP form by visible light irradiation.
The self-cleaning behavior observed for this system, where the surface
became less hydrophilic but also experienced a morphological change,
demonstrates a novel mechanism that has a mechanical component in
addition to the changes in hydrophilicity. It is also the first report,
to our knowledge, of self-cleaning performance accompanied by a decrease
in hydrophilicity
Hydrophobic Antifouling Electrospun Mats from Zwitterionic Amphiphilic Copolymers
A porous
material that is both hydrophobic and fouling-resistant is needed
in many applications, such as water purification by membrane distillation.
In this work, we take a novel approach to fabricating such membranes.
Using the zwitterionic amphiphilic copolymer polyÂ(trifluoroethyl methacrylate-<i>random</i>-sulfobetaine methacrylate), we electrospin nonwoven,
porous membranes that combine high hydrophobicity with resistance
to protein adsorption. By changing the electrospinning parameters
and the solution composition, membranes can be prepared with a wide
range of fiber morphologies including beaded, bead-free, wrinkly,
and ribbonlike fibers, with diameters ranging between ∼150
nm and 1.5 μm. The addition of LiCl to the spinning solution
not only helps control the fiber morphology but also increases the
segregation of zwitterionic groups on the membrane surface. The resultant
electrospun membranes are highly porous and very hydrophobic, yet
resist the adsorption of proteins and retain a high contact angle
(∼140°) even after exposure to a protein solution. This
makes these materials promising candidates for the membrane distillation
of contaminated wastewater streams and as self-cleaning materials