Supporting Information
Nanoprecipitation for Ultrafiltration Membranes
Qifeng Wang,†,‡ Sadaki Samitsu,† Yoshihisa Fujii,† Chiaki Yoshikawa,§ Toyohide Miyazaki,†
Hidekuni Banno,† and Izumi Ichinose*,†
†
Polymer Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan
§
Biomaterials Unit, National Institute for Materials Science, 2-1 Sengen, Tsukuba, Ibaraki
305-0047, Japan
‡
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306,
United States (Present address)
E-mail: ICHINOE.Izumi@nims.go.jp
Phase separation in nanoprecipitation
In general, the use of a very good solvent for a polymer brings about delayed demixing of
the polymer solution and leads to coarsening of the phase separation structure. This has been
widely investigated with regard to the fabrication of porous membranes. As an example, the
binodal curves of the polymer/NMP/water
and
polymer/THF/water
systems
are
illustrated in the diagram to the right
(Mulder,
M.,
Basic
Principles
of
Membrane Technology, 2nd ed.; Kluwer
Academic Publishers, 1997; pp 123-140).
When using NMP, demixing of the
polymer solution occurs rapidly with the
addition of a small amount of water and
the resulting polymer-rich phase contains
a small amount of NMP. In contrast,
demixing
of
the
polymer
in
tetrahydrofuran (THF) proceeds slowly and the polymer-rich phase contains a significant
amount of THF, as shown by the tie line in the phase diagram. In our nanoprecipitation
process, delayed demixing produced coarse polymer nanoparticles that contained a large
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amount of THF. Conversely, NMP induced instantaneous demixing of the polymer solution
and generated small, rigid nanoparticles that assembled into fibrous nanoparticle fibers.
Similarly, polystyrene (PS; Mw 480,000 by GPC) nanoparticle fibers were obtainable using a
NMP/ethanol system but were not achieved using THF. In the present study, therefore, we
frequently used NMP as a good solvent for polymers.
Preparation of ultrafiltration membranes
In a typical experiment, a dispersion of polymer nanoparticle fibers containing 0.1 mg of
the polymer was filtered through a CA filter with an effective area of 2.27 cm2. The polymer
weight (0.1 mg) corresponds to volumes of 5, 2 and 30 mL of the PVPh, PVDF and PANI
solutions, respectively. The filtration volume could be varied based on the effective area of
the CA filter or the desired thickness of the ultrafiltration membrane.
A scanning electron microscope (SEM)
image of the surface of a CA filter with a 200
nm cut-off is shown to the right. This image
was obtained using a Hitachi S-4800 after
coating the specimen with a 2 nm thick
platinum layer to prevent electric charging
effects. The filter evidently has many pores
several micrometers in diameter, and these
pores can be well covered by filtering a
dispersion of polymer nanoparticle fibers.
PVDF nanoparticle fibers were usually prepared from 1 mL of a NMP solution of PVDF
(1.0 mg mL-1) and 19 mL of ethanol, using this dispersion for the preparation of the
nanoparticle membranes. The choice of ethanol as a poor solvent was important so as to
realize a high rejection rate for 5 nm Au nanoparticles. Figure S1 shows SEM images of
PVDF nanoparticle membranes prepared using water and a water/ethanol mixture as the poor
solvents as well as an image of a typical membrane obtained with ethanol. When using a 1:1
water/ethanol mixture, the membrane exhibited large pores, while employing water as the
poor solvent gave a more defined nanoparticle morphology with increased porosity. PVDF
nanoparticles prepared with water seemed to be the most rigid, while the nanoparticles
prepared with ethanol as the poor solvent were relatively soft and sticky. In the latter case, the
PVDF nanoparticles likely contained small amounts of NMP and ethanol.
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A
B
C
Figure S1. SEM images of PVDF nanoparticle membranes prepared using different poor
solvents: (a) water, (b) water/ethanol (1:1 v/v) and (c) ethanol.
Evaluation of filtration performance
Figure S2 shows the changes in the UV-vis absorption spectra of 10 nm Au nanoparticle
solutions before and after filtration. The red spectra with an absorption peak near 520 nm
correspond to the original feed solutions. In the cases of PVPh and PVDF nanoparticle
membranes, this peak almost disappeared in the filtrates (blue spectra), indicating 99% and
98% rejection. The filtrate from a PANI nanoparticle membrane retained a very weak peak
due to the 10 nm Au nanoparticles, from which the rejection rate was calculated to be 95%.
As shown by the green spectra, the concentration of the Au nanoparticles was approximately
doubled when half of the feed solution had been filtered, indicating that the removal of the Au
nanoparticles was achieved by filtration, not by adsorption.
Figure S2. UV-vis spectra of 10 nm Au nanoparticle solutions before and after filtration with
PVPh, PVDF and PANI nanoparticle membranes. Red, blue, and green spectra are feed,
filtrate and concentrated solutions, respectively. The spectra were obtained when 4.91, 5.04
and 4.75 mL of 10 nm Au nanoparticle solutions (10 mL) were filtered by PVPh, PVDF and
PANI membranes, respectively.
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Pressure resistance of nanoparticle membranes
Figure S3 shows the water fluxes of PVPh, PVDF and PANI nanoparticle membranes at
high pressures. These membranes were prepared using 0.1 mg of the corresponding polymer
on a CA filter (effective area: 2.27 cm2). The membrane thicknesses are shown in Figure S4.
The water flux of the PVPh nanoparticle membrane increased linearly up to 0.4 MPa with a
slight change in slope at this pressure, after which the flux again increased linearly up to 2.0
MPa. The PVDF nanoparticle membrane showed linear flux increases up to 0.5 MPa, and
then up to 1.4 MPa with a slightly decreased slope. However, the flux obtained from this
membrane at a pressure difference of 2.0 MPa deviated considerably from linearity, likely due
to the decrease in porosity of the membrane at such a high pressure. The PANI nanoparticle
membrane showed the same tendency as the PVPh membrane up to 0.9 MPa, although a large
increase in flux was observed at 1.4 MPa, most probably resulting from the formation of small
cracks in the membrane and the associated increase in leakage. However, it is safe to say that
none of these nanoparticle membranes exhibited significant structural changes up to a
pressure of 0.9 MPa.
Figure S3. Water fluxes of polymer nanoparticle membranes at high pressures.
Control of membrane thickness
Figure S4 summarizes the relationship between the thicknesses of PVPh, PVDF and PANI
nanoparticle membranes and the polymer mass used in the preparation of the corresponding
polymer nanoparticle fibers. As discussed in our communication, some portion of the polymer
is lost in the filtration process, judging from the thickness and density of the porous
membranes. Since the CA filters had pores several micrometers in diameters, it is possible for
fragments of the polymer nanoparticle fibrous network to pass into or through the filter.
However, since a portion of each polymer dispersion forms an extended network structure, the
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thickness of the nanoparticle membranes still increases as the polymer mass used to produce
the nanoparticle dispersion increases.
Figure S4. Thicknesses of polymer nanoparticle membranes prepared using various polymer
masses.
Water fluxes under different pH conditions
Figure S5 presents the water fluxes of PVPh, PANI and PVDF nanoparticle membranes
over the pH range of 2.0 to 12.0. These membranes were prepared in the same manner as
those summarized in Table 1 and the pH was adjusted using either HCl or NaOH. The water
flux was measured at a pressure difference of -0.08 MPa. The water flux of the PVDF
nanoparticle membrane did not show any remarkable changes over this pH range, while the
PANI membrane flux decreased slightly at lower pH values. In contrast, the PVPh membrane
flux was stable at low pH, but unstable at a pH of 12.0. It is well known that the phenol
groups of PVPh deprotonate at high pH values and so this membrane is likely to be slightly
expanded at pH 12.0 such that the pore sizes are increased. Notwithstanding the flux
variations, these three membranes were all stable over the range of pH 2.0 – 10.0.
Figure S5. Water fluxes of polymer nanoparticle membranes under different pH conditions.
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Figure S6 shows the reversible color change of a PANI nanoparticle membrane. The color
of this membrane changed from blue to green when water with a pH of 2.0 was filtered
through it and returned to blue when water at a pH of 3.0 was filtered (note that, due to the
reflection of light from the membrane, the photographic images appear to have different
colors).
Figure S6. Photographic images of a PANI nanoparticle membrane at pH 2.0 and 3.0.
Pore size control using nanoparticle porogens
Ultrafiltration (UF) membranes with relatively large pores were prepared by mixing
porogen nanoparticles into the dispersions of polymer nanoparticle fibers. In one trial, 4 mL
of an ethanol solution of PVPh (0.5 mg mL-1, 50 °C) were added to 96 mL of water (30 °C)
with vigorous stirring. After 5 min the volume was adjusted to 100 mL with water. In addition,
1 mL of a NMP solution of PVDF (1.0 mg mL-1, 25 °C) was mixed with 19 mL of ethanol
(25 °C) in a similar manner. The resultant dispersions of PVPh nanoparticle fibers (10 mL)
and PVDF nanoparticle fibers (10 mL) were mixed, after which 2 mL of the mixed dispersion
was filtered through a CA filter (effective area: 2.27 cm2). The resulting PVPh/PVDF
nanoparticle membrane was treated with 20 mL of ethanol to dissolve the PVPh in the
membrane and then immersed in boiling water for 10 min to improve the mechanical
properties of the membrane.
The above UF membrane prepared by the removal of PVPh nanoparticles showed an
extremely high water flux of 3270 L m-2h-1 at a pressure difference of -0.08 MPa, and its
rejection of 40 nm Au nanoparticles was 98% or greater. In sharp contrast, the rejection of 20
nm Au nanoparticles was less than 10%, indicating that many pores greater than 20 nm were
formed by the removal of the PVPh nanoparticle porogen. This membrane was stable for
drying and storage at an ambient temperature.
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Large-area nanoparticle membrane
Our filtration-based process for the fabrication of polymer nanoparticle membranes has also
been applied to the manufacture of large-area ultrafiltration membranes. Figure S7 shows a
photographic image of a PVDF nanoparticle membrane with an area of approximately 1000
cm2. We have confirmed that all areas of this membrane show excellent rejection properties,
as reported in our communication. To increase the ease of handling of this material, the
membrane was frequently treated with hot water or hot glycerin.
Figure S7. Photographic image of a large-area PVDF nanoparticle membrane.
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