Introduction
With the recent advances in surface functionalization methods, the
fabrication of porous domains with special wettability has become one of
the leading research areas in the field of membrane
science.1–7 Currently, one of the primary topics in
this area is the fabrication of Janus membranes −membranes with
asymmetric wettability. Janus membranes bring novel functionalities, not
achievable using conventional membrane processes, to membrane-based
processes.8–11 Given this asymmetric wettability,
Janus membranes offer an additional means to regulate the transport of
species in a designated direction.12 This unique
feature makes these membranes an attractive candidate for many
applications such as ion gating,4 oil/water
separation,3 interfacial mass
transport,13 energy harnessing,14anti-gravity capillary water pumps,15 bubble
aeration,16 and water
desalination.17
To date, extensive research exists on the fabrication of Janus membranes
for different applications.18–22 Fabrication
methodologies are classified into two main categories, interfacial and
non-interfacial strategies.23 Interfacial methods
typically add a new layer or interface to the substrates. As a result,
additional resistance is introduced to the transport of species across
the membrane. In comparison, non-interfacial strategies are not limited
to the surface. The non-interfacial strategies refer to the techniques
in which asymmetric wettability is formed within the porous substrate.
Chemical vapor deposition and atomic layer deposition are among the
promising non-interfacial strategies for altering the wettability of
porous media.23,24 However, it is still challenging to
limit the wettability modification to the surface or a specific region
within the porous domains.23,25 Therefore, despite the
promising advantages of Janus membranes, the complexities involved with
the fabrication schemes of these membranes have hindered their
development on a large scale.
Here, we developed a simple method, based on the selective deposition of
fluoropolymers onto porous media. We utilized initiated chemical vapor
deposition (iCVD) to synthesize polymeric domains from the vapor phase
and coated the outermost layers of various hydrophilic porous
substrates. By infiltrating nonvolatile liquids containing
polymerization inhibitors, we limited the deposition of the
fluoropolymer to the surface and prohibited the bridging of polymer
films over the pores of the substrate. We examined the performance of
the fabricated Janus membranes in both static and dynamic tests.
Experimental Methods
Materials and chemicals
Hydrophilic polyvinylidene fluoride (Durapore®, 0.22 μm, 47 mm) and
cellulose acetate (0.45 μm, 47 mm) membranes were purchased from
Millipore Sigma. Perfluorobutane sulfonyl fluoride (PBSF, 96%) and
hydroquinone (HQ) (>99%) were purchased from
Sigma-Aldrich. Isopropyl alcohol (IPA, ACS grade), glycerol (certified
ACS), and sodium chloride (NaCl, ACS grade) were purchased from Fisher
Scientific. Ethylene Glycol (99.5%) was purchased from ACROS Organics.
Hexafluoropropylene oxide (HFPO) was purchased from Oakwood chemical.
Deionized (DI) water was obtained from a Simplicity® ultrapure water
purification system (Millipore, Billerica, MA). Ultrapure nitrogen was
purchased from Matheson Gas Company.
Initiated chemical vapor deposition (iCVD) of PTFE
The PTFE coating was deposited onto hydrophilic substrates using a
custom-made iCVD reactor with a volume of 3600 cm3.
For each deposition, a hydrophilic membrane, prefilled with
glycerol/inhibitor mixture (5000 ppm), was placed on the reactor stage,
which was cooled to 15 oC via a recirculating chiller
(Thermo Fisher Scientific). The monomer, HFPO, was metered into the
reactor using a mass flowmeter (MKS Instruments) with a flow rate of 12
sccm. The PBSF jar was maintained at 27 oC to provide
a constant flow rate of 1 sccm, adjusted using a precision needle valve
(Swagelok). A pressure transducer (MKS Instruments) and an automated
butterfly valve, connected to a vacuum process controller (MKS
Instruments), were used to maintain the reactor pressure at the set
point (300-1200 mTorr). For activating the initiator through thermal
pyrolysis, a filament array of phosphor bronze (Goodfellow), suspended 2
cm above the substrate, was heated resistively to 350oC.
Membrane characterization
Scanning electron microscopy (SEM) images were taken using the FEI
Helios NanoLab 660 microscope. To reduce the moisture content, we dried
the samples in a vacuum oven set at 60 oC for three
hours. After drying, the samples were cut to size, mounted on the SEM
stub, and coated with 60 nm of gold, using a Ted Pella sputtering
machine (108-Auto). Fourier-transform infrared spectroscopy (FTIR)
measurements were performed using the attenuated total reflection (ATR)
module of Bruker Alpha-p. A small (1 cm × 1 cm) sample was cut to size
and placed on the diamond crystal of the ATR module. The FTIR
measurements were performed using 24 high-resolution scans on each
sample with a resolution of 4 cm-1. The liquid contact
angles on the membrane surfaces were measured using an optical
tensiometer (Rame-hart, Model 590) and the sessile drop method. A 5 μL
liquid droplet was placed on the dried membrane sample. We performed the
measurements on three random points on each sample and presented the
data with one standard deviation. The liquid entry pressure (LEP) and
gas permeation measurements were performed using a custom-made porometer
setup described elsewhere.26 The overhead space of the
membrane in the filter holder was filled with DI water. Subsequently,
the pressure behind DI water was increased gradually. The LEP is
reported as the pressure at which gas flow is detected by the flowmeter,
placed inline at the outlet of the filter holder. We performed three
measurements and presented the data with one standard deviation. For the
gas permeation test, we flowed ultrapure nitrogen into the membrane at
different pressures ranging from 2 to 40 kPa. The nitrogen permeation
through the membrane was recorded using a digital flowmeter (Omega
Engineering, FMA1820A), connected to a computer. We characterized three
membranes and presented the data with one standard deviation. The DCMD
performances for the Janus membranes were evaluated using a
laboratory-scale DCMD unit described elsewhere.27 We
placed the membrane into a custom-built cell with a channel dimension of
26 mm in length, 26 mm in width, and 3 mm in depth. The effective
membrane area exposed to feed and distillate streams was about 6
cm2, with the hydrophilic side facing the feed stream.
The temperatures of feed and permeate streams were maintained at 70 °C
and 20 °C, respectively. The water vapor flux across the membrane (J)
was determined by the gravimetric method.27
Results and Discussion
Figure 1 illustrates the representative images of the membrane surfaces
and the schematic of the procedure used to impart asymmetric wettability
into hydrophilic substrates. Our fabrication scheme had three
consecutive steps, shown in Figure 1 (a). In Step (i), the support was
filled with glycerol/inhibitor (HQ) mixture. The detailed procedure for
filling the support with liquid mixture can be found in Section S1,
Supporting Information. Subsequently, in Step (ii), the filled membrane
was placed on the cooled stage (15 oC) of the iCVD
reactor, and the deposition of polytetrafluoroethylene (PTFE) is
performed at a pressure of 900 mTorr for different times (e.g., five
minutes). After the PTFE deposition, in Step (iii), the
glycerol/inhibitor mixture was removed from the sample by placing the
substrate in an isopropyl alcohol (IPA) bath. To visualize the
glycerol/inhibitor removal, we placed the coated support on the top of
an IPA/Water (50/50 v.%) bath, shown in Movie S1, Supporting
Information. It can be clearly seen that the glycerol/inhibitor mixture
is diffusing from the membrane into the bath. Figure 1 (b) shows a
top-down scanning electron microscopy (SEM) image of the PVDF support.
In that image, the top surface of the PVDF support has an open and
interconnected structure. Figures 1 (c) and (d) also show the top-down
and cross-section SEM images of the dried Janus membrane, respectively,
fabricated through the described process. As shown, the deposited PTFE
has a particulate-like structure on the top of the sponge-like structure
of hydrophilic PVDF support; in the image we separated these areas using
a dashed line. Moreover, the SEM images show that the top surface of
membranes remained open after PTFE deposition. As a control experiment,
we performed the same operation on porous PVDF supports, under the same
operational conditions for iCVD, without wetting the membranes. The
results indicate that the substrate, not prefilled with our liquid
mixture, was blocked, and nitrogen did not flow through these
substrates.