Figure 2. (a) Comparison of FTIR spectra of glycerol and
glycerol/inhibitor mixture before (control) and after 5 min PTFE
deposition. Here, to make the substrate, an equal amount (30 μl) of
corresponding solution was spin-coated on plasma-cleaned Si substrates,
with comparable areas. Then, PTFE deposition was performed in the iCVD
reactor within 5 min at TS=15 oC and
P=300-1200 mTorr. (b) The reaction pathway for thermal hemolysis
activation of the initiator in the gas phase followed by quenching the
primary free radicals on the liquid surface.
As mentioned earlier, for the samples prewetted by the glycerol/HQ, both
chemical and physical mechanisms are in action. The deposition was
limited to the top surfaces and the liquid prevents the reactants from
coating the inner walls of the porous support. At the same time, HQ
quenches the free radicals absorbed into glycerol. Figure 2 (b) shows
the pathway for thermal hemolysis (activation) of the initiator
(perfluorobutane sulfonyl fluoride),31 and the
proposed reaction for chemical inhibition. We propose that the primary
radicals (), adsorbed on the surface of glycerol/inhibitor mixture, are
quenched by HQ, through electron transfer.32,33 The
FTIR results of the glycerol/inhibitor mixture after primary radical
adsorption also showed that HQ is reduced to quinone, confirming that HQ
inhibits polymerization by quenching the radicals; see Section S3,
Supporting Information.
To investigate the effectiveness of physicochemical inhibition, we
evaluated the surface porosity of PVDF support by measuring nitrogen
permeation at various transmembrane differential pressures. The PTFE
deposition was performed at different pressures ranging from 300 to 1200
mTorr. Figure 3 (a) compares the nitrogen permeation for the pristine
PVDF support with those of coated samples prepared at different iCVD
pressures. As shown, we did not observe any significant reduction in
nitrogen permeation after depositing of PTFE. This observation indicates
the deposition was limited to the solid domains, and the surface
porosity of the membranes was preserved.
To measure the wettability of the substrates, we performed the water
contact angle (CA) measurement on the top and bottom surface of the
processed membranes. Figure 3 (b) shows that the water CA on the top
surface of all samples was above 140 degrees. In contrast, the water
droplet placed on the back side of the membrane wicks through. We note
that the contact angle of water on the top surface of the Janus
membranes increases with increasing the pressure of iCVD processing.
Section S4, Supporting Information, includes the SEM images showing the
effect of varying the deposition pressure on the morphology of Janus
membrane surfaces. Here we attribute the observed trend in the
membranes’ CA to the induced roughness as a result of a transitionig the
deposition process from the adsorption limited to a gas-phase limited
one 34 Figure 3 (c) also shows the optical image of
water and ethylene glycol droplets placed on the top and the back side
of the Janus membranes. As shown, both liquids formed contact angles
above 140 degrees on the top surface of the Janus membrane. However, the
contact angle of all liquids on the back side was zero. This observation
shows that the back side of the membrane remained pristine. The X-ray
photoelectron spectroscopy (XPS) C1s core electron spectra collected
from the bottom surface of the coated membrane also matched that of a
pristine PVDF; see Section S5, Supporting Information.
To examine the underwater oleophobic properties of the hydrophilic side
of the Janus membranes, we measured the contact angle for oil droplets
when the hydrophilic side is immersed in water. Figure 3 (d) displays
the side-view optical image from a fabricated Janus membrane placed on
the water surface with the hydrophilic side facing the water. As shown,
the oil droplet has a contact angle of ~140 degrees with
the membrane and does not wet the surface, confirming underwater
oleophobicity of the untreated side. To illustrate this functionality
more clearly, we recorded a video of the testing procedure of a Janus
membrane, shown in Movie S2, Supporting Information. As shown, after
suspending a Janus membrane on the top surface of a water bath, water
penetrates the Janus membrane such that the membrane becomes
semi-transparent. However, the thin PTFE layer (2-3 μm) hinders water
from reaching the top surface. As a result, when water droplets impinged
on the top surface of the Janus membrane, they bounced back off the
surface due to the air gap formed within the PTFE domain; see Movie S2,
Supporting Information.