Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion

Sung Joon Park; Jung Hyun Lee

doi:10.1016/j.memsci.2020.118449

Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion

Sung Joon Park, Jung Hyun Lee

Department of Chemical and Biological Engineering

Research output: Contribution to journal › Article › peer-review

20 Citations (Scopus)

Abstract

Dual-layer slot coating (DSC) is a state-of-the-art technique that can fabricate thin film composite membranes by simultaneously spreading two monomer solutions to form an unsupported ultrathin polyamide (PA) selective layer, which is subsequently adhered to a support. To demonstrate its versatility, DSC was applied to polyethylene and polysulfone supports modified with O₂ plasma and/or polydopamine (PDA) coating for the fabrication of high-performance reverse osmosis (RO) membranes. PDA coating enabled the uniform and robust PA deposition by uniformly hydrophilizing supports and reinforcing PA-support interfacial adhesion through the introduction of oxygen-containing and amine groups that promote hydrogen bonding with the PA layer, thus achieving good RO performance. The O₂ plasma treatment on PDA-coated supports further strengthened PA-support interfacial adhesion by increasing the number of carboxyl groups with a higher hydrogen bonding ability, hence fabricating long-term stable, high-performance RO membranes that outperform a commercial RO membrane. This superior RO performance was enabled by the extremely thin (~7 nm) and highly crosslinked PA structure as well as strong PA-support interfacial adhesion. The surface tension analysis suggested that the work of adhesion at the PA-support interface of >~110 mJ m⁻² is required to achieve high membrane performance.

Original language	English
Article number	118449
Journal	Journal of Membrane Science
Volume	614
DOIs	https://doi.org/10.1016/j.memsci.2020.118449
Publication status	Published - 2020 Nov 15

Keywords

Dual-layer slot coating
Interfacial adhesion
Interfacial polymerization
Reverse osmosis
Thin film composite membranes

ASJC Scopus subject areas

Biochemistry
Materials Science(all)
Physical and Theoretical Chemistry
Filtration and Separation

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10.1016/j.memsci.2020.118449

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@article{4cd9842cfeba4784a17676d09ed22793,

title = "Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion",

abstract = "Dual-layer slot coating (DSC) is a state-of-the-art technique that can fabricate thin film composite membranes by simultaneously spreading two monomer solutions to form an unsupported ultrathin polyamide (PA) selective layer, which is subsequently adhered to a support. To demonstrate its versatility, DSC was applied to polyethylene and polysulfone supports modified with O2 plasma and/or polydopamine (PDA) coating for the fabrication of high-performance reverse osmosis (RO) membranes. PDA coating enabled the uniform and robust PA deposition by uniformly hydrophilizing supports and reinforcing PA-support interfacial adhesion through the introduction of oxygen-containing and amine groups that promote hydrogen bonding with the PA layer, thus achieving good RO performance. The O2 plasma treatment on PDA-coated supports further strengthened PA-support interfacial adhesion by increasing the number of carboxyl groups with a higher hydrogen bonding ability, hence fabricating long-term stable, high-performance RO membranes that outperform a commercial RO membrane. This superior RO performance was enabled by the extremely thin (~7 nm) and highly crosslinked PA structure as well as strong PA-support interfacial adhesion. The surface tension analysis suggested that the work of adhesion at the PA-support interface of >~110 mJ m−2 is required to achieve high membrane performance.",

keywords = "Dual-layer slot coating, Interfacial adhesion, Interfacial polymerization, Reverse osmosis, Thin film composite membranes",

author = "Park, {Sung Joon} and Lee, {Jung Hyun}",

note = "Funding Information: Both top and bottom surfaces of the isolated DSC-PA layer exhibited highly uniform, smooth (rms roughness below 3 nm) and dense morphologies (Fig. 3a and b), which were qualitatively consistent with those of the DSC-PA layer prepared using a low concentration MPD solution [13]. The symmetric and uniform structure of the DSC-PA layer was remarkably different from that of the conventional IP-assembled PA layer with a highly heterogeneous structure consisting of significantly rough top and porous bottom surfaces [33]. The DSC-DPPE membrane had a slightly rougher surface (rms roughness of 17.1 ± 0.7 nm) than the isolated DSC-PA layer, reflecting the textural roughness of the underlying DPPE support (Fig. 3c). Nevertheless, the surface roughness of the DSC-DPPE membrane was one order of magnitude lower than that of a commercial RO membrane (SWC4+) with a typical ridge-and-valley morphology (rms roughness = 101.8 ± 0.8 nm) (Fig. 3c). The DSC-PA layer exhibited extremely thin thickness (~7.2 nm) (Fig. 3d), which was approximately two orders of magnitude lower than those of conventional IP-assembled PA layers (~100 nm) [1,6,29]. There have been innovative attempts to fabricate sub-10 nm thick PA layers, but their somewhat sophisticated strategies make their commercialization difficult [ 34–36]. In this regard, our approach enables the fabrication of an ultrathin, sub-10 nm thick PA membrane in a much simpler and more commercially viable manner. The maximum sample size that can be processed with the current lab-scale DSC set-up is 10 × 20 cm2. To commercialize the DSC technique, the scaling-up issues, including the size of the dual slot die and plates and operating parameters, should be addressed.To identify the PA-support adhesion mechanism of the DSC-TFC membrane, the chemical and physical properties of each TFC component (support and PA layer) were characterized separately. Fig. 5a presents the FT-IR spectra of pristine and modified PE supports. Pristine PE showed strong peaks at 1464 cm−1 (C–H2 deformation), and 1474 cm−1 (C–H2 bending) [14,15]. For the PPE support, new peaks appeared at 1278 cm−1 (C–O stretching, carboxyl), 1632 cm−1 (O–H stretching, hydroxyl/carboxyl) and 1720 cm−1 (C[dbnd]O stretching, carbonyl/carboxyl) [ 47–49]. This result indicates that the O2 plasma treatment on pristine PE produces oxygen-containing groups, including hydroxyl, carbonyl and carboxyl groups [50,51]. Compared to pristine PE, the DPE support exhibited newly appeared peaks at 1610 cm−1 (C[dbnd]C resonance vibration, aromatic ring), 1632 cm−1, 1720 cm−1 and 3400 (N–H/O–H stretching vibration), which were characteristic PDA peaks [48,49, 52–54]. PDA coating is known to introduce oxygen-containing (hydroxyl and carbonyl) and amine groups [16,52,53]. In comparison with DPE, the DPPE support showed more pronounced peaks at 1632 cm−1 and 1720 cm−1 with a new peak at 1278 cm−1, indicating the further generation of oxygen-containing groups on the PDA coating by the plasma treatment [55].Fig. 5b shows the FT-IR spectra of the representative DSC-DPPE membrane in comparison with the DPPE support. The DSC-DPPE membrane exhibited characteristic PA peaks at 1668 cm−1 (amide I, C[dbnd]O stretching), 1542 cm−1 (amide II, N–H in-plane bending) and 1610 cm−1 (hydrogen-bonded C[dbnd]O), which were not detected for the DPPE support, verifying the formation of the PA layer on the support via DSC [11, 13–16,29].The chemical and physical properties of support and PA layer surfaces were analyzed using XPS and zeta potential characterization, as summarized in Table 2. The PPE support surface contained a discernible amount of oxygen (O), which was not detected by XPS for pristine PE, due to its plasma-generating oxygen-containing groups. The DPE support had higher O and nitrogen (N) contents than pristine PE due to the introduction of oxygen-containing and amine groups by PDA coating [56]. The DPPE support exhibited higher O but lower N contents than DPE, indicating that the O2 plasma treatment on the PDA coating increased the amount of oxygen-containing groups and/or converted pre-existing oxygen-containing groups to more-oxidized groups [55].Based on the chemical and physical properties of PA and support layers, the adhesion mechanism at the PA-support interface of the TFC membrane can be speculated. Interfacial adhesion mechanisms can include mechanical coupling and chemical interactions [66,67]. Mechanical coupling represents the adhesion-reinforcing mechanism by physical structures on two contacting surfaces interlocked with each other, which has been known as one of important interfacial adhesion mechanisms for in-situ IP-assembled, conventional TFC membranes [6,40,68]. However, this mechanical interlocking would not be significant for the DSC-TFC membrane, in which a preformed ultra-smooth PA layer is adhered to a support surface. Chemical interactions at the PA-support interface can include electrostatic, vW force (dispersion force), polar and hydrogen bonding interactions [12, 69–71]. Particularly, in aqueous media, the electrostatic interaction can, to some extent, contribute to interfacial adhesion [72]. Both PA bottom and support top surfaces had distinct negative charges, which would cause electrostatic repulsion during the PA deposition onto the support. Furthermore, despite its higher negative surface charge, the DPPE support showed better adhesion with the PA layer than DPE, as evidenced by the higher NaCl rejection of the DSC-DPPE membrane than that of DSC-DPE (Table 1). This result suggests that chemical interactions other than the electrostatic interaction primarily govern PA-support interfacial adhesion. Of these chemical interactions, hydrogen bonding would play a more predominant role in interfacial adhesion because its energy (10–50 kJ mol−1) is significantly higher than vW (~1 kJ mol−1) and polar (2–8 kJ mol−1) interaction energies [28,73]. The PA bottom surface was characterized as the amide chemistry with unreacted amine and carboxyl groups, which can form hydrogen bonding with various functional groups on the support surface [74,75]. For example, the DPE support surface contains hydroxyl, carbonyl and amine functional groups, which formed strong hydrogen bonding with the PA layer, thus resulting in the good NaCl rejection (>98.5%) of the DSC-DPE membrane. The XPS analysis revealed that the plasma treatment on the DPE support predominantly increased the number of more-oxidized surface carboxyl groups. The total number of hydrogen acceptor/donor sites of the carboxyl group (5 = acceptor (4)/donor (1)) is larger than those of other oxygen-containing groups (hydroxyl: 3 = acceptor (2)/donor (1), carbonyl: 2 = acceptor (2)/donor (0)), indicating its higher hydrogen bonding ability [71,76,77]. Lamellar et al. have also reported that the hydrogen bonding strength of the carboxyl group is stronger than that of the hydroxyl group [78]. In addition, the carboxyl group with both a hydrogen acceptor and donor would more strongly form hydrogen bonding than the carbonyl group with only a hydrogen acceptor [79]. Hence, the increased amount of carboxyl groups on the DPPE support would significantly increase the hydrogen bonding strength with the PA layer, predominantly reinforcing PA-support interfacial adhesion, which can explain the superior NaCl rejection of the DSC-DPPE membrane.This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2019R1A2C1002333, 2019M3E6A1064103and2018R1A4A1022194). Funding Information: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government ( 2019R1A2C1002333 , 2019M3E6A1064103 and 2018R1A4A1022194 ). Publisher Copyright: {\textcopyright} 2020 Elsevier B.V.",

year = "2020",

month = nov,

day = "15",

doi = "10.1016/j.memsci.2020.118449",

language = "English",

volume = "614",

journal = "Jornal of Membrane Science",

issn = "0376-7388",

publisher = "Elsevier",

}

TY - JOUR

T1 - Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion

AU - Park, Sung Joon

AU - Lee, Jung Hyun

N1 - Funding Information: Both top and bottom surfaces of the isolated DSC-PA layer exhibited highly uniform, smooth (rms roughness below 3 nm) and dense morphologies (Fig. 3a and b), which were qualitatively consistent with those of the DSC-PA layer prepared using a low concentration MPD solution [13]. The symmetric and uniform structure of the DSC-PA layer was remarkably different from that of the conventional IP-assembled PA layer with a highly heterogeneous structure consisting of significantly rough top and porous bottom surfaces [33]. The DSC-DPPE membrane had a slightly rougher surface (rms roughness of 17.1 ± 0.7 nm) than the isolated DSC-PA layer, reflecting the textural roughness of the underlying DPPE support (Fig. 3c). Nevertheless, the surface roughness of the DSC-DPPE membrane was one order of magnitude lower than that of a commercial RO membrane (SWC4+) with a typical ridge-and-valley morphology (rms roughness = 101.8 ± 0.8 nm) (Fig. 3c). The DSC-PA layer exhibited extremely thin thickness (~7.2 nm) (Fig. 3d), which was approximately two orders of magnitude lower than those of conventional IP-assembled PA layers (~100 nm) [1,6,29]. There have been innovative attempts to fabricate sub-10 nm thick PA layers, but their somewhat sophisticated strategies make their commercialization difficult [ 34–36]. In this regard, our approach enables the fabrication of an ultrathin, sub-10 nm thick PA membrane in a much simpler and more commercially viable manner. The maximum sample size that can be processed with the current lab-scale DSC set-up is 10 × 20 cm2. To commercialize the DSC technique, the scaling-up issues, including the size of the dual slot die and plates and operating parameters, should be addressed.To identify the PA-support adhesion mechanism of the DSC-TFC membrane, the chemical and physical properties of each TFC component (support and PA layer) were characterized separately. Fig. 5a presents the FT-IR spectra of pristine and modified PE supports. Pristine PE showed strong peaks at 1464 cm−1 (C–H2 deformation), and 1474 cm−1 (C–H2 bending) [14,15]. For the PPE support, new peaks appeared at 1278 cm−1 (C–O stretching, carboxyl), 1632 cm−1 (O–H stretching, hydroxyl/carboxyl) and 1720 cm−1 (C[dbnd]O stretching, carbonyl/carboxyl) [ 47–49]. This result indicates that the O2 plasma treatment on pristine PE produces oxygen-containing groups, including hydroxyl, carbonyl and carboxyl groups [50,51]. Compared to pristine PE, the DPE support exhibited newly appeared peaks at 1610 cm−1 (C[dbnd]C resonance vibration, aromatic ring), 1632 cm−1, 1720 cm−1 and 3400 (N–H/O–H stretching vibration), which were characteristic PDA peaks [48,49, 52–54]. PDA coating is known to introduce oxygen-containing (hydroxyl and carbonyl) and amine groups [16,52,53]. In comparison with DPE, the DPPE support showed more pronounced peaks at 1632 cm−1 and 1720 cm−1 with a new peak at 1278 cm−1, indicating the further generation of oxygen-containing groups on the PDA coating by the plasma treatment [55].Fig. 5b shows the FT-IR spectra of the representative DSC-DPPE membrane in comparison with the DPPE support. The DSC-DPPE membrane exhibited characteristic PA peaks at 1668 cm−1 (amide I, C[dbnd]O stretching), 1542 cm−1 (amide II, N–H in-plane bending) and 1610 cm−1 (hydrogen-bonded C[dbnd]O), which were not detected for the DPPE support, verifying the formation of the PA layer on the support via DSC [11, 13–16,29].The chemical and physical properties of support and PA layer surfaces were analyzed using XPS and zeta potential characterization, as summarized in Table 2. The PPE support surface contained a discernible amount of oxygen (O), which was not detected by XPS for pristine PE, due to its plasma-generating oxygen-containing groups. The DPE support had higher O and nitrogen (N) contents than pristine PE due to the introduction of oxygen-containing and amine groups by PDA coating [56]. The DPPE support exhibited higher O but lower N contents than DPE, indicating that the O2 plasma treatment on the PDA coating increased the amount of oxygen-containing groups and/or converted pre-existing oxygen-containing groups to more-oxidized groups [55].Based on the chemical and physical properties of PA and support layers, the adhesion mechanism at the PA-support interface of the TFC membrane can be speculated. Interfacial adhesion mechanisms can include mechanical coupling and chemical interactions [66,67]. Mechanical coupling represents the adhesion-reinforcing mechanism by physical structures on two contacting surfaces interlocked with each other, which has been known as one of important interfacial adhesion mechanisms for in-situ IP-assembled, conventional TFC membranes [6,40,68]. However, this mechanical interlocking would not be significant for the DSC-TFC membrane, in which a preformed ultra-smooth PA layer is adhered to a support surface. Chemical interactions at the PA-support interface can include electrostatic, vW force (dispersion force), polar and hydrogen bonding interactions [12, 69–71]. Particularly, in aqueous media, the electrostatic interaction can, to some extent, contribute to interfacial adhesion [72]. Both PA bottom and support top surfaces had distinct negative charges, which would cause electrostatic repulsion during the PA deposition onto the support. Furthermore, despite its higher negative surface charge, the DPPE support showed better adhesion with the PA layer than DPE, as evidenced by the higher NaCl rejection of the DSC-DPPE membrane than that of DSC-DPE (Table 1). This result suggests that chemical interactions other than the electrostatic interaction primarily govern PA-support interfacial adhesion. Of these chemical interactions, hydrogen bonding would play a more predominant role in interfacial adhesion because its energy (10–50 kJ mol−1) is significantly higher than vW (~1 kJ mol−1) and polar (2–8 kJ mol−1) interaction energies [28,73]. The PA bottom surface was characterized as the amide chemistry with unreacted amine and carboxyl groups, which can form hydrogen bonding with various functional groups on the support surface [74,75]. For example, the DPE support surface contains hydroxyl, carbonyl and amine functional groups, which formed strong hydrogen bonding with the PA layer, thus resulting in the good NaCl rejection (>98.5%) of the DSC-DPE membrane. The XPS analysis revealed that the plasma treatment on the DPE support predominantly increased the number of more-oxidized surface carboxyl groups. The total number of hydrogen acceptor/donor sites of the carboxyl group (5 = acceptor (4)/donor (1)) is larger than those of other oxygen-containing groups (hydroxyl: 3 = acceptor (2)/donor (1), carbonyl: 2 = acceptor (2)/donor (0)), indicating its higher hydrogen bonding ability [71,76,77]. Lamellar et al. have also reported that the hydrogen bonding strength of the carboxyl group is stronger than that of the hydroxyl group [78]. In addition, the carboxyl group with both a hydrogen acceptor and donor would more strongly form hydrogen bonding than the carbonyl group with only a hydrogen acceptor [79]. Hence, the increased amount of carboxyl groups on the DPPE support would significantly increase the hydrogen bonding strength with the PA layer, predominantly reinforcing PA-support interfacial adhesion, which can explain the superior NaCl rejection of the DSC-DPPE membrane.This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2019R1A2C1002333, 2019M3E6A1064103and2018R1A4A1022194). Funding Information: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government ( 2019R1A2C1002333 , 2019M3E6A1064103 and 2018R1A4A1022194 ). Publisher Copyright: © 2020 Elsevier B.V.

PY - 2020/11/15

Y1 - 2020/11/15

N2 - Dual-layer slot coating (DSC) is a state-of-the-art technique that can fabricate thin film composite membranes by simultaneously spreading two monomer solutions to form an unsupported ultrathin polyamide (PA) selective layer, which is subsequently adhered to a support. To demonstrate its versatility, DSC was applied to polyethylene and polysulfone supports modified with O2 plasma and/or polydopamine (PDA) coating for the fabrication of high-performance reverse osmosis (RO) membranes. PDA coating enabled the uniform and robust PA deposition by uniformly hydrophilizing supports and reinforcing PA-support interfacial adhesion through the introduction of oxygen-containing and amine groups that promote hydrogen bonding with the PA layer, thus achieving good RO performance. The O2 plasma treatment on PDA-coated supports further strengthened PA-support interfacial adhesion by increasing the number of carboxyl groups with a higher hydrogen bonding ability, hence fabricating long-term stable, high-performance RO membranes that outperform a commercial RO membrane. This superior RO performance was enabled by the extremely thin (~7 nm) and highly crosslinked PA structure as well as strong PA-support interfacial adhesion. The surface tension analysis suggested that the work of adhesion at the PA-support interface of >~110 mJ m−2 is required to achieve high membrane performance.

AB - Dual-layer slot coating (DSC) is a state-of-the-art technique that can fabricate thin film composite membranes by simultaneously spreading two monomer solutions to form an unsupported ultrathin polyamide (PA) selective layer, which is subsequently adhered to a support. To demonstrate its versatility, DSC was applied to polyethylene and polysulfone supports modified with O2 plasma and/or polydopamine (PDA) coating for the fabrication of high-performance reverse osmosis (RO) membranes. PDA coating enabled the uniform and robust PA deposition by uniformly hydrophilizing supports and reinforcing PA-support interfacial adhesion through the introduction of oxygen-containing and amine groups that promote hydrogen bonding with the PA layer, thus achieving good RO performance. The O2 plasma treatment on PDA-coated supports further strengthened PA-support interfacial adhesion by increasing the number of carboxyl groups with a higher hydrogen bonding ability, hence fabricating long-term stable, high-performance RO membranes that outperform a commercial RO membrane. This superior RO performance was enabled by the extremely thin (~7 nm) and highly crosslinked PA structure as well as strong PA-support interfacial adhesion. The surface tension analysis suggested that the work of adhesion at the PA-support interface of >~110 mJ m−2 is required to achieve high membrane performance.

KW - Dual-layer slot coating

KW - Interfacial adhesion

KW - Interfacial polymerization

KW - Reverse osmosis

KW - Thin film composite membranes

UR - http://www.scopus.com/inward/record.url?scp=85089255966&partnerID=8YFLogxK

U2 - 10.1016/j.memsci.2020.118449

DO - 10.1016/j.memsci.2020.118449

M3 - Article

AN - SCOPUS:85089255966

SN - 0376-7388

VL - 614

JO - Jornal of Membrane Science

JF - Jornal of Membrane Science

M1 - 118449

ER -

Fabrication of high-performance reverse osmosis membranes via dual-layer slot coating with tailoring interfacial adhesion

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