Graphene Quantum Dots-Doped Thin Film Nanocomposite Polyimide

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Applications of Polymer, Composite, and Coating Materials

Graphene Quantum Dots Doped Thin Film Nanocomposite Polyimide Membrane with Enhanced Solvent Resistance for Solvent Resistant Nanofiltration Shuxuan Li, Can Li, Xiaojuan Song, Baowei Su, Bishnupada Mandal, Babul Prasad, Xueli Gao, and Congjie Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19834 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Materials & Interfaces

Graphene Quantum Dots Doped Thin Film Nano-composite Polyimide Membrane with Enhanced Solvent Resistance for Solvent Resistant Nanofiltration

Shuxuan Li1, Can Li1, Xiaojuan Song 1, Baowei Su1,*, Bishnupada Mandal2,*, Babul Prasad2, Xueli Gao1, Congjie Gao1

1

Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of China), Ministry of Education, Qingdao 266100, China

2

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India

*Corresponding authors; Phone / Fax: +86 - 532 - 66786371, Email : [email protected] (B. Su); Phone: +91 3612582256 / +91 - 9957181980, Fax: +91 361 2582291, E - mail: [email protected] (B.Mandal)

KEYWORDS: Solvent resistant nanofiltration (SRNF); Graphene Quantum Dots (GQDs); Interfacial polymerization (IP); Chemical imidization; Chemical crosslinking.

ABSTRACT: The core of the organic solvent nanofiltration (OSN) technology are solvent resistant nanofiltration (SRNF) membranes. Till now, relative poor performance of solvent 1

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resistance is still the bottleneck of industrial application of SRNF membranes. This work reports a novel polyimide (PI)-based thin film nano-composite (TFN) membrane which was embedded with graphene quantum dots (GQDs) nanoparticles and showed an improved solvent resistance for OSN application. This kind of SRNF membrane, termed as (PI-GQDs/PI)XA, was synthesized via a serial processes of interfacial polymerization (IP), imidization, crosslinking and solvent activation. The IP process was performed between an aqueous m-phenylenediamine (MPD) solution doped with GQDs nanoparticles having an average size of 1.9 nm, and an 1,2,4,5-benzenetetracarboxylic acyl chloride (BTAC) n-hexane solution on PI substrate surface. The prepared (PI-GQDs-50/PI)X SRNF membranes without organic solvent activation achieved an ethanol permeance of nearly 50 % higher than those of the GQDs-free membranes under the same preparation conditions, while no compromise of the dye rejection was observed. Further, after the solvent activation using N, N-dimethylformamide (DMF) at 80 ºC for 30 min, the ethanol permeance achieved about an 8 - folds increment, from 2.84 to 22.6 L m−2 h−1 MPa−1. Interestingly, the rejection of RDB also increased from 97.8 to 98.6 %. A long-term permeation test of more than 100 h using Rose Bengal (RB, 1017 Dalton)/DMF solution at room temperature demonstrated that the synthesized (PI-GQDs-50/PI)XA membranes could maintain the DMF permeance and the RB rejection as high as 18.3 L m−2 h−1 MPa−1 and 99.9 %, respectively. Moreover, immersion test of the prepared (PI-GQDs-50/PI)XA 2


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SRNF membranes in both DMF and ethanol at room temperature for about one year also demonstrated the long-term organic solvent stability, indicating their good potential for OSN application.

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1. INTRODUCTION Substantial amount of organic solvents was consumed in various industrial fields including chemical, petroleum, pharmaceutical, and food production. Traditional separation processes represented by distillation are quite energy and cost intensive, in chemical and related industries which usually amount to about 40 ~ 70 % of operating costs.1 Comparatively, membrane processes have great potential applications in these separation aspects due to their lower operating temperature and less energy consumption.2 Solvent resistant nanofiltration (SRNF), also termed as organic solvent nanofiltration (OSN), is a promising technique which can separate and purify organic mixtures at molecular level.3 SRNF membranes have a molecular weight cut-off (MWCO) between 200 and 1000 Dalton (Da) and require a trans-membrane pressure difference as driving force,4 similar to nanofiltration (NF) membranes applied in aqueous systems. SRNF membranes have enormous potential in the separation of organic substances from organic solvents which were widely applied in food,5-7 petrochemical,8-10 pharmaceutical,11-12 and fine chemical industries.13-14 Most SRNF membranes reported in literatures as well as those used commercially were integrally skinned asymmetric (ISA) ones fabricated via phase inversion.15-16 These SRNF membranes are essentially made by polyimide (PI) polymer, since it has excellent thermal stability and good resistance to ordinary solvents such as toluene, methanol, ethanol, ethyl 4


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acetate, etc.17-19 The SRNF membranes made by PI polymer can be further crosslinked to maintain a stable separation performance even in more harsher solvent environments such as strong polar N, N-dimethylformamide (DMF) solutions.17, 19-21 Recently, interfacial polymerization (IP) method, which is widely applied for the preparation of NF and reverse osmosis membranes using in aqueous solution separation, has gradually gained attention for the fabrication of SRNF membranes.22-23 TFC membranes usually have higher fluxes than ISA membranes, while have essentially the same selectivity.24 Most TFC SRNF membranes investigated so far were prepared via IP on the substrates such as crosslinked PI or hydrolyzed polyacrylonitrile (PAN-H) by the reaction between diamine monomer and trimesoyl chloride (TMC) to form a polyamide (PA) skin layer.20,

23-24

Jimenez - Solomon et al.24-26 synthesized a series of PA TFC

membranes on crosslinked PI substrates and proved that they were stable in DMF. They further conducted a solvent activation procedure to prepare OSN membranes which exhibited much higher permeabilities for polar solvents such as DMF, than commercial ISA SRNF membranes, and yet had comparable rejections.25 They also reported a new method to design rigid polymer skin layer by manipulating the molecular structure in the IP process. The composite SRNF membranes they prepared had enhanced micro-porosity and reduced thickness (~20 nm) of the skin layer, and displayed superior permeability and selectivity as compared to commercially available membranes.26 Hermans et al.27 5



synthesized another TFC SRNF membranes by combining crosslinking and impregnation of the substrate together with phase inversion during the phase inversion procedure via using a diamine mixture in the coagulation bath and then followed by an IP reaction, thus greatly shortened the whole preparation procedure. The ethanol permeance almost doubled, from 0.9 to 1.7 L m−2 h−1 MPa−1, while the rejection of Rose Bengal (RB, 1017 Da) increased from 94.0 % to 97.5 %. The TFC SRNF membranes reported so far still have relative low permeance, which need to be increased significantly. Recently, nanoparticles incorporation or deposition has been proved to be an efficient method to enhance the separation performance of various membranes such as RO,28 NF,29 forward osmosis (FO),30 and so on. This is mainly attributed to the structure modification of the polymer skin barrier layer by nanoparticles, since some filler particles could hydrate and result in local generation of heat during the IP process and influence the IP kinetics, some filler materials even have catalytically active surfaces and could possibly influence the IP as well.31 Commonly, nano SiO2,32 nano TiO2,33 carbon dots (CDs),34 metal-organic frameworks (MOFs),35 and graphene oxide (GO),36 covalent organic framework (COFs)37 have been used for preparing thin film nano-composite (TFN) SRNF membranes. Li et al.32 prepared a PAN/PA SRNF membrane via the IP reaction between an aqueous polyether imide (PEI) solution doped with functional SiO2 nanospheres and an organic TMC solution, and 6


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realized a 50% increase of the ethanol permeance from 21.2 to 30.8 L m−2 h−1 MPa−1, and a PS (990 Da) rejection of above 92.8%. The fabricated PAN/PEI-SiO2-Py membranes exhibited a stable SRNF performance during a 12 h test. Sorribas et al.38 demonstrated that with the MOFs incorporation, the TFN SRNF membranes showed a 160 % increase of methanol (MeOH) permeance, reaching 39 L m−2 h−1 MPa−1, while retaining a MWCO less than 232 g mol−1 for polystyrene (PS) oligomers MeOH solution. Li et al.37 successfully prepared a COFs (SNW-1) incorporated OSN membranes and achieved a Rhodamine B (RDB, 479 Da) rejection of 99.4% and an ethanol permeance of about 80 L m−2 h−1 MPa−1. Shao et al.39 synthesized a kind of GO doped polypyrrole (PPy) SRNF membrane and achieved an ethanol permeance of about 78 L m−2 h−1 MPa−1 and a rejection of around 99% for RB. Recently, graphene quantum dots (GQDs), also termed as carbon quantum dots (CQDs), graphene oxide quantum dots (GOQDs) or CDs,40 have gained much attention for membranes fabrication.40 Compared with those sheet, spherical, and tubular fillers in the graphene family, GQDs are zero dimensional (0 D) nanomaterials and possess superior properties such as larger edge effects, good thermal and chemical stability, lower cytotoxicity than micron size graphene oxide sheet, etc.41-43 GQDs incorporation have been proved to increase the water recovery and power density of pressure - retarded osmosis (PRO) membranes for osmotic power generation.44-45

Zhang et al.46 reported a TFN

7



membrane which comprised of GOQDs incorporated tannic acid (TA) skin layer, and showed an 50% increase of the pure water flux, reaching 23.3 L m−2 h−1 at 0.2 MPa, compared to that of the pristine TA TFC membrane, while maintained a Congo red (696 Da) rejection of 99.8 %. Xu et al.30 revealed that under appropriate content GQDs incorporation enhanced the water permeability, surface hydrophilicity and antifouling property, and decreased the surface roughness of the prepared TFN FO membranes, demonstrating the advantages of GQDs in membranes fabrication. Considering that even smaller fillers have ultrahigh specific surface area, in this work, we put forward the preparation of a novel GQDs doped TFN SRNF membrane. We incorporated GQDs into the aqueous MPD solution and used tetra-acyl chloride instead of TMC as the organic monomer to synthesize an active skin layer of polyamic acid (PAA) via IP reaction followed by an imidization, and a subsequent chemical crosslinking reaction to strengthen the integral stability of the prepared SRNF membrane. We further adopted an organic solvent activating procedure according to Jimenez-Solomon et al.25 and Mariën et al.47 to adjust their integral separation performance. 2. EXPERIMENTAL SECTION 2.1 Materials. PI polymer (Lenzing P84, in granule form) was bought from HP Polymer GmbH (Austria). 1,2,4,5-benzenetetracarboxylic acyl chloride (BTAC, 97%) was purchased from Zhiya Pharmaceutical Technology Co., Ltd (Guangzhou, China). Citric 8


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acid (99%), acetic anhydride (98%), triethylamine (TEA, 98%), acetone (98%), 1,6hexanediamine (HDA, 99%), isopropanol (IPA, 99.7%), Rose Bengal (RB, 1017 Da), Rhodamine B (RDB, 479 Da), sodium hydroxide (NaOH, 98%), N, N-dimethylformamide (DMF), polyethylene glycol 400 (PEG 400), n-hexane (99.7%), ethanol (EtOH, 99.7%) and hydrochloride (HCl, 35%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). M-phenylenediamine (MPD, 99.5%) was provided by Tianjin Guangfu Fine Chemical Research Institute (China). All the reagents were used without further purification. Polyethylene terephthalate (PET) nonwoven fabric was bought from Teijin Co., Ltd. (Japan). Deionized (DI) water was purified from an ultra-pure water machine (FDY1002UV-P, Qingdao Flom Technology Co., Ltd. China). DL NF membrane (MWCO of 300 Da) was bought from GE Co., Ltd. (USA). Polyacrylonitrile (PAN) UF membrane with MWCO of 50,000 Da was procured from Beijing RisingSun Membrane Technology Co., Ltd. (China). 2.2 Synthesis of GQDs nanoparticles. GQDs nanoparticles were synthesized according to the literature.48 We purified the synthesized GQDs solution by using the PAN UF membrane to remove any impurities with particles size larger than about 10.0 nm. This could be achieved since filtration cake can be easily formed on the UF membrane surface and the initial turbid UF filtrate was returned back to the raw solution until the UF filtrate 9



became completely clear. The UF filtrate was then purified by the DL NF membrane using DI water under diafiltration mode to remove ions and any impurities with particles size smaller than about 1.0 nm until the conductivity of the NF permeate decreased down to about 10 µS cm−1. Finally, the NF concentrate was freeze-dried and the obtained GQDs powder was used for further characterization. 2.3 Preparation of the PI UF substrate membrane. A detailed description of the PI UF membrane can be found in our previous work.49 Briefly, P84 (20.0 wt %) granules was dissolved in DMF with PEG 400 (1.0 wt %) as a pore former, and was stirred overnight to achieve a homogeneous dissolution. Thus prepared dope solution was centrifuged to remove air bubbles prior to the casting onto a piece of polyester nonwoven fabric taped on a glass plate. The gap between the casting knife and the glass plate was set at 230 μm, and the casting velocity was set at 1.2 m min−1. Immediately after the casting, the cast fabric was immersed into a DI water coagulation bath (thermostated at 25 ºC) for 10 min to form the PI UF substrate. Afterward, it was rinsed and then was immersed in DI water for at least 12 h for further analysis and SRNF membrane preparation. 2.4 Preparation of GQDs doped nano-composite membranes. Prior to the preparation, 2.0 wt % aqueous MPD solution with certain amount of GQDs was dispersed by an ultrasonicator (JT-410HT, Jituo Co, Shenzhen, China) of 240 W under N2 atmosphere for 60 min. 0.3 wt % BTAC in n-hexane was stirred by a magnetic stirrer under 10


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N2 atmosphere for 30 min. The PI substrate was attached to a glass plate after being rinsed with DI water for about one minute. Afterward, we poured the aqueous MPD solution onto the substrate surface and lasted for 8 s. Then the excess solution was removed with the help of a squeeze roller followed by drying in the air until all droplets vanished. At this stage, the substrate was denoted as (MPD-GQDs-c)/PI, in which the character “c” means the GQDs content (mg L−1) in the aqueous MPD solution. When “c” is zero, it means that no GQDs is doped. Subsequently, the BTAC solution was poured onto the above substrate surface and was lasted for 6 s to form a GQDs embedded active layer of polyamic acid (PAA). Afterward, it was oven dried for 5 min at 80 ºC, and denoted as (PAA-GQDs-c)/PI. It was then imidized using a mixture of acetic anhydride, TEA and acetone (volumetric ratio of 3:1:10)50 at 50 ºC for 0.5 h, and was termed as (PI-GQDs-c)/PI. Afterwards, it was crosslinked using HDA/IPA solution (10.0 wt %) at 60 ºC for certain time, and was denoted as (PI-GQDs-c/PI)X, in which the subscript “X” means crosslinking. The crosslinking time was set at 1.0 h without specification. The activation of (PI-GQDs-c/PI)X membrane was done using DMF for 0.5 h at 80 ºC, and was then soaked in ethanol for more than 3 h at room temperature to replace completely the remaining DMF in the membrane matrix, and was denoted as (PI-GQDs-c/PI)XA, in which the additional subscript “A” means the solvent activation process. The steps of the SRNF membrane preparation process are summarized in Table 1. 11



Table 1. The steps of the SRNF membranes preparation process.

Step

Membranes

Detail description of the steps

Phase inversion of the casting solution ( 20.0 wt % P84 1

PI substrate and 1.0 wt % PEG 400)in DMF) Contact the PI substrate with an aqueous solution (2.0 wt

2

(MPD-GQDs-c)/PI % MPD and a certain amount of GQDs) for 8 s IP with an organic solution (0.3 wt % BTAC in n-hexane)

3

(PAA-GQDs-c)/PI for 6 s Imidization in a mixture of acetic anhydride, TEA and

4

(PI-GQDs-c)/PI acetone (volumetric ratio of 3:1:10) at 50 ºC for 0.5 h Chemically crosslinking in HDA/IPA solution (10.0 wt

5

(PI-GQDs-c/PI)X %) at 60 ºC for a certain time

6

(PI-GQDs-c/PI)XA

DMF activation at 80 ºC for 0.5 h

The reaction equations for the synthesis of the PI skin layer and the possible reaction equations for the synthesis of the (PI-GQDs) function layer and the GQDs-embedded wholly crosslinked (PI-GQDs-c/PI)X SRNF membranes are shown in Figure 1. 12


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NH2 Cl n

O

O

C

C

HO

+ 2n H2O Cl

C

C

O

O

O

C

C

OH

C

C

NH

O

O

Cl

+ n NH2

O

Cl

+ 4n HCl

NH

n

(a)

(b) NH2

n GQDs

n

n

O Cl C

O C Cl

NH2 Cl C O

2n H2O

C Cl O

O

O

C

C OH

4nHCl

O C NH

C

O

(GQDs) O

NH

n

(c) O C

O C OH

HO C O

C NH O

O O TEA + 2nH3C C O C CH3 NH

O C N C O

n

(d)

13


O C N C O

+4n CH3COOH n


PI substrate O

O

C

C

Skin layer

O

C

C

O

O

O C

O C

C O

C O

N

N

N

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m

Crosslinker

N

NH2

H2N

n

Crosslinking

PI substrate crosslinking O

O

C

C

O

O C NH O C

N

NH O C

C

m1

O

HN

O C

O

C

C

C O

n1

HN

O HN

N C

C

O

O

O C NH O C

N

NH O

Skin layer crosslinking

Interface crosslinking

O

O

C

C

N C O

n2

HN

NH O C

O N

HN

N

m2

O C

C

C

O

O

m3

C O

NH C O HN C O

n3

(e) Figure 1. Reaction equations for the synthesis of the PI skin layer (a, d)51-52 and the possible reaction equations for the synthesis of the (PI-GQDs) function layer (b, c)30, 53 and for the formation of the GQDs-embedded (PI-GQDs-c/PI)X SRNF membrane by the wholly crosslinking reaction (e).49, 54

2.5 GQDs and membrane characterizations. The synthesized GQDs nanoparticles and membranes were characterized by using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet Magna-560, USA), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA), Raman spectrum (Thermo

14


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Scientific DXR Raman microscope, USA), scanning electron microscopy (SEM, S-4800, Hitachi, Japan), atomic force microscopy (AFM, Agilent 5400,USA), transmission electron microscopy (TEM, HT-7700, Hitachi, Japan). ATR-FTIR was used to research the chemical composition of the GQDs and the membranes surface. Raman spectrum was used to investigate the existence of GQDs55-58 in the synthesized (PI-GQDs-50/PI)XA membranes. SEM was used to observe the surface morphology of the (PI-GQDs-c/PI)X membranes. The surface roughness of the (PI-GQDsc/PI)X membranes as well as the height of the GQDs nanoparticles were measured using AFM. The morphological structure of the synthesized GQDs nanoparticles was characterized by TEM. At least six hundred GQDs nanoparticles on the TEM image were used to calculate the particle size distribution. The elemental composition and chemical bonding of both the GQDs nanoparticles and the membranes surface were determined using XPS. 2.6 Membrane surface hydrophilicity characteristics. The contact angle hydrophilicity of the surface of the fabricated PI/PI and (PI-GQDs)/PI membranes before and after the chemical crosslinking was measured using a contact angle goniometer (DSA100, Kruss, Germany) with the static sessile drop method. Five drops of DI water (each drop was 1.0 µL) were dropped onto the surface of each membrane sample sheet.

15



The contact angle of each drop was measured and the average value of the five independent measurements was used for each sample. 2.7 Membrane separation performance. A cross-flow pressure filtration apparatus described in our earlier research work59 was used to access the membranes permeability. The picture and the flow diagram of the setup for the evaluation of the membrane separation performance were shown in Figure S1. 100 mg L−1 RDB (479 Da)/ethanol solution was used as test solution for the membrane separation performance measurement. The standard curve of the RDB/ethanol solution was shown in Figure S2 (a). To evaluate the long-term solvent resistance, we further conducted a permeation test using 100.0 mg L−1 RB (1017 Da) / DMF solution. We selected RB instead of RDB in the DMF solution for the long-term permeation test since we found that the color of the bulk RDB/DMF solution gradually receded during the long-term permeation. The standard curve of the RB/DMF solution is shown in Figure S2 (b). All the above tests were performed at 1.0 MPa and room temperature. After the monitored permeation flux reached a steady state for each test, the dye concentrations of both the feed and the permeate solutions were measured separately by a UV-Vis spectrophotometer (UV-5100, Shanghai Metash instrument Co., Ltd., China) at a wavelength of 544 nm (for RDB / ethanol solution) or 552 nm (for RB/DMF solution). The solvent permeance (P) was calculated by measuring the mass (Δm) of the permeate through 16


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the effective membrane area (A) during the testing time interval (Δt) with trans-membrane differential pressure (Δp) according to Eq. (1):

P  1000 

Δm s  A  Δt  Δp

(1)

The dye rejection (R) was calculated using Eq. (2), where Cp and Cf are the dye concentrations of the dye in the permeate and the feed solutions, respectively.

 Cp  R  1   100%  Cf 

(2)

Sometimes we prepared membrane samples of the same preparation conditions on different days to verify the reproducibility. The standard deviations of the permeance and the rejection values under each condition were considered by averaging of at least two or three pieces of each samples. 2.8 Membranes pore size prediction. According to Santos et al.,60 the term “pore” through which transport can take place is conceived as void spaces which originated from the irregular packing of the polymer. The pore size distribution of the NF membrane skin layer was calculated using the log-normal probability density function (Eq. 3) described by Belfort et al.,61 Zydney et al.,62 and Bowen et al.63 All the related equations were summarized in our previous work49. Recently, Campbell et al.15 also used these equations to predict the pore size distribution of OSN membranes.

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   r b  2   log *    2   1  r   fR r   exp-   2b r 2b      

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(3)

3. RESULTS AND DISCUSSION 3.1 Characteristics of GQDs. The thickness of the synthesized GQDs was in the range of 0.8 ~ 2.0 nm, with an average thickness of 1.8 nm, as can be seen from Figure 2 (a) and (b). This suggested that most of the GQDs nanoparticles were comprised of 1 ~ 3 layers of graphene.64 This might be very beneficial for the preparation of a uniform membrane skin layer compared with using other nanomaterials, since most of the nanoparticles used for fabricating TFN membranes are not sheet-like and usually have a particle size of much higher than 50 nm, which may generate many defects between the nanoparticles and the hosting polymer,46 and result in the decrease of solute rejection. The TEM results in Figure 2 (c) and (d) indicated that the GQDs nanoparticles showed a narrow size distribution in the range of 1.0 and 3.0 nm, with an average particle size of 1.9 nm. This small average size greatly improved the specific surface area, hence more additional pathways for the solvent permeation could be provided. The FTIR spectrum of the synthesized GQDs can be seen in Figure S3. The transmittance peak at 1569 cm−1 belongs to the stretching vibration of the C–C backbone 18


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of the aromatic hydrocarbon. The peaks at 1645 and 1409 cm−1 correspond to the stretching vibration of C=O groups and symmetric vibration of carboxyl groups, respectively; The peak at 1099 and 3350 cm−1 corresponds to the stretching vibration of –OH groups.55, 65 The above peaks demonstrated the presence of aromatic groups, phenolic hydroxyl groups and carboxyl groups (–COOH) on the prepared GQDs nanoparticles, which was further confirmed by the C1s and O1s partial XPS analysis of the synthesized GQDs, as shown in Figure 2 (e) and (f). The XPS results suggested that the prepared GQDs have good hydrophilicity, which would be very beneficial for the dispersion of the GQDs in the aqueous MPD solution and for further incorporating them into the TFN SRNF membrane’s skin layer via the IP process.

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(e)

(f) Experimental data Fitting curve Baseline

Experimental Data Fitting Curve Baseline

C-C

-COOH

C-OH

C-OH

292

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290

288

286

284

538

282

536

-COOH

C=O

534

532

530

528

Binding Energy (eV)

Binding Energy (eV)

Figure 2 (a) AFM image of GQDs; (b) the corresponding height profile along the blue lines shown in the panel (a); (c) TEM image of GQDs; (d) the size distribution of

20


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GQDs shown in the panel (c); (e) The C1s partial XPS spectrum of GQDs; (f) The O1s partial XPS spectrum of GQDs.

3.2 Characteristics of the SRNF membranes 3.2.1 FTIR and Raman analysis. The FTIR spectra of the PI substrate, the prepared (PAA-GQDs-50)/PI, (PI-GQDs-50)/PI and (PI-GQDs-50/PI)X composite membranes are shown in Figure 3 (a). These spectra revealed distinct peaks features of the amide and imide groups, which clearly exhibited the effective conversion between the amide and the imide groups during the membrane preparation process. The peaks at 1720 and 1779 cm−1 which are due to the stretching vibration of the −C=O in the imide groups of the polyimide polymer,66 and the characteristic peaks at 1558 and 1654 cm−1 are due to the characteristic vibration of amide I and amide II of the polyamide polymer.67 The peaks of the imide groups were much clear on the spectrum of the PI substrate. However, after the IP process, the imide peaks decayed significantly, and those of the amide peaks emerged on the spectrum of (PAA-GQDs-50)/PI, which confirmed the amidation reaction in Figure 1 (a).

21



Figure 3. FTIR spectra of the substrate and the prepared (PAA-GQDs-50)/PI, (PI-GQDs50)/PI, (PI-GQDs-50/PI)X TFN SRNF membranes (a), and Raman spectra of the (PIGQDs-50/PI)XA SRNF membranes (b).

After the subsequent imidization process, the peaks of the imide groups reappeared, as can be seen on the spectrum of (PI-GQDs-50)/PI, which demonstrated the successful 22


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imidization reaction between the acetic anhydride and the PAA of the skin layer in Figure 1 (d). Furthermore, after the crosslinking procedure, the characteristic bands of the amides groups strengthened and the characteristic bands of the imide groups weakened, as shown on the spectrum of the (PI-GQDs-50/PI)X membrane, which confirmed the crosslinking reaction between the HDA and the PI imide ring 68 as shown in Figure 1 (e). As ATR-FTIR could not distinguish the GQDs nanoparticles scattered inside the skin layer of the PI composite membranes, we used Raman spectrum to investigate the existence of GQDs in the (PI-GQDs-50/PI)XA membrane. As depicted in Figure 3 (b), the spectrum of the (PI-GQDs-50/PI)XA membrane surface exhibited three main Raman peaks at 1350 cm−1 (D peak), 1593 cm−1 (G peak), and 1780 cm−1, which were in accordance with the research work of Liu et al.69 and Seki et al.70 As both D and G peaks are the characteristic peaks of GQDs in the Raman spectrum, they demonstrated that GQDs had adhered successfully when MPD/GQDs solution encountered the PI substrate surface. Further, they remained stable during the subsequent interfacial polymerization, chemical imidization, chemical crosslinking, and organic solvent activation processes. 3.2.2 XPS analysis. Figure 4 shows the XPS spectra of the surface elements of the prepared composite membranes with and without GQDs incorporation. It can be seen from the deconvolution of the N1s XPS spectra, Figure 4 (a) and (b), that both imide and amide groups71 were found on the surface of the OSN membranes after crosslinking, but amide 23



groups were dominant, whether with or without GQDs incorporation, indicating that most imide groups were converted to amide groups. Nevertheless, essentially no difference could be visible from the deconvolution of the N1s XPS spectra and the calculated contents of the N-related groups for the prepared (PI-GQDs-0/PI)X and (PI-GQDs-50/PI)X membranes, as also shown in the Table S1. However, for O1s spectra, we can see from Figure 4 (c) and (d) that there is apparent difference between the two kinds of SRNF membranes. For (PI-GQDs-50/PI)X membranes, there is a larger content of the carboxyl (O=C-O) groups compared with the GQDs-free membrane, as can also be seen in the O column of Table S1. This was partially attributed to the incorporation of GQDs nanoparticles since there exist carboxyl groups on the GQDs surfaces, as has been demonstrated by the XPS and FTIR analysis, which were discussed in Section 3.1. The incorporated GQDs, from another aspect, could hinder the diffusion of MPD molecules,46 and hence hinder the IP process to some extent. Thus, more -COCl groups of the TMC would have to hydrolyze to form -COOH groups.72

24


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(a) (PI-GQDs-0/PI)X

HN-C=O (amide)

O=C-N (imide)

402

N1s

HN-C=O (amide)

O=C-N (imide)

R-NH2

400

398

R-NH2

396

404

402

Binding Energy (eV) (c) (PI-GQDs-0/PI)X

400

398

O=C-O (carboxyl)

Fitting Curve Baseline Experimental Data

(d) (PI-GQDs-50/PI)X O1s

Intensity

HN-C=O (amide)

HN-C=O (amide) O=C-O (carboxyl)

N-C=O (imide)

536

534

532

396

Binding Energy (eV) Fitting Curve Baseline Experimental Data

O1s


Intensity

Intensity

N1s

404

(b) (PI-GQDs-50/PI)X


Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


530

528

N-C=O (imide)

536

534

532

530

528

Binding Energy (eV)

Binding Energy (eV)

Figure 4 Deconvolution XPS spectra of N1s and O1s for (a, c) (PI-GQDs-0/PI)X and (b, d) (PI-GQDs-50/PI)X membranes.

3.2.3 SEM and AFM analysis. The surface morphologies of the SRNF membranes under different GQDs loading were shown in Figure 5. The (PI-GQDs-0/PI)X membrane surface was full of pits and crumples (Figure 5 (a)). With the increase of the GQDs loading, the pits and crumples reduced gradually (Figure 5 (b) - (d)), indicating that the addition of GQDs was very beneficial for forming the uniform surface morphology of the SRNF 25



membranes. This suggested that GQDs have

steric hindrance which could impede the

diffusion of MPD and retard the formation of the PAA layer,46 resulting in the formation of a relatively smoother surface.

26


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Figure 5. SEM and AFM images of the surface of the prepared (PI-GQDs-0/PI)X (a, e), (PI-GQDs-25/PI)X (b, f), (PI-GQDs-50/PI)X (c, g), and (PI-GQDs-75/PI)X (d, h) TFN SRNF membranes.

Figure 5 (e) - (h) exhibit the AFM images of the synthesized TFC and TFN membranes. The surface of the TFN membranes appears flatter than that of the TFC membrane. The surface roughness (Ra) values of the prepared TFC and TFN membranes are quite similar to each other, from 27.3 nm to 24.3 nm as the GQDs content increased from 0 to 75 mg L−1, which indicates that the addition of GQDs has a marginal influence on the surface roughness of the membrane.

3.2.4 Contact angle analysis. Figure S4 shows the contact angle of the surface of the prepared composite membranes. It increased marginally by incorporating GQDs nanoparticles and decreased only slightly by the chemical crosslinking, indicating that both GQDs and chemical crosslinking have little effect on the hydrophilicity of the membrane surface. 3.3 Effect of chemical crosslinking. The separation performance of the fabricated GQDs-free membrane after chemical crosslinking is illustrated in Figure 6.

27



5

100

4

RDB Rejection Ethanol Permeance

95

3 90 2

RDB Rejection (%)

Ethanol Permeance (L m-2 h-1 MPa-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 67

85 1

0

80 0

20

40

60

80

100

120

Crosslinking Time (min)

Figure 6. The effect of the crosslinking on the ethanol permeance and the RDB rejection of the prepared (PI-GQDs-0/PI)X TFC SRNF membrane without solvent activation.

When the crosslinking time was in the range of 30 to 90 min, the RDB rejection remained at about 98%; the ethanol permeance decreased at first and then remained at about 2.0 L m−2 h−1 MPa−1 after 60 min. The decrease in permeance could be attributed to the densification of the skin layer and the reduction of the interstitial space among the polymer chains after introducing the crosslinker.19, 73 As the extension of the crosslinking time can be beneficial for strengthening the solvent resistance of the SRNF membranes, 60 min was selected as the optimal crosslinking time for further investigation. 28


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The SEM images of the cross-sectional morphologies of the prepared (PI-GQDs-0)/PI, (PI-GQDs-0/PI)X and (PI-GQDs-50/PI)X membranes are shown in Figure 7.

Figure 7. SEM images of the cross - sectional morphologies of the prepared (PI-GQDs0)/PI (a), (PI-GQDs-0/PI)X (b), and (PI-GQDs-50/PI)X (c) TFN SRNF membranes.

Before crosslinking, the area between the red lines in Figure 7 (a) clearly showed that the active layer has an average thickness of about 160 ~ 170 nm. However, after the crosslinking, the interface between the active layer and the substrate became almost invisible, as presented in Figure 7 (b) and (c). This was attributed to the fact that the active layer after imidization has the same imide groups as the substrate, hence the crosslinking reaction occurs not only in the active surface layer and in the PI substrate, but also on the interface between the two layers. By comparing Figure 7 (b) and (c), we can see that the addition of GQDs has no significant influence on the cross-sectional morphology of the membrane. 29



3.4 The effect of GQDs loading. The effect of the GQDs loading on the separation performance of the (PI-GQDs-c/PI)X SRNF membranes without solvent activation is shown in Figure 8.

Figure 8. The effect of GQDs content on the separation performance of the prepared (PIGQDs-c/PI)X TFN SRNF membranes.

The ethanol permeance of all the TFN SRNF membranes increased compared to the GQDs-free membrane. The ethanol permeance showed maximal at GQDs content of 50 mg L−1. The increase of the ethanol permeance at the GQDs content range of 0 ~ 50 mg L−1 could be attributed to various factors such as additional narrow passageways provided 30


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by GQDs for the permeation of the solvent molecules, and the mobilization of GQDs which disrupts the polymer chain packing and leads to an increased system free volume.73-74 However, with the doping content of GQDs higher than 50 mg L−1, GQDs may agglomerate and heterogeneously distribute in the PA matrix,75 leading to less additional narrow passageways for the permeation of the solvent molecules. Moreover, a resent research work on incorporation of nitrogen-doped GOQDs for the preparation of RO membranes demonstrated that high loading of nitrogen-doped GOQDs in PA skin layer changed the thin film structure from polyamide to polyester, and decreased simultaneously the water permeability and the salt rejection.76 This suggested that high GQDs content may also cause change of polymer structure from PA to polyester, and decrease simultaneously the ethanol permeance and the RDB rejection. As an overall result, (PI-GQDs/PI)X TFN membranes with high GQDs loading exhibited both low solvent permeability and low solute rejection.

Figure 9 illustrates the effect of the GQDs content on the pore size probability function curves and the surface porosity of the prepared (PI-GQDs-c/PI)X membranes which were calculated using the related equations listed in Table S2.

31



Figure 9. The pore size distribution (a) and porosity (b) of the surface of the (PI-GQDs/PI)X SRNF membranes at different GQDs content in the aqueous solution.

For the GQDs content between 25 and 75 mg L−1 (Figure 9 (a)), the pore size probability density curves shifted towards a smaller mean pore size and a relative higher probability density. This indicated that the GQDs embedment could promote the generation of smaller pores and narrower pore size distribution. Several aspects can illustrate the above effect of the GQDs nanoparticles. The presence of delocalized π electrons on GQDs surface facilitates the interaction between GQDs and the aromatic backbone structure of the membrane polymer through non-covalent π - π stacking.77 Meanwhile, some of the carboxyl groups on the GQDs nanoparticles could react with the amine groups of MPD, while some of the remaining carboxyl groups could conduct condensation reactions with the terminal carboxyl groups of BTAC during the IP

32


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process.78 Therefore, GQDs can have much closer attractive interaction with the membrane polymer chains, thus resulting in relative smaller void spaces inside the skin layer of the (PI-GQDs/PI)XA membrane, i.e., smaller pores and narrower pore size distribution of the pore size, as compared with the GQDs-free membranes. The surface porosity, mean pore size, standard deviation, and surface pore density of the membranes calculated using the pore size distribution model were shown in Table S2. When the content of GQDs was in the range of 0 to 50 mg L−1, the mean pore size decreased from 0.77 to 0.67 nm with increasing GQDs content, while the surface pore density and porosity increased, suggesting that the appropriate increase of the GQDs content was beneficial for adjusting the structure of the skin layer. However, with the further increase of the GQDs content from 50 to 100 mg L−1, the mean pore size increased up to 0.78 nm, while the surface porosity and the surface pore density decreased, suggesting that GQDs occurred agglomeration and decreased their specific surface area, thus the GQDs nanoparticles would not function well at such higher content. This has been well demonstrated recently by Hassanzadeh et al.79 who found that a content between 50 mg L−1 and 70 mg L−1 can be considered as the “critical association concentration” for GQDs in which range the GQDs had been demonstrated to associate into larger particles of approximately 80 nm in diameter. Therefore, the (PI-GQDs-c/PI)X membranes achieved the lowest mean pore size but the largest surface pore density and surface porosity at 50 33



mg L−1 GQDs content, as shown in Figure 9 (b). This indicated that the solvent permeance and the solute retention can be enhanced simultaneously by incorporating GQDs of up to 50 mg L−1. 3.5 The effect of solvent activation. The effect of DMF activation on the transport properties of the synthesized SRNF membranes are shown in Figure 10. Whether with or without GQDs incorporation, all the rejections increased after the DMF activation, and the ethanol permeance of the membranes even increased vigorously to about 7.0 ~ 8.0 folds that of the membranes before DMF activation, indicating the spectacular effect of solvent activation.

34


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Figure 10. The effect of GQDs incorporation and the solvent activation post-treatment in DMF at 80 oC for 30 min on the ethanol permeance (a) and the RDB rejection (b) of (PIGQDs-0/PI)X membrane and (PI-GQDs-c/PI)X TFN SRNF membranes.

The increments were even larger at GQDs content of 50 - 75 mg L−1, as shown in Figure 10 (a), while no compromising rejections were observed, as shown in Figure 10 (b). This indicated a synergetic effect of the GQDs incorporation and the solvent activation, demonstrating the promising prospect of the GQDs embedment for the improvement of the SRNF membrane performance. Take the (PI-GQDs-0/PI)X and the (PI-GQDs-50/PI)X membranes as examples, the ethanol permeance of the former increased from 1.93 to 13.4 L m−2 h−1 MPa−1 after DMF activation at 80 ºC for 0.5 h, while that of the latter increased 35



from 2.84 to 22.6 L m−2 h−1 MPa−1. Meanwhile, the RDB rejection of the former increased from 97.2 % to 98.5 % after DMF activation at 80 ºC for 0.5 h, while that of the latter increased from 97.8 % to 98.6%. The permeance of the (PI-GQDs-50/PI)XA SRNF membrane was the highest, about twice that of the GQDs-free membrane, indicating that the embedment of GQDs could benefit greatly the permeance of the TFN SRNF membrane. The observed increase in RDB rejection after DMF activation could be attributed to the fact that the PA layer swelled when it was exposed to DMF, which could eliminate the imperfections inside the skin layer. Meanwhile, the enhanced ethanol permeance could be due to the dissolution of those PA fragments with lower molecular weight by DMF, thus many blocked membrane pores could be opened24-25, 80, and both the surface pore density and the surface porosity increased, as shown in Table S3. Therefore, the ethanol permeance increased vigorously. This in turn, could lower down the concentration of the solute in the permeate from the dissolution-diffusion model,81 and result in a further increased solute rejection. Figure 11 exhibits the effect of solvent activation on the pore size probability function curves and the surface porosity of the prepared (PI-GQDs-50/PI)X SRNF membranes which were calculated using the pore size distribution model.

36


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Figure 11. The effect of DMF activation post - treatment on the pore size (a) and surface porosity (b) of the prepared (PI-GQDs/PI)X TFN SRNF membranes.

It can be seen from Figure 11 (a) and Table S3 that the GQDs incorporated membranes consistently showed a narrower pore size distribution, a smaller pore size and a higher probability density as compared to those of the GQDs-free SRNF membranes. Moreover, the peaks of the pore size probability density curves, average pore sizes and the deviation of the pore sizes of both the (PI-GQDs-50/PI)X and (PI-GQDs-50/PI)XA SRNF membranes were essentially the same. This confirmed that the solvent activation had a marginal effect on the pore size distribution of the TFN membranes, indicating that GQDs embedment can help to stabilize the pore size distribution. We attributed this result to the synergistic effect of the covalent π - π stacking and the covalent interaction between the −COOH on GQDs with the NH2 of MPD, as illustrated in Section 3.4. 37



3.6 Long-term stability of (PI-GQDs/PI)X TFN SRNF membrane. The long-term permeation test result of the prepared SRNF membranes at room temperature were shown in Figure 12 (a). The DMF permeance increased gradually during the initial 50 h and then reached a relatively stable level at about 18.3 L m−2 h−1 MPa−1. The rejection of RB was almost stable at 99.9% during the 100 h test, demonstrating that the (PI-GQDs/PI)X membranes have excellent solvent resistance in very harsh polar DMF.

38


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39



Figure 12. The long-term solvent resistance of (PI-GQDs-50/PI)XA membranes. (a) During 100 h cross-flow filtration with RB/DMF solution (100.0 mg L−1) as feed at room temperature, (b) Static immersion for more than 120 h in DMF at 80 ºC, (c) Static immersion test in organic solution at room temperature for nearly one year. A: before immersion in DMF; B: after static immersion in DMF for 356 days; C: before immersion in ethanol; D: after static immersion in ethanol for 363 days.

To accelerate the testing procedure of the solvent resistance and to widen the application of the fabricated SRNF membrane to higher temperature systems, we immersed the prepared (PI-GQDs-50/PI)XA SRNF membranes in DMF at 80 ºC for more than 120 h. During this period, the membranes were taken out several times and the separation performances were measured to evaluate their solvent resistance. The results are shown in Figure 12 (b). The RDB rejection decreased marginally from 98.3% to 96.1% after being immersed for the initial 8 h. However, it remained at about 94% after being immersed for more than 100 h, proving the excellent solvent resistance to DMF at relatively high temperature. It is noteworthy to mention that the tolerance of extremely harsh DMF at such high temperature and for such long time has seldom been reported in literature so far.

40


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A long time solvent resistance ability of the prepared (PI-GQDs-50/PI)XA SRNF membranes at room temperature has also been performed. We statically immersed the membranes in DMF and ethanol, respectively, at room temperature for about one year, and tested the separation performance afterwards. The results are shown in Figure 12 (c), and the pictures of the membranes can be found in Figure S5. It can be seen that the (PI-GQDs50/PI)XA TFN SRNF membranes had stable ethanol permeance and RDB rejection after being immersed in DMF and ethanol for such long time, indicating their excellent solvent resistance. It was worth mentioning that we also did another long-term static immersion test using the same kind of (PI-GQDs-c/PI)XA membranes but prepared under different IP reaction conditions with different GQDs content, as shown in Figure S6. The results demonstrated that they also exhibited excellent solvent resistance, which clearly proved the excellent reproducibility of the (PI-GQDs-c/PI)X SRNF membranes.

3.7 Benchmark. The separation and solvent resistance performance of the SRNF membranes synthesized in the present work and those reported in the latest literature works were listed in Table 2. It should be mentioned that, in most cases, the solvent permeance of SRNF membranes needs to be specified to certain solvent system, temperature, and solute concentration for comparison. Therefore, we only selected those separation performances of the SRNF membranes tested using polar solvents such as IPA, ethanol, 41



MeOH, as well as DMF for the comparison of the solvent resistance. The SRNF membranes synthesized in the present work exhibited excellent solvent permeance and solute rejection compared with those in the literatures. Moreover, there are seldom any SRNF membranes reported in literatures that could tolerate DMF at temperature as high as 80 oC for more than 120 h. This is a strong indication that the SRNF membranes we prepared have excellent solvent resistance.

42


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Table 2. The comparison of the separation performance and the long-term solvent resistance performance of the SRNF membranes prepared in the current work with those of the membranes in the literature works.

Separation performance test Membranea

Long-term test

Solute

P

R

Solvent

Solute

Testing

P

R

(MW: Da)

(b)

(%)

(Temperature)

(MW: Da)

time

(b)

(%)

Ref.

Solvent

(PA/SiO2)/PAN

(PA/TiO2)/PAN

IPA

IPA

PEG (1000) 14.5

PEG (200)

9.9

99

95

PI/POSS

EtOH

RB (1017)

12.6

99

PAN-

EtOH

BB (854)

19

>95

-

-

-

PEG

650

IPA (c)

DMF(c) -

43


-

[33]

6.1

99

(1000)

min

RB (1017)

48 h

-7.0

-98

[82]

-

-

-

[83]

-

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Pebax®/GO

DMF

BB (854)

10

95

(PA/MIL-53)/PI

MeOH

PS (1000)

23

99 -

Page 44 of 67

-

-

-

-

[38]

[84]

(PA/ZIF-8)/PI

MeOH

PS (1000)

25

99

PVDF-PS20

EtOH

RB (1017)

25

91

EtOH(c)

RB (1017)

24 h

25 -20

91

EtOH

MB (800)

9.1

99

EtOH (25 oC)

RB (1017)

48 h

-10

99 [85]

PDA/PI DMF APTMS/PI

Acetone

RB (1017) -

6.3

99

DMF (25 oC)

9

d

DMF (100 oC)

RB (1017) -

48 h

-6

99

6h

2

d

[86]

2 PA/PAN

DMF

RB (1017)

9

94

DMF

(c)

-

un-dissolved months

44


[20]

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EtOH

RDB (479)

22.6

98

DMF (c)

RB (1017)

100 h

RDB DMF (80 oC)

18.3

99.9

27 100 h

(479)

94.0 (EtOH)

(PI-GQDs50/PI)XA

This RDB DMF

RB (1017)

18.3

99.9

DMF

EtOH

PAN: polyacrylonitrile; APTMS: 3-Aminopropyl trimethoxysilane

b

unit: L m−2 h−1 MPa−1

c

room temperature

d MWCO

22 98.9

(479)

days

RDB

363

(c)

= 236 Da 45


(EtOH)

21 (479)

a

356

(c)

days

98.8

work

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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4. CONCLUSIONS We synthesized a novel GQDs incorporated (PI-GQDs-c/PI)XA TFN SRNF membrane via a serial processes of interfacial IP, imidization, crosslinking and solvent activation. The synthesized GQDs nanoparticles with an average size of 1.9 nm have been embedded successfully in the skin layer of the TFN membrane. The solvent activation using DMF substantially improved the ethanol permeance by 7 to 8 folds. The GQDs embedment further enhanced the separation performance of the SRNF membrane. In the experimental range of the GQDs content (0 to 300 mg L−1), 50 mg L−1 was investigated to be the optimal content. The prepared (PI-GQDs-50/PI)X TFN membranes displayed an ethanol permeance of up to 22.6 L m−2 h−1 MPa−1 after DMF activation, while the RDB rejection maintained above 97.6%. The prepared (PI-GQDs-50/PI)XA SRNF membrane demonstrated high solvent resistance in strong polar solvents such as DMF. During the 100 h filtration test, the DMF permeance and the RB rejection maintained as high as 18.3 L m−2 h−1 MPa−1 and 99.9 %, respectively. Even after being immersed in DMF at 80 ºC for more than 100 h, the RDB rejection still remained at about 94%, demonstrating the excellent organic solvent resistance and the high temperature tolerance of the prepared SRNF membrane. Moreover, the prepared (PI-GQDs-c/PI)XA SRNF membranes maintained stable ethanol permeance and RDB rejection after being immersed in DMF and ethanol, respectively, at room 47



temperature for about one year, proving great potential for OSN applications in strong polar solutions.

NOMENCLATURE fR(r)―Log-normal probability density function, m−1 r ―Membrane pore radius, m

r * ―Mean pore radius, m

ASSOCIATED CONTENT Supporting Information. The flow diagram and the actual photograph of the cross-flow filtration evaluation platform. The standard curve of the RDB/ethanol and the RB/DMF solutions. FTIR spectrum of GQDs. The surface contact angle of the composite membranes. The photograph of the prepared membranes before and after being statically immersed in ethanol for nearly one year; The long-term static immersion of the prepared membranes in DMF at room temperature for nearly one year; Surface chemical species and their contents. The mean pore size and standard deviation of the pore size, surface porosity and surface pore density of the prepared SRNF membranes with the GQDs content and solvent 48


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activation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (B. Su); *E-mail: [email protected] (B.Mandal) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by Natural Science Foundation of China (No. 21476218) and the Fundamental Research Funds for the Central Universities of China (No. 201822012). This is MCTL Contribution No. 193. The authors thank Dr. Prof. W. S. Winston Ho for his kind guidance when Prof. Baowei Su worked as a visiting scholar in The Ohio State University.

REFERENCES 49



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Graphene Quantum Dots-Doped Thin Film Nanocomposite Polyimide

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