Asymmetric forward osmosis membranes from p-aramid nanofibers

Asymmetric forward osmosis membranes from p-aramid nanofibers

Journal Pre-proof Asymmetric forward osmosis membranes from p-aramid nanofibers Lei Miao, Tingting Jiang, Shudong Lin, Tao Jin, Jiwen Hu, Min Zhang, ...

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Journal Pre-proof Asymmetric forward osmosis membranes from p-aramid nanofibers

Lei Miao, Tingting Jiang, Shudong Lin, Tao Jin, Jiwen Hu, Min Zhang, Yuanyuan Tu, Guojun Liu PII:

S0264-1275(20)30125-8

DOI:

https://doi.org/10.1016/j.matdes.2020.108591

Reference:

JMADE 108591

To appear in:

Materials & Design

Received date:

8 December 2019

Revised date:

19 February 2020

Accepted date:

20 February 2020

Please cite this article as: L. Miao, T. Jiang, S. Lin, et al., Asymmetric forward osmosis membranes from p-aramid nanofibers, Materials & Design(2020), https://doi.org/10.1016/ j.matdes.2020.108591

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© 2020 Published by Elsevier.

Journal Pre-proof

Asymmetric Forward Osmosis Membranes from p-Aramid Nanofibers Lei Miao,‡,a,b Tingting Jiang,‡,b,c,d,e Shudong Lin,b,c,d,e Tao Jin,*,b,c,d,e Jiwen Hu,*,b,c,d,e Min Zhang,a Yuanyuan Tu,b,c,d,e and Guojun Liub,c,d,f School of Materials and Energy Engineering, Foshan University, Foshan, P. R. China, 528000

b

Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou, P. R. China, 510650

c

Key Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of Sciences, P. R. China, 510650

d

Guangdong Provincial Key Laboratory of Organic Polymer Materials for Electronics, P. R. China, 510650

e

The University of Chinese Academy of Sciences, Beijing, P. R. China, 100039

f

Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6

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p-Aramid is an ideal building block for forward osmosis (FO) membranes due to its

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Abstract.

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Correspondence to: Jiwen Hu (Tel: 86-020-85232307, E-mail: [email protected])

extraordinary thermal resistance, chemical stability, and mechanical properties.

However, existing

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aramid membranes have certain limitations such as large pore diameters and low salt rejection rates.

In

this work, we describe a facile solvent exchange-delay phase inversion strategy to prepare p-aramid

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nanofibrous membranes that would be suitable for FO applications.

In this strategy, p-aramid

nanofibers with an average diameter of 16 ± 4 nm and an average length of 382 ± 89 nm were employed as membrane matrices.

Prior to the immersion of the cast film into a coagulation bath, a

pre-evaporation protocol was carefully designed and introduced to provide a slower exchange rate between the good solvent and the non-solvent, which delayed the demixing process between p-aramid nanofibers and thus yielded an asymmetric membrane with a denser active layer as well as a loose

*

Corresponding author. Prof. Dr. Jiwen Hu and Prof. Jin Tao, Email addresses: [email protected]



These two authors have contributed equally to this work.

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The resultant membrane showed excellent FO water flux, NaCl rejection ratios, tensile

strength, thermal properties, and solvent resistance.

The membrane reported in this work may provide

a promising candidate for separation applications and the results reported herein will facilitate the development of high-performance nanofibrous membranes.

Keywords: forward osmosis membranes, phase inversion, p-aramid nanofibers

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1. Introduction Forward osmosis (FO) processes have gained much attention in recent years [1, 2].

These

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processes utilize an osmotic pressure difference to drive water transport through a FO membrane from In

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the lower osmotic pressure side to the higher osmotic pressure side of the membrane [3].

comparison with the traditional pressure-driven membrane processes (i.e., nano/microfiltration or

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reverse osmosis) [4], a FO process requires minimal hydraulic pressure, which results in lower energy

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consumption and less deposition of contaminants onto a FO membrane [2, 5, 6]. Therefore, FO processes have demonstrated great potential and numerous advantages in various applications, such as

thermolabile biomedicines [9].

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wastewater or saline water treatment [3], power generation [7], food processing [8], or the enrichment of

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The FO membrane that serves as a barrier between a feed solution and a draw solution is the most important component in a typical FO process [1].

Usually, FO membranes consist of a thin dense

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active layer and a thick porous layer. The thin dense active layer determines the selectivity and water flux of the membranes, while the thick porous layer provides mechanical support [1, 5]. FO membranes exist.

Two types of

One is known as a skinned asymmetric structure consisting of a thin

skinned-layer and a porous support, which is usually prepared via a one-step phase inversion process. The dense skin layer and porous layer are both composed of the same material such as cellulose ester,[10] polybenzimidazole [11], or polyamide-imide [12].

Alternatively, the other type is a thin film

composite (TFC) membrane that is obtained from the initial fabrication or selection of a proper supporting substrate prior to the coating of a thin selective layer onto the support via spin-coating [13], interfacial reactions [14], or dip-coating processes [15].

Journal Pre-proof Ideally, FO membranes should have several key properties such as high water flux [5], high salt-selectivity [2], desirable stability (mechanical, thermal, and chemical) [16, 17], as well as a low internal concentration polarization (ICP) [5].

However, due to the modest performances of FO

membranes and the limited variety of available draw solutions, the commercial applications of FO membranes remain limited [18, 19]. Although FO membranes that are prepared via the phase inversion of cellulose ester, polybenzimidazole, or polyamide-imide have relatively high salt selectivities [10-12], their water flux as well as their mechanical, thermal, and chemical stability are unsuitable for

well as the high tortuosity of their supporting layers.

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applications due to the intrinsic properties of these membranes that include relatively low porosity as Although the performance of TFC FO

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membranes can be independently tailored by independently optimizing the structures of both the porous

complex preparation conditions.

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support layers and the selective dense layer, they are prepared via multiple step procedures or involve Thus, the development of facile and effective strategies for the

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preparation of high performance FO membranes is highly desirable but presents a significant challenge

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[20].

Scheme 1. (A) Structure of a p-aramid repeat unit. (B) Schematic illustration depicting the current strategy for the preparation of FO membranes derived from nanofibers.

Poly(p-phenylene terephthalamide) (PPTA, or p-aramid, which is shown in Scheme 1A) is a typical aromatic polyamide with ultrahigh crystallinity.

Due to the regular arrangement of p-aramid molecules

as well as the existence of strong intermolecular hydrogen bonds between amide groups, this family of polymers usually has excellent thermal stability, solvent-tolerability, and tensile moduli, as well as the ability to resist deformation under high pressure [14, 21]. Some previous studies have demonstrated that p-aramids could be fabricated into nanofibers when the hydrogen bonds were partly disrupted via

Journal Pre-proof the cleavage of protons from their respective amide groups [22].

Moreover, the excellent thermal

stability and tensile properties of the individual p-aramid polymers were still retained by the resultant nanofibers [23, 24].

Our previous investigation and those conducted by other groups revealed that

p-aramid nanofibers (ANFs) could be used to prepare highly stable asymmetric membranes with a nanoporous thin layer and a loosely fibrous nanofiber support via a solvent-induced phase inversion process [25, 26].

It is widely known that nanofibrous membranes usually have large surface areas as

well as highly porous structures [27].

Meanwhile, it has been demonstrated that the disrupted

amide groups that had participated in these interactions [26].

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hydrogen bonds between ANFs can be regenerated when the protons are returned to the respective Therefore, we hypothesized that an FO

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membrane bearing a thin dense active layer and a thick porous support can be prepared via a

the fabrication ANFs-based FO membranes.

This hypothesis motivated us to develop strategies for

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hydrogen bonding interactions between ANFs.

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solvent-induced phase inversion process, in which the thin dense active layer can be formed via

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The present contribution describes our current endeavour to develop an alternative strategy for the preparation of ANFs-based FO membranes consisting of a dense layer and a nanofibrous support via a

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phase inversion process instead of that employed for the preparation of a porous membrane with a thin nanoporous layer along with a loosely packed nanofiber-based support which was discussed in our

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previous report [25]. The most critical issue is that a nanofiber-based dense layer that meets the requirements of FO applications cannot readily be obtained from nanofibers via the existing phase

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inversion methods [28]. To address this issue, we proposed a solvent exchange-delay phase inversion method to slow down the demixing of ANFs in this work, which not only promoted the creation of a dense ANFs-based active layer, but also retained the loose support layer. We are not aware of any reports describing asymmetric FO membranes that are entirely prepared from ANFs.

The resultant FO membrane as well as its preparation method therefore represents the first

example of the preparation of a FO membrane from nanofibers rather than from individual polymer molecules.

The numerous existing nanofiber-based FO composite membranes are typically prepared

through similar steps involving the weaving of micro- or nanofibers that are obtained via electro-spinning to form a support layer prior to the chemical deposition of a dense polymer layer (as shown in Scheme 1B) [29]. Our current strategy is quite different from these reported strategies with

Journal Pre-proof regard to both the precursor materials as well as the preparation strategies.

We found that the

ANFs-based membranes prepared via our solvent exchange-delay phase inversion method have excellent potential as FO membranes with distinctive integrated properties such as high water flux, excellent NaCl-selectivity, as well as desirable mechanical, thermal, and chemical stability.

2. Experimental Section 2.1 Membrane Preparation

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Dried [email protected] fibers (5.0 g, 0.021 mol of repeat units, DupontTM, Mw = 4.0 × 104 g/mol, diameter = 12.0 ± 1.4 μm) that had been cut into pieces with length of 5 mm, potassium tert-butoxide

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(KOtBu, 5.0 g, 0.045 mol, Aladdin Reagent Co., Ltd., Shanghai, China), methanol (6.3 mL, 0.16 mol),

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and dimethyl sulfoxide (90.9 mL, 1.28 mol) were thoroughly mixed and mechanically stirred at room temperature over a period of 12 h to obtain a homogenous and dark p-aramid dispersion (containing 5 wt% This dispersion was sealed and stored overnight to allow the complete release of any air

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of ANFs).

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bubbles. Subsequently, the p-aramid dispersion was cast onto a thoroughly cleaned glass plate via the use of a film coater (BGD 209/2, Biuged (Guangzhou) Co., Ltd. of China) with different gate sizes The cast film was placed into an oven

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(which corresponded to the thickness of the resultant cast film).

for various pre-determined durations (defined as the pre-evaporation time). The relative humidity in

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the oven was kept at ~40% in the presence of a mixture of H2SO4/H2O (v/v = 60/100). was then immersed in an aqueous HCl solution with a pH of 2.0 over a duration of 8 h.

The cast film Subsequently,

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the membranes were successively soaked in ethanol and n-hexane over a duration of 8 h, and then annealed in a vacuum oven at various temperatures (defined as the thermal annealing temperature) for 48 h.

The side of the membrane which faced the air and the coagulation bath was denoted as the front

side, while the other side that faced the glass plate was denoted as the back of the membrane.

Details

regarding each of the membranes and their corresponding preparation parameters are summarized in Table 1. Table 1. Preparation parameters for various ANFs-based FO membranes Sample code P0 P1

Preparation parameters Cast film thickness (μm) Pre-evaporation time (s) Annealing temperature (°C) 0 300 50 10

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200 300 400 500 600

50

30

25 50 65 80 95 120

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300

30

P2, C2, and A1 were prepared using the same conditions and parameters.

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30 50 70 90

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P2 * P3 P4 P5 C1 C2 * C3 C4 C5 A0 A1* A2 A3 A4 A5

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2.2 Performance Tests An apparatus incorporating a draw cell and a feed cell (as shown in Scheme 2) was employed to

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evaluate the FO water flux (J, L/(m2·h)) and the NaCl rejection ratio (R) of the membrane. mechanism of our test was the same as that for the widely used cross-flow setting [30].

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membrane sample was fixed in place at the junction between the draw cell and the feed cell.

The Each

The front

side (or active side) of the membrane faced the feed solution while the back of the membrane faced the An aqueous sucrose solution (0.5 M)

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draw solution (this arrangement was denoted as the AL-FS mode).

and a NaCl solution (0.1 M) were selected as the draw solution and the feed solution, respectively.

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FO water flux measurements were conducted at room temperature with magnetic stirring.

The

Changes in

the volume of the draw solution were recorded after an initial equilibration time of 30 min.

J was

∆𝐴

calculated via the equation 𝐽 = 𝑆∆𝑡, where ΔA (L) corresponds to the incremental change in volume of the draw solution within 0.5 h, while Δt (h) denotes the duration of the test (0.5 h), and S (4.2 m2) represents the effective area of the membrane.

These water flux tests were performed five times under

the same conditions and the average value was subsequently calculated and reported.

During the test of

R, 20 mL of an aqueous sucrose solution (0.5 M) and a 20 mL NaCl solution (0.1 M) were placed into the draw cell and the feed cell, respectively.

Until the osmotic pressures of both sides were equal, or

the volumes of liquid in both cells became constant, the equilibrium concentration of NaCl in the draw solution (𝐶𝑑𝐸 ) or the feed solution (𝐶𝑓𝐸 ) were respectively determined via conductivity measurements

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(DDS-307, Leici, Shanghai, PR China). 𝑅=

𝐶𝑓𝐸 (𝑉𝑓0 −∆𝑉) 𝑉𝑓0 𝐶𝑓0

R was calculated via 𝑅 = 1 −

𝐶𝑑𝐸 (𝑉𝑓0 +∆𝑉) 𝑉𝑓0 𝐶𝑓0

× 100% or

× 100%, where ΔV (L) corresponds to the incremental change in volume of the draw

solution, 𝑉𝑓0 (mL) denotes the initial volume of the feed solution (20 mL), and 𝐶𝑓0 corresponds to the

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initial concentration of the aqueous NaCl solution in the feed cell (0.1 M).

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Scheme 2. Photograph of the apparatus used to measure the water flux and NaCl rejection rate in the AL-FS mode (the gradual grey arrow in the inset image indicates the direction of the water flow).

2.3 Characterization To prepare ANFs samples for IR characterization, a dispersion of ANFs was diluted with DMSO to a concentration below 0.01 wt%.

Subsequently, 10.0 μL of this diluted dispersion of ANFs was added

dropwise onto a KBr pellet and subsequently dried in an infrared oven. The FT-IR spectra of the ANFs dispersions were recorded with a TG209F3 infrared spectrometer (Netzsch, Germany).

The FT-IR

spectra of the dried ANFs-based FO membranes were directly recorded by this instrument with the use of an attenuated total reflectance (ATR) detector.

The morphologies of the ANFs were observed with

an AFM (Multi-mode 8, Bruker, United State) system that was operated in the tapping-mode.

Images

of the surface (or front side), back, and cross-section of the membrane were captured with a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) at an accelerating voltage of

Journal Pre-proof 2.0 kV. The tensile strength of each of the dried samples with dimensions of 10.0 cm × 1.0 cm was determined using a Universal Testing Machine (CMT7503, Shenzhen SANS Testing Machine Co., Ltd., Shenzhen, China) at a tensile rate of 20.0 mm/min.

The tensile strengths of three samples that had

been prepared via same conditions were measured and the average value was calculated.

The

measurement of the water contact angles (WCAs) of the dried membrane was performed using a goniometer (Powereach Digital Technology Equipment Co., Ltd., Shanghai, China).

To evaluate the

membrane’s thermal and chemical stability, the membrane was heated to various temperatures and kept at these temperatures for various pre-determined times, or immersed into different solvents for 24 h.

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The water flux, NaCl rejection, and tensile strength of the thermal treated-membrane were subsequently 𝑣

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re-evaluated. The variation ratio (VR) was calculated via 𝑉𝑅 = (𝑣𝑥 − 1) × 100%, where vx denotes 0

the values of the water flux, NaCl rejection rate, or tensile strength of the membrane after heating or

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immersion, and v0 represents these corresponding values for the membrane prior to heating or

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immersion.

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3. Results and Discussion 3.1 ANFs Dispersion

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A p-aramid polymer solution or melt is necessary for the preparation of nanofibers. However,

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commercial p-aramid is insoluble in most organic solvents. This behavior can primarily be attributed to the strong intermolecular hydrogen bonds that are formed between different amide groups in two

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neighboring polymer molecules [31, 32].

KOtBu, methanol, and DMSO were used in our study to

disrupt the hydrogen bonds existing between the p-aramid fibers in order to obtain well-dispersed ANFs. As a strong Lewis base, tBuO- is able to scavenge protons from the amide groups in p-aramid backbones but KOtBu cannot undergo complete dissociation in DMSO, which is a widely used membrane-forming aprotic solvent [33]. ions.

However, it is critical that DMSO can solvate K+ ions more strongly than tBuO-

On the other hand, methanol not only serves as a proton transfer agent [34] but also promotes

ion-pair dissociation of KOtBu [35].

Therefore, the concentration of tBuO- ions will increase and the

deprotonation of the amide groups in the aramid molecules will thus be accelerated.

With the

disruption of the hydrogen bonds as well as the formation of amide anions, the electrostatic interactions can cause the p-aramid chains to repel each other, thus yielding well-dispersed ANFs.

AFM

Journal Pre-proof characterization indicated that the desired ANFs were successfully prepared.

Based on measurements

that were performed at 30 different locations, the average diameter and the length of the ANFs were 16 ± 4 and 382 ± 89 nm, respectively (Figure 1).

This nanofiber structure was consistent with other

reports indicating that the p-aramid exists as polymeric crystalline arrays, rather than as individual

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polymer molecules [22].

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Figure 1. A) An AFM image as well as plots of B) the diameter and C) the length distribution of the ANFs reported herein.

Figure 2. FT-IR spectra of: A) ANFs and B) the dry ANFs-based membrane. Although the bending vibration of the free N-H moieties in the amide groups was observed at ~3400 cm-1, the bending vibration absorbance of the hydrogen-bonded N-H moieties had shifted to a lower

Journal Pre-proof wavenumber as had been described in previous reports [36].

As shown by the FT-IR spectra of the

ANFs dispersion (red curve in Figure 2), a prominent absorbance corresponding to free N-H stretching vibrations was observed at 3410 cm-1 for our nanofiber dispersion.

Meanwhile, vibrations

corresponding to weakly hydrogen-bonded N-H moieties could still be detected at 3320 cm-1. These results revealed that most of the hydrogen bonds were disrupted in this nanofiber dispersion but a certain degree of hydrogen bonding did still exist. In addition, it could be concluded that most of the amide bonds in the p-aramid backbones were not disrupted in the ANFs dispersion when KOtBu was present

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based on the absence of the typical carbonyl absorbance that would otherwise be observed at ~1650

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cm-1.

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3.2 Membrane Formation

An ideal FO membrane requires both a dense active layer as well as a porous support layer.

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However, it was not possible to obtain a dense active layer that would be suitable for FO processes from In a conventional phase inversion process,

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ANFs dispersion via conventional phase inversion methods.

a polymer that is dispersed in a good solvent (known as a casting solution) is directly immersed into a In this process, rapid solvent-exchange between the good

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non-solvent (known as a coagulation bath).

solvent and the non-solvent firstly occurs on the interface of the coagulation bath and the casting

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solution, which results in instant demixing of the polymer on the surface of the casting solution. phenomenon can be considered as a kinetically controlled process [25].

The conformations of the

polymer molecules are immediately locked in an instantaneous demixing process.

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This

In this case, the

conformational entropy is still preserved and a desirable entropy reduction cannot be realized. Consequently, this process usually yields a broad pore size distribution on the active layer, which will result in a low salt rejection ratio in FO applications.

As polymeric aggregates, polymer nanofibers

also have higher weights than individual polymer molecules, which imparts them with higher rigidity and less mobility [37], thus also facilitating the formation of a nonuniform pore structure.

The surface

morphology of the membrane that was prepared via a rapid solvent-exchange process is shown in Figure 3A, where a relatively rough surface bearing prominent cracks (insert in Figure 3A) was observed. expected, this membrane exhibited an excellent water flux of 31.2 ± 3.1 L/(m2·h).

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However, its

rejection ratio for 0.1 M NaCl was merely 88.6 ± 0.4%, which was much lower than those of most

Journal Pre-proof existing FO membranes [11, 38-42]. In order to prepare a nanofiber-based membrane with a dense active layer which had a narrow pore size distribution, it was crucial that the ANFs had enough time to release conformational entropy. Therefore, the demixing of the nanofibers had to be slowed down during the early stage of the phase inversion process, and thus the pre-evaporation procedure was of critical importance.

Due to the

insolubility of p-aramids in water, the ANFs residing on the surface of the film would slowly form larger aggregates to release entropy, whilst the aggregation and entanglement of the ANFs on the surface

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would result in a higher viscosity at the surface of the cast film than is encountered within its interior. In our experiments, a dispersion of ANFs was placed in an oven with a constant humidity of 40%.

Due

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to the good miscibility between water and DMSO, very few water molecules diffused into the surface of

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the cast film, and thus the surface of that film had a high viscosity. Subsequently, when this cast film was immersed into a coagulation bath, the demixing of the nanofibers on the surface of the cast film was

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slowed down significantly (i.e., polymer precipitation was delayed), thus yielding a narrow pore size

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distribution and a dense active layer [43]. Meanwhile, the formation of uniform macrovoid structures in the support layer was also promoted due to the gradual demixing process.

Thermal annealing also

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played an important role in further optimizing the packing arrangement of the nanofibers on the active layer, which promoted the formation of a denser active layer with a high salt rejection ratio [44].

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Our solvent exchange-delay phase inversion process improved the structures of the resultant nanofiber-based FO membranes.

Compared with the relatively rough porous surface of sample P0 that was prepared

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surface structures.

First, this process yielded ANFs-based membrane with enhanced

without pre-evaporation treatment (Figure 3A), a short pre-evaporation treatment of 10 s for sample P1 effectively enhanced the surface morphology and some smooth areas became visible, which are marked by red circles in Figure 3B.

A dense and smooth layer was obtained when we used our optimized

preparation conditions, in which the pre-evaporation time was extended to 30 s (Figure 3C). However, extending the pre-evaporation time further may yield a denser active layer, which would weaken the water flux performance.

The pore structure was not visible in Figure 3C because the average pore

diameter was in the range of 3-5 Å for a typical FO membrane, and they would thus be too small for observation due to the limited resolution of the SEM images.

Second, larger pores with an average

diameter of 90 ± 16 nm were readily visible on the back of the membrane (Figure 3D).

Third, a

Journal Pre-proof hierarchical structure is clearly visible in the cross-sectional view (Figure 3E), which displays a thin active layer (marked by red arrows in Figure 3E) with a thickness of ~30 nm as well as a porous support layer that is perforated by large pores.

The thinness of the membrane’s active layer could help to

enhance its water flux performance [45, 46].

Meanwhile, the porous support layer was formed by

ANFs (see the insert in Figure 3F) and AFM characterization revealed that the diameter of an individual nanofiber was 18 ± 3 nm.

Fourth, the total thickness of the dried membrane was only 23 ± 2 μm

(Figure 3F), which was much thinner than bulk or blend polymeric membranes (as shown in Table 2).

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Cross-sectional SEM characterization revealed that a continuous porous structure existed in the aramid nanofibrous membrane’s support layer (inset image in Figure 3F), which promoted the rapid permeation It was demonstrated that the presence of this continuous porous structure along with the

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of water.

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thinness of the layer could both help to inhibit undesirable ICP processes when the membrane was operated in the AL-FS mode (with the active layer facing the feed solution and the support layer facing

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the draw solution) [47], which is a key requirement for an ideal FO process [2].

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We reached two conclusions from the FT-IR spectrum of the dried membrane shown in Figure 2 (blue curve). First, we believed that amorphous (non-hydrogen bonded areas) and crystalline regions

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(hydrogen bonded areas) co-existed in the dried membrane. The enhancement of the N-H stretching vibration at 3320 cm-1 (peak 1) as well as the appearance of the C=O stretching vibration at 1610 cm-1

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(peak 2, inset in Figure 2) indicated that a re-formation of the hydrogen bonds had taken place [36, 48]. Meanwhile, it was noteworthy that the dried membrane also exhibited a free N-H absorbance at 3410

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cm-1 (peak 3) and a stretching vibration corresponding to free C=O moieties at 1650 cm-1 (peak 4, inset in Figure 2), which revealed that free ANFs were also present in the membrane.

Crystalline regions are

desirable because they enhance the mechanical properties of the membrane and promote salt rejection. Thus, thermal annealing treatment was required to obtain p-aramid-based membranes with enhanced properties.

In this process, the random agglomerations of ANFs would further regulate their

conformations to form a more robust structure that would offer enhanced mechanical properties along with better salt rejection performance. On the other hand, we had some evidence demonstrating that the C-N bonds in the amide groups were partly disrupted by KOtBu, as can be seen in Figure 2B.

First, broad N-H and O-H stretching

signals were observed instead of a sharp peak of N-H stretch at 3400 cm-1 (peak 3) [49]. Second, weak

Journal Pre-proof and broad OH peaks corresponding to carboxyl group associations appeared in the range of 2400-2700 cm-1 (peak 5). Third, characteristic peaks corresponding to O-H bending vibrations in carboxyl groups were visible at 1400 and 910 cm-1 (peaks 6 and 7).

During use in FO applications, the presence of a

small amount of anionic carboxyl groups will significantly enhance the membrane’s hydrophilicity as

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well as its NaCl rejection performance.

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Figure 3. SEM images of: A) the front side of sample P0 that was prepared without pre-evaporation treatment, B) the front side of sample P1 that was prepared with 10 s of pre-evaporation treatment, C) the front side, D) the back, while images E) and F) show the cross-section of an A3 membrane that was prepared via our optimized conditions. Higher magnification versions of each image are shown in the insets.

3.3 Water Flux and NaCl Rejection A trade-off between water flux and selectivity is usually required for an effective membrane [16]. It is difficult to simultaneously enhance these two properties if an FO membrane has been prepared from the same polymer.

To balance these two performances via the three-step approach used herein, we

optimized the pre-evaporation time, the thickness of the cast film, and the thermal annealing temperature, which were all important parameters influencing both the water flux and the salt rejection ratio.

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Figure 4. Variations in the water flux and NaCl rejection ratios of samples that were prepared with various: A) pre-evaporation times (P0-P5), B) cast film thicknesses (C1-C5), and C) annealing temperatures (A0-A5).

For a FO membrane with a nanofibrous or continuous support layer, the FO performances were An active layer with an “isoporous” structure and a

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primarily dominated by the active layer [50].

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narrow pore size distribution was desirable for the promotion of water flux as well as the salt rejection

ANFs-based membranes.

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rate [16]. Regulating three preparation parameters were found to help enhance the FO performance of These parameters included the pre-evaporation time, cast film thickness, and

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the thermal-annealing temperature.

Pre-evaporation can effectively slow down the solvent-exchange rate in a phase inversion process With

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to promote the formation of a dense active layer on the surface of aramid nanofibrous membrane.

regard to the pre-evaporation process, we assumed that a longer pre-evaporation time would facilitate

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the formation of a denser active layer with smaller pore diameters, which might enhance the salt rejection performance but diminish the water flux. shown in Figure 4A.

This hypothesis was confirmed by the results

When a pre-evaporation time of 30 s was selected, the water flux and NaCl

rejection ratio both increased to desirable values of 20.9 ± 1.3 L/(m2·h) and 96.7 ± 0.1%, respectively. However, with a longer pre-evaporation time, the water flux decreased dramatically whereas only a modest increase in the NaCl rejection ratio was observed. A thicker cast film would result in a thicker dried membrane and consequently a thicker membrane would facilitate a higher salt rejection ratio due to the preferential formation of a denser active layer. However, this would also weaken the water flux performance [51]. the results shown in Figure 4B.

This deduction was confirmed by

We measured the thicknesses of the dried membranes that were

Journal Pre-proof prepared from cast films with thicknesses of 200, 300, 400, 500, and 600 μm, which were respectively found to be 18 ± 2, 23 ± 2, 35 ± 2, 54 ± 3, and 66 ± 3 μm.

The NaCl rejection ratio increased from

96.2 ± 0.1% to 96.7 ± 0.1% when the thickness of the cast film was increased from 200 to 300 μm, whilst an undesirable degradation of the water flux from 28.8 to 20.9 ± 1.3 L/(m2·h) was also observed. It is noteworthy that the tensile strength of the dried membrane (Sample A3) with a thickness of 23 ± 2 μm reached up to 20.30 ± 0.02 MPa.

This tensile strength was significantly higher than that of the

dried membrane (Sample C1) with a thickness of 18 ± 2 μm, which was merely 11.20 ± 0.02 MPa.

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Thermal annealing facilitated the formation of crystalline regions, which would further enhance the salt rejection ratio and tensile strength, while causing degradation of the water flux.

As shown in

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Figure 4C, the water flux and NaCl rejection ratio of the membrane that was annealed at room

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temperature (or understood as receiving no thermal annealing) were 26.8 ± 1.4 L/(m2·h) and 91.2 ± 0.1%, respectively, while the NaCl rejection ratio of a thermally annealed membrane increased up to This improved NaCl rejection performance was achieved with acceptably affective the

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96.7 ± 0.1%.

It is noteworthy that a higher thermal annealing

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water flux, which decreased to 20.9 ± 1.3 L/(m2·h).

temperature (such as 120 °C) resulted in only a minimal increase in the NaCl rejection ratio, but the

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water flux decreased significantly to 15.8 ± 0.3 L/(m2·h). In comparison with other phase inversion FO membranes which were prepared from widely known

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commercial polymers [11, 12, 38, 39], from a novel synthetic polymer [40], or from a polymer/additive matrix [41, 42], our optimized ANFs-based membranes had remarkable FO water flux performance

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without adversely affecting the NaCl rejection ratio (Table 2).

Table 2. Comparison of the FO membrane performance via phase inversion Support/Active layer

ANFs

Membran e thickness (μm)

~23

Surfac e WCA (°)

~40.5

Cellulose triacetate/ cellulose acetate

~160

51.3

Cellulose triacetate

~50

--

Feed/Dra w solution 0.1 M NaCl/0.5 M Sucrose 0.1 M NaCl/ 1.0 M KBr 0.1 M

FO water flux (Jw, L/(m2·h) )

Salt rejection ratio (%)

Strength (MPa)

19.9 ± 0.6

98.6 (0.1 M NaCl)

20.30

9.27

99.5 (0.1 M NaCl)

--

[38]

21.1

97.4 (0.1

29.57

[39]

Ref.

This work a

Journal Pre-proof

--

Carbon nanofiber/ cellulose

--

53.5

Hydrophilized polybenzimidazole

~120

57-75

Poly(amide-imide)

140-200

--

Poly(triazole-co-oxadiazole-co-hydrazin e)

5-15

59.463.7

--

[41]

18.0

--

4.0

[42]

2.0-6.0

96.5-99. 0 (0.1 M NaCl)

--

[11]

9.70

[12]

59.1-71. 2

[40]

8.36-9.7 4

1.9-5.8

49.0 (5.3 mM MgCl2) 98.1-98. 7 (1.0 g/L Na2SO4)

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Data in this line correspond to A3.

98.3 (0.1 M NaCl)

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a

11.5

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--

M NaCl)

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Cellulose triacetate/ ZnCl2/ lactic acid

NaCl/2.0 M glucose 0.1 M NaCl/2.0 M glucose 0.01 M NaCl/1.0 M NaCl 0.1 M NaCl/2.0 M NH4HCO3 DI water/0.5 M MgCl2 DI water/0.5 M Na2SO4

Figure 5. A) Stress-strain curve and B) water contact angle of the ANFs-based membrane (Sample A3). 3.4 Other Properties The membrane that was prepared via our optimized conditions (Sample A3) also exhibited an excellent tensile strength of 20.30 ± 0.02 MPa.

Three stages were readily visible in the stress-strain

curve of our ANFs-based membrane (Figure 5A).

During the first stage, the curve exhibited typical

elastic behavior in response to stretching of the membrane. Subsequently, the strain became constant and due to the existence of semi-crystallized nanofibers in the support layer, ductile behavior began to be observed and a yield point was reached.

In contrast with other common ductile materials, strain

Journal Pre-proof hardening was observed during the third stage, which might be caused by orientation and alignment of ANFs in the direction of the load.

Both the strength and stiffness of the membrane (in the stretching

direction) were enhanced during this stage.

As the membrane was deformed further, the crystalline

nanofibers also became damaged, which finally resulted in the cleavage of the membrane. Our p-aramid-based flat membrane exhibited considerable hydrophilicity, with a static WCA of only ~ 40.5 ± 1.8° (shown in Figure 5B). This highly desirable result was to be expected, and may be attributed to two factors. First, the carboxyl groups that were obtained via the hydrolysis of the amide

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moieties enhanced the hydrophilicity of the membrane. Second, the presence of non-hydrogen bonded amide groups was also demonstrated, which have been reported to effectively enhance the hydrophilicity

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of polymeric membranes [52].

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We also evaluated the performance of ANFs-based membranes with various heating times. According to the results shown in Figure 6, the p-aramid membrane’s water flux remained virtually

As discussed earlier, the membrane’s NaCl rejection ratio was slightly

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durations at this temperature.

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unchanged after heating at 95 °C for 24 h and began to diminish slightly when it was heated for longer

enhanced due to the rearrangements of the ANFs.

The membrane’s tensile rejection was also enhanced

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after heating at 95 °C, but decreased rapidly at 120 °C due to thermal-oxidation of the ANFs in the

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presence of oxygen and water molecules [53, 54].

Figure 6. Variations in the: A) water flux, B) NaCl rejection ratio, and C) tensile strength of the ANFs-based membrane (Sample A3) during heating at various temperatures for pre-determined times.

On the other hand, the p-aramid membrane exhibited highly reproducible performance with regard

Journal Pre-proof to its weight, water flux, NaCl rejection, as well as its tensile strength, even if it was soaked in different solvents for 24 h (as shown in Table 3). These results revealed that heating or soaking had negligible influence on the surface or internal structure of the p-aramid membrane and that it exhibited robust FO performance.

Table 3. The variation ratio of our ANFs-based membrane’s (Sample A3) performances after soaking in various solvents for 24 h.

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THF -0.05 0.02 -0.01 0.01

Ethanol

Acetone

-0.02 0.01 0 0

-0.03 0.01 -0.01 0.01

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Mass Water flux NaCl rejection Tensile strength

Aqueous NaOH solution (1.0 M) -0.06 0.02 -0.01 -0.02

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Aqueous HCl solution (1.0 M) -0.01 -0.04 0.02 0.03

Variation ratio (%)

4. Conclusion process.

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In this work, we have prepared a free-standing aramid nanofibrous membrane via a phase inversion First, a dispersion of ANFs with an average diameter and length of 16 ± 4 and 382 ± 89 nm,

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respectively, was prepared from Kevlar 49 fibers to provide a casting solution.

Pre-evaporation

treatment was employed to delay the demixing of the ANFs and thus yielded a membrane with a dense

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active layer as well as a porous supporting layer.

The optimized membrane fabrication conditions were

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found to include the use of cast films with a thickness of 300 μm, a pre-evaporation time of 30 s, and thermal annealing treatment at 80 °C for 48 h.

The dried membrane that was prepared via these

optimized conditions had a thickness of 23 ± 2 μm, as well as a dense active layer and a porous support layer with a pore diameter of 90 ± 16 nm.

The FO water flux, NaCl rejection ratio, and the tensile

strength of the optimized ANFs-based membrane was 19.9 ± 1.1 L/(m2·h), 98.6 ± 0.1%, and 20.30 ± 0.02 MPa, respectively.

These performances surpassed those of most reported FO membranes that

were prepared via phase inversion. of ~ 40.5 ± 1.8°.

The membrane was highly hydrophilic, with a water contact angle

The FO properties of the membrane were also evaluated at various operating

temperatures and the obtained results revealed that the membrane exhibited robust FO performances after heating at up to 80 °C for 48 h.

The fabrication strategy and the resultant membrane described

Journal Pre-proof herein may have significant potential for applications in the separations field and have relevance in the food, cosmetics, and pharmaceutical sectors as well as the area of water treatment.

In addition, this

work may lead to the development of FO membranes with enhanced performance and greater durability.

Acknowledgements Dr. Jiwen Hu wishes to thank the National Natural Science Foundation of China (No. 51173204, 21404121, and 51503124), the Pearl River Novel Science and Technology Project of Guangzhou (No.

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201506010031), the Development Fund for Special Strategic Emerging Industry in Guangdong Province (2015B090915004), the Guangdong Natural Science Foundation (2015A030313799, 2015A030313822

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and 2016A030313163), the Science and Technology Program of Guangzhou City (201510010128), the

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Science Research Special Project of Guangzhou City (2014J4100216), and the Production Education Research Project in Guangdong Province (2015B090915004, 2015B010135002) for providing financial Dr. L. Miao wishes to thank the Foshan Functional Polymer Engineering Center (No.

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support.

2016GCZX008), and

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2016GA10162), the key Project of the Department of Education of Guangdong Province (No. the Guangdong Natural Science Foundation (No. 2018A030313717,

1.

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Author Contributions Lei Miao: Writing-Original Draft, Methodology, Funding acquisition Tingting Jiang: Validation, Formal analysis, Investigation Shudong Lin: Resources, Validation Tao Jin: Conceptualization, Writing - Review & Editing

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Jiwen Hu: Conceptualization, Writing-Review & Editing, Resources, Project administration, Funding

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acquisition

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Min Zhang: Formal analysis

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Yuanyuan Tu: Resources, Investigation

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Guojun Liu: Supervision, Writing - Review & Editing

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered

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as potential competing interests:

Journal Pre-proof Highlights 1. Asymmetric and free-standing FO membrane was prepared from p-aramid nanofibers. 2. A dense active layer as well as a loose substrate layer was yielded via solvent exchange-delay phase inversion strategy. 3. p-Aramid nanofibers-based FO membrane showed excellent FO water flux, NaCl rejection ratios,

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tensile strength, thermal properties, and solvent resistance.