Effect of trifunctional planar monomer on the structure and properties of polyamide membranes

Journal Pre-proofs Full Length Article Effect of trifunctional planar monomer on the structure and properties of polyamide membranes Lianrui Zhao, Songmiao Liang, Yan Jin, Zhuyuan Wang, Lijie Hu, Yan Kang, Jian Tao, Wei Peng PII: DOI: Reference:

S0169-4332(19)33231-3 https://doi.org/10.1016/j.apsusc.2019.144415 APSUSC 144415

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

29 May 2019 11 October 2019 15 October 2019

Please cite this article as: L. Zhao, S. Liang, Y. Jin, Z. Wang, L. Hu, Y. Kang, J. Tao, W. Peng, Effect of trifunctional planar monomer on the structure and properties of polyamide membranes, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144415

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© 2019 Published by Elsevier B.V.

Effect of trifunctional planar monomer on the structure and properties of polyamide membranes Lianrui Zhao, Songmiao Liangƾ, Yan Jin, Zhuyuan Wang, Lijie Hu, Yan Kang, Jian Tao, Wei Peng Vontron Membrane Technology Co., Ltd., Guiyang 550018, Guizhou, P.R. China ƾ

Corresponding author: [email protected] (Songmiao Liang); Tel: +86-851-6270267; Fax: +86-851-6270267

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Abstract: Pararosaniline (PAR) was successfully introduced into polyamide network aiming to optimize the structure and properties of polyamide nanofilms. The synthesis of the free-standing polyamide nanofilms was performed on agarose hydrogel supports by interfacial polymerization using 1,3-phenylenediamine (MPD) and PAR as aqueous-phase monomers. The structure and properties of the resultant polyamide nanofilms were systematically analyzed. Atomic force microscopy analysis indicated that PAR can greatly improve the mechanical strength of the polyamide nanofilms. Young’s modulus of the polyamide (PA) nanofilms increases about 1.5 times with the introduction of PAR units in the polyamide network. Strikingly, scratching resistance of the composite membranes was obviously improved through incorporating the polyamide nanofilms containing PAR units. The loss in salt rejection of the composite membranes after scratching is approximately three times lower than that of the membranes without using PAR. Both of the nanofilm thickness and contact angle increase with the increase of PAR, while the effect of PAR on the surface roughness behaves opposite. Using porous polysulfone (PS) membranes as supports, composite membranes comprising the polyamide nanofilms prepared from 0.015 wt% PAR shows higher water flux and salt rejection as compared to that of the nanofilms without PAR.

Keywords: Interfacial polymerization; Polyamide nanofilm; Trifunctional planar monomer; Elastic modulus; Reverse osmosis membrane

2

1. Introduction Reverse osmosis (RO) technique has grown as a promising solution to relieve the global risk of water scarcity and water pollution. One of the most used materials in this field is the polyamide thin film composite (TFC) membranes as a result of its high stability to pH (3-11), chemical and physical impact [1-2]. Using a porous substrate, commercial thin film composite (TFC) membranes are generally fabricated by in situ interfacial polymerization between 1,3,5-benzenetricarbonyl trichloride (TMC) and 1,3-phenylenediamine (MPD) [3-8]. Factors, such as the Young’s modulus of the polyamide (PA) nanofilms, the structure and morphology of the polyamide nanofilms, thickness, roughness and wettability strongly affect the performance of RO membranes [9-20]. In such case, designing promise polyamide nanofilms through optimizing the reaction conditions becomes hot topics to obtain highly performance RO membranes [21-26]. However, although a plenty of efforts have been paid on the improvement of the separation performance of the RO membranes, there are few papers had focused on the mechanical properties of the polyamide nanofilms [27]. The RO membranes generally have low tolerance to various styles of mechanical hurt as a result of the nano-scale thickness and low elastic modulus of the polyamide nanofilm. As the key separation materials in desalination system, the RO membranes need to withstand the harsh impact causing by cross-flow system. In such case, high speed flush of the water flow would cause inreparable damage on the polyamide network of the selective layer. It greatly depresses the long-term stability of the RO 3

membranes operating under high applied pressure. Therefore, it is very necessary to prepare a durable reverse osmosis membrane. Pararosaniline (PAR) as a rigid trifunctional monomer has a great influence on the three-dimensional structure of the polyamide nanofilm, which can form large intramolecular free-volume voids while increasing the thickness of the polyamide nanofilm [5-6, 28]. However, its roles and the effect of its structure in the interfacial polymerization process have received little attention. To our knowledge, this is the first study using PAR as one of building blocks for interfacial polymerization, enhancing the properties of polyamide film. Schematic diagram of interfacial polymerization between PAR, MPD and TMC is shown in Figure 1. In this work, the elastic modulus of polyamide nanofilm added PAR in aqueous solution has a tremendous promotion compared with conventional polyamide nanofilm of RO membrane. Compared with the original membrane, the scratch resistance of the membranes adding PAR increased obviously. It is widely accepted that the rigid trifunctional monomer PAR has great application prospects in optimizing the performance of RO membrane.

4

NH2 Cl O

H2N

+ H2N

Cl

OH

O N H

O N H

NH

O

O

O N H

N H

NH

O N H

N H

HO

NaOH

Cl

O

O

NH2

+

O O

NH2

HN

Cl

N H

O

Fig. 1. Schematic diagram of interfacial polymerization between Pararosaniline, 1,3-phenylenediamine and 1,3,5-benzenetricarbonyl trichloride.

2. Material and methods 2.1 Materials Polysulfone (PSF) ultrafiltration membrane was supplied by the Vontron Corporation. The membrane has a MWCO of 30 kDa and the PSF layer sustained by a polyester nonwoven fabric material. The membrane was cut into rectangle films (length: 30 cm, width: 10 cm), and consecutively washed by deionized (DI) water and ethanol at room temperature. N-hexane (Fisher Scientific), m-phenylenediamine (MPD) (Sigma-Aldrich), trimesoyl chloride (TMC) (Sigma-Aldrich) were used during interfacial polymerization. Pure pararosaniline (PAR) was purchased from Acros. 5

NaCl with purity over 99% were purchased from Sigma-Aldrich. Scratch tests were carried out using the coefficient of friction tester (Labthink MXD-02). 2.2 Membrane preparation A synthetic method for forming polyamide active layer based on conventional interfacial polymerization has been described by elsewhere [29-30]. Salt-rejection polyamide nanofilms were prepared using MPD, PAR and TMC as monomers. Firstly, the PSF ultrafiltration membrane adhered to the organic plate was immersed in an aqueous solution containing different concentration of MPD and PAR for 10 seconds: 3.5% + 0%, 3.49% + 0.01%, 3.485% + 0.015%, 3.48% + 0.02%. Subsequently, the PSF supporting membrane soaked with MPD/MPD+PAR was placed on a stainless steel plate and the excess aqueous solution on its surface was removed by a rubber roller. Then the PSF membrane socked with MPD/MPD+PAR aqueous solution was immersed in a hexane solution of TMC with 0.2 wt% concentration for 10 second. After removing excess hexane solution on the membranes, the treated membranes were allowed to dry naturally for a few minutes at ambient temperature. In the end, the prepared RO membranes were laid in DI water for further use. The RO membranes with the concentrations of PAR in an aqueous solution for 0, 0.01wt%, 0.015 wt% and 0.02 wt% were designed as RO-0, RO-0.01, RO-0.015 and RO-0.02. 2.3 Membrane characterization Before microscopic characterization the membranes were dried in ambient temperature for 24 h in a vacuum desiccator. The characterization of surface chemical compositions of the RO membranes was imaged using FT-IR spectrometer and XPS. 6

The surface of the RO membranes was observed using FE-SEM (Hitachi S-4300) made by Japan. The back surface of polyamide nanofilm was obtained by the following method. First of all, the nonwoven fabric was stripped from the RO composite membrane. Then, the polyamide nanofilm was attached to the glass plate with the polysulfone support layer facing up. And the polysulfone layer was washed off by DMF several times until no white precipitate was visible. After removing the residual DMF the samples on the glass plate can be observed in SEM. AFM (Bruke) was used to measure the roughness and thickness of the RO membranes. A 3 x 3 μm area was captured and eight replicates of each sample were tested. A sampling resolution of at least 512 points per area with the speed of 0.2-1 Hz. The surface roughness is expressed as Ra and Rq. Different substrates were utilized to estimate the roughness parameters, surface morphology and the thickness from the AFM scans of nanofilms. The free-standing polyamide nanofilms were transferred to silicon wafers and dried at 45 oC in vacuum oven to measure the thickness of nanofilms from AFM. A cut was performed to expose the surface of the wafer and offer measurement of the height from the surface of the upper polyamide nanofilm to the surface of silicon wafer. The thickness of polyamide nanofilm was measured from the difference of height between the polyamide nanofilm and the silicon wafer using a one dimensional statistical function. To investigate the surface wetting properties of polyamide nanofilms, the water contact angle (WCA) was measured at room temperature using a contact angle meter (DSA 30, Kruss Gmblt, Germany). 7

2.4 Membrane separation properties The fabricated composite RO membranes with different concentrations of PAR were tested in cross-flow cells, using 1500 ppm NaCl solution at 150 psi and 25 oC. The RO membrane samples were stressed at 150 psi for 30 minute before any data was collected. The water flux Jw (L m-2 h-1) was calculated by Eq. (1): [31-33] Jw =

V A´ t

(1)

where Jw is the permeated water flux (L m-2 h-1),V is the volume of permeate water (L), A is the area of the RO membrane, and t is the collected time (h). The salt rejection (R) was tested by Eq. (2): æ Cp ö ÷ ´100% R(%) = ç1 ç C ÷ f ø è

(2)

where Cf (mg/L) and Cp (mg/L) are the feed and permeate concentration, respectively. The concentration of feed and permeate solution were texted using a conductance meter ( Elmeiron conductivity meter CC-501, Poland). 2.5 Wrinkling-based measurement of the elastic properties of the polyamide films Santanu et al [sub-10nm] had presented a very simple wrinkle-based measurement of the elastic modulus of the polyamide nanofilms. The polyamide nanofilms of the elastic modulus can be written as Eq. (3): [34]

(1 - v ) æç 2

Enf = 3Es

nf

1 - vs

2

l ç 2ph nf è

ö ÷ ÷ ø

3

(3)

where Enf and Es are the elastic modulus of the nanofilm and the substrate. The v nf is the Poisson’s ratio of the nanofilm and the vs is the Poisson’s ratio of the substrate. λ is 8

the wavelength of the wrinkle, and the thickness of the polyamide nanofilm is hnf. A novel method for preparing the free-standing polyamide nanofilms has been described by elsewhere [35]. Firstly, aqueous solution containing 2% low melting-point agarose was heated to 80 oC stirring until the agarose dissolved completely. Then the solution was poured into a metal gasket on a nonwoven plate gradually transferred into gel state after 30 min. The polyamide nanofilms were prepared on the gel support membrane using interfacial polymerization. As mentioned above, MPD, PAR and TMC were used as monomers to prepare polyamide nanofilms. At first, the gel support membranes were immersed in an aqueous solution containing 3.5 wt% MPD for 10 seconds. Meanwhile PAR was added to aqueous solution with the concentration of 0.015%. Subsequently, the excess aqueous solution on MPD soaked gel supporting membranes were removed by nitrogen. The gel soaked in the MPD/MPD+PAR aqueous solution were then immersed in a 0.2 wt% concentration of TMC in hexane solution for 10 seconds. Then the excess hexane solution was poured off. The handled free-standing polyamide nanofilms were allowed to dry naturally for a few minutes at ambient temperature. At last, the prepared free-standing polyamide nanofilms were vertically immersed into 70 oC hot water for 5 min absolutely removing the gel. The polyamide nanofilms were then covered to another support membrane (PDMS) or silicon wafer for further characterization. 3. Results and discussion 3.1 Membrane chemical characterization The RO membrane surface chemical compositions with different concentrations 9

of PAR were characterized by ATR-FTIR. Fig. 2 shows the ATR-FTIR spectra of the RO-0, RO-0.01, RO-0.015 and RO-0.02 membranes. The absorptions at 1664, 1609 and 1543 cm-1 represent C=O stretching (amideϨ), hydrogen bonded C=O stretching (amideϨ) and the stretching vibration of N-H (amide ΙΙ), respectively. As the reaction between PAR and TMC also generated the aromatic amide bonds (-CONH-), no new adsorption band appear in FTIR adsorption spectra. In the process of interfacial polymerization, the conjugated six-membered ring hydrochloride structure of PAR can introduce more MPD into the n-hexane oil phase according to similar compatibility principles. Compared with the RO-0 membrane, it can be seen that the absorption at 1543 cm-1 of the RO-0.015 membrane was strengthened. The absorption at 1543 cm-1 was caused by the stretching vibration of N-H(amide ĉ). Some studies demonstrated that the enhancement of the FTIR absorption always caused by the increase of film

Absorbance (%)

thickness. [36]

1664

1609 1543

d

c

b a

1800

1600

1400

1200

1000

Wavenumber (cm-1)

Fig. 2. ATR-FTIR spectra of skin-layers of different membranes: (a) RO-0 original membrane, (b) 10

RO-0.01, (c) RO-0.015, (d) RO-0.02.

The surface chemical compositions of the RO-0, RO-0.01, RO-0.015 and RO-0.02 membranes were further studied by XPS. Three major emissions could be seen from all of the wide spectra at 532.1 eV, 399.3 eV and 284.4 eV represent the O-1s, N-1s and C-1s, respectively. Table 1 shows the elemental composition of the membrane surface. Compared with the RO-0 membrane, the content of the N element of all modified membranes increased dramatically after introducing the PAR, which corresponded well with the FTIR. The N/O atomic ratio of RO-0.02 membrane is the highest, followed by RO-0.015, RO-0.01 and RO-0. According to the previous work on the molecular structure of PA layer prepared by MPD and TMC, a higher N/O ratio means a higher crosslinking degree of the polyamide membrane [37]. Though the introduction of PAR does change the N/O ratio of PA layer in this paper, the effect is negligible concerning that the content of PAR addition is extremely low ( less than 0.02% ). When PAR is added to the aqueous phase, more MPD can enter the oil phase and react with the TMC to increase the degree of crosslinking of the polyamide nanofilm. Table 1 Elemental surface composition of RO-0, RO-0.01, RO-0.015 and RO-0.02 membranes determined by XPS.

Element ( atom % ) Membrane

N/O C

N

O

RO-0

62.54

14.59

22.87

0.638

RO-0.01

62.73

14.81

22.46

0.659

RO-0.015

62.28

15.40

22.32

0.69

11

RO-0.02

62.14

15.63

22.23

0.703

3.2 Membrane surface hydrophilicity A static water contact angle (WCA) measurements was used to characterize the hydrophilicity of the membrane surface. Surface roughness, porosity, pore size, hydrophilicity and its distribution etc have a great influence on the WCA of the membrane surface [38]. The obtained values are presented in Fig. 3. It can be seen that the highest WCA value of 87.4o was obtained for the RO-0.02 membrane. In the process of interfacial polymerization, the conjugated six-membered ring hydrochloride structure of PAR is transformed into a benzene ring structure. The polyamide nanofilm is composed of cross-linked portion and the linear portion. The linear portion contains a carboxylic acid group, and the higher the degree of crosslinking, the less the content of the carboxylic acid group, and the poorer the hydrophilicity of the membrane surface. Hence, the hydrophobic of the polyamide layer was enhanced with the increasing of PAR concentration.

12

100

Contact angle (o)

90 80 70 60 50 40 30

RO-0

RO-0.015

RO-0.01

RO-0.02

Fig. 3. The static contact angles of original membrane (RO-0) and the membranes modified by PAR with different concentration (0.01%, 0.015%, and 0.02%).

3.3 The surface of the fully-aromatic polyamide (FAPA) film porous structure The RO membrane utilize a thin film composite design, and nonwoven polyester is used as the bottom layer, which is composed of a FAPA selective film on porous polysulfone (PSF) support layer [5]. In order to obtain the FAPA film, the nonwoven fabric polyester backing was first peeled off. The remaining RO membrane was placed on a steel mesh(mesh number:400)with the PSF support membrane facing up and washed with DMF to completely remove the PSF support membrane. The DMF washing procedure was repeated several times until no white precipitate was visible. SEM images of the top layers and the back surfaces of polyamide nanofilms are presented in Fig. 4. It can be seen from the SEM images of top layers of the FAPA films that the surface becomes smooth with the concentration of PAR. From the SEM images of the back sides of the FAPA films, the surfaces become roughness and appear 13

a lot of pores with increasing the concentration of PAR. Previous work has shown that the introduction of rigid parts into polyamide nanofilms by interfacial polymerization enhances the porosity of TFC films [39].

Fig. 4. SEM images of (A) the RO-0 of FAPA film top surface; (B) the RO-0.01 of FAPA film top surface; (C) the RO-0.015 of FAPA film top surface; (D) the RO-0.02 of FAPA film top surface; (A’) the RO-0 of FAPA film back surface; (B’) the RO-0.01 of FAPA film back surface; (C’) the RO-0.015 of FAPA film back surface; (D’) the RO-0.02 of FAPA film back surface.

Cross-sectional SEM images of polyamide membranes based on glass plate are presented in Fig. 5. The thickness of the FAPA films increased with the PAR concentration from 118 nm of the RO-0 membrane to 138 nm of the RO-0.02 membrane. The main reason is due to the addition of trifunctional planar monomer, which forms a three-dimensional structure in the polyamide nanofilm. At the same time, according to the principle of similarity and compatibility, amine monomer is more likely to diffuse into the oil phase, thus increases the thickness of the polyamide film.

14

Fig. 5. SEM cross-section images of (A) RO-0, (B) RO-0.01, (C) RO-0.015, (D)RO-0.02.

The TFC membrane’s surface morphology was investigated using AFM. Compared with the original TFC membrane, the surface morphology of the modified membranes has a slight change. Fig. 6 shows the average surface roughness (Ra) of the TFC membranes from 80.7 to 65.9 nm. By adding PAR, the modified membranes become much smooth. This is due to the cross-linking structure of membranes increased and became more dense and smooth than the original membrane.

15

105 90

Ra (nm)

75 60 45 30 15 0

RO-0

RO-0.015

RO-0.01

RO-0.02

Fig. 6. Roughness values of unmodified and modified membranes obtained from AFM.

3.4 Membrane permeation properties The separation performance of the prepared TFC membranes for the NaCl solution was studied and compared as shown in Fig. 7. Compared with the corresponding data, it can be seen that RO-0.015 exhibits very good permeation property. The rejections for the NaCl in the case of RO-0.01, RO-0.015 and RO-0.02 increase in some degree compared with RO-0 composite membrane. Especially for RO-0.015 membrane, the desalination rate was as high as 99.4% and the relatively high flux was 24.55 L m-2 h-1 at the operating pressure of 150 psi. The rejections of composite membranes increase first and then decrease because of the addition of PAR. With the increase of the amount of PAR, the FAPA forms a tighter three-dimensional network structure, and the salt rejection increases. When the content is up to 0.02%, the three-dimensional mesh pores increase, resulting in the rejection of NaCl decreases. 16

Due to the conventional FAPA RO-0 membrane, the intramolecular nanospace of cross-linked FAPA layer has a better three-dimensional structure. The RO-0.01, RO-0.015 membranes exhibit high water permeability at an ultralow pressure. However, the water permeation of the RO-0.02 membrane decreases with the increase of the FAPA layer thickness. The significant improvements in membrane permeability are due to the unique three-dimensional network structure. In which, the rigid intramolecular contort three-dimensional structures of PAR and TMC can reduce the vibration and rotation of molecular chains, thereby endowing the FAPA layer with excellent salt rejection.

30

99.4 99.2

24 21

99.0

18 15

R (%)

-2

-1

Flux (L m h )

27

98.8

12 9

98.6

6 3

98.4

0 RO-0

RO-0.015

RO-0.01

RO-0.02

Fig. 7. The water flux and salt rejection of the RO-0, RO-0.01, RO-0.015, RO-0.02 membranes (testing conditions: 150 psi, 1500 ppm NaCl solution, 25 oC and pH of 7.0).

3.5 Polyamide nanofilms elastic properties For pressure driven membrane applications, the elastic modulus is a very important factor of the mechanical property in thin film geometries. The elastic properties of the polyamide nanofilm obtained by dissolving PSf from the nonwoven 17

fabric with DMF may be destroyed, because DMF can also dissolve off the linear polyamide structure.[40] Therefore, it is necessary to prepare a free-standing polyamide nanofilm. Santanu et. al presented a very simple wrinkle-based method of the elastic modulus of the polyamide nanofilms[27,34]. A novel method for preparing the free-standing polyamide nanofilms has been described by elsewhere [35]. The free-standing polyamide nanofilms with the concentrations of PAR from 0 to 0.015 wt% were designed as PA-0 and PA-0.015. Polyamide nanofilms covered on silicon wafer were characterized in terms of thickness and roughness by AFM. To measure very precisely the thickness and surface morphology of the free-standing polyamide nanofilms from AFM images, it is necessary to transfer them to a silicon wafer, which has very low average roughness. As shown in Fig. 8, it is obvious that the PA-0.015 polyamide nanofilm is thicker and smoother than the PA-0. The surface of PA-0 has more peak-valley structure than the PA-0.015. The thickness of the free-standing polyamide nanofilms raised with the increasing concentration of PAR is consistent with polyamide nanofilm of the reverse osmosis membrane. And, the tendency of the roughness is the same with the thickness. 18.8 nm thick PA-0 polyamide nanofilm and 29.2 nm thick PA-0.015 nanofilm were measured on silicon wafer. Note that, the addition of PAR increased the thickness of polyamide nanofilm, consistent with the above illustration as Fig. 5.

18

14

20

A: Thickness ~ 18.8nm 12

B: Thickness ~ 29.2nm hnf

18

hnf

16 14 No. of events

No. of events

10 8 6

12 10 8 6

4

4 2 2 0 200

250

300

0 150

350

200

250

300

Height (nm)

Height (nm)

Fig. 8. Cross-sectional AFM images and height profiles of polyarylate nanofilms supported on PDMS. A, PA-0; B, PA-0.015.

Tensile testing of the nanofilms was measured as the previous article reported [26-27]. A sheet of PDMS with a thickness of 1.5 mm (20 mm×15 mm) was stretched 10% using the stretching tool. The free-standing polyamide nanofilms was transferred on to stretched PDMS strips following a procedure similar to previously described [40-41] and dried at room temperature. The elongated PDMS sheet covered with polyamide nanofilm on the top was released slowly from the holder, which generated compressive force in the polyamide nanofilm. As reported [27], the Young’s modulus of PDMS is about 1.86 MPa, and the Poisson’s ratio of polyamide and PDMS are 0.39 and 0.49 respectively. As illustrated in Fig.9, the wrinkle-forming AFM graphics were formed when the FAPA free standing nanofilms are transferred onto an elastomeric substrate and subjected to applied compressive stress: A, PA-0; B, PA-0.015. The counted values of Young’s modulus are given in table 2. Compared with the PA-0, the 19

Young’s modulus of PA-0.015 increases by 55.4% from 3.43 to 5.33 GPa. It was confirmed that the addition of PAR enhanced the elastic modulus of the polyamide nanofilm.

Fig. 9. AFM images of the wrinkle-formed FAPA free standing nanofilms: A, PA-0; B, PA-0.015. Table 2 Mechanical properties of polyamide nanofilms fabricated by interfacial polymerization measured from the wrinkling experiment.

Polyamide

Wavelength of the

Thickness of the

Young’s modulus of

nanofilms

wrinkling pattern

nanofilm

the nanofilm

¬(nm)

hnf(nm)

Enf(GPa)

PA-0

676

18.8

3.43

PA-0.015

1216

29.2

5.33

3.6 Scratch resistance 20

The SEM and the AFM of the scratch surfaces in different membranes under same measure conditions are shown in Fig. 10. The elongated strip geometry that recurs in the direction of motion of the scratch indenter is characteristic of all photomicrographs. Fig.9 a, b, c, d reveals the effect of normal loading on the microscopic features of the scratched surface. It is obvious that the distance between adjacent strip grooves increased and the strip grooves were shallowed with the increased of PAR from 0 to 0.015%. The observation can be contributed to the fact that higher scratch resistance translates into a lighter scratch groove.

Fig. 10. The SEM and the AFM of the scratch surface in different membranes 21

As shown in Fig.11, scratch resistance also be clearly reflected from the re-test data of scratched membranes. The water flux of the scratched membrane adding PAR is slightly higher than the original membrane. And, the salt rejection of membranes decreased significantly after scratched. The salt rejection of the RO-0 membrane reduced most significantly from 99.29% to 96.87%. The salt rejection of the RO-0.015 membrane reduced least obviously from 99.40% to 98.60%. The results indicated that the RO-0.015 had the highest scratch resistance. And, the scratch resistance of membranes depended on the strength of polyamide nanofilm elastic modulus. Original membranes Scratched membranes

21

Original membranes Scratched membranes

100

99

15

R (%)

J (GFD)

18

12 9

98

97

6 3

96

0

RO-0

RO-0.01

RO-0.015

RO-0.02

RO-0

RO-0.01

RO-0.015

RO-0.02

Fig. 11. The original and re-test data of water flux and salt rejection of the RO-0, RO-0.01, RO-0.015, RO-0.02 membranes (testing conditions:1500 ppm NaCl aqueous solution, 150 psi, 25 oC and pH of 7.0).

4. Conclusions In summary, by using rigid trifunctional planar monomer PAR as one of the monomers in interfacial polymerization to fabricate TFC membrane, we have proved the formation of three-dimensional structure in the polyamide nanofilm of the enhanced elastic modulus. The water permeance of the RO-0.015 was 14.44 L m-2 h-1 and salt rejection was over 99.4% for 1500 ppm sodium chloride solutions under a pressure of 150 psi, compared with the original membrane water permeance of the 22

RO-0 was 11.41 L m-2 h-1 and salt rejection was 99.29%. The characterization results indicated that the RO-0.015 membrane had obtained a highly cross-linked and smooth skin layer with high elastic modulus. The resulting PA-0.015 with the thickness of 29.2 nm has higher Young’s modulus (5.33 GPa) than the Young’s modulus of the PA-0 (3.43 GPa) with the thickness of 18.8nm. And, the RO-0.015 offers a slight increase in permeation properties, where it’s both more permeable and more selective than the RO-0. Scratching resistance of the composite membranes was obviously improved through incorporating the polyamide nanofilms containing PAR units. The loss in salt rejection of the composite membranes after scratching is approximately three times lower than that of the membranes without using PAR. This work might inspire interfacial synthesis of the spanking growing family of trifunctional planar monomers to obtain high elastic modulus polyamide nanofilm with great potential for application in molecular separation, including water purification and desalination. Acknowledgements This work was supported by National Key Technology Research and Development Program (2014Bab06b01, China) and National Program on Key Research Project (2016YFE0118800-06, China). References [1] D. Li, Y. Yan, H. Wang. Recent advances in polymer and polymer composite membranes for reverse and forward osmosis processes. Prog. Polym. Sci. 61 (2016) 104–155. [2] J. R. Werber, C. O. Osuji, M Elimelech. Materials for next-generation desalination and water 23

purification membranes. Nat. Rev. Mater. 1 (2016) 1-15. [3] Y. Lia, S. Lic, K.S. Zhang. Influence of hydrophilic carbon dots on polyamide thin film nanocomposite reverse osmosis membranes. J. Membr. Sci. 537 (2017) 42-53. [4] H.M. Park, K.Y. Jee, Y.T. Lee. Preparation and characterization of a thin-film composite reverse osmosis membrane using a polysulfone membrane including metal-organic frameworks. J. Membr. Sci. 541 (2017) 510-518. [5] H. Yan, X.P. Miao, J. Xu, G.Y. Pan, Y. Zhang, Y.T. Shi, M. Guo, Y.Q. Liu. The porous structure of the fully-aromatic polyamide film in reverse osmosis membranes. J. Membr. Sci. 475 (2015) 504–510. [6] T. Fujioka, N. Oshima, R. Suzuki, W.E. Price, L.D. Nghiem. Probing the internal structure of reverse osmosis membranes by positron annihilation spectroscopy: Gaining more insight into the transport of water and small solutes. J. Membr. Sci. 486 (2015) 106–118. [7] S.J. Parka, W. Choia, S.E. Namb, S. Hongc, J.S. Leed, J.H. Lee. Fabrication of polyamide thin film composite reverse osmosis membranes via support-free interfacial polymerization. J. Membr. Sci. 526 (2017) 52–59. [8] Q. Shi, L. Ni, Y.F. Zhang, X.S. Feng, Q.H Chang, J.Q. Meng. Poly(p-phenylene terephthamide) embedded in polysulfone as substrate for improving compaction resistance and adhesion of thin film composite polyamide membrane. J. Mater. Chem. A. 5 (2017) 1-29. [9] A.F. Ismail, M. Padaki, N. Hilal, T. Matsuura, W. J. Lau. Thin film composite membrane - Recent development and future potential. Desalination. 356 (2015) 140-148. [10] T.H. Lee, M.Y. Lee, H.D. Lee, J.S. Roh, H.W. Kim, H.B. Park. Highly porous carbon nanotube/polysulfone nanocomposite supports for high-flux polyamide reverse osmosis membranes. J. Membr. Sci. 539 (2017) 441-450. [11] M. Safarpour, V. Vatanpour, A. Khataee, H. Zarrabi, P. Gholami, M.E. Yekavalangi. High flux and fouling resistant reverse osmosis membrane modified with plasma treated natural zeolite. Desalination. 411 (2017) 89-100. [12] M. Shafiqa, A. Sabira, A. Islama, S.M. M. Khana, S.N. Hussainb, M.T.Z. Z. Buttc, T. Jami. Development and performance characteristics of silane crosslinked poly(vinyl alcohol)/chitosan membranes for reverse osmosis. J. Ind. Eng. Chem. 48 (2017) 99-107. [13] M. Asadollahi, D. Bastani, S.A. Musavi. Enhancement of surface properties and performance of reverse osmosis membranes after surface modification: A review. Desalination. 420 (2017): 330-383. [14] F.F Shao, L.F Dong, H.Z Dong, Q. Zhang, M. Zhao, L.Y Yu, B.L Pang, Y.J Chen. Graphene oxide modified polyamide reverse osmosis membranes with enhanced chlorine resistance. J. Membr. Sci. 525 (2017) 9-17. 24

[15] R. Verbeke, V. Gómez, I. F. J. Vankelecom. Chlorine-resistance of reverse osmosis (RO) polyamide membranes. Prog. Polym. Sci. 72 (2017) 1-15. [16] M. Carta, R.M. Evans, M. Croad, Y. Rogan, J.C. Jansen, P. Bernardo, F. Bazzarelli, N.B. McKeown. An efficient polymer molecular sieve for membrane gas separations. Sci. 339(2013) 303-307. [17] W. Lia, X. Sua, A. Palazzoloa, S. Ahmedb, E. Thomasa. Reverse osmosis membrane, seawater desalination with vibration assisted reduced inorganic fouling. Desalination. 417 (2017) 102-114. [18] H.B. Park, C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E.V. Wagner, B.D. Freeman, D.J. Cookson. Polymers with cavities tuned for fast selective transport of small molecules and ions. Sci. 318 (2007) 254-258. [19] V. Vatanpour, N. Zoqi. Surface modification of commercial seawater reverse osmosis membranes by grafting of hydrophilic monomer blended with carboxylated multiwalled carbon nanotubes. Appl. Surf. Sci. 396 (2017) 1478-1489. [20] Y. Zhao, C.Q. Qiu, X.S. Li, A. Vararattanavech, W.M Shen, J. Torres, C.H. Nielsen, X. Hu, A.G. Fane, C.Y.Y. Tang. Synthesis of robust and high-performance aquaporin-based biomimetic membranes

by

interfacial

polymerization-membrane

preparation

and

RO

performance

characterization. J. Membr. Sci. 423 (2012) 422-428. [21] Y.J. Zeng, L.H. Wang, L. Zhang, J.Q. Yu. An acid resistant nanofiltration membrane prepared from a precursor of poly(s-triazine-amine) by interfacial polymerization. J. Membr. Sci. 546 (2018) 225-233. [22] K.P. Leea, G. Bargemanb, R. Rooijb, A.J.B. Kempermana, N.E. Benes. Interfacial polymerization of cyanuric chloride and monomeric amines: pH resistant thin film composite polyamine nanofiltration membranes. J. Membr. Sci. 523 (2017) 487-496. [23] K.P. Lee, J. Zheng, G. Bargeman, A.J.B. Kemperman, N.E. Benes, pH stable thin film composite polyamine nanofiltration membranes by interfacial polymerisation, J. Membr. Sci. 478 (2015) 75-84. [24] K.P. Lee, G. Bargeman, R. de Rooij, A.J.B. Kemperman, N.E. Benes. Interfacial polymerization of cyanuric chloride and monomeric amines: pH resistant thin film composite polyamine nanofiltration membranes. J. Membr. Sci, 523 (2017) 487-496. [25] Q.L Song , S. Jiang , T. Hasell , M. Liu , S.J Sun, A.K. Cheetham, E. Sivaniah, A.I. Cooper. Porous Organic Cage Thin Films and Molecular-Sieving Membranes. Adv. Chem. 28 (2016) 2629–2637. [26] M.F.J. Solomon1, Q.L. Song, K.E. Jelfs, M.M. Ibanez1, A.G. Livingston. Polymer nanofilms with enhanced microporosity by interfacial polymerization. Nat. Mater. 15 (2016) 760-767. [27] S. Karan, Z.W Jiang, A.G. Livingston. Sub–10 nm polyamide nanofilms with ultrafast solvent 25

transport for molecular separation. Sci. 348 (2015) 1347-1351. [28] Y.Q. Li, M.M. Klosowski, C.M. McGilvery, A.E. Porter, A.G. Livingston, J.T. Cabral. Probing flow activity in polyamide layer of reverse osmosis membrane with nanoparticle tracers. J. Membr. Sci. 534 (2017) 9-17. [29] Y. Jin, S.M. Liang, Z.C Wu, Z.Q. Cai, N. Zhao. Simulating the growth process of aromatic polyamide layer by monomer concentration controlling method. Appl. Surf. Sci. 314 ( 2014) 286-291. [30] S.M. Liang, G.Y. Xu, Y. Jin, Z.C. Wu, Z.Q. Cai, N. Zhao, Z.L. Wu. Annealing of supporting layer to develop nanofiltration membrane with high thermal stability and ion selectivity. J. Membr. Sci. 476 (2015) 475-482. [31] M. Kumar, D. McGlade, J. Lawler. Functionalized chitosan derived novel positively charged organic–inorganic hybrid ultrafiltration membranes for protein separation. RSC Adv. 4 (2014) 21699-21711. [32] E. Eren, A. Sarihan, B. Eren, H. Gumus, F.O. Kocak. Preparation, characterization and performance enhancement of polysulfone ultrafiltration membrane using PBI as hydrophilic modifier. J. Membr. Sci. 475 (2015) 1-8. [33] N. Ma, L.R. Zhao, X.Y. Hu, Z. Yin, Y.F. Zhang, J.Q. Meng. Protein transport properties of PAN membranes grafted with hyperbranched polyelectrolytes and hyperbranched zwitterions. Ind. Eng. Chem. Res. 56(2017) 1-35. [34] C.M. Stafford, C. Harrison, K. Beers, A. Karim, E.J. AMIS, M.R. Vanlandingham, H. Kim, W. Volken, R.D. Miller, E.E. Simony. A buckling-based metrology for measuring the elastic moduli of polymeric thin films. Nat. Mater. 3 (2004) 545-550. [35] S.M. Liang, Z.Y. Wang, Y. Jin, N. Zhao, L.J. Hu, Y. Hou, J. Xu. Polyester Nanofilms with enhanced polyhydroxyl architectures for the separation of metal ions from aqueous solutions. ACS Appl. Nano Materials. 11 (2018)6176-6186. [36] Y. Liu, B.Q He, J.X. Li, R.D. Sanderson, L. Li, S.B. Zhang. Formation and structural evolution of biphenyl polyamide thin film on hollow fiber membrane during interfacial polymerization. J. Membr. Sci. 373 (2011) 98-106. [37] S. Kim, S.Y. Kwak, T. Suzuki. Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane. Environ. Sci. Technol. 39 (2005) 1764-1770. [38] D. Rana, T. Matsuura. Surface modifications for antifouling membranes. Chem. Rev. 110 (2010) 2448-2471. [39] H.D. Qian, J.F. Zheng, S.B. Zhang. Preparation of microporous polyamide networks for carbon 26

dioxide capture and nanofiltration. Polym 54 (2013) 557-564. [40] P. Marchetti, M.F.J. Solomon, G. Szekely, A.G. Livingston. Molecular separation with organic solvent nanofiltration: a critical review. Chem. Rev. 114 (2014) 10735–10806. [41] M. D. Guiver, Y.M. Lee. Polymer rigidity improves microporous membranes. Science 339 (2013) 284–285.

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Highlights for Effect of trifunctional planar monomer on the structure and properties of polyamide membranes Lianrui Zhao, Songmiao Liangƾ, Yan Jin, Zhuyuan Wang, Lijie Hu, Yan Kang, Jian Tao, Wei Peng Vontron Membrane Technology Co., Ltd., Guiyang 550018, Guizhou, P.R. China

ƾ

Corresponding author: [email protected] (Songmiao Liang); Tel: +86-851-6270267; Fax: +86-851-6270267

Highlights: 1. This is the first study on enhancing the elastic modulus of polyamide nanofilms, using pararosaniline as one of building monomers for interfacial polymerization. 2. The scratch resistance of polyamide layers in reverse osmosis membranes was systematically estimated using atomic force microscope. 3. The Young’s modulus of the polyamide nanofilms increased from 3.43 GPa to 5.33 GPa as increasing the pararosaniline concentration. 4. The effect of pararosaniline on the microstructure and performance of reverse osmosis membranes was also investigated.

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Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

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