High solvent-resistant and integrally crosslinked polyimide-based composite membranes for organic solvent nanofiltration

Journal of Membrane Science 564 (2018) 10–21

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High solvent-resistant and integrally crosslinked polyimide-based composite membranes for organic solvent nanofiltration ⁎

Can Lia,b, Shuxuan Lia,b, Li Lva,b, Baowei Sua,b, , Michael Z. Huc,

T

⁎⁎

a

Key Laboratory of Marine Chemistry Theory and Technology of Ministry of Education, Ocean University of China, 238 Songling Road, Qingdao 266100, China College of Chemistry & Chemical Engineering, Ocean University of China, 238 Songling Road, Qingdao 266100, China c Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Organic solvent nanofiltration (OSN) Dopamine Polyimide Interfacial polymerization (IP) Imidization

This paper reports a new class of integral polyimide (PI)-based thin film composite (TFC) membranes (with improved solvent resistance in both the skin layer and the substrate) for organic solvent nanofiltration (OSN). The OSN membrane was prepared via interfacial polymerization (IP) onto a PI ultrafiltration (UF) substrate, followed by an imidization, a chemical crosslinking, and a solvent activation process. During the IP process, mphenylenediamine (MPD), dopamine (DA), and 1,2,4,5-benzene tetracarboxylic acyl chloride (BTAC) were respectively used as an aqueous monomer, an aqueous additive, and an organic monomer. We proved that amide bonding formed between MPD and the PI substrate during the immersion of the substrate in the aqueous MPD solution, thus providing a strong binding between the substrate and the subsequently formed skin layer of polyamide acid (PAA), which was formed due to reaction between MPD and BTAC during the IP process. DA contains amine group and could also help build a strong binding between the substrate and the skin layer during the same IP process. The subsequent imidization step converted the PAA molecules of the skin layer into PI polymers, which was quite similar to that of the substrate. The final crosslinking step crosslinked not only the inner molecules of the skin layer, and the inner molecules of the substrate, but also the interface molecules between the skin and substrate so as to form an integral composite membrane with improving solvent resistance. The fabricated OSN membranes under optimal preparation conditions exhibited an ethanol permeance of 2.03 L m−2 h−1 bar−1 with a rejection of 98% for Rhodamine B (479 Da) and exhibited an outstanding organic solvent resistance without compromising separation performance during the persistent immersion in DMF at 80 °C for two weeks, indicating a promising prospective in OSN applications.

1. Introduction Organic solvent nanofiltration (OSN), also termed as solvent resistant nanofiltration (SRNF), is an emerging separation technology for mixtures of organic molecules by simply applying pressure differential across the functional OSN membrane. OSN is of great significance owing to the vast amount of solvents used in industries [1,2]. OSN applications range from refining industry to production of fine chemicals and cover processes such as edible oil refining and degumming, catalysts recovery, solvents recycling, polymers fractionation and solvents exchanges [3]. OSN process is much more energy-efficient and typically consumes an order of magnitude less energy than those

conventional solvent purification and recovery processes such as evaporation and distillation that are known to consume a lot of energy because of the high heating demand [2,4]. Furthermore, OSN can be combined with other purification techniques for process intensification and can lead to greener processes with improved yields, purity and energy efficiency [5]. OSN membrane is the core component of the OSN separation technology. An ideal OSN membrane should possess not only high permeance and high rejection but also excellent chemical stability. Polyimide (PI) has been the mostly often used material for OSN membrane development [6–8], and the PI-based membranes have shown high solvent permeance and solvent resistance in organic solutions [9,10].

Abbreviations: OSN, organic solvent nanofiltration; SRNF, solvent resistant nanofiltration; TFC, thin film composite; IP, interfacial polymerization; UF, ultrafiltration; MPD, m-phenylenediamine; DA, dopamine; BTAC, 1,2,4,5-benzene tetracarboxylic acyl chloride; PI, polyimide; PA, polyamide; PAA, polyamide acid; RDB, Rhodamine B (479 Da); RB, Rose Bengal (1017 Da); EtOH, ethanol; IPA, isopropyl alcohol; DMF, N, N-dimethyl formamide; NMP, N-methyl pyrrolidone; HDA, 1,6-hexanediamine ⁎ Corresponding author at: Key Laboratory of Marine Chemistry Theory and Technology of Ministry of Education, Ocean University of China, 238 Songling Road, Qingdao 266100, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (B. Su), [email protected] (M.Z. Hu). https://doi.org/10.1016/j.memsci.2018.06.048 Received 4 March 2018; Received in revised form 10 June 2018; Accepted 24 June 2018

Available online 26 June 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

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Nomenclature di Di,∞ dp dsolvent fR(r) kc kd Δp Pe ' r V(r)

Vsolute Vs Δx Y

Molecular diameter of solute, m The diffusion of solute in dilute solution, m2 s−1 Pore diameter, m Molecular diameter of solute, m Log-normal probability density function, m−1 Hindrance factor of the convex, dimensionless Hindrance factor of the diffusion, dimensionless Transmembrane pressure, Pa Peclet number, the ratio of convex and diffusion, dimensionless Membrane pore radius, m Mean velocity in membrane pore, m s−1

The molar volume of the solute at boiling point, m3 mol−1 Partial molar volume of solute, m3 mol−1 Membrane skin layer thickness, m parameter, dimensionless

Greek letter

r* σ* η(r) η0 ρ ε

Mean pore radius, m Distribution standard deviation, m Viscosity of the liquid in membrane pores, Pa s Viscosity of the bulk liquid, Pa s Pore density, m−2 Surface porosity, dimensionless

fabrication of a PI skin layer on a PI substrate. Moreover, few studies have investigated the “interfacial connection strategy” between the skin layer and the substrate to improve the solvent resistance of the OSN membranes. Recently, dopamine (DA) has drawn intensive attention due to the fact that it can spontaneously oxidize and self-polymerize to form a thin polydopamine (PDA) layer onto nearly all kinds of substrates [24–26]. DA has been separately coated onto a PI support to form a self-polymerized PDA layer as an OSN membrane with good performance for dyes separation from solvents [27]. PDA layer was also studied as an intermediate layer for trimesoyl chloride (TMC) grafting followed by a further polyethyleneimine (PEI) deposition to construct a hierarchically multilayer-structured NF separation membrane [28]. However, there have been no reports found on using DA as an additive during the IP process for the synthetic fabrication of TFC type of OSN membranes. Taking advantages of the exceptional adhesion performance of DA on a variety of substrate materials and the promising chemical stability of PI polymers in OSN applications, here in this paper we report a new class of DA-containing integral PI-based OSN membranes (consisting of both the PI skin layer and the PI substrate materials) via IP reactions and a further imidization process. As both the skin layer and the substrate had essentially similar PI molecular structures, we further applied a crosslinking procedure to crosslink not only the PI molecules of the skin layer but also those of the substrate. Moreover, the PI molecules at the interface between the substrate and the skin layer could also be crosslinked, which means that the skin layer and the substrate could be strongly bonded together by the covalent force to form a wholly crosslinked (or termed as “integrally”) PI-based composite OSN membrane. We also investigated the effects of DA addition and the crosslinking reaction conditions on the separation performance of our novel integrally crosslinked PI-based composite OSN membranes. We further conducted a long-time test to evaluate the solvent resistance of our OSN membranes by both high-temperature immersion and long-term filtration with strong polar solvents that were widely used in chemical industries and were quite difficult to so far reported conventional OSN membranes.

Most of the currently developed PI-based OSN membranes are integrally skinned asymmetric (ISA) and are prepared by the LoebSourirajan (L-S) phase inversion method [2,11]. Livingston et al. [12–14] and Vankelecom et al. [8,15,16] have carried out many research studies on the preparation of crosslinked PI OSN membranes based on the L-S phase inversion method. However, ISA membranes suffered from flux limitations in OSN applications due to their much tight membrane structure [10,17–19]. Interfacial polymerization (IP) process which was first developed by Cadotte [20] can overcome the drawbacks of phase inversion membranes. The prepared thin film composite (TFC) membranes via IP consist of an ultra-thin ‘‘separating barrier layer’’ on the top of a chemically different porous substrate. Recently, Livingston et al. [4,17,18,21] have conducted some research on the fabrication of OSN membranes by the IP method. They reported a new approach through controlling the IP process by using an intermediate layer to form a layer of free-standing polyamide (PA) nanofilm which was less than 10 nanometer in thickness. They incorporated them as separating layers in composite membranes and achieved more than two orders of magnitude higher the permeance than that of commercially available membranes, while maintaining about the equivalent solute retention [21]. They also prepared TFC PA membranes via IP using crosslinked PI UF substrates followed by N, N-dimethyl formamide (DMF) activation and thus significantly increased the solvent permeance without sacrificing rejections [17]. So far, most of the studies on TFC OSN membranes focused on the PA skin layer preparation via IP reaction between MPD and trimesoyl chloride (TMC) on a crosslinked PI UF substrate. However, the formed PA skin layer and the crosslinked PI substrate are not similar in molecular structure. As the PI substrate was already crosslinked prior to the IP reaction, surely there were no strong interactions such as covalent bonding between the skin layer and the underline substrate [22,23]. This might have no significant effect for nanofiltration (NF) membranes applied in aqueous solutions that could not cause swelling or delamination phenomenon. However, for OSN membranes used in organic solutions (especially very harsh polar organic solutions), the skin-substrate interfacial bonding strength affect significantly the solvent resistance, which in turn, would greatly affect the membrane separation performance. For an ideal OSN membrane prepared by the IP process, the enhanced solvent resistance should come from not only the skin layer but also the substrate, as well as the interfacial material compatibility between these two membrane components. It is very important to keep the interfacial bonding strength between the skin layer and the substrate. Inspired by the fact that PI has excellent solvent resistance, we suggest that a PI skin layer might be more compatible with the PI substrate and the formed integral PI-based OSN membrane might be more resistant to organic solvents. However, to the best of our knowledge, there was no research found so far on the chemical synthesis

2. Material and methods 2.1. Material Polyimide (PI, Lenzing P84) was purchased from Inspec Fibers GmbH, Lenzing (Austria). Polyester (PE) non-woven fabric was purchased by Japan Teijin Co., Ltd. 1,2,4,5-Benzene tetracarboxylic acyl chloride (BTAC, 97%) was provided by Guangzhou Zhiya Pharmaceutical Technology Company. Dopamine hydrochloride (DA) was purchased from Chengdu Aikeda Chemical Co., Ltd. Acetic anhydride, 1,6-hexanediamine (HDA, 99%), polyethylene glycol (PEG) 400, N-methyl pyrrolidone (NMP), m-phenylenediamine (MPD, 99.5%), 11

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Fig. 1. Schematic illustration for the synthetic fabrication of the integrally crosslinked PI-based composite OSN membranes.

(c) The PI-PAA membrane was immersed in a 250 ml solution of imidization reagent (volumetric ratio of TEA : acetic anhydride : acetone = 1:3:10 [30]) at 50 °C for 30 min to convert the PAA molecules of the skin layer to PI structure. The membrane was then taken out and rinsed with IPA, and was further immersed into IPA at 60 °C for 30 min to remove any residual acetone inside it. The resulting integral PI TFC membrane in this stage was referred to as PI-PI.

Rhodamine B (RDB, 479 Da) and Rose Bengal (RB, 1017 Da) were purchased from Sinopharm Chemical Reagent Co., Ltd. Acetone, trimethylamine (TEA), n-hexane, ethanol, isopropyl alcohol (IPA) and N, N-dimethyl formamide (DMF) were supplied by Tianjin Fuyu Fine Chemical Co., Ltd. All the solvents and chemicals were used without further purification. Deionized (DI) water was used for membrane rinsing and preparation of aqueous solutions. 2.2. Preparation of the PI UF membrane The asymmetric PI UF substrate was fabricated via the L-S phase inversion method. Briefly, a casting solution with 20.0 wt% P84 and 1.0 wt% PEG 400 in DMF was prepared and continuously stirred for one day to dissolve completely the P84 polymer. Afterwards, the casting solution was kept still overnight to disengage any air bubbles. The bubble-free casting solution was then cast on the PE non-woven fabric that was tapped on a glass plate, using a continuous casting machine (HLK GM3125, Suzhou Hol Ykem automatic Technology Co., Ltd.) with an adjustable knife set at 250 µm gap and a casting speed of 0.1 m s−1. The casting layer was allowed to evaporate in the air for 15 s, then was immersed together with the glass plate into a DI water coagulation bath to perform the phase inversion. After 10 min, the formed PI UF membrane was transferred to a water bath for further use. 2.3. Preparation of the integrally crosslinked PI-based composite OSN membranes The fabrication of the integrally crosslinked PI-based composite OSN membranes includes the following four steps. (a) The PI UF membrane was taped onto a glass plate and immersed in an aqueous monomer solution containing 2.0 wt% MPD and different concentrations (0, 10.0, 20.0, 30.0, 40.0 and 50.0 mg L−1, respectively) of DA for 8 s under ambient conditions. After that, the excess solution on the PI substrate surface was removed by a squeeze roller. The amine-loaded substrate was dried in the air for about one minute and was termed as PI-(MPD+DA) for the purpose of surface chemical characterization. (b) The PI-(MPD+DA) membrane was contacted with the organic monomer solution containing 0.36 wt% BTAC in hexane for 6 s according to Ho et al. [29] to perform the IP reaction to form a skin layer of polyamide acid (PAA), then the excess organic solution on its surface was drained off and the pristine membrane together with the glass plate was cured immediately in an oven at 80 °C for 5 min. The formed TFC membrane in this stage was termed as PI-PAA.

Fig. 2. The chemical reaction mechanism of the interfacial polymerization: (a) MPD+BTAC; (b) the possible reaction: MPD/DA + BTAC; (c) imidization of MPD+BTAC; (d) imidization of MPD/DA+BTAC [30,31]; (e) the possible mechanism for the crosslinking. 12

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Fig. 2. (continued)

not specified. Subsequently, the solvent activated membrane was immersed in ethanol at room temperature for 3 h to remove the residual DMF completely. The final membrane was termed as (PI-PI)xa.

(d) The prepared PI-PI TFC membrane was further immersed in 10.0 wt % HDA IPA solution to perform crosslinking according to Livingston et al. [14]. The temperature of the crosslinking agent was increased up to 60 °C to accelerate the crosslinking process. The crosslinked membrane was termed as (PI-PI)x.

2.5. Membranes characterizations

The fabrication route for the integrally crosslinked PI-based composite OSN membrane is shown in Fig. 1. The reaction mechanism for the fabrication of the integrally crosslinked PI-based composite OSN membrane is shown in Fig. 2.

The variation of the chemical groups of the membranes in different fabrication steps was detected using Attenuated Total Reflectance Fourier Transform Infrared spectrometer (ATR-FTIR, Thermo Nicolet Magna-560) and was quantitatively analyzed using Eq. (1) [32]:

log I0 / I = k⋅c⋅L

2.4. Solvent activation post treatment

(1)

where I0 and I are the incident intensity and the transmission intensity for certain wave number of the ATR-FTIR, respectively; c is the component concentration (g L−1) in the sample; L is the thickness of the absorption cell (cm); k is the absorption coefficient. Form this equation it can be seen that log(I0/I) is proportional to the concentration of the specific component in the sample. The change in chemical structure of the membrane surface in each

The final crosslinked (PI-PI)x composite membrane was immersed into DMF to conduct a solvent activation to dissolve and wash away some low-molecular-weight crosslinked polyamide fragments for further enhancing the solvent permeance according to Livingston et al. [17]. We increased the DMF temperature up to 80 °C to speed up the activation process. The activation time was fixed as 30 min by default if

13

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2.6. Filtration experiments

groups) [41] increased obviously, indicating that partial PI molecules of the substrate had reacted with MPD molecules to form amide groups. Detail information was summarized by the quantitative analysis to demonstrate the fact, as shown in Table 2. For the PI substrate, log(I0/I)1657/ log(I0/I)1722 and log(I0/I)1549/ log(I0/I)1722 were 0.061 and 0.005, respectively. After immersing into the aqueous MPD solution, they increased up to 0.090 and 0.019 (Table 2), respectively, which were about 1.5 times and 4 times of the PI substrate. This is a strong indication that chemical reaction occurred between the PI substrate and the MPD molecules to form amide covalent bonding, which could be very beneficial for strengthening the binding force between the subsequent IP skin layer and the substrate.

The separation performance of the prepared composite OSN membranes were evaluated in terms of solvent permeance and solute rejection using a laboratory-made filtration system [33]. The effective area of each membrane sheet was 28.26 cm2. Membrane sheets were placed into four crossflow membrane cells in series. The filtration test was carried out using a 100.0 mg L−1 dyes solution under the ambient temperature. Permeate sample was collected as a function of time and was weighted by an electric balance to determine the solvent permeance (P, L m−2 h−1 bar−1) according to Eq. (2):

3.1.2. ATR-FTIR analysis of the membranes The spectra of ATR-FTIR are shown in Fig. 4. For the PI substrate, typical bands were observed at 725, 1363, 1722 and 1784 cm−1, corresponding to the carbonyl group stretch, C-N stretch vibration, C˭O symmetric stretch vibration and C˭O asymmetric stretch vibration of the imide group, respectively [16,42,43]. After the immersion of the PI substrate into the aqueous MPD/DA solution, in addition to the changes as illustrated in Fig. 3 of Section

modification process was further confirmed by using Raman (Thermo Scientific DXR) microscope. The element composition and chemical bonding of the membrane surface were analyzed by an X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, US). The surface and cross-sectional morphologies of each membrane sample were observed by a scanning electron microscopy (SEM, S-4800SEM, Hitachi, Japan). The surface roughness of the prepared TFC OSN membranes was characterized by an atomic force microscopy (AFM, Agilent 5400) equipped with a dimension AFM scan head and was further analyzed by PICOVIEW software.

P=

ΔV A⋅Δt⋅Δp

Table 1 Pore size distribution model and the related equations [34–39].

(2)

where ΔV (L) is the volume of the permeate collected in the time period Δt (h) under a trans-membrane pressure Δp (bar), A (m2) is the effective membrane area. The solute (dye) rejection was calculated from Eq. (3):

Cp R = ⎜⎛1− ⎟⎞ × 100% ⎝ Cf ⎠

(

⎧ ⎡log r + ⎪ r* 1 fR (r ) = ⋅exp − ⎣ ⎨ 2b r 2πb ⎪ ⎩

b 2

2

) ⎤⎦

⎫ ⎪ ⎬ ⎪ ⎭

(4)

2

σ* b = log ⎡1.0 + ⎛ ⎞ ⎤ ⎢ ⎝ r* ⎠ ⎥ ⎦ ⎣

(3)

where Cp and Cf denote the dye concentrations in the permeate and in the feed solution, respectively. The dye concentration was measured by using an ultraviolet and visible spectrophotometer (METASH UV-5100, China). The membranes were conditioned at 1.0 MPa using the test solution for about half an hour until the permeance reached a steady value prior to the test, and then the permeance and the rejection were periodically measured. The permeance and the rejection values as well as the standard deviations of them were calculated from the average values of at least two pieces of different membrane sheets.

R (r ) = 1 −

[kC (r ) + Y (r )]⋅φ (r ) 1 − {1−[kC (r ) + Y (r )]⋅φ (r )}⋅exp[−Pe′ (r )]

(6)

Y (r ) =

8η (r )⋅kD (r )⋅Di, ∞⋅Vs RT2⋅r 2

(7)

Pe (r ) =

[k C (r ) + Y (r )]⋅r 2⋅ΔP 8⋅η (r )⋅kD (r )⋅Di, ∞

(8)

λ (r ) =

2.7. Theoretical analysis of the skin layer pore size distribution Log-normal probability density function was employed to describe the pore size distribution (the average pore radius r* and the pore diameter deviation σ*) on the skin layer of the OSN membranes [34–39]. The formulas for the pore size distribution model and the related equations used in the theoretical analysis of the skin layer pore size distribution can be seen in Table 1.

(5)

di d = i dp 2r

(9)

kC = A + Bλ (r ) + Cλ2 (r ) + Dλ3 (r )

(10)

kD = E + Fλ (r ) + Gλ2 (r ) + Hλ3 (r )

(11)

Di, ∞ =

1.173 × 10−16 (ϕ⋅Msolvent )0.5⋅T 0.6 η0⋅Vsolute

(12) 2

η (r ) d d = 1 + 18 ⎛⎜ solvent ⎞⎟ − 9 ⎛⎜ solvent ⎞⎟ η0 ⎝ dp ⎠ ⎝ dp ⎠

3. Results and discussion 3.1. Mechanism of the membrane formation

V (r ) =

3.1.1. The interaction between MPD and the PI substrate After the PI substrate was immersed in the aqueous MPD solution, the color of the substrate surface quickly changed from light cyan to mustard, indicating that MPD reacted with PI molecules of the substrate surface quickly. This phenomenon was confirmed by the ATR-FTIR spectra, as shown in Fig. 3. Although the PI characteristic peak (C˭O symmetric stretch of imide groups) at 1722 cm−1 [40] remained unchanged after the immersion, however, the amide band at 1657 cm−1 (C = O stretch of amide groups) and 1549 cm−1 (N-H stretch of amide

JV =

R=

14

r 2⋅Δp 8η (r )⋅Δx

πN0⋅Δp 8⋅Δx

∫0

(13)

(14) ∞ fR (r )⋅r 4

η (r )

⋅dr

(15)

∫0∞ fR (r )⋅r 4⋅R (r )⋅dr ∫0∞

fR (r ) ⋅ r 4 ⋅dr η (r )

(16)

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Fig. 3. ATR-FTIR spectra of the PI substrate before and after immersion in the aqueous MPD solution.

Fig. 5. Raman spectra of the PI substrate, the PI-(DA+MPD), the PI-PI and the (PI-PI)x membranes.

Table 2 Quantitative analysis of the groups of the PI substrate before and after immersion in the aqueous MPD solution.

composite OSN membrane was a hybrid matrix of polyamide and polyimide, and the majority of the skin layer is a kind of highly crosslinked polyamide from polyimide.

log(I0/I)

1722 cm−1 1657 cm−1 1549 cm−1

PI substrate PI substrate+ MPD

0.932 0.914

0.057 0.083

0.005 0.017

log (I0 / I )1657 log (I0 / I )1772

log (I0 / I )1549 log (I0 / I )1772

0.061 0.090

0.005 0.019

3.1.3. Raman spectroscopy The Raman spectra of the membranes in different preparation stages are shown in Fig. 5. The PI substrate showed characteristic peaks at 1375, 1618, and 1782 cm−1, which were attributed to the C-N-C band vibration of the imide groups, C˭C vibration of the aromatic rings, and C˭O vibration of the imide groups, respectively [48,49]. After the PI substrate was coated with the DA and MPD mixture aqueous solution, all the peaks of the PI substrate became very weak, except a broad peak emerging at around 3000 cm−1, as can be seen from the spectrum of the PI-(DA+MPD) membrane in Fig. S2. It is obvious that this broad peak can be assigned to the -OH stretching vibration of DA and the N-H stretching vibration of DA and MPD. This was in agreement with the analysis of the ATR-FTIR spectra. However, the PI characteristic peaks at 1375, 1618 and 1782 cm−1 appeared again after the IP and the subsequently imidization processes, as can be seen on the spectrum of the PI-PI membrane. After crosslinking by HDA, the characteristic peaks of imide became weak, as can be seen on the spectrum of the (PI-PI)x OSN membrane, which surely demonstrated the conversion of imide group to amide group. This confirmed our analysis in Section 3.1.2 that the fabricated integrally crosslinked PI-based composite OSN membrane was a hybrid matrix of polyamide and polyimide.

Fig. 4. ATR-FTIR spectra of the PI substrate, the PI-(DA+MPD), the PI-PAA, the PI-PI and the (PI-PI)x membranes.

3.1.1, a broad band that appeared at around 3000–3600 cm−1 was observed on the spectrum of the PI- (DA+MPD) membrane (Fig. S1). This band was assigned to the combination of -OH stretching vibration of DA and N-H stretching vibration of DA and MPD [44,45], suggesting the co-deposition of DA and MPD on the PI substrate surface. After the IP process, the characteristic peaks of the imide groups diminished significantly, and two new bands were observed at 1657 and 1549 cm−1 on the spectrum of the PI-PAA membrane, which were attributed to the C˭O stretching vibration (amide I) and the N-H stretching vibration (amide II), respectively [30,41]. Another band was observed at 1608 cm−1, which is attributed to the carboxylate group, indicating the formation of a layer of PAA with carboxylic acid groups on the PI support [46]. After the subsequent imidization, the peaks that associate with the imide groups (1784, 1722 and 1363 cm−1) recovered [46], as can be seen on the spectrum of the PI-PI membrane. After the final crosslinking with HDA for 1 h, almost all the characteristic peaks of imide vanished except that only one less intense peak at 1722 cm−1 remained, as can be seen on the spectrum of the (PI-PI)x membrane. The amide band at 1657 cm−1 and 1549 cm−1 increased obviously, which confirmed the successful crosslinking reaction [16,47]. This indicated that the integrally crosslinked PI-based

3.1.4. XPS spectroscopy As ATR-FTIR could not distinguish clearly the peaks of DA and MPD, we used XPS to further illustrate the detail information of the prepared DA-containing OSN membrane. O atom was selected to analyze the detail coating mechanism since the chemical circumstance of O atom is much simpler than N atom in this kind of membrane. The high-resolution O1s XPS spectra of both the PI substrate and the PI(DA+MPD) membrane was deconvoluted and fitted, as shown in Fig. 6. As shown in Fig. 6(a), there were two fitting peaks on the O1s spectra of the PI substrate, which were assigned to the oxygen characteristic binding energy in the imide group (O˭C-N-C˭O) and the ketone group (C˭O) of PI polymer [41], respectively. After aqueous MPD/ DA coating, two new binding energy peaks emerged at 530.6 eV and 532.5 eV, as shown in Fig. 6(b). The peak at 530.6 eV was assigned to the amide covalent bond (O˭C-NH) [50] formed by the reaction between MPD and the PI substrate, which indicated that the interfacial binding force could be greatly strengthened. The peak at 532.5 eV was assigned to the C-OH bond of dopamine [27,50], indicating that DA successfully coated on the surface of the PI substrate after immersing the substrate in the aqueous MPD/DA solution. 15

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the higher probability of the existence of smaller pores in the thin dense selective layer [52,53], proving that DA could adjust the pore size and microstructure of the (PI-PI)xa OSN membranes. However, when the concentration of DA increased further from 30.0 to 50.0 mg L−1, both the permeance and the rejection declined, as shown in Fig. 7, suggesting that the competition between DA and MPD interfered the main IP reaction [29,54] even apparently at higher DA concentration and resulted in larger pore size but lower pore density, as shown in Table S1, as well as looser surface, as shown in Fig. 9(d). This deteriorated the subsequently formed PAA skin layer structure and the final separating performance of the final OSN membrane. Therefore, the optimal DA concentration in the aqueous phase solution was 30.0 mg L−1 considering the permeance and the rejection of the (PIPI)xa OSN membranes. The SEM images shown in Fig. 9(e)-(h) revealed that the addition of DA at different concentrations have little effect on the cross-sectional morphology of the membrane. The AFM topographical images of the (PI-PI)xa OSN membranes with/without DA incorporation are shown in Fig. 10. It can be seen that the surface of both membranes showed typical ridge-valley morphology. Adding 30.0 mg L−1 DA into the aqueous MPD solution made the surface of the (PI-PI)xa OSN membrane a little smoother than that without DA incorporation, with an average surface roughness reducing from 33.5 nm to 27.6 nm. This might be due to the competition between DA and MPD during the IP process which could slow down the rapid IP reaction between MPD and BTAC to some extent, thus decrease the surface roughness [55]. 3.3. Effect of crosslinking time on the membrane performance The thickness of the skin layer of the PI substrate is about 220 nm according to Fig. 11(a). The cross-sectional SEM image of the DA-incorporating PI-PI membranes before crosslinking (Fig. 11(b)) clearly showed the skin layer become much thicker to about 420 nm, and this skin layer is relative uniform. Surely, this skin layer is the combination of the IP skin layer and that of the substrate. The thickness of the IP skin layer was about 200 nm after subtracting the thickness of the skin layer of the PI substrate. The interface between the IP skin layer and the skin layer of PI substrate was almost invisibility because the IP skin layer after the imidization is quite similar to the PI substrate in molecular nature. This is an indication of the good compatibility between the two layers [27,56], which is beneficial for the further crosslinking step to crosslink molecules between the two layers to form a kind of wholly crosslinked (PI-PI)x OSN membrane. The interface between the adjacent layers (the skin layer and the sublayer of PI substrate) also became obscure after crosslinking (Fig. 11(c)) and further increased the integrity of the prepared membrane.

Fig. 6. High-resolution and deconvolution of O1s XPS spectra of the PI substrate (a) and the PI-(DA+MPD) membrane (b).

3.2. Effect of the DA concentration on the membrane performance The effect of the DA concentration in the aqueous MPD monomer solution on the separation performance of the (PI-PI)xa OSN membranes is shown in Fig. 7. The ethanol permeance and RDB rejection were enhanced apparently when the DA concentration in the MPD solution increased from 0 to 30.0 mg L−1. Afterward, the ethanol permeance and RDB rejection decreased gradually with the further increase of the DA concentration. This similar trend was also reflected from the analyzed probability density function curves of the pore size in the skin layer (Fig. 8) according to the numerical fitting of the pore size distribution model in Table 1 with the ethanol permeance and RDB rejection data, and the analyzed mean pore diameters (Table S1). At low DA concentrations (0–30.0 mg L−1), the incorporation of DA could strengthen the transport of solvent molecules by increasing the hydrophilicity of the (PI-PI)xa OSN membrane due to the hydroxyl group of DA [51]. The single amine group in DA molecule would terminate the PAA network during the IP process, reduce the crosslinking degree of the (PI-PI)xa OSN membrane by competing with MPD [29] (Fig. 2) and contribute to forming a looser active layer on the substrate surface, as shown in Fig. 9(a)-(c). Thus, both the pore density and the porosity increased, as shown in Fig. 8 and Table S1, which were also beneficial for the solvent permeance. It is apparent that when the DA concentration was in the range of 0–30.0 mg L−1, the peaks of the probability density function curve for the pore size became higher and the average pore size shifted from 0.75 nm down to 0.68 nm, quite similar to that reported by Livingston et al. which was 0.82 nm for hybrid polymer/MOF OSN membranes [34] and to that reported by Tai-Shung Chung et al. which was 0.74 nm for PI OSN membrane [52]. The shift of the mean pore size to a lower value resulted in a higher rejection due to

Fig. 7. Effect of DA concentration in the aqueous MPD solution on the separation performance of the (PI-PI)xa OSN membranes. 16

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3.4. Stability and long-term performance of the integrally cross-linked PIbased composite OSN membranes 3.4.1. Effect of DA addition on the solvent resistance In order to investigate the effect of DA addition on the solvent resistance of the prepared (PI-PI)xa OSN membrane, the membranes containing DA and those without DA were immersed in two kinds of aprotic solvents (DMF and NMP) at 80 °C for a period of time. The results are shown in Fig. 13. As shown in Fig. 13(a), the RDB rejection of the (PI-PI)xa OSN membranes prepared with/without DA incorporation slightly decreased and the ethanol permeance continuously increased when the membranes were persistently immersed in 80 °C DMF for 10 days, which indicated a certain degree of swelling, dissolution and weight loss. However, the rejection could still maintain as high as 98% for RDB (479 Da) after persistent immersion in DMF at 80 °C for 14 days, which was seldom seen in literatures for OSN membranes immersed at such high temperature DMF for such long time. This indicated that the (PIPI)xa OSN membranes we prepared exhibited excellent solvent stability. This was mainly due to the crosslinking effect, as well as the powerful binding interaction between MPD molecules and the PI substrate which formed a strong covalent bond between the IP skin layer and the PI substrate. Both the solvent permeance of the (PI-PI)xa OSN membranes with DA incorporation and that without DA incorporation dramatically increased during the long-term immersion, from about 1.74 and 2.04 L m−2 h−1 bar−1 at the beginning to 3.06 and 3.13 L m−2 h−1 bar−1 at the end of the test period, respectively. This can be attributed to the fact that some oligomers fragments with small polymer chains which remained in the skin layer after crosslinking would be dissolved in DMF [17], thus resulting in a higher permeance. Meanwhile, only a quite slightly decrease of the rejection after the long-term immersion in DMF, indicating that the prepared OSN membrane can tolerate such high temperature DMF to some extent. There was only quite slight degradation on the membrane functional skin layer. This might be attributed to the annealing effects of DMF during the swelling process which can eliminate imperfections inside the top skin layer [17], thus retarding the degradation of the skin layer. It was worth mentioning that the (PI-PI)xa OSN membrane with DA incorporation had higher rejection and slight higher or similar permeance compared with those of the membrane without DA-incorporation during the long term

Fig. 8. Probability density function curves for the pore size (dp) of the (PI-PI)xa OSN membranes with different DA-incorporation concentration in the aqueous MPD solution.

The performance of the fabricated (PI-PI)xa OSN membranes with crosslinking time is shown in Fig. 12. Both the rejection and the permeance increased when the crosslinking time increased up to 60 min. The increase of the ethanol permeance can be attributed to the fact that the introduction of HDA rendered the PI membrane more hydrophilic [57]. Meanwhile, the increase of the crosslinking time can make the membrane skin layer relatively more “tighter” which was beneficial for the dye rejection. However, with the further extension of the crosslinking time up to 120 min, both the dye rejection and the ethanol permeance decreased. This might be due to the higher degrees of crosslinking which limited the flexibility and rearrangement of the polymer chains and resulted in an even tighter membranes [58], causing the falling down of the ethanol permeance inside the denser membranes, and decreasing the effect of the subsequent DMF activation. Hence the ethanol permeance decreased apparently, resulting in the decrease of the dye rejection according to the non-equilibrium thermodynamics model [59]. Therefore, the performance of the (PI-PI)xa membranes shows optimal at 60 min crosslinking time.

0 mg L−1 Surface

20.0 mg L−1

a

10 μm

1 μm

40.0 mg L−1

c

b

10 μm

10 μm

e

Cross

30.0 mg L−1

g

f

1 μm

1 μm

d

10 μm

h

1 μm

Fig. 9. Surface and cross-sectional SEM images of the (PI-PI)xa OSN membranes with various DA concentration. (a, e): 0 mg L−1; (b, f): 20.0 mg L−1; (c, g): 30.0 mg L−1; (d, h): 40.0 mg L−1. 17

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Fig. 10. AFM topographical images of the (PI-PI)xa OSN membranes: without DA incorporation (a) and with 30.0 mg L−1 DA incorporation (b).

shown in Fig. 13(b). We emphasize NMP since it is an even harsher strong polar organic solvent that can dissolve PI polymer easily, and there has been no report on PI OSN membranes that could tolerate NMP at such a high temperature and for such a long time. Both the (PI-PI)xa

immersion. This means that DA incorporation is beneficial for the solvent resistance of the prepared (PI-PI)xa OSN membranes. Similar results can also be seen from the persistent immersion of the prepared (PI-PI)xa OSN membranes in NMP at 80 °C for 14 days, as

a2

a1 ~ 220 nm m

~ 2220 nm

5 μm

b1

1 μm μ

b2

~ 4220 nm

~ 420 nm

1 μm

5 μm

c1

c2

1 μm

5 μm

Fig. 11. Cross-sectional SEM images of PI substrate (a1, a2), the DA-incorporating PI-PI membranes before crosslinking (b1, b2) and after crosslinking (c1, c2). 18

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2.0 1.5 1.0 Ethanol Permeance RDB Rejection

0.5 0.0

0

30 60 90 120 Crosslinking time (min)

Permeance (L m-2 h-1 bar-1)

2.5

Rejection, 0 mg L-1 DA Rejection, 30 mg L-1 DA

3 96 2 94 1

6 (b)

92

0.5 d 1.5 d 2.5 d 3.5 d 4.5 d 5.5 d 7 d 10 d Time of immersing in DMF (day) Permeance, 0 mg L-1 DA Permeance, 30 mg L-1 DA

14 d

Rejection, 0 mg L-1 DA

2

80 70

1

60

0

50 20 40 60 80 100 120 140 160 180

3.4.2. Long-term filtration test During the long-term filtration test, DMF was selected as solvent to verify the stability of the membranes. However, we found that RDB as the solute is instability in DMF with the increase of filtration time, the reason is not clear and a further research on this phenomenon is beyond the scope of this work so that RDB was substituted by RB (1017 Da) in the long-term filtration test according to other studies [27,60,61]. Longterm filtration performance of the DA-incorporating (PI-PI)xa OSN membrane using 100.0 mg L−1 RB DMF solution at room temperature can be seen from Fig. 14. The RB rejection kept up to 99.9% during the long-term running for 7 days, indicating that the prepared (PI-PI)xa OSN membranes exhibited excellent solvent resistance. After an initial swelling caused by DMF, the DMF permeance decreased gradually and reached a constant value of about 1.15 L m−2 h−1 bar−1 which is relatively high as compared with those of other research works, as shown in Table 3.

90

100

4

96

3

94

2

92

1

90

14 d

Rhodamine B Rejection (%)

98

0.5 d 1.5 d 2.5 d 3.5 d 4.5 d 5.5 d 7 d 10 d Time of immersing in NMP (day)

3

DMF, indicating that the addition of DA is beneficial for the solvent resistance.

Rejection, 30 mg L-1 DA

5

0

90 DMF Permeance RB Rejection

Fig. 14. Long-term filtration performance of the DA-incorporating (PI-PI)xa OSN membrane using 100.0 mg L−1 Rose Bengal (1017 Da) DMF solution at room temperature.

100

98

0

EtOH permeance (L m-2 h-1 bar-1)

Permeance, 30 mg L-1 DA

4

Time (h)

Rhodamine B Rejection (%)

EtOH permeance (L m-2 h-1 bar-1)

4

Permeance, 0 mg L-1 DA

100

0

Fig. 12. Effect of crosslinking time on the separation performance of the (PIPI)xa OSN membranes with DA incorporating. (When the crosslinking time was 0 min, the membrane was not activated because it could be dissolved by DMF.).

(a)

5

RB Rejection (%)

100 95 90 85 80 75 70 65 60

3.0

Rhodamine B Rejection (%)

Permeance (L m-2 h-1 bar-1)

C. Li et al.

3.5. Comparison with other OSN membranes Table 3 listed the separation performance of the fabricated (PI-PI)xa OSN membranes with the incorporation of 30.0 mg L−1 DA in this work and those of some other OSN membranes reported in the state-of-the-art literatures which use the same or similar testing systems. It should be mentioned that the direct quantitative comparison of the separation performance is quite difficult due to the difference in polymer materials and filtration conditions among different studies, as well as the solvent resistance performance of each fabricated OSN membrane. Compared to the OSN membranes in the state-of-the-art literatures, the (PI-PI)xa OSN membranes showed significant advantages on the solvent permeance and the solute rejection. Take dye/ethanol as the test solution, although we used RDB with a smaller molecular weight (479 Da), we obtained a dye rejection as high as 98% and an ethanol permeance as high as 2.03 L m−2 h−1 bar−1. We also obtained a rejection as high as 99.9% for RB (1017 Da) and a DMF permeance of 1.15 L m−2 h−1 bar−1, which is also excellent compared with other research works using RB (1017 Da)/DMF as the test solution. Moreover, this kind of DA-incorporating (PI-PI)xa OSN membrane has remarkable solvent resistance, which is much promising for commercial OSN applications.

88

Fig. 13. The solvent resistance of the (PI-PI)xa OSN membranes prepared with/ without DA incorporation after persistent immersion in DMF (a) and NMP (b) at 80 °C for different time.

OSN membranes with/without DA incorporation could tolerate the high-temperature NMP to some extent. Again, the DA-incorporating (PI-PI)xa OSN membrane has a higher rejection and a similar reduction trend in the rejection after the long-term immersion in the high-temperature NMP compared with that of the (PI-PI)xa OSN membrane without DA-incorporating, quite similar to the result of long-term immersion in DMF. The variation of the ethanol permeance also shows about the same trend with the result of the long-term immersion in

4. Conclusions We have successfully developed a novel class of (PI-PI)xa OSN membranes with integrally crosslinked skin-substrate PI polymers via 19

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Table 3 Comparison of the separation performance between OSN membranes fabricated in this work with those reported in the state-of-the-art literatures. Membrane materiala

Solvent

Permeance (L m−2 h−1 bar−1)

Solute and MW (Da)

Rejection (%)

Ref.

(PI-PI)xa

EtOH DMF EtOH DMF EtOH EtOH EtOH DMF DMF

2.03 1.15 0.91 0.63 0.6 2.7 1.12 3.1 1.5

RDB (479) RB (1017) MB (800) RB (1017) RB (1017) RB (1017) RB (1017) RB (1017) Styrene oligomers (236)

98 99.9 99 99 84 100 99 93 91

This work

c(PDA/PI) (Catechol/PEI)/PAN PA/crosslinking Matrimid PI Crosslinking polyimidePI (DA/TMC)/cPAN PA/crosslinking P84 PI a

[27] [62] [63] [64] [65] [17]

PAN, PEI are abbreviations of polyacrylonitrile and polyethyleneimine.

interfacial polymerization between DA-incorporated aqueous MPD solution and organic BTAC solution using PI UF substrate, followed by a subsequent imidization process, a crosslinking process (to create integral crosslinking near PI-based skin-substrate interface area), and a solvent activation process. The PI UF substrate and the PI skin layer were bonded tightly together by the combination of the covalent bond between MPD and the PI substrate, the adhesion of DA, and the crosslinking effect of the crosslinker. DA not only played an important role in enhancing the solvent resistance of the prepared integral crosslinked PI-based composite OSN membranes, but also significantly optimized the membranes surface morphology and reduced the roughness of the composite membranes surface. The DA-incorporating (PI-PI)xa OSN membranes possessed excellent separation performance and solvent resistance even after the long-term immersion in very harsh strong polar solvents such as DMF and NMP at 80 °C for two weeks, as well as the filtration test for RB/DMF solution at room temperature for 7 days, proving a promising prospect in OSN applications.

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[50]

[51]

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[56]

[57] [58]

[59]

[60]

[61]

[62]

[63]

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