Polyacrylonitrile-supported self-aggregation crosslinked poly (vinyl alcohol) pervaporation membranes for ethanol dehydration

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Polyacrylonitrile-supported self-aggregation crosslinked poly (vinyl alcohol) pervaporation membranes for ethanol dehydration ⁎

Meisheng Li , Jie Wang, Shouyong Zhou, Ailian Xue, Feiyue Wu, Yijiang Zhao

⁎

School of Chemistry and Chemical Engineering, Huaiyin Normal University, Jiangsu Engineering Laboratory for Environmental Functional Materials, Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, No.111 West Changjiang Road, Huaian 223300, Jiangsu Province, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Pervaporation dehydration Water-selective membrane Surface modification Poly(vinyl alcohol) Polyacrylonitrile

Highly water-selective poly(vinyl alcohol) (PVA)/polyacrylonitrile (PAN) pervaporation membranes were prepared by combining self-aggregation crosslinking reactions for PVA through ammonium persulfate (APS) initiations and interface physicochemical structure regulations for PAN supports. The membrane physicochemical properties were tested and the pervaporation performances were investigated by separation of 95 wt% ethanol aqueous. The results showed that the swelling degree and crystalline region of PVA membranes decreased with the increasing of APS content. On the contrary, the gel fraction of PVA membranes increased. Interestingly, the interface physicochemical structure between the PAN support and PVA separation layer played an important role in pervaporation processes. When the PAN supports were treated by sodium hydroxide (NaOH) and tannic acid (Ta), the water fraction in permeation always kept over 99.99 wt%, especially after Ta treatment. After optimization, the membrane based on PVA content of 8 g, 1.25 g self-crosslinking agent of APS and spin-coated onto H2-PAN (PAN modified by NaOH and Ta respectively), showed good permeability-selectivity and long-term performance stabilities. The total flux and water content ratio in permeation were 117.6 g/(m2h) and 99.99 wt% respectively by using the PVA8-1.25/H2-PAN membrane. After running for 120 h, the membranes still displayed a good stability for the ethanol dehydration in the pervaporation process. Importantly, the ethanol solution could be concentrated to over 99.99% using this polyacrylonitrile-based self-aggregation crosslinked poly (vinyl alcohol) pervaporation dehydration membrane.

1. Introduction Poly(vinyl alcohol) (PVA) has a strict linear and soft structure, and a large amount of lateral hydroxyl groups (eOH) in the molecular chain, which is well known as a synthetic biodegradable polymer and possesses excellent mechanical properties [1,2]. For dehydration membranes, PVA is a widely used polymer material for the preparation of pervaporation membranes because of its high hydrophilicity, good heat resistance and stable chemical properties, resistance to organic solvents and substances, good film-forming properties, and low price [3,4]. However, since PVA is prone to swelling when being exposed to water, PVA must be modified to achieve the desired separation performance. Crosslinking is an effective modification method [5–8]. Currently, PVA pervaporation membranes are prepared based on a hydroxyl crosslinking reaction. Conventional crosslink agents included aldehydes, inorganic salts, organic acids and their derivatives, for example, Sulfosuccinic acid [9–11], succinic acid [3,5], amic acid [6], maleic acid [12–14], glutaraldehyde [7,8], lithium chloride (LiCl) [15], and

⁎

fumaric acid [16–18]. The key problem is that this cross-linking methods convert a large amount of hydroxyl groups in PVA chain into ester or ether groups, decreasing the membrane hydrophilicity, as well as the water selectivity and permeability. Are there any ways to obtain crosslinked PVA while retaining enough hydroxyl groups? Bezuidenhout et al. [19] and Miranda et al. [20] introduced a PVA cross-linking by using potassium persulfate and ultraviolet light to achieve self-crosslinking while retaining hydroxyl groups, respectively. In 2014, Gu et al. [21] prepared cross-linked PVA pervaporation membranes by using ammonium persulfate (APS) for the separation of 95 wt% ethanol–water system. The flux and separation factor reached 319.8 g/(m2h) and 3751.8, respectively. Thus, the use of non-hydroxyl crosslinked PVA membranes for pervaporation dehydration attracted researchers’ attentions. Besides, the interface physicochemical structures between supports and PVA dehydration separation layers play an important role in pervaporation processes. In previous reports, PAN supports were simply treated by water [22,23], even using the sodium hydroxide (NaOH)

Corresponding author. E-mail addresses: [email protected] (M. Li), [email protected] (Y. Zhao).

https://doi.org/10.1016/j.eurpolymj.2019.109359 Received 30 September 2019; Received in revised form 28 October 2019; Accepted 6 November 2019 0014-3057/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Meisheng Li, et al., European Polymer Journal, https://doi.org/10.1016/j.eurpolymj.2019.109359

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8400 Stirred Cell, Merck Millipore, Germany) [28]. All wet membranes were compacted with water at 0.25 MPa for 30 min, and then tested at 0.2 MPa and ambient temperature (25 °C). The pure water flux (PWF) was calculated by the equation below:

[24], which were just to remove the oil and introduce carboxyl group (eCOOH) onto PAN surfaces. However, in the course of our studies, the support could be further modified to improve membrane performances. Among different surface modification agents, Tannic acid (Ta) is a kind of natural polyphenols being rich in phenolic hydroxyl groups (eOH), which can bind with polar groups in PVA chains. Then, it can be used to modify PAN substrate through extensively functional reactivity of hydrogen bonds [22] and chemical bonds, like ester bonds between eOH and eCOOH groups. In fact, Ta has been selected as a crosslinker to improve the properties of membranes and other materials. For example, Rubentheren et al. [25,26] synthetized a novel chitosan film with nanocrystalline cellulose as a physical reinforcement and Ta as a crosslinker. Insertion of Ta into the nanocomposite film further improved the tensile strength by 82.6%. In addition, Ta made the structure transfer into an anhydrous crystalline conformation, when compared with neat chitosan film. In 2015, Luo et al. [27] developed a green one-step strategy to fabricate three-dimensional (3D) graphene-based multifunctional material with the aid of Ta. The Ta retained in the skeleton of 3D graphene functions as a biofunctional component, which endows the graphene hydrogel (TA-GH) with good antibacterial capability and efficient adsorbents in water purification. From above, PVA initiated by APS can self-agglomerate, even forming a large network structure, like a pocket. If the supports are modified by Ta, the phenol hydroxyl in Ta can react with the PVA in the hydrophilic pocket, so that PVA can fit closely with Ta. Then, the interfaces between supports and PVA layers can be regulated. Therefore, based on the combination of self-aggregation crosslinking reactions in PVA through APS initiation and the interface physicochemical structure regulation for PAN supports using Ta, the pervaporation dehydration performance of membranes would be improved helpfully. Herein, highly water-selective PVA/PAN pervaporation membranes were prepared successfully by self-aggregation crosslinking reactions in PVA through APS initiations. The detailed effect of PVA and APS contents on membrane properties were studied, especially the effect of different physicochemical structures of PAN supports. Also, a 95 wt% ethanol/water was selected to investigate the separation performances for the as-prepared membranes.

J1 =

V A1 t1

(1)

Where J1 is the PWF (L/(m2h)), V is the permeate volume (L) , t1 is the permeate time (h) and A1 is the actual membrane area (m2). The PWF is averaged from 5 times to reduce errors. The overall porosity (ε, %) [29] was determined using the gravimetric method at ambient temperature (25 °C) in the following equation:

ε=

Ww − Wd × 100% ρwater A2 L

(2)

Where Ww denotes the wet weight of the membrane (g), Wd denotes the dry weight of the membrane (g), ρwater denotes the Wahaha pure water density (0.998 g/cm3) at room temperature (25 °C), A2 is the effective membrane area (cm2), and L is the wet membrane thickness (cm) following water immersion for 48 h. Each measurement was carried out three times to reduce error. The static contact angles of PAN, H1-PAN and H2-PAN were measured by a Drop Shape Analyzer (DSA, OSA200-T), and the pore size distributions were analyzed by a liquid–liquid displacement technique (isobutanol-water system) (Pore-size Distribution Analyer, PSDA-10). 2.3. Fabrication of pure PVA and crosslinked PVA membranes As we know, the mass fractions of PVA and APS have important impacts on the swelling degree, hydrophilicity-hydrophobicity performance, viscosity, as well as the pervaporation. In this work, the following experiment methods were provided to ensure the optimum addition of PVA and APS. The detailed experiment conditions for different PVA membranes were given in table 1. To be specific, a certain mass fraction of PVAX (Χ = 5, 6. 7, 8, 9, 10 (%)) solution was prepared at 90 °C. Then, after degassing at room temperature, a certain amount of APS was dispersed in PVA solution at 25 °C for 6 h to obtain PVAX-Y (Y = 0, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75 and 2.0 (%)) casting solution. The composite membranes were prepared by spin-coating (Vacuum Spin Coater, VTC-100PA) each solution (2 mL) onto PAN, or H1-PAN or H2PAN substrates at 70 °C and removing the solvent under ambient condition (25 °C) for 10 min. The spin-coating conditions were 30 s

2. Experimental 2.1. Materials Poly (vinyl alcohol) (PVA) (1750 ± 50), sodium hydroxide (NaOH) and ethanol (≥was 99.7 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tannic acid (Ta) and ammonium persulfate (APS) were supplied by Aladdin Industrial Co., Ltd. (Shanghai, China). Polyacrylonitrile (PAN) ultrafiltration membranes with a molecular weight cut-off of 50000 Da were obtained from Beijing Saipuruite Equipment Co., Ltd. (Beijing, China). All the materials and reagents were of analytical grade and used without further purification. Deionized water was used throughout the experiments.

Table 1 The detailed prepared conditions for different PVA membranes. Membranes PVA Y

2.2. Modification and characterization of PAN

PVA5-1.0 PVA6-1.0 PVA7-1.0 PVA8-1.0 PVA9-1.0 PVA10-1.0 PVA8-1.0 PVA8-1.0 PVA8-0 PVA8-0.5 PVA8-0.75 PVA8-1.0 PVA8-1.25 PVA8-1.5 PVA8-1.75 PVA8-2.0

PAN supports were modified via layer-by-layer self-assemble method. Firstly, PAN ultra-filtration membranes (7 cm × 7 cm) were immersed in absolute ethanol and Wahaha purified water for 24 h, respectively. This treatment was repeated by 3 times to ensure the fully adsorption equilibrium to remove glycerin from the surfaces and then fully dried, which was marked as PAN. Secondly, the PAN was soaked in 2 mol/L sodium hydroxide (NaOH) for 30 min at 65 °C, recorded as H1-PAN. Thirdly, the H2-PAN was obtained through immediately immersing H1-PAN in 2 g/L Ta for 30 min at 65 °C, and being dried in an oven at 80 °C. The filtration performances of PAN, H1-PAN and H2-PAN membranes were evaluated by a 400 mL dead-end filtration module (Model 2

X-

The mass ratio of PVA (X)

The mass ratio of APS (Y)

The mass ratio of H2O

wt. %

wt. %

wt. %

5 6 7 8 9 10 8 8 8 8 8 8 8 8 8 8

1 1 1 1 1 1 1 1 0 0.5 0.75 1.0 1.25 1.5 1.75 2.0

94 93 92 91 90 89 91 91 92 91.5 91.25 91 90.75 90.5 90.25 90

Support types

PAN

H1-PAN H2-PAN H2-PAN

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permeate side was below 0.1 kPa. The effective membrane area was 12.57 cm2. The permeation was collected by a liquid nitrogen cold trap and analyzed by gas chromatography (GC-2014C, SHIMADZU). The chromatograph operated with column and detector temperatures were 120 °C and 150 °C. Then, a gasification chamber temperature of 160 °C, a bridge current of 100 mA, a carrier gas flow rate of 40 mL/min and an injection volume of 1 μL were set respectively. The analysis methods and the standard curves of water and ethanol are shown in fig. S1 (Supplemental Information). As can be clearly seen from the fig. S1, both the linear fit coefficients of water and ethanol concentration were higher than 0.999, indicating that the standard curves obtained were suitable for the following analysis. The permeation flux (J2, g/(m2h)) and separation factor (α) were calculated as:

duration, 500 rpm rotation speed and 100 rpm/s Acc/Dec. That is, the spin-coating machine takes 5 s to achieve a steady speed of 500 rpm, and then runs for 30 s. In the end, the membranes were cross-linked by heating in oven at 80 °C for 30 min. On the other hand, the homogeneous membranes were prepared via scraping the same casting solution on the clean glass, which can be used for characterization. 2.4. Characterizations The membrane morphology was analyzed by scanning electron microscopy (SEM, QUANTA FEG450), and the membrane cross-section should be brittle using liquid nitrogen. The surface roughness of all the membranes were investigated by atomic force microscope (AFM, Bruker, USA). The images were calculated by the software to obtain the average roughness (Ra), root average square of Z data (Rq) and the maximum difference between the highest peak and the lowest valley (Rz) at a scan area of 3.0 × 3.0 μm. The crystalline structures of PVA membranes were confirmed by Xray diffraction (XRD, Switzerland ARL/X, TRA), and the experimental conditions of 2 theta were scanned from 5° to 80°. Fourier transform infrared spectra (FT-IR, Nicolet is50) was used to study the chemical structure of PVA membranes, which were investigated by a BRUKER Vertex 50 FTIR spectrometer equipped with a horizontal attenuated transmission accessory. In testing, KBr was selected for reference. The scanning range, resolution and frequency were 4500–400 cm−1, 4 cm−1 and 32, respectively. Stress–strain curves of the membranes were obtained using an electronic stretching machine (AGS-X, 10KN, STD, SHIMADZU) at room temperature with a strain rate of 10 mm/min. The dimension of the samples was approximately 10 × 50 mm. Drop Shape Analyzer (DSA, OSA200-T) was used to test static contact angle. The droplet size was 2 µL. To reduce errors, five different places were selected in each test and the average value was calculated.

J2 =

Q A3 t2

(4)

α=

YW / YE XW / XE

(5)

where Q (g) is the mass of the permeate during operation time t2 (h), and A3 (m2) is the effective membrane area. Y and X are the mass fractions of water (W) and ethanol (E) in permeate and feed, respectively. 3. Results and discussion 3.1. Interface structure regulations between PAN supports and PVA layers To regulate the interface physicochemical structure between PAN supports and PVA layers, three different PAN supports were prepared. The preparation strategy was shown in Scheme 1. The reaction mechanisms between PAN and NaOH has been studied by various analytical techniques. As shown in Scheme 1, 13C NMR studies showed that the eCN groups of PAN were transformed by NaOH into more hydrophilic functional groups, such as eCONH2 and eCOO-Na+ [30,31].

2.5. Viscosity of casting solution

3.1.1. Characterization of three different PAN supports From table 2, it is clearly that the hydrolysis of PAN in NaOH can effectively enhance the surface hydrophilicity of PAN (the contact angles dropped from 50.8° to 22.8°) and the porosity (from about 46.97% to 78.98%). The direct result was that the PWF increased approximately 2 times as well. After further modification by Ta, from Fig. 1(f), the membrane density increased evidently. Then, the PWF decreased to one quarter of H1-PAN and one seventh of H2-PAN. The digital photos (Fig. 1(a-c)) and SEM surface images (Fig. 1(d1f1)) of PAN, H1-PAN and H2-PAN described the change of surface morphologies clearly. Besides, the cross-section images (Fig. 1(d2-f2)) of PAN, H1-PAN and H2-PAN also explained that the modified membranes had complete and uniform cross-section images with no obvious defects. In order to further confirm the modification extents by NaOH and Ta, the mass changes were measured, which was marked as Δm1 and Δm2 (Fig. 1(a-c)). The surface etching of PAN by NaOH could increase the mass, while surface modified by Ta made the mass significantly lighter than initial PAN. This could be used to explain why the surface from PAN, H1-PAN to H2-PAN became denser and smoother. Moreover, the AFM specific surface roughness parameters (including Ra, Rq and Rz) were determined and the results are shown in Fig. 2 and table 3. From table 3, compared with original PAN membrane, the roughness of H1-PAN was increased greatly, while the H2-PAN deceased sharply close to that of the original PAN. However, compared with Fig. 2 (a) and Fig. 2(c), the H2-PAN membrane was denser than PAN. From the above results, the following two conclusions could be obtained: (1) NaOH played a role of strong oxidation etching, making the surface of PAN uneven, and forming a fish-scale-like gully surface. (2) Ta could react further with fish-scale-like fragments, resulting in a smooth surface, and further increasing the surface densification. The

The viscosity of PVAX-Y casting solutions were measured by digital viscometer (DV-2, Jing Tian, China), which was produced by Shanghai Jingtian Electronic Instrument Co., Ltd. The sample was tested at 25 °C, SPL3 rotor, 100.0 RPM, and 20 ~ 80% percentage torque (%). Each test should be be stable, and took three values to calculate the average. 2.6. Swelling degree and gel fraction The swelling degree (SD, %) of the membranes were calculated by the following equation: W −W SD = SW D × 100% (2) D The dry free-standing membrane was weighted (WD), which was heated in oven for 2 h. After soaking in 95 wt% ethanol aqueous solution at 75 °C for 48 h, the swollen membrane was reweighted (WS) after quickly removing the surface liquid [5]. Each measurement was carried out three times to reduce error and the average value was calculated. The gel fraction (G) of the membranes [21] were calculated by the next equation: M G = M1 × 100% (3) 0 Firstly, the dry PVA homogeneous membrane was weighted (M0). After condensing and re-fluxing for 12 h at 100 °C, the gelatinous membrane was reweighted (M1) after heating in oven for 6 h at 60 °C. Each measurement was carried out for three times. 2.7. Pervaporation separation experiments The pervaporation separation equipment was described previously [3], which was self-built in our laboratory. Feed was circulated at a flow rate of 3.0 mL/min under normal pressure, and the pressure of 3

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Hydrogen bond

Scheme 1. The proposed modification mechanism of PAN in NaOH and Ta at 65 °C.

mechanism of PAN in NaOH and Ta at 65 °C in Scheme 1 was reasonable. As we know, the eCOOH can be introduced into the PAN [30,31,34], that is, the eCN in PAN will be replaced by eCOOH successfully, which proved that the hydrophilicity of H1-PAN was improved (Table 2). Furthermore, the introduction of Ta, which is rich in eOH group, caused a slight decrease in hydrophilicity conversely because of the benzene ring in Ta and formation of ester bonds (eOeC] O). Moreover, from Scheme 1, it is implied that hydrogen bonds can also be formed in large quantities between eCOOH, eCN and eOH groups.

Table 2 The performance of PAN, H1-PAN and H2-PAN membranes. Membranes

Contact angle/(°)

PWF/(L/(m2·h))

Thickness/(cm)

Overall porosity/(%)

PAN H1-PAN H2-PAN

50.8 ± 0.4 22.8 ± 0.1 31.8 ± 0.1

42.78 ± 0.15 79.61 ± 0.19 12.25 ± 0.11

0.016 0.017 0.015

46.97 ± 0.24 78.98 ± 0.17 57.34 ± 0.15

surface roughness of H2-PAN membrane was reduced, thereby the hydrophilicity decreased slightly. However, due to a lot of eOH in Ta, the hydrophilicity of H2-PAN was also far better than that of PAN. To investigate the changes of surface chemical structures, the infrared spectra of the PAN substrates before and after modified were compared in Fig. 3. The characteristic peaks at 2243 cm−1 and 1452 cm−1, which were due to the eCN group. Meanwhile, the peak at 1714 cm−1 attributed to the eC]O group could not be seen clearly in H1-PAN, which suggested that the PAN substrate was modified by NaOH. The peaks at 2965 cm−1 and 1666 cm-1was CeH stretching vibration and eNH2 stretching vibration in eCONH2 group, respectively. However, after further modification by Ta (H2-PAN in Fig. 3), the peak at 1714 cm−1 ascribed to eOeC]O groups appeared again. Furthermore, the peak intensity became stronger and the peak position shifted slightly to the left. Meanwhile, the new peak at 1472 cm−1 was assigned to skeleton stretching vibration of benzene ring, which also proved that Ta was successfully introduced to the surface. Moreover, the peak at 3294 cm−1 in Fig. 3 indicated that the membrane surface has large numbers of eOH and eNH groups [32,33]. From above analysis, it’s inferred that the proposed modification

3.1.2. Effect of different PAN supports on separation performances To study the effect of PAN, H1-PAN and H2-PAN on the pervaporation performances, the total flux and the content of permeation water were tested in 95 wt% ethanol dehydration experiments. As shown in Fig. 4, the total fluxes decreased form 259.9, 115.9 to 103.8 g/ (m2h) and the permeation water contents increased from 95.80, 99.41 to 99.99 wt% for the PAN, H1-PAN and H2-PAN membranes, respectively. From above surface characterizations (Fig. d1–f1), from PAN, H1-PAN to H2-PAN, the supports densification increased obviously. Moreover, the adhesion or interface reaction between the modification substrate and PVA casting solution was enhanced (Scheme 1), which made non-selective interface pores less. Namely, the compactness and high-performance stability of the pervaparation membranes surface were improved due to the stepwise modification of NaOH and Ta (Fig. 1(d–f)), which could react with PVA layers. 3.2. Characterization of self-aggregation crosslinked PVA membranes APS was selected to initiate the self-aggregation crosslinking 4

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Fig. 1. (a-c) digital photos of PAN, H1-PAN and H2-PAN membranes, and SEM surface (1) and cross-section (2) images of (d) PAN, (e) H1-PAN and (f) H2-PAN.

Fig. 2. AFM images of membrane top surfaces: (a) PAN; (b) H1-PAN; (c) H2-PAN. 5

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%, respectively. Besides, the typical peak at 19.7° in Fig. 5(b) was ascribed to self-aggregation crosslinked PVA membranes. As the increasing in PVA fractions, the peak strength of self-aggregation crosslinked PVA membranes gradually weakened. It is implied that the crystallinity of PVA membranes decreased as well. However, the APS content in casting solution was constant. As the amount of PVA increased further, the added APS amount was not sufficient to crosslink PVA completely, thereby reducing the viscosity of the PVAX-1.0 casting solution (Table 4). The decreases in viscosity is negative for membrane density, affecting the properties of the membranes. Therefore, the PVA8-1.0 membrane owned a suitable membrane performance.

Table 3 The top surface roughness parameters of PAN, H1-PAN and H2-PAN membranes. Membranes

PAN H1-PAN H2-PAN

Roughness parameters Ra (nm)

Rq (nm)

Rz (nm)

10.60 ± 1.41 184.52 ± 5.21 11.53 ± 1.02

13.83 ± 1.22 226.17 ± 10.14 14.84 ± 1.52

143.05 ± 9.17 1621.54 ± 27.82 121.33 ± 2.74

3.2.2. Effect of the APS mass contents Fig. 6(a) indicated the equilibrium swelling behavior of PVA8-Y membranes in 95 wt% ethanol-water solution. With the increase of APS, the swelling degree of crosslinked PVA membranes in ethanol solution decreased gradually. This was due to the self-aggregation crosslinking of PVA initiated by APS, which reduced the flexibility of PVA chains and decreased the swelling degree of the composite membranes in ethanol aqueous solution. When the mass fraction of APS was increased from 0 to 1.25 wt%, the swelling degree of PVA8-Y membranes in ethanol aqueous solution dropped sharply. Then, when the content of APS increased continuously, the self-aggregation crosslinking degrees changed slowly, which made the swelling degrees of the membranes trend to be stable. This results was consistent with the change trend of the crystallinity in Fig. 5(b). As we know, PVA is a kind of water-soluble polymer material. When PVA is crosslinked, the crosslinked part is insoluble in water. Then, the gel fraction of the product is generally modeled in the chain crosslinking system. Thus, the degree of crosslinking can be characterized by measuring the solubility of PVA membrane in hot water. The effect of APS content in casting solution on the gel fraction of cross-linked PVA8Y membranes was provided in Fig. 6(b). It can be seen that with the increase of APS, the gel fraction of cross-linked PVA8-Y membranes increased sharply. When the APS mass fraction was more than 1.0 wt%, the gel fraction tended to be stable, which was consistent with the swelling properties of the PVA8-Y membranes (Fig. 6(a)). Due to the free radical crosslinking reaction of PVA induced by APS, PVA was insoluble in water. The higher the crosslinking degree, the less PVA soluble in water. These resulted in higher gel fractions of self-aggregation PVA membranes directly. The mechanical properties of the membranes were described in Fig. 7(a). With the increasing of APS content from 0.5 to 1.25 g, both the tensile strength and tensile modulus of PVA8-Y membranes were greatly improved compared with the pure PVA8-0 membrane. Further increasing in the APS content would cause a decrease in the mechanical properties. Also, Fig. 7(b) reflected the effect of APS content in casting solution on the pervaporation performance of cross-linked PVA8-Y/H2PAN membranes. When the APS content increased from 0 to 0.5 g, the total flux varied from 62.9 to 31.6 g/(m2h), while the water mass fraction in permeation was increased from 95.90 to 97.02 wt%. However, as the APS content increased to 0.75 g, the total flux and permeation water content reached 87.0 g/(m2h) and 99.20 wt%, respectively. Furthermore, the fluxes of crosslinked PVA8-Y membranes increased continuously when the APS content changed from 0.75 g to 1.25 g. Then, the total fluxes decreased and tended to stabilize when the APS contents were more than 1.25 g. Interestingly, the permeation water contents changed from 99.20 wt% to 99.99% when the APS contents increased from 0.75 g to 2.0 g. In the chromatographic analysis, there was only a water peak in the sample on the permeate side, and no characteristic peak of ethanol appeared (Fig. S2 in the Supplemental Information). In the crosslinked membranes, the APS played the following major roles. Firstly, the self-crosslinking reactions of PVA initiated by APS decreased in the crystallinity of PVA (Fig. 8) and increased the amorphous region in PVA. Besides, the cross-linking reactions made the amorphous region denser, reducing the creativity of

Fig. 3. FT-IR analysis spectra of PAN, H1-PAN and H2-PAN membranes.

Fig. 4. Effect of PAN, H1-PAN and H2-PAN on the pervaporation performance of PVA8-1.0 membranes.

reaction of PVA. It is suggested that a free radical (NH4SO2-O*) arising from APS would abstract a tertiary hydrogen atom from the PVA chain (at a CeH linkage) to yield a polymeric radical. This radical reacts with eOH groups giving rise to the formation of ether bonds between PVA chains, leading to the crosslinking of PVA [19,20]. The proposed selfaggregation crosslinking mechanisms of PVA initiated by APS were given in Scheme 2. 3.2.1. Effect of PVA contents The effect of PVA mass fraction on membrane performances was investigated systematically, while the APS initiator was fixed at 1.0 wt %. The results were shown in Fig. 5. From Fig. 5(a), the increase in PVA mass fraction, the chains of polymer PVA vary in length, which affects the membranes density and surface hydrophilicity, thereby changing pervaporation performance (Fig. 5(c)). From Fig. 5(a, c), the PVA8-1.0 membrane had the best hydrophilicity, which directly resulted in the best pervaporation performances. The relative flux, separation factor and permeation water content were 259.9 g/(m2h), 1191 and 95.80 wt 6

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Scheme 2. The proposed self-aggregation crosslinking mechanisms of PVA initiated by APS.

3.3. Long-term operating stability of the membranes

the PVA chain and the swelling degree of PVA to ethanol-water solution, which was fully illustrated in Fig. 6(a). Secondly, when the contents of APS were low, the crosslinking degrees of PVA were also not high, and the crystallinity was not obvious. Thus, the permeation fluxes were only slightly increased. When the APS mass fractions were greater than 1.0 wt%, the crystallinity changed more obviously. The amorphous region of the membrane increases more, and the permeation flux of the membrane increased rapidly. When the mass fraction of APS reached 1.25 wt%, the membrane has the highest permeation flux. However, the continuous increase in APS contents could no longer change the PVA crystallinity. Then, the dense structure produced by membrane cross-linking resulted in a decrease in membrane flux.

Long-term operating stabilities are very important and critical in membrane applications. The long-term separation performance of PVA8-1.25/H2-PAN for 95 wt% ethanol dehydration at 75 °C was examined and shown in Fig. 9. Both the total flux and the water ratio in permeation remained stable for running up to 120 h. The total flux exhibits a little initially decline mainly due to the relaxation of pure PVA chains and finally stabilized at about 117 g/(m2h). Importantly, the water ratio in permeation always remained beyond 99.99 wt%. After 120 h, the water ratio in feed was almost zero. It means that the ethanol solution can be concentrated to over 99.99% using this polyacrylonitrile-based self-aggregation crosslinked poly (vinyl alcohol) 7

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36.2±0.6

θ

Fig. 5. Effect of PVA mass fraction on (a) contact angle, (b) XRD and (c) the pervaporation performance of PVAX-1.0/PAN membranes. Table 4 The viscosities of different PVAx-Y casting solutions. Samples

Viscosity/(mPa⋅s)

Percentage torque/(%)

Samples

Viscosity/(mPa⋅s)

Percentage torque/(%)

PVA5-1.0 PVA6-1.0 PVA7-1.0 PVA8-1.0 PVA9-1.0 PVA10-1.0 PVA8-0

96.0 ± 0.3 104.6 ± 0.7 152.6 ± 0.5 271.6 ± 0.7 251.6 ± 0.5 242.1 ± 0.6 212.7 ± 1.1

20.2 28.6 25.4 22.7 21.7 20.9 20.3

PVA8-0.5 PVA8-0.75 PVA8-1.0 PVA8-1.25 PVA8-1.5 PVA8-1.75 PVA8-2.0

294.9 287.8 271.6 262.3 260.6 253.6 247.4

21.6 22.2 22.7 25.6 21.3 20.6 22.5

± ± ± ± ± ± ±

0.3 0.2 0.5 0.7 0.4 0.3 0.3

± ± ± ± ± ± ±

1.1 0.5 0.7 0.9 0.6 0.8 0.9

± ± ± ± ± ± ±

0.8 1.0 0.7 0.9 0.5 0.4 0.8

interface physicochemical structure regulations for PAN supports. (1) Three different PAN supports were prepared through chemical modifications. During the modification, NaOH played a role of strong oxidation etching, forming a fish-scale-like gully surface. Also, Ta reacted further with fish-scale-like fragments, resulting in a smooth surface, and further increasing the surface densification. The hydrophilicity of H2-PAN membrane was slightly lower than that of H1-PAN. However, due to a lot of eOH on the surface of Ta, the hydrophilicity of H2-PAN was still far higher than that of PAN. After that, a PVA separation layer was casted onto the different PAN support. The compactness and high-performance stability of the pervaparation

pervaporation dehydration membrane. Also, it can be easily seen from Fig. 9 that the PVA8-1.25/H2-PAN membrane run stably over 120 h. It confirmed that the membrane showed good thermal and mechanical stabilities, which translated to a high swelling resistance.

4. Conclusions A new highly water-selective polyacrylonitrile-based self-aggregation crosslinked poly (vinyl alcohol) pervaporation dehydration membrane was prepared by combining self-aggregation crosslinking reactions for PVA through ammonium persulfate (APS) initiations and 8

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(a)

(b)

Fig. 7. Effect of APS mass addition on the (a) tensile strength and (b) pervaporation performance for PVA8-Y/H2-PAN membranes. Fig. 6. Effect of APS mass addition on the (a) swelling degree and (b) gel fraction for PVA8-Y/H2-PAN membranes.

membranes surface were improved due to the stepwise modification of NaOH and Ta, which could react with PVA layers. (2) For the PVA layers, ammonium persulfate (APS) was added to initiate the self-aggregation crosslinking reaction among PVA chains. The crosslinked PVA membranes displayed good mechanical, and antiswelling properties. The total flux and water content ratio in permeation were 117.6 g/(m2h) and 99.99 wt% respectively by using the PVA81.25/H2-PAN membrane. After running 120 h, the membranes still displayed a good stability for the ethanol dehydration in the pervaporation process. Importantly, the water ratio in feed was almost zero after 120 h. It means that the ethanol solution can be concentrated to over 99.99% using this polyacrylonitrile-based self-aggregation crosslinked poly (vinyl alcohol) pervaporation dehydration membrane.

θ

Declaration of Competing Interest

Fig. 8. Effect of APS mass addition on the XRD spectra for PVA8-Y/H2-PAN membranes.

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 “Polyacrylonitrile-supported self-aggregation crosslinked poly (vinyl alcohol) pervaporation membranes for ethanol dehydration (authors: Meisheng Li, Jie Wang, Shouyong Zhou, Ailian Xue, Feiyue Wu, Yijiang Zhao) ”.

Acknowledgements The authors are grateful for the financial supports of National Natural Science Foundation of China (21878118, 21978109), Jiangsu Province Natural Science Foundation (BK20171268), the Key University Science Research Project of Jiangsu Province (18KJA530003, 19KJA150009), Jiangsu Province Qing Lan Project.

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Fig. 9. Long-term operating stability of PVA8-1.25/H2-PAN for the dehydration of 95 wt% ethanol/water mixtures at 75 °C.

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