Interpenetrating Nanofibrous Composite Membranes for Water

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Interpenetrating Nanofibrous Composite Membranes for Water Purification Xiangxiang Liu, Hongyang Ma, and Benjamin S. Hsiao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00565 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 17, 2019

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Interpenetrating Nanofibrous Composite Membranes for Water Purification

Xiangxiang Liua, Hongyang Maa,b,*, and Benjamin S. Hsiaob,* a State

Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

b Department

*

of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, USA

Corresponding authors

Phone: +1(631)229-6899 (H.M.); +1(631)632-7793 (B.S.H). Fax: (631)632-6518 E-mails: [email protected] (H.M.); [email protected] (B.S.H.)

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Abstract Free-standing interpenetrating nanofibrous composite membranes were fabricated by two-nozzle electrospinning approach, where polyvinyl alcohol (PVA) and polyacrylonitrile (PAN) nanofibers were integrated. The membrane contained two components: PVA nanofibers with thicker diameter as the skeleton scaffold and PAN nanofibers with thinner diameter as the functional scaffold. The geometrical characteristics of the composite membrane were determined in terms of fiber diameter, pore size, and their distributions, which were affected by electrospinning parameters. The mechanical properties and durability of the composite membrane,

containing

crosslinked

PVA

skeleton

(with

glutaraldehyde

(GA))

and

interpenetrating ultra-fine PAN scaffold, were drastically enhanced as compared to the single component membrane. A model of interpenetrating nanofibrous networks was proposed to describe the structural feature of the composite membrane. The resulting composite membrane exhibited high water permeability as well as high adsorption of chromium (VI) from contaminated water, after functionalization of the PAN component by surface grafting of positively charged species.

Keywords: interpenetrating nanofibrous network; electrospinning; microfiltration membrane; heavy metal ion; adsorption.



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1. Introduction

Contamination of water resources by toxic heavy metal ions, such as chromium (VI) ions, is an urgent problem for human health. Chromium (VI) ions are typically produced from mineral, textile and paint industries, and they impose high toxicity and carcinogenicity and should be addressed on a priority basis. 1 Electrospun nanofibrous membrane with quasi-three dimensional structure exhibited great advantages over conventional membranes for water purification. 2,3 In specific, the high porosity and highly interconnected porous structure of the electrospun membrane offer low hydraulic resistance for water transportation, and thus high throughput and low energy consumption. Meanwhile, the unique characteristics of electrospun membrane, such as high surface area, adjustable pore geometry, and ease to be functionalized, can further enable the creation of efficient filtration membranes with high adsorption capability.

4-6

Recently,

electrospun nanofibrous membranes have been demonstrated as effective media for a wide range of filtration applications, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, and membrane distillation. 7-10

A two-nozzle electrospinning setup has been demonstrated to fabricate nanofibrous composite membrane system, which offers great advantages over conventional single-jet electrospinning process. 7, 11 There are some earlier works dealing with the use of multi-jet multicomponent process, although the membrane applications were not designed for filtration. These works included the constructions of (1) artificial tissue scaffolds by mixing nano- and microscaled polyurethane (PU) and poly(ethylene oxide) (PEO) fibers;

11

(2) macro-porous scaffolds

by leaching out one of the two components, such as PEO from PU/PEO or poly(ε-caprolactone)


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PCL/PEO composite fibrous mat;

12

(3) composite nanofibrous yarn products;

13

(4) functional

composite media, such as pH-sensors, uranium ion-absorbents, superhydrophobic films, as well as catalytic, biologically active mats.

14-18

The basic design of these composite membranes is

similar, but each nanofibrous component would play a different role. In specific, one component usually serves as a scaffold and the other component provides functionality to the system. In the current study, we demonstrate that the two-nozzle electrospinning approach can effectively fabricate filtration membranes, suitable for water purification, whereas the interpenetrating nanofibrous networks offer composite membranes with the following advantages: (1) enhanced mechanical properties associated with the direct usage of electrospun nanofibrous membranes for practical water filtration applications under medium to high pressure; (2) adjustable pore size and distribution associated with the increase in filtration efficiency; and (3) selective functionalization of one nanofibrous component of the membrane to achieve high retention of heavy metal ions.

In this study, free-standing interpenetrating nanofibrous composite membranes, containing polyvinyl alcohol (PVA) and polyacrylonitrile (PAN) nanofibers, were fabricated by the two-nozzle electrospinning process. By changing the concentration of each component, the composition of electrospun PVA/PAN membranes was adjusted systematically. The mechanical properties of the composite membrane could be drastically increased (as seen by tensile testing), which indicated the enhanced durability. The geometrical characteristics of the electrospun composite membrane, including fiber diameter, pore size, and its distribution were correlated with the fabrication parameters. In this membrane system, PVA nanofibers after crosslinked with GA were served as a skeleton, which provided the mechanical strength to the membrane without



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additional substrate. In a way, we “implanted” the substrate into the nanofibrous layer of the membrane. Finer PAN nanofibers were further modified by surface-grafting to provide positively charged functional groups. The final nanofibrous composite membranes were challenged with chromium (VI) contaminated water to test the water permeability and retention of chromium ions.

2. Experimental

2.1

Materials

Polyacrylonitrile (PAN) (Mw = 1.5 105 Da) and polyvinyl alcohol (PVA) (Mw = 7.3 × 104 Da, 98% hydrolyzed) were purchased from Macklin Incorporation (China) and Aladdin Industrial Corporation (China), respectively. Triton X-100 surfactant was purchased from SigmaAldrich. Glutaraldehyde (GA, 50 wt% in water) was received from Macklin Incorporation. Polyvinylamine (PVAm, Mw = 3.4 × 104, 90% hydrolyzed) was provided by Jinjile Chem, Ltd. (Shanghai). Other chemicals, including acetone, N,N-dimethyl formamide (DMF), and hydrochloric acid (36.5 % aqueous solution), were obtained from Beijing Chemical Works. All chemicals were used as received without purification unless noted.

2.2

General electrospinning process

Electrospun PAN, PVA, and PVA/PAN composite membranes were prepared by a custom build two-nozzle electrospinning setup (Supporting Information Figure S1). In brief, 5 ACS Paragon Plus Environment

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PAN solutions with different concentrations (8 wt% and 10 wt%) were prepared by dissolving appropriated amounts of PAN powder in DMF at 60 oC oil bath for 24 h. Similarly, PVA aqueous solution with different concentrations (8 wt%, 10 wt%, 12 wt%, and 13 wt%) were prepared at 90 oC (120 oC for 13 wt%) for 48 h, where 0.5 v/w% of Triton-100 was added to assure the homogeneous dissolution. The composition and viscosity of PVA/PAN electrospinning solution systems are illustrated in Table 1.

Table 1 Chemical composition and viscosity of PVA/PAN electrospinning solution systems Samples

PVA solution (H2O, wt%)

PVA solution

PAN solution (DMF, wt%)

(Pa·s)

PAN solution (Pa·s)

PVA12%

12

3.90

0

N/A

PAN10%

0

N/A

10

0.56

PVA8%/PAN10%

8

0.42

10

0.56

PVA10%/PAN10%

10

0.82

10

0.56

PVA12%/PAN10%

12

3.90

10

0.56

PVA13%/PAN10%

13

8.62

10

0.56

PVA8%/PAN12%

8

0.42

12

0.88



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PVA10%/PAN12%

10

0.82

12

0.88

PVA12%/PAN12%

12

3.90

12

0.88

PVA13%/PAN12%

13

8.62

12

0.88

These polymer solutions were uploaded into two syringe pumps on opposite sides of the collecting drum and the delivery rate by each pump was adjusted separately between 12 and 60 L/min, depending on preparation requirement. Typically, the solution delivery rate was fixed at 20 L/min. The electrospinning voltages could also be separately applied to either spinneret in the range of 10 - 25 kV, where 20 kV was chosen in our experiments. The distance between the spinneret and drum could be adjusted between 12 and 16 cm for either side of electrospinning. The inner diameter of the spinneret was 0.6 mm, and the volume of the polymer solution used was 2.6 mL for both PAN and PVA sides. The rotation speed of the drum was kept at 60 rpm and the traveling distance on the drum was set as 12 cm. A closed chamber was used to control the electrospinning environment, where temperature was kept at 25  2 oC and the relative humidity was 20 - 40 %. Electrospun nanofibrous membranes were deposited on an aluminum foil wrapped on the drum, where the membrane was peered off from the foil after preparation.

2.3

Leaching out the PVA nanofibrous component

To explore the structure and morphology of the electrospun PVA/PAN nanofibrous composite membrane, each component including PAN and PVA scaffolds could be leached out 7 ACS Paragon Plus Environment

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by using the solvent extraction method. In this task, we were particularly interested in evaluating of the PAN scaffold morphology. For this purpose, four pieces nanofibrous composite membrane (0.3 g, 7 × 7 mm square) was immersed in 1000 mL of water and the system was heated at 90 oC for 4 h before immersion in fresh water. This treatment was repeated for 3 times. The dried composite membrane was analyzed by elemental analysis to confirm that all PVA nanofibers were removed, leaving behind only the PAN scaffold. The PAN scaffold was dried in an oven at 60 oC for 24 h.

2.4

Crosslinking PVA nanofibers in acetone with GA

To extract PAN nanofibers from the PVA/PAN composite membrane, DMF could be used. Unfortunately, PVA nanofibers could also be dissolved partially in DMF. Therefore, a GA aqueous solution was used to cross-link PVA before the DMF-extraction treatment, as shown in Scheme 1. In brief, 1 g of nanofibrous composite membrane was immersed in 300 mL of acetone solution containing 0.03 mol/L of GA and 0.01 mol/L of HCl. The membrane was remaining in the reaction system for 24 h before washing with a large amount of pure water for 3 times. Subsequently, the membrane was dried in a vacuum oven at 40 oC for 24 h.

O

O

OH

O

O

OH

GA n

OH

H+

Scheme 1 Cross-linking reaction of PVA with GA 8 ACS Paragon Plus Environment


2.5

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Leaching out of PAN nanofibrous component

Similar to the extraction of PVA nanofibers, the removal of PAN nanofibers from the PVA/PAN composite membrane was carried out by immersing 0.3 g of the composite membrane in 500 mL of DMF for 2 h before changing to fresh DMF solvent. This process was repeated for 3 times until all PAN nanofibers were leached out, which was confirmed by the elemental analysis. The membrane was washed with fresh water for 3 times and dried in a vacuum oven at 40 oC for 24 h.

2.6

Membrane characterization

Scanning electron microscopy (SEM) was carried out using a JEOL JSM-7800F microscope (JEOL, Japan) to investigate the membrane morphology. Both cross-sectional and top views of the samples were investigated. The cross-sectioned samples were prepared by fracturing the water-wetted membrane, frozen in a liquid nitrogen bath. The fiber diameter and its distribution of the membrane samples were analyzed through SEM images using the customdeveloped Leika software in our lab (www.dell.chem.sunysb.edu).

A pore-size distribution analyzer (JW-PD200, JWGB Sci. &Tech.) was used to measure the bubble point, mean pore size and pore size distribution. In this test, a wetting agent GQ-16 with a surface tension of 16 dynes/cm was used. 9 ACS Paragon Plus Environment

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Tensile test was conducted by using a tensile apparatus (UTM5205XHD, SUNS, Shenzhen) under asymmetric deformation. All membrane samples were cut into a dog bone shape with dimensions of 50 mm  10 mm. The samples were uniaxially stretched at room temperature. The initial length between the Instron clamps was 30 mm and the stretching rate was maintained at 5 mm/min.

2.7 Grafting of PVAm onto PAN nanofiber surface

A pre-determined amount of GA-crosslinked PVA/PAN composite membrane (0.5 g) was immersed in a 500 mL of PVAm solution (3.0 %) containing 0.5 g of anhydrous AlCl3. The grafting reaction (Scheme 2) was carried out by heating the system at 90 oC for 4 h. The PVAmgrafted membrane was rinsed with DI (de-ionized) water thoroughly to remove unreacted PVAm and the resulting membrane was dried in an oven at 60 oC for 24 h.

PVAm n

CN

AlCl3

m

HN

NH

CN

n-m

p

Scheme 2 Grafting of PVAm onto PAN nanofiber



2.8

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Pure water flux of nanofibrous composite PVA/PAN membrane

A dead-end microfiltration setup was employed to investigate the pure water flux of the membrane. In this study, 50 mL of distilled water was filled up the filtration cell, which was connected to a nitrogen cylinder. A membrane disc with 45-mm diameter was used and the operating pressure was kept at 2.0 psi. The filtration experiments were carried out 3 times, where the average value was taken. The pure water flux was determined by the direct measurement of the permeate volume collected over a certain period and calculated by Eq. (1),

(1) where J (L/m2h) was the pure permeation flux , V (L) was the volume of permeate, A (m2) was the effective membrane area, and t (h) was the collection time.

2.9

Dynamic adsorption of nanofibrous composite PVA/PAN membrane

In this test, a membrane sample disc with 25-mm diameter was latched in a microfiltration cell and loaded on a syringe pump to obtain the breakthrough curve for dynamic adsorption evaluation. The injection rate was set as 0.8 mL/min (100 L/m2h) and the pressure drop was recorded from the pressure gauge. About 100 mL of feed solution (1.0 ppm) was used to determine the adsorption capacity of the membrane. A breakthrough curve was plotted by metal ion concentration vs. volume of the permeation solution. The rejection ratio was evaluated using the Eq. (2),


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(2) where Cp and Cf were the Cr (VI) concentration of permeate and feed solutions, respectively, measured by ICP (Agilent 7700).

3.

Results and Discussion

3.1

Morphology of electrospun nanofibrous composite membranes

The free-standing electrospun nanofibrous composite PVA/PAN membranes were fabricated by a two-nozzle electrospinning setup, where the concentrations of PAN and PVA solutions were 10 wt% and 13 wt%, respectively. Morphologies of the resulting membranes were examined by SEM, and the results are shown in Figure 1.

(A)

2 µm

(B)

2 µm



(C)

(D)

2 µm

10 µm

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Figure 1 SEM images of electrospun (A) 10 wt% of PAN, (B) 13 wt% of PVA, (C) and (D) PVA13%/PAN10% nanofibrous composite membranes with top and cross-sectional views, respectively.

In Figure 1 (A), it was seen that the average fiber diameter of electrospun PAN nanofibers was 145.7 ± 24.8 nm. As a comparison, the diameter of PVA nanofibers was 991.4 ± 133.1 nm (Figure 1 (B)), which was about 7-times larger than that of PAN nanofibers. This could be attributed to the viscosity difference between the PAN and PVA solutions, which were 0.56 Pa·s and 8.62 Pa·s, respectively. Figures 1 (C) and (D) clearly exhibited the interpenetrating morphology of the PVA/PAN nanocomposite membrane, with two distinct fiber diameters (PAN and PVA).

The fiber diameter and its distribution were found to change systematically when the PVA concentration varied from 8 wt%, 10 wt%, 12 wt%, to 13 wt%. The results are shown in Figure 2. Meanwhile, the corresponding viscosities of PVA solutions were also increased from 0.42 Pa·s, to 0.82 Pa·s, 3.90 Pa·s and 8.62 Pa·s, respectively (Table 1). This indicates the 13 ACS Paragon Plus Environment

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viscosity is strongly dependent on the solution concentration, where tremendous viscosity

PVA13%/PAN12%

80

80

PVA12%/PAN12%

0

0

PVA10%/PAN12%

80

PVA8%/PAN12%

0

0

PVA13%/PAN10%

80

80

PVA12%/PAN10%

0

0

PVA10%/PAN10%

80

80

80 0

PVA8%/PAN10%

0

Frequency (%)

difference exits among these solutions.

Frequency (%)

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


0

400 800 1200 Fiber Diameter (nm)

Figure 2 Fiber diameter distribution in PVA/PAN nanofibrous composite membranes, as functions of composition.

It was seen that the fiber diameter of PVA in PVA/PAN nanofibrous composite membranes increased rapidly with increasing PVA concentration and viscosity of solutions. When the PAN concentration was set at 10%, the fiber diameter was 171.58 ± 48.20 nm, 410.78



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± 63.82 nm, 715.63 ± 127.08 nm and 1026.92 ± 196.75 nm for the PVA concentration of 8 wt%, 10 wt%, 12 wt% and 13 wt%, respectively. However, the corresponding fiber diameter of PAN increased from 171.6 ± 48.2 nm to 185.4 ± 32.3 nm, when the PAN concentration changed from 10 wt% to 12 wt%. The difference of fiber diameter was negligible between PVA and PAN nanofibers when the PVA concentration was 8 wt% and the PAN concentration was either 10 wt% or 12 wt%. In fact, in SEM images of this composite, we could not distinguish which nanofiber was PAN or PVA. The resulting fiber diameter exhibited a single distribution, as shown in Figure 2. However, when the PVA concentration increased to 10 wt%, 12 wt%, and 13 wt%, two separated fiber diameter distributions were seen (Figure 2), where a new fiber diameter distribution was separated from the original distribution and shifted gradually to higher values depending on the PVA concentration. Partial overlap was still observable when the PVA concentration was 10 wt% and that of PAN was 10 wt% or 12 wt%, where the fiber diameter distributions were completely separated when the PVA concentration was above 12 wt%. The fiber diameter distribution became broader, when the PVA fiber diameter increased. It was interesting to note that the PVA fiber diameter was less affected by changing the PAN concentration of solutions, which indicated that the two nozzles worked independently. It was also evident that the fiber diameters and their distributions in pure PAN and PVA nanofibers were quite similar to those of the PAN and PVA nanofibrous components in the composite membranes. Thus, the fiber diameters and their distributions of PAN and PVA nanofibrous components could be adjusted separately to meet the requirements of different applications. In this study, we designated the thinner PAN nanofibers for functionality (to adsorb metal ions) and larger PVA nanofibers for providing the mechanical strength to the composite membrane.


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It is well known that the pore size of a nonwoven nanofibrous membrane depends empirically on its fiber diameter, where the larger fiber diameter results in bigger pore size and broader pore size distribution.

19

Therefore, it was interesting to see how does the varying fiber

diameter affect on the pore size and pore size distribution of a nanofibrous composite membrane with an interpenetrating structure. The mean pore size and pore size distribution of electrospun PVA/PAN nanofibrous composite membranes prepared with different concentrations of PAN and PVA solutions are shown in Figure 3. It was seen that the mean thicknesses of these membranes were about 60.0 μm, while the porosities of the membranes remained about the same at 84.6 ± 0.2 %.

500

500 (A)

(B)

400

400 300 200 PVA8%/PAN10%

100

PVA10%/PAN10%

Frequency (a. u.)

Frequency (a. u.)

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


300 200 PVA8%/PAN12% PVA10%/PAN12% PVA12%/PAN12% PVA13%/PAN12%

100

PVA12%/PAN10%

0 0.2

PVA13%/PAN10%

0.4

0.6 0.8 Pore size (m)

1.0

1.2

0 0.2

0.4

0.6 0.8 Pore Size (m)

1.0

1.2

Figure 3 Mean pore size and pore size distribution of electrospun PVA/PAN nanofibrous composite membranes: (A) 10 wt% of PAN and (B) 12 wt% of PAN.

In Figure 3, the mean pore size of electrospun PVA/PAN nanofibrous composite membranes was found to increase with increasing PVA concentration, when the concentration of



Page 18 of 36

PAN solution was fixed. This trend was similar to the increase in PVA fiber diameter with the PVA concentration. Thus, an empirical relationship between the mean fiber diameter and mean pore size could be established. It was seen that the concentrations of PVA and PAN exhibited different capabilities to affects the pore size of the composite membranes. The mean pore size of the nanofibrous composite membranes ranged from 0.49 ± 0.02 μm to 0.77 ± 0.02 μm (i.e., a 57 % increase), when the diameter of PAN nanofibers changed from 171.6 ± 48.2 nm to 1026.9 ± 196.8 nm (i.e., a 498 % increase) at the constant PAN concentration of 10 %. However, the mean pore size changed drastically from 0.49 ± 0.02 μm to 0.63 ± 0.02 μm (i.e., a 30 % increase), when the diameter of PAN nanofibers changed only from 171.6 ± 48.2 nm to 185.4 ± 32.3 nm (i.e., a 8 % increase) at the constant PVA concentration of 8 %. The same trend was also observed from the maximum pore size and most probable pore size (Figure S2, Supporting Information). Consequently, we could conclude that the pore size of the nanofibrous composite membranes was mainly affected by the diameter of PAN nanofibers.

To further investigate the nanofibrous composite membrane structure and learn how the two nanofibrous components interpenetrate each other, we designed a series of experiments to remove one nanofibrous component (i.e., PVA or PAN nanofiber) from the composite membrane and determined the pore size and pore size distribution of the residual membrane. Based on the good solubility of PVA in hot water, especially in the nanofiber format, the nanofibrous composite membrane was immersed in water at 90 oC for 12 h to completely remove the PVA component (the removal was also confirmed by the elemental analysis, as seen in Table S1, Supporting Information). On the other hand, the PAN component was removed from the


Page 19 of 36

nanofibrous composite membrane using DMF, after the PVA component of the composite membrane was crosslinked first (PVA without crosslinking could be partially dissolved in DMF).

500 (A)

Frequency (a. u.)

400 300

PVA8%/PAN10% PVA8%/PAN10%-W PVA8%/PAN10%-DMF PVA10%/PAN10% PVA10%/PAN10%-W PVA10%/PAN10%-DMF PVA12%/PAN10% PVA12%/PAN10%-W PVA12%/PAN10%-DMF PVA13%/PAN10% PVA13%/PAN10%-W PVA13%/PAN10%-DMF

200 100 0 500

0

1

2

3 4 5 Pore size (m)

6

7

(B)

400

Frequency (a. u.)

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


300

PVA8%/PAN12 PVA8%/PAN12-W PVA8%/PAN12-DMF PVA10%/PAN12 PVA10%/PAN12-W PVA10%/PAN12-DMF PVA12%/PAN12 PVA12%/PAN12-W PVA12%/PAN12-DMF PVA13%/PAN12 PVA13%/PAN12-W PVA13%/PAN12-DMF

200 100 0

0

1

2

3 4 5 Pore size (m)

6

7

Figure 4 Pore size distribution of nanofibrous composite membranes after treatment of water and DMF. The nanofibrous composite membranes were prepared by 10 wt% of PAN solution (A) and 12 wt% of PAN solution (B), respectively, and the PVA component of the membrane was crosslinked.



Page 20 of 36

The pore size and pore size distribution of the composite membrane residues, containing only the PAN or PVA component, are shown in Figures 4 (A) and 4 (B), respectively. It was interesting to see that the pore size distribution of the membrane residual moved to a lower value when the PVA component was eliminated, where that moved to a higher value when the PAN component was eliminated. In Figure 2, it was seen from that PVA nanofibers exhibited a larger mean fiber diameter, which defined larger pores in the membrane. It was thus understandable that removal of PVA nanofibers reduced the larger pores in the membrane, where the resulting pore size decreased and the pore size distribution moved to lower values (Figure 4). In a way, the nanofibrous composite membrane lost its skeleton and PAN nanofibers collapsed and decreased the membrane pore size. On the other hand, PAN nanofibers defined smaller pores in the membrane due to the finer fiber diameter. After removing the PAN component, the pore size of the composite membrane increased with increasing PVA fiber diameter. This could also be understood based on the following consideration. In the nanofibrous composite membrane, fine PAN nanofibers interpenetrated into the PVA nanofibrous network, and created ultra-fine nanofibrous network with the PVA scaffold. Once the fine PAN nanofibers were removed, the resulting pore size, defined only by PVA nanofibers, became larger and the pore size distribution moved to higher values. In another words, the pore size of the nanofibrous composite membrane was dominantly contributed by the fine nanofibrous components (i.e., PAN nanofibers). Actually, the weight coefficient of PAN nanofibrous component was 10-times higher than that of PVA component by calculation based on Figure 4. The above structure could be confirmed directly from SEM images of the nanofibrous composite membranes, as shown in Figure 5.


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(A)

(B)

(C)

4 µm

4 µm

(D)

(E)

(F)

2 µm

2 µm

2 µm

4 µm

Figure 5 SEM images of nanofibrous composite membranes of prepared by (A) 12 wt% of PVA solution and 10 wt% of PAN solution (PVA12%/PAN10%), (B) GA-crosslinked PVA12%/PAN10%, (C) cross-sectional view of GA-crosslinked PVA12%/PAN10%, (D) magnified GA-crosslinked PVA12%/PAN10%, (E) PVA12%/PAN10% after water treatment, and (F) GA-crosslinked PVA12%/PAN10% after DMF treatment.

In Figure 5 (A), an interpenetrating nanofibrous network was observed in the membrane, which contained large and fine fibers mingled together. It was seen that the GA-crosslinking treatment did not alter the structure of the skeleton (Figure 5(B)). In Figure 5(C), the crosssectional view of the GA-crosslinked PVA12%/PAN10% exhibited clearly that the PVA component (fiber diameter = 715.6 ± 127.1 nm) formed a supporting skeleton, which the PVA diameter was about 6-times higher than that of the PAN diameter. The PAN component created an ultra-fine nanofibrous network structure, which interpenetrated in the PVA skeleton. The composite structure of the PVA/PAN nanofibrous membrane was further dissected into two 20 ACS Paragon Plus Environment


Page 22 of 36

separate components: the nanofibrous PAN component after leaching out PVA and the PVA nanofibrous component after crosslinking followed by leaching out PAN. These results are shown in Figures 5 (E) and 5 (F), respectively. It was found that PAN nanofibers collapsed and became curving due to the removal of PVA skeleton using the hot water treatment. As a result, the porosity of the resulting PAN residual membrane decreased to 79.5 ± 1.1 % (the porosity of the original PVA/PAN nanofibrous composite membrane was 84.3 ± 0.3 %), i.e., about 5 % decrease in porosity (Figure S3, Supporting Information). However, the morphology of the crosslinked PVA residual membrane exhibited almost no change after the removal of the ultrafine PAN network. For example, the porosity of the PVA residual membrane was 84.5 ± 0.3 %, which was almost the same as that of the original composite membrane. The thicknesses of these membranes were also measured before and after the two leaching treatments. As expected, a notable change was observed in the residual membrane after the removal of PVA, but very little change after the removal of PAN (Figure S3).

Based on the above results, we proposed the two possible models to describe the nanofibrous composite membrane structure. These two model structures (interpenetrating structure and layered structures) are illustrated in Figure 6. (A)

(B)


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Figure 6 Representatives of electrospun nanofibrous composite membranes: interpenetrating structure (A) and layered structure (B).

In both structures, the larger fiber can form the skeleton of the membrane and provide mechanical strength; and the smaller fibers can create a finer network and provide desired functionalities to the composite membrane. Both fibrous components can integrate together, but in a different manner, producing a bi-continuous structure. The interpenetrating structure assumes the two electrospinning processes operate in the same delivery manner, where deposition of each fiber component has the same rate; whereas the layered structure indicates the two electrospinning processes operate almost in a sequential manner. Based on the above results, we conclude that the electrospun nanofibrous composite membrane exhibited the integrated critical structure of Figure 6 (A) under the conditions outlined in our two-nozzle electrospinning approach. This is because the terminal speeds of the two depositing fibers were quite different, but these speeds were significantly higher than that of the collector rotation. The general properties of the resulting composite membranes, including pore size and pore size distribution thus can be adjusted accordingly by fine-tuning each nanofibrous component, although their effects are different as noted earlier.

3.2

Mechanical properties of nanofibrous composite membranes



The mechanical properties of typical electrospun membranes need to be enhanced with a non-woven polyethylene terephthalate (PET) mat for practical filtration under operating pressure 3,20.

However, the poor adhesion between the PET substrate and the electrospun nanofibrous

scaffold could drastically decrease the durability and robust of the membrane

21-23.

One way to

address the issue of adhesion strength is by constructing symmetrical free-standing electrospun nanofibrous membranes with adequate strengths.

The mechanical properties of interpenetrating PVA/PAN nanofibrous composite membranes were evaluated by tensile experiments. The corresponding strain-stress curves are illustrated in Figure 7.

10

PVA nanofibers PAN nanofibers PVA/PAN nanofibers Cross-linked PVA/PAN

8 Stress (MPa)

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

Page 24 of 36

6 4 2 0

0

20

40 60 Strain (%)

80

100

Figure 7 Strain-stress curves of electrospun PVA12%, PAN10%, PVA12%/PAN10%, and cross-linked PVA12%/PAN10% nanofibrous membranes


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The tensile results verified that electrospun PVA/PAN nanofibrous composite membrane exhibited more enhanced mechanical properties than pure PAN nanofibrous membrane. A yield point was observed in electrospun PVA, PAN, and PVA/PAN nanofibrous membranes, which indicated that these membranes probably possessed two types of deformations: sliding between the nanofibers and stretching of the nanofibrous network(s). The sliding deformation should occur before the yielding point, resulting the reorientation of electrospun nanofibers. However, the network stretching should take place after the yield point. The corresponding stress increased continuously until the membrane fracturing, which defined the ultimate tensile strength of the membrane. It was found that the PAN nanofibrous membrane was relatively rigid and the PVA membrane was quite flexible (as indicated by the large elongation-to-break ratio). As a result, the PVA/PAN composite membrane exhibited a higher Young’s modulus than that of PVA membrane and a higher elongation-to-break ratio than that of PAN membrane. However, after GA crosslinking of PVA, the yield point of the membrane disappeared, indicating that the nanofibrous structure became highly immobilized and only stretching deformation of the network was observed. We noted that the ultimate tensile strength and Young’s modulus of PVA/PAN nanofibrous membrane was about 1.5-times higher than that of PAN membrane, but the GA crosslinked PVA/PAN composite membrane exhibited even higher ultimate tensile strength and Young’s modulus (132.7 ± 6.3 and 8.1 ± 0.1 MPa, respectively) that were about 2times higher than those of PAN membrane. Based on the above results, it could be concluded that interpenetrating PVA/PAN nanofibrous composite membranes could be used as freestanding membranes without the support of PET non-woven substrate.

3.3

Filtration performance of nanofibrous composite membranes 24 ACS Paragon Plus Environment


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The selective functionalization of the PAN component of the PVA/PAN nanofibrous composite membrane was conducted, aiming to enhance the functionality of the membrane for water purification. In this work, polyvinylamine (PVAm) was used to functionalize the PAN surface by addition reaction of the amino group (in PVAm) and cyano group (in PAN) to produce an amidine linkage (the reaction was catalyzed with AlCl3). This scheme effectively grafted PVAm onto the PAN nanofiber surface, confirmed by elemental analysis data (Table S2, Supporting Information). 20, 21 The amino groups in the grafted chains should work as adsorption sites to attract negatively charged heavy metal ions, such as chromium (VI) which exhibit high toxicity and carcinogenicity and is commonly found in water sources contaminated by industrial pollutions from mineral, textile, and paint industries.

22, 23

The water permeability of PVA/PAN

composite membranes before and after grafting was depicted in Table 2.

Table 2 Characterizations of nanofibrous composite membranes prepared by 12 wt% of PVA solution and 10 wt% of PAN solution (PVA12%/PAN10%) Membrane

Fiber diameter (PVA/PAN, nm)

Maximum pore size (µm)

Mean pore size (µm)

Porosity (%)

Water peremability (L/m2h/psi)

PVA/PAN membrane

824 ± 138/ 217 ± 50

1.15 ± 0.02

0.83 ± 0.01

84.0 ± 1.0

-

GA crosslinked PVA/PAN membrane

866 ± 79/ 216 ± 33

0.98 ± 0.11

0.67 ± 0.02

79.0 ± 1.0

1368 ± 824

PVAm-graftedPVA/PAN membrane

838 ± 141/ 190 ± 33

0.79 ± 0.01

0.61 ± 0.01

73.0 ± 1.0

1030 ± 28


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The nanofibrous composite membrane has to be cross-linked with GA before use for water purification. It was clear to see from Table 2 that the pore size of the membrane decreased obviously after cross-linking reaction, and even further decreased by surface modification, which however, will increase the rejection ratio of the filtration expectedly. The water permeability of these membranes after chemical modifications was also decreased slightly when compared to that of the original membrane without modification, probably due to the decrease in pore size and porosity. However, the surface-modified composite membrane exhibited still very high water flux (1030 ± 28 L/m2h/psi), indicating that it is a good candidate for microfiltration, capable of removing any particulates with size larger than 0.6 µm.

7, 24

Meanwhile, these

membranes also can adsorb heavy metal ion, such as Cr (VI), from contaminated water.

Both static and dynamic adsorption experiments were carried out to determine the filtration performance of the PVAm-g-PVA/PAN nanofibrous composite membrane. In the static experiment, the adsorption capacity of the membrane approached the equilibrium state after 20 min. The maximum adsorption capability of the composite membrane could be determined as 66.5 mg/g for Cr (VI) (Table S3, Supporting Information), based on the weight of the composite membranes (the PVA nanofibrous component was served as a skeleton support only); and the maximum adsorption capability became doubled, i.e., 133.0 mg/g for Cr (VI), when the weight of PAN nanofibrous component was used instead of the total weight of the composite membrane. The evaluated maximum adsorption capacity of nanofibrous composite membranes was compared with that of reported and commercially available adsorbents, and the results are shown in Figure 8 (A). 22, 23, 25-36



(A)

E-spun/nanocomposite PAN/PVA membrane E-spun absorbents

300 200 100 0

0

0.5 Concentration of Cr (VI)

400 Adsorption Capacity (mg/g)

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

Page 28 of 36

(B)

0.4 0.3 0.2 0.1 0.0

10 20 30 40 50 Absorbents of Cr (IV)/Cr (III)

0

20

40 60 Volume (mL)

80

Figure 8 Static Cr (VI) adsorption capacity of different adsorbents (A) and breakthrough curve of adsorption of Cr (VI) ion (B) using the modified PVA12%/PAN10% nanofibrous composite membrane.

It was interesting to see that the adsorption capacity of PVAm-g-PVA/PAN nanofibrous composite membrane was several times higher than that of most known adsorbents, such as activated carbon. On the other hand, the PVAm-g-PVA/PAN membrane also exhibited excellent adsorption capacity in the dynamic adsorption process, as shown in the breakthrough curve (Figure 8 (B)), while the pristine PVA/PAN membrane showed negligible adsorption. It was seen that a large amount of contaminated water could be purified with the demonstrated nanofibrous composite membrane. For example, a disc of PVA/PAN composite membrane (with 25-mm diameter) could purify up to 40 mL of Cr (VI)-contaminated water at a concentration of 1.0 ppm, where the rejection ratio could remain at ~ 100 %. Meanwhile, the permeation flux could be remained as 100 L/m2h and the corresponding pressure drop was remained at about 0.01 27 ACS Paragon Plus Environment

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MPa. Combining the high filtration performance of the membrane and the cost-effectiveness of the electrospinning production, we argue that the free-standing nanofibrous composite membrane will be a good candidate for industrial wastewater treatment.

4. Conclusions

Free-standing PVA/PAN nanofibrous composite membranes with interpenetrating nanofiber structure have been fabricated using the two-nozzle electrospinning technique. The membrane consisted of integrated PVA and PAN nanofibrous networks, where the PVA component possessed a larger diameter, serving as the skeleton, and the PAN component possessed a smaller diameter, serving as a functional nano-web scaffold for adsorption. The geometric characteristics of the PVA/PAN composite membranes, including fiber diameter, pore size, and their distributions were investigated systematically. By varying the PVA concentration from 8 wt% to 13 wt%, the fiber diameter increased drastically and two types of nanofibers, e.g., PVA and PAN, could be clearly identified. The pore size and pore size distribution could be affected by the diameters of the two nanofibrous components, especially by the component with small fiber diameter (PAN), where the integration of two different fibers narrowed the pore size distribution. Therefore, by adjusting the diameters of two nanofibrous components, the pore size and pore size distribution of the composite membrane could be controlled at a broad range. Further understanding of the integration of two nanofibrous components were achieved by selectively leaching out of one component, allowing the interpenetrating nanofibrous structure in the composite membrane to be confirmed. The mechanical properties of the PVA/PAN 28 ACS Paragon Plus Environment


Page 30 of 36

composite membrane could be drastically enhanced by cross-linking of the PVA component, indicated that free-standing nanofibrous membrane with sufficient mechanical strength could be created. The application of the PAN modified composite membranes, as microfiltration filters with metal adsorption capability was demonstrated. In addition to a high water permeability, the positively charged PVAm-g-PVA/PAN membrane exhibited the adsorption capacity of 133 mg/(g PAN nanofibers) against Cr (VI) ions, which was several times higher than those of commercially available adsorbents, such as activated carbon.

Supporting Information

The Supporting Information is available free of charge on the ACS Publication website. Two-nozzle electrospinning set-up; the pore sizes of nanofibrous composite membranes versus polymer concentrations; porosity and thickness of the membranes before and after water/DMF treatments; elemental analysis data; Langmuir and Freundlich isotherms of the nanofibrous membrane.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51673011), the State Key Laboratory of Organic-Inorganic Composites at Beijing University of


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Chemical Technology (oic-201503004), the Fundamental Research Funds for the Central Universities (buctrc201501), and the National Science Foundation (DMR-1808690).

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Interpenetrating Nanofibrous Composite Membranes for Water

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