Tuning the Performance of Composite Membranes by Optimizing

Kazem Shahidi and Denis Rodrigue. Industrial & Engineering Chemistry Research 2017 56 (5), 1306-1315. Abstract | Full Text HTML | PDF | PDF w/ Links ...
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Tuning the Performance of Composite Membranes by Optimizing PDMS Content and Cross-Linking Time for Solvent Resistant Nanofiltration Yujing Zhang, Haoqin Zhang, Yifan Li, Heng Mao, Guanghui Yang, and Jingtao Wang* School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China ABSTRACT: Herein, a series of composite membranes with optimized solvent permeance and rejection are prepared by combining the advantages of hybridization and cross-linking techniques. Polyethylenimine (PEI) and hydroxyl terminated trifluoride polydimethylsiloxane (PDMS) are cross-linked with trimesoyl chloride as the skin layer, which is isotropic rather than hierarchical. The chain mobility of PEI is inhibited upon hybridization and cross-linking, affording enhanced solvent resistance and thermal/mechanical stabilities. The composite membrane achieves high rejection ability with the rejection of PEG 1000 of about 100%. Additionally, the synergy of hydrophilic PEI and hydrophobic PDMS segments gives acceptable solvent permeances for acetone (up to 2.7 L m−2 h−1 bar−1) and ethyl acetate (up to 1.4 L m−2 h−1 bar−1). The membrane microstructures are facilely tuned by regulating PDMS content and cross-linking time, allowing the efficient optimization of solvent resistant nanofiltration performances. Moreover, the operational stability and the separation of lotus seedpod proanthocyanidins−ethanol/water mixtures are investigated to evaluate the practical application of the composite membrane.

1. INTRODUCTION Solvent resistant nanofiltration (SRNF), an emerging energyefficient separation process, is an excellent candidate for separating and purifying organic mixtures.1−3 As the kernel of SRNF, the SRNF membrane should possess high rejection, acceptable solvent flux, and adequate solvent resistance. A challenge for the as-developed SRNF membranes is to meet the above criteria simultaneously.4 This is because the membranes could only transport either nonpolar or polar solvents, and meanwhile they usually suffer from the inherent drawback of the trade-off effect between flux and rejection.5−7 Presently, the development of alternative SRNF membranes that possess elevated levels of rejection and acceptable solvent permeate flux is in urgent demand for the SRNF technique. To this end, tremendous efforts have been devoted to rectifying the trade-off between permeance and rejection. Among these various attempts, elaboration of the hydrophilic− hydrophobic hybrid membrane by integrating hydrophilic and hydrophobic blocks into one architecture is supposed to be a convenient and efficient approach to overcome the inherent shortcomings of SRNF membranes, owing to the ingenious synergy in physicochemical characteristics and solvent permeation properties.8,9 Moreover, the multidimensional interconnected networks can be toughly constructed to enhance the structural stability of the polymer membrane through a chemical cross-linking reaction.10−12 Recently, a study by See Toh et al.13 described the analogous preparation of PI-(Lenzing P84) based SRNF membranes cross-linked by aliphatic amines. The obtained membranes were stable in polar aprotic solvents with a DMF permeance of 1−8 L m−2 h−1 bar−1 and a MWCO value between 250 and 420 g mol−1. Katrien Vanherck et al. found that cross-linking asymmetric Matrimid based polyimide membranes with p-xylylenediamine endowed membranes with permeabilities up to 5.4 L m−2 h−1 bar−1 and rejections up to 98% for low molecular weight dyes (Rose Bengal and Methyl © 2015 American Chemical Society

Orange) in dimethylformamide, N-methylpyrrolidinone, dimethylacetamide, and dimethyl sulfoxide.14 Chemical cross-linking implies that the polymer chains are covalently bound together to achieve uniform cross-linking throughout the membrane. However, the cross-linking reduces the free volume and chain mobility of the polymer matrix, which raises the transfer resistance for solvent molecules and hence decreases the permeate flux.15−17 It can be envisaged that introducing flexible chain segments would afford the membrane higher fractional free volume.18 With appropriate control over the microstructures, the membranes might exhibit a higher permeability while maintaining selectivity. A more detailed discussion on these aspects can be found in a recent review by Marchetti et al.19 Recently, poly(ether imide) (PEI) has been utilized as a generic membrane material for ultrafiltration and solvent resistant nanofiltration due to its excellent solvent resistance and mechanical/thermal stabilities.20,21 The affluent hydrophilic amine groups on PEI chains can be easily cross-linked.22 Meanwhile, polydimethylsiloxane (PDMS), the most commonly used hydrophobic material, contains a flexible siloxane (SiO) backbone substituted with methyl groups and is reported to be chemically stable in all organic solvents after cross-linking.23 Accordingly, introducing PDMS exerts the function of cross-linking to create hydrophilic−hydrophobic hybrid networks and endows the membrane with a favorable nanofiltration property. In a previous study,8 we carried on a preliminary exploration and attempt on PAN/PEI−TMC− PDMS which introduced PDMS to cross-link with PEI, and the preliminary results were obtained. Despite the relatively high Received: Revised: Accepted: Published: 6175

April 2, 2015 May 17, 2015 May 26, 2015 May 26, 2015 DOI: 10.1021/acs.iecr.5b01236 Ind. Eng. Chem. Res. 2015, 54, 6175−6186

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Industrial & Engineering Chemistry Research

(0.05 g) was prepared and then dip-coated onto the surface of PAN support for 3 min at 25 °C to obtain a thin and defect-free PEI layer. Afterward, a certain amount of PDMS was dissolved into n-hexane solution (X%, 50.0 g) with cross-linker (0.5 g, TMC) under stirring for 2 h until a homogeneous solution was obtained. After degassing, the above solution was cast onto the PEI-coated PAN support for a certain time to perform the interfacial polymerization and the cross-linking reaction. Finally, the resulting membranes were dried in air atmosphere for 10 min and then in a 60 °C oven for another 2 h. The composite membranes were designated as PAN/PEI−PDMS-X % and PAN/PEI−PDMS-Ys, where X (X = 0.5, 1.0, 1.5, and 2.0) represented the PDMS content in n-hexane solution and Y (Y = 30, 60, 90, 120, and 180) represented the cross-linking time. It should be noted that, during the investigation of the influence of PDMS content, the cross-linking time was set to 120 s, while the PDMS content was set to 1.0% during the investigation of the influence of cross-linking time. 2.3. Characterization of the Membranes. Fourier transform infrared (FTIR) spectra of the membranes were collected using a Nicolet MAGNA-IR 560 instrument in the range of 4000−700 cm−1 with the resolution of 4 cm−1 at room temperature. The surface morphologies of the membranes were observed with a scanning electron microscope (SEM, JSM7500F) instrument after being freeze-fractured in liquid nitrogen and then sputtered with Au. The thermal properties of the membranes were probed by thermogravimetric analysis (TGA, TGA-50 SHIMADZU) testing from 30 to 800 °C at 10 °C min−1 under nitrogen atmosphere. The mechanical properties of the membranes were analyzed by the tensile test on an Instron Mechanical Tester (Testometric 350 AX) with the size of 1.0 × 4.0 cm2, at an elongation rate of 2 mm min−1 under room temperature. 2.4. Measurement of Solvent Uptake and Area Swelling. The membrane sample was kept in an oven at 60 °C for 48 h to be fully dried and then tailored into a squaredshaped sample (about 1.50 × 1.50 cm2). The weight (Wdry, g) and area (Adry, cm2) were measured accurately. Then, the sample was immersed in a certain pure solvent for 48 h at room temperature for a complete equilibrium. After removing the residual solvent on the membrane surface, the weight (Wwet, g) and area (Awet, cm2) were remeasured. The solvent uptake and area swelling were obtained from eqs 1 and 2, respectively:

isopropanol permeance and acceptable solvent resistant ability of the membrane, the benefits from hydrophilic−hydrophobic hybrid networks have not been sufficiently explored, due to the insufficient attention focused on investigating the effects of PDMS content and cross-linking time on the interfacial polymerization process and membrane microstructure. Accordingly, the microstructures of membrane were difficult to regulate, resulting in relatively poor thermal stability, mechanical strength, and solvent permeability (especially for butanone and ethyl acetate). Additionally, apparently separated layers of PDMS and PEI were observed, which was envisaged to lower the hybridization level of PDMS and PEI segments. In this study, a series of composite membranes with crosslinked PEI−PDMS hybrid active layer were prepared via interfacial polymerization. By optimizing the polymerization process, the hybridization of PDMS and PEI was achieved in molecular scale without layering. The microstructures and physicochemical properties of the membranes were regulated by altering the polymerization conditions, including PDMS content and cross-linking time. The solvent resistance and SRNF performances of the membranes in terms of solvent uptake, area swelling, permeance, and solute rejection were investigated in detail. Moreover, the operational stability and the separation of lotus seedpod proanthocyanidins (LSPC)− ethanol/water mixtures were evaluated as well.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. PEI (Mw 20 kDa), hydroxyl-terminated PDMS (5000 mPa s), and TMC were purchased from Alfa Aesar (Tianjin, China) without further purification. Sodium dodecyl sulfate (SDS) was supplied by Yingpeng Additives Chemical Engineering Co., Ltd. (Shanghai, China). PAN support with the molecular weight cutoff (MWCO) of 100 kDa was supplied by MegaVision Membrane Engineering & Technology Co., Ltd. (Shanghai, China). Acetone, ethyl acetate, and n-hexane were obtained from Tianjin Kewei Chemistry Co., Ltd. Polyethylene glycol oligomers (PEG, Mw’s 200, 400, 600, 800, and 1000) were supplied by Guangfu Fine Chemical Research Institute (Tianjin, China). Deionized water was used in all experiments. 2.2. Preparation of the Membranes. The preparation procedure of the composite membrane was illustrated in Scheme 1. PAN support was immersed into water for 30 min to reduce the intrusion of PEI chains into PAN pores. PEI (2.0 g) aqueous solution (50.0 g) containing surface active agent SDS Scheme 1. Preparation Procedure of the Composite Membrane and the Corresponding Microstructures

solvent uptake (%) = (Wwet − Wdry )/Wdry × 100

(1)

area swelling (%) = (A wet − Adry )/Adry × 100

(2)

All measurements were carried out at least three times, and the average value was taken as the final value. 2.5. Nanofiltration Performances of the Membranes. Nanofiltration experiments were carried out in a stainless steel dead-end pressure cell with the available membrane area of 18.2 cm2 and volume of 185 cm3. During the test, the pressurized N2 was utilized to drive the feed solution to get through the membrane. Pure acetone and ethyl acetate were used as organic solvents for the permeance test. The membrane sample was immersed into the corresponding solvent for 48 h for a complete equilibration. The feed solution was poured into the cell, and then the sample was precompacted with pure solvent at 2 bar until steady state permeance was achieved (about 30 min). Under different pressures (4 and 10 bar), the amount of permeation that transported through the membrane was measured by weighing the liquid collected in a flask. The 6176

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Industrial & Engineering Chemistry Research permeance (P, L m−2 h−1 bar−1) was calculated by the following equation:

P = V /At Δp

(3)

where V, A, t, and Δp were the permeate volume (L), effective membrane area (m2), testing time (h), and transmembrane pressure (bar). For the rejection measurement, a series of PEG oligomers with the molecular weights ranging from 200 to 1000 Da were dissolved in water as feed solutions (500 mg L−1). The transmembrane pressure was 10 bar. The speed of stirrer was controlled at 300 rpm to minimize concentration polarization.1,24 The concentration of PEG in the permeate solution was analyzed by utilizing a UV−vis spectrophotometer (PerkinElmer Lambda 25) after being colored by BaCl2/I2. The level of rejection was obtained from the following equation: R = (1 − Cp/Cf ) × 100

Figure 1. FTIR spectra of PAN support, PAN/PEI, and PAN/PEI− PDMS-X%.

(4)

Cf and Cp indicate the solute concentrations in the feed and permeate solutions, respectively. The rejection was the average of three measurements, with the error less than 5%.

resultant active layer might be isotropic rather than a multilayered structure.8 The surface morphologies of the as-prepared membranes were observed by SEM, and the images are shown in Figure 2. It could be found that all the active layers were steady and equably coated onto PAN support without obvious defects. Compared with PAN/PEI, the presence of PDMS made the surface of the composite membrane rough due to the elastic chain structure and restricted chain motion. For PAN/PEI− PDMS-X%, the membrane surface tended to be homogeneous with the increase of PDMS content. Such phenomena were mainly attributed to the fact that more PDMS chains were cross-linked with PEI chains with the increase of PDMS content from 0.5% to 2.0%, forming the uniform hybrid networks. The cross-sectional image reveals the structure of the composite membrane, which is composed of a porous support layer and an isotropic PEI−PDMS active layer. The thickness of the whole composite membrane was around 110 μm, and the thickness of the active layer was about 1.2 μm. In addition, it should be noted that, when further elevating PDMS content, the PDMS chains would self-polymerize into a PDMS layer with the aid of water molecules. Therefore, the PDMS content was kept below 2.0% in this study. The thermal properties of the membranes were investigated by TGA, and the results were depicted in Figure 3a. It could be found that all the membranes underwent a three-stage weight loss. The first stage was attributed to the evaporation of water from the membrane around 30−200 °C. The second stage was the degradation of the branched chains on PAN and PEI around 360−450 °C, and the third stage was the pyrolyzation of the polymer backbones around 450−600 °C. For the first stage, incorporating PDMS gave a significant decrease in weight loss, and increasing PDMS content further reduced the weight loss. Such findings inferred that the presence of hydrophobic PDMS lowered the water-absorbing ability of composite membranes.26,27 For the second stage, PAN/PEI exhibited a decomposition temperature (TD) value of 334 °C. By comparison, the incorporation of PDMS elevated the TD values to 355 and 359 °C for PAN/PEI−PDMS-0.5% and PAN/PEI−PDMS-1.0%, respectively. This should be ascribed to the fact that the mobility of PEI chains was restrained after cross-linking with PDMS. This also afforded PAN/PEI−

3. RESULTS AND DISCUSSION 3.1. Preparation of the Composite Membranes. In this study, a series of composite membranes with hybridized and cross-linked active layer were prepared by interfacial polymerization (Scheme 1). Aqueous PEI solution was dip-coated onto PAN support, followed by the dip-coating of PDMS−TMC nhexane solution. The NH/NH2 on PEI and OH on PDMS were cross-linked by COCl on TMC through nucleophilic substitution reaction, forming cross-linked PEI− PDMS networks. During the preparation, the same amount of PEI was utilized to ensure close thicknesses of the active layer for all the membranes. Under such conditions, the membrane performances were mainly dependent on the inner morphology of the active layer, and they were optimized by varying the PDMS content and cross-linking time. 3.2. Membrane Microstructures and SRNF Performances of PAN/PEI−PDMS-X%. 3.2.1. Characterization of the PAN/PEI−PDMS-X%. To investigate the chemical structures within the active layer, the FTIR spectra of PAN support, PAN/PEI, and PAN/PEI−PDMS-X% were obtained as shown in Figure 1. The strong adsorption band of PAN support at 2245 cm−1 was attributed to the stretching vibration of C N. By comparison, this band disappeared in the spectrum of PAN/PEI. Meanwhile, two new bands around 1355 and 1552 cm−1 were observed, corresponding to the stretching vibration of the CN and NH groups. Such a phenomenon might indicate that the PAN surface was fully covered by a PEI layer.25 Additionally, CONH groups were formed, as evidenced by the CO stretching vibration band at 1623 cm−1. The formation of CONH groups was probably attributed to the cross-linking between NH/NH2 on PEI and COCl on TMC. The characteristic band of Si CH3 at 1267 cm−1 was observed in the spectra of PAN/PEI− PDMS-X%. Compared with the spectrum of PAN/PEI, the characteristic bands of CN and NH groups on PEI were also observed in the spectra of PAN/PEI−PDMS-X%. This finding might suggest that the cross-linking at molecular scale between PDMS and PEI should be obtained, and the 6177

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Figure 2. SEM images of the surfaces of (a) PAN/PEI, (b) PAN/PEI−PDMS-0.5%, (c) PAN/PEI−PDMS-1.0%, and (d) the cross-section of PAN/ PEI−PDMS-1.0%.

Figure 3. (a) TGA and (b) stress−strain curves of PAN/PEI and PAN/PEI−PDMS-X%.

mechanical stability, with a Young’s modulus of 700 MPa, along with a tensile strength of 36.1 MPa and an elongation at break of 23.2%. By comparison, introducing PDMS endowed the membrane with enhanced mechanical stability, achieving the Young’s modulus of 940 MPa and the tensile strength of 47.7 MPa at the elongation of 19.7% for PAN/PEI−PDMS-0.5%. Such a phenomenon was reasonably ascribed to the inhibited chain mobility of the cross-linked networks.27 For PAN/PEI−

PDMS-X% higher char yields when compared with that of PAN/PEI. For instance, the char yield of PAN/PEI was 18.5%. When the PDMS content was elevated from 0.5% to 1.0%, the char yield increased from 23.4% to 28.5%. Mechanical stability is another parameter for the operation of a membrane. The mechanical properties of the membranes were probed via their stress−strain curves, as shown in Figure 3b. It was found that PAN/PEI possessed typically good 6178

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Figure 4. Influence of PDMS content on (a) solvent uptake and (b) area swelling of the membranes in acetone and ethyl acetate at room temperature.

Figure 5. Influence of PDMS content on permeance of (a) acetone and (b) ethyl acetate at room temperature and under the transmembrane pressures of 4 and 10 bar. (c) Rejection curves of PAN/PEI−PDMS-X% under the transmembrane pressure of 10 bar, and the membranes were equilibrated in ethyl acetate for 48 h prior to testing.

3.2.2. Solvent Uptakes and Area Swellings of the PAN/ PEI−PDMS-X%. Solvent uptake and area swelling play crucial roles in SRNF performances, as they reflect the free volume and solvent resistant properties of a membrane.28−30 Herein, two commonly used solvents were selected for the solvent resistance measurements, including acetone and ethyl acetate,

PDMS-X%, increasing the PDMS content from 0.5% to 1.0% elevated the Young’s modulus from 940 to 1040 MPa, along with a tensile strength improvement from 47.7 to 50.1 MPa. Collectively, these results suggested that the hybrid networks donated enhanced thermal and mechanical stabilities to PAN/ PEI−PDMS-X% when compared with those of PAN/PEI. 6179

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Industrial & Engineering Chemistry Research with the dipole moments of 9.0 × 10−30 and 6.3 × 10−30 C m, respectively. Figure 4 reveals that PAN/PEI displays the solvent uptakes of 45.9% and 50.9% in acetone and ethyl acetate, respectively. By comparison, the corresponding area swellings were much lower (0.66% and 2.74%). Such a phenomenon indicated that the adsorbed solvent molecules were mainly stored in the pores of the PAN support. A similar observation had been reported in other composite SRNF membranes.31 Compared with PAN/PEI, the solvent uptakes and area swellings of PAN/PEI−PDMS-X% in acetone and ethyl acetate were obviously reduced. For example, the solvent uptakes of PAN/PEI−PDMS-0.5% in acetone and ethyl acetate were 38.1% and 46.6%, and the area swellings were 0% and 2.28%, respectively. The restrained segment mobility should be responsible for this reduction, which reduced the adsorption capacity of the active layer for solvent molecules. Under identical conditions, the solvent uptakes and area swellings in acetone were lower than those in ethyl acetate for all the membranes. This could be attributed to the hydrophobic nature of PDMS chains, which were applicable to transporting solvent molecules with low polarity. For PAN/PEI−PDMS-X%, the solvent uptakes were only slightly altered by increasing the PDMS content, and this was probably because most of the adsorbed solvent molecules were stored in the pores of the PAN support. In comparison, the area swellings were obviously reduced when the PDMS content increased from 0.5% to 1.5%, due to the decrease of chain mobility in the active layer. The area swellings in ethyl acetate gradually reduced from 2.28% to 0.67% with the PDMS content increasing from 0.5% to 1.5%. When further increasing the PDMS content to 2.0%, the area swelling in ethyl acetate was elevated to 2.01%, probably due to the aggregation of PDMS segments. Notably, the area swellings for all the PAN/PEI−PDMS-X% were below 2.8%, suggesting that the as-prepared composite membranes possessed excellent solvent resistance for SRNF application.8,32 3.2.3. Nanofiltration Performances of the PAN/PEI− PDMS-X%. Nanofiltration performances of the as-prepared membranes were evaluated in terms of permeance and rejection. A previous study had testified that the PEI-based SRNF membrane displayed favorable permeance for isopropanol (up to 5.6 L m−2 h−1 bar−1), helped by the presence of hydrophilic groups (NH/NH2) on PEI, whereas acetone and ethyl acetate showed relatively low permeance. Accordingly, acetone and ethyl acetate were employed as solvents in this study, and an attempt was made to enhance their transfer abilities by regulating the microstructures of the active layer. As presented in Figure 5a,b, PAN/PEI attained permeances of about 2.7 and 1.4 L m−2 h−1 bar−1 for acetone and ethyl acetate under 4 bar, respectively. Incorporating PDMS decreased the transfer ability for both acetone and ethyl acetate. For example, the permeances of PAN/PEI−PDMS0.5% for acetone and ethyl acetate were 2.3 and 1.1 L m−2 h−1 bar−1, respectively. The reduction of permeance should be ascribed to the following factors: (i) the restrained chain mobility in the active layer, which would weaken the diffusion ability of polymer chains, and (ii) the reduced area swelling of the membrane, which would raise the transfer resistance for solvent molecules. These influences became stronger with the increase of PDMS content, and consequently the permeance continuously reduced. For instance, the permeance of acetone under 4 bar decreased from 2.3 to 1.5 L m−2 h−1 bar−1 as the PDMS content increased from 0.5% to 2.0%, and meanwhile the permeance of ethyl acetate decreased from 1.1 to 0.2 L m−2

h−1 bar−1. Compared with ethyl acetate, acetone exhibited higher permeation ability in all the membranes. Such a phenomenon was ascribed to its smaller molecular size and lower viscosity, both of which would confer a faster transfer rate on acetone molecules through the active layer. This finding was inconsistent with the behaviors of solvent uptake and area swelling, which were static adsorption equilibria. In this case, the mutual interactions between solvent and polymer chain played a significant role. In addition, it could be found that the permeance of acetone was efficiently decreased by elevating the transmembrane pressure. As the pressure increased from 4 to 10 bar, the permeances of PAN/PEI−PDMS-1.0% for acetone reduced from 2.2 to 1.3 L m−2 h−1 bar−1, respectively. This finding should be attributed to membrane compaction. It should be noted that the permeabilities of the active layer in PAN/PEI−PDMS-1.0% under 4 bar for acetone and ethyl acetate were 2.6 × 10−3 and 9.1 × 10−4 L m−1 h−1 bar−1, respectively. In addition, the permeance of n-heptane was also tested, and it was found it was almost 0 L m−2 h−1 bar−1 (that is, no transport of n-heptane). Despite the permeance reduction, PAN/PEI−PDMS-X% possessed acceptable transport abilities when compared with the data obtained in the literature.33,34 The rejection ability is reflected by MWCO, which refers to the molecular weight of solute when the rejection is 90%. Figure 5c depicts the rejection curves of PAN/PEI and PAN/ PEI−PDMS-X% using PEG oligomers (Mws, ranging from 200 to 1000) as solutes. It was found that all the membranes displayed continuous increase of rejection with the elevating of the Mw of PEG. For instance, the rejection of PAN/PEI elevated from 91.0% to 92.8% as the PEG Mw increased from 200 to 1000. Compared with PAN/PEI, the incorporation of PDMS obviously enhanced the rejection abilities of PAN/PEI− PDMS-X%. For example, the rejection of PAN/PEI for PEG 400 increased from 90.9% to 97.2%, 94.9%, 93.5%, and 94.6% when the PDMS contents were 0.5%, 1.0%, 1.5%, and 2.0%, respectively. This phenomenon was reasonably ascribed to the inhibited chain mobility and reduced membrane swelling upon the hybridization of PDMS, which raised the transfer resistance for PEG oligomers.35 With the increase of PDMS content, the rejection properties of PAN/PEI−PDMS-X% were slightly altered. Figure 5c also revealed that the rejections of PAN/ PEI−PDMS-X% for PEG 200 were all above 90%, indicating that their MWCOs were below 200 Da. Accordingly, the asprepared membranes displayed high rejection ability. 3.3. Membrane Microstructures and SRNF Performances of PAN/PEI−PDMS-Ys. In the previous section, we optimized the performances of PAN/PEI−PDMS-X% by regulating PDMS content. Considering the comprehensive performances of permeance and rejection, the PDMS content of 1.0% was chosen in the following study, in which the membrane microstructures and performances were optimized by tuning cross-linking time. A series of composite membranes with different cross-linking times were fabricated and designed as PAN/PEI−PDMS-Ys, where Y (Y = 30, 60, 90, 120, and 180) represented the cross-linking time. 3.3.1. Characterization of the PAN/PEI−PDMS-Ys. The chemical structures and morphologies of PAN/PEI−PDMS-Ys were investigated by FTIR and SEM, respectively. Figure 6 presents the FTIR spectra of PAN/PEI and PAN/PEI−PDMSYs. Similarly, the characteristic bands for SiCH3 (1267 cm−1) and CONH (1623 cm−1) appeared in the spectra of PAN/ PEI−PDMS-Ys, which might confirm the hybridization and cross-linking of PDMS and TMC. During the cross-linking 6180

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alteration of cross-linking time, mainly due to the identical PDMS content for all the PAN/PEI−PDMS-Ys. The influence of cross-linking time on the morphologies of PAN/PEI−PDMS-Ys was probed by the SEM images in Figure 7. Similar to the phenomenon observed in PAN/PEI−PDMS-X %, the surface of PAN/PEI−PDMS-Ys became rough after incorporating PDMS. With the same PDMS content, the surface of the active layer tended to be smooth as the crosslinking time increased from 60 to 180 s. This observation was probably attributed to the fact that more PEI chains were crosslinked with PDMS chains through TMC with the increase of cross-linking time. This tendency was in accordance with that of the influence of PDMS content on the surface morphologies of PAN/PEI−PDMS-X%. Moreover, Figure 8 revealed that all the active layers were uniformly deposited onto PAN support without obvious defects. The thermal and mechanical properties of PAN/PEI− PDMS-Ys were investigated, and the results are shown in Figure 8. Figure 8a indicates that PAN/PEI and PAN/PEI− PDMS-Ys undergo a similar three-stage weight loss, including the evaporation of water, the degradation of the branched chains, and the pyrolyzation of polymer backbones. It could be found that more hydrophilic NH/NH2 groups on PEI were reacted into hydrophobic CONH as the cross-linking time increased, lowering the water adsorption ability of the membrane. As a result, the weight loss of PAN/PEI−PDMS-Ys

Figure 6. FTIR spectra of PAN/PEI and PAN/PEI−PDMS-Ys.

reaction, the NH/NH2 on PEI reacted with the  COCl on TMC, forming CONH groups. It was found that more TMC was consumed with the increase of cross-linking time. As a result, more CONH groups were formed and the intensity of the corresponding band (1623 cm−1) was enhanced. Additionally, it could be found that the intensities of SiCH3 bands remained almost unchanged with the

Figure 7. SEM images of the membrane surface: (a) PAN/PEI, (b) PAN/PEI−PDMS-60s, (c) PAN/PEI−PDMS-120s, and (d) PAN/PEI−PDMS180s. 6181

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Figure 8. (a) TGA and (b) stress−strain curves of PAN/PEI and PAN/PEI−PDMS-Ys.

Figure 9. Effect of cross-linking time on (a) solvent uptake and (b) area swelling of the membranes in acetone and ethyl acetate at room temperature.

in the first stage was reduced when increasing the cross-linking time. In the second stage, the incorporation of PDMS elevated the TD values of PAN/PEI−PDMS-Ys (above 346 °C) compared to that of PAN/PEI (342 °C). Meanwhile, the TD values of PAN/PEI−PDMS-Ys slightly increased from 346 to 351 °C as the cross-linking time increased from 60 to 180 s. Such a phenomenon was reasonably ascribed to the increase of cross-linking degree, which caused a gradual enhancement of the suppression of chain mobility in the active layer.36 Such a feature meanwhile afforded the PAN/PEI−PDMS-Ys a slight increase of char yield from 22.1% to 28.5% with the crosslinking time elevating from 60 to 180 s. Figure 8b shows the mechanical properties in the form of stress−strain curves, which reveals that the presence of PDMS endows PAN/PEI−PDMS-Ys with improved mechanical stabilities. Extending the cross-linking time afforded the membranes a continuous increase in tensile strength and Young’s modulus. For instance, when the cross-linking times were 60, 120, and 180 s, the Young’s modulus values of PAN/ PEI−PDMS-Ys were 720, 1040, and 1050 MPa, and the tensile strengths were 46.1, 50.1, and 52.6 MPa, whereas the corresponding elongations were 19.7%, 19.5%, and 19.2%, respectively. These observations indicated that the mechanical

stabilities of PAN/PEI−PDMS-Ys could be enhanced by increasing the cross-linking degree, although the membranes became brittle. In summary, Figure 9 suggested that elevating the cross-linking degree of hybrid networks could enhance the thermal and mechanical stabilities of PAN/PEI-PDMS-Ys by increasing cross-linking time. 3.3.2. Solvent Uptakes and Area Swellings of the PAN/ PEI−PDMS-Ys. The influence of cross-linking time upon the solvent resistant ability of PAN/PEI−PDMS-Ys was investigated by testing their area swellings and solvent uptakes. The results in Figure 9 reveal that all the membranes display relatively high solvent uptakes but low area swellings, inferring that the adsorbed solvent molecules are mainly in the pores of PAN support. Compared with PAN/PEI, the presence of PDMS endowed the membranes with enhanced solvent resistant ability, as testified by the reduced solvent uptakes and area swellings for PAN/PEI−PDMS-Ys. It was found that the solvent uptakes and area swellings of PAN/PEI−PDMS-Ys were slightly altered when varying the cross-linking time. The low area swellings (below 2.8%) implied that the as-prepared composite membranes possessed excellent solvent resistant properties, which promoted the potential for practical application in SRNF. 6182

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Figure 10. Influence of cross-linking time on permeance of (a) acetone and (b) ethyl acetate at room temperature and under the transmembrane pressures of 4 and 10 bar. (c) Rejection curves of PAN/PEI−PDMS-Ys under the transmembrane pressure of 10 bar, and the membranes were equilibrated in ethyl acetate for 48 h prior to testing.

3.3.3. Nanofiltration Performances of the PAN/PEI− PDMS-Ys. The nanofiltration performances of PAN/PEI and PAN/PEI−PDMS-Ys were systematically evaluated in terms of permeation and rejection ability, and the results are shown in Figure 10a,b. It could be found that PAN/PEI−PDMS-Ys displayed similar behaviors to PAN/PEI−PDMS-X%: (i) the presence of PDMS in the active layer lowered the transfer ability for both acetone and ethyl acetate; (ii) the acetone permeance of the membrane was obviously decreased by elevating the transmembrane pressure while the permeance of ethyl acetate improved; and (iii) acetone exhibited higher permeation ability than ethyl acetate for all the membranes. In addition, it could be found that increasing the cross-linking time weakened the transfer abilities of PAN/PEI−PDMS-Ys. Such a phenomenon should be associated with the enhancement of the suppression of chain mobility in the hybrid networks, which increased the diffusion resistance for solvent molecules.37 For instance, the permeance of acetone under 4 bar decreased from 2.7 to 2.3 L m−2 h−1 bar−1 with the reduction of 14.8% when the cross-linking time increased from 30 to 180 s, while the ethyl acetate permeance decreased from 1.1 to 0.7 L m−2 h−1 bar−1 with the reduction of 36.4%. Figure 10c presents the rejections of PAN/PEI and PAN/ PEI−PDMS-Ys. It was found that PAN/PEI−PDMS-Ys attained higher rejections than PAN/PEI. Increasing crosslinking time would elevate the cross-linking degree of the active layer, in turn enhancing the rejection ability. For instance, the

MWCO of PAN/PEI−PDMS-30s was about 440 Da. When increasing the cross-linking time to above 60 s, the rejections of composite membranes for PEG 200 were higher than 90%, which meant that the MWCOs were less than 200 Da. Moreover, increasing the molecular weight of PEG (solute molecule) weakened the influence of cross-linking time on rejection of PAN/PEI−PDMS-Ys, which attained close rejection values. The low MWCOs suggested the adequate rejection abilities of PAN/PEI−PDMS-Ys for SRNF application. 3.4. Operational Stability of the Membrane. To evaluate the operational stabilities of the as-prepared membranes in organic solvents, a filtration experiment was carried out for a period of 720 min according to the literature.38 PEG 1000 and ethyl acetate were utilized for the test of rejection and permeance stabilities, respectively. Considering the optimum SRNF performances, PAN/PEI−PDMS-1.0% was chosen as representative and the results were depicted in Figure 11. It was found that the permeance of ethyl acetate reduced from 1.1 to 0.9 L m−2 h−1 bar−1 with the reduction of 18.2% during the initial 240 min. This reduction was reasonably ascribed to the compaction of membrane and the pore blockage by solvent molecules. Thereafter, the permeance tended to be stable with a constant value of around 0.8 L m−2 h−1 bar−1. Similar to the tendency of permeance, the solute rejection slightly decreased from 98.2% to 95.0% during 720 min with the reduction of 3.3%. This decrease was probably due to the 6183

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Figure 11. Operational stability of PAN/PEI−PDMS-1.0% in ethyl acetate solution at room temperature under the transmembrane pressure of 10 bar.

Figure 12. Photographs and concentrations of LSPC−ethanol/water solution before and after filtration using PAN/PEI−PDMS-1.0% as SRNF membrane.

increase of PEG concentration at the feed side and consequently increased osmotic pressure. Despite the reductions of permeance and rejection, the composite membranes showed a promising operation stability when compared to the other literature.39,40 3.5. Concentration of Lotus Seedpod Proanthocyanidins. Proanthocyanidins, natural antioxidants that possess the ability to scavenge oxygen free radicals and stimulate antioxidant enzyme activity, exhibit potent antioxidant capacity and possible protective effects on human health in reducing the risk of chronic diseases.41 It has been demonstrated that the main components in LSPC are monomers, dimers, trimers, and tetramers of proanthocyanidins, in which the catechin and epicatechin are the basic units and the dimers are the main component. LSPC powder is easily dissolved in organic solvents, such as methanol, ethanol, acetone, and so on.42 After solvent extraction with an organic solvent/water mixture, LSPC is usually concentrated using macroporous adsorption resin or membrane separation. Herein, PAN/PEI−PDMS-1.0% was employed as a representative to purify LSPC from an LSPC−ethanol/water mixture considering that the molecular weight of LSPC was above 230 Da. Primarily, LSPC was extracted from lotus seedpod by ethanol−water solution at room temperature. The obtained solution was brown, as shown in Figure 12. The concentration was calculated to be 835 mg L −1 by UV−vis using anillin−hydrochloric acid as a chromogenic agent. Afterward, the feed solution was concentrated with PAN/PEI−PDMS-1.0% under 10 bar. After filtration, the concentration of LSPC in the permeation solution was 45.9 mg L−1. The rejection for LSPC was 94.5%. In addition, the permeance of PAN/PEI−PDMS-1.0% was 1.0 L m−2 h−1 bar−1. These data were comparable with or higher than those obtained by other techniques in the literature,43 and therefore, this study may offer a promising strategy to pursue the concentration of LSPC in an organic solvent system.

indicated (i) that incorporating PDMS donated enhanced thermal/mechanical stabilities to the composite membrane by restricting the polymer chain mobility and (ii) that the crosslinked networks reinforced the solvent resistant and rejection ability of the composite membrane. Particularly, the composite membranes attained the MWCOs below 440 Da along with the acceptable solvent permeances. Meanwhile, area swelling below 2.8% was acquired for the composite membranes. The microstructures of the active layer were tuned by PDMS content and cross-linking time: (i) elevating the PDMS content could generate more hybrid networks between PEI and PDMS, giving a reduction of chain mobility, and (ii) elevating the cross-linking time could increase the cross-linking degree of the hybrid networks, affording a denser active layer. Consequently, tunable SRNF performances were obtained. Moreover, the composite membrane displayed promising operational stability and reliable rejection/purification abilities for LSPC.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-371-63887135. Fax: +86-371-63887135. E-mail: [email protected] (J.T.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (21206151 and U1407121) and the China Postdoctoral Science Foundation (2014T70687).

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4. CONCLUSIONS In summary, a series of PAN/PEI−PDMS composite membranes were prepared via interfacial polymerization. During the preparation, PEI (NH/NH2) and PDMS (terminated OH) chains were cross-linked by TMC, forming cross-linked and hybrid networks. The resulting active layer was isotropic rather than separated layers (i.e., PEI layer and PDMS layer). The systematical characterizations and measurements 6184

NOMENCLATURE Mw = molecular weight (g mol−1) wt = weight fraction (%) Wdry = weight of the dry membrane (g) Wwet = weight of the membrane after immersion (g) Adry = area of the dry membrane (cm2) Awet = area of the membrane after immersion (cm2) F = solvent flux (L m−2 h−1) V = infiltration volume (L) A = membrane surface area (m2) t = infiltration time (h) Δp = transmembrane pressure (bar) DOI: 10.1021/acs.iecr.5b01236 Ind. Eng. Chem. Res. 2015, 54, 6175−6186

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R = rejection (%) Cp = solute concentrations in permeate (mg L−1) Cf = solute concentrations in the feed (mg L−1) TD = decomposition temperature (°C) Subscripts

w = weight dry = dried membrane wet = immersed membrane p = permeate f = feed D = decomposition

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