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Novel charged chitosan composite nanofiltration membranes containing chiral mesogenic group.

INTRODUCTION

Nanofiltration (NF) technology has been used in diverse fields including concentration, purification, and fractionation of various products, etc [1, 2]. Due to its high separation efficiency and low energy expenditure required in operation, NF has gained more and more attention [3-5]. The NF membrane can be divided into neutral NF membrane, positively charged NF membrane and NF negatively charged membrane. The separation mechanism involves sieving effect and electrical (Donnan) effect. This combination allows NF membranes to be effective for a range of separation of mixtures of organic molecules (neutral or charged) and salts [6]. Donnan potential is created between the charged ions in the NF membrane and the coions in the influent. Because of this potential, coions in the influent are rejected. Because of the electro-neutrality requirements in the influent, the counter-ions are rejected as well [7]. The excellent performance can be achieved by improving membrane preparation process or altering the membrane properties. This article is focusing on the latter approach to achieve high flux by adjusting the membrane hydrophobicity, and high rejection of multivalent ion by adjusting the surface charge.

Chitosan has been routinely used in membrane preparation due to its abundance, hydrophilicity and environmental benignancy, which can be modified by hydroxylation and amination reactions [2, 8-12]. Employing chitosan and its modified derivatives, various NF membranes were reported by surface crosslinking, blending and ultraviolet irradiation preparation methods [2, 10-13].

In this article, charged composite NF membrane was designed with polysulfone UF membrane as the base to provide mechanical strength and the modified chitosan/chitosan mixture as the top layer to provide filtration function, aiming to achieve high rejection with high flux. As the helical structures existed in chiral liquid crystals [14-16] and charged compounds could adjust surface charge of NF membranes, the chiral mesogenic compound and the charged compounds were grafted to chitosan through hydroxylation to provide the change of the structure and the surface charge, hence performance of the NF membrane [17-19]. Series of NF membranes were prepared with this design. The rejection reached the maximum of 95.7% for Ca[Cl.sub.2] with [P.sub.2-7] composite NF membrane, corresponding flux was 3155 [Lm.sup.-2] [h.sup.-1]. The rejection reached the maximum of 93% for [Na.sub.2]S[O.sub.4] with [P.sub.3-5] composite NF membrane, corresponding flux was 3879 [Lm.sup.-2] [h.sup.-1]. These excellent performances were the result of the membrane structure and charged modification through the introduction of a chiral mesogenic compound and charged compounds. The membranes were typical charged NF membranes.

EXPERIMENTAL

Materials and Methods

A self-made test equipment was adopted for the membrane performance test, as shown in Fig. 1. DDS-307 conductivity meter (Shanghai Leid Instrument Factory) was used to evaluate the conductivity of solution; Spectrum One infrared spectrometer (PerkinElmer) was used to test the chemical composition of monomers and polymers; differential scanning calorimetry (DSC) analytical meter (NETZSCH DSC-204) was used for the measurement of thermal transition properties of monomers; Polarized Optical Microscopy (POM; Leica) was used for the observation of the texture of monomers; the Perkin Elmer 341 polarimeter was used to performed the optical rotation of monomers and SSX-550 type scanning electron microscope (SEM, Shimadzu) was used for the observation of the morphology of NF membrane.

Chitosan ([M.sub.w] [greater than or equal to] 20,000 Da, degree of deacetylation [greater than or equal to] 90%); triethylamine, Sulfanilic acid, Phenol, N-methyl pyrrolidone, and polyvinylpyrrolidone were analytical grade and purchased from Sinopharm Chemical Reagent; chloroacetic acid, acetic acid, hexanedioic acid, acetone, glutaraldehyde, and polyvinyl alcohol were obtained from Shenyang Xinxi reagent Factory; Cholesterol was purchased from Henan xiayi Bell biological Products and polysulfone was purchased from Shanghai Shuguang chemical plastics industrial corporation.

Preparation of Modified Chitosan

The structure of the chiral mesogenic compound [M.sub.1], the positively charged compound [M.sub.2] and the negatively charged compound [M.sub.3] were shown in Figs. 2-A.

Chitosan was grafted polymerization with three monomers; hence, polymers of [P.sub.1]; [P.sub.2], and [P.sub.3] were obtained. The polymerization schemes were shown in Figs. 5-7.

General Procedure for Mesogenic Compounds Modified Chitosan Preparation

[M.sub.1], [M.sub.2], and [M.sub.3] compound acid chloride derivatives were prepared by reacting [M.sub.1], [M.sub.2], and [M.sub.3] with SO[Cl.sub.2] in at 50[degrees]C for 6 h. The products were purified by distillation.

The acyl chlorides obtained from last step were dissolved in chloroform and added to the chitosan methanesulfonic acid solution dropwise over 3.5 h. The ratio between chitosan and [M.sub.1], [M.sub.2], and [M.sub.3] acyl chloride was listed in Tables (1-3). Once the reaction was over, the reaction mixture was cooled down to 4[degrees]C for 10 h before acetone precipitation treatment. The products were then filtered twice and allowed to dry under vacuum.

Preparation of Composite NF Membranes

A porous polysulfone UF membrane was used as the substrate. The polysulfone membrane was prepared by phase inversion process using N-methylpyrrolidone as the solvent and polyvinyl pyrrolidone and acetone as the additive [20-23].

The mixture of modified chitosan and chitosan was dissolved in 2.5 ml 6% acetic acid solution with 0.06% polyvinyl alcohol (porogen). The casting solution was obtained by deaerating the above solution. The polysulfone UF membrane was fixed on the glass. The casting solution was then applied to coat the UF membrane. The newly formed membrane was vaporized for 60 s at room temperature, and then crosslinked by 1% glutaraldehyde. The composite membrane was ready after 14 h at ambient temperature [24-28].

Permeation Experiment

Flux and rejection were calculated based on the Eqs. 1 and 2.

F = V/At (1)

where F is the flux; V is the volume of the permeating fluid passing through the membrane; A is the effective area of membrane (0.93 [cm.sup.2]); and t is the time for permeation.

R = (1 - [C.sub.p]/[C.sub.0]) x 100% (2)

where R is the rejection; and [C.sub.p] and [C.sub.0] are the concentrations of the permeated fluid and feed, respectively.

The concentration was replaced by the conductivity of salt solutions because the solution was very dilute in this study.

RESULTS AND DISCUSSION

The Characterization of the Compounds and the Chitosan Derivatives

Infrared Analysis. Spectrum One infrared spectrometer was used to test the chemical compositions of the chiral mesogenic compound, charged compounds, and the chitosan derivatives.

[M.sub.1]. The infrared spectrum of [M.sub.1] was shown as Fig. 8. The absorption band at 3,490 [cm.sup.-1] was associated with the stretching vibration of hydroxyl of carboxylic acid; the absorption band at 2,943 and 2,867 [cm.sup.-1] was associated with the stretching vibration of the methyl and the methylene; and the absorption band at 1,722 and 1,694 [cm.sup.-1] was associated with the stretching vibration of carbonyl.

[M.sub.2]. The infrared spectrum of [M.sub.2] was shown as Fig. 9. The absorption band at 3,433 [cm.sup.-1] was associated with the stretching vibration of hydroxyl of carboxylic acid; the absorption band at 3,200-2,510 [cm.sup.-1] was associated with the stretching vibration of association hydroxyl of carboxylic acid; the absorption band at 1,690 [cm.sup.-1] was associated with the stretching vibration of carbonyl; and the absorption band at 1,034, 1,239 [cm.sup.-1] was associated with the stretching vibration of C--N in quaternary ammonium group.

[M.sub.3]. The infrared spectrum of [M.sub.3] was shown as Fig. 10. The absorption band at 3,485 [cm.sup.-1] was associated with the stretching vibration of hydroxyl of carboxylic acid; the absorption band at 2,910-2,500 [cm.sup.-1] was associated with the stretching vibration of association hydroxyl of carboxylic acid; the absorption band at 1,690 [cm.sup.-1] was associated with the stretching vibration of carbonyl; the absorption band at 1,270-1,250 [cm.sup.-1] was associated with the stretching vibration of sulfonic group; and the absorption band at 1,131 [cm.sup.-1] was associated with the stretching vibration of azobenzene groups; the absorption band at 1,238-1,150 [cm.sup.-1] was associated with the stretching vibration of ether linkage.

[P.sub.1]. The infrared spectrums of polymers [P.sub.1] was shown in Fig. 11. Comparing with the raw chitosan, the ester carbonyl absorption peak appeared at about 1,730 [cm.sup.-1], and the absorption peak of ester carbonyl increased with the ratio increase of monomer and primary hydroxyl of chitosan from bottom to top.

[P.sub.2] and [P.sub.3]. The infrared spectrums of polymers [P.sub.2] and [P.sub.3] were shown in Figs. 12 and 13. An overlying ester carbonyl absorption peak appeared at about 1,730 [cm.sup.-1] and it also increased from bottom to top.

Thermal Analysis. The phase transitions and corresponding enthalpy changes of the chiral mesogenic compound and the charged compounds were characterized by DSC.

The DSC curve of Monomer [M.sub.1] was shown in Fig. 14. The heating curve had two endothermic peaks, respectively, representing the melting transition ([T.sub.m] = 137[degrees]C) and the clearing transition ([T.sub.i] = 147[degrees]C), the endothermic enthalpy of which were [DELTA][H.sub.m] = 72.37 J/g and [DELTA][H.sub.i] = 1.15 J/g.

The DSC curve of Monomer [M.sub.2] was shown in Fig. 15. The heating curve had one endothermic peaks, representing the melting transition ([T.sub.m] = 200.4[degrees]C), the endothermic enthalpy of which were [DELTA][H.sub.m] = 351.3 J/g. [M.sub.2] began to decompose when the temperature raised.

The DSC curve of Monomer [M.sub.3] was shown in Fig. 16. The heating curve had one endothermic peak, representing the melting transition ([T.sub.m] = 301[degrees]C), the endothermic enthalpy of which was [DELTA][H.sub.m] = 113.3 J/g. [M.sub.3] began to decompose when the temperature rised.

Textures Analysis. The optical textures of the chiral mesogenic compound were studied by the POM with a hot stage under a nitrogen atmosphere.

The mesomorphism and textures of monomer [M.sub.1] were shown in Fig. 17. The POM of monomer M, revealed that [M.sub.1] exhibited an enantiotropic oily streak texture and a broken focal conic texture during its heating and cooling cycles. When M] was heated to about 137[degrees]C, it began to melt. At 142[degrees]C, the oily streak texture of the cholesteric phase appeared as shown in Fig. 17a, and the birefringence totally disappeared at 147[degrees]C. When cooled to 145.2[degrees]C from isotropic, the broken focal conic texture displayed, as shown in Fig. 17b. It was a typical chiral liquid crystal monomer.

Optical Rotation. Optical rotation of monomer [M.sub.1] was performed by the Perkin Elmer 341 polarimeter. [M.sub.1] was optically active in the light of a Na-lamp at [lambda] = 589 nm at ambient temperature. The specific rotation was -2.4. So [M.sub.1] was a chiral compound with helical structures [15, 16]. When it was gafted to chitosan, the structure was changed, hence the performance of NF membranes.

Effect of the Degree of the [P.sub.1] on the Rejection and Flux of Composite NF Membrane

The test of [P.sub.1] composite NF membrane was conducted after a prepressure at 0.4 Mpa for 0.5 h. The relationships between grafting degree and membrane performance were shown in Figs. 18 and 19. The rejection and flux both increased till 5% with [P.sub.1-3] composite membrane. The rejection reached the maximum with [P.sub.1-4] composite membrane when the grafting degree of [M.sub.1] to chitosan was 10%. The maximum rejections were 63, 66.1, and 66.8% for NaCl, [Na.sub.2]S[O.sub.4], and Ca[Cl.sub.2], and corresponding fluxs were 2,134, 2,002, and 2,026 [Lm.sup.-2] [h.sup.-1]. When the grafting degree was higher than 10%, the rejection dropped and flux increased, indicating the filtration performance deteriorating. The data indicated the structure of [M.sub.1] and its appropriate composition in the chitosan derivative were crucial for the composite membrane performance. The reason might be as follows: with the right grafting degree, the helical structure of [M.sub.1] made the tortuosity of the pore increase. While the pore became larger when the percentage of [M.sub.1] exceeded 10%, which might result from the bulk of [M.sub.1]. The difference of those three types of solution was very small. The reason was that the ions of those three types of solution were the same size as [10.sup.-10] m. Therefore, the 10% graft degree of Mt to chitosan was selected for further experiment.

Effect of the Degree of the [P.sub.2] on the Rejection and Flux of Composite NF Membrane

The test was the same as before. The relationships between grafting degree and membrane performance were shown in Figs. 20 and 21. With the increase of [M.sub.2] grafting degree to the chitosan, the rejection increased for Ca[Cl.sub.2] except for degree of 95%, the rejection reached the maximum of 95.7% when the grafting degree was 70% ([P.sub.2-7] composite NF membrane), corresponding flux was 3,155 [Lm.sup.-2] [h.sup.-1]. While the rejection of NaCl and [Na.sub.2]S[O.sub.4] decreased. The rejection of [Na.sub.2]S[O.sub.4] was smaller than NaCl. The flux increased all the time as shown in Fig. 21. The data indicated the charged characteristic of [M.sub.2] and right percentage in the chitosan derivative was crucial for the composite membrane performance. The function of [M.sub.2] was Donnon exclusion [29, 30]. Because the active layer had quaternary ammonium group distribution and allowed a stronger repulsion for [Ca.sup.2+] than [Na.sup.+]. The reason of NaCl > [Na.sub.2]S[O.sub.4] was that the attraction was stronger for SO[4.sup.2-] than [Cl.sup.-] [29-33]. The order of rejection was Ca[Cl.sub.2] > NaCl > [Na.sub.2]S[O.sub.4]. The trend of the Ca[Cl.sub.2] rejection could be explained as follows: the electrostatic effect strengthened with the increase of the graft degree, which led to the increase of the rejection. Effect of the porosity caused by [M.sub.2] played a leading role when the grafting degree exceeded 70%. Therefore, the rejection began to decrease. The trend of the flux could be explained as follows: the volume of [M.sub.2] led to increasing of the porosity along with the grafting degree increased. It was typical positively charged NF membrane.

Effect of the Degree of the [P.sub.3] on the Rejection and Flux of Composite NF Membrane

The performance of [P.sub.3] composite NF membrane was shown in Figs. 22 and 23. With the increasing of [M.sub.3] grafting degree to the chitosan, the rejection first increased and then decreased for [Na.sub.2]S[O.sub.4]. The rejection reached the maximum of 93% when the grafting degree of [M.sub.3] to chitosan was 20%, corresponding flux was 3,879 [Lm.sup.-2] [h.sup.-1]. While the rejection of NaCl and Ca[Cl.sub.2] decreased. The flux increased all the time. The data indicated the charged characteristic of [M.sub.3] and right percentage in the chitosan derivative was crucial for the composite membrane performance. The function of [M.sub.3] was Donnon exclusion [29, 34-36]. Negatively charged NF membranes had high rejection for high-valence anions, and had no effect on univalence anions [36, 37]. Because the active layer had sulfate group distribution and allowed a stronger repulsion of S[O.sup.2-.sub.4] than [Cl.sup.-]. The reason of NaCl > Ca[Cl.sub.2] was stronger attraction forces for [Ca.sup.2+] than [Na.sup.+]. Therefore, the order of rejection was [Na.sub.2]S[O.sub.4] > NaCl > Ca[Cl.sub.2]. The conclusion was as expected. The trend of [Na.sub.2]S[O.sub.4] could be explained as follow: at beginning, the electrostatic effect strengthened with the increase of the graft degree till 20%, which led to the increase of the rejection. Effect of the porosity caused by [M.sub.3] played a leading role when the grafting degree exceeded 20%. Therefore, the rejection decreased. The trend of the flux could be explained as follow: the modified chitosan and chitosan formed a large network structure. The porosity increased with the increasing of [M.sub.3] grafting degree to the chitosan. As a result, the flux increased. It was a typical negatively charged NF membrane.

Structure Characteristic of Composite Membrane

The cross-section and surface of the membranes were characterized by a SSX-550 SEM as shown in Fig. 24. (a) was the cross-section photograph of [P.sub.2-7] composite NF membrane, (b) was the surface photograph, (c) was the cross-section photograph of [P.sub.3-5] composite NF membrane (d), and was the surface photograph. The composite membrane cross-section was magnified 2,000 times. The surface was 20,000 times. The surface of composite membrane was smooth, which formed the active layer of composite membrane. The cross-section of composite membrane had two layers. The upper was the thin and dense crosslinking layer; the lower was the polysulfone support layer with a finger-like structure. The combination of the dense function layer and the loose supporting layer allowed the composite membrane maintaining high rejection with high flux.

CONCLUSIONS

The composite NF membranes reported in this article were structured with two layers: the upper was the thin and dense crosslinked layer, which played a crucial role in separation; the lower was the polysulfone support layer, with a finger-like porous texture. The upper layer was prepared with a homogenous mixture of modified chitosan and chitosan. A chiral mesogenic compound [M.sub.1], a positively charged compound and a negatively compound were grafted to chitosan, respectively. The helical structure of [M.sub.1] made the tortuosity of the pore increase. The positively charged compound and the negatively compound adjusted the surface charge of NF membrane. The rejection reached the maximum of 95.7% for Ca[Cl.sub.2] with [P.sub.2-7] composite NF membrane, corresponding flux was 3,155 [Lm.sup.-2] [h.sup.-1]. The rejection reached the maximum of 93% for [Na.sub.2]S[O.sub.4] with [P.sub.3-5] composite NF membrane, corresponding flux was 3,879 [Lm.sup.-2] [h.sup.-1]. These excellent performances were the result of the membrane structure and charged modification through the introduction of chiral mesogenic compound and charged compounds. Comparing with conventional NF membranes, the membrane was used in low pressure with high flux, especially for the separation of high-valence ions from solution. The membranes were typical charged NF membranes.

ACKNOWLEDGMENTS

Financial support from the Scientific research program of Yingkou Institute of Technology (No. YYL201507) and the Scientific research program in general of Department of education of Liaoning Province (No. L2015552) are gratefully acknowledged.

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Tao Mu, Guanglei Tan, Guofeng Du, Lijie He, Zhen Li, Xuelei Li

Department of Chemical Engineering, Yingkou Institute of Technology, Yingkou 115014, People's Republic of China

Correspondence to: T. Mu; e-mail: mutao780920@126.com

DOI 10.1002/pen.24381

TABLE 1. Polymerization feeding of [P.sub.1].

Polymer 1      [m.sub.cts] (g)   [m.sub.M1] (g)   A (a)

[P.sub.1-0]         1.09               0            0
[P.sub.1-1]         1.09             0.026        0.01
[P.sub.1-2]         1.09             0.051        0.02
[P.sub.1-3]         1.09             0.129        0.05
[P.sub.1-4]         1.09             0.257         0.1
[P.sub.1-5]         1.09             0.514         0.2
[P.sub.1-6]         1.09             1.258         0.5
[P.sub.1-7]         1.09             2.056         0.8
[P.sub.1-8]         1.09              2.57         1.0

(a) A: the molar ratio of monomer [M.sub.1] and primary hydroxyl.

TABLE 2. Polymerization feeding of P2.

              [m.sub.cts]   [m.sub.M1]   [m.sub.M2]
Polymer 2        (g)          (g)          (g)       A (a)   B (b)

[P.sub.2-0]     1.09         0.257          0         0.1      0
[P.sub.2-1]     1.09         0.257        0.0098      0.1    0.01
[P.sub.2-2]     1.09         0.257        0.0196      0.1    0.02
[P.sub.2-3]     1.09         0.257        0.0489      0.1    0.05
[P.sub.2-4]     1.09         0.257        0.0978      0.1    0.1
[P.sub.2-5]     1.09         0.257        0.1955      0.1    0.2
[P.sub.2-6]     1.09         0.257        0.4888      0.1    0.5
[P.sub.2-7]     1.09         0.257        0.6843      0.1    0.7
[P.sub.2-8]     1.09         0.257        0.8798      0.1    0.9

(a) : the molar ratio of monomer [M.sub.1] and primary hydroxyl.
(b) : the molar ratio of monomer [M.sub.2] and primary hydroxyl.

TABLE 3. Polymerization feeding [P.sub.3].

               [m.sub.cts]   [m.sub.M1]   [m.sub.M3]
Polymer 3          (g)          (g)          (g)       A (a)   B (b)

[P.sub.3-0]       1.09         0.257          0         0.1      0
[P.sub.3-1]       1.09         0.257        0.0168      0.1    0.01
[P.sub.3-2]       1.09         0.257        0.0336      0.1    0.02
[P.sub.3-3]       1.09         0.257        0.084       0.1    0.05
[P.sub.3-4]       1.09         0.257        0.168       0.1     0.1
[P.sub.3-5]       1.09         0.257        0.336       0.1     0.2
[P.sub.3-6]       1.09         0.257         0.84       0.1     0.5
[P.sub.3-7]       1.09         0.257        1.176       0.1     0.7
[P.sub.3-8]       1.09         0.257        1.512       0.1     0.9

(a) A: the molar ratio of monomer [M.sub.1] and primary hydroxyl.
(b) B: the molar ratio of monomer [M.sub.3] and primary hydroxyl.


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Author:Mu, Tao; Tan, Guanglei; Du, Guofeng; He, Lijie; Li, Zhen; Li, Xuelei
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jan 1, 2017
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