preparation and characterization of polyacrylonitrile ultrafiltration membranes

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Journal of Membrane Science 222 (2003) 87–98 Preparation and characterization of polyacrylonitrile ultrafiltration membranes Sen Yang, Zhongzhou Liu Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, PR China Received 12 December 2002; accepted 5 March 2003 Abstract Asymmetric polyacrylonitrile (PAN) ultrafiltration (UF) membranes are prepared from three kinds of coagulant: water, aqueous solution of sodium chloride (NaCl) and aqueous solution of sodium carbonate (Na 2 CO 3 ), using dimethylacetamide (DMAC) as solvent and calcium chloride (CaCl 2 ) as additive by phase inversion method. The membranes are characterized in terms of the pure water flux, molecular weight cut-off (MWCO) profile and direct field emission scanning electron microscopy (FESEM) observations. The addition of CaCl 2 to the casting solution, up to 3 wt.%, increases the resulting membrane permeability while maintaining their retentive properties. A decrease of the permeability and retention properties is observed when the coagulation bath is aqueous solution of NaCl. Na 2 CO 3 in the coagulation bath reacts with CaCl 2 in the casting solution and produces precipitate of CaCO 3 . The effect of Na 2 CO 3 concentration on the PAN membrane permeability and retention properties is examined. It is found that the pure water fluxes of the PAN membranes increase drastically when the concentration of Na 2 CO 3 is high enough. © 2003 Elsevier B.V. All rights reserved. Keywords: Chemical reaction; Coagulant; Additive; Ultrafiltration; Polyacrylonitrile 1. Introduction The majority of polymeric membranes with asym- metric structure have been prepared by nonsolvent induced phase inversion (NIPI) process [1]. In this process, a homogeneous polymer solution is spread directly onto a suitable support by using a casting knife, and then immersed into a nonsolvent coagu- lation bath. The casting solution phase separation, responsible for a membrane formation occurs by Corresponding author. Tel.: +86-10-62849195; fax: +86-10-62923563. E-mail addresses: [email protected] (S. Yang), [email protected] (Z. Liu). the diffusional exchange of solvent and nonsolvent across the interface between casting solution and nonsolvent. It is well known that the formulation of the cast- ing solution and the coagulation bath can affect the final membrane structure and properties. Introduction of suitable additive to casting solution is one conve- nient and efficient method to obtain membranes with special properties. The additive may be water [2,3], inorganic salts [4,5], low molecular weight organ- ics [6–8], surfactants [9], polymer [10,11], mineral fillers [12–14], or mixtures of them [3,11]. There are several potential mechanisms through which such additives can affect the resulting membrane via 0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00220-5

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Page 1: Preparation and characterization of polyacrylonitrile ultrafiltration membranes

Journal of Membrane Science 222 (2003) 87–98

Preparation and characterization of polyacrylonitrileultrafiltration membranes

Sen Yang, Zhongzhou Liu∗Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,

P.O. Box 2871, Beijing 100085, PR China

Received 12 December 2002; accepted 5 March 2003

Abstract

Asymmetric polyacrylonitrile (PAN) ultrafiltration (UF) membranes are prepared from three kinds of coagulant: water,aqueous solution of sodium chloride (NaCl) and aqueous solution of sodium carbonate (Na2CO3), using dimethylacetamide(DMAC) as solvent and calcium chloride (CaCl2) as additive by phase inversion method. The membranes are characterizedin terms of the pure water flux, molecular weight cut-off (MWCO) profile and direct field emission scanning electronmicroscopy (FESEM) observations. The addition of CaCl2 to the casting solution, up to 3 wt.%, increases the resultingmembrane permeability while maintaining their retentive properties. A decrease of the permeability and retention propertiesis observed when the coagulation bath is aqueous solution of NaCl. Na2CO3 in the coagulation bath reacts with CaCl2 in thecasting solution and produces precipitate of CaCO3. The effect of Na2CO3 concentration on the PAN membrane permeabilityand retention properties is examined. It is found that the pure water fluxes of the PAN membranes increase drastically whenthe concentration of Na2CO3 is high enough.© 2003 Elsevier B.V. All rights reserved.

Keywords:Chemical reaction; Coagulant; Additive; Ultrafiltration; Polyacrylonitrile

1. Introduction

The majority of polymeric membranes with asym-metric structure have been prepared by nonsolventinduced phase inversion (NIPI) process[1]. In thisprocess, a homogeneous polymer solution is spreaddirectly onto a suitable support by using a castingknife, and then immersed into a nonsolvent coagu-lation bath. The casting solution phase separation,responsible for a membrane formation occurs by

∗ Corresponding author. Tel.:+86-10-62849195;fax: +86-10-62923563.E-mail addresses:[email protected] (S. Yang),[email protected] (Z. Liu).

the diffusional exchange of solvent and nonsolventacross the interface between casting solution andnonsolvent.

It is well known that the formulation of the cast-ing solution and the coagulation bath can affect thefinal membrane structure and properties. Introductionof suitable additive to casting solution is one conve-nient and efficient method to obtain membranes withspecial properties. The additive may be water[2,3],inorganic salts[4,5], low molecular weight organ-ics [6–8], surfactants[9], polymer [10,11], mineralfillers [12–14], or mixtures of them[3,11]. Thereare several potential mechanisms through whichsuch additives can affect the resulting membrane via

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0376-7388(03)00220-5

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88 S. Yang, Z. Liu / Journal of Membrane Science 222 (2003) 87–98

changing solvent capacity[15], precipitation kineticsand thermodynamic properties[10,16–18].

The addition of organic or inorganic componentsto a nonsolvent coagulation bath is another impor-tant method used in membrane modification. By thechoice of different nonsolvent (coagulant), the poly-meric membrane can be changed from asymmetric tosymmetric. Usually, water as the nonsolvent coagu-lant is the most common choice in preparing asym-metric membranes. Adding solvent into the water isa common method to suppression the formation ofmacrovoids (finger-like pores), particularly in prepar-ing hollow fiber membranes[19–21]. Chun et al.[22]reported that increasing the amount of dimethylac-etamide (DMAC) in the coagulation bath could delaythe demixing phase inversion, increase the stabilityof the polymer ternary system, and reduce the porespecific volume of sub-layer and the pore size in theskin layers.

Some researchers used the interaction betweenadditive in the casting solution and the coagulationmedium to explain the effect of additive on the prop-erties and morphology of membranes. Chuang et al.[23] proposed a mechanism describing the affinitybetween additive and coagulation medium to investi-gate the effect of dextran and poly(vinyl pyrrolidone)(PVP) additives on the formation of poly(vinyl al-cohol) (PVA) membranes. However, most of theseinteractions are physical interactions in nature. Toour knowledge there is no report on the preparationof asymmetric polymeric membrane by a chemicalreaction between additive and coagulant.

Polyacrylonitrile (PAN) is one of the versatile poly-mers that are widely used for making membranes dueto its good solvent resistance. PAN has been usedas a substrate for ultrafiltration (UF), microfiltration(MF) and reverse osmosis (RO)[24–26]. Schamagland Buschatz[25] reported pure water flux of PANmembranes was 688 and 6150 l/m2 h bar with 20,000and 15.7×106 u (20,000 and 15.7×106 Da) of molec-ular weight cut-off (100%).

In the present work, the polyacrylonitrile mem-branes were prepared from three kinds of coagulant:water, aqueous solution of sodium chloride (NaCl)and aqueous solution of sodium carbonate (Na2CO3),utilizing calcium chloride (CaCl2) as additive anddimethylacetamide as solvent. When the casting filmwas immersed in the aqueous solution of sodium

carbonate, the following chemical reaction occurred:

Na2CO3 + CaCl2 = CaCO3 ↓ +2NaCl

The effects of additive, coagulant and chemical re-action on the membrane properties and morphologywere investigated.

2. Experimental

2.1. Materials

Polyacrylonitrile was purchased from ShanghaiJinshan Chemical Industry Factory and dimethylac-etamide from Shanghai Organic Chemical IndustryFactory. Sodium carbonate, sodium chloride and cal-cium chloride were obtained from Beijing ChemicalIndustry Factory. Lysozyme (Shanghai Lizhu), pepsin(TBO, Tokyo), albumin egg (Sigma) and bovineserum albumin (BSA) (Beijing Shuangxuan) wereused in the retention test.

2.2. Viscosity studies

The viscosity of salt-free and salt-containingsamples of solvent and dilute polymer solutions(≤0.3 wt.% PAN) was measured using a Ubbelohdecapillary viscometer in a thermostatted water bath at20 ± 0.1◦C. For each polymer solution, two rela-tive viscosity (η) values were obtained. One relativeviscosity value was determined by normalizing theefflux time measured for the polymer solution withrespect to the efflux time of pure DMAC. The othervalue was obtained by normalizing the solution effluxtime with respect to that of CaCl2-containing DMAC.The viscosity of 12 wt.% PAN solutions with andwithout CaCl2 was obtained by using a Falling Ballviscometer (Thermo Haake) at 20± 0.1◦C.

2.3. Preparation of membranes

The casting solutions consisted of 12 wt.% solu-tion of PAN in DMAC to which quantity of CaCl2was added. Solutions were allowed to stand overnightbefore casting and then cast at room temperature ona glass plate by spreading them between thin wires(0.195 mm in diameter) with a glass knife in order tocontrol the thickness of the films without a preceding

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dry phase inversion in atmosphere; and then the filmswere immediately immersed in a coagulation bath at20◦C. The coagulation mediums were demineralizedwater or aqueous solutions of sodium salt (NaCl orNa2CO3) at different concentrations. After immersion,the membranes were kept in the coagulation bath forat least one night to complete the formation process.In order to remove the remaining solvent, additive orthe CaCO3 settlings out of the membrane structure,the membranes were rinsed with demineralized waterand wet stored until tested. The actual thickness of themembrane was measured using micrometer.

2.4. Membrane characteristics

2.4.1. Membrane structureThe morphology of the prepared membranes was

inspected with field emission scanning electron mi-croscopy (FESEM, AMARY, 1910FE). For this pur-pose, all samples were soaked in 40 vol.% glycerolaqueous solution for 24 h, dried in vacuum, frozen inliquid nitrogen and fractured. After plated with gold,they were transferred into the microscope.

2.4.2. Flux and separation experimentsA common permeation test apparatus was used to

measure pure water flux and protein separation ofPAN membranes. The obtained membrane sheets werecut into circle membrane species of 3.35 cm diame-ter before use. The pure water flux and protein reten-tion were measured at 100 kPa, room temperature and471.24× 10−1 rad/s (450 rpm). The filtration was re-peated three times on different membrane sheets sothat the total analyzed surface was 0.0079 m2. Threesets of membrane samples were made for each castingcondition specified in this paper and the average fluxand solute separation data were reported. After eachrun the whole test apparatus was rinsed thoroughlywith demineralized water and membrane was washedto remove any deposition.

Each membrane sample was initially compactedat 200 kPa with deionized water, until three suc-cessive flux measurements were constant. Flux wasdetermined by mass collected over a measured time(1–10 min).

Ultrafiltration experiments at 100 kPa and roomtemperature were carried out in the same assemblywith (i) lysozyme (0.05 g/dl), (ii) pepsin (0.1 g/dl),

(iii) albumin egg (0.1 g/dl) and (iv) bovine serum al-bumin (0.1 g/dl) solutions prepared in distilled water.The protein concentrations in the feed and permeatesamples were determined using a spectrophotometer(Shimadzu, UV-120-02) at 280 nm. And the molecu-lar weight cut-off (MWCO) profiles were constructed.

3. Results and discussion

3.1. Effect of different coagulation mediums onmembrane performance

3.1.1. Viscosity of the casting solutionViscosity of the casting solution can hinder severely

the exchange rate of solvent and nonsolvent duringphase inversion process, and therefore, it can be usedas an important parameter to influence the precipi-tation kinetics and thus, the formation of resultingmembrane morphology[18]. The viscosities of thesalt-free and salt-containing PAN solutions were nor-malized with respect to the pure DMAC solution vis-cosity and the relative viscosity (ηr) trends presentedin Fig. 1a. The curves plotted inFig. 1ashow that therelative viscosity of the PAN solutions increases withincreasing polymer concentration. At given PAN con-centration, the relative viscosity also increases withincreasing CaCl2 concentration.

However, when the PAN solution viscosities werenormalized with respect to the CaCl2-containingDMAC viscosity instead of the salt-free DMAC vis-cosity (η′

r), the curves plotted inFig. 1bshow that therelative viscosity data for all PAN solutions contain-ing CaCl2 now lie practically on the same curve. Thisresult suggests that at a given polymer concentration,the increase in the viscosity of PAN solutions withincreasing salt content is determined predominantlyby the viscosity characteristics of the salt–solventmedium. That is, the CaCl2 interacts more stronglywith DMAC solvent than with the PAN polymerchains, and it suggests that the effective solvatingpower of DMAC for PAN is progressively reducedwith increasing salt concentration. This is also consis-tent with the observation[27] that at a given polymerconcentration, the increase in the viscosity of thePAA–NMP solutions with increasing LiCl contentis mainly determined by the viscosity characteristicsof the salt–solvent mixture medium. Lee et al.[27]

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Fig. 1. Relative viscosity of PAN–DMAC solutions as a function of PAN and CaCl2 concentrations. Polymer solution efflux times werenormalized to (a) the efflux time of pure DMAC solvent (ηr), and (b) the efflux times of CaCl2-containing DMAC (η′

r). CaCl2 concentrationis 0, 1, 2 and 3 wt.% from bottom to top.

thought the reason is that LiCl interact more stronglywith NMP than with PAA, leading to the formationof LiCl–NMP complexes and, hence, a decrease inthe solvation power of NMP for PAA.

The viscosities (η) of the casting solutions areshown inTable 1. The addition of CaCl2 to 12 wt.%PAN/DMAC solution causes a decrease in viscosity,and the viscosity is the lowest (651 mPa s) when CaCl2concentration is 2 wt.%. This is consistent with thedecrease of solvent power of DMAC/CaCl2 for PAN.

3.1.2. Water as the coagulantFour polyacrylonitrile membranes, designated as

PAN1a to PAN4a were prepared using different con-centrations of CaCl2. The compositions of casting

Table 1PAN membranes coagulated in water

Membrane Composition of casting solution (wt.%) Solution viscosityη (mPa s) Total membrane thickness (�m)

P S A

PAN1a 12 88 – 1399 135PAN2a 12 87 1 963 125PAN3a 12 86 2 651 120PAN4a 12 85 3 708 117

P, polyacrylonitrile; S, DMAC; A, CaCl2. Coagulation bath: demineralized water at 20◦C.

solution and total membrane thickness are given inTable 1. The cast thickness of all the four membranes,before gelling, is 195�m based on the thickness ofcast knife and the final membrane thickness is foundto be thinner than the cast thickness indicating den-sification of polymer network during gelation[28].Table 1 shows that the PAN membranes preparedfrom CaCl2 is thinner than that of the PAN mem-branes prepared from neat DMAC, and the thicknessof PAN membrane decreases with increasing of CaCl2concentration.

Fig. 2 shows the relative pure water fluxes in thevarious membranes. PAN membranes prepared fromCaCl2 have considerably higher flux than membranesprepared from neat DMAC. PAN2a, PAN3a, and

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Fig. 2. Pure water flux of PAN membranes prepared from different concentrations of CaCl2 (wt.%).

PAN4a have 191, 229 and 289% of the PAN1a flux,respectively. The results indicate that the pure waterflux of PAN membranes can be increased by additionof CaCl2 as additive, and increasing the content ofCaCl2 in the casting solution from 1 to 3 wt.%, thepure water permeability rate increases almost linearly.

3.1.3. Aqueous solution of NaCl as the coagulantThe effect of aqueous solution of NaCl as the coag-

ulant on the membrane performance is studied. NaClconcentration varies from 1 to 20 wt.%, and the com-position of the casting solution keeps unchanged. Thecomposition of the coagulation bath, the gelation timemeasured by stopwatch (the gelation time is definedas from the casting solution being put into the coag-ulation bath to the appearance of the totally opaque

Table 2PAN membranes coagulated in NaCl solution

Membrane Composition of the coagulation bath (wt.%) Gelation time (s) Total membrane thickness (�m)

NaCl Water

PAN1b 1 99 10 125PAN2b 5 95 12 128PAN3b 10 90 17 210PAN4b 20 80 25 291

The composition of the casting solution: polyacrylonitrile, 12 wt.%; DMAC, 85 wt.%; CaCl2, 3 wt.%. Temperature of coagulation bath:20◦C.

film) and the thickness of the membranes are shownin Table 2. The gelation time increases from 10 to 25 sas NaCl concentration increases from 1 to 20 wt.%,and the membrane prepared from aqueous solution ofNaCl is thicker than the membrane prepared from wa-ter, especially when the concentration of NaCl is morethan 5 wt.%.

A method for decreasing activity of the aqueous co-agulation bath is addition of an inorganic salt to thecoagulation bath[29]. Addition of inorganic salt to thecoagulation bath reduces the chemical potential (µ)of water due to the salt effect, and then reduces thedriving force for the film precipitation. Inflow of H2Oand NaCl (coagulant) and the outflow of DMAC (sol-vent) therefore become slow. This is consistent withthe result that NaCl as additive in the coagulation bath

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Fig. 3. Pure water flux of PAN membranes prepared from different concentration of aqueous solution of NaCl () and Na2CO3 ( ) (wt.%).

decreases the gelation rate. When the concentration ofNaCl is above 5 wt.% the thickness of the PAN mem-brane become thicker, resulting in a loose morphologyof membrane (Fig. 7b).

The pure water fluxes of PAN membranes preparedfrom different concentration of NaCl are shown inFig. 3. It can be seen that the pure water fluxes areapproximately equal to that of the membrane preparedfrom water when NaCl concentration is lower than5 wt.%. The pure water flux decreases from 688 to470 l/m2 h while the concentration of NaCl increasesfrom 5 to 20 wt.%.

Table 3PAN membranes coagulated in Na2CO3 solution

Membrane Composition of the coagulation bath (wt.%) Gelationtime (s)

The amountof precipitate

Total membranethickness (�m)Na2CO3 Water

PAN1c 0.1 99.9 11 Little 134PAN2c 1 99 10 Little 130PAN3c 2 98 10 Much 104PAN4c 5 95 6 More 88PAN5c 10 90 5 More 96PAN6c 20 80 3 Morea 162

The composition of the casting solution: polyacrylonitrile, 12 wt.%; DMAC, 85 wt.%; CaCl2, 3 wt.%. Temperature of coagulation bath:20◦C.

a The precipitate is locked in the membrane.

3.1.4. Aqueous solution of Na2CO3 as the coagulantCaCl2 in the casting solution can react with Na2CO3

in the coagulation bath to produce CaCO3 precipitate.The effects of this chemical process on the PAN mem-brane performance are investigated.

Composition of the coagulation bath, thickness ofthe membrane, gelation time and amount of precip-itate are shown inTable 3. In this system, Na2CO3as additive in the coagulation bath should reduce thedriving force for the membrane formation and the dif-fusion rate between solvent and nonsolvent just likeNaCl in the coagulation bath. However, the gelation

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rate accelerates with the increase of Na2CO3 concen-tration in the coagulation bath. That can be explainedby the reaction between CaCl2 and Na2CO3. Increas-ing Na2CO3 concentration, the chemical reaction be-come faster, the gelation time become shorter and themembrane become thinner except that the membranebecame thicker dramatically when the concentrationis 20 wt.%. An interesting phenomenon is noticed thatthe gelation rate of PAN membrane is so quick that theprecipitate cannot diffuse out of the membrane struc-ture and is locked in it when Na2CO3 concentrationis 20 wt.%. This can explain that the membrane be-come thicker and rough slightly compared with othermembranes.

Fig. 3 shows the relative water fluxes of variousmembranes. It can be seen that the pure water fluxalso increases as Na2CO3 concentration increase inthe coagulant. Compared to PAN membrane preparedfrom water with the same composition of casting so-lution, the pure water fluxes of PAN1c, PAN2c andPAN3c are lower and those of PAN4c, PAN5c, andPAN6c are higher. These results may be due to thecompetition between the salt effect and the effect ofthe chemical reaction. An interesting phenomenon isnoticed that the low Na2CO3 concentration shows asame effect on the permeation of PAN membranes justlike that of high NaCl concentration. Further studiesare in progress to explain the interesting results.

Fig. 4. MWCO profile of PAN membranes prepared from different concentrations of CaCl2. CaCl2 concentration: (a) 0 wt.%; (b) 1 wt.%;(c) 2 wt.% and (d) 3 wt.%.

3.2. Membrane characterization

3.2.1. Molecular weight cut-off profileThe molecular weight cut-off profiles are con-

structed for PAN membranes by measuring soluteseparation for four proteins of molecular weight rang-ing from 14,400 to 67,000 u (14,400 to 67,000 Da).The results are given inFigs. 4–6.

For the various membranes prepared from CaCl2as additive and water as the coagulant, the molec-ular weight cut-off profiles (Fig. 4) are superposedat molecular weight of protein higher than 35,000 u(35,000 Da). The results indicate that CaCl2 as addi-tive can increase the pure water flux of PAN mem-brane and does not change the average pore sizesignificantly compared with the PAN membrane pre-pared from water as the coagulant and without anyadditive in the casting solution. Shinde et al.[30] ex-amined the effect of various inorganic halides (LiCl,ZnCl2 and AlCl3) added to a casting solution of PANin N,N-dimethyl formamide (DMF). They reachedthe same conclusion that addition of di- and trivalentsalts resulted in membranes with a pore size similarto the membrane prepared without any additive.

ComparingFig. 4 with Fig. 5, an interesting resultcan be drawn that the aqueous solution of NaCl as thecoagulant reduces the protein retention slightly. Desh-mukh and Li [31] studied the effect of coagulation

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Fig. 5. MWCO profile of PAN membranes prepared from aqueous solution of NaCl. NaCl concentration: (a) 1 wt.%; (b) 5 wt.%; (c)10 wt.% and (d) 20 wt.%.

medium, ethanol (10–50%) and water (90–50%),on the PVDF hollow fiber membranes. They foundthat the presence of ethanol in the coagulation bathreduced the polymer precipitation rate in phase inver-sion process, and the effective porosity of the result-ing membranes decreased as ethanol concentrationin the coagulation bath increased. This can explainthe slightly decreased retention capability and the

Fig. 6. MWCO profile of PAN membranes prepared from aqueous solution of Na2CO3. Na2CO3 concentration: (a) 1 wt.%; (b) 5 wt.%; (c)10 wt.% and (d) 20 wt.%.

drastically reduced permeation property when NaClconcentration is high enough.

The retention properties of PAN membranes pre-pared from aqueous solution of Na2CO3 are showedin Fig. 6. PAN membranes prepared from the lowNa2CO3 concentration (1 and 5 wt.%) have the similarMWCO profiles compared with PAN membranes pre-pared from water as coagulant (Fig. 4). That means the

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Fig. 7. FSEM-pictures of the cross-sections of different membranes prepared from the systems 12/3/85 (w/w/w) PAN/CaCl2/DMAC. Thecompositions of the coagulant: (a) demineralized water; (b) 10 wt.% NaCl; (c) 1 wt.% Na2CO3; (d) 5 wt.% Na2CO3 and (e) 20 wt.% Na2CO3.

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low Na2CO3 concentration (1 and 5 wt.%) has littleeffect on the pore size distribution. PAN membranesprepared from the high Na2CO3 concentration (10 and20 wt.%) has the similar MWCO profiles, it becomesflat compared with PAN membranes prepared fromwater as coagulant (Fig. 4) and the higher the concen-tration, the flatter the MWCO profile. That means thereare some big defects on the membrane surface. Thesedefects are caused by some solid particles (CaCO3)formed in the membrane after the solidification takesplace. This explains the fact that the membrane be-comes thicker and rough slightly as compared withother membranes.

It can be seen fromFigs. 4–6that the separationrate increases with increase in molecular weight ofprotein solute and the profiles are not sharp but aregradual and diffusive for PAN membranes preparedfrom different coagulation mediums. The cut-off ofmost PAN membranes is found to be around 60,000 u(60,000 Da).

3.2.2. Membrane morphologyIn order to compare the morphology of PAN mem-

branes prepared from different coagulation mediums,the structural changes of the membrane are observedby using FESEM.

Fig. 7 shows some images of PAN membranecross-sections. PAN membranes (Fig. 7a) preparedfrom water exhibits a typical asymmetric structure[32], composed of a thin and dense skin layer and aporous bulk that contains independent finger-like cavi-ties enclosed in a porous solid matrix. The skin layer isresponsible for the permeation or retention of soluteswhereas the porous bulk acts as a mechanical support.Young and Chen[33] reported that the skin layerbecame less dense and the finger-like macrovoids be-came less evident, as the DMSO (solvent) content inthe coagulation bath increased. The macrovoids evencan be eliminated when the bath contains a significantamount of solvent. NaCl in the coagulation bath showsa similar effect on the structure of PAN membranes.When the content of NaCl is low in the coagulationbath, the structure of PAN membranes does not showany obvious change (the pictures are not shown); whenthe content is more than 10 wt.% the structure exhibitsa kind of transition from a typical asymmetric struc-ture to a structure between the asymmetric and thesymmetric (untypical asymmetric structure). FESEM

(Fig. 7b) shows that the macrovoids are lessened dras-tically and the surface layers become somewhat porousin the membranes prepared from 10 wt.% NaCl.

The change of PAN membrane structure shows a re-verse tendency while the coagulation bath is aqueoussolution of Na2CO3. The FESEM pictures (Fig. 7c–e)show the transition from an untypical asymmetricstructure to a typical asymmetric structure with in-creasing content of Na2CO3. This is consistent withthe acceleration of the reaction rate. These pictures(Fig. 7c and d) illustrate that the membrane thicknessdecreases when more Na2CO3 is added to the coagu-lation bath; this is consistent with the thickness resultof various membranes. This indicates a faster trans-portation of solvent/nonsolvent during the membraneformation process when Na2CO3 is added to the co-agulation bath, and the transportation is even fasterwhile more Na2CO3 is added to the coagulation bath.

Combining with the cut-off, the similar structure ofPAN membranes explains the almost same permeationprepared from water, 1 and 2 wt.% of NaCl and 5 wt.%of Na2CO3. The finger-like pores decrease in size andnumber, the sponge-like structure of the less denseskin layer explains the low permeation capability ofPAN membranes prepared from high concentration ofNaCl and low concentration of Na2CO3.

4. Conclusions

1. Membranes with high permeation can be preparedfrom polyacrylonitrile polymer with CaCl2 as ad-ditive, DMAC as solvent and water as the coagula-tion bath. The permeation of PAN membrane canbe controlled by the concentration of CaCl2.

2. Changing concentration of CaCl2 from 1 to 3 wt.%increases the membrane permeation and has littleinfluence on the average pore radius.

3. The aqueous solution of NaCl as the coagulationbath decreases the permeability of PAN membrane,and the higher the NaCl concentration, the morethe permeability decrement.

4. The chemical reaction between the additive andthe coagulation medium play a significant role inPAN membrane performance. The reaction rate cancontrol the membrane performance. The higher rateresults in the higher permeation. PAN membranes

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with big pore size can be fabricated by acceleratingthe chemical reaction.

5. FESEM pictures clearly show that the membranesall have an analogous typical asymmetric struc-ture, whenever the membranes were prepared fromlow concentration of NaCl, high concentrationof Na2CO3 or water; and the membranes pre-pared from the high concentration of NaCl andlow concentration of Na2CO3 have the analogousstructure—a structure between the asymmetric andthe symmetric.

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