Desalination 286 (2012) 131–137

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Desalination

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Preparation and characterization of PSf/clay nanocomposite membranes withPEG 400 as a pore forming additive

Yuxin Ma a,⁎, Fengmei Shi b, Zhengjun Wang a, Miaonan Wu a, Jun Ma b, Congjie Gao c

a College of Architecture and Civil Engineering, Heilongjiang University, Harbin 150080, PR Chinab School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, PR Chinac College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, PR China

⁎ Corresponding author at: College of Architecture andUniversity, Harbin 150080, PR China. Tel.: +86 451 8660

E-mail address: oucmyx@126.com (Y. Ma).

0011-9164/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.desal.2011.10.040

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 August 2011Received in revised form 13 October 2011Accepted 28 October 2011Available online 5 December 2011

Keywords:ClayNanocomposite membranePSfUltrafiltration

Flat sheet asymmetric PSf/clay nanocomposite membranes with different clay dosage were prepared byphase inversion method. Dimethyl acetamide was used as a solvent and water was used as a coagulant.PEG 400 was used as a pore forming additive in the casting solution. The morphology and structure of mem-branes were analyzed by scanning electron microscope, transmission electron microscope and X-ray diffrac-tometer. The performance of membranes was evaluated in terms of pure water flux (PWF), protein rejection,porosity, contact angle, tensile strength and elongation at break respectively. Results showed that clay had agood dispersion in the PSf matrix. The addition of clay additive increased the ratio of large pore in the skinlayer and weakened the tensile strength. PWF and porosity of membranes increased with the increase ofclay dosage. With increase in clay dosage from 0 wt.% to 6 wt.%, the PWF increased from 342 L m−2 h−1 to382 L m−2 h−1.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polymer-inorganic nanocomposite membranes (PINMs), also calledmixed matrix membranes (MMMs), which were formed by inorganicparticles uniformly dispersed in a polymer matrix, have receivedmuch attention in thefield of gas separation, pervaporation and ultrafil-tration (UF) membranes for many years. They present an interestingapproach to improve the separation properties of polymer membranesbecause they possess properties of both organic and inorganicmembranes such as good permeability, selectivity, mechanical strength,and thermal and chemical stability [1]. The structure and performanceof nanocomposite membranes are generally a function of the physicaland chemical properties of the polymer matrix and nanoparticles aswell as the method of nanoparticle incorporation.

Depending on how many dimensions in the nanometer range,inorganic particles dispersed in the PINMs can be distinguished as isodi-mensional nanoparticles, nanotubes or nanowires, clays when thedimensions in the nanometer range are three, two, one respectively.Most of the recent studies on PINMs have focused on introduction ofisodimensional nanoparticles such as TiO2 [2–8], Al2O3 [9–11], ZrO2

[12], SiO2 [2,13–15] nanoparticles to better membrane skin properties(thickness, overall porosity, and surface porosity) and the macrovoidmorphology of the support layer. The PINMswere prepared by blendingpolymer solutions with nanoparticles (ex situ) [1–3,5–10,12–14] or

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with their precursors (in situ) [4,15]. Carbon nanotubes (CNTs) areoften employed asfiller materials to prepare porous polymer nanocom-posite hydrophobic membranes [16,17] or gas separation membranes[18]. However, the inorganic ingredients are liable to agglomerate inthe membranes because of the huge difference between the polymerand inorganic materials in their properties and strong agglomerationof the nanofillers [19,20]. The agglomeration of nanofillers in themembrane weakens the mechanical strength of the membrane.

Smectite-type clays, such as hectorite, montmorillonite (MMT), andsynthetic mica are commonly used as fillers to enhance the propertiesof polymers [21]. Smectite type clays have a layered structure. Eachlayer is constructed from tetrahedrally coordinated Si atoms fusedinto an edge shared octahedral plane of either aluminumormagnesiumhydroxide. The layer thickness is around 1 nm, and the lateral dimen-sions of these layers may vary from 30 nm to several microns or larger,depending on the particular layered silicate. Hundreds or thousands ofthese layers are stacked together with weak van der Waals forces toform a clay particle. With such a configuration, it is possible to tailorclays into various different structures in a polymer. The use of clay inthe polymer–clay nanocomposite technology can be considered animportant technique to produce polymer inorganic nanocompositematerials, since by adding very low clay content (b10 mass% clay) tothe polymers, several properties such as mechanical, thermal, optical,electric, flammability and barrier properties can be enhanced [22–28].

Modified MMT (protoned MMT, sulfonated MMT or organophilicMMT) were used in the production of proton exchange membranemembranes for fuel cell as polymer additives because of their nanome-ter size, proton conductivity and potential to reduce fuel (e.g.methanol)

Table 1Composition of the casting solution.

Membranes PSf (g) DMAc (g) Clay (g) PEG 400 (g) wclay* (wt.%)

PNM-0 13.50 61.50 0 6 0PNM-1 13.50 61.50 0.135 6 1PNM-2 13.50 61.50 0.27 6 2PNM-3 13.50 61.50 0.405 6 3PNM-4 13.50 61.50 0.54 6 4PNM-5 13.50 61.50 0.675 6 5PNM-6 13.50 61.50 0.81 6 6

*wclay is the mass ratio of clay to PSf.

132 Y. Ma et al. / Desalination 286 (2012) 131–137

permeability [29–33]. Incorporation of nanoclay can be considered asan effective method to improve the mechanical strength in porousmembrane supports [33]. Picard et al. [34] and Alexandre et al. [35]studied the barrier properties of polyamide/montmorillonite nanocom-posite membranes. Picard et al. found nanocomposite membranesexhibited superior barrier properties to gas and water than neat poly-amide film. Alexandre et al. found that nanocomposite films had a per-meability transition at low silicate content of 2.5 wt.%. Up to 2.5 wt.% ofthe C30B (organophilic MMT) nanofiller, the water permeability de-creases with the increase in the nanofiller volume fraction. Koh et al.[36] proved the particular barrier property through intercalation ofclay in the Poly(lactic acid) matrix and found that both gas and vaporpermeabilities of the prepared nanocomposite membranes decreasedwith the increase of clay content.

Polysulfone (PSf) membranes have been widely used asmicrofiltra-tion and ultrafiltration membranes in many industrial fields for theirlow cost, superior film forming ability, good mechanical and anti-compaction properties, strong chemical and thermal stabilities andoutstanding acidic and alkaline resistance. However their hydrophobicnature that often results in severemembrane fouling and decline of per-meability has been a barrier for their application of water treatment[37]. OrganophilicMMTwas commonly used as additives in the PSf/claynanocomposites [38–40]. A few papers about PSf/clay nanocompositemembranes were presented. Monticelli et al. [41] prepared porousmembranes and dense films with polysulfone solutions in N-methyl-2-pyrrolidone (NMP) containing different types and amounts of clay(unmodified or organically modified) without any other additives.They found that Cloisite Na and Cloisite 93A formed microaggregates,Cloisite 30B yielded nanostructures composed of both single sheetsand well-ordered multilayer silicate clusters. The addition of Cloisite30B to the casting solution influenced the phase-separation process inthe coagulation bath. Cloisite 30Bwas also found to improve the wetta-bility and mechanical properties of dense films. Anadão et al. [42] alsoprepared PSf/MMT nanocomposite membranes with a combination ofthe solution dispersion method and the final step of the wet-phaseinversionmethod. They found that Na-MMT intercalated and exfoliatedin the PSfmatrix. Thermal stability and hydrophilicity of nanocompositemembranes were improved by the incorporation of MMT. In addition,clay minerals are organophobic in nature, therefore organic modifica-tion is required to provide a microchemical environment for intercala-tion with hydrophobic polymers [44,45]. Intergallery distance ofunmodified MMTs such as Na-MMT is relatively small and will affectthe distribution state of MMT in the nanocomposites. The optimumorganoclay contents in the nanocomposites were commonly 3 wt.% ofthe polymers [38–40]. The clay contents used in the study of Monticelliet al. were only 0.5–1.25 wt.% of the polymer. Additives such as PEG,PVP and LiCl were often needed in the preparation of membrane asthe pore forming agent and PSf membrane prepared without additiveshad lower permeability [41,43]. It is necessary to study the effect ofclay dosage on the structure and performance with the addition ofadditives.

This study aims to prepare flat sheet PSf/clay nanocomposite mem-branes by phase inversion method. N,N-dimethyl acetamide (DMAc),deionized water and PEG 400 were used as solvent, coagulant andpore forming agent respectively. Effects of clay (organophilic MMT)dosage on morphology, the permeation characteristics and mechanicalproperties of the prepared membrane were investigated in detail.Morphology of each membrane was analyzed by scanning electron mi-croscopy (SEM). The performance of the membranes was investigatedby water permeation and bovine serum albumin (BSA) and pepsinrejection behavior. Mechanical properties were determined by tensilestrength and elongation at break. X-ray diffraction (XRD) patterns,transmission electronmicroscopy (TEM) and scanning electronmicros-copy (SEM) provided information about nanocomposite structurewhich was useful to understand changes in thermal and mechanicalresistances in comparison to the pure PSf membrane features.

2. Experimental

2.1. Materials

PSf (Udel P3500, obtained from its manufacturer) was used as thebase polymer in the membrane casting solution. DMAc (AP, TianjinBodi Chemicals Co., Ltd., PR China) was used as solvent. PEG 400(CP, Tianjin Kemio Chemicals Co., Ltd., PR China) was used as the non-solvent pore forming additive in the casting solution. Clay (organophilicMMT,MMTmodified by long chain alkyl (C16–C18) quatemary ammoni-um salt) was provided by Zhejiang Anji Tianlong Organic Bentonite Co.,Ltd. which has been commercially on the market under the trade nameof BT-880 and used as received. Deionized water was used as the non-solvent in the coagulation bath. BSA (MW=67000 Da) and pepsin(MW=35000 Da) were used in the solute rejection test.

2.2. Membrane preparation

PSf membrane and PSf/clay nanocomposite membranes wereprepared by phase inversion method. Certain amount of clay(0 wt.%–6 wt.% of PSf dosage) mixed with measured amount of DMAcand magnetically stirred for 0.5 h at ambient temperature. PSf andPEG 400 were added to premixed solutions and dissolved at 60 °C.The solution was magnetically stirred for at least 12 h to guaranteecomplete dissolution of the polymer. These prepared solutions werekept for at least 24 h without stirring at room temperature to removeair bubbles in the solution. The homogeneous casting solutions werecast uniformly onto a glass substrate by means of a hand-casting knifewith a knife gap set at 200 μm and immediately immersed into awater bath. An overview of the experimental conditions was reportedin Table 1.

2.3. Characterization of membranes

2.3.1. Permeation flux (PWF) and rejection (R)The permeation flux and rejection of the preparedmembraneswere

measured by an UF cross flow filtration experimental set-up fed withdistilled water at a transmembrane pressure of 100 kPa after pre-pres-surized for 30 min at 200kPa. The schematic of UF cross flow filtrationexperimental set-up was presented in our previous study [43]. Thepermeation flux were defined as formula (1).

PWF ¼ VA� t

ð1Þ

where PWF is the purewater flux (L m–2 h–1), V is the permeate volume(L), A is the membrane area (m2) and t is the time (h).

Rejection was characterized with 200 mg/L BSA aqueous solutionand 200 mg/L pepsin aqueous solution respectively after themembranewas previously filtered with pure water until flux was steady. The con-centrations of BSA and pepsin in permeate and feed were determined

133Y. Ma et al. / Desalination 286 (2012) 131–137

by an UV-spectrophotometer (Shimadzu UV-2450, Japan). It wascalculated according to formula (2).

R ¼ 1−Cp

Cfð2Þ

where Cp and Cf are the concentrations of protein in permeate and initialfeed, respectively.

2.3.2. Porosity (P) and Contact angle (CA)Membrane porosity was measured in the method of dry–wet

weight. The membrane maintained in distilled water was weighedafter mopping superficial water with filter paper. Then the wet mem-brane was placed in an air-circulating oven at 60 °C for 24 h and thenfurther dried in a vacuum oven at 80 °C for 24 h before measuring thedry weight. From the two weights (wet sample weight and dry sampleweight), the porosity of membrane was calculated using formula (3) as

P %ð Þ ¼ Ww−Wd

ρW � A� δ� 100 ð3Þ

where P is the porosity of membrane,Ww is the wet sample weight (g),Wd is the dry sampleweight (g), ρw is the density of purewater(g/cm3),A is the area of membrane in wet state (cm2) and δ is the thickness ofmembrane in wet state (cm). In order to minimize experimentalerror, each membrane was measured for three times and calculatedaverage.

The contact angle measurements were carried out with a contactangle meter (DSA100, KRÜSS). A water droplet was placed onto a flathomogeneous membrane surface and the contact angle of the dropletwith the surface was measured. The reported values were the averagesof the contact angles of five droplets.

2.3.3. Morphological studiesThe morphologies of the membranes were observed with a JEOL

JEM-6700F SEM. The observationswere carried out on the cross-sectionand upper surface of membranes broken in liquid nitrogen and coatedwith gold by sputtering. The top of the cross-section photographspresented in this article is the skin layer of the membrane (that is tosay the side of the membrane in direct contact with water in thecoagulation step).

The membranes to be examined were embedded into the epoxyresin. The epoxy resin was microtomed with a Leyca Ultracut-R into60–90-nm-thick slices in liquid nitrogen and then the slices wereobserved with a JEOL-JEM-2010, which has an acceleration voltage of200 kV.

2.3.4. Liquid–liquid displacement porosimetry (LLDP)Computerized analysis of SEM image is a standard and widely used

method for the investigation of perforated materials [46,47]. However,the morphological parameters such as pore size, pore number, etc. isdifficult to be measured from the SEM photographs as almost all thepores are in the ultrafiltration range [43,48]. Such practice would onlygive rough approximation of the pore size by overestimating thesmallest pores on the surface and also by considering dead end(blocked) pores along with the open pores [49]. Liquid displacementmethod was adopted to compare the morphology of differentmembranes.

The average pore size as well as the pore number and pore areadistribution of the preparedmembranes were determined by the liquiddisplacement method [49], also known as combined bubble pressureand solvent permeability method. In this method, the membrane iswetted previously with an appropriate penetrating liquid and then animmiscible liquid that does not wet the membrane is pressurized topass through the pores displacing the previous liquid which is alreadyoccupying the pores. In this work, the alcohol-rich phase of water–

isobutanol–methanol (25:15:7, v/v) mixture was taken as a wettingliquid, and aqueous-rich phase of the mixture was used as a displacingliquid. The contact angle between the liquid–liquid interface andmembrane material can be assumed as zero, the pore size wascalculated by Cantor's equation as follows [48,50].

r ¼ 2σΔP

ð4Þ

Here r is the radius of the wetting membrane, (m), σ is the surfacetension of the liquid–liquid displacement mixture at 20 °C, (mN/m),△Ρ is the pressure drop across the membrane, (N/m2).

The flowrate Q through a pore with the radius of r under a pressureof △Ρ was calculated by Hagen–Poiseuille equation.

Q ¼ π � r4 � Δp8μ � L� τ

ð5Þ

Here μ is the viscosity of the displacing liquid, L is the thickness of themembrane and τ is the tortuosity of the pores.

The pore size distribution function was given by Lloyd et al. [51]:

f rð Þ ¼ dQd Δpð Þ−

QΔp

� �Δp5

C15C2

ð6Þ

Here C1=2σcosθ, θ is the contact angle, C2 ¼ NTπ8μLτ and NT was the

total number of pores. In this study, surface tension of the system was0.35 mN/m at 20 °C, and τ was 1 [50,52].

2.3.5. Tensile strength and elongation at breakTensile strength and elongation at break of membranes were deter-

mined by a universal electronic strength measurement (AGS-J,Shimadzu). Measurements were carried out at room temperature anda strain rate of 20 mm/min was employed. The reported values werethe averages of at least eight samples.

2.4. Cloud point and ternary phase diagram determination

Cloud point determination method was reported in Blanco's study[53]. In this study, PSf/DMAc mixtures were kept 100%, clay additivesdosagewere based on the PSf/DMAcmixtures. DMAc solutionswith dif-ferent polymer contents (5, 10, 15, 20 wt.%) and different clay dosage(0, 0.4, 0.8 wt.%) were placed in tubes at a constant temperature(30 °C). Small volumes of a water/DMAc mixture (10/90 in weight)were added to the tubes until turbidity occurred (detected by visualobservation). The tubes were heated to 70 °C to dissolve the formedphase, then cooled down to 30 °C. The cloud point composition wascalculated from the mass balance in the system corresponding to theadded volume at which turbidity started to form upon cooling. Theprotocol was chosen because phases often separate locally at the spotwhere the non-solventmixture hits the polymer solution; if the systembecomes limpid after a heating–homogenization–cooling sequence,then the cloud point is not reached at the system composition and stud-ied temperature. Another volume of non-solvent mixture was added tothe polymer dope and the temperature sequence was repeated untilobservation of a persistent turbidity. Such a protocol is valid formixtures presenting an upper critical solution temperature, as thosein the present study.

3. Results and discussion

3.1. Morphological study

SEM analysis is an important technique to study the membranemorphology and qualitative information regarding surface and cross-sectional morphology of the membranes can be obtained. The SEM

PNM-0 PNM-5PNM-3

Fig. 1. Cross-section SEM images of PSf/clay nanocomposite membranes with different wClay.

PSf

DMAc

0 wt% Clay 0.4 wt% Clay 0.8 wt% Clay

Water

0.20

0.15

0.10

0.05

0.000.00 0.05 0.10 0.15 0.20

0.95

1.00

0.90

0.85

0.80

Fig. 3. Ternary phase diagram with cloud points for PSf/DMAc/water system withdifferent wClay.

134 Y. Ma et al. / Desalination 286 (2012) 131–137

images of the cross-section of different PSf/clay nanocompositemembranes prepared with different clay dosage were shown in Fig. 1. Itwas seen from Fig. 1 that themembranes were having asymmetric struc-ture consisting of a dense top surface layer (skin layer, air side) and aporous sublayer (support layer, glass side). The skin layer acts as a separa-tion layer and the support layer provides the mechanical strength. Thesublayer seemed to have finger-like cavities beneath the top surfacelayer and large voids near the bottom surface layer. When the dosage ofclay increased, the number of finger-like pores was found to increase. Atthe same time, the length of finger-like cavities was found shorter, butthe large voids were found larger and prominent.

The change in the size of pores due to the presence of clay in thepolymer structure can be explained as follows. Due to high mutualaffinity of DMAc for water, instantaneous demixing resulted in theformation of finger-like cavities in the sublayer of the preparedmembranes [49]. The existence of clay (organophilicMMT) in themem-brane casting solutions had two effects. (1) The clay existed in a state ofsolid. The membrane dope became thermodynamically less stable,which resulted in rapid instantaneous demixing when the membranedope was immersed in the coagulation bath. (2) Clay acted as a surfac-tant and decreased the tension of interface between the water andmembrane dope and affected the exchange rate of solvent and non-sol-vent during phase inversion process, and influenced the precipitationkinetics and the formation of resulting membrane morphology conse-quently. The clay mineral platelets was hydrophilic while the alkyl(C16–C18) groups adhered to the clay surface was hydrophobic. Whenthe nascent membrane was immersed in the non-solvent (water), theclay mineral platelets could enhance the inflow rate of water diffusionbecause of its hydrophilicity, the alkyl (C16–C18) groups had a goodcompatibility with the solvent (DMAc) and restrained the exchangeprocess between the non-solvent in the coagulation bath and thesolvent in polymer dope. This increased the porosity of membrane.

The SEM images of the upper surface of PSf/clay nanocompositemembranes prepared with different clay dosage were shown in Fig. 2.

PNM-0 PNM

Fig. 2. Upper surface SEM images of PSf/clay nan

The surface of thesemembranes is very dense. The pores of thesemem-branes were too small to be seen in 30000magnifications. The pore sizeof the UF membranes is usually characterized by their pore size distri-bution or molecular weight cut-off (MWCO). This type surfacemorphology may be due to the instantaneous demixing of membranecasting solution and rapid precipitation of polymer matrix.

The changes in the phase border lines (binodal curves) in PSf/DMAc/water systemwith different clay dosage were shown in Fig. 3. With the

PNM-5-3

ocomposite membranes with different wClay.

135Y. Ma et al. / Desalination 286 (2012) 131–137

increase of clay content, the path taken by the polymer solution of fixedstarting composition to reach the phase border line would be a littleshorter for a more clay dosage sample. The longer the solventexchanged through the skin formed when the dope was immersed inwater, themore developed the processes of polymer-lean phase growthand coalescence, thus the larger the finger-like pores. Indeed, theincrease of clay content increased the inflow rate of water diffusion inthe polymer solution film because of its hydrophilicity and led tomore large finger-like pores and large voids.

3.2. Structure of PSf/clay nanocomposite membranes

The XRD patterns of clay and PSf/clay nanocomposite membraneswere compared in Fig. 4. The organically modified clay exhibited threewell-defined peaks at d-spacings of 3.00, 2.08, and 1.48 nmcorrespond-ing to d001, d002, and d003 planes. No obvious peak of the clay was foundin the XRD diffractograms of PNM-3 and PNM-5 nanocomposite mem-branes. The results of XRD analysis inferred that the clay minerals wereexfoliated and had a good dispersion in the PSfmatrix. Theywere differ-ent from the results of Monticelli et al. [41]. The clay used in the exper-iment had the similar structure with Cloisite 93A, the membraneprepared with PSf /Cloisit 93A and NMP had a interlayer distance of2.91 nm. This showed the DMAc solvent was helpful to the exfoliationof the organoclay. With the clay addition in the PSf/DMAc solution,these clay minerals dispersed in the DMAc solvent. The organophilictreatment reduced the attractive forces between the clay layers, therebyfacilitating the intercalation of PSf in the intergallery space. The interca-lation led to the disordering of the layered clay structure and the stirringof membrane dope exfoliated the layered structure of clay. When the

200 nm

Fig. 5. TEM images of PSf and PSf/c

1 2 3 4 5 6 7 8 9 10 110

200

400

600

800

1000

1200

1400

1600

1800

2000

2 THETA

Organoclay PNM-0 PNM-3 PNM-5

Fig. 4. XRD diffractograms of PSf membrane and PSf/clay anocomposite membraneswith various clay contents.

membranes were immersed in the non-solvent bath, the solvent wascontinuously exchanged by water with a decreasing diffusion rate,since the coagulated surface could hamper the diffusion and the layers,filledwith polymer chains, were linked up forming this exfoliated struc-ture [42]. Nanocomposite membranes with a good dispersion of claywere formed. The result was different from the study of Anadão et al.,which had a partial intercalation/exfoliation structure [42].

TEM technique was carried out to characterize the morphology ofthe nanocomposite membranes. TEM images of PSf and nanocompositemembranes were shown in Fig. 5. The nanocomposite membraneprepared with 3.0 wt.% clay dosage showed individual clay mineralplatelets dispersed in the PSf matrix. Therefore, TEM result alsoevidenced that clay had a good dispersion in the PSf matrix.

3.3. Effect of clay dosage on membrane hydrophilicity and porosity

Hydrophilicity and porosity of the membrane are two importantparameters in the membrane permeation and the separation processand have a close relationship with PWF and the morphology of mem-branes. The contact angle (CA) is often used to describe the surfacehydrophilicity [54,55]. In general, membrane hydrophilicity is higherwhile its contact angle is smaller.

The contact angle (CA) and porosity of PSf/clay nanocompositemembranes with different clay dosage were shown in Fig. 6. It can beseen from Fig. 6 that with increase in the clay dosage, the porosityincreased from 45.2% for PNM-0 membrane to 48.1% for PNM-6 mem-brane and the contact angle had no obvious change. The addition ofclay to the membrane dope enhanced the inflow rate of water and hin-dered the exchange process between the non-solvent in coagulation

100 nm

lay nanocomposite membrane.

-1 0 1 2 3 4 5 6 7

45

46

47

48

75

76

77

78

79

80

81

82

83

84

85

CA

( o

)

Porosity

Po

rosi

ty (

%)

wClay

(%)

CA

Fig. 6. Porosity and contact angle of PSf/clay nanocomposite membranes with variouswClay.

0 10 20 30 40 50 60 70 80 90-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

F (

r)(1

/nm

)

R (nm)

PNM-0

PNM-3

Fig. 8. Pore size distribution of PSf membrane and PCM-3 nanocomposite membrane.

4.8

5.0

5.2

20

22Tensile strength

Pa)

Elongation

-1 0 1 2 3 4 5 6 765

70

75

80

85

90

95

340

350

360

370

380

390

PW

F (

Lm

-2h

-1)

Rej

ecti

on

(%

)

wClay (wt%)

RPepsin

RBSA

PWF

Fig. 7. Effect of wClay on the pure water flux and protein rejection of PSf/clay nanocom-posite membranes.

136 Y. Ma et al. / Desalination 286 (2012) 131–137

bath and solvent in polymer dope. This increased the ratio of watercontent in the nascent membrane and increased the porosity of mem-brane prepared.

-1 0 1 2 3 4 5 6 7

3.6

3.8

4.0

4.2

4.4

4.6

12

14

16

18

Elo

ng

atio

n (

%)

Ten

sile

str

eng

th (

M

wClay (%)

Fig. 9. Tensile strength and elongation at break of PSf/clay nanocomposite membraneswith various wClay.

3.4. Effect of clay dosage on membrane permeability, solute rejection andpore size distribution

Pure water flux (PWF) and pore size distribution are considered tobe the key specification factors for porousmembranes. PWF has a directrelationship with the number of pores and the pore size on themembrane surface (top layer porosity) [56]. The effect of clay dosageon PWF and protein rejection of PSf/clay nanocomposite membraneswere shown in Fig. 7. It can be seen from Fig. 7 that PWF increasedfrom 342 L m−2h−1 for pure PSf membrane to 382 L m−2h−1 forPNM-6 membrane. The PWF was higher than that of the membranesof Monticelli et al. because of the addition of PEG 400 additives [41].BSA rejection had no obvious change while pepsin rejection decreasedfrom 72.1% for PNM-0 membrane to 66.3% for PNM-6 membrane withthe increase of clay dosage. The increase of PWF may be ascribed tothe increase ofmembrane porosity. Another reasonmay be the increaseof pore number in the skin layer arising from the increase of porosity.

Liquid–liquid displacement porosimetry (LLDP) is a very suitableand accurate characterization technique, whose information is very im-portant in the case of UF membranes [50]. The pore size distribution ofPSf membrane and nanocomposite membrane was shown in Fig. 8. Itwas shown in Fig. 8 that the addition of clay had little effect on thepore size of membrane, the size of pores in PSf membrane and PNM-3membrane was in the size of 1–25 nm. The predominant pores were3 nm for the two membranes. The addition of clay increased the ratioof large pore in the skin layer. This is the main reason for PWF increasewith the increase of clay dosage.

3.5. Effect of clay dosage on membrane mechanical properties

Tensile strength and elongation at break are two important param-eters to describe the mechanical properties of membranes. The effectof clay dosage on the mechanical strengths of the PSf/clay nanocompo-site membranes was shown in Fig. 9. With the increase in clay dosage,the tensile strength at break decreased from 5.06 MPa for PNM-0mem-brane to 4.01 MPa for PNM-6 membrane and elongation at breakdecreased from 20.2% for PNM-0 membrane to 12.2% for PNM-6

membrane. This is maybe due to the increase of porosity weakenedthe strength of the membrane.

4. Conclusions

Flat sheet PSf/clay nanocomposite membranes with different claydosage were prepared by diffusion induced phase-separation methodwith PEG 400 as a pore forming agent. Effect of the clay dosage on themorphology, structure, pore size distribution and properties such aspermeability, protein rejection, porosity, contact angle and mechanicalproperties were studied. The results can be summarized as follows.

1) All the membranes are found to have asymmetric structure asseen from SEM photographs.

2) Clay has a good dispersion in the form of exfoliated platelets in thePSf matrix.

3) The addition of clay additive can increase the ratio of large pore in theskin layer and has little effect on the hydrophilicity of membranes.

4) The increase of clay dosage in the membrane dope can increase theporosity, and the PWF of nanocomposite membranes prepared.

5) The addition of clay additive weakens the mechanical propertiesof membranes.

137Y. Ma et al. / Desalination 286 (2012) 131–137

Acknowledgements

The authors are grateful for the financial support by the NationalNatural Science Foundation of China (Grant no. 50978067).

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