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J. of Supercritical Fluids 56 (2011) 312–321
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids
journa l homepage: www.e lsev ier .com/ locate /supf lu
ntifouling performance of poly(acrylonitrile)-based membranes: From greenynthesis to application
elma Barroso, Márcio Temtem, Teresa Casimiro, Ana Aguiar-Ricardo ∗
EQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal
r t i c l e i n f o
rticle history:eceived 21 May 2010eceived in revised form7 September 2010ccepted 20 October 2010
eywords:upercritical carbon dioxideANEGAraft polymersembranes
oulingrotein filtration
a b s t r a c t
In order to develop clean ultrafiltration membranes able to prevent the fouling of biological compounds infiltration processes, poly(ethylene glycol) methyl ether acrylate (PEGA) was grafted to poly(acrylonitrile)(PAN) by free-radical polymerization in supercritical carbon dioxide (scCO2) and the grafted copolymerwas blended with PAN to fabricate porous membranes using scCO2-induced phase inversion method.Fourier transform infrared (FT-IR) analysis, 1H nuclear magnetic resonance (1H NMR) and differentialscanning calorimetry (DSC) confirmed that the poly(acrylonitrile)-graft-poly(ethylene oxide) (PAN-g-PEO) was successfully synthesized, for the first time, in scCO2. The effect of increasing PEGA content on theinitial monomer feed mixture on graft polymer morphology and average molecular weight was studied.Blended membranes with different PEGA contents were investigated by scanning electron microscopy(SEM), mercury porosimetry and dynamical mechanical analysis (DMA) to characterize their morpholog-ical, physico-chemical and mechanical properties. Moreover, water contact angle measurements, purewater permeability and filtration experiments were performed to evaluate membrane hydrophilicity and
fouling resistance properties. Permeation experiments of model foulants, bovine serum albumin (BSA)and starch solutions were used to investigate antifouling character of blend membranes at different pHs.PAN:PAN-g-PEO (70:30) showed to be the ultrafiltration membrane with best performance. Furthermore,comparing with conventional technologies blended membranes of PAN:PAN-g-PEO prepared by a scCO2-assisted process showed enhanced hydrophilicity, larger protein and starch solution permeabilities andgood resistance to irreversible fouling, indicating that the technology is an efficient process to preparenes f
fouling resistant membra. Introduction
Membrane fouling is one of the most important challenges facedn membrane operations [1,2]. Fouling results in flux decline, whichncreases the energy demand for filtration [3]. To counteract thisroblem, membranes are cleaned, often with aggressive chemi-als, to remove the foulants adsorbed on membrane surfaces. Whenleaning becomes ineffective, the membranes must be replaced [4].ouling by proteins, other biomolecules, and organic matter is gen-rally attributed to the hydrophobic nature of membrane materials,hich leads to a high interfacial energy with water-rich media that
s reduced upon biomolecules adsorption [5].Poly(acrylonitrile) (PAN)-based membranes combine sufficient
hemical stability with good membrane performance in aqueousltration applications [6], but still suffer from significant fouling3]. A promising alternative approach to PAN membrane surface
odification that avoids extra manufacturing steps and provides
∗ Corresponding author. Tel.: +351 212 949 648; fax: +351 212 948 550.E-mail address: [email protected] (A. Aguiar-Ricardo).
896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2010.10.035
or biomacromolecule separations.© 2010 Elsevier B.V. All rights reserved.
modification of membrane internal pores involves blending anamphiphilic copolymer with the hydrophobic base material, whichsegregates to the membrane surface during casting by immersionprecipitation (phase inversion) to produce a hydrophilic surface[5]using common organic solvents. Poly(ethylene oxide) [7] andpoly(ethylene glycol) methacrylate [3] are two common polymersthat have been used to copolymerize and graft with PAN, poly-sulfone (PS) [6] or poly(vinylidene fluoride) (PVDF) [6] in orderto prepare filtration membranes and silicone sensors [8] withimproved performance. Compared with unmodified controls, filtra-tion membranes incorporating surface-segregating comb additivesexhibited increased porosity, wetability and resistance to fouling byproteins, which translates into large flux enhancements [5].
Specially in the case of PAN incorporation, due to its high meltingpoint, high melt viscosity and poor thermal stability, the incor-poration should be made in the synthesis step, it means, that
acrylonitrile must be copolymerized with the other monomers inorder to achieve desirable polymer properties. Different polymer-ization and copolymerization routes have been followed, somereported works use traditional organic solvents such as, N, N-dimethylformamide (DMF) [9] and dimethylsulfoxide (DMSO)
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10]. Xu et al. [11] aiming to increase the hydrophilicity ofoly(acrylonitrile)-based membranes developed a water-phaserecipitation polymerization that is a “greener” route to copoly-erize the acrylonitrile monomer with water soluble monomers
ut this polymerization still involves costly purification and dry-ng processes; a potential alternative is the synthesis of PAN-basedolymers in scCO2. Different acrylonitrile copolymers were syn-hesized by atom transfer radical polymerization performed inupercritical media [12–14]. The use of scCO2 offers many advan-ages over conventional solvents: CO2 is nontoxic, nonflammable,nexpensive and readily available in high purity from a variety ofources [15]. Since it is a gas at normal pressure by simply reduc-ng the pressure of the system, it is possible to easily separatehe solvent and residues from the polymer, leading to highly pure
aterials [16].Poly(ethylene glycol) methyl ether acrylate (PEGA) is a macro-
olecule usually grafted with other polymers such as, PAN [3],oly(dimethylsiloxane) (PDMS) [17] and acrylates [18], in order to
mprove the properties of synthesized copolymers, namely theirouling resistance [3,18], surfactant capacity [17], the ability toerve as supports for solid-phase synthesis [19], hydrogels, etc.
PAN grafted with PEGA has been investigated in order to pro-uce antifouling ultra and microfiltration membranes [3,6]. Theouling is usually described by the formation of a “cake” on mem-rane surface [20]. The protein adhesion is affected by interactionsetween molecules and membrane surface, solution chemistry,H, ionic strength and membrane morphology [5]. An adjustmentf these parameters could be the key to solve the antifoulinghenomena. In food and biotechnological industries, ultra andicrofiltration processes besides proteins often deal with polysac-
harides. In fouling investigations, polysaccharide solution can bef great interest, namely starch that is an important dietary energyource and frequently present in complex mixtures of biomacro-olecules.In this work, the goal was to investigate the synthesis of PAN
rafted with PEGA using supercritical fluid technology and alsoo prepare PAN:PAN-g-PEO membranes by scCO2-induced phasenversion [21,22], in order to further evaluate the fouling resis-ance of the prepared membranes [23]. Filtration of bovine serumlbumin (BSA) and starch solutions was performed at differentHs to reveal if the environmental-friendly scCO2-assisted methodeveloped in this work is more effective in improving the foulingesistance of PAN:PAN-g-PEO membranes.
. Experimental
.1. Materials
Acrylonitrile (AN, purity 99%), N,N-dimethylformamideDMF, purity > 99), dimethylsulfoxide (DMSO, purity > 99%),
oly(ethylene glycol) methyl ether acrylate (PEGA, purity > 99%),,2′-azobis(isobutyronitrile) (AIBN, purity ≥ 98%), calcium car-onate, bovine serum albumin (BSA) (Mw = 66 kDa, purity ≥ 98%)nd starch (starch soluble, purity ≥99%) were purchased fromigma–Aldrich. Dimethylsulfoxide d6, was purchased from Cam-Scheme 1. Schematical representa
l Fluids 56 (2011) 312–321 313
bridge Isotope Laboratories, Inc. Calcium chloride (CaCl2, purity99.5%) was purchased from Merck and the sodium chloride PA(NaCl) was purchased from Panreac. Carbon dioxide (CO2) wassupplied by Air Liquide with 99.998% purity. All reagents wereused without any further purification.
2.2. Polymers synthesis
The synthesis of the PAN and PAN-g-PEO polymers followedthe procedure described by Temtem et al. [15]. The polymerizationreactions were performed in a 33 mL stainless steel high-pressurecell equipped with two aligned sapphire windows in both topssealed with Teflon o-rings. The cell is immersed in a thermostaticwater bath with ±0.01 ◦C of stability. Temperature control wasdone with a RTD with a precision of ±0.001 ◦C in a probe contactingthe cell, connected to a Hart Scientific PID controller. The internalagitation is assured by magnetic stirring. The pressure is controlledwith a manometer (SETRA, model Datum 2000) with a precision of±0.01 MPa.
The cell was charged with AN and PEGA (typically 3.8 g of feedmonomer mixture), in composition ratios ranging from 70–100 wt%of AN and 0–30 wt% of PEGA, and initiator (2 wt% of AIBN withrespect to total mass of reactants). The reactions were performed at65 ◦C and 25 MPa during 24 h under stirring, according to Scheme 1.At the end of the reactions the resulting polymeric materials wereslowly washed with fresh high-pressure CO2 for 1 h in order toremove any unreacted monomer.
2.3. Polymers characterization
Particle morphology observations were based on scanning elec-tron microscopy (SEM) data. SEM micrographs were obtained on aHitachi S-2400, with an accelerating voltage set to 15 kV. All sam-ples were gold coated before analysis.
Average molecular weights of the polymer samples were deter-mined by gel permeation chromatography (GPC) using a KNAUERsystem with an evaporative light scatter detector from Polymerlaboratories (temperature of evaporator: 175 ◦C, temperature ofnebulisator: 85 ◦C and the eluent rate flow was 1 mL/min of DMF).Two Polypore columns were used in series.
1H NMR spectra were recorded on a Bruker ARX 400 MHz spec-trometer. Approximately 0.010 g of sample was dissolved in 500 �Lof deuterated dimethylsulfoxide.
FT-IR measurements were performed using Winfirst Lite equip-ment (16 scans and 1 cm−1 resolution). Pellets containing finelygrounded powder of a small amount of each copolymer mixed withdried KBr (1:5 mass ratio) were made before recording.
Thermal analysis was carried out on a differential scanningcalorimetry (DSC) instrument from Setaram (Model DSC 131) witha scan rate of 10 ◦C min−1 and a temperature range of 25–250 ◦C.
Approximately 0.010 g of sample was used. The glass transitiontemperature is taken at the midpoint of the heat capacity transitionbetween the upper and lower points of deviation from the extrap-olated glass and liquid lines. The equipment was calibrated withindium (Tm = 156.6 ◦C and �Hm = 28.4 J mol−1) as the standard.tion of PAN-g-PEO synthesis.
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.4. Membranes preparation
PAN:PAN-g-PEO membranes were prepared following the pro-edure described in detail by Temtem et al. [21], using aigh-pressure cell optimized for membrane preparation withcCO2-induced phase inversion method. The casting solutions wereoaded in Teflon disks (with a diameter of 68 mm and 1.5 mmeight) and placed inside the high-pressure vessel. Casting solu-ions were prepared by blending PAN and PAN-g-PEO (50:50 wt%)n DMSO with the following composition: 20, 30 and 40 wt%. CO2 isdded until the desired pressure, with an exact flow, using a Gilson05 piston pump. After reaching the normal operational pressure,he supercritical solution passes through a back pressure regulatorJasco 880-81) which separates the CO2 from the solvent. All thesexperiments were performed at 20.0 ± 0.7 MPa with a CO2 flowf 10.0 g/min during 1 h. At the end, the system is depressurizeduring 10 min and a thin homogeneous membrane was obtained.
.5. Membranes characterization
Membranes morphology studies were based on SEM data. SEMicrographs were obtained on a Hitachi S-2400, with an accelerat-
ng voltage set to 15 kV. All samples were coated with gold beforenalysis.
Membrane porosity and pore size distribution was determinedy mercury porosimetry in a Micromeritics Auto Pore IV equipmentsing sample weights between 0.080 and 0.130 g in order to obtaingood distribution. These analyses were performed in two steps,rst applying low pressure at 345 KPa and in the second applyingigh-pressure at 223 MPa. The results were treated using SigmaPlotrogram.
Membrane hydrophobicity was evaluated through the measure-ent of the contact angles of water droplets Goniometer MODEL
AM 100.The tensile properties of the membranes were determined with
ensile testing equipment (MINIMAT firm-ware v.3.1) at room tem-erature. The samples were cut into strips with 2 mm × 15 mm. The
ength between the clamps was set at 5 mm and the speed of test-ng set at 0.1 mm min−1. A full scale load of 20 N and maximumxtension of 90 mm were used. The elastic modulus was calculatedrom the slope of the linear portion of the stress–strain curve. Allamples were tested in dry state at room temperature.
Load extension graphs were obtained during testing and con-erted to stress–strain curves applying equations:
F �L
tress = � =AStrain = ε =
L,
here F is the applied force, A the cross-sectional area, �L is thehange in length and L is the length between clamps.
able 1etails of feed composition, sample designation for graft polymers prepared and experim
Graft polymera ANb (wt%) Yieldc PAN-
PAN 100 87 AggrePAN-g-PEO (90:10) 90 80 AggrePAN-g-PEO (80:20) 80 74 AggrePAN-g-PEO (70:30) 70 73 Aggre
a The reactions were carried out at 25 MPa at 65 ◦C for 24 h in a 33 mL cell. 3.8 g of feedb Weight of AN with respect to total weight of AN + PEGA.c Determined gravimetrically.d As determined from SEM micrographs.e Appearance of the polymer after venting.f Determined by 1H NMR.
l Fluids 56 (2011) 312–321
2.6. Filtration experiments
The permeability to pure water was determined by measuringthe water flux trough the membranes using a 10 mL filtration stain-less steel high-pressure cell with an effective area of 4.1 cm2. All theexperiments were carried out varying the applied hydrostatic pres-sure from 0 to 0.18 MPa. At least three measurements of distilledwater flux were performed for each membrane. The permeabil-ity of the membranes was obtained by the slope of linear relationbetween flux and pressure, and it is given by Darcy Law:
F = Lp · �p
where F is the flux that passes through the membrane (L/(m h)), Lpis the permeability (L/(m h MPa)) and �p is the drop of pressure(MPa).
Fouling experiments were performed on 25 mm diameter mem-branes using the same filtration stainless steel high-pressure celldescribed above. The filtration cell was stirred at 500 rpm using astir plate to minimize concentration polarization. Fouling experi-ments were performed at 0.4 MPa. Firstly, distilled water (0.01 L)passed through the membrane until the flux remained stable. Sec-ondly, feed solutions containing BSA or starch (1 g/L) were filtrated,at different pHs (4.5, 7.4 and 10). Permeate samples of 0.001 Lwere collected along the permeation of a total of 0.030 L of foulantsolutions. BSA and starch retention values were obtained by mea-suring the foulant concentrations in the permeated samples byUV–visible spectroscopy using a Helios Alpha Double-Beam UV–visspectrophotometer. BSA and hydrolysed starch [24] concentrationswere obtained by measuring absorbances at 280 nm and 590 nm,respectively. To evaluate the water recovery property of these BSAand starch-permeated membranes, pure water flux was measuredagain after cleaning. Cleaning was performed by operating themembrane test unit with 0.020 L of distilled water until absorbanceat both wavelengths became zero. The accumulated concentrationsof BSA and starch recovered during the cleaning procedure allowus to determine the reversibility of fouling.
The BSA or starch rejection ratios were calculated by the follow-ing equation:
R(%) =(
1 − Cpermeate
Cfeed
)× 100,
where Cpermeate and Cfeed are the BSA or starch concentrations inpermeate and feed solutions, respectively, quantified by UV–visspectrophotometry.
The relative flux reduction (RFR) and the flux recovery ratio(FRR) were calculated as follows [25]:
(Jp
)
RFR(%) = 1 −JW
FRR(%) =(
JRJW
),
ental results obtained in the synthesis of PAN-g-PEO in scCO2.
g-PEO morphologyd Producte Graft conversion (%)f
gated Powder –gated Powder 40gated Powder 50gated Powder 73
monomer mixture and 2 wt% of AIBN with respect to total mass were used.
T. Barroso et al. / J. of Supercritical Fluids 56 (2011) 312–321 315
Fig. 1. SEM images of the synthesized graft polymers: (a) PAN (100); (b) PAN-g-PEO (90:10); (c) PAN-g-PEO (80:20); (d) PAN-g-PEO (70:30).
Table 2Graft polymers composition, molecular weights, polydispersity and molecular weight distributions.
Graft polymer PAN-g-PEO composition (%)a Mnb Mw
b Mw/Mnb Mw distributionsb
PAN (100) 100 180433 151656 1.19 UnimodalPAN-g-PEO (90:10) 92−8 136047 108629 1.25 BimodalPAN-g-PEO (80:20) 80−20 111756 70440 1.58 Bimodal
67
wow
3
3
ve
PAN-g-PEO (70:30) 73−27 824
a Obtained by integration of 1H NMR spectra.b Obtained by GPC.
here JW is the pure water flux of the membrane, JP is the fluxbtained during the loading stage and JR is the flux during theashing stage.
. Results and discussion
.1. Graft polymer synthesis
The graft polymerization of AN and PEGA was investigatedarying the initial reactants ratio composition. The reaction is a het-rogeneous precipitation polymerization following a free-radical
Fig. 2. FT-IR spectra of synthesized polymer
59469 1.18 Bimodal
mechanism. All experiments were performed at 25 MPa and 65 ◦C,ensuring initial homogeneous conditions. The reactions proceededfor 24 h, and at the end the cell was depressurized at the same tem-perature for 20 min. Table 1 presents the details of feed compositionand sample designation of the synthesized polymers. Four differ-ent initial monomer compositions were used, varying AN from 70to 100 wt% with respect to the total mass of reactants. At these con-
ditions, initial homogeneous mixtures were obtained. High yieldswere obtained for all the tested compositions and the polymer mor-phology was similar in all experiments. SEM images, presented inFig. 1, show that the graft polymers are formed by agglomeratedparticles. Similar structures were already obtained for other poly-s: (a) PAN and (b) PAN-g-PEO (70:30).
3 critical Fluids 56 (2011) 312–321
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16 T. Barroso et al. / J. of Super
ers synthesized by precipitation and dispersion polymerizationsn scCO2 [16,26].
Table 2 summarizes the average molecular weights, polydis-ersity (Mw/Mn), molecular weight distributions and calculatedopolymer compositions of the synthesized graft copolymers. Theverage molecular weight decreases when the ratio of PEO inraft copolymer increases. This could be explained by the higherolecular weight of PEGA macromonomer, which might lead to
igher molecular weight of the growing polymer chains in therst stages of polymerization. Monomers or oligomers with higherolecular weights being less soluble in scCO2 cannot sustain so
fficiently in the supercritical medium and consequently, copoly-ers with lower molecular weights and lower yields are obtained
27].Thermal analysis of synthesized polymers was also performed
y DSC. While PAN exhibited a glass transition temperature of 94 ◦Cn accordance with reported values in literature [28], for graft poly-
ers no glass transition temperatures could be determined in theemperature range of −130 ◦C to 200 ◦C, due to a highly mobilityindrance caused by the chemical grafting.
The synthesis of PAN and PAN-g-PEO was confirmed both byT-IR and NMR spectra analysis. Fig. 2a presents the FT-IR spec-ra for PAN, showing absorption bands at 2928 cm−1 (symmetricaltretching band –CH2–), 2239 cm−1 (symmetrical stretching bandCN) and 1452 cm−1 (asymmetrical stretching band –CH). As anxample, Fig. 2b shows the PAN-g-PEO (70:30) FT-IR spectra, wherepart from characteristic bands of PAN, also appear two absorp-ion bands at 1731 cm−1 (ester carbonyl absorption) and 1103 cm−1
asymmetrical C–O–C stretching) of PEO [9].Fig. 3 shows the 1H NMR spectra of PAN homopolymer (a), PAN-
-PEO (90:10) (b), PAN-g-PEO (80:20) (c) and PAN-g-PEO (70:30)d). In Fig. 3a it is possible to observe the characteristic peaks ofAN at 3.12 ppm (peak 1) and 2.02 ppm (peak 2) corresponding tohe � and � protons to the CN group, respectively. Comparing thepectra depicted in Fig. 3b–d we notice evident modifications inpectra due to the graft polymerization of PAN with PEGA. Besideshe characteristic proton signals of PAN (peak 1 and 2), two neweaks appear around 3.49 ppm (peak 3) and 3.65–3.66 ppm (peak). Peak 3 corresponds to the � proton to the CN group, which suf-ered a shift (�ı ∼0.5 ppm) due to graft linkage between PAN andEO. Peak 4 is the characteristic peak of PEO. Graft conversion wasetermined by integration of peaks 1 and 3. The values of graft con-ersion are presented in Table 1 where it is visible that the increasef PEO content in the graft polymerization leads to higher graftonversions [29]. This is in agreement with molecular weight dis-ributions obtained by GPC, shown in Table 2. As it can be seen PANomopolymer presents a unimodal distribution while PAN graftedith PEGA (for all PEO contents) shows a bimodal distribution due
o a percentage of PEO homopolymer.
.2. PAN:PAN-g-PEO membranes
In this work, the process parameters that influence the mem-rane formation – temperature, pressure and depressurizationime – were fixed at 40 ◦C, 20 MPa and 10 min, respectively. Allxperiments were performed using a CO2 flow of 10 mL/min forh. Initially, three different copolymer compositions in DMSOasting solutions were tested: 20, 30 and 40 wt% of mixtures ofynthesized PAN and synthesized PAN-g-PEO. However, the onlyomposition that showed to be able to form a regular membraneas the casting solution with 20 wt% of PAN(50):PAN-g-PEO(50)
ecause with other compositions irregular and stiff structures werebtained. Due to this fact, all the membranes produced and char-cterized in this work were obtained using casting solutions with0 wt% of PAN(50):PAN-g-PEO(50) for all PAN-g-PEO graft poly-ers synthesized. As it can be seen in Fig. 4 membranes show
Fig. 3. 1H NMR spectra of the synthesized polymers: (a) PAN homopolymer; (b)PAN-g-PEO (90:10); (c) PAN-g-PEO (80:20); (d) PAN-g-PEO (70:30).
quite different morphologies depending on the PAN-g-PEO poly-mer content in the casting solution. The PAN membrane presentsa more regular surface with no visible pores, but observing thecross-section morphology nanopores can be visualized. Blendedmembranes of PAN with graft-polymers PAN-g-PEO showed thatwith the increase of PEO ratio in the graft polymer, membrane poresat surface and cross-section became much larger. While PAN:PAN-
g-PEO (90:10) present sub-micron spherical pores, PAN:PAN-g-PEO(70:30) exhibit large pores as tubular macrovoids.Mercury intrusion porosimetry data showed distinct averagepore size diameters and different pore size distributions, depend-
T. Barroso et al. / J. of Supercritical Fluids 56 (2011) 312–321 317
F polymg O (80c
iaFbia
ig. 4. SEM images of the PAN and PAN:PAN-g-PEO membranes. Variation of the graft-PEO (90:10) cross-section; (d) PAN:PAN-g-PEO (90:10) surface; (e) PAN:PAN-g-PEross-section; (h) PAN:PAN-g-PEO (70:30) surface.
ng on the PEO content in the polymer used in the casting solution,s can be concluded from data summarized in Table 3 and shown in
ig. 5. PAN:PAN-g-PEO (80:20) and PAN:PAN-g-PEO (70:30) mem-ranes show very close porosity and pore size diameters, as its evident from Fig. 5, where half part of pore size distributionsre overlapped. The average pore size diameter and the porosity
er used in the casting solution: (a) PAN cross-section; (b) PAN surface; (c) PAN:PAN-:20) cross-section; (f) PAN:PAN-g-PEO (80:20) surface; (g) PAN:PAN-g-PEO (70:30)
increase for membranes with higher content of PEO in polymercomposition and these results are in agreement with SEM results
discussed above and with the literature data [3].The strong morphology changes due to the increase of PAN-g-PEO content in membrane composition lead to very high pure waterfluxes. Comparing with PAN:PAN-g-PEO (90:10) membrane, the
318 T. Barroso et al. / J. of Supercritical Fluids 56 (2011) 312–321
Table 3Membranes porosity and average pore size diameter of PAN and PAN:PAN-g-PEOblend membranes with different PAN-g-PEO contents.
Membrane Porosity (%) Average pore sizediameter (�m)
PAN 31 0.10
wPopwwbhhPbsmutbobbatgltmseasstthi(i
Fc
Table 4Effect of PAN-g-PEO composition on membranes’ permeability, contact angle andYoung modulus.
Membrane PermeabilityL/(m2 h MPa)
Contact angle (◦) Young modulus(kPa)
PAN 590a 100 ± 5 389
PAN:PAN-g-PEO(90:10)
1580 42 ± 5 209
PAN:PAN-g-PEO 2430 25 ± 5 158
PAN:PAN-g-PEO (90:10) 56 0.26PAN:PAN-g-PEO (80:20) 61 2.43PAN:PAN-g-PEO (70:30) 63 2.68
ater flux increased approximately four times the one observed forAN:PAN-g-PEO (70:30) membrane (refer to Table 4). The increasef PEO content in membranes composition enlarged their averageore size diameters, pores interconnectivity and thus membraneater permeabilities. Another relevant aspect to discuss relatedith membrane’s permeability is the hydrophilicity of the mem-
ranes surface. It is reported in literature that PAN membranesave a great performance in aqueous filtration applications [6],owever in this work, the measurement of water permeability forAN membrane was impossible to perform. PAN surface showed toe hydrophobic, only by changing to chloroform, a hydrophobicolvent, it was possible to perform the permeability measure-ents. This fact can be explained by the scCO2-assisted process
sed in this work to fabricate the membranes. Usually, the produc-ion of PAN membranes described in the literature is performedy traditional phase inversion method [3] using different polarrganic solvents [6] that can influence the polarity of the mem-rane. In this work, the phase inversion method was inducedy supercritical carbon dioxide which is nonpolar and acts asn anti-solvent to the polymer, so during polymer precipitationhe molecular arrangement should direct the more hydrophobicroups to the outer surface of pores and surface walls while theess CO2 philic groups tend to be more in the inner structure. Inhe case of plain PAN membranes, CO2-assisted phase inversion
ethod is able to produce membranes with more homogeneousurfaces than the traditional methods, leading to membranes withnhanced hydrophobic properties. Temtem et al. [21] described annalogous result obtained with polysulfone membranes using theame supercritical CO2-assisted method. The interactions betweenupercritical carbon dioxide molecules and the polymer chains,hat are precipitating from the casting solution, guide the struc-ural organization of polymer chains in order to produce a more
ydrophobic membrane surface. Contrarily, membranes with PEOn their composition, PAN:PAN-g-PEO (90:10), PAN:PAN-g-PEO80:20) and PAN:PAN-g-PEO (70:30), showed a good performancen water flux measurements substantiating the importance of
ig. 5. Pore size distributions obtained from different ratios of graft copolymer usingasting solutions with a concentration of 20 wt%.
(80:20)PAN:PAN-g-PEO
(70:30)5840 14 ± 5 91
a Measurement with chloroform.
membrane surface nature in terms of hydrophilicity or hydropho-bicity as well as the importance of interactions between polymer,solvent and anti-solvent during the process of membranes produc-tion [21]. The PEO chains from the blend polymer can enrich thepores surface walls and endow the membranes with sufficient andstable hydrophilic behaviour. The pure water permeability (PWP)exhibited by PAN:PAN-g-PEO membranes prepared in this workvaried from 1580 to 5840 L/(m2 h MPa) depending on PEO contentof graft polymer (Table 4). Asatekin et al. [3] prepared similar blendmembranes of PAN:PAN-g-PEO by conventional phase inversionachieving PWPs of 1590 L/(m2 h MPa) for a membrane with 20% ofPAN-g-PEO, approximately one-third of the highest PWPs achievedin this work with PAN:PAN-g-PEO (70:30). The increased water fluxis a significant potential advantage of these blended membranes forcommercial filtration processes [3,5]. In the case of PAN:PAN-g-PEO(70:30), the PWP is equal to the reported by Belfer et al. [30] for PANmembranes grafted with HEMA and is fivefold the value reportedby Ulbricht et al. [31] for membranes of PAN grafted with PEGAprepared by conventional phase-inversion methods.
Contact angle measurements are the most convenient wayto investigate the hydrophilicity and wetting characteristics ofpolymeric surfaces [3,32].Water contact angles of PAN:PAN-g-PEOmembranes are included in Table 4. Pure PAN membranes havethe highest pure water contact angle (100◦). As it was expectedmembranes with more PEO ether groups can establish morehydrogen-bond interactions with water and, consequently, havelower contact angles. The large pores on the surface of membranesproduced by graft polymers with more PEO content are anotherexplanation to the decrease of contact angles values [3].
DMA was used to characterize the viscoelastic properties ofthe prepared membranes. Tensile tests provide an indication ofthe strength and elasticity of the membranes which are impor-tant factors to consider in potential applications [33]. Fig. 6 showsthe correlation between membranes strain and the applied stress.
Fig. 6. Mechanical analysis of PAN and PAN:PAN-g-PEO membranes.
T. Barroso et al. / J. of Supercritical Fluids 56 (2011) 312–321 319
AN-g-
Atatmstcm
3
bwpwbbo7gct(ds(c(mtdmPstao
aPht
doibtr
mobility that starts suffering hydrolysis. At pH 4.5 and 7.4 insignif-icant differences in filtration profiles are observed. At both pHsapproximately 0.65 g/L of starch was permeated through the mem-brane. Comparing the antifouling profiles of both foulant model
Table 5Rejection, recovery and retention ratios of model foulants permeated through allblend PAN:PAN-g-PEO membranes at pH 7.4.
Membrane Foulant Rejection(%)a
Recovery(%)b
Retention(%)c
PAN:PAN-g-PEO(70:30)
BSA 79 21 <0.1
PAN:PAN-g-PEO(80:20)
BSA 63 14 23
PAN:PAN-g-PEO(90:10)
BSA 27 9 64
PAN:PAN-g-PEO(70:30)
Starch 60 10 30
PAN:PAN-g-PEO(80:20)
Starch 45 11 44
PAN:PAN-g-PEO Starch 35 15 50
Fig. 7. Antifouling profiles for all PAN:P
ll membranes presented a plastic behaviour, what means thathere is a proportional dependence of their stress at brake and theirchieved elongation. The slope of stress–strain curves gives us alsohe Young modulus that is a measure of membrane stiffness. Young
odulus data for all prepared membranes are presented in Table 4,howing that membrane’s stiffness decreases for higher PEO con-ent ratio in membrane composition. Surprisingly, no comparisonan be made with literature data as, to the best of our knowledge, noechanical properties of PAN:PAN-g-PEO have ever been reported.
.3. Fouling experiments in PAN:PAN-g-PEO membranes
In order to evaluate the antifouling capacity of blended mem-ranes, filtration assays with BSA and starch buffered solutionsere performed. PAN membrane prepared by scCO2-assistedhase-inversion process showed to be nonpermeable to pureater, so no fouling experiments could be performed with both
iomacromolecule solutions. By increasing PEO content on mem-ranes blend composition, different antifouling properties werebserved for BSA and starch. Fig. 7 shows the filtration profiles at pH.4 using membranes with different PEO contents. The PAN:PAN--PEO (70:30) membrane showed to be the best membrane in bothase-studies, BSA and starch, due to its less resistance in filtra-ion assays. In case of BSA (Fig. 7a) filtration with PAN:PAN-g-PEO70:30) membrane, the BSA concentration decreased at the mid-le of assay (0.8 g/L) and remains constant until the end of loadingtage. Besides, using PAN:PAN-g-PEO (80:20) and PAN:PAN-g-PEO90:10) membranes the BSA concentration stabilized at lower con-entrations, 0.5 and 0.25 g/L, respectively. In polysaccharide assaysFig. 7b), the starch concentration during the loading stage was
ore regular but more starch retention was observed compara-ively with BSA, for PAN:PAN-g-PEO (90:10) there was a significantecrease in starch concentration. For PAN:PAN-g-PEO (70:30)embrane the starch concentration stabilizes at 0.5 g/L while in
AN:PAN-g-PEO (80:20) and PAN:PAN-g-PEO (90:10) membranestabilizes at 0.45 and 0.2 g/L, respectively. All starch concentra-ion values are lower than BSA concentration values in loadingnd washing stages meaning that a more irreversible adsorptionf starch occurs.
Table 5 shows the rejection, recovery and retention values of BSAnd starch for all membranes. The values suggest that PAN:PAN-g-EO (70:30) is a promising membrane to prevent fouling although,ighly more efficient to prevent BSA fouling (retention value:<0.1%)han starch (retention value:<30%).
In order to improve the antifouling capacity of this membrane,ifferent pH conditions were explored to understand the influence
f this parameter on fouling phenomena. It is already describedn the literature that proteins and mineral salts adsorption coulde minimized by adjusting the pH of the feed composition nearhe iso-electric point (IEP) of the proteins [34]. For polysaccha-ides, namely starch that has no IEP, another strategy is adopted,PEO membranes: (a) BSA and (b) starch.
as the increase of feed solution pH. At very high pH conditionsstarch hydrolyses and its chain conformation is easily modifiedand consequently, the starch structure changes reducing the foul-ing [35].In this work, BSA and starch were permeated through themembrane at three different pHs (4.5, 7.4 and 10) leading to dif-ferent filtration profiles as plotted in Fig. 8. For BSA (Fig. 8a) thebest antifouling profile was observed at pH 4.5 (corresponding toits IEP) and the worst at pH 10. Increasing the solution pH awayfrom the IEP, the retention is significantly increased and the BSAconcentration collected in the permeate (loading stage) stabilizesat approximately 0.5 g/L (50% of feed solution BSA concentration).Besides, near the IEP (pH 4.5) when the protein has no charge, thefouling is less evident and a constant filtration profile (constant BSAconcentration in loading stage) is visible, near to 1 g/L, the concen-tration of the feed solution. At pH 7.4 a better profile is observedcomparing with pH 10 but it is at pH 4.5 that the protein adsorp-tion is practically negligible. These results are in agreement withthe literature because it is already described that the maximumpermeation of protein occurs when pH of buffered solution equalsprotein IEP [34]. The starch permeation (Fig. 8b) improved signif-icantly at pH 10, where the concentration in permeate stabilizedat approximately 0.8 g/L, only 20% reduction of the concentrationon feed solution (1 g/L). Starch is a complex and rigid molecule dueto the aromatic rings brought together in its chains but, as it waspreviously described and demonstrated in the literature [35], in abasic medium the starch fouling is reduced due to higher chains
(90:10)
a Values obtained during the loading stage in the filtration assays.b Values obtained during the washing stage in the filtration assays.c Values obtained by difference between the feed solution and rejection and
recovery values.
320 T. Barroso et al. / J. of Supercritical Fluids 56 (2011) 312–321
Fig. 8. Antifouling profiles at different pHs using the PAN:PAN-g-PEO (70:30) membrane: (a) BSA and (b) starch.
Table 6Model foulants rejection, recovery and retention ratios on PAN:PAN-g-PEO (70:30) blend membrane as a function of pH permeation conditions.
Solution conditions BSA Starch
Rejection (%)a Recovery (%)b Retention (%)c Rejection (%)a Recovery (%)b Retention (%)c
pH 4.5 96 4 0 56 9 35pH 7.4 79 21 0 60 10 30pH 10 56 9 35 75 15 10
very values.
smtbrtpattit
parsomatte
Table 7Relative flux reduction and flux recovery ratio of model foulants at different pHsusing the PAN: PAN-g-PEO (70:30).
Solution conditions BSA Starch
a Values obtained during the loading stage in the filtration assays.b Values obtained during the washing stage in the filtration assays.c Values obtained by difference between the feed solution and rejection and reco
olutions it can be stated that PAN:PAN-g-PEO (70:30) membrane isore resistant to protein deposition than to polysaccharide deposi-
ion. After permeation experiments, the membranes were cleanedy water rinsing to quantify foulants recovered and irreversiblyetained by the membranes. The results in terms of foulant rejec-ion, recovery and retention ratios are presented in Table 6 for allH conditions studied. The results obtained indicate that at pH 4.5nd 7.4 all BSA is recovered independently of the amount rejectedhrough the membrane, simply by PBS rinsing, meaning that athese conditions only reversible fouling occurs. Contrarily, somerreversible starch deposition occurs evidenced by the high reten-ion values included in Table 6.
To confirm the water recovery property of these BSA and starch-ermeated membranes, pure water fluxes were measured againfter cleaning by washing with deionized water. The relative fluxeduction (RFR) and the flux recovery ratio (FRR) values are pre-ented in Table 7. During the filtration, protein molecules depositedr adsorbed on the surface or inside membrane pores lead to a dra-
atic loss of flux specially at pH 4.5 (RFR = 41%) but the flux can belmost recovered (FRR = 88%) after cleaning, suggesting that pro-ein fouling is reversible. Starch permeation experiments showedhat, in this case, the pH increase leads to a lower RFR (55%) butven after PBS washing the flux can be only recovered to 71%.
Fig. 9. Flux profiles at different pHs using the PAN:P
RFR (%) FRR (%) RFR (%) FRR (%)
pH 4.5 41 88 74 49pH 7.4 62 72 79 57pH 10 82 34 55 71
Fig. 9 shows the pH-dependent flux profiles of BSA and starchpermeations. During the loading stage a flux decrease is registereddue the high amount of BSA and starch in contact with mem-branes. However, during the washing stages which were performedwith PBS, BSA and starch adsorbed on the membrane surface couldbe significantly removed and almost complete recovery of initialfluxes are attained at pH 4.5 and 7.4 after BSA solution filtrationand at pH 10 after starch solution filtration. All these results sus-tain the important role of pH in preventing fouling phenomena in
biomacromolecules filtration processes. Changing the pH of thesolutions higher fluxes can be achieved during loading stages iffouling is minimized. In the case of BSA solutions faster and effi-cient filtration profiles can be obtained at pH 4.5 while starch ismore recovered at pH 10.AN-g-PEO membrane: (a) BSA and (b) starch.
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T. Barroso et al. / J. of Super
The introduction of PAN-g-PEO content in blend membranesodified dramatically the membranes morphology as well as its
ydrophilicity. The combination between the controlled mem-rane morphology induced by scCO2 processing, the adequatelend composition and the optimum permeation conditions are keyspects to find the ideal membrane with higher fouling resistanceor a specific biofoulant that we might need to avoid.
. Conclusions
In order to prevent the fouling in filtration processes, PAN-g-EO polymers were successfully synthesized in scCO2 and blendedembranes of PAN and PAN-g-PEO were prepared to assure no
races of organic solvents or contaminations. By varying the graftolymer concentration in the casting solution, it was possible toontrol the membrane’s properties in terms of mean pore size,ydrophilicity, mechanical behaviour, permeability and foulingesistance. PAN:PAN-g-PEO (70:30) showed to be the membraneith best performance in filtration studies. These ultrafiltrationembranes resisted completely to irreversible fouling, at pH
.5 and 7.4, by 1 g/L of BSA recovering 90% of initial flux afterashing the membrane with no need for aggressive cleaning pro-
edures and resisted to irreversible fouling by 1 g/L of starch butith lower recovery of water flux (71%). Furthermore, comparingith conventional technologies blended membranes of PAN:PAN-
-PEO prepared by a scCO2-assisted process showed enhancedydrophilicity, larger protein and starch solution permeabilitiesnd good resistance to irreversible fouling, indicating that theechnology is an efficient process to prepare fouling resistant mem-ranes for biomacromolecule separations.
cknowledgements
The authors would like to thank the financial support fromundacão para a Ciência e Tecnologia (FCT-Lisbon) throughontracts PTDC/CTM/70513/2006, MIT-Pt/BS-CTRM/0051/2008,octoral grant SFRH/BD/62475/2009 (T.B.) and National NMRetwork REDE/1517/RMN/2005, MIT-Portugal Program, Bioengi-eering Systems Focus Area, Fundacão Calouste Gulbenkian, FEDERnd FSE. We wish to thank the Analytical Services Laboratory ofEQUIMTE. AAR is grateful to V.D.B. Bonifácio for his help anddvice.
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