journa l homepage: www.e lsev ier .com/ locate /memsci
n experimental investigation of evaporation time and the relative humidity on aovel positively charged ultrafiltration membrane via dry–wet phase inversion
i Gao, Beibei Tang ∗, Peiyi Wu ∗
he Key Laboratory of Molecular Engineering of Polymers (Ministry of Education) and Department of Macromolecular Science and Advanced Materials Laboratory,udan University, Shanghai 200433, People’s Republic of China
r t i c l e i n f o
rticle history:eceived 6 July 2008eceived in revised form 1 September 2008ccepted 26 September 2008vailable online 5 October 2008
eywords:ry–wet phase inversion
n situ amination
a b s t r a c t
In this paper, a novel positively charged asymmetrical membrane was manufactured from bromi-nated polyphenylene oxide (BPPO)/N-methyl-2-pyrrolidone (NMP)/H2O via in situ amination withtriethanolamine (TEOA) and a dry–wet phase inversion. The casting solution was exposed to the humidsurroundings before immersing into the coagulation bath. The positive charge character of the membranesurface was examined by streaming potential and the effect of the evaporation time and the relativehumidity (RH) on the membrane properties and microstructure were investigated, respectively. It wasinterestingly found that the role of evaporation time and the relative humidity on the membrane per-formance and morphologies for a positively charged casting system was different from the conventional
ositively chargedvaporation timeelative humidity
rule. This was mainly due to the competition of two influence factors, i.e., evaporation of solvent andwater absorption of the casting solution. The results were conformed to SEM observation and pore sizedistribution. Furthermore, the process of water absorption of the casting solution was monitored by atten-uated total reflectance infrared (ATR-FTIR) spectroscopy technique. Additionally, in order to compare tothe dry–wet phase inversion method, the membranes were obtained by prolonging the exposure time tomore than 12 h (which was similar to vapor-induced phase inversion) at different RH. Polymer nodules
and a
cettmeommmsres
on the membrane surface
. Introduction
Polymeric membranes are nowadays used for a wide range ofndustrial separation applications extending from microfiltrationo reverse osmosis [1]. Their structures generally consist of a densekin supported by a porous sublayer. These asymmetric morpholo-ies can be produced by a phase separation process being a resultf either a temperature or a nonsolvent addition. There are gener-lly four phase inversion processes: immersion precipitation [2,3],hermally induced phase separation [4], vapor-induced phase sep-ration [5] and dry-casting of a polymer solution [6]. By combiningwo methods of immersion precipitation and the dry-casting pro-ess, a new process called dry–wet phase inversion is initiated inhich the dope solution is exposed to a nonsolvent vapor (usually
ater) for a time interval prior to immersion into a coagulationath. It has been well established that the properties of the mem-rane can be modified by varying the casting solution and theormation conditions by changing the main factors, such as the
omposition of the polymer solution (additives, etc.), the solventvaporation temperature, as well as the nature and temperature ofhe coagulation media [7]. Besides, the factors of the evaporationime and the relative humidity also have a great influence on the
orphology and property of the membranes. For example, Paulsent al. had investigated duration of the evaporation step on theccurrence and prominence of macrovoid pores in cellulose acetateembranes via the dry–wet phase inversion process [8]. Further-ore, Tsay and McHugh [9] had developed a ternary diffusionodel to analyze the mass transfer dynamics of the evaporation
tep. In addition, there had been several morphological studieselating the relative humidity of the humid surroundings in sev-ral recent papers. Sun et al. had pointed out that the pore size wastrongly affected by the relative humidity (RH) in the environment10]. And a symmetric morphology and the similar influence of theH were also reported by Park et al. [11] for the polysulfone/NMPystem and Caquineau et al. [12] for polyetherimde/NMP system.t should be noted that the above systems relating the relative
umidity were about the vapor-induced phase inversion. In fact,he relative humidity is also significant to membrane performancesnd morphologies during dry–wet phase inversion, especially, forhydrophilic casting system. However, it is seldom to be reported
hat the influence of relative humidity and evaporation time under
2.4.3. Pore size distributionPore size distribution of all membrane samples were deter-
mined by liquid–liquid interfacial contact method [16]. Theexperimental runs were carried on with the apparatus schemat-ically shown in Scheme 1. The membrane was equilibrated in
L. Gao et al. / Journal of Mem
certain RH on membrane performance and morphologies for aharged casting system.
In our previous studies [13], a new route was initiated forreparing a positively charged asymmetric membrane by in situmination and phase inversion, where amination process was per-ormed in the casting solution by directly adding trimethylamineTMA) organic solution into the brominated poly(2,6-dimethyl-1,4-henylene oxide) (PPO) casting solution. In our recent research,
t was found that triethanolamine (TEOA) was a better aminationgent than TMA because there are three hydroxyl groups on TEOAolecule structure, which could increase the hydrophilicity and
nti-fouling property of the membrane.Therefore, to that end and also as an extension of the research of
ur group on the positively charged membranes, it was examinedn detail to study the effect of relative humidity and evaporationime under a certain RH on the performance and morphologies ofhe positively charged membrane during dry–wet phase inversion.articularly, the positively charged membrane was prepared fromrominated polyphenylene oxide (BPPO)/N-methyl-2-pyrrolidoneNMP)/H2O by in situ amination in which triethanolamine (TEOA)sed as amination agent. Additionally, in order to compare to thery–wet phase inversion method, we also manufactured mem-ranes by prolonging the exposure time for more than 12 h (whichas similar to vapor-induced phase separation). A series of inter-
sting results were obtained.
. Experimental
.1. Materials
Ploy(2,6-dimethyl-1,4-phenylene oxide) (PPO) of intrinsic vis-osity equal to 0.57 × 10−3 m3 kg−1 in chloroform at 25 ◦Cas obtained from Institute of Chemical Engineering of Bei-
ing; Triethanolamine, chlorobenzene, bromine and N-methyl-2-yrrolidone (NMP) were all of analytical grade. Poly(ethylenelycol) (PEG) of molecular weight of 400 g/mol was chosen asrganic additive in casting solution and de-ionized water as nonsol-ent (coagulation bath). Brovine serum albumin (BSA) with averageolecular weight of 69,000 g/mol supplied by Sinopharm Chemical
eagent Co. Ltd. was used as a probe molecule for solution rejectionest.
.2. Bromination
PPO was dissolved in chlorobenzene to form about 8.0 wt%olution and this solution was subject to bromination by addinghlorobenzene-diluted bromine. The extent of bromination wasontrolled by the amount of bromine being added, while the sub-titution position (benzyl or aryl) was controlled by temperature.he final solution was precipitated with methanol, washed andried at 60 ◦C for at least 20 h to obtain the brominated poly-er.
.3. Membrane preparation
The brominated PPO was dissolved in NMP to form about 20 wt%omogeneous solution. Then, a given amount TEOA-NMP solutionnd additive PEG-400 were added into the polymer solution. Theolution was kept for some time at 40 ◦C to accelerate reactionetween TEOA and BPPO and then cooled down and agitated for cer-
ain time. Then, the casting solution was cast onto a clean glass platefter removed air bubbles. After exposed to a certain RH surround-ng for a period of time, the glass plate covered with casting solutionas horizontally immersed into de-ionized water for at least 24 h to
emove the solvent and solidify the membrane structure. Then the
Ssm
Science 326 (2009) 168–177 169
embranes were washed with de-ionized water repeatedly andetly stored.
.4. Membrane characterization
.4.1. Flux and separation experimentsPure water flux and protein rejection test were conducted with
cross-flow membrane module, as shown in our recent article14], which could offer a membrane area of 60 cm2. Before mea-urement, all the membranes were pretreated with high pressurerop (0.4 MPa) for about 20 min and then the pure water flux andejection were measured at 0.2 MPa operation pressure difference.
ater flux was achieved by measuring the volume of permeate thatenetrated the unit area membrane per unit time and calculateds F = V/At, where V is the total volume of pure permeated duringhe experiment, A represents the membrane area and t denoteshe operation time. De-ionized water was used to measure theure water flux of the membrane. The rejection rate was mea-ured with 0.5 g/L BSA (average molecular weight is 69,000 g/mol)olution. And the operation pressure was 0.2 MPa. The concentra-ion of the permeation and feed solution was determined by usingltraviolet–visible spectrophotometer (Lambda 35, PerkinElmer,SA) at 280 nm. The rejection, R, was calculated by R = 1 − (Cp/Cf),here Cp and Cf were the concentration of the permeation and the
eed solution, respectively.
.4.2. Streaming potentialThe setup to determine streaming potential was almost similar
o that described by our previous paper [15]. Reversible Ag/AgCllectrodes, placed on both sides of the membrane, were used toeasure the resulting electrical potential difference (�E) as the
ressure difference across the membrane (�P) changed through aigital electrometer (VC 890D, Shenzhen Victor Hi-tech Co. Ltd.).hen the streaming potential was calculated as SP = �E/�P. Theressure difference ranged from 0 to 4 × 105 Pa. The 0.1 mol/m3 KClolution was put in the unit by N2 pressure controlled with a gauge.
cheme 1. Apparatus for pore size distribution (1, N2 steel bottle; 2 and 4, pres-ure valve; 3 and 5, pressure gauge; 6, i-pentanol container; 7, i-pentanol phase; 8,embrane; 9, water phase; 10, micropipette; 11, liquid valve).
1 brane Science 326 (2009) 168–177
ielpiTwfiprwprmPHapafaccpsc[
2
tawtwlrwow
2
abTtg
3
bot
eqspttoad
Fig. 1. The values of streaming potential of the prepared membrane.
wne
3
3
p2ca
increased, the pure water flux of the membranes continuouslyincreased from the initial lower value of 169 L/h m2 without expo-sure time quickly to a relative higher value of 1236 L/h m2 with3.0 min evaporation time. The experimental result of the pure water
70 L. Gao et al. / Journal of Mem
-pentanol saturated water (water phase). At the start of thexperiment all pores of the membranes were filled with a wetiquid (i-pentanol saturated water). In brief, water saturated i-entanol (i-pentanol phase) was used to displace water phase
n prewetted membrane with constant increment of pressure.he upper section of the measurement cell was filled withater phase, and the lower was connected with a reservoir,lled with i-pentanol phase. The interfacial tension value of i-entanol towards water was 4.8 × 10−5 N cm−1 (4.8 dyne cm−1),eferring to Fu et al.’s measurement [17]. The two sectionsere connected to the pressure control device. The flux of i-entanol was measured by means of micropipette. The poreadii (r) and the pore size distribution function were deter-ined using Cantor equation r = 2� cos �/p and expression f (r) =
ii(Pi−1Qi − PiQi−1)/Pi−1
∑mi=1Pi/Pi−1(Pi−1Qi − PiQi−1) derived from
agen–Poiseuille’s equation, respectively [18]. Here, r, �, �, P and Qre the pore radii, the water/i-pentanol interfacial tension, waterhase/polymer contact angle, applied pressure and the perme-te flow rate, respectively. As mentioned above, the two phasesor liquid–liquid replacement is water saturated with i-pentanolnd i-pentanol saturated with water. Thus, the difference in theontact angle for each membrane is minor. For simplifying thealculation it was assumed that the liquid wetted the material com-letely, i.e., the contact angle was assumed to be zero. To obtainmoothing curves through the flow-pressure data, a smoothedubic spline method was used and executed in Matlab program19].
.4.4. ATR-FTIRFTIR spectra of the pure NMP and the casting solution exposed
o surroundings with RH about 20% were accomplished by attenu-ted total reflectance infrared (ATR-FTIR) spectroscopy instrumentith Nicolet Nexus Smart ARK Fourier transform infrared spec-
rometer. The samples were directly cast on the ATR crystal cellith a thickness of about 2 �m. Then, the spectra were col-
ected every 1 min. The total exposure time was 10 min and theoom temperature was 15 ◦C. The measured wave number rangeas between 4000 and 400 cm−1 at a resolution of 4 cm−1. Allriginal spectra were baseline corrected using the Omnic 6.1 soft-are.
.4.5. Membrane morphologyThe cross-sections of obtained membranes were observed using
scanning electron microscopy (XL 30 Philips). Air-dried mem-rane samples were cryogenically fractured in liquid nitrogen.hen photographs were obtained on a SEM instrument afterhe membranes were coated with conductive layer of sputteredold.
. Results and discussion
To examine the electrical properties of the prepared mem-ranes, their steaming potentials were commonly measured. Onef the membranes was randomly chosen to be characterized andhe result was illustrated in Fig. 1.
The value of the streaming potential indicates the sign of thelectrical charges of the membranes and its magnitude can reflectualitatively these relative magnitudes [20]. Fig. 1 clearly demon-trates that the streaming potential value of the membrane wasositive, implying that we finally successfully prepared a posi-
ively charged membrane. Using TEOA as the amination agent,he results showed great agreement with those investigated inur previous paper in which trimethylamine was the aminationgent [21]. The reaction equation between TEOA and BPPO wasrawn in Scheme 2. As showed, tertiary amine (TEOA) reacted
F(
Scheme 2. Quaterisation formula between TEOA and BPPO.
ith the bomomethlylated groups to form quaternary ammo-ium salt in the casting solution and a positively charged solutionmerged.
.1. Evaporation time on membrane properties and morphologies
.1.1. Pure water flux and rejectionTo investigate the influence of evaporation time on membrane
erformances and morphologies, evaporation time of 0, 0.5, 1.0, 1.5,.0, 2.5 and 3.0 min were respectively applied to prepare positivelyharged membrane at RH about 60%. The results of pure water fluxnd rejection of the membrane were presented in Fig. 2.
It could be clearly seen from Fig. 2 that as evaporation time
ig. 2. Effect of evaporation time on the pure water flux (-�-) and BSA rejection-�-) of positively charged membranes at 0.2 MPa operating pressure.
brane
flmgp[itsoifiwcoawsisrbdsvwtito
t
mppt
otadttrewwtrttewiviibw
L. Gao et al. / Journal of Mem
ux was opposite to the conventional rule that the membrane per-eation is decreased with an increase in evaporation time. It has
enerally been accepted that the evaporation step in the membranereparation procedure is used to make more retentive membrane22], which is also clearly certified in our previous study [13]. Thiss because along with the solvent evaporating, the polymer concen-ration in the surface layer increases which would lead to a denseurface layer. Moreover, this layer would inhibit the exchange ratef solvent and nonsolvent through the membrane surface duringmmersion process [23] and thereby resulting in smaller pores andnally the lower pure water flux. However, in our system, thereere two aspects of factors on the structures of the positively
harged membranes during the evaporation step. Besides the effectf solvent evaporation of the casting solution, there was a process ofbsorption of water from the vapor surroundings. The absorption ofater would decrease the thermodynamics stability of the casting
olution, leading to a rapid phase inversion which results in form-ng larger pores and macrovoids in the membranes. Specially, in ourtudy, the effect of absorption of water might play a more importantole on membrane structure, especially on the sublayer structure,ecause a high boiling point of NMP (about 203 ◦C) should hin-er its evaporation and a high affinity between hydrophilic castingolution and water should increase the water inflow from theapor phase (the water absorption of NMP and casting solutionere discussed in Section 3.1.2 which monitored with ATR-FTIR
echnique). Therefore, as the evaporation time increased, the cast-ng solution would absorb more water, thus leading to decreasing
hermodynamics stability and an increasing pure water flux wasbserved.
The measurement of the pure water flux was agreement withhese morphology observations of the cross-section in Fig. 3. The
t
to
Fig. 3. Morphologies changes in the membranes prepared by varying
Science 326 (2009) 168–177 171
orphology changes were quite noticeable when the membranesrepared with various evaporation time. It could be seen that theore size in sublayer of the membrane become larger and larger ashe evaporation time increased.
As shown in Fig. 2, it was also interesting to find that the rejectionf the membrane firstly increased from 73.7% to 90% when evapora-ion time increased from 0 to 0.5 min, and then was maintaining atbout the same value from 0.5 to 2.0 min exposure time, and finallyecreased to 62.1% when evaporation time continuously increasedo 3.0 min. The change in rejection with an increase in evaporationime was due to the competition of two influence factors, i.e., evapo-ation of solvent and water absorption of the casting solution. Whenvaporation time was lower than 0.5 min, evaporation of solventas the major influence on the membrane surface layer becauseater absorption of the casting solution is very little in such a short
ime. Therefore, the membrane surface layer with 0.5 min evapo-ation time was denser than that without evaporation time. Thus,he rejection increased when evaporation time prolonged from 0o 0.5 min. During the evaporation time from 1.0 to 2.0 min, theffect of solvent evaporation appeared to counteract the effect ofater absorption. Then, the pore size seemed to be the same dur-
ng this period of time which attributed to the constant rejectionalue. However, after the evaporation time was more than 2.0 min,t was speculated that the water absorbing effect become the mainnfluence factor during exposure process which might form mem-ranes with larger pores and thus the rejection decreased. Thatas the reason why the rejection decreased after 2.0 min exposure
ime.To further clarify the membranes structure, the pore distribu-
ion of membranes were presented in Fig. 4. It could be clearlybserved that the pore radius corresponding to the maximum
evaporation time: (a) 0 min, (b) 1 min, (c) 2 min and (d) 3 min.
172 L. Gao et al. / Journal of Membrane Science 326 (2009) 168–177
F 2 min,f
pwcht
3
piroww
srwstaats
ig. 4. Pore size distribution for different evaporation time: (a) 0 min, (b) 1 min, (c)our membranes.
ore size distribution with evaporation time from 0.5 to 2.0 minas almost keeping at a constant value about 30 nm which was
oincident with the constant rejection results, and the followingigh value of 35 nm at 3.0 min evaporation time further verifiedhe correspondingly lower rejection value.
.1.2. The ATR-FTIR spectraTo confirm the effect of water absorption, the spectra of the
ure NMP and the casting solution exposed to humid surround-
ngs for 10 min were collected every 1 min with ATR-FTIR and theesults were shown in Fig. 5(a) and (b), respectively. We focusn investigating the spectra region between 3800 and 3100 cm−1
hich is generally assigned to hydroxyl stretching vibration. Itas observed from Fig. 5(a) that the intensity of the hydroxyl
s
aac
(d) 3 min and (e) pore radius corresponding to maximum pore size distribution for
tretching absorption was increasing as the time enhanced. Theesults indicated the solvent NMP was definitely absorbing waterhen it contacted to the surroundings and the longer the expo-
ure time was, the more water it absorbed. Furthermore, in ordero simulate the real preparation conditions, the casting solutionbsorbing water was monitored by ATR-FTIR. As shown in Fig. 5(b),n increase in intensity of the hydroxyl stretching (�(OH)) absorp-ion implied that the amount of water absorbed in the castingolution increased simultaneously with the prolonging of the expo-
ure time.
In addition, the peak area between 3800 and 3100 cm−1 (A) asfunction of exposure time was plotted in Fig. 6. It was interestingnd obvious to see that variation rate of the �(OH) intensity of theasting solution was quicker than that of pure NMP when evapora-
L. Gao et al. / Journal of Membrane Science 326 (2009) 168–177 173
Ft
tciantts
Fp
F(
citocd
3m
vmsrft
wbtmt
ig. 5. FT-IR spectra of (a) pure NMP and (b) the casting solution directly contactingo the air.
ion time was less than 4 min, which meant that the capability ofasting solution absorbing water was stronger than that of NMP atnitial stage of evaporation. This was because both the hydrophilicdditive PEG-400 and the positively charged quaternary ammo-
ium groups in the casting solution could also absorb water fromhe vapor surroundings. On the contrary, after 4 min exposure time,he rate of water absorption by the casting solution seemed to belower. The reason was that there would be a liquid layer above the
ig. 6. Peak area of 3100–3800 cm−1 (A) as a function of exposure time (min) in theure NMP and the casting solution systems.
twibcmitvTRsRlto
tswoft
ig. 7. Effect of the relative humidity on the pure water flux (-�-) and BSA rejection-�-) of positively charged membranes at 0.2 MPa operating pressure.
asting solution after a period of water absorption which wouldnhibit the water from flowing into the solution. Menut et al. iden-ified the existence of the liquid layer with the observation of anptical microscopy [24]. The analysis of the experimental spectraertified the fact that water absorption was playing a main roleuring the exposure process.
.2. The relative humidity on the membrane properties andorphologies
As discussed by Park et al. [11], the relative humidity played aery important role in the phase inversion kinetics of the polymerembrane. During the evaporation step where the initial casting
olution contacts with the humid surroundings, the increase of theelative humidity would enhance the driving force for a net dif-usion into and accumulation of the water in the casting solutionhereby inducing a fast phase inversion.
In our system dry–wet casting process was applied duringhich evaporation time was 2.0 min. Five ultrafiltration mem-
rane samples were prepared during various relative humidity ofhe evaporation surroundings. Pure water flux and rejection of
embranes were presented in Fig. 7. The results demonstratedhat the pure water flux values were significantly affected byhe relative humidity in the environment, that was, the pureater flux increased with an increase in the relative humid-
ty. The increase of the permeability of the membrane coulde attributed to a more porous structure. During exposure pro-ess of the casting solution, the evaporation of NMP proceededore quickly at a relative lower RH evaporation surround-
ng, which would form a denser surface layer. Furthermore,he denser layer would hinder the nonsolvent into the sol-ent while precipitating and lead to a slow phase inversion.hen membrane was obtained with smaller pores. As for highH, water absorption had more influence on the membranetructure than the evaporation rate of NMP. That was to say, highH could favor the pore forming in the membranes which finally
ead to the increasing pure water flux. However, this pore size men-ioned here was related to pores under the membrane surfaces, inther words, across the membrane.
To further investigate the effect of the RH on the structure ofhe positively charged membrane, the morphologies of the cross-
ection of membranes prepared during different relative humidityere illustrated in Fig. 8. It was clearly observed that the trend
f morphological changes with an increase in RH completely con-ormed to that with the increasing of evaporation time. That waso say, all the membranes had an asymmetrical structure and as
174 L. Gao et al. / Journal of Membrane Science 326 (2009) 168–177
ing th
tsc
wB9tssawrortts
3(
3
vmsFsrbt
Fig. 8. Morphologies changes in the membranes prepared by vary
he increasing RH, the pore size was clearly increasing in the cross-ections of membranes. The observation of the SEM images wasoincident with the determination of the pure water flux.
The rejection of the membranes to BSA was examined, whichas also shown in Fig. 7. It could be seen that the rejections to theSA were all high and keeping almost at the same value of about7%. The results were strongly certified by the pore size distribu-ion of membranes prepared during different relative humidity. Ashown in Fig. 9, it could be observed that the pore radius corre-ponding to maximum pore size distribution was almost the same,bout 30 nm, conforming to the constant rejection value. It wasorth noting that the measure of pore size distribution was mainly
elated to the surface pore of the membrane. Combining the results
f the permeation flux and rejection, the relative humidity waseally influencing pore size which was across the membrane andhus high RH lead to high pure water flux. And the constant rejec-ion value was due to the constant pore size on the membraneurface.
sgdsa
e relative humidity: (a) 28%, (b) 40%, (c) 54%, (d) 74% and (e) 90%.
.3. Membranes prepared by vapor-induced phase inversionVIPS)
.3.1. Morphology and mechanism of the symmetric structureCompared to the above-mentioned asymmetrical membranes
ia a dry–wet phase inversion, membranes were obtained by aethod of prolonging the exposure time to about 12 h. The cross-
ection morphologies of the membranes could be observed inig. 10. It was clearly seen that the obtained membranes wasymmetrical morphologies. The same symmetric structure waseported for the polycarbonate and polysulfone membranes madey VIPS [25,26], which was believed to be due to the flat concentra-ion profiles of solvent, nonsolvent and polymer cross the casting
olution. According to Strathmann [27], asymmetric morphologyenerally resulted from the steep concentration and activity gra-ients of nonsolvent, solvent and polymer across the polymerolution. Because of the rapid exchange of solvent with nonsolventt interface, the chemical composition at the membrane surface
L. Gao et al. / Journal of Membrane Science 326 (2009) 168–177 175
F , (e) 9m
rwres
ig. 9. Pore size distribution during different RH (a) 28%, (b) 40%, (c) 54%, (d) 74%embranes.
eached a value leading to phase inversion in a very short time,hile it was still far below the binodal point in the membrane inte-
ior. Moreover, the formed dense skin layer served to hinder thexchange of solvent and nonsolvent which could make the phaseeparation proceed for a longer time as going away from the inter-
fHcls
0% and (f) pore radius corresponding to maximum pore size distribution for five
ace. Therefore, an asymmetric structure of membrane was formed.owever, in the case of VIPS, the concentration profiles of the threeomponents were flat across the casting solution because the rate-imiting step for the nonsolvent (water) transport into the castingolution was the slow diffusion in the vapor phase adjacent to the
176 L. Gao et al. / Journal of Membrane Science 326 (2009) 168–177
Fp
mva
pisibtRpit
3
3mu
dsabpipuipfif
Fi
isFc
4
spvpgiqfcrt
ig. 10. SEM images of cross-section of membranes prepared via vapor-inducedhase separation at two different relative humidity (a) 30% and (b) 60%.
embrane surface. Therefore, the casting solution precipitated atirtually the same time over the entire membrane cross-sectionnd thereby resulting in the symmetric structure.
It could be indicated from Fig. 10 that the pore size of membranerepared at RH 30% was bigger than that prepared at RH 60%. Sim-
lar rules was also justified by Park et al. [11] that the average poreize made from water vapor at RH 100% were 5.5 nm, whereas itncreased to 11.8 nm at RH 70%. These results could be explainedy following reasons. In case of vapor-induced phase separation,he phase separation would proceed more slowly and longer as theH become lower because of the lower water activity in the vaporhase corresponding to the lower water concentration in the cast-
ng solution. The slower phase separation could offer more time forhe coarsening of the polymer-poor phase and yield larger pores.
.3.2. Nodules and pinholes on top of the membranesCareful examination of the membranes prepared via VIPS at RH
0% and RH 60%, as indicated in Fig. 11, showed that the poly-er nodules existed on the membrane surface. These nodules were
sually not observed on the surface by the dry–wet phase inversion.Menut et al. [24] had proposed that the existence of liquid layer
ue to water absorption on the casting solution could give a rea-onable explanation. That is to say, on membrane surface there wasnother solution where polymer concentration was lower than theulk casting solution. Then, with polymer in the surface liquid layer,hase separation would still occur when enough water was drawn
nto the layer. And the phase separation was called “secondaryhase separation”, compared to the bulk phase separation occurred
nder the liquid layer. Besides, because the polymer concentration
n the liquid layer was low, the structure in this region resulted fromhase separation might be different from that in the bulk of the castlm. Then polymer beads or nodules emerged on membrane sur-
ace through a nucleation and growth of the polymer-rich phase
pwsts
ig. 11. SEM images of surface morphologies of membranes prepared via vapor-nduced phase separation at the relative humidity of (a) 30% and (b) 60%.
n liquid layer. A deeper examination of the membrane surfacehowed polymer pinholes and large round-shaped pores, as well.ormation of such pinholes could arise from incomplete closure ofells during coalescence [24].
. Conclusions
Positively charged asymmetric membranes, as justified by mea-urement of streaming potential, were prepared by brominatedolyphenylene oxide (BPPO)/N-methyl-2-pyrrolidone (NMP)/H2Oia in situ amination with triethanolamine (TEOA) and a dry–wethase process. The membrane properties and morphologies werereatly influenced by evaporation time and the relative humid-ty. As evaporation time enhanced, the pure water flux increaseduickly, whereas the rejection of the membrane increased firstlyrom the evaporation time from 0 to 0.5 min and then almost keptonstant when the evaporation time from 0.5 to 2 min; and theejection had a distinct decrease if the evaporation time was largerhan 2 min. The reasons were believed to be attributed to the com-etition of two influence factors, i.e., evaporation of solvent and
ater absorption of the casting solution. Additionally, ATR-FTIR
pectra of the casting solution further verified the phenomena ofhe pure NMP and casting solution absorbing water during expo-ure process. The increase of the relative humidity could make
brane
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A
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L. Gao et al. / Journal of Mem
ure water flux increase but had almost no influence on the BSAejection. This was because of the slower NMP evaporation as wells the slower and longer phase separation when the RH becameower.
Furthermore, the pore size distribution and the SEM imagesere examined and the results were consistent with the mea-
urements of pure water flux and rejection. By comparing to thery–wet process, a symmetric membrane was obtained whilehe exposure time to the humid surroundings was more than2 h because of the flat concentration of the solution cross-ection. Interestingly, morphologies of nodules and pinholes werelearly observed on the top surface of the symmetric mem-ranes which might not be observed during the dry–wet phase
nversion.Therefore, by properly controlling evaporation time and the rel-
tive humidity, a desired asymmetric membrane with high pureater flux and relative high rejection could be manufactured. And
lso membranes with various morphologies could be obtained viaapor-induced phase inversion.
cknowledgements
The authors gratefully acknowledge the financial support by theational Science of Foundation of China (NSFC) (Nos. 20706009,0876028, 20425415), the “Leading Scientist” Project of ShanghaiNo. 07XD14002), the National Basic Research Program of China2005CB623800), PHD Program of MOE (20050246010) and Chinaostdoctoral Science Foundation (20060400617).
eferences
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