Hc

CJa

b

c

d

a

ARR1AA

KOSWIMW

1

wiplw

0d

Journal of Membrane Science 392– 393 (2012) 157– 166

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

jo u rn al hom epa ge: www.elsev ier .com/ locate /memsci

ydrophilic silica additives for disulfonated poly(arylene ether sulfone) randomopolymer membranes

hang Hyun Leea, Wei Xieb, Desmond VanHoutena, James E. McGratha,∗, Benny D. Freemanb,ustin Spanoc, Sungsool Wic, Chi Hoon Parkd, Young Moo Leed

Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USADepartment of Chemical Engineering, Center for Energy and Environmental Resources, University of Texas at Austin, Austin, TX 78758, USADepartment of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USAWCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea

r t i c l e i n f o

rticle history:eceived 19 April 2011eceived in revised form3 December 2011ccepted 15 December 2011vailable online 24 December 2011

eywords:rganic–inorganic nanocompositeilicaater/salt selectivity

onic conductivityembrane desalinationater purification

a b s t r a c t

Hydrophilic silica (SiO2) nanoparticles (average size = 7 nm), which act as inorganic acids at low pH (<2),were added together with a PEO–PPO–PEO triblock copolymer dispersant to a random disulfonatedpoly(arylene ether sulfone) copolymer in the potassium salt ( SO3

−K+) form in order to control perme-ation and rejection characteristics of the homopolymer. The dispersants (shell) absorbed on the surfaceof SiO2 nanoparticles (core) formed a distinctive core–shell structure. The PEO units located at the outsideof the dispersant formed complexes with SO3

−K+ groups of BPS-20 via ion–dipole interactions. Theseinteractions induced a compatible binary system following the Flory–Fox equation associated with glasstransition temperature (Tg) depression and prevented extraction of the water-soluble dispersant evenunder aqueous conditions. The ion–dipole interaction, combined with hydrogen bonding between SiO2

and the dispersant, caused SiO2 nanoparticles to be well distributed within the BPS-20 matrix up to a limitof 1 wt.% of SiO2 and minimized the formation of non-selective cavities within the matrix’s hydrophilicwater channels. The resulting BPS-20 SiO2 nanocomposites showed improved salt rejection and reducedionic conductivity. These trends are analogous to those of disulfonated copolymer systems, with polargroups creating hydrogen bonding or acid–base complexation with SO3

−K+ groups in BPS copolymers.Well dispersed SiO2 nanoparticles in highly water-permeable desalination membranes are expected toresult in an increase in salt rejection but very little change in water permeability. The addition of nanopar-ticles to desalination membranes may offset the permeability-selectivity tradeoff observed in polymermembranes. Above 1 wt.%, SiO2 nanoparticles increased both the interchain distance between polymerchains and the water uptake. However, the increased hydrophilicity due to high SiO2 content did not

yield improved water permeation of the nanocomposite membranes. The SiO2 nanoparticles acted asbarriers, hindering water passage (restrictive diffusion) and lowering water permeability. Meanwhile,acidic hydroxyl groups (OH2

+) on the SiO2 surface in the sulfonate matrix led to improved ionic con-ductivity, but NaCl rejection capability decreased because the concentration of SO3

−K+ was diluted byhighly absorbed water molecules, resulting in weakened Donnan exclusion. The mechanical propertiesand chlorine resistance of all BPS-20 SiO2 nanocomposites were comparable to those of BPS-20.

. Introduction

The production of freshwater via desalination of salty/brackishater is becoming an increasingly attractive approach for address-

ng the global water shortage [1]. There are diverse desalination

rocesses already in operation, including multi-stage flash distil-

ation (MSF) which has the highest market share (>85%) in theorld [2] but consumes huge amounts of energy [3]. Recently,

∗ Corresponding author. Tel.: +1 540 231 5976; fax: +1 540 231 8517.E-mail address: jmcgrath@vt.edu (J.E. McGrath).

376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.12.015

© 2012 Elsevier B.V. All rights reserved.

membrane desalination has garnered attention as the most promis-ing desalination process [4] because of its much greater energyefficiency.

A key component in membrane desalination is a semi-permeable membrane capable of simultaneous high water fluxand ion rejection. The state-of-the-art desalination membraneis interfacially polymerized polyamide (PA), which meets thepermeation and NaCl rejection requirements irrespective of mea-

sured pH conditions [5]. However, PA dramatically loses itssalt rejection characteristics when even low level of chlorine(e.g., sodium hypochlorite) is used as an oxidizing biocide dur-ing disinfection [6]. Disulfonated poly(arylene ether sulfone)

158 C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166

re of B

cbaBiDiw[

amdaincwbtmpmlwtim

datiacn[bmthla

nutb

ucstotbc

Fig. 1. Chemical structu

opolymers (BPS-XX, XX = degree of sulfonation (DS), Fig. 1) haveeen developed as alternative desalination membrane materi-ls with high chlorine tolerance [7]. According to its DS values,PS-XX offers a reasonable trade-off between water permeabil-

ty and salt rejection. Highly water-permeable BPS-XX with highS generally shows low salt rejection, and vice versa [7,8]. It

s difficult to control both water permeability and salt rejectionith membranes made only of sulfonated polymeric materials

3,7–9].This paper describes a new platform to form water perme-

ble, ion-selective channels within a relatively hydrophobic BPS-20atrix. Incorporating inorganic oxide nanoparticles with pH

ependent-electrochemical surfaces (e.g., silica (SiO2)) may offer means to effectively overcome the flux-selectivity tradeoff thats generally observed in polymeric membranes. Organic–inorganicanocomposites have drawn interest for a wide range of appli-ations, such as pervaporation [10,11], gas separation [12,13],ater purification [14–16], and fuel cell applications [17–20],

ecause the materials show excellent chemical, mechanical, andhermal stability. Inorganic nanoparticles incorporated into poly-

eric matrices can improve these materials’ performance for waterurification, including salt removal. For example, nanocompositeembranes fabricated from carbon nanotubes [21–23] and zeo-

ite [15] offer extremely fast water transport. Bio-fouling resistanceas enhanced after adding titanium oxide (TiO2) owing to its pho-

obactericidal effect [14]. However, there have been no reports ofon-selectivity improvement without significant loss of water per-

eability.Our current challenge is to achieve homogeneous inorganic

istribution; fine inorganic particles spontaneously agglomeratend can form non-selective cavities or heterogeneous distribu-ion in polymer matrices. Inorganic concentration also stronglynfluences both mechanical properties and free volume elementsssociated with ion and water transport [24,25]. High inorganiconcentration (>15% of inorganic content) may result in high desali-ation performance not achieved by pure polymeric materials26]. However, high concentration often induces an incompati-le interface between the inorganic particles and the polymeratrices, and the resulting hybrids may be mechanically brit-

le. The best strategy for avoiding these problems is to maximizeybrid membrane performance by homogenously distributing a

imited number of inorganic nanoparticles with large surfacerea.

In this study, we used a non-ionic surfactant to distribute SiO2anoparticles, which act as acids at low pH. A similar method wassed in the fabrication of nanocomposites for fuel cell applica-ions [18–20,27]. How the inorganic nanoparticles are compatiblyound in polymer matrices containing fixed charged ions (e.g.,SO3

−M+ groups, M+ = Na+ or K+) has not, to date, been clearlynderstood. Thus, a primary goal of the present study was toomprehend the distribution of inorganic nanoparticles. This workystematically investigated the effects of nano-sized SiO2 par-icles on the macroscopic characteristics of the corresponding

rganic–inorganic nanocomposite desalination membranes, par-icularly those properties associated with permeation/rejectionehavior. Finally, the chlorine tolerance of the nanocomposites wasompared with that of PA.

PS-20 (x = 0.2, M+ = K+).

2. Experimental

2.1. Materials

The polymer matrix used in this study was BPS-20 (DS mea-sured using 1H NMR = 20.1 mol%) in potassium salt form ( SO3

−K+),which was synthesized on a 1 kg scale by Akron Polymer Sys-tem (Akron, OH, USA) following published procedures [28–30].The intrinsic viscosity (IV) was 0.82 dL g−1 in NMP containing0.05 M LiBr at 25 ◦C. Pluronic® L64 (PEO13–PPO30–PEO13, meanmolecular mass = 2900 mol kg−1), a commercially available tri-block copolymer surfactant composed of polyethylene oxide (PEO,

CH2CH2O ) and poly(propylene oxide) (PPO, CHCH3O ) blocks,was obtained from BASF (Ludwigshafen, Germany) and used asa dispersant for inorganic distribution. Hydrophilic fumed SiO2,Aerosil® 380 (Degussa Chemical Co., Dusseldorf, Germany) with

Si OH concentration of 1.48 mmol g−1 was dried at 80 ◦C for twodays before use. Its surface area and average particle size were380 m2 g−1 and 7 nm, respectively. The solvent dimethyl acetamide(DMAc) was purchased from Aldrich Chemical Co. (WI, USA) andused without additional purification.

2.2. Nanocomposite (BPS-20 SiO2) fabrication

After vacuum drying at 110 ◦C for one day, BPS-20 (2 g) was dis-solved in DMAc (10 wt.%) and filtered using 0.45 �m PTFE syringefilter to remove impurities in the solution state. A mixture of 1 wt.%Aerosil® 380 (0.02 g) and 3 wt.% L64 (0.06 g) in DMAc was made byfollowing a previously published procedure [18]. In detail, 0.02 gof Aerosil® 380 powder was added to 0.06 g of L64 and mechani-cally stirred at room temperature for 4 h. Next, DMAc was addedin order to make a 4 wt.% solution. After mixing for one day, theresulting solution was added to the BPS-20 solution and mechan-ically stirred for an additional day at ambient temperature. Then,the corresponding BPS-20 SiO2 film in SO3

−K+ form was fabri-cated via solution-casting: the cast solution was dried for 4 h at90 ◦C, then heated to 150 ◦C for one day under vacuum. After it waspeeled off, the film was kept in deionized water for two days toremove residual solvent. Other BPS-20 SiO2 nanocomposite filmswith different SiO2 concentrations (0.5, 2, 3 and 5 wt.% per BPS-20 weight) were also made following this procedure. The nominalmembrane thickness of all films was about 30–40 �m. In all thefilms, the weight ratio of SiO2 to L64 was constant (1/3). The toughand ductile BPS-20 SiO2 films are denoted as BPS-20 SiO2 concen-tration (wt.%) (Table 1). For example, BPS-20 SiO2 0.5 wt.% filmdenotes a BPS-20 film containing 0.5 wt.% SiO2 and 1.5 wt.% L64. Inaddition, reference BPS-20 films were prepared with either 3 wt.%L64 (BPS-20/L64) or 1 wt.% SiO2 (BPS-20/SiO2 1 wt.%).

2.3. Characterization

The surface potential (zeta (�) potential) of Aerosil® 380was measured using Zetasizer 3000 HAS (Malvern Instruments,

Worcestershire, UK) over a pH range of 0.5–9 at 30 ◦C. The zetapotential of each sample was measured at least three times. Priorto measurement, Aerosil 380 was suspended at a constant concen-tration of 10 mg L−1 in aqueous media adjusted to pH 0.5 using 1 M

C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166 159

Table 1Basic characteristics of BPS-20 and BPS-20 SiO2 nanocomposites.

Sample L64 content(wt.%)

SiO2 content(wt.%)

MeasuredTg (◦C)a

PredictedTg (◦C)b

Tg difference(◦C)c

Saltpermeability(cm2 s−1)

Tensilemodulus(MPa)

Tensilestrength(MPa)

Elongation(%)

BPS-20 277 3.75 × 10−11 1580 ± 10 56 ± 3 25 ± 18BPS-20/SiO2 1 wt.% 0 1 279 –d 1180 ± 120 47 ± 7 5 ± 4BPS-20/L64 3 247 253 −6 –d 2230 ± 160 85 ± 4 10 ± 4BPS-20 SiO2 0.5 wt.% 1.5 0.5 277 264 13 –d 2000 ± 90 67 ± 5 14 ± 8BPS-20 SiO2 1 wt.% 3 1 276 253 23 –d –d –d –d

BPS-20 SiO2 2 wt.% 6 2 271 233 38 1.85 × 10−11 –d –d –d

BPS-20 SiO2 3 wt.% 9 3 270 215 55 –d 1930 ± 160 65 ± 5 7 ± 2BPS-20 SiO2 5 wt.% 15 5 255 186 69 1.06 × 10−11 2130 ± 110 74 ± 3 6 ± 1

a Measured by DMA.b Calculated via Flory–Fox equation considering only L64 content (1/Tg = WBPS-20/Tg,BPS-20 + WL64/Tg,L64) in the mixed matrices. Wi represents weight fraction of component

i. Tg of L64 = −55 ◦C [48].

H0tlm

ss1

firarwasss4asu

b6m

(Isaw2

rTbc

sosid

pSs

c Measured Tg − theoretical Tg.d Not measured.

3PO4. Afterward, the test medium pH was adjusted to 9.0 using.5 M NaOH. Moreover, the same apparatus was used to measurehe SiO2 particle size in the presence or absence of L64 via dynamicight scattering (DLS, angle = 90◦) in DMAc medium at pH 4. The

easurement was repeated at least nine times.Cross-polarization magic-angle spinning (CPMAS) 13C NMR

pectra were taken with a Bruker Avance II 300 MHz wide-borepectrometer operating at Larmor frequencies of 75.47 MHz for3C and 300.13 MHz for 1H nuclei. Samples (50–60 mg) in thinlm forms were cut into small pieces for packing into 4 mm MASotors for MAS experiments. Spin-lock pulses of 1 ms duration werepplied along both 1H and 13C channels for CP, employing a 50 kHzf-field at the 13C channel and a ramped rf pulse at the 1H channelhose rf-field strength changes linearly over a 25% range centered

t 38 kHz. A pulse technique known as total suppression of spinningide bands (TOSS) [31] was combined with a CP sequence to obtainideband-free 13C MAS spectra at 6 kHz MAS spinning speed. NMRignal averaging was achieved by co-adding 2048 transients with a

s acquisition delay time. 1H and 13C �/2 pulse lengths were 4 �snd 5 �s, respectively. Small phase incremental alternation with 64teps (SPINAL-64) [32] decoupling sequence at 63 kHz power wassed for proton decoupling during 13C signal detection.

SiO2 distribution in the BPS-20 matrix was observed usingoth field emission scanning microscopy (FE-SEM, JOEL Model JSF340F, Tokyo, Japan) and high-resolution transmission electronicroscopy (HR-TEM, JOEL3010, Tokyo, Japan).The glass transition temperature (Tg) of BPS-20 SiO2 films

width = 4 mm) was measured using a TA DMA 2980 (TAnstrument, DE, USA) in the multi-frequency tension mode. Mea-urements were conducted in the temperature range 0–300 ◦C with

ramp of 5 ◦C min−1 in nitrogen atmosphere, where each sampleas subjected to a preload force of 0.025 N with an amplitude of

5 �m at a frequency of 1 Hz.The d-spacing value (A) was obtained from a wide angle X-

ay diffraction (WAXD, Rigaku Denki Model RAD-C diffractometer,okyo, Japan) pattern in the range of 5◦ ≤ 2� ≤ 50◦. This value cane used to monitor the change in average distance between BPS-20hains according to the SiO2 concentration.

Water uptake (%) of BPS-20 SiO2 films (dimen-ion = 5 cm × 5 cm) was determined by subtracting the weightf the dry sample from that of the fully hydrated sample. Eachample was dried under vacuum at 110 ◦C for one day andmmersed in deionized water at 25 ◦C for one day to obtain theirry and wet weights, respectively.

The ionic conductivity (�, mS cm−1) of each 1 cm × 4 cm sam-le was calculated from the equation � = l/RS. Here, l (cm) and

(cm2) are the distance of reference electrodes and cross-ectional area, respectively. Film resistance (R, �) was measured in

deionized water at 30 ◦C using four-point probe alternating current(a.c.) impedance spectroscopy [33]. The electrode system, which isset up in a thermally controlled water reservoir, was connected toa frequency response analyzer (Solartron 1252A) and an electro-chemical interface (Solartron 1287).

Pure water permeability (Pw, L �m m−2 h−1 bar−1) of BPS-20 SiO2 films was evaluated at 25 ◦C under cross-flow filtrationwith a feed of deionized water at a feed pressure of 800 psig. Theequation Pw = (V × T)/(A × t × �P) was used, where V is the volumeof water permeated across the sample with active area A and thick-ness T at steady state per unit time, t, under a pressure difference(�P). Salt rejection, Rs, was calculated from the difference of NaClconcentration in feed (Cf) and in permeate (Cp), using the equa-tion Rs = (Cf − Cp)/Cf × 100. The permeate through BPS-20 SiO2 filmswas collected in a cross-flow filtration system fed by a 2000 ppmNaCl aqueous solution (feed pressure = 800 psig and pH 6.5–7.5).Feed and permeate salt conductivity values were measured usingan Oakton Con 110 conductivity meter (Cole Parmer, Vernon-Hills,NJ, USA), and a calibration curve was used to calculate salt concen-tration from solution conductivity.

Mechanical properties of BPS-20 SiO2 films in dogbone shape(50 mm long, minimum width 4 mm) were assessed using anInstron 5500R universal testing machine (Instron, Norwood, MA,USA) equipped with a 200 lb load cell at 30 ◦C and 44–54% relativehumidity (RH). Prior to measurement, all specimens were dried at110 ◦C for one day and then equilibrated at 4% RH at 30 ◦C. At leastfive measurements were taken and an average value was calcu-lated.

3. Results and discussion

The surface properties of inorganic oxides are influenced morestrongly by pH than by the concentration of salts in their environ-ment [34]. Aerosil® 380, the inorganic oxide used in this study,has a hydrophilic surface nature due to its surface silanol groups( Si-OH). Its electrochemical potential is altered spontaneouslydepending on pH, as shown in Fig. 2. The surface potential ofAerosil® 380 was positive at low pH (<∼2) and negative at highpH (>∼2), since Si-OH groups can take either H+ or OH− ions:

Acidic : Si-OH + H+ → Si-OH2+ (aquo state)

or

Basic : Si-OH + OH− → Si-O− (oxo state) + H O

2

Similar amphoteric properties have been reported in diversemetallic oxides such as tungsten (VI) oxide, vanadium (V) oxide, zir-conium (IV) oxide, chromium (III) oxide, and copper (II) oxide [35].

160 C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166

Fig. 2. Surface potential of Aerosil® 380 suspension in aqueous medium at 30 ◦C.The concentration of L64 was 30 mg L−1, which was consistent with the relativec

A�pl3i2igaavr

aaStca[3witoc

poowii

®

within the hydrophilic domains of BPS-20, where SiO2 nanoparti-

omposition of Aerosil® 380–L64 mixture used in this study.

erosil® 380’s isoelectric point (IEP), defined as the pH at which = 0 or the surface potential is neutral, was observed at pH 2.1. TheH values of sulfonate polymer matrices (0-1) [36] as solid acids are

ower than the IEP of Aerosil® 380. This fact suggests that Aerosil®

80 can selectively take and transport cations such as H+ and Na+

n a BPS-20 SiO2 medium, as sulfonate groups ( SO3−K+) in BPS-

0 do. Usually, charged surfactants can be strongly adsorbed ontonorganic particles in mixed media, profoundly changing the inor-anic surface potential, even when small amounts of surfactantsre used [34]. However, when L64 surfactant was added into anqueous Aerosil® 380 suspension, the surface potential changedery little. This small change in electrochemical property may beelated to L64’s non-ionic character.

L64’s amphiphilic nature derives from its hydrophilic PEO blocksnd a hydrophobic PPO block. This balance between hydrophilicnd hydrophobic properties was expected to minimize undesirableiO2 agglomeration. The L64 concentration used for SiO2 distribu-ion was 16 g L−1, which is much higher than its critical micelleoncentration (3.8 g L−1) in DMAc. Thus, L64 exists in micelle formt our chosen concentration, as is often observed in aqueous media37]. Fig. 3 presents the average Z-diameter of L64 micelles, Aerosil®

80, and a mixture of L64 and Aerosil® 380. The Z-diameter of L64as smaller than that of Aerosil® 380 aggregated due to physical

nteraction between Si-OH groups on the surface of each nanopar-icle (7 nm). However, when the surfactant (shell) was adsorbedn the hydrophilic SiO2 (core) surface, its average size increased,reating a peculiar core–shell structure.

L64 is located at the outside of the core–shell structure sus-ended in polar DMAc. When mixed with BPS-20 solution, ethylenexide ( CH2CH2O ) units in L64 can make complexes via intra-r intermolecular ion–dipole interaction with potassium (K+) ions

−

hich maintain strong electrostatic interaction with SO3 groupsn BPS-20 (i.e., SO3

−K+) (Fig. 4(a)) [38]. The ion–dipole interactions similar to those reported in crown ethers, particularly those with

Fig. 3. Z-diameter of L64 surfactant, Aerosil 380 and the mixture measured inDMAc at pH 4 as a function of temperature. Here, Z-diameter was obtained via DLSmeasurement and pH was adjusted with 0.1 M HCl.

long CH2CH2O units (>9), under both aqueous and non-aqueousfree alkali metal cation conditions [39–41].

Fig. 4(b) presents the solid state 13C NMR spectra measured onthe BPS-20 derivatives including BPS-20/L64. The membrane sam-ple was kept in water at 30 ◦C for 10 days to test the extent ofextraction of L64 under aqueous conditions. After 10 days in water,no noticeable changes were observed in the areas of aliphatic car-bon peaks, 75 ppm for PEO and 24 ppm for PPO derived from L64when compared to that of non-water treated reference sample.Based on these NMR spectra, we conclude that L64 is not extractablefrom the BPS-20 matrix even under fully hydrated conditions. Inaddition, all characteristic peaks of the aromatic phenylene groupsof pure BPS-20 are unaffected by L64 addition.

We also investigated the plasticization effect of L64 with andwithout co-adding SiO2 in the BPS-20 polymer matrix. Fig. 4(c)shows 1H T1 data measured on BPS-20 derivative membranes.The method incorporates an inversion-recovery sequence in solidstate that is implemented on the standard 1H–13C CP scheme forsite-specific signal detection along the 13C channel. We employedthe 13C peak at 129 ppm to encode the 1H T1 signal of directlybonded methine protons in aromatic phenylene rings. The strong1H–1H homonuclear dipolar interactions among many differentproton sites form a dipolar coupling reservoir that facilitates rapidequilibration between different 1H sites, resulting in an identi-cal T1 value. 1H T1 data obtained are plotted using the equationln[1 − M(�)] = −�/T1 + ln 2, where M(�) is the signal intensity mea-sured at a mixing time �. Presented in Fig. 4(c) are plots, their bestleast-square-fit equations, and 1H T1 values calculated from theslope of each least-square-fit equation. As shown by the T1 param-eters specified in each plot, L64 addition decreases T1 from 0.66 s(pure BPS-20) to 0.61 s (BPS-20/L64); addition of SiO2 decreases itfurther to 0.59 s. The reduced T1 observed after adding L64 can beattributed to the presence of K+-ethylene oxide ion–dipole interac-tions that improve compatibility between BPS-20 polymer chainsand L64 molecules. The 1H T1 data confirm our prediction thatL64 molecules do act as plasticizers, resulting in a softer polymermatrix that provides shorter motional correlation time of molecu-lar segments and, thus, a shorter T1 time. However, co-adding SiO2did not significantly modify the motional characteristics of aro-matic phenylene rings in the polymer matrix, as can be deducedfrom the measured T1 value (0.59 s). The ion–dipole interactionsbetween L64 and BPS-20 position the L64-SiO2 micelles primarily

cles are expected to influence water transport. SiO2 nanoparticlespositioned at a micelle core do not directly interact with BPS-20polymer chains; therefore, SiO2 has only marginal effects on 1H T1

C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166 161

, (b) solid-state 13C NMR spectra, and (c) T1 plot data of BPS-20 derivatives.

tin

pBLtp(eTcctwtc(tFati

101

102

103

104

Tg (4)

= 270.4 oC

Tg (5)

= 255 oC

Tg(2)

= 246.7 oC

Tg (3)

= 276 oC

BPS-20 (1) BPS-20/L64 (2) BPS-20_SiO2 1 wt.% (3) BPS-20_SiO2 3 wt.% (4) BPS-20_SiO2 5 wt.% (5)

Stor

age

mod

ulus

[MPa

]

Tg (1)

= 277 oC

0.0

0.1

0.2

0.3

0.4

0.5

Tan

Fig. 4. (a) Postulated ion–dipole interaction between BPS-20 and L64

ime of aromatic phenylene groups. The BPS-20/L64 sample placedn water for 10 days had a slightly lower T1 value (0.59 s) but showedo decrease in the L64 peak intensity.

Both L64 and Aerosil® 380 influenced the glass transition tem-erature (Tg) of BPS-20. Each BPS-20 derivative containing L64 (e.g.,PS-20/L64 and BPS-20 SiO2) shows a single Tg, irrespective of its64 contents (Fig. 5). This is very different from the immiscible sys-em, which has two independent Tg values, one derived from eacholymeric component. After the addition of 3 wt.% of L64 to BPS-20BPS-20/L64), Tg was depressed to 247 ◦C due to the plasticizationffect of L64 on physically crosslinked ionic domains in BPS-20.he reduced Tg value is close to theoretical prediction (253 ◦C)alculated from the Flory–Fox equation [38,42]. This reduction indi-ates that ion–dipole interactions between BPS-20 and L64 makehe binary system compatible. Although a similar Tg depressionas observed for BPS-20 SiO2 systems with high L64 concentra-

ions, its measured Tg value was higher than a theoretical valuealculated via the Flory–Fox equation with L64 contents in BPS-20Table 1). The Tg difference grows with increasing SiO2 concen-ration, suggesting that Aerosil® 380 retards plasticization by L64.

urthermore, it means that another strong interaction between L64nd SiO2 (e.g., hydrogen bonding) is observed in BPS-20 SiO2 sys-ems in addition to the ion–dipole interaction. These combinednteractions appear to enhance compatibility between BPS-20 and

500 100 150 200 250 300

Temperature [ºC]

Fig. 5. DMA profile of BPS-20, BPS-20/L64, and BPS-20 SiO2 films.

162 C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166

Fig. 6. SEM images of (a) BPS-20/SiO2 1 wt.% without L64 and (b) BPS-20 SiO2 1 wt.%, and (c) energy-dispersive spectrometer (EDS) Si-mapping image of BPS-20 SiO2 1 wt.%under the same magnification (10,000×).

, (b) 1

Sc

Sbslpo(ifibtSIawaaac

2wrtwdp2Sbtp2S

ter of the inorganic oxide resulted in improved water uptake,but water permeability decreased. These results verify that SiO2nanoparticles with hydrophilic surfaces trap water molecules

Fig. 7. TEM images of BPS-20 SiO2 (a) 0.5 wt.%

iO2 nanoparticles and minimize the formation of non-selectiveavities within hydrophilic water channels of BPS-20.

Fig. 6 shows SEM images of BPS-20 films containing 1 wt.%iO2 nanoparticles. The SiO2 distribution state was determinedy the presence of L64 dispersant. In BPS-20/SiO2 without L64,pontaneous SiO2 agglomeration was easily observed even at aow SiO2 concentration (Fig. 6(a)). In contrast, the dispersant sup-ressed SiO2 aggregation in BPS-20 SiO2 1 wt.%. The presencef SiO2 (bright dot) was confirmed using only EDX Si-mappingFig. 6(c)). TEM images, Fig. 7, provided information on the changen SiO2 distribution with increasing SiO2 content in BPS-20 SiO2lms. SiO2 nanoparticles are generally observed in dark spotsecause their electron density (∼0.3 electron A−3) [43] is higherhan other elements in the composites. In BPS-20 SiO2 0.5 wt.%,iO2 nanoparticles were distributed in localized cluster phases.nterestingly, the average size of the dark spots is about the sames the individual particles, indicating that SiO2 particles are fairlyell distributed and do not agglomerate. On the other hand,

dding over 0.5 wt.% SiO2 induced some SiO2 agglomerates (aver-ge size = ∼25 nm) in the localized cluster phases (Fig. 7(b) and (c)),ltering the water/ion transport properties through the resultinghannels.

Fig. 8 shows the change in average distance between BPS-0 polymer chains after adding L64 and/or SiO2. The samples,hich were dried under ambient conditions, can be used to indi-

ectly understand changes in polymer chain spacing, particularly inheir hydrophilic domains. The widest polymer interchain distanceas observed in BPS-20/SiO2 1 wt.% without L64, since irregularlyistributed SiO2 aggregation prevented compact polymer chainacking. L64 reduced the average d-spacing value between BPS-0 chains via strong ion–dipole interactions (BPS-20/L64). AfteriO2 incorporation (BPS-20 SiO2 1 wt.%), the interchain distanceecame narrower than that of BPS-20/L64 with the same L64 con-

ent. However, continued addition of SiO2 nanoparticles (>1 wt.%)hysically enlarged the interchain distance of corresponding BPS-0 SiO2 nanocomposites, which may be associated with localizediO2 agglomeration, as shown in Fig. 7.

wt.% and (c) 3 wt.% (magnification: 100,000×).

Both L64 and SiO2 influenced water uptake and water perme-ability of BPS-20 mixed matrices (Fig. 9). The narrowed interchaindistance following L64 addition caused BPS-20/L64 to exhibit lowerwater uptake and permeability than those measured for BPS-20.When 1 wt.% SiO2 nanoparticles were added, the resulting mem-brane was less swollen and less water permeable, as inferredfrom the d-spacing trend between BPS-20/L64 and BPS-20 SiO21 wt.%. Above 1 wt.% of SiO2, the pH-dependent acidic charac-

Fig. 8. Average d-spacing value of BPS-20 SiO2 films as a function of SiO2 contentsand water channel structure change inferred from the values.

C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166 163

543210

2.8

3.0

3.2

3.4

3.6

3.8

4.0

BPS-20_SiO2 nanocomposites

BPS-20/L64

Wat

er u

ptak

e [%

]

SiO2 content [wt%]

BPS-20

(a) (b)

5432100.010

0.015

0.020

0.025

0.030

0.035

BPS-20

Wat

er p

erm

eabi

lity

[Lm

m-2 h

r-1 b

ar-1]

SiO2 content [wt.%]

BPS-20/L64

erme

wtwowti

scuctgSrpi

R

Ha

pP2Tlaphnc(ioirbwpwc

Fig. 9. (a) Water uptake and (b) water p

ithin the hydrophilic water channels, and physically prevent theirransport through the channels. This behavior is very consistentith the characteristics of nanosized SiO2 particles reported in

rganic–inorganic fuel cell membranes [18,20]: in terms of (1)ater retention capability and (2) barrier property to aqueous mix-

ures (e.g., CH3OH–H2O complexes). A similar trend was observedn salt permeability (Table 1).

L64 and SiO2 contributed differently to ionic conductivity andalt rejection of the resulting composite membranes (Fig. 10). Ioniconductivity measured in the fully hydrated state has been widelysed to electrochemically evaluate how easily ion species in theomplex form with water molecules can be transported throughest membranes (e.g., ion exchange membranes with fixed chargedroups such as COO−M+ and SO3

−M+, M = H+, Li+, Na+, K+) [33].alt rejection (Rs) is defined as the percentage of aqueous ionsemoved from the feed water and can be influenced by both waterermeability (Pw) and salt permeability (Ps) as shown in the follow-

ng equation [9]:

s = (Pw/Ps)(VW /RT)(�p − ��)1 + (Pw/Ps)(Vw/RT)(�p − ��)

(1)

ere VW, �P, and �� represent water volume, pressure differencepplied to the test cell, and osmotic power, respectively.

In spite of their conceptual correlation, no studies have com-ared ionic conductivity and salt rejection. L64 – particularly itsEO units – can play two important roles in BPS-20/L64 and BPS-0 SiO2 nanocomposites: (1) ionic conductor and (2) plasticizer.he ion-conducting capability of PEO units has been reported inithium ion (Li+) battery applications employing PEO derivativess polymer electrolytes [44,45], where Li+ ions are easily trans-orted via oxygen lone pair electrons in CH2CH2O units withigh mobility even in the solid state. A comparable transport phe-omenon was observed in fully hydrated BPS-20/L64. Its ioniconductivity was improved after the addition of L64 to BPS-20Fig. 10(a)). Simultaneously, the plasticization of L64, particularlyts PEO units, on physically crosslinked hydrophilic ionic domainsf the BPS-20 weakened electrostatic interactions between the K+

ons and SO3− groups in BPS-20. These diminished interactions

educed ion rejection (Fig. 10(b)), much as happens in BPS-20 PEOlend systems [38]. However, when <1 wt.% SiO2 nanoparticles

ere added with the L64, membrane water swelling was sup-ressed and NaCl rejection improved, while ionic conductivityas reduced. Analogous behaviors were reported in disulfonated

opolymer systems with polar groups (e.g., benzoxazole [46],

ability of BPS-20 SiO2 nanocomposites.

benzimidazole [47], or triphenyl phosphine oxide [7]) in theirhydrophobic segments, which form hydrogen bonding or acid–basecomplexation with SO3

−H+ groups. Note that SiO2 nanoparticlesat concentrations less than 1 wt.% were fairly well distributed inthe BPS-20 matrix. These well dispersed SiO2 nanoparticles, in con-junction with CH2CH2O–SO3

−M+ complexes, appear to promotewater/salt selectivity, just as the polar groups do. But, BPS-20 SiO2nanocomposites with high SiO2 contents (>1 wt.%) exhibit reducedNaCl rejection that arises from enhanced water uptake due tothe surface hydrophilicity of SiO2 and, consequently, weakenedDonnan exclusion [3] (Fig. 10(b)). Interestingly, salt rejection ofthe nanocomposites changed rapidly depending on SiO2 content,while the effect of SiO2 on water permeability was far less signif-icant. The trend was quite different from that for BPS copolymers.Thus, we anticipate that well-distributed SiO2 nanoparticles in ahighly water swollen BPS copolymer with a high DS (e.g., BPS-40)may improve salt rejection in the corresponding nanocompositemembranes without a big loss in water permeability. The ionic con-ductivity trend in the system was opposite to that of NaCl rejection.

Plasticizers generally give rise to reduced mechanical proper-ties. The negative effect on membrane toughness may be severein mixed matrices containing inorganic additives with incompat-ible interfaces. Fortunately, membrane toughness and ductility ofBPS-20 SiO2 nanocomposites were very similar to those of BPS-20(Table 1).

Chlorine resistance is an important factor associated withdesalination membrane lifetime. To assess Cl resistance, eachsample was immersed in a buffered aqueous solution of sodiumhypochlorite (NaOCl) with different concentrations (1000 and10,000 ppm) at pH 4 ± 0.3 and 25 ◦C. After immersion for eitherthree days or one week and washing in deionized water for atleast two days, we measured the change in membrane character-istic peaks with solid state 13C NMR spectroscopy (Fig. 11). PA,obtained via interfacial polymerization of m-phenylenediaminein aqueous phase and trimesoyl chloride in organic phase, wasused for comparison. Even chlorine exposure at a low concen-tration (e.g., 100 ppm) for one day was enough to convert C Hbonds in the PA phenyl rings into C Cl bonds. The carbonyl peakintensity decreased as chlorine concentration increased. Surpris-ingly, BPS-20 derivative copolymers, including BPS-20/L64 and

BPS-20 SiO2 nanocomposites, exhibited no significant change ineither peak intensity or position under harsh chlorine conditions(e.g., 10,000 ppm). This result suggests that the absence of amidelinkages vulnerable to chlorine attack allows superior chlorine

164 C.H. Lee et al. / Journal of Membrane Science 392– 393 (2012) 157– 166

54321010

12

14

16

18

20

22

24 BPS-20 SiO

2 nanocomposites

BPS-20/L64

Ioni

c co

nduc

tivity

@30

o C [m

S cm

-1]

SiO2 content [wt.%]

BPS-20

In fully hydrated state(a) (b)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.790

91

92

93

94

95

96

97

98

99

1000.5 wt.%

1 wt.%

2 wt.%

5 wt.%

BPS-20/L64

"SiO2 content control"

"Sulfonation degree control"

Water permeability [L m m-2 hr-1 bar-1]

NaC

l rej

ectio

n [%

]

20 mol.%

30 mol.%

35 mol.%

40 mol.%

Fig. 10. (a) Ionic conductivity and (b) NaCl rejection of BPS-20 SiO2 nanocomposites.

f (a) P

rp

4

lcaWdP2

Fig. 11. Chlorine resistance o

esistance in the disulfonated polymer systems, irrespective of theresence of L64 and/or SiO2.

. Conclusions

SiO2 nanoparticles with acidic surface characteristics at pHower than 2 were added to a disulfonated poly(arylene ether)opolymer (e.g., BPS-20) in the potassium salt form, together with

polymeric dispersant composed of PEO and PPO units (L64).

hen L64 (shell) was mixed with SiO2 (core), the mixture pro-

uced a distinctive core–shell structure. In the BPS-20/L64 blend,EO units in L64 formed complexes with SO3

−M+ groups in BPS-0 via ion–dipole interactions, which induced a Fox-Flory-like,

A and (b) BPS-20 SiO2 1 wt.%.

progressive Tg depression and, then, made the two componentscompatible. The complexes were strong enough to prevent L64from being extracted even in the fully hydrated state. Mean-while, SiO2 nanoparticles formed strong hydrogen bonds with L64,which compromised the Tg depression derived from the com-plexes between L64 and BPS-20. These interactions combined toyield a SiO2 phase distributed within BPS-20, particularly in itshydrophilic domains. In the BPS-20 SiO2 nanocomposites withSiO2 content less than 1 wt.%, well distributed SiO2 nanoparticlesreduced the average distance between BPS-20 chains and lowered

the water uptake, in spite of the nanoparticles’ hydrophilic sur-face characteristics. Consequently, both water permeability andionic conductivity decreased, but salt rejection increased. At higherSiO2 concentrations (>1 wt.%), a somewhat aggregated SiO2 phase

rane S

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C.H. Lee et al. / Journal of Memb

adically altered membrane characteristics. The average spacingetween BPS-20 chains increased and water uptake continuously

ncreased. Unexpectedly, water permeability was reduced; the SiO2anoparticles served as barriers to trap water molecules. Unlikehe usual permeation/rejection tradeoff, reduced water permeabil-ty was not accompanied by improved ion-selectivity. Though SiO2anoparticles with acidic surface characteristics dispersed in a BPS-0 matrix gave rise to improved ion conductivity, high water uptakeuppressed NaCl rejection, owing to weakened Donnan exclu-ion. Here, the ion conductivity trend was opposite to that of saltejection. Interestingly, the reduction in water permeability accom-anying improved NaCl rejection was much smaller in BPS-20 SiO2anocomposites than in polymeric materials. The result suggestshat the same concept may be employed in highly water perme-ble polymer systems to control both permeability and selectivity.he resulting BPS-20 SiO2 nanocomposite exhibited toughness anductility as well as chlorine resistance comparable to BPS-20.

The addition of hydrophilic SiO2 together with L64, whichorms hydrolytically durable ion–dipole interaction with salt formPS copolymers, is expected to impact fouling resistance, another

mportant desalination material requirement, as reported forydrophilic TiO2-polymer composites [14]. Finally, the long-termurability of nanosized SiO2 particles in the nanocomposite shoulde monitored, especially in case where the resulting membranesre used for drinkable water purification. Our on-going studiesre focused on demonstration of highly water permeable and salt-elective nanocomposites based on highly sulfonated polymers andeared toward practical water purification.

cknowledgements

This work was supported by Dow Water & Process Solutions.his work was also supported in part by the National Scienceoundation (NSF)/Partnerships for Innovation (PFI) Program (Granto. IIP-0917971) and by the Korean Foundation for Internationalooperation of Science & Technology (KICOS) through a grant pro-ided by the Korean Ministry of Education, Science & TechnologyK20701010356-07A0100-10610). This material is partially basedpon work supported by the National Science Foundation underrant No. DMR-0923107.

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