Structural Characterization of Thin-Film Polyamide Reverse Osmosis

Article pubs.acs.org/IECR

Structural Characterization of Thin-Film Polyamide Reverse Osmosis Membranes Jonathan Albo,† Hideaki Hagiwara,‡ Hiroshi Yanagishita,‡ Kenji Ito,‡ and Toshinori Tsuru†,* †

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagayami-yama, Higashi-Hiroshima 739-8527, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305−8565, Japan

‡

S Supporting Information *

ABSTRACT: This study aims to explore the structural characteristics of the inhomogeneous top layer of thin-film composite membranes when pretreated by different methods: room temperature−oven, ethanol−hexane in a solvent exchange process, and freeze-drying. An evaluation of the nano-order free-volume pore size of the polyamide samples was carried out by nanopermporometry (NPP) and was quantitatively compared with the free-volume pore estimated from normalized Knudsenbased permeance (NKP) and with positron annihilation characterization (PALS). NPP results denoted a bimodal polyamide membrane structure described by a dense matrix and highly permeable regions. The application of different condensable vapors (water, hexane, and isopropanol) resulted in a free-volume pore size smaller than dp = 0.6 nm for dense regions, which was confirmed after NKP and PALS. In addition, the influence of highly permeable regions on permeance decreased in the following order: ethanol−hexane > freeze-drying > room temperature−oven samples, demonstrating an effective membrane structure alteration after different pretreatments.

1. INTRODUCTION The use of thin-film composite membranes (TFC) for reverse osmosis (RO) processes has been widely extended due to their advantageous high flux and rejection provided by the thin aromatic polyamide (PA) separating layer. Most commercially available RO TFC membranes are formed in situ by the interfacial polymerization of an aromatic polyamine such as mphenylenediamine (MPD) with one or more aromatic polyacyl halides (for example, trimesoyl chloride (TMC)). These chemical and mechanical resistant aromatic-based membranes1 exhibit excellent performance in many desalination and water purification applications and are already in mass production.2,3 However, for an optimized separation performance, a clear understanding of PA membrane characteristics at not a uniquely macroscopic level (physical and mechanical properties) but at a nanoscale one (local spaces and distribution) is demanded. The concept that stipulates that it is the local spaces (free volume) around the permeating molecule that determine the diffusion coefficient is the key to understanding diffusion in polymer membranes. Polymers are normally divided into two broad categories, rubbery and glassy, where transport can be described by a solution-diffusion model. In a rubbery polymer, the polymer chains can rotate more freely, making the polymer soft and resulting in higher diffusion coefficients. On the other hand, in a glassy polymer, steric hindrance along the polymer prohibits the rotation of the polymer segments, resulting in a rigid polymer with low diffusion coefficients. However, due to an extraordinarily high and interconnected free volume, some polymeric membranes, such as PIM (polymer intrinsic microporous), can also act as porous materials with pores ranging from 0.5 to 1.5 nm.2,4,5 In this case, pore-flow transport occurs through the so-called free-volume pore spaces of the membrane and separation can be primarily explained by © 2014 American Chemical Society

molecular sieving. Furthermore, membrane structures with small (0.42 nm) and also larger (1.2−1.4 nm) free-volume diameter elements have been reported for PTMSP high-freevolume polymer,6 where a crossover from solution-diffusion to Knudsen transport seems to occur. As a rule of thumb, the transition from solution-diffusion and pore-flow transport mechanism is in a diameter range of 0.5−1 nm2. In this region a dual-mode transport model is the most appropriate description for permeances and selectivities.7,8 Positron annihilation lifetime spectroscopy (PALS) has drawn much attention recently in polymeric membrane research due to its great ability to explore free-volume holes at the molecular level.9−12 The application of PALS to PAbased-TFC RO membranes has resulted in free-volume hole sizes ranging from 0.4 to 0.8 nm in diameter.13−15 Therefore, in PA membranes with subnano holes, the transition between pore-flow and solution-diffusion transport seems to occur in a porous−nonporous material. In our previous study, gas permeation results revealed that the dry PA layer consisted of a dense matrix where chain mobility with temperature enabled permeation by the activated diffusion of small gases, such as He, and highly permeable regions where larger species, such as N2, could permeate exclusively via the Knudsen mechanism.16 Lately, in the following work, it was found that the transport of an isopropanol (IPA)/water mixture in pervaporation also responded to a two-region structure, where the free volume arising from the wetting of the dense and highly permeable polymer chains, together with the higher affinity of IPA Received: Revised: Accepted: Published: 1442

October 10, 2013 December 19, 2013 January 7, 2014 January 7, 2014 dx.doi.org/10.1021/ie403411w | Ind. Eng. Chem. Res. 2014, 53, 1442−1451

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic drawing of the NPP experimental setup.

molecules and PA, defined the separation performance.17 Both studies resulted in a comparatively high flux and selectivity and demonstrated the applicability of the composite PA membranes for high-temperature separation processes. Additionally, the previous results suggested that different pretreatments for the membrane could alter gas and vapor permeances. Consequently, for further discussion of the effect of membrane pretreatments, free-volume pore size and the distribution in the aromatic PA membrane structure would enhance the understanding of the dominated transport mechanism. Nanopermporometry (NPP) is a methodology that typically has been used to measure pore sizes of less than 50 nm.18,19 This methodology offers pore size and distribution evaluation, similarly to bubble point, where the permeate gas flow rate through a wet porous membrane is measured by increasing the pressure difference across the membrane,20 and biliquid permporometry, in which a liquid is used to displace another liquid from a porous membrane.21 One of the advantages of the present NPP methodology is the ability to measure nanosized pores ranging from 0.6 to 30 nm, while bubble-point and biliquid permporometry have been used to characterize pore sizes in the microfiltration (larger than 100 nm) and ultrafiltration (10−100 nm) ranges, respectively. The type of vapors applied in NPP definitely affected the membrane structural characterization due to different molecule sizes, polarity, and interactions with the membrane surfaces. To date, water, alcohol, cyclohexane, or carbon tetrachlorides have been mainly applied.22−24 A previous work suggested that the vapors of water and nonpolar compounds were appropriate for measuring free-volume pore size distributions smaller than 1 nm in microporous ceramic membranes, in contrast to the utilization of vapors with a larger molecular size and some affinity to the membrane material.19 A clear understanding of the effect of vapors applied on NPP and their use for freevolume pore size determination has not been fully achieved for polymeric materials. In the present study, NPP is used for the first time to examine the membrane structure and the free-volume pore size of PA-based RO membranes under different pretreatment procedures. The results are compared quantitatively with freevolume pore sizes estimated from normalized Knudsen-based permeance (NKP), which is based on the permeation of gas molecules and their kinetic diameter,25 and from PALS, which relies on the diffusion of a gas into the surface accessible spaces in the membrane structure. Finally, the separation performance of the PA-based membranes is evaluated for water permeability and salt rejection. The results were compared to gas permeation and discussed in terms of the membrane local spaces. The present study will aid in an understanding of the

transport mechanisms for compounds in the bimodal structure of PA membranes for the development of high-performance membrane separation processes.

2. EXPERIMENTAL SECTION 2.1. Materials. CPA5, high-rejection RO membranes were kindly provided by Nitto Denko (Osaka, Japan) and adopted in the present study. The membrane consisted of a TFC with a top-skin aromatic PA layer (∼200 nm), a middle microporous polysulfone (∼40 μm), and a bottom poly(ethylene terephthalate) layer (∼120 μm). The specific chemical composition of the PA layers is proprietary information of the supplier. Membranes were delivered in a polyethylene bag containing less than 1% sodium m-bisulfite solution and were kept in a refrigerator at 4 °C before the analysis. All high-purity chemicals of analytical grade used in this study were purchased from Sigma Aldrich (Tokyo, Japan). 2.2. Membrane Pretreatment Methods. Commercial PA membranes were tested immediately after common membrane pretreatment procedures. A detailed description of the procedures is shown in the Supporting Information (Appendix 1). (1) Room temperature−oven (RTO): Membranes were dried at room temperature and then in an oven at 120 °C. (2) Ethanol−hexane (EH): Membranes were dried in ethanol−hexane by a solvent exchange process. (3) Freeze-drying (FD): Membranes were in tert-butanol and dried in freeze-drying equipment. 2.3. Membrane Sorption. Sorption experiments were performed at room temperature (25 ± 2 °C) in a Shimadzu TGA-50 apparatus with a sensitivity of ±0.001 mg. A N2 flow rate of 50 mL/min was introduced in the TG equipment after bubbling through a humidifier containing pure water, hexane, or IPA. Prior to the measurement, membranes were pretreated under RTO procedure to remove the sorbed water from within the membrane structure. The sorption uptake, S, in the PA sample was gravimetrically measured and calculated as follows: ⎛ m − md ⎞ S=⎜ s ⎟ × 100 ⎝ md ⎠

(1)

where ms and md are the weight of the sorbed and dry membranes, respectively. 2.4. Contact Angle. The membrane wettability testing was carried from sessile water drops using a goniometer equipped with a camera device (Kyowa, Tokyo, Japan) at room temperature (25 ± 2 °C) in air. Contact angles were measured 1443

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component for the lifetime data by using that for a Kapton foil. An average hole radius rPs (nm) was calculated from τPs (ns) based on the following equation:9−12

by defining a circle around the drop and recording the tangent angle formed at the substrate surface. 2.5. Nanopermporometry. Free-volume pore size in the membrane samples (2.21 cm2) was tested in a NPP experimental apparatus, as schematically represented in Figure 1. He and N2 were used as noncondensable gases, while the liquids used as condensable vapors were water, hexane, and IPA. The gas flow rate from the cylinder was regulated by a mass flow controller (STEC, Kyoto, Japan). The feed was at atmospheric pressure, and the pressure difference across the membrane ranged from 10 to 20 kPa, monitored by a pressuredifference sensor (Sunx, Tokyo, Japan). The temperature of the humidifier was controlled at 40 °C, while the apparatus was at room temperature (25 ± 2 °C). Prior to testing, He was fed through to remove the vapor inside the system. NPP was initiated by measuring the steady permeance of the dry gas as a reference initial point. The vapor pressure was then increased gradually by controlling dry gas, Qd, via MF-1, and wet gas, Qw, via MF-2 mass flow controllers, until gas permeation was blocked by capillary-condensation or adsorption-induced swelling. The partial pressure of vapor in the feed stream, p, was determined based on the flow rate of the dry gas, Qd, and the wet gas, Qw, and the total pressure of the feed stream, pT, which is the sum of atmospheric pressure, p0, and the pressure difference, Δp, across the membrane assuming that the liquid was under saturated vapor pressure, ps, after the mist trap (5 in Figure 1), and that the vapor pressure was sufficiently low compared with pT: p=

Qw Qw + Qd

ps

−1 ⎡ ⎛ 2πrPs ⎞⎤ rPs 1 + τPs = 0.5⎢1 − sin⎜ ⎟⎥ ⎢⎣ rPs + 0.166 2π ⎝ rPs + 0.166 ⎠⎥⎦

(5)

2.7. RO Experiments. LP and salt rejection, R, were tested in a stainless dead-end cell in a single-solute system. The effective membrane area was 2.21 cm2. Rejection and water permeance in single-solute systems were measured through the pretreated PA-based membranes and calculated according to the following equations: LP =

R=1−

(2)

Qi ΔpA

(3)

(7)

3. RESULTS AND DISCUSSION 3.1. Nanopermporometry Results. In our previous work, gas permeation tests demonstrated that the separation characteristics of the TFC RO commercial membranes were mainly attributed to the top aromatic PA layer, which consisted of two different structures.16 First, there was a dense matrix that enabled the permeation of small gases, such as He. Second, there were highly permeable regions where larger species, such as N2, could permeate via Knudsen mechanism.16 In order to estimate these different free-volume pore sizes, He and N2 have been selected for NPP as noncondensable gases. Figure 2 shows the NPP experimental curve for a CPA5RTO sample. He and N 2 dimensionless permeances, normalized with the lowest humidity, are plotted as a function of the relative water humidity in the feed stream (p/ps). A detail of the experimental procedure is shown in the Supporting Information (Appendix 2). As observed, the permeance of He was decreased at a p/ps < 20%, while N2 remained invariable. After p/ps = 90%, the permeance of both gases rapidly decreased, which can be explained as water vapor partial condensation and/or sorption in the induced-swelling membrane at higher humidity, blocking the permeation of the noncondensable gases (both He and N2). The two decreasing regions in the He curve may denote a bimodal structure described by two different free-volume pore

where Qi is the permeate flow rate and A is the membrane area. In a small free-volume pore size (diameter dp), vapor sorption and condensation at vapor pressure, p, lower than ps, occur, as it is represented in the Kelvin equation: ⎛P⎞ σ cos θ RT ln⎜ ⎟ = 2υ rk ⎝ Ps ⎠

CP Cf

(6)

where Δv is the permeate volume, Δt is the permeation time, A is the effective membrane area, Δp is the operation pressure, and ΔΠ is the osmotic pressure difference. In eq 7, CP, is the permeate concentration and Cf is the feed concentration. In order to maintain constant ΔΠ, across the membrane over the course of the experiment, the feed solution was changed periodically after the measurements (every 30 min). The osmotic pressure was calculated from NaCl Cf. Water permeances and rejection values were stable after 2 h of experimental time. The operation pressure was set at 1.5 MPa, and temperature was maintained at 25 °C in a water bath. The values were obtained for a NaCl concentration of 2,000 ppm (wt) in deionized water. Membranes were immersed in the 2,000 ppm NaCl aqueous solution and then set in the dead-end cell prior to use. The electric conductivity was evaluated using an ES-51 conductivity meter (Horiba, Kyoto, Japan).

Gas permeance, Pi, was calculated using the following equation: Pi =

Δv ΔtA(Δp − ΔΠ)

(4)

where υ, σ, and θ are the molar volume, surface tension, and contact angle, respectively. This equation permits calculation of the Kelvin radius, rk. 2.6. Positron Annihilation Lifetime Spectroscopy. Positron annihilation lifetime measurements were carried out at various positron incident energies, E, ranging from 1.8 to 10 keV by utilizing a 22Na-RI-based pulsed-positron beam system (PALS-200A Fuji Imvac, Yokohama, Japan) with a time resolution of approximately 290 ps full width at half-maximum at the prompt peak of the obtained data. The lifetimes of positrons were recorded as the time difference between a pulsing trigger and the corresponding detection timing of the annihilation radiation, and the annihilation events were accumulated with total counts of 2.0 million. Multiexponential analysis was applied to the recorded lifetime data to deduce the average lifetime of the long-lived positronium (Ps), τPs, for the films. In the analysis, care was taken of the background 1444



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Table 1. Physicochemical Properties of the Solvents vapor

M (g/mol)a

σ(30°C) (mN/m)b

kinetic diameter (nm)

water hexane IPA

18.01 86.18 60.09

71.2 18.4 23.1

0.3 0.51 0.47

θ (deg)c S (%)d 52 0 0

12.1 20.8 46.4

M = molecular weight. bσ = surface tension. cθ = contact angle. dS = sorption uptake.

a

8.8, which is close to the solubility parameter value of the aromatic PA.26 It should be noted that the IPA sorption uptake obtained in this work, S = 46.4%, is in good agreement with that value obtained in the literature for TFC PA-based membranes, S = 49.5%, where attenuated total reflection− Fourier transform infrared (ATR-FTIR) analysis suggested the preferential sorption of alcohols in the PA top layer.26 The PA sorption uptake with time and water contact angle view is presented in the Supporting Information (Appendix 3). Generally speaking, and according to the figure, hexane and IPA may be candidates for measurements of larger free-volume pores in the membrane, while water can be used to determine small spaces. It should be noted that the Kelvin equation cannot be applied for a pore diameter of less than 2 nm. However, the measurement by NPP showed a reasonable correlation with the separation performances of the porous membranes having pore sizes as small as 0.5 nm,19 and, therefore, this may reveal the probable structural characteristics of the PA membrane, which is the main objective of the present work. Figure 4 shows the dimensionless permeance curve for He as a function of p/ps when using water, hexane, and IPA as condensable vapors.

Figure 2. Dimensionless permeance as a function of relative humidity (p/ps) for a CPA5-RTO membrane (vapor: water).

size regions. The bimodal structure is consistent with our previous results on gas separation16 and pervaporation/vapor permeation.17 N2 (0.36 nm) is able to permeate only through highly permeable regions (large free-volume pores), and its permeation is not reduced at low relative humidities. Alternatively, He (0.26 nm) first showed a decrease in permeation because vapor blocked local spaces in the dense matrix (available for permeation of small gases, such as He), which was followed by a second decrease after p/ps = 90%, where highly permeable regions can be partially plugged. This bimodal structure is in agreement with the findings for PTMSP high-free-volume polymer, with small and larger free-volume element diameters.6 3.2. NPP Using Different Solvents As Condensable Vapors. The molecular size of vapors and their interaction with the membrane may affect the free-volume pore size measurements.19 Figure 3 represents the Kelvin diameter, dk, of

Figure 4. He dimensionless permeance as a function of relative humidity (p/ps) with water, hexane, and IPA as condensable vapors in CPA5-RTO.

These results show how the application of water and hexane as condensable vapors resulted in similar tendencies, despite their differences in hydrophobicity and polarity. Alternatively, IPA was able to wet the membrane and completely block the gas permeation at p/ps = 20%. The nature of physisorption processes of condensable vapors is usually divided into the sorption of vapor molecules to the free-volume pore wall at low vapor pressure and capillary condensation afterward.24 In the case of water as a condensable vapor, since the molecular size is relatively small compared with

Figure 3. Kelvin diameter as a function of relative humidity (p/ps) for the three solvents applied.

the applied condensable vapors (water, hexane, and IPA) as a function of p/ps. The curves are defined by the molar volume, surface tension, and contact angles of every vapor at the same p/ps, eq 4. The physicochemical properties of the solvents are presented in Table 1. A higher sorption rate for IPA was obtained, probably because the solubility parameter of IPA is 1445



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permeation of He and N2, giving a clear difference between the two membrane structures: dense and highly permeable regions. IPA effectively reduces the permeation of the noncondensable gas, and the permeance reduction is more likely related to the sorption of the vapor in the material, instead of to capillary condensation. In any case, the application of the three condensable vapors showed an important decrease in dimensionless permeance up to dk = 0.6 nm and then established a stable value. Therefore, permeation of He at dk < 0.6 nm can be assumed to correspond to free-volume pores in the dense regions of the membrane, while dk > 0.6 nm may be associated with highly permeable regions. 3.3. Effect of Membrane Pretreatments on NPP. Membrane pretreatment prior to use can produce shrinkage and swelling of the PA membrane structure.16,17 Thus, it is important to understand the effect of pretreatment procedures on the free-volume pore size, since it determines the performance characteristics. Figure 6 shows the dimensionless permeance of He (a) and N2 (b) for the membrane after different pretreatments (RTO, EH, and FD). Water is used as condensable vapor, since low sorption in the PA material is expected (S = 12.1%) and may give clearer information on the local space distribution of the two membrane regions. As shown in Figure 6a, the initial He permeation was reduced for RTO and FD samples with increasing p/ps of up to 20%. This relative humidity may be enough to block dense membrane permeable spaces, thereby limiting He permeation. However, in the EH sample, He permeance did not vary before p/ps = 90%. This may indicate that gas permeation through samples treated under ethanol−hexane is controlled by the free-volume pore characteristics of the highly permeable regions. On the other hand, N2 (Figure 6b) remained in a stable value before p/ps = 90%, with slight differences between membranes pretreated under different procedures. Besides, RTO showed the smallest dimensionless permeance at the end of the test, in comparisons with FD and EH, which is attributed to the reduced number and size (width and length) of highly permeable regions of the PA selective layer after drying at high temperature. Figure 7 graphically represents the transport of He and N2 through the dense and highly permeable membrane regions.

hexane and IPA, and there is a reduced affinity to PA membranes, a reduction in dimensionless permeance may directly indicate the free-volume pore size distribution of the membrane. Hexane, which is hydrophobic and nonpolar, presented a sorption value that was similar to that of water. The size of hexane molecules after sorption and condensation is thought to be very thin, resulting in a dimensionless permeance curve that is similar to that of water.19 IPA, however, with a kinetic diameter of 0.47 nm and the highest degree of sorption uptake for a PA membrane, S = 46.4%, is expected to be sorbed into the wall of the highly permeable region and onto the membrane surface at a low relative pressure, blocking the permeation of the noncondensable gas. Finally, Figure 5 shows the dimensionless permeance of He as a function of dk (eq 4) for the three condensable vapors

Figure 5. He dimensionless permeance as a function of Kelvin diameter with water, hexane, and IPA as condensable vapors for CPA5-RTO.

applied. The curves may correspond to a free-volume pore size distribution curve. As shown in the figure, IPA presents a lower dimensionless permeance than either water or hexane at the same Kelvin diameter. No He permeation at dk > 1.7 nm (corresponding to p/ps = 20%) was observed when using IPA, while water and hexane still showed permeation at p/ps = 90%. Consequently, water and hexane as condensable vapors in NPP are able to partially plug membrane permeable regions and reduce the

Figure 6. Dimensionless permeance of He (a) and N2 (b) in samples after different pretreatments (RTO, EH, and FD) as a function of relative humidity (p/ps) for water as a condensable vapor. 1446



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membrane region to the overall He permeation and give an approximation of the effect of different pretreatments in the membrane structure. As shown in Table 2, the RTO procedure increased the relative contribution of dense regions on gas permeance by comparison with the EH and FD procedures, which can be attributed to the membrane shrinkage that occurred after drying at high temperature.16 In the EH procedure, alcohol may have swelled the PA chains26 and removed the plugging compounds (from membrane preparation) from within permeable regions;27 both effects would have resulted in the membrane permeation being controlled by the highly permeable regions. These results were consistent with Figure 6a and confirmed the bimodal membrane structure. In the FD procedure, the membrane morphology is fixed with a minimized distortion that may have occurred during normal drying, and thus low membrane shrinkage was expected,28 which resulted in an intermediate influence of highly permeable regions on gas permeation. The averaged free-volume pore size in the samples may be reduced in the following order: EH > FD > RTO. Generally, membranes with larger average free-volume pore sizes would also have higher permeances, and low permeation discrimination to different molecule sizes, resulting in lower separation factors, as observed with the He/N2 gas selectivity tendency in the samples (Table 2). 3.4. Normalized Knudsen-Based Permeance. To further explore the PA membrane structure, a normalized Knudsen-based permeance was then applied. NKP is a simple and effective method25 to evaluate the average free-volume pore size based on the modified gas-translation model originally proposed by Xiao and Wei29 and Shelekhin et al.30 NKP is defined as the ratio of the permeance of component i, Pi, to that calculated using He permeance and molecular weight, M, under the Knudsen diffusion mechanism:

Figure 7. He and N2 permeation through dense and highly permeable membrane regions of the membrane.

The transport through these subnano free-volume pores of PA membranes may be described by size exclusion (molecular sieving), solubility differences (solution-diffusion) and Knudsen diffusivity in the local spaces (free-volume pores) of the porous−nonporous membrane. The relative contribution of dense and highly permeable regions to overall He permeance, PHe, is presented in Table 2. Table 2. Relative Contribution to Gas Permeance of the Membrane Regions relative He permeation contribution (%)

P (10−8 mol/(m2·s· Pa))a

a

CPA5 pretreatment

He

N2

αHe/N2b

DRc

HRd

RTO EH FD

4.97 21.3 12.5

1.28 7.58 3.66

3.88 2.81 3.41

31.8 5.7 23.6

68.2 94.3 77.4

P = permeance. bα = gas selectivity for He/N2. cDR = dense region. HR = highly permeable region.

PHe

NKP =

(8)

where PN2 is the N2 permeance and M is the molecular weight of the gases. The relative contribution of highly permeable regions (HR) on He permeation can be calculated as the ratio of He permeation through highly permeable regions, PHe,HR, and overall He permeation in the membrane, PHe, according to the following equation: HR/% =

PN2 PHe

M N2 MHe

(10)

(1 − dk, i /d p)3 (1 − dk,He/d p)3

(11)

For an estimation of dp, first, the NKP for each gas can be measured experimentally according to eq 10 and then dp can be obtained by fitting NKP as a function of dk,i, eq 11. According to gas permeation values from our previous work16 presented in Table 3, Figure 8 shows gas permeances reported for RTO, EH, and FD membranes, as a function of gas kinetic diameters. The continuous curves show predicted

M N2 MHe

Mi MHe

NKP can be analyzed using the kinetic diameter of permeating molecules, dk, as follows:

The calculated values were based on dry gas permeation (p/ps = 0%) results in NPP. The estimation assumes that He permeated both membrane regions, while N2 permeated only highly permeable regions via Knudsen diffusion.16 Therefore, the permeation of He through highly permeable regions (large free-volume pores) can be estimated according to Knudsen diffusion mechanism: PHe,HR = PN2

Pi

NKP =

d

Table 3. Gas Permeation through the Membranes after Different Pretreatments P (10−8 mol/(m2·s·Pa))a

× 100

pretreatment

He

H2

CO2

O2

N2

C3H8

SF6

RTO FD EH

4.79 11.8 20.8

5.07 13.5 29.7

0.89 2.75 6.38

1.03 3.24 7.72

1.11 3.53 7.92

0.89 2.78 6.47

0.47 1.46 3.44

(9)

The relative contribution of dense regions (DR) on He permeation was calculated as the difference in the overall permeation. The results may reveal the contribution of each

a

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Figure 8. Gas permeance as a function of kinetic diameter. Continuous line shows predicted permeance using Pi = PHe(Mi/MHe)1/2. Points are experimental data: (a) RTO, (b) EH, and (c) FD.

permeances (Pi = PHe(Mi/MHe)1/2), based on He permeance under the Knudsen diffusion mechanism, since He is the smallest molecule with no sorption properties. The experimentally obtained permeances agreed well with the predicted ones in the EH sample (Figure 8b) and deviated for the larger molecules in the cases of RTO and FD. Furthermore, the figure shows the initial decreases in the NKP values at lower kinetic diameters for the RTO and FD samples, probably due to the influence of molecular sieving in dense regions. These results may indicate that small gases, such as He and H2, are able to permeate through dense matrix and highly permeable regions, compared with larger species, such as N2, that permeate exclusively via Knudsen mechanism through larger free-volume pores.16 On the other hand, the constant NKP value for the EH sample may indicate that the transport was controlled by Knudsen diffusion through highly permeable regions, in accordance with the NPP results (Figure 6 and Table 2). Therefore, the averaged free-volume pore size in the samples may be reduced in the following order: EH> FD> RTO. Additionally, Figure 9 shows normalized Knudsen-based permeance as a function of molecular size for the sample after the different drying procedures. The dotted line represents the NKP calculations based on eq 11 using dp = 0.6 nm, and it is included as a reference. The ratio of experimentally obtained permeance to Knudsen-based predicted permeance indicates the degree of permeance reduction in dense regions in the following order: RTO > FD > EH. This is inversely proportional to the relative contribution to gas permeance of highly permeable regions: EH > FD > RTO (Table 2). From the experimental curves, a bimodal structure of the PA membrane was confirmed. The dense regions may consist of a structure with free-volume pore sizes smaller than dp = 0.6 nm according to NPP and NKP results, which is in good agreement

Figure 9. NKP as a function of kinetic diameter. Points are experimental, and the reference curve is calculated based on eq 11 using dp = 0.6 nm (dotted line).

with free-volume pore size diameters ranging from 0.4 to 0.8 nm based on PALS characterization13−15 for reverse osmosis PA-based thin-film composite membranes. 3.5. Positron Annihilation Lifetime Spectroscopy. Figure 10 shows the variation of the Ps lifetime, τPs, and the average diameter, 2rPs, calculated from eq 5, for the membranes after the different pretreatments (RTO, EH, and FD) as a function of positron incident energy E. The overall tendency was similar for all samples, that is, τPs increased from ∼1.8 to ∼2.2 ns with increasing E from 1.8 to 10 keV. The obtained τPs is in agreement with that observed for a NF membrane with similar chemistry (1.93 ns at E = 2.0 keV) reported previously.31 By considering the positron mean implantation depth, the outer layer for each membrane should be associated with an E range between 1.8 and 3 keV, giving an average diameter below 0.6 nm. A comparison of rPs among the three membranes signifies that the hole size in the outer layer was smaller in the order of EH > FD > RTO, which is 1448



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Figure 11. CPA5 and SCW5 membrane performance in samples without pretreatment (no pretreatment, NP), and after RTO, FD, and EH procedures (25 °C, 1.5 MPa, and 2,000 ppm).

for an elevated molecular packing and therefore a small variation in the membrane structure after pretreatment. Alternatively, water permeance in the CPA5 sample was increased by factors of 1.94 and 1.29 for EH samples compared with those permeances in RTO and FD samples, respectively, confirming that different pretreatments effectively produced alterations in desalination performance. Nevertheless, water permeance for samples dried under the three procedures was below those permeances obtained in samples that were not dried, NP. This can be attributed to the negative impact of the drying procedures on membrane−water interactions due to the dehydration of water-swollen hydrogel that fills the membrane pores.17 Upon immersion of dried samples in the aqueous solution, water may be unable (or only slowly able) to access interchain hydrogen bonds formed during drying. Besides, a previous report probed, via PALS and water transport tests in dried and hydrated poly(arylene ether sulfone) polymers, that sorption of water in dried samples may alter the free-volume pore sizes of the polymer. Water molecules may occupy the free volume of the dried membrane, reducing the cavity-volume size.32 As a result, water cannot easily permeate polymer chains. As expected, slightly higher rejections were observed from Figure 11 when using CPA5 (high-rejection RO membrane) compared with SWC5 (seawater). In addition, despite the differences in LP, produced by the influence on membrane performance of the different pretreatments, R remained almost invariable for each membrane type, with only slight decreases in samples dried at room temperature−oven. Thus, clearly both dense and highly membrane permeance regions of the membrane (detected by NPP, NKP, and PALS) are effective for NaCl rejection. If cracking of the membranes upon drying would occur, a remarkable alteration of rejection would be expected. However, this was not observed. The stable NaCl rejections, despite the differences in water permeance, may also denote the effective separation of NaCl by molecular sieving in the pretreated samples. Then, the results may suggest that the structure of the dried PA layer can be representative of the structure in the hydrated state. In addition, the rejection in RO is not uniquely described by the sieving effect, but also for membrane-charged function groups33,34 due to the presence of carboxylic groups generated by the unreacted groups of TMC from membrane synthesis.35 Consequently, the pretreatments applied might not alter the chemically charged structures that are effective for a NaCl sizeexclusion mechanism.

Figure 10. Variation of τPs for the membranes with the different pretreatments (RTO, EH, and FD) as a function of positron E. Average diameter 2rPs obtained from τPs using eq 5 is shown on the right-hand axis. On the upper axis, positron mean implantation depth was calculated from 40/ρE1.6 with ρ = 1 g/cm3.

consistent with the above argument. Furthermore, the obtained size range, i.e., the diameters smaller than dp = 0.6 nm, agreed well with that from the permeance properties (NPP and NKP). 3.6. Effect of Membrane Structure on RO Performance. In order to correlate the membrane structure with the separation ability, a CPA5 membrane was evaluated for water permeance and salt rejection. The RO performance may offer criteria for evaluating the free-volume pore size distribution of membranes pretreated differently. Membrane samples were treated under RTO, EH, and FD procedures and compared with samples without pretreatment (no pretreatment, NP), which were washed only in pure water. Table 4 summarizes the Table 4. Comparison of Gas and Water Permeance in RO after Different Pretreatments

a

CPA5 pretreatment

PHe (10−8 mol/(m2·s·Pa))a

RTO EH FD NP

4.97 21.3 12.5

LP (L/(m2·h·bar))b 0.82 1.59 1.23 4.37

± ± ± ±

0.15 0.12 0.09 0.31

P = permeance. bLP = water permeance.

comparison of water permeance, LP, with the permeance of dry He in NPP (Table 2). The smaller the He permeance, the smaller the water permeance through the membrane, which shows the correlation between the permeation properties of the membrane in RO and gas permeation after different pretreatments. Figure 11 presents LP and R for the CPA5 (high-rejection membrane). The results are compared with a SWC5 seawater PA membrane (Nitto Denko), which is considered to possess a more rigid structure for high-pressure applications,16,17 and, therefore, the membrane structure is expected to be affected less by different pretreatments. As observed, SWC5 showed no remarkable differences in water permeance after the different procedures, which may be attributed to the high degree of chain rigidity, which accounts 1449



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understanding of compound transport mechanisms in the inhomogeneous structure of aromatic polyamide membranes for the development of a new generation of membranes with improved separation performance.

In short, the water permeance results from RO tests confirmed the average free-volume pore size tendency observed from NPP, NPK, and PALS (EH > FD > RTO) and demonstrated that different membrane pretreatments may affect the characteristics of dense matrix and highly permeable regions in the bimodal PA structure.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSION In this work, polyamide membranes were pretreated and their free-volume pore sizes were estimated by nanopermporometry (NPP), normalized Knudsen-based permeance (NKP), and positron annihilation lifetime spectroscopy (PALS). The main conclusions of the work are as follows. (1) In NPP, the permeance of He was decreased at p/ps = 0−20%, while N2 remained invariable. At p/ps = 90%, the permeance of both gases rapidly decreased. The two decreasing regions in the dimensionless permeance curve denoted a bimodal membrane structure, which consisted of dense and highly permeable regions. The application of water as a condensable vapor, with a comparatively small molecular size and a reduced affinity to polyamide materials, indicated the free-volume pore size distribution of the membrane. The reduction in He permeance for the three applied condensable vapors (water, hexane, and isopropanol) at dp = 0.6 nm may indicate the size of the local space in dense regions, which was confirmed after NKP tests. (2) The application of different pretreatment procedures influenced the membrane structure. In particular, the influence of highly permeable regions on permeance was found to decrease in the following order: ethanol−hexane > freezedrying > room temperature−oven samples. The comparatively reduced average free-volume pore size in oven pretreated samples was attributed to membrane shrinkage during hightemperature drying, while in ethanol−hexane treated polyamide chains, swelling and plugging compounds might have been removed, thereby decreasing the membrane resistance to gas permeation. (3) In PALS, dependence of the Ps lifetime on positron incident energy clearly showed that the holes in the outer layer of the present membranes were smaller in the order of ethanol−hexane > freeze-drying > room temperature−oven samples and that the estimated hole sizes were below 0.6 nm in diameter, which is in good agreement with the expected local space size in the dense regions. (4) Water permeance in RO for the CPA5 membrane was increased by factors of 1.94 and 1.29 for samples treated in ethanol−hexane compared with those treated at room temperature−oven and freeze-drying, respectively. This demonstrated the correlation between the permeation ability of the membranes in RO and gas permeation systems. On the other hand, salt rejection remained almost invariable after the different drying procedures despite the differences in membrane structure, probably due to membrane shrinkage, which produce an effective separation of NaCl by molecular sieving in the pretreated samples, but also due to the chemically charged structures that were effective for a NaCl exclusion mechanism, that were not altered after different pretreatments. In summary, this work proposed for the first time the application of the NPP technique for the structure characterization of polyamide membranes. The obtained results were consistent with NKP and PALS data, especially in the size range below 0.6 nm, and showed a bimodal free-volume pore size distribution in the membrane. The results aid in the

Text describing membrane pretreatment methods, nanopermporometry measurements, and physicochemical properties of the solvents and figures showing the time course for He permeance, water, hexane, and IPA sorption uptake in the RO polyamide membrane, and the side view of a water drop on the CPA5 PA membrane after RTO drying. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 824 24 7714. Fax: +81 824 22 7191. E-mail: tsuru@ hiroshima-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Japan Society for the Promotion of Science, under the Postdoctoral Fellowship for Foreign Researchers FY2012.

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A C dk dp E L m M p P Q rk rPs R S t v

NOMENCLATURE membrane area concentration Kelvin diameter free-volume pore size positron incident energy water permeance mass molecular weight pressure permeance flow rate Kelvin radius hole radius salt rejection sorption uptake time volume

Greek Letters

θ Π σ τPs υ

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contact angle osmotic pressure surface tension positronium lifetime molar volume

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Structural Characterization of Thin-Film Polyamide Reverse Osmosis

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