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Sep 11, 2017 - affected the viscosity of the blend solutions but not the cloud point. The membrane ... Membrane- based technology has shown attractive...
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Effect of Molecular Weight of Sulfonated Poly(ether sulfone) (SPES) on the Mechanical Strength and Antifouling Properties of Poly(ether sulfone)/SPES Blend Membranes Li-Feng Fang, Hui-Yan Yang, Liang Cheng, Noriaki Kato, Sungil Jeon, Ryosuke Takagi, and Hideto Matsuyama* Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, Rokkodaicho 1-1, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: To improve the mechanical strength and antifouling properties of poly(ether sulfone) (PES)/sulfonated poly(ether sulfone) (SPES) blend membranes, three kinds of SPES with different molecular weights were attempted as additives. This is the first time that SPES with both high molecular weight (Mw = 141 000 g mol−1) and a high degree of sulfonation (DS, 30%) was used for membrane fabrication. It is also the first investigation of the effect of molecular weight of SPES on the properties of the blend membranes. The results show that the compatibility between PES and SPES was very high. The molecular weight of SPES affected the viscosity of the blend solutions but not the cloud point. The membrane morphology, surface chemical compositions, and mechanical strength were investigated. In addition, the surface hydrophilicity, surface charge, pore structures, and antifouling properties of the fabricated membranes were evaluated. The membrane fabricated with a casting solution containing SPES of high molecular weights (Mw = 110 000 and 141 000 g mol−1) with a PES/SPES ratio of 9/1 showed high mechanical strength and the superior antifouling potential. This was due to the formation of the thick sponge-like structures, and the hydrophilic and negatively charged surfaces of the membranes.

1. INTRODUCTION

ding, and blending, are widely used to enhance surface hydrophilicity of PES membranes.5−7 Among the modification methods, blending sulfonated poly(ether sulfone) (SPES) into PES membranes is one of the popular methods, due to the hydrophilic and negatively charged sulfonate groups on the SPES chains.8−10 Generally, the hydrophilicity of the SPES membrane increases with the degree of sulfonation (DS), while the tensile strength decreases with higher DS.11 For PES/SPES blend systems, the same hydrophobic component in SPES as PES improves their compatibility. Moreover, the hydrophilicity of the PES/SPES blend membrane is enhanced by increasing the SPES ratio in the casting solution. The pore size and sublayer porosity are enlarged by the addition of SPES.12 In addition, the protein (e.g., bovine serum albumin (BSA)) adsorption is suppressed and biocompatibility is improved.13,14 Due to the negative charge on the membrane surface, the protein separation can be achieved via strong electrostatic attraction/exclusion to charged proteins.15 Although various kinds of PES/SPES membranes with different PES/SPES ratios and different DS of SPES have been fabricated and compared,11,12 the effect of the molecular

Due to the growing population and rapid development of industry, drinking water shortages and water contamination have become the serious problems; therefore, clean and sustainable water sources are urgently required. Membranebased technology has shown attractive features for water treatment in terms of high efficiency, low energy consumption, low footprint, and environment-friendliness, compared with traditional separation techniques. In particular, ultrafiltration (UF) membranes, whose pore size is in the range of 2−100 nm,1 can effectively remove the bacteria and biomacromolecules with high water permeability from the feed source (e.g., river water and wastewater) to provide clean and secure water. However, membrane fouling and insufficient mechanical strength restrict their application in the water treatment field, especially under harsh conditions.2,3 Poly(ether sulfone) (PES) is commonly used as a commercial membrane material due to its excellent thermal, chemical, and oxidation resistance and rigid mechanical properties.4 However, when this membrane is applied to water treatment, severe fouling inevitably occurs due to its intrinsic hydrophobicity. Consequently, hydrophilic modification of this membrane is required for the improvement of its performance in water purification applications. Many approaches, including surface grafting, surface coating, embed© XXXX American Chemical Society

Received: Revised: Accepted: Published: A

July 20, 2017 September 7, 2017 September 11, 2017 September 11, 2017 DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

SPES with high molecular weight was manufactured by the patented method (WO2013/094586, US9228060, and JP5824734). Three types of SPES with the different molecular weights (45 000, 110 000, and 141 000 g mol−1) were labeled as SPES-L, SPES-M, and SPES-H, respectively. Dimethylacetamide (DMAc, Wako Pure Chemical Industries, Japan) and sodium alginate (SA, Wako Pure Chemical Industries) were used as the solvent and model foulant, respectively. Polyethylene glycol (PEG, Mw = 8000 g mol−1) was obtained from MP Biochemicals, LLC, France. PEGs (Mw = 20 000 and 35 000 g mol−1) and poly(ethylene oxide)s (PEOs, Mw = 100 000, 200 000, and 400 000 g mol−1) were purchased from Sigma-Aldrich Co. LLC, Germany. PEGs and PEOs were used as probe molecules for the determination of the membrane pore size and its distribution. Milli-Q water (Milli-Q integral 3, Millipore SAS, France) was used in this study. 2.2. Fabrication of the Pure PES Membrane and PES/ SPES Blend Membranes. The pure PES membrane and PES/SPES blend membranes were fabricated by the nonsolvent induced phase separation (NIPS) process. First, the PES resin or a series of PES/SPES blends were dissolved in DMAc with a determined concentration at room temperature for 8 h to form a homogeneous casting solution. After, the solution was kept still to release the bubbles at ambient temperature overnight, was then cast on the neat nonwoven fabric placed on the glass plate with a 200-μm-thick applicator at ∼20 °C, and then immersed in the coagulation bath (i.e., deionized water, ∼20 °C) to solidify. Finally, the membranes were rinsed thoroughly in Milli-Q water several times and kept in Milli-Q water for further measurement. For the mechanical strength measurement, the self-standing pure PES membrane and blend membranes were fabricated by the same method but without nonwoven supporting. 2.3. Cloud Point. The cloud-point curve for the PES/SPESDMAc-water ternary system was measured by titration method, as described in detail elsewhere.23 Briefly, the sample solutions were prepared with the different concentrations in the same way as in section 2.2. The Milli-Q water was then slowly and continuously added into the sample solution by a pipet with magnetic stirring until the clear solution became cloudy. Then, the cloudy solution was kept stirring and more water was added into the solution to examine if the sample solution became clear again after 30 min. The weight of the added water was recorded when the turbid solution did not clarify within 30 min, and this point was recognized as the cloud point. The composition of the cloud point was determined by calculating the weight percentage of PES/SPES, DMAc, and water in the bottle. 2.4. Viscosity of Casting Solutions. The viscosity of the casting solutions was measured by modular compact rheometer (MCR302, Anton Paar, Japan) with a rotator (CP50-1; diameter, 50 mm; angle, 1°; truncation, 102 μm) at 25 °C. A series of the viscosities were recorded by increasing the shear rate stepwise from 0.1 to 1000 s−1. 2.5. Characterizations of the Membranes. 2.5.1. Surface Chemical Compositions and Morphology. The surface chemical compositions of the membranes were characterized by X-ray photoelectron spectroscopy (XPS, JSP-9010MC, JEOL, Japan) with Al Kα excitation radiation (1486.6 eV). The detecting depth was set as ∼6 nm, and the binding energy was calibrated from the existing C 1s (285 eV). The membrane morphology was observed by a scanning electron microscope (SEM, JSF-7500F, JEOL, Japan). The cross-sectional samples for the SEM observation were obtained by fracturing in liquid

weight of SPES on the structures and properties of the blend membranes has not yet been systematically investigated. For the PES/SPES blend membrane, a critical issue that should be considered is that the mechanical strength of the fabricated membrane will be reduced by blending hydrophilic components, such as SPES.16,17 The molecular weight of the blending additive affects the membrane structures and performance as a result of the phase separation process. It has been reported that the pore size, porosity, and water permeability of the membranes generally increase with an increase in molecular weight of the blending additives such as polyethylene glycol (PEG) and polyvinylpyrollidone (PVP), due to the pore-forming effect.18,19 However, additives with higher molecular weight would significantly enhance the kinetic hindrance for the phase separation process, leading to the suppression of macro-void formation in the cross section.20,21 Another factor affecting the membrane properties is the retention of additives in the blend membranes. Generally, the higher the molecular weight of the additives, the more they are retained in the membrane, resulting in the increased hydrophilicity but decreased water flux.21 Therefore, the molecular weight of blending additives is a critical factor that affects the membrane structures and properties. In this study, SPES was used as a blending additive with the hydrophilic and negatively charged sulfonated groups. Thus, it was important to find a suitable molecular weight and fraction of SPES for membrane modification resulting in higher performance in terms of the enhanced mechanical strength and excellent antifouling properties. Three kinds of SPES with different molecular weights (45 000, 110 000, and 141 000 g mol−1) but the same DS (30%) were adopted and blended with PES to fabricate flat sheet membranes. As far as we know, this study is the first time to use SPES with high molecular weight (141 000 g mol−1) and high DS (30%) for membrane fabrication. First, the compatibility between SPES and PES was studied based on the Schneier theory.22 Then, the casting solutions with different molecular weights of SPES were compared via the cloud point of the PES/SPES-dimethylacetamide (DMAc)-water ternary system and the viscosities. The properties of the blend membranes including the structures, surface chemical compositions, morphology, surface hydrophilicity, surface charge, and mechanical strength were evaluated. Finally, the dependency of antifouling properties on the molecular weight of SPES was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Poly(ether sulfone) (PES, Ultrason E6020P, Mw = 58 000 g mol−1, BASF Co., Germany) was used for membrane fabrication. Sulfonated poly(ether sulfone) (DS = 30%, Mw = 45 000, 110 000, and 141 000 g mol−1, Figure 1) was kindly supplied by Konishi Chemical Ind. Co., LTD, Japan, and was used as the membrane additives.

Figure 1. Chemical structures of PES and SPES. (a) PES, n = ∼250; (b) SPES, x/y = 7/3. B

DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research nitrogen. Before measurement, all of the surfaces and cross sections were coated with a thin OsO4 layer using an osmium coater (Neoc-STB, MEIWAFOSIS Co. Ltd., Japan). The surface roughness was measured by atomic force microscope (AFM, SPI3800N/SPA400, SII Co., Japan) in tapping mode. The data were collected in an area of 500 nm × 500 nm. 2.5.2. Mechanical Strength Measurement. The mechanical strength of the membranes was tested on a tensile strength tester (AGS-J, Shimadzu Co., Japan) according to ASTM D638-14. Before the measurement, the membranes were dried at the ambient temperature for 1 d. The self-standing flat sheet samples were cut into a dumbbell shape (Figure S1), and the tensile rate was set at 20 mm min−1. The thickness of the sample was measured by the micrometer (MCD130-25, Niigataseiki Co., Japan). 2.5.3. Surface Hydrophilicity and Charge. The surface hydrophilicity of the membranes was evaluated by the air bubble contact angle using a contact angle goniometer (Drop Master 300, Kyowa Interface Science Co., Japan). Different from the water contact angle measurement, the air bubble contact angle was measured directly in water, which eliminates the chemical and physical change of the membrane surface due to drying. The detailed test procedure used in this study was described in detail elsewhere.24,25 The surface zeta potential was measured using a surface streaming potential electrokinetic analyzer (SurPASS, Anton Paar GmbH, Austria) according to the Helmholtz−Smoluchowski equation26,27 with 1 mmol L−1 KCl solution as the background solution at ambient temperature. The pH of the background solutions was in the range of 2.5−6, which was adjusted by adding 0.1 mol L−1 HCl to the KCl solution. 2.5.4. Membrane Pore Structures and Bulk Porosity. The pore size and its distribution of the membranes were determined by the solute transport method.28 PEGs (Mw = 8000, 20 000, and 35 000 g mol−1) and PEOs (Mw = 100 000, 200 000, and 400 000 g mol−1) were used as probe molecules. All PEGs or PEOs were dissolved in Milli-Q water to a concentration of 200 ppm. The Stokes radii (a, nm) of PEGs and PEOs can be obtained from their molecular weights (Mw) according to eqs 1 and 2.28,29 For PEG a = 16.73 × 10−3M w 0.557

Assuming the relation between the rejection of the solute and the solute diameter satisfies the log-normal probability function, the mean solute size (μs) was calculated as the solute diameter corresponding to 50% rejection. The geometric standard deviation (σs) about the mean diameter was determined from the ratio of the solute diameter at the rejection of 84.13% and at 50%. Ignoring the dependence of solute separation on the steric and hydrodynamic interaction between solute and pore, the mean pore size (μp) and the geometric standard deviation (σp) of the membrane can be considered to be the same as μs and σs. Then, the pore size distribution (f(dp)) can be expressed by following probability density function (eq 4).28,30 df (d p) dd p

⎛ m − m2 ⎞ bulk porosity(%) = ⎜1 − 1 ⎟ × 100 Vρ ⎠ ⎝

(5)

−3

where ρ (1.0 g cm ) is the water density. 2.5.5. Water Permeability, Rejection, and Antifouling Properties. The water permeability and antifouling properties of the membranes were evaluated with a cross-flow membrane cell (refer to Figure S2, effective area: 22 cm2, the tunnel height: 1.9 mm) and a boost pump (CDP8800, Water Quality, U.S.A.). The flow rate was maintained at 0.2 L min−1 in all filtration tests. The pure water filtration test was carried out until the flux became stable under a transmembrane pressure of 0.1 MPa using Milli-Q water. In the fouling test, the initial water flux (Ji) was set at 200 L m−2 h−1 by changing the operating pressure (Pi) using Milli-Q water. When the water flux became stable at 200 L m−2 h−1, the feed solution was changed to a 1000 ppm of SA solution, and the filtration test lasted for 1 h. SA rejection (R, %) was determined by eq 3. The SA concentrations of permeate and feed were determined by UV−vis spectrometry (U-2000, Hitachi Co., Tokyo, Japan) at a wavelength of 220 nm. The membrane was then backflushed with Milli-Q water at a pressure of 0.01 MPa for 2 min, and the pure water flux (Jr) at Pi was measured again. The flux recovery ratio (%) was obtained with eq 6 and was used to evaluate the membrane antifouling properties.

(1)

(2)

Thus, the Stokes radii of the solutes used in this experiment were 2.5, 4.2, 5.7, 9.0, 13.5, and 20.3 nm, respectively. Pure water filtration was first carried out under a transmembrane pressure of 0.1 MPa at a flow rate of 160 mL min−1 with MilliQ water using a cross-flow membrane cell (effective area, 8.04 cm2). After the water flux became stable, the PEG/PEO solution was fed as the solution, and the filtration process was conducted at a transmembrane pressure of 0.1 MPa. After excluding the residual water in the outlet tunnel, the permeate solution was collected. The concentrations (ppm) of PEGs and PEOs in the feed (cf) and permeate (cp) were determined by a total organic carbon analyzer (TOC-V CSH, Shimadzu, Japan). The rejection (R, %) was obtained using eq 3. ⎛ cp ⎞ R(%) = ⎜1 − ⎟ × 100 cf ⎠ ⎝

(4)

The bulk porosity was measured by first taking the wet membrane out from the water bath followed by quick removal of the excess water on the membrane surface by wiping with a tissue. The volume (V, cm3) and weight (m1, g) of the membrane were the measured. The membrane was again weighed (m2, g) after being freeze-dried overnight. The bulk porosity was then obtained:

For PEO a = 10.44 × 10−3M w 0.587

⎡ (lnd − lnμ )2 ⎤ p 1 p ⎥ = exp⎢ − ⎢⎣ d p(ln σp) 2π 2(lnσp)2 ⎥⎦

flux recovery ratio(%) =

Jr Ji

× 100 (6)

3. RESULTS AND DISCUSSION 3.1. Compatibility of PES/SPES Blend Systems. For the blend system, the compatibility between all components is one of the critical factors which affects the membrane fabrication and its final properties. Here, the compatibility between PES and SPES was evaluated based on Schneier theory.22 In this theory, the compatibility is evaluated by the mixing enthalpy, ΔHm (eq 7), calculated based on Flory−Huggins theory.

(3) C

DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research According to the Schneier theory, ΔHm over 0.01 cal mol−1 means that the blend system is incompatible, and the blend system is compatible when ΔHm is lower than 0.01 cal mol−1. ⎡ ⎤1/2 X2 2 ⎥ ΔHm = ⎢X1M1ρ1(δ1 − δ2) ⎢⎣ X1M 2ρ2 + X 2M1ρ1 ⎥⎦

recipes for the fabricated pure PES membrane and PES/SPES blend membranes are shown in Table 2. To simplify their Table 2. Compositions of the Pure PES Membrane and PES/ SPES Blend Membranes

(7)

Here, subscripts 1 and 2 denote polymer 1 and polymer 2, respectively. X is the weight fraction of the polymer in the blend, and X1 + X2 = 1. M is the molecular weight of the repeating monomer unit (g mol−1). ρ and δ are the density (g cm−3) and solubility parameters ((cal cm−3)1/2) of the polymer, respectively. In eq 7, ΔHm is affected by the molecular weight of the repeating unit but not by the molecular weight of the polymer. Therefore, the compatibility of the PES/SPES blend system will not depend on the molecular weight of SPES. Table 1 summarizes the pertinent information regarding PES and SPES used. The relationship between ΔHm and additive

chemicals PES SPES (DS = 0.3)

molecular weight of repeated monomer units/g mol−1

solubility parameter/ cal cm−31/2

1.37 1.25

232 256

11.031,32 11.631,32

SPES

L-23-10 M-23-10 H-23-10 L-23-30 M-23-30 H-23-30 PES-23

SPES-L SPES-M SPES-H SPES-L SPES-M SPES-H -

polymer concentration/%

SPES/PES ratio

DMAc/%

23

1/9

77

23

3/7

77

23

0/1

77

description, the blend membranes are named as L/M/H-a-b. Here, L, M, and H represent SPES-L, SPES-M, and SPES-H in the blend membranes, respectively. “a” represents the polymer concentration in the casting solution, and “b” stands for the SPES percentage in the polymer blend. PES-23 stands for the pure PES membrane with a polymer concentration of 23%. 3.2. Cloud Point of the PES/SPES Solution. The phase diagram of the PES/SPES-DMAc-water ternary blend system was measured to determine the cloud-point curves with different molecular weights of SPES. In this experiment, the 10% SPES fraction was chosen for cloud-point determination because 10% SPES fraction is completely within the compatible region. As shown in Figure 3, the cloud-point curves for all PES/SPES systems are almost the same, indicating that these systems show similar thermodynamic stability, as expected from eq 7. Moreover, due to the strong affinity between the sulfonate acid group and water, the casting solutions with higher SPES ratios in the blend system are expected to possess the higher tolerance of water. 3.3. Viscosity of Casting Solutions. As shown in Figure 3, there is no obvious effect of the molecular weight of SPES on the thermodynamic stability of the PES/SPES solution. However, the molecular weight of SPES affects the viscosity of the casting solutions, and therefore, it affects the phase separation kinetics. As shown in Figure 4a, it is found that the viscosity of the casting solutions scarcely depends on the shear rate, which is similar to a Newtonian fluid. Therefore, all viscosity data reported here are the average of three independent measurements at a shear rate of 100 s−1. It is clear that the viscosity of the casting solution increases with the increase of molecular weight of SPES, as shown in Figure 4b. The increased viscosity is mainly caused by the higher molecular weight of SPES and a higher degree of entanglement between the longer SPES chains and the PES chains. In addition, as can be seen in Figure 4b, the viscosity of casting solution decreases with the increasing SPES fraction in the blend system. For example, the viscosity of L-23-10 is 1543 mPa s, while that of L-23-30 is 877 mPa s. Generally, increased viscosity of the polymer solution would restrain the solvent and nonsolvent exchange rate during the phase separation process, further suppressing the formation of macro-void structures in the membrane33,34 and enhancing the mechanical strength.35 Therefore, a higher molecular weight of SPES and a lower SPES fraction will be better for the fabrication of a membrane with high mechanical strength. 3.4. Characterizations of the Pure PES Membrane and PES/SPES Blend Membranes. 3.4.1. Surface Chemical

Table 1. Density, Molecular Weight of the Repeated Monomer Unit, and Solubility Parameters of PES and SPES density/ g cm−3

membrane code

fraction (i.e., SPES fraction) in the blend system is shown in Figure 2. It was found that SPES shows the much better

Figure 2. Mixing enthalpy of the PES/SPES blend system. The dotted line represents the critical boundary between compatibility and incompatibility of the systems based on Schneier theory. The area above the dotted line represents the incompatible region, while that below the line is the compatible region.

compatibility with PES compared to other common additives (e.g., PEG and PVP; Table S1 and Figure S3). When fraction of additive is 10%, ΔHm in the SPES/PES blend system was 0.00955 cal mol−1, while those in PEG/PES and PVP/PES blend systems were 0.0577 and 0.0195 cal mol−1, respectively. Therefore, SPES is a better choice than PEG or PVP, from the viewpoint of the compatibility with PES, to fabricate PES blend membranes. For the PES/SPES blend system, the compatible region is lower than 11% or higher than 62% of the SPES fraction. Then, 10% and 30% of SPES fractions were chosen as the typical membrane fabrication conditions, representing the PES/ SPES compatible and incompatible areas, respectively. The D

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Figure 3. Phase diagram of the PES/SPES-DMAc-water system (a, PES/SPES-L; b, PES/SPES-M; and c, PES/SPES-H). SPES fraction in the PES/ SPES blend system was 10%.

Figure 4. (a) Shear rate dependency of viscosity of the casting solutions (PES-23, L/M/H-23-10). (b) Viscosity of the casting solutions (PES-23, L/ M/H-23-10 and L/M/H-23-30) at a shear rate of 100−1. The codes for the casting solutions are the same as those listed in Table 2.

Compositions and Morphology. The surface chemical compositions were determined by XPS. As shown in Table 3,

surface segregation found in other blend systems, such as polyvinyl chloride/methacryloyloxyethylphosphorylcholine-copoly(propylene glycol) methacrylate24 and poly(vinylidene fluoride) (PVDF)/PVDF-g-poly(oxyethylene methacrylate)36 systems. This may be due to the rigid structures of hydrophilic sulfonated groups of SPES when they directly covalently bond with benzene rings, which limits their movement to the top surface of the membrane during the phase separation process. Compared with the grafting copolymer, the short side chains (i.e., sulfonated groups) cannot flexibly migrate due to the large polymer backbones. Another reason for the unapparent surface segregation is attributed to the relatively excellent compatibility between PES and SPES (Figures 2 and S3, comparing the PES/ PEG and PES/PVP blend systems). The highly entangled PES and SPES chains limit the migration of the hydrophilic groups to the surface. This result also implies that the hydrophilic components are uniformly distributed in the membrane matrix, which is expected to be stable during long-term application. Figure 5 shows membrane morphology of the pure PES membrane and PES/SPES blend membranes. It can be seen that all surfaces (Figure 5a) are dense, which is similar to other reports in the literature.12 The cross sections consist of a dense layer, an open sponge-like sublayer, and a finger-like supporting layer (Figures 5b,c and S4). The tickness of the sponge-like sublayer increases with SPES ratio in both the blend and the molecular weight of SPES. Specifically, the thickness of the sponge-like sublayer for H-23-30 is ∼32 μm. Due to the hydrophilicity of SPES, the increase of the SPES fraction in the blend system increases the water tolerance during the phase separation process, which leads to the deeper position where the phase separation occurs. The incorporated water would

Table 3. Surface Chemical Compositions of the Pure PES Membrane and PES/SPES Blend Membranes theoretical surface chemical compositions/%a

experimental surface chemical compositions/%

membrane ID

C

O

S

C

O

S

PES-23 L-23-10 M-23-10 H-23-10 L-23-30 M-23-30 H-23-30

75.0 74.5

18.8 19.1

6.2 6.4

73.4

19.9

6.7

73.3 73.2 73.4 73.2 72.6 72.2 72.6

19.3 19.0 18.9 19.1 19.6 19.9 19.7

7.4 7.8 7.7 7.7 7.8 7.9 7.7

a

Theoretical surface chemical compositions were calculated according to the initial PES/SPES ratio in the polymer blend during membrane fabrication on the assumption that the elements are uniformly distributed in/on the membrane matrix/surface.

the differences between the experimental and theoretical S and C percentages in all membranes are less than 2%. The theoretical surface chemical compositions were calculated according to the initial PES/SPES ratio in the polymer blend during membrane fabrication. In theory, all of the elements are supposed to uniformly distribute in/on the membrane matrix/ surface. Moreover, all elemental fractions on the surface of the pure PES membrane are similar to the blend membranes. This indicates that all elements are uniformly distributed in/on the membrane matrix/surface. This result is quite different from the E

DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Tensile strength and elongation at break of the pure PES membrane and PES/SPES blend membranes.

sublayer thickness in the cross sections (Figure 5) increases the membrane mechanical strength.40 One thing should be pointed out that the mechanical strength of the PES/SPES membranes is reduced when the SPES fraction in the polymer blend increases (e.g., H-23-10 vs H-23-30), though the thickness of the sponge-like sublayer increases. As in previous studies,43 the most important comparison for evaluating the effect of the thickness of the sponge-like sublayer is between the same or similar membrane material. In this study, the mechanical strength of the PES/ SPES blend with a larger sponge-like sublayer (H-23-30) was weakened, due to the softness of SPES.11 As shown in Figure 2, compatibility of PES and SPES worsens as the fraction of SPES approaches 30%. This lower compatibility results in the lower molecular entanglement between PES and SPES, which may be another reason for relative weakness of H-23-30. Thus, the increase of the sponge-like sublayer thickness does not result in the predicted increase in mechanical strength of H-23-30. As mentioned in section 3.3, the high molecular weight of SPES helps to maintain the high viscosity of the casting solution when the polymer concentration is constant, which leads to an increase in the mechanical strength of the membrane. Another way to increase the viscosity of the casting solution is by increasing the polymer concentration, which also results in the increased mechanical strength. However, the latter method consumes more polymer and blending additive. Therefore, blending additives with higher molecular weight are a more cost-effective method. 3.4.3. Surface Properties and Pore Structures of the Blend Membranes. As discussed in section 3.4.2, L/M/H-23-10 show much better mechanical strength than L/M/H-23-30. Therefore, the pure PES membrane (PES-23) and PES/SPES membrane with 10% SPES fraction (L/M/H-23-10) were then characterized and compared. The surface hydrophilicity of the membranes was characterized by the air bubble contact angle. Compared with the water contact angle measurement, the air bubble contact angle is conducted in water, which eliminates the chemical and physical changes of the membrane surface due to drying. Higher air bubble contact angles indicates increased hydrophilicity.24 It can be seen in Figure 7 that the air bubble contact angles for the PES/SPES blend membranes (L/M/H-23-10) are higher than that for the pure PES membrane (PES-23), indicating the improved surface hydrophilicity due to the introduction of hydrophilic SPES. For the blend membranes with different molecular weights of SPES, the air bubble contact angles were similar, but that of H-23-10 was a little higher than the others.

Figure 5. (a) Top surfaces and (b) cross sections of the pure PES membrane and PES/SPES blend membranes. (c) Compositions of the cross section (H-23-30).

sharply increase the viscosity of the casting solution before gel formation occurs, which suppresses the formation of the fingerlike structures and further results in the thicker sponge-like sublayer structures.17 The increasing molecular weight of SPES results in the increased viscosity of the casting solution, as shown in Figure 4b. This increased viscosity causes a deceleration of the solvent and nonsolvent exchange when the cast membrane is immersed in the coagulation bath and prevents the formation of the macro-void structures according to the nucleation and growth mechanism.37−39 The detailed formation mechanism will be systematically studied in future research. Furthermore, the thicker sponge-like sublayers likely provide the membranes with higher mechanical strength and higher pressure resistance when compared with the finger-like structures.40 Thus, it is clear that the higher molecular weight SPES is better for the fabrication of membranes with high mechanical strength. 3.4.2. Mechanical Strength. High mechanical strength of the polymer membranes is desirable, especially when the membranes are applied in harsh conditions, such as in aeration and back flushing processes.16 As shown in Figure 6, the tensile strength and elongation at break of the PES/SPES blend membranes are lower than those of the pure PES membrane, and L/M/H-23-30 are much weaker than L/M/H-23-10. This may be due to the aggregative state of PES being decreased by the introduction of the highly polar sulfonic acid. The introduced sulfonic acid also leads to the expansion of the polymer matrix and thus an increase in chain movement.11 Both factors increase the flexibility of the normally rigid polymer, which is similar to other blend systems.41,42 Therefore, the lower SPES fraction in the membranes is preferable for the higher mechanical strength. It should also be noted that the tensile strengths at break of M-23-10 and H-2310 are similar (∼82% of PES-23) and higher than that of L-2310 (∼62% of PES-23). This can be attributed to the differences in membrane structure, that is, the increase of the sponge-like F

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pure PES membrane. This phenomenon is similar to the data reported by Rahimpour et al.37 There are several factors that influence the surface roughness of the membrane, such as the interactions between components in the casting solution and phase inversion kinetics. However, it is difficult to determine which factor is dominant for the tested membranes, due to the differing compositions and viscosities of the casting solutions. Even though PES-23 and M-23-10 have similar viscosities of the casting solutions, the stronger affinity between SPES and water decreases the phase separation rate, resulting in a smoother surface of M-23-10 compared to that for PES-23.37 Moreover, it was also found that the surface roughness decreases slightly with the increasing molecular weight of SPES, that is, 1.84 nm for L-23-10, 1.77 nm for M-23-10, and 1.54 nm for H-23-10. Generally, a smoother surface is beneficial for the antifouling properties of the membrane The pore sizes and their distribution of the blend membranes were also characterized by the solute transport method and solute rejection data, as shown in Figure S4. It can be seen from Table 4 and Figure 9 that the pore size (μp) of the blend

Figure 7. Air bubble contact angles for pure PES membrane and PES/ SPES blend membranes with 10% SPES fraction (L/M/H-23-10).

Surface charge is another critical parameter for membrane separation and antifouling performance. As shown in Figure 8,

Figure 8. Surface zeta potential as a function of pH on the pure PES membrane and PES/SPES blend membranes.

Figure 9. Pore size and its distribution of the PES/SPES blend membranes (L-23-10, M-23-10, and H-23-10) calculated with eq 4 using the μp and σp values shown in Table 4.

the isoelectric point (IEP) for PES-23 is pH ∼3.5, while the blend membranes show a significantly negative charge even at pH 2.7, due to the sulfonic acid groups on the membrane surfaces. M-23-10 and H-23-10 possess a similar surface charge, while L-23-10 is slightly more negative. These results are consistent with surface contact angle data (Figure 7). The evenly distributed sulfonic acid groups endow the membranes with enhanced hydrophilicity and more negative surface charge. It is likely that the hydrophilicity and surface charge would be maintained even if the top surface is destroyed by some harsh conditions, such as chemical cleaning. The surface roughness of the membranes was measured by AFM. As shown in Table 4, the root-mean-square roughness of the PES/SPES blend membranes is smaller than that of the

membrane decreases with the increasing molecular weight of SPES (L-23-10 > M-23-10 > H-23-10). This is because the viscosity of the casting solution increased with the increase of molecular weight of SPES, and the phase separation rate decreased, resulting in smaller pore size. 3.4.4. Water Permeability and Antifouling Properties of the Membranes. As shown in Table 4, the pure water permeability of the PES/SPES blend membranes is significantly increased compared with that of the pure PES membranes. For instance, the pure water permeability of M-23-10 is 233 L m−2 h−1 bar−1, compared to that for PES-23 which is 20 L m−2 h−1 bar−1. This can be attributed to the improved hydrophilicity (Figure 7) and increased bulk porosity (Table 4) of M-23-10.

Table 4. Surface Roughness and Pore Size Distribution of the Pure PES Membrane and PES/SPES Blend Membranes membrane codes PES-23 L-23-10 M-23-10 H-23-10

RRMS/nma

μp/nmb

σpb

± ± ± ±

12.9 11.1 9.7

1.57 1.80 1.68

2.30 1.84 1.77 1.54

0.24 0.08 0.06 0.11

bulk porosity/%c 19.9 43.9 45.4 52.3

± ± ± ±

2.7 4.8 4.3 3.2

pure water permeability/L m−2 h−1 bar−1 20 242 233 107

± ± ± ±

4 28 29 10

a c

RRMS: The root-mean-square roughness. bThese values were determined using the solute transport method as described in section 2.5.4. Calculated from eq 5. G

DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. (a) Relative flux as a function of operation time using SA solution (1000 ppm) as model foulant, and (b) the flux recovery ratios of the pure PES membrane and PES/SPES blend membranes (model foulant, 1000 ppm of SA solution).

Among the PES/SPES blend membranes, the flux recovery ratio of L-23-10 (∼84%) is lower than the others (M-23-10 and H-23-10, ∼90%) even though the negative surface charge density of L-23-10 is higher (Figure 8) and the hydrophilicity similar (Figure 7). This is likely due to the pore plugging taking place on the L-23-10 surface, since the pore size of L-23-10 is larger and SA can easily enter and plug the pores. This explanation is supported by the SA rejection data of the membranes. In summary, the PES/SPES blend membranes show significantly improved antifouling properties, which is further enhanced by using SPES with higher molecular weights.

For the PES/SPES blend membranes with different molecular weights of SPES, the permeability decreases with an increase of the molecular weight of SPES (L-23-10 > M-23-10 > H-23-10), due to the decreased surface pore size of the membranes (Table 4). Furthermore, increasing the thickness of the sponge-like sublayer also played a role in the decrease of the membrane permeability. Due to their hydrophilic and negatively charged surfaces of membranes, the PES/SPES blend membranes are expected to have improved antifouling properties in the context of wastewater treatment. To reduce the effects of water permeability of the membranes on the fouling properties, a pure PES membrane with high water permeability was prepared. The new membrane, PES-16 (16% PES in the casting solution), has a water permeability of 205 ± 10 L m−2 h−1 bar−1, which is similar to those of L-23-10, M-23-10, and H23-10. Also the pore size of PES-16 was characterized (Figures S5 and S6), and the mean pore size (μp) and the geometric standard deviation (σp) of the membrane are 8.4 nm and 2.02, respectively. The SA rejections for PES-16, L-23-10, M-23-10, and H-23-10 are 84.9 ± 0.2%, 91.4 ± 11.6%, 95.7 ± 4.9%, and 99.6 ± 0.1%, respectively. Figure 10 shows time depended relative flux during fouling measurements and the flux recovery ratios of the pure PES membrane (PES-16) and the PES/SPES blend membranes (L/ M/H-23-10). The flux recovery ratio determined by eq 6 reflects the antifouling properties of membranes. It is clear that the flux recovery ratios of the blend membranes (L/M/H-2310) are much higher than that of the pure PES membrane (PES-16), indicating the improved antifouling properties. From the pore size results of all membranes (Figure S6), the pure PES membrane (PES-16) has a smaller pore size than the PES/ SPES blend membranes. However, due to the lack of the strong negative charge on the surface and pore walls, which cannot effectively prevent the negative charged foulant from entering the membrane matrix, the SA rejection for PES-16 is relatively lower. This is one reason for the severe fouling of the pure PES membrane. Moreover, a hydrated layer, formed on the hydrophilic surface, creates a physical/chemical barrier to prevent foulants from approaching the hydrophobic matrix.44 In addition, the electrostatic repulsion between the negatively charged surface and the negatively charged foulants impedes the foulant adsorption.14,15 Therefore, the improved antifouling properties are attributed to the improved hydrophilicity and increased negative charge of the PES/SPES blend membranes.

4. CONCLUSIONS The PES/SPES blend membranes with different molecular weights of SPES were fabricated to improve their mechanical strength and the antifouling properties. The molecular weight of SPES significantly affected the viscosity of the casting solutions but not the cloud point of the polymer solutions. The increased viscosity of the casting solutions resulted in the suppression of the diffusional exchange during the phase separation process, and a dense layer and thicker sponge-like sublayer were formed, as seen in the cross sections of the blend membranes. With the increase of molecular weight of SPES, the mechanical strength of the membranes was increased. The hydrophilicity and the negative charge density of the membranes were also increased by blending SPES. The improved surface hydrophilicity and negative charge led to enhanced antifouling properties of the blend membranes. M23-10 and H-23-10 showed superior antifouling properties. Therefore, it can be concluded that SPES with higher molecular weights (110 000 and 141 000 g mol−1) is a better additive for blend membrane fabrication.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02996. Schematic of the dumbbell shape for the mechanical strength measurement, schematic of the flat sheet membrane cell for the water permeability and fouling tests, the mixing enthalpy of the PES blend system, the upper interface of the pure PES membrane and PES/ SPES blend membrane in large magnification, solute rejections for the PES/SPES blend membranes (L-23-10, H

DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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M-23-10, and H-23-10) vs the molecular weight of PEGs/PEOs, pore size and its distribution of the pure PES membrane (PES-16) and the PES/SPES blend membranes (L-23-10, M-23-10, and H-23-10), and density, molecular weight of repeated monomer units, and solubility parameters of PES, SPES, and PEG and PVP. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hideto Matsuyama: 0000-0003-2468-4905 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Grants-in-Aid from the Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction, Kobe) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, for the financial support. We also thank Prof. Bao-Ku Zhu, Mr. Ming-Yong Zhou, and Ms. Zhi-Ying Liang (Zhejiang University, China) for the zeta potential measurement.

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DOI: 10.1021/acs.iecr.7b02996 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX