Preparation and characterization of PVDF-P(PEGMA-r-MMA) ultrafiltration blend membranes via simplified blend method

Preparation and characterization of PVDF-P(PEGMA-r-MMA) ultrafiltration blend membranes via simplified blend method

Desalination 319 (2013) 47–59 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Prepa...

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Desalination 319 (2013) 47–59

Contents lists available at SciVerse ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Preparation and characterization of PVDF-P(PEGMA-r-MMA) ultrafiltration blend membranes via simplified blend method Ping-Yun Zhang, Zhen-Liang Xu ⁎, Hu Yang, Yong-Ming Wei, Wen-Zhi Wu, Dong-Gen Chen State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

H I G H L I G H T S • Simplified blend method was beneficial to micro-stricture adjustment of PVDF casting solution. • PVDF membranes possessed narrow distribution pore size and molecular weight cut off (MWCO). • Ethanol coagulant resulted in obvious tunable morphologies and hydrophilicity improvement.

a r t i c l e

i n f o

Article history: Received 23 November 2012 Received in revised form 27 March 2013 Accepted 1 April 2013 Available online 30 April 2013 Keywords: Amphiphilic copolymer P(PEGMA-r-MMA) Simplified blend Blend membranes Coagulants

a b s t r a c t P(PEGMA-r-MMA) amphiphilic copolymer with Mn of 66,500 g/mol and Mw of 34,200 g/mol was successfully synthesized via free radical polymerization. And polyvinylidene fluoride (PVDF)-P(PEGMA-r-MMA) blend membranes were fabricated from water and ethanol coagulants via simplified blend method by directly blending PVDF and P(PEGMA-r-MMA) amphiphilic copolymer solution (including the reaction mixture) to form casting solution. The formation of the supramolecular aggregates in PVDF solution containing the copolymer were confirmed by dynamic light scattering and scanning electron microscopy. This contributed to the micro-structure adjustment of PVDF solution and resulted in its decreasing surface tension, accelerating precipitation rate and increasing viscosity with trivial strain thinning behavior. Furthermore, the effects of the variations in dopant contents and coagulant compositions on the performances of those blend membranes were investigated. All PVDF-P(PEGMA-r-MMA) blend membranes possessed narrow distribution mean effective pore size (μ), molecular weight cut off (MWCO), improved recovery water flux after filtration experiments of bovine serum albumin and tuned configurations. Compared with the instantaneous demixing process in water coagulant, the delayed demixing process in ethanol favored the pore-forming and surface segregated of the polar head group of the copolymer, which induced the increasing μ, MWCO, tunable morphologies and hydrophilicity improvement. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Porous polymeric membranes have widespread applications in water and wastewater purification, desalination and gas separation. PVDF is advantageous over other polymeric materials due to its high mechanical strength and excellent chemical resistance, and thus these properties make it suitable for purification and separation. However, the functions of PVDF membranes are greatly influenced by their physical morphologies and chemical compositions. The optimized-structure of PVDF in terms of the membrane surface properties, morphologies and mechanical properties are anticipated for various applications in areas of wastewater treatment. Special emphasis is given on the effects of various parameters involved in the ⁎ Corresponding author. Tel.: +86 21 64253061; fax: +86 21 64252989. E-mail address: [email protected] (Z.-L. Xu). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.04.001

preparation of PVDF membranes with hydrophilicity modification from phase inversion process [1]. Precise control over the porous morphology of the membranes to meet the specific requirements via non-solvent induced phase separation (NIPS) process can be achieved by tuning various experimental parameters, including the ratio of polymer dope formulation, the type of solvent and additive, the amount of additive, preparation temperature and composition of coagulant. Synthetic pore-forming amphiphiles agents with segregated hydrophilic and hydrophobic moieties in the structure have attracted many researchers' attention in recent years [2–5]. Even the simplest class of the amphiphiles, the head or tail surfactants, can form a wide variety of self-assembled structures, depending on the relative volumes of the hydrophilic and hydrophobic components [6]. Usually, the amphiphilic polymers have been used as the models to study the microphase separation in the bulk and self-assembling behavior in the solution because of the nonionic and highly crystalline

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nature of poly(ethylene glycol) (PEO) [7]. P(PEGMA-r-MMA) amphiphilic copolymers prepared via free radical polymerization are among them with the common head or tail surfactants. It is speculated that the self-assembled structures of the P(PEGMA-r-MMA) amphiphilic copolymers in polymer solution might play an important role in the micro-structure modulation of PVDF membranes. Besides, a good solvent is essential in formulating a uniform polymer solution, and in obtaining the membranes with tunable morphologies and performances. It has been reported that N,N-dimethylacetamide (DMAc) and triethyl phosphate (TEP) are solvents for PVDF. PVDF polymer dissolved fairly easily in DMAc, but it did not easily dissolve in TEP [8]. As a strong solvent, when DMAc is used as solvent, PVDF membrane always exhibits finger-like macrovoids structure. Nevertheless, the membranes cast with TEP as solvent exhibit symmetry sponge structure and excellent mechanical property [9]. TEP-DMAc has been adopted as co-solvent to prepare PVDF membranes with anticipation properties [10]. In this study, TEP-DMAc (70:30, mass ratio) is used as co-solvent to obtain targeting PVDF membrane with sponge-like cross sectional structure and increase pore size with narrow distribution in its top-surface. Additionally, as for coagulants, water is well-known the harsh non-solvent, which has high diffusive rate with many solvents. Methanol, ethanol and 1-octanol are the common soft nonsolvents [11,12]. As soft non-solvent, ethanol is not only used as the coagulant to slow down the precipitation rate, but also to post-treat the pristine membranes [13]. Based on the synergetic consideration of the mass transfer rate between the coagulants and the casting solutions, water and ethanol are chosen as coagulants to provide the information on the modulation route of P(PEGMA-r-MMA) amphiphilic copolymer during various demixing processes. In this study, we aim to obtain tuned behavior of the supramolecular aggregates via simplified blend method by directly blending PVDF and the P(PEGMA-r-MMA) amphiphilic copolymer solution (including the reaction mixture) to form the PVDF casting solution [14]. Thus, the simplified blend method is expected to provide the morphology modulation of the PVDF membranes by tuning the micro-structure of the polymer solution with the presence of the P(PEGMA-r-MMA) amphiphilic copolymer. The reasons that the simplified blend method prevails in comparison to those regular blend methods are as follows. On one hand, the simplified blend method is environment-friendly; it is utilized to reduce the overall costs associated with the membrane preparation process, avoiding further surface modifications of the PVDF membranes. On the other hand, the excess or unreacted PEGMA and MMA monomers can act as poreforming agents in the membranes and are expected to be washed out during the NIPS process [15]. Furthermore, DLS and SEM are utilized to investigate the conformation adjustment of PVDF in the solution containing P(PEGMA-r-MMA) amphiphilic copolymer. And the influences of the variations dopant contents and coagulant compositions on the performances of the PVDF-P(PEGMA-r-MMA) blend membranes were investigated.

and MMA were purified by basic Al2O3, while AIBN was recrystallized in ethanol. All other chemicals used in the experiments were analytical grade and were used without further purification. 2.2. Synthesis of P(PEGMA-r-MMA) amphiphilic copolymer The typical synthesis route of P(PEGMA-r-MMA) amphiphilic copolymer was as follows. PEGMA (120.0 g, 0.1 mol), MMA (6.6 g, 0.06 mol) and TEP (70.4 g) mixture was purified by 4.0 wt.% basic Al2O3, and then the mixture was charged into a conical flask with plug and stored in a dark place for further use. The nitrogen gas was bubbled through the reaction solution, and the reaction vessel was sealed with a septum. A certain amount of recrystallized AIBN ([AIBN]/[MMA + PEGMA + TEP] = 1/100, mass ratio) was added to the corresponding solution and then the reaction solution was heated to 70 °C; the polymerization was carried out at 70 °C with stirring for 2 h. The synthesis route of P(PEGMA-r-MMA) was shown in Fig. 1. Then, some amount of reaction solution mixtures were cooled to ambient temperature and dialyzed by a bag filter (RC-44-7K, Greenbird Science & Technology Development Co., Ltd, Shanghai) of 7000 molecular weight cut off (MWCO). After being dialyzed in the stirring deionized water for two days (water was changed every 3 h), the copolymer was recovered by freeze drying (FD-1C-50, Beijing Boyiyang Instrument, CO. LTD). 2.3. Characterization of P(PEGMA-r-MMA) amphiphilic copolymer After purification, the construction of P(PEGMA-r-MMA) amphiphilic copolymer was characterized by 1H NMR, gel permeation chromatograph (GPC) and Fourier Transform Infrared Spectrometer (FT-IR). 1H NMR spectrum was recorded on a Bruker BioSpin GmbH (Germany) operated at 400 MHz, using D6-DMSO and tetramethylsilane (TMS) as solvents and internal standard, respectively. Molecular weight and molecular weight distribution of styrene/purification P(PEGMA-r-MMA) amphiphilic copolymer were investigated by a GPC system of a Waters 1525 (Waters CO., USA). Tetrahydrofuran (THF) was used as eluent with a flow rater of 1.0 mL/min at 35 °C, and the concentration of sample was 0.5 wt.%. Fourier Transform Infrared spectra of the copolymer samples were measured on a Bruker Vector 22 FT-IR Spectrometer (Switzerland). 2.4. Preparation of PVDF-P(PEGMA-r-MMA) blend membranes Various concentrations of P(PEGMA-r-MMA) solution (including the reaction mixture) were directly blended with PVDF to form casting solution in co-solvent. After PVDF was completely dissolved at 80 °C, and standing time of the casting solution was at least 12 h

2. Experimental 2.1. Materials Polyvinylidene fluoride (PVDF, Solef® 6010) was purchased from Solvay Advanced Polymers, L.L.C (Alpharetta GA, USA). DMAc, MMA, Al2O3 (basic), ethanol, triethyl phosphate (TEP), azobisisobutyronitrile (AIBN) were purchased from Shanghai Sinopharm Chemical Reagent Co. LTD (China). Polyethylene glycol monomethyl ether methacrylate (PEGMA, Mn = 1200, industrial grade) was a mahogany liquid and was supplied by Taijie Chemical Co. LTD (China, Shanghai). Bovine serum albumin (BSA) (MW 67 K) and Dextran (MW 40 K, 70 K, MW 100 K, MW 500 K and MW 2000 K) were purchased from Lianguan Biochemical Reagent Company of Shanghai and Sigma-Aldrich Co., respectively. Deionized water was prepared by our own lab. PEGMA

Fig. 1. The synthesis route of P(PEGMA-r-MMA) amphiphilic copolymer.

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to eliminate the inside bubbles. Then the casting solutions at ambient temperature were cast onto a glass plate at 25 °C and 60% relative humidity by means of a glass rod, and then were immediately immersed into the water-ethanol (50:50, mass ratio) coagulation bath (at 25 °C). The pristine membranes were maintained in fresh water for one week, and the deionized water was changed three times per day to ensure the complete removal of the residual solvent. The preparation conditions for the membranes are summarized in Table 1. 2.5. Characterization of PVDF casting solutions 2.5.1. Conformation and morphology of the PVDF casting solutions containing P(PEGMA-r-MMA) amphiphilic copolymer Dynamic light scattering (DLS) measurements were performed using a noninvasive back-scatter (NIBS) apparatus with a constant 173° scattering angle at 25 °C ± 0.1 °C. The morphology of the copolymer in the selective co-solvent was detected at first (Zatasizer Nano ZS, Malvern Instruments Ltd., U.K). Then the conformation of 1.0 wt.% PVDF solution with the presence of P(PEGMA-r-MMA) amphiphilic copolymer was investigated. The sample volume used for analysis was 2 ml, and all the samples presented absorption at 633 nm 0.01. A total of 15 scans with an individual duration of ~ 5 min were obtained for each sample. Measurement for each set of aliquots was performed in triplicate and the lowest polydispersity index (PdI) value was taken. The scattering intensity data were derived by the auto-measure software (Malvern Instruments, Malvern UK) to obtain the harmonic intensity weighted average hydrodynamic diameter (Rh). The mean diameter of the ensemble of particles, also referred to as the z-average diameter (Dh) of the reverse micelles, was derived from the slope of the linearized form of the correlation function [16]. Then the conformation of 18.0 wt.% PVDF in TEP-DMAc-PEGMAMMA system via simplified blend was further validated. The co-solvent of some amount of PVDF casting solutions with various compositions was quickly volatized at 80 °C in DHG Series Heating and Drying Oven (Jing Hong Laboratory Instrument Co., Shanghai), and then the samples were sputtered with gold; the morphology of those casting solution were obtained using scanning electron microscope (SEM, S-3400N, Hitachi). 2.5.2. Surface tension The surface tension of the casting solution was measured by a JK99C Automatic Surface and Interface Tension Measure Instrument (Shanghai Zhongcheng Digital Technology Apparatus Co. Ltd., China) via the Wilhelmy plate method at ambient temperature. 2.5.3. Viscosity The viscosities of the PVDF casting solutions were obtained with a DV-II + PRO Digital Viscometer (Brookfield, USA) at 298 K, controlled

Table 1 Resultant PVDF-P(PEGMA-r-MMA) blend membranes and their corresponding preparation conditions. Membrane no.

Polymer content (wt.%)

Co-solvent composition TEP/DMAc (7:3, mass ratio)

P(PEGMA-r-MMA) amphiphilic solution (wt.%)

Coagulant

MPEGMAW-0 MPEGMAW-1 MPEGMAW-2 MPEGMAW-3 MPEGMAW-3ma MPEGMAE-0 MPEGMAE-1 MPEGMAE-3 MPEGMAE-3ma

18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0

57.4/24.6 56.7/24.3 56.0/24.0 55.3/23.7 55.3/23.7 57.4/24.6 56.7/24.3 55.3/23.7 55.3/23.7

0.0 1.0 2.0 3.0 3.0a 0.0 1.0 3.0 3.0a

Water Water Water Water Water Ethanol Ethanol Ethanol Ethanol

a

3.0 wt.% of PEGMA, MMA and TEP mixture without polymerization.

49

by water bath. The reported data are the curves of viscosity relative to shear rate. 2.5.4. Precipitation kinetics of casting solutions Dynamic light transmittance experiment was utilized to investigate the precipitation kinetics of the casting solutions in water and ethanol coagulants, carried out using a self-made device, as described by Zhang et al. [17]. A collimated laser was directed toward the glass plate, immersed in the coagulation bath. The light intensity was captured by the detector and then was recorded in the computer. The precipitation rate of the PVDF casting solution in the coagulation bath was characterized by the curve of light transmittance relative to the immersion time. 2.6. Membrane performance 2.6.1. Morphologies of PVDF membranes Top surface and cross sectional morphologies of PVDF-P(PEGMAr-MMA) blend membranes (dried under ambient temperature) were observed using field-emission scanning electron microscopy (FESEM) (S-4800, HI-9054-0006). The samples were immersed in liquid nitrogen and fractured and then were sputtered with gold; the cross section and top surface configurations of the samples were obtained. 2.6.2. Mechanical properties of PVDF membranes The mechanical properties (i.e., the break strength, elongation at break and Young's modulus) of the PVDF membranes were conducted by using Microcomputer-Digital Display Integrative Control Testing Machine (QJ210A, Shanghai Qingji Instrument Sciences and Technology Co. Ltd., China) at ambient temperature. The flat sample with the settled width of 15 cm was clamped at both ends and pulled in tension at a constant elongation speed of 50 mm/min with an initial length of 25 cm. The mechanical properties were obtained from the stress–strain curves through the average of at least five measurements. 2.6.3. Dynamic contact angle (DCA) A contact angle analyzer (JC2000D1, Shanghai Zhongchen Digital Technology Apparatus Co. Ltd., China) was used for the determination of the dynamic contact angles (θ) of the PVDF membranes at ambient temperature. A water droplet of 0.2 μL was dispersed from a needle tip onto the membrane surface. The machine was coupled with a camera, enabling image capture at 10 frames/s. The contact angles were determined from these images using the specific calculation software. To ensure that the results were sufficiently credible, the experimental errors in measuring the θ values were evaluated to be less than ±0.5°. The measurement for each set of samples was performed in triplicate, and the average data of contact angles was taken. 2.6.4. Filtration properties of resultant membranes The filtration properties of the membranes were characterized by determinations of the permeation flux (J), porosity (ε), flux recovery ratio (FRR) and pore size distribution. A round-shaped membrane with a constant membrane area A = 2.289 × 10 −3 m 2 was installed into a cell, and the pressure in the cell was maintained at 0.1 MPa. Next, pure water was forced to permeate through the membrane, and the flux was recorded as Jw after membrane compaction was achieved with the stable flux. Additionally, a 300 mg/L BSA was forced to permeate through the membrane at the same pressure, and the flux was recorded as JB. To confirm the water flux recovery property of these BSA permeated membranes, pure water flux (JR) was measured after washing with pure water three times in 0.5 h. The pore size distribution of membranes under completely wet condition was characterized by solute transport of 300 mg/L dextran (MW 40 K, MW 70 K, MW 100 K, MW 500 K and MW 2000 K) solution via ultrafiltration experiments; the mean effective pore size (μ),

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the geometric standard deviation (σ) and MWCO were obtained according to the method described by Yang et al. [18]. The permeation flux (J) and FRR were calculated as follows [17,19]. J¼

V At

ð1Þ 

FRRð% Þ ¼

JR JW

  100%

ð2Þ

Where J is the flux of membrane for pure water or BSA solution (L/(m 2 · h)), V is the permeate volume of the pure water or BSA solution (L), A is the membrane area and t is the microfiltration time (h). The porosity (ε) of the resultant membranes was defined by following equation [10]:

peak at 1250.7 cm−1 and 1351.7 cm−1 were ascribed to the C\O symmetric stretching vibration and asymmetric stretching vibration, respectively. The absorption peak at 1453.7 cm−1 of the spectrum was assignable to the CH2 group, while that peak of 2872.8 cm−1 corresponded to the C\H stretching vibration of the CH2 groups of the PEGMA. The broad adsorption band at 3495.7 cm−1 and absorption peak at 1647.8 cm−1 could be ascribed to the H2O molecules adsorbed on the PEG segments of the P(PEGMA-r-MMA). It is very difficult to completely remove H2O from the copolymer because the PEO blocks have an affinity for water [22]. 3.2. Performances of PVDF-P(PEGMA-r-MMA) blend membranes

The chemical construction of P(PEGMA-r-MMA) copolymer was analyzed based on the identified chemical shift of the characteristic group. Fig. 2 showed the 1H NMR peaks with the chemical shifts of P(PEGMA-r-MMA). The resonance δ = 3.5 could be assigned to the H– of the methoxyl group of the monomers of MMA and PEGMA; the ether group in the PEGMA segment showed a chemical shift of δ = 3.4. The H– of the methylene groups connected to the oxygen atom of the ester group in the lateral chain of the PEGMA segment showed a chemical shift of δ = 4.0. Besides, δ = 0.5–2.0 was the H– in the main chains of the copolymer [20,21]. GPC result showed that the Mn and Mw of the P(PEGMA-r-MMA) were 66,500 g/mol and 34,200 g/mol, respectively (Multi-polydispersity index = 1.94). Besides 1H NMR analysis, the chemical composition of the P(PEGMAr-MMA) amphiphilic copolymer was further confirmed by FT-IR spectroscopy measurements as shown in Fig. 3. The spectrum of the copolymer indicated the expected absorbance for the ether groups (1108.7 cm−1) and the characteristic absorbance for the carbonyl groups of the ester at 1728.4 cm−1. Furthermore, the absorption

3.2.1. Morphologies and mechanical properties of the resultant blend membranes The tuned morphologies of the top surface and cross sectional of PVDF-P(PEGMA-r-MMA) blend membranes prepared from water and ethanol coagulants were investigated by FESEM images, and were shown in Figs. 4 and 5, respectively. All membranes revealed spongelike cross sectional with various morphologies of the top surface. The morphologies of the membranes prepared from water coagulant were examined at first. Pure PVDF exhibited dense morphology of the top surface, which became denser as the addition of the monomers without polymerization. However, PVDF-P(PEGMA-r-MMA) blend membranes possessed the typical wrinkle top surface. A careful inspection suggested that those wrinkles were substantially dense as increasing concentration of the copolymer. Examined in detail, the differences of the sponge-like cross section caused by adding the P(PEGMA-r-MMA) amphiphilic copolymer could be obtained. Macrovoids appeared, and their size enlarged as increasing concentration of the copolymer. The cross section under high magnification was used to elucidate the interior structure of the membrane bulk. It could be observed that pure PVDF nanograins arranged in a form of wide stripe-shape, while they were replaced by the cauliflower type after the addition of the monomers without polymerization. And the slim stripe-shape nanograins constructed the membrane bulk of PVDF-P(PEGMA-r-MMA) blend membranes. The morphologies of the PVDF-P(PEGMA-r-MMA) blend membranes prepared from the ethanol coagulant were significantly different with that prepared from the water coagulant. Fig. 5 showed that the soft coagulant was beneficial to the formation of the porous top surface. Pure PVDF membranes displayed porous top surface with the wide stripe-shape nanograins of cross section. The cauliflower type arrangement of the nanograins with loose packing constructed the whole thickness of the membranes prepared using the monomers without polymerization as additives. The porous top surface morphology of those blend membranes increased as increasing concentration of the

Fig. 2. 1H NMR spectrum of P(PEGMA-r-MMA) amphiphilic copolymer (Solvent: d6-DMSO).

Fig. 3. FT-IR spectra of P(PEGMA-r-MMA) amphiphilic copolymer.

ðm1 m2 Þ

ε ¼ ðm

1 m2 Þ

 ρG

. ρG

þ m2 =ρp

 100%;

ð3Þ

where m1 is the weight of wet membrane (g), m2 the weight of dry membrane (g), ρG the ethanol density (0.789 g/cm 3), and ρP the polymer density (1.765 g/cm 3). 3. Results and discussion 3.1. Characterization of P(PEGMA-r-MMA) amphiphilic copolymer

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Fig. 4. FESEM images of various PVDF-P(PEGMA-r-MMA) blend membranes prepared from water coagulant.

copolymer. Besides, the nanograins that constructed the membranes bulk were modulated from the stripe-shape to the interconnected styli-type with the existence of P(PEGMA-r-MMA) amphiphilic copolymer. Additionally, those styli-type nanograins became slimmer as increasing concentration of the copolymer. The influence of the P(PEGMA-r-MMA) copolymer on membrane performance was further elucidated by mechanical properties as shown in Table 2. Table 2 revealed that the PVDF-P(PEGMA-r-MMA)

blend membranes prepared from water coagulant possessed poorer mechanical properties when compared with that of pure PVDF. Reason behind that was the formation of the macrovoids in the sponge-like cross section as Fig. 4 shown. And the enlarging size of the macrovoids as the increase concentration of the copolymer explained the further decreasing mechanical properties of the blend membranes. The cauliflower type nanograins through the whole thickness of the MPEGMAW3m and the MPEGMAE-3m explained their poor mechanical properties.

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Fig. 5. FESEM images of various PVDF-P(PEGMA-r-MMA) blend membranes prepared from ethanol coagulant.

However, the PVDF-P(PEGMA-r-MMA) blend membranes prepared from ethanol coagulants exhibited significantly enlarged break strength and Young's modulus, which further improved as increasing concentration of the copolymer. The superior ductile behavior of the MPEGMAE-1 was attributed to the stripe-shape solid entities of the top surface and the interconnected styli-type nanograins with similar size in the membrane bulk. While the sharply decreasing elongation at break of the MPEGMAE-3 was ascribed to the slimmer styli-type nanograins that constructed the membranes cross section. 3.2.2. Filtration properties of PVDF-P(PEGMA-r-MMA) blend membranes To investigate the relationship between the membrane morphologies and the performances in detail, the filtration properties of the resultant membranes were studied, and the results were shown in Table 3. The FRR data in Table 3 revealed that all PVDF-P(PEGMA-rMMA) blend membranes had good recovery water flux after the filtration experiments with BSA. And the higher concentration of the copolymer was, the larger FRR was. The MPEGMAW(E)-3m revealed that the addition of the PEGMA and MMA monomers led to a decrease

flux when compared with that of MPEGMAW(E)-0. A comparison of the MPEGMAW(E)-3 and the MPEGMAW(E)-3m suggested that the presence of the copolymer improved the flux of the blend membranes. During the demixing process, the copolymer diffused from the interior of the casting solution and worked as pore-forming additive; this was

Table 2 Mechanical properties of various PVDF-P(PEGMA-r-MMA) membranes. Membrane list.

Break strength/MPa

Elongation at break/%

Young's modulus/MPa

MPEGMAW-0 MPEGMAW-1 MPEGMAW-2 MPEGMAW-3 MPEGMAW-3m MPEGMAE-0 MPEGMAE-1 MPEGMAE-3 MPEGMAE-3m

4.4 4.0 3.1 2.7 2.0 4.6 7.9 11.0 1.8

219 104 74.6 23.8 29.1 204 348 24.7 82.5

87.7 81.8 70.3 63.3 40.8 90.3 189 245 25.7

± ± ± ± ± ± ± ± ±

0.3 0.1 0.2 0.1 0.2 0.2 1.0 0.2 0.2

± ± ± ± ± ± ± ± ±

7.2 0.9 0.9 1.4 0.8 9.5 8.7 2.8 8.4

± ± ± ± ± ± ± ± ±

8.9 2.0 3.0 5.7 1.7 3.6 9.2 5.5 3.5

P.-Y. Zhang et al. / Desalination 319 (2013) 47–59 Table 3 Filtration properties of various PVDF-P(PEGMA-r-MMA) membranes. Membrane list.

JW/(L.m−1.h−1)

JB/(L.m−1.h−1)

FRR/(%)

ε/(%)

MPEGMAW-0 MPEGMAW-1 MPEGMAW-2 MPEGMAW-3 MPEGMAW-3m MPEGMAE-0 MPEGMAE-1 MPEGMAE-3 MPEGMAE-3m

45.0 15.0 27.0 44.0 33.1 1320 628 978 675

21.0 11.0 25.0 35.0 8.4 371 423 956 549

60.1 76.5 79.5 82 25.5 29.4 85.6 97.4 25.8

78.8 70.3 71.2 78.2 74.6 85.6 84.1 85.1 84.3

± ± ± ± ± ± ± ± ±

0.6 0.6 4.1 1.8 1.8 6.3 6.8 7.7 5.4

± ± ± ± ± ± ± ± ±

0.3 2.3 4.4 1.1 1.2 3.9 5.0 7.3 6.1

beneficial to the increase of JW and JB. However, JW and JB of those blend membranes decreased and then increased with the increasing concentration of the copolymer. The reasons for the downward and upward trends of JW and JB in the presence of the copolymer were as follows. During the demixing process, the copolymer diffused from the interior of the casting solution, which explained the increase flux of the blend membranes. However, with low concentration of the copolymer, the existence of the monomers limited the pore-forming of the copolymer and decreased the flux. The JW and JB decreased at first, whereas, with increasing concentration of the copolymer, more copolymer diffused from the interior of the casting solution and contributed to the increase flux of the blend membranes.

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Moreover, comparing with those prepared from the water coagulant, the membranes prepared from the ethanol coagulant exhibited better FRR and the enlarged fluxes of water and BSA. The coagulant compositions changed the liquid–liquid and solid–liquid demixing dynamics of the casting solutions. Casting solutions in the water coagulant exhibited an instantaneous demixing process, resulting in the formation of the typical dense wrinkle top surface. This explained the low flux of the resultant PVDF-P(PEGMA-r-MMA) blend membranes. While the delayed demixing process in the ethanol coagulant was beneficial to the formation of the porous top structures, and contributed to the increasing flux [23]. The cumulative pore size distributions and probability density function curves of the PVDF blend membranes prepared from water and ethanol coagulants were given in Figs. 6 and 7, respectively. The corresponding μ, σ and MWCO were also given in the two figures. It could be obviously observed from Fig. 6 that the MPEGMAW-3's distribution of pore size was narrow, with decreasing μ and MWCO. This was because that its top surface structure was originally occupied by the typical wrinkle configuration as the FESEM images shown. Apparently, Fig. 7 exhibited that the PVDF-P(PEGMA-r-MMA) blend membranes possessed narrow distribution pore size, increasing μ and MWCO. The consequence was induced by the formation of the porous structure during the delayed demixing process. Furthermore, the increasing μ and MWCO of the MPEGMAW(E)-3 were attributed to the pore-forming of the P(PEGMA-r-MMA) amphiphilic copolymer during demixing process when compared with that of the MPEGMAW(E)-3m.

Fig. 6. Cumulative pore size distribution curves and probability density function curves of various PVDF-P(PEGMA-r-MMA) blend membranes prepared from water coagulant.

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Fig. 7. Cumulative pore size distribution curves and probability density function curves of various PVDF-P(PEGMA-r-MMA) blend membranes prepared from ethanol coagulant.

3.2.3. Dynamic contact angle of PVDF-P(PEGMA-r-MMA) blend membrane DCA was utilized to provide some information on the hydrophilicity change caused by the addition of the P(PEGMA-r-MMA) amphiphilic copolymer. The DCA of the top-surface (T) and the bottom-surface (B) of the resultant membranes were characterized by the curves of water drop angle relative to contact time as shown in Fig. 8. The curves indicated that the both surfaces of PVDF membranes had excellent hydrophobic when the PEGMA and MMA monomers without polymerization were added. The excellent hydrophobic property of the membranes could be explained by the fact that the hydrophobic group of the monomers might have been imparted during the demixing process. This was consistent with the decrease flux of the resultant PVDF membranes. And the PVDF-P(PEGMA-r-MMA) blend membranes prepared from the water coagulant showed limited hydrophilicity improvement. While both surfaces of the PVDF-P(PEGMA-r-MMA) blend membranes prepared from the ethanol coagulants had decreasing start contact angle θs (or advancing contact angle) and steady receding contact angle θE (or equilibrium contact angle). And the θs and θE further decreased as increasing concentration of the P(PEGMA-r-MMA) amphiphilic copolymer. Moreover, it revealed that the hydrophilicity improvement of the top-surface was better than that of the bottom. Combining the porosity data of the PVDF-P(PEGMA-r-MMA) blend membranes as shown in Table 3, it was concluded that the hydrophilicity improvement didn't originate in porosity, but for other reasons [24]. It could account for this that the PEO groups of the P(PEGMA-

r-MMA) amphiphilic copolymer surface segregated in both surfaces of the blend membranes during the delayed demixing process [25]. And the delay demixing process in ethanol coagulant was beneficial to the surface segregation of the polar groups of P(PEGMA-r-MMA) amphiphilic copolymer and significant hydrophilicity improvement. With respect to the aforementioned FRR of the PVDF-P(PEGMA-rMMA) blend membranes, it was worthwhile to mention that the hydrophilicity improvement of those membrane were consistent with the good recovery water flux after the filtration experiments with BSA as shown in Table 3. Additionally, the increase of flux of the blend membrane might be related to the hydrophilicity modification of inner pores of the PVDF membrane facilitating the water flux or pore forming at the top surface. 3.3. Determination of physical–chemical properties of casting solutions 3.3.1. Precipitation kinetics of casting solutions by dynamic light transmittance Demixing kinetics of various casting solutions in the water and ethanol coagulants was shown in Fig. 9. It demonstrated that the delayed demixing process appeared in ethanol coagulant. Besides, the curves confirmed that the presentence of P(PEGMA-r-MMA) amphiphilic copolymer in casting solution accelerated the demixing process in both the two coagulants. And the precipitation rate further speeded up as the increasing concentration of the copolymer. The PEO groups of the P(PEGMA-r-MMA) were to form the inner core of the reverse micelle in the co-solvent, which led to the surface of the

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Fig. 8. The dynamic contact angles of various PVDF-P(PEGMA-r-MMA) blend membranes (T: top-surface; B: bottom-surface).

casting solution was studded with the hydrocarbon functional group of the copolymer. This contributes to increasing the immiscibility between the casting solution and the coagulant, enhancing the precipitation rate [26]. Based on the comprehensive consideration of the performances of the PVDF-P(PEGMA-r-MMA) blend membranes as mentioned above, it was believed that the delayed demixing process was beneficial to the formation of the porous structures and the hydrophilicity improvement. The demixing kinetics of the casting solutions containing P(PEGMA-r-MMA) amphiphilic copolymer in water coagulant explained the downward and upward trends of JW and JB of the blend membranes as Table 3 shown. With low concentration of the copolymer, the existence of the monomers decreased the flux. The accelerated precipitation rate caused by the presence of the copolymer led to the decreasing JW and JB at first, whereas, with increasing concentration of the copolymer, more copolymer diffused from the interior of the casting solution and worked as the poreforming additive. This explained the increase flux of the blend membranes. The delayed demixing process in the ethanol coagulant

contributed to crystallization prior to liquid-liquid demixing, which favored the formation of porous top structures [23]. Comprehensive analysis of the membrane performances mentioned above suggested that the delay demixing process was beneficial to the increasing flux, μ and MWCO.

3.3.2. Surface tension of casting solution Fig. 10 exhibited the surface tensions of the various casting solutions containing the P(PEGMA-r-MMA) amphiphilic copolymer. It displayed that the surface tension decreased as increasing concentration of the copolymer. The amazing point was that the surface tension sharply increased as the addition MMA and PEGMA monomers without polymerization. Because surface tension was related to the surface arrangement of the molecules at the interface between air and liquid, the impartation of the hydrophilic functional group of the monomers without polymerization on the surface of the solution explained the increasing surface tension. The hydrocarbon functional group of the copolymer that

Fig. 9. Precipitation rates of various PVDF casting solutions in water (a) and ethanol (b) coagulants.

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Fig. 10. Surface tensions of various PVDF casting solutions.

covered the surface of the casting solution contributed to the decreasing surface tension.

Fig. 12. Intensity-weighted micelle size distribution of P(PEGMA-r-MMA) in TEP-DMAc co-solvent.

DLS was widely used to probe the size and size distribution of the submicron-sized particles, proteins and reverse micelles [27]. It could struggle with finding correct particle size distributions in systems

containing polydispersed particles or droplets, where their sizes ranged over several orders of magnitude. Due to the sensitivity of the equipment, only the solution with a low polymer concentration could be measured. However, it was possible to provide some information regarding the morphology modulation of the casting solutions with the presence of the P(PEGMA-r-MMA) amphiphilic copolymer. Figs. 12 and 13 showed the intensity-weight of the supramolecular aggregates of the P(PEGMA-r-MMA) and the polymer–copolymer in the co-solvent, respectively. The detailed information for the intensityweighted particles of Figs. 12 and 13 was provided in Table 4. Fig. 12 illustrated the structures in the solution containing the PEGMA and MMA monomers without polymerization were very complex with both monomers and monomer aggregates existing in solution. Additionally, a more complex population of monomer aggregates formed with the addition of 1.0 wt.% PVDF, as shown in Fig. 13. Moreover, Fig. 13 revealed that the dominating populations were the aggregates of the polymer–monomers and the monomers themselves. However, the intensity-weighted aggregates in Figs. 12 and 13 obtained via simplified blend method, revealed novel results. A comparison between PEGMA-3 and PEGMA-3m showed that the distribution of the supramolecular aggregates in the solution was distinctly narrow with the presence of the P(PEGMA-r-MMA) copolymer. Interpeak (with one peak extension to interior of the other peak) appeared, and became divided as increasing concentration of the amphiphilic copolymer. Combined with Table 4, it could be deduced that the Rh of the dominating population of aggregates increased with increasing

Fig. 11. Shear viscosities of various PVDF casting solutions.

Fig. 13. Intensity-weighted particle size distribution of 1.0 wt.% PVDF in TEP-DMAc co-solvent containing various concentrations of P(PEGMA-r-MMA).

3.3.3. The viscosities of various casting solutions Fig. 11 presented the viscosities as a function of the shear rates for the PVDF casting solutions. All PVDF solution exhibited trivial strain thinning behavior. The interesting observation was that PVDF solutions containing the P(PEGMA-r-MMA) amphiphilic copolymer revealed increasing shear viscosities. This implied that the strong molecular interconnection between the copolymer and PVDF dominated the increasing viscosities with the chain stretching and sliding under shear. Besides, the hydrophobic groups of the monomers without polymerization also had some influence on the increasing viscosities [17]. Considering the accelerated precipitation rate of casting solution after the addition of the copolymer as shown in Fig. 9, it revealed that the increasing viscosity was not the dominant factor in determining the precipitation rate. To provide direct evidences of the micro-structure adjustment of the PVDF solutions containing the P(PEGMA-r-MMA) amphiphilic copolymer, DLS and SEM measurements were carried out. 3.4. Micro-structure adjustment of the casting solution by the existence of P(PEGMA-r-MMA)

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Table 4 Detail information for intensity-weighted particles of various solution systems. List

Peak 1

PEGMA-1 PEGMA-2 PEGMA-3 PEGMA-3ma PEGMA-0-1.0% PEGMA-1-1.0% PEGMA-2-1.0% PEGMA-3-1.0% PEGMA-3m-1.0%a a

Peak 2

Peak 3

Z-Average Dh (d. nm)

Rh (d. nm)

Intensity (%)

Rh (d. nm)

Intensity (%)

Rh (d. nm)

Intensity (%)

167 137 19.5 1068 610 213 293 271 7.4

47.9 62.2 37.5 35.0 70.5 97.2 93.0 86.6 42.2

36.6 19.2 115 4597 4998 4668 17.2 32.6 3383

47.6 31.7 33.1 23.6 22.6 2.8 4.0 9.5 35.3

6.8 4925 2780 0.8 10.3

3.2 6.1 29.4 21.7 6.9

4837 4584 34.99

3.1 3.9 12.4

46.19 45.31 47.48 595.8 609.7 134.0 145.4 155.9 112.4

3% of PEGMA, MMA and TEP mixture without polymerization.

concentration of the copolymer. The domination populations of the supramolecular aggregates of the copolymer in the co-solvent were the reverse micelles (size: 10–100 nm) and the nanospheres (size: 100–200 nm), which was consistent with their significantly decreasing Dh [28,29]. This evidenced the formation of the small-sized copolymer aggregates with the presence of the copolymer. The DLS data of the 1.0 wt.% pure PVDF solution indicated that the PVDF chains were in a coiled state and tended to extend before becoming entangled with neighboring chains in the TEP-DMAc co-solvent [30]; the corresponding Dh was 609.7 nm. And the Dh significantly decreased with the presence of the copolymer. The sharply decreased Dh of the PVDF–copolymer via simplified method was attributed to the additional enhancement of the lowly coiled state and increased entanglement with the neighboring chains of PVDF, which was caused by the formation of the supramolecular aggregates of the PVDF–copolymer [31]. And the Dh of the dominating population of the aggregates increased with increasing copolymer concentration in the solution. According to Fig. 13 and Table 4, the Rh of the reverse micelles and the nanospheres of the copolymer in the selective

co-solvent transferred to the narrowly distributed nanocapsules (size: 200–300 nm) with the addition of 1.0 wt.% PVDF. The supramolecular aggregates of the PVDF–copolymer contributed to the enlarged Rh of the PVDF–copolymer system, which suggested the micro-structure adjustment of the polymer solution and the conformation of PVDF. It was believed that the hydrophobic group of the copolymer contributed to either the adsorption of polymer over the copolymer layer or the formation of mixed adsorption layers [32]. Additionally, the formation of the supramolecular aggregates of the PVDF–copolymer with narrow distribution confirmed the existence of the hydrophobic groups of the copolymer and the conformation adjustment of PVDF. With only 1.0 wt.% of PVDF, it was not possible to fully mimic the micro-structure adjustment of 18.0 wt.% PVDF because there was a significant viscosity difference between the two solutions; additionally, the conformation of the 18.0 wt.% PVDF was further evidenced by the morphology images collected for various the PVDF casting solutions, as shown in Fig. 14. Pure PVDF exhibited the characteristic lobed-structure morphology in the co-solvent, with the characteristic

Fig. 14. Influence of P(PEGMA-r-MMA) amphiphilic copolymer on tuned morphology of various PVDF casting solutions (CS: casting solution).

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lobed-structure of the polymer domains being replaced by an irregular meshlike network of continuous domains. The shift from a lobedstructure to a meshlike structure was consistent with the existence of the aggregate of the polymer–monomers and the monomers themselves as the addition of the monomers without polymerization. The results also were consistent with the decreasing Rh and Dh in the DLS data. The existence of P(PEGMA-r-MMA) amphiphilic copolymer yielded the series of continuous and regular wavelike domain. In combination with DLS results, this confirmed the formation of the supramolecular aggregates of the PVDF–copolymer in a case of domination with narrow distribution. This interesting and exciting finding of DLS and SEM was directly related to the change of physical–chemical properties of the casting solutions via simplified method as discussed in above section. With respect to the aforementioned performances of the PVDF-P(PEGMA-r-MMA) blend membranes, it was worthwhile to mention that those narrow distribution supramolecular aggregates of the PVDF–copolymer were consistent with their narrow distribution pore size, MWCO and the regular slim stripe-shape/styli-type nanograins which constructed the membranes bulk. And those supramolecular aggregates might work as ‘template’ to form macrovoids in the cross sectional of the blend membranes prepared under instantaneous demixing process in the water coagulant. Due to the confine of the narrow distribution supramolecular aggregates of the PVDF–copolymer during the delayed demixing process in the ethanol coagulant, those PVDF-P(PEGMA-r-MMA) blend membranes possessed the size similar interconnected styli-type nanograins that constructed the membranes bulk [23]. 4. Conclusion P(PEGMA-r-MMA) amphiphilic copolymer was successfully synthesized via free radical polymerization using PEGMA and MMA as reactant, adopting AIBN as evocating agent. PVDF-P(PEGMA-r-MMA) blend membranes were fabricated by adopting TEP-DMAc (70:30, mass ratio) as co-solvent via simplified blend method from water and ethanol coagulants, respectively. DLS and SEM results confirmed the micro-structure adjustment of PVDF caused by the formation of the supramolecular aggregates in the solution with the presence of PVDF and the amphiphilic copolymer. Those aggregates induced the change of the physical–chemical properties of those casting solutions in terms of the decreasing surface tension, accelerating precipitation rate and increasing viscosity with trivial strain thinning behavior. The tuned conformation of PVDF and the different compositions of the coagulants modulated the filtration performances and changed the configurations of the resultant PVDF-P(PEGMA-r-MMA) blend membranes. All those blend membranes exhibited narrowed distribution pore size and improved recovery water flux after the filtration experiments with BSA. The instantaneous demixing process in the water coagulant resulted in the formation of the typical dense wrinkle top surface. This explained the low flux, decrease μ and MWCO of resultant blend membranes. While the crystallization of the casting solutions occurred prior to liquid-liquid demixing in the ethanol coagulant and the confine of the narrow distribution supramolecular aggregates during demixing process contributed to the formation of the porous top surface and the size similar styli-type morphologies of the blend membranes bulk. This was consistent with their increasing flux, μ and MWCO. Besides, those membranes' interconnected styli-type configurations guaranteed their superior mechanical properties in terms of the enlarged break strength, high elongation at break and increasing Young's modulus. Additionally, the delayed demixing process was beneficial to the poreforming and surface aggregates of the polar head groups of P(PEGMAr-MMA) amphiphilic copolymer, which explained the increasing μ, MWCO and hydrophilicity improvement of the resultant PVDFP(PEGMA-r-MMA) blend membranes. The newly developed hydrophilic ultrafiltration PVDF-P(PEGMA-r-MMA) blend membranes prepared from ethanol coagulant with low-fouling of BSA and tunable

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