Supplementary Information (SI)

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1 Supplementary Information (SI) Nanofiber based triple layer hydro-philic /-phobic membrane - a solution for pore wetting issue in membrane distillation J. A. Prince a, D. Rana b, *, T. Matsuura b, N. Ayyanar a, T. S. Shanmugasundaram a, and G. Singh a, * a Environmental & Water Technology Centre of Innovation, Ngee Ann Polytechnic, 535 Clementi Road, Singapore , Singapore b Industrial Membrane Research Institute, Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, KIN 6N5, Canada 1. Experimental 1.1. Materials Poly(vinylidene fluoride) (PVDF) Kynar 761 grade with a melting point of C was purchased from Arkema Pte. Ltd., Singapore. Poly(vinyl pyrrolidone) (PVP) K17 technical grade was purchased from Shanghai Welltone Material Technology Co. Ltd., Shanghai, PR China. Methylene bis(p-phenyl isocyanate) (diphenylmethanediisocyanate, MDI, 98%), polyethylene glycol (PEG, typical Mn200 Daltons), poly(propylene glycol) (PPG, typical Mn 425 Daltons) were supplied by Sigma-Aldrich, Inc., St. Louis, MO, USA. Zonylfluorotelomer intermediate, 2-(Perfluoroalkyl)ethanol, (oligomericfluoro-alcohol, OFA, BA-L of average Mn 443 Daltons and 70 wt% fluorine) is a DuPont product supplied by Aldrich Chemical Company, Inc., Milwaukee, WI, USA. Ethanol, acetone, N,N -dimethyl acetamide (DMAc) and sodium chloride were of analytical grade from Sigma, Singapore. The water used was distilled and de-ionized (DI) with a Milli-Q plus system from Millipore, Bedford, MA, USA. 1

2 SMM synthesis The surface modifying macromolecules (SMM) were synthesized using a two-step solution polymerization method for hydrophobic SMM (hereafter called BSMM) or using a single-step solution polymerization method for hydrophilic SMM (hereafter called LSMM). For BSMM, the first polymerization step was conducted in a solution with a predetermined composition to form polyurethane for the reaction of MDI with PPG as a pre-polymer. In the second polymerization step, the pre-polymer was end-capped by the addition of OFA, resulting in a solution of BSMM. The molar composition of BSMM is MDI: PPG: OFA = 2: 1: 2.For LSMM, the polymerization step was conducted in a solution with a predetermined composition to form polyurethane for the reaction of MDI with PEG.The molar composition of LSMM is MDI: PEG = 3: 4. The BSMM (or LSMM) solution was then added drop-wise to a mixture of ice and water to precipitate the polymer. The polymer was kept in distilled water for 1 day to leach out residual solvent. Finally, the BSMM (or LSMM) was dried in an air circulation oven for 3 days. The chemical structures of the SMMs are shown in Fig. 1. The names of the BSMM and LSMM are poly(4,4 -diphenylenemethylene propylene-urethane) with both ends capped by OFA and poly(4,4 -diphenylenemethylene ethylene-urethane) with both ends capped by PEG, respectively. Fig. 1 The chemical structure of BSMM and LSMM SMM characterization The elemental analysis of fluorine content in the BSMM was carried out using a standard method in ASTM D3761. A specific weight (10-50 mg) of sample was placed into oxygen flask bomb combustion (Oxygen Bomb Calorimeter, Gallenkamp). After pyro-hydrolysis, the fluorine (ion) was measured by ion chromatography (Ion Chromatograph, Dionex DX1000). 2

3 The number average molecular weight, M n and polydispersity index (PDI, M w /M n, where M w is the weight average molecular weight) of BSMM were determined by gel permeation chromatography (GPC) (Waters Associates, Milford, MA). The system was equipped with three Waters UltraStyragel packed columns: Waters 600 Fluid Unit, Waters 600 Controller, and Waters 410 RI detector. Tetrahydrofuran (THF) was used as the mobile phase at 40 ºC and a flow rate of 0.3 ml/min. The calibration curve was performed using polystyrene (Shodex, Tokyo, Japan) standards with different molecular weights between 1.3 x 10 3 and 3.15 x 10 6 g/mol. The Waters Millenium 32 software was used for data acquisition and calculation of the polymer molecular weights. Characterization of LSMM was analyzed by the nuclear magnetic resonance (NMR) spectroscopy. A concentrated sample for NMR analysis was prepared by dissolving as much LSMM sample in DMSO-d 6. All spectra were acquired on a Bruker AVANCE 400 NMR spectrometer. The 1 H NMR spectrum was acquired in 32 scans using a 30 pulse. The acquisition time was 3.8 seconds. The 1 H- 13 C gradient enhanced HMQC spectrum was acquired using 16 scans for each of 256 slices. The recycle delay was set at 1 second and the acquisition time was 96 msec. The spectral width in the 13 C domain was set at 170 ppm. The 13 C NMR spectrum was acquired using WALTZ 16 proton decoupling in 10,420 scans using a 30 excitation pulse. The acquisition time was 1.0 seconds and the recycle delay was 1 second Membrane preparation Preparation of membranes by phase inversion The surface modified PVDF and control PVDF membranes were prepared for the intermediate layer by the phase inversion method. PVDF was used as the base polymer, DMAC was the base solvent, ethanol was used as a non-solvent, PVP was used as pore former and hydrophobic BSMM was added to increase the surface hydrophobicity of the membrane. The composition of the dope solutions is shown in Table 1 together with the membrane code. PVP powder was first added into the mixed DMAC and Ethanol solution in a media bottle and the solution was stirred by a magnetic stirrer for at least 1 h to dissolve 3

4 PVP powder. Then the BSMM and the PVDF were added and the suspension was stirred for at least 24 h at 80 o C and 250~350 rpm to obtain a completely dissolved polymeric solution. The polymeric solution was casted at room temperature by immersion precipitation process. The dope solution was casted onto a glass plate at around 25 o C and around 60% relative humidity by means of a casting rod with a gap of 100 microns and 50 microns. When the casting is completed, the cast film was left out for 30 s to let the SMM migrate to the membrane surface before immersing the cast film together with the glass plate into water for coagulation. The membrane turned opaque soon after coming into contact with water, which indicates that the coagulation and precipitation of PVDF from the solution was started, and finally a translucent, white flat sheet membrane was formed. The membrane was kept in water for 24 h to remove the residual DMAC. The membrane was further immersed progressively into DI water, ethanol and n-hexane, before it was hung at ambient temperature for at least one day for drying. To investigate the effect of hydrophobic BSMM on the surface modified PVDF composite membrane performance, four different BSMM concentrations in the casting dope were tried. Table 1: Composition of the dope solutions for the the preparation of PVDF membranes by the phase inversion technique Sample ID PVDF- Kynar- 761(wt%) PVP- K30 (wt%) Hydrophobic BSMM (wt%) DMAC (wt%) Ethanol (wt%) S S S S S Preparation of membranes by electrospinning PVDF-LSMM(Hydrophilic) composite electrospun nanofiber was coated by electrospinning on the low contact angle side of S3 and S5 membranes (see Table 1) to prepare the hydrophilic layer of the triple layer S6 and S7 membrane. As Table 2a indicates the spinning dope includes hydrophilic LSMM which renders the coated layer hydrophilic. For electrospinning, the spinning dope was stirred at 80⁰C for h to make the solution 4

5 homogeneous. A 10 ml of the spinning dope was electrospun at a rate of 1 ml/h by applying a voltage of 22KV between the tip of the spinneret and the rotating metal drum (collector) with a distance of 150mm to obtain a uniform mat (coating) of thickness 30±5 µm on the surface of substrate S3 or S5 wrapped around the rotating drum. After electrospinning, the nanofibers were dried for a day at room temperature for the solvents to evaporate. The nanofibers together with the substrate were then heat pressed at 140 C and 200 kpa to ensure good binding. Then, highly hydrophobic PVDF nanofibers were spun on the other side (hydrophobic side) of the substrate S3 or S5 to fabricate triple layer hydrophobic-hydrophilic membranes. The composition of the casting dope for electrospinning is presented in Table 2b. Dope preparation, electrospinning and heat pressing were made under the same conditions as those for the hydrophilic nanofiber spinning. The heat pressed membranes were stored in a desiccator before use. Table 2a: Composition of the PVDF-Hydrophilic LSMM electrospinning dope solutions for the bottom side (low contact angle side) Sample PVDF (wt%) LSMM-SMM (wt%) DMAC (wt%) Acetone (wt%) S S Table 2b: Composition of the PVDF electrospinning dope solutions for the top side (high contact angle side) Sample PVDF (wt%) DMAC (wt%) Acetone (wt%) S S

6 1.3. Membrane characterization The water contact angles (CA w ) of the membranes were determined using the VCA Optima Surface Analysis System (AST Products, Inc., Billerica, MA, USA). The membrane sample was placed on the glass sample plate and fixed with tape. The syringe filled with distilled water was installed to stand vertically. One μl of distilled water was deposited on the membrane surface. The contact angle was measured at five different points on each sample to obtain the average contact angle. The liquid entry pressure of water (LEP w ) was measured by placing the membrane in a dead end cell, which was loaded with DI water. Compressed nitrogen was used to apply pressure in the cell. The pressure was noted when the first drop of water came from the cell. The experiment was carried out thrice using three different membranes fabricated under the same condition. The results were averaged to obtain the final LEP w. The pore sizes of the membranes were measured using the Porometer 3G instrument from Quantachrome. The membrane sample of 25mm diameter was wetted with the Porofil liquid before analysis and then placed in the holder. The instrument used 3GWin Control software to calculate the pore size of the samples. The pore size was measured three times to obtain the average pore size for each sample. The membrane samples were sputter coated with gold, using a JFC-1600 auto fine coater (JOEL, Tokyo, Japan) and the morphology was observed by using scanning electron microscope (SEM) (JOEL JMS-6400, Japan). The effects of SMMs on the morphology were evaluated Membrane performance by DCMD The membranes were tested individually in DCMD setup with an effective membrane area of 117 cm 2 under identical operating conditions. The schematic diagram of the DCMD is shown in Fig. 3. The membrane performance was compared in terms of permeate flux and 6

7 salt rejection factor. Warm saline solution was supplied from the feed tank to the feed chamber of the membrane distillation cell by a circulation pump at a flow rate of 1.2 L/m (0.16m/s). Similarly, cooling water was recycled back to the cooling tank at a flow rate of 1.2 L/m (0.16m/s). The membranes were placed between the two chambers.. The inlet and the outlet temperatures of both the feed and the permeate solutions were measured. The DCMD module and all tubes were insulated to prevent heat loss. The permeate flux was measured by monitoring the water weight in both the feed and permeate side. The loss in the amount of water in the feed container should be equal to the gain in the amount of water in the permeate container when there is no leakage of water in the DCMD setup. The feed was 3. 5 wt% aqueous NaCl solution. Multiple tests were run on the membrane with different sets of temperature ranging from 40 o C to 80 o C for the hot feed and a standard of 17±2 o C for the cold feed. Fig. 2 Schematic diagram of the DCMD set-up. 7

8 2. Results and Discussion 2.1. SMM characterization The wt% of fluorine content of the BSMM was It had M n of 4.46x10 3 Daltons and polydispersity of Referring to Fig. 1, the value of p, number of repeat unit of CF 2, was calculated from the molecular weight of OFA to be As for LSMM, the value of n, number of repeat unit of PPG, was calculated from the average molecular weight of PPG to be Based on the chemical structure and the value of M n, the number of repeat unit, m, was calculated to be The 1 H NMR spectrum for LSMM is shown in Fig. 3.The characteristic peaks of the 1 H and 13 C NMR are listed in Table 3 and Table 4, respectively. Due to the absence of a correlation in the HMQC spectrum, as shown in Fig. 4, the NH proton can be assigned at 8.51 and 9.63 ppm. Fig. 3 1 H NMR spectrum of LSMM. 8

9 Table 3: The assignments of 1 H NMR characteristic peaks of LSMM. Assignment H (ppm) -CH 2 -CH 2 -O C 6 H 4 -CH 2 -C 6 H CH 2 -CH 2 -O OH C 6 H , NH-CO-O- 8.51, , 3.35 ppm peaks due to DMSO-d 6 Table 4: The assignments of 13 C NMR characteristic peaks of LSMM. Assignment H (ppm) -C 6 H 4 -CH 2 -C 6 H CH 2 -CH 2 -O , 69.17, 70.26, (CH) 4 C , (CH) 4 C NH-CO-O ppm peaks due to DMSO-d 6 C 9

10 Fig. 4 1 H- 13 C HMQC spectrum of LSMM. The presence of the hydroxyl group of the LSMM sample was confirmed by HMQC spectra in DMSO-d 6 solvent as peak was observed at about 4.58 ppm in the 1 H spectra but no signal appeared in the HMQC spectrum. The value of the n was calculated from the average molecular weight of PEG 200 Daltons which is The value of m was calculated using the 1 H NMR. The ratio between the integrated peak area of the signals corresponding to the two hydrogen of both hydroxyl end-group (AH end-groups ) and the peak area of the aromatic signal (AH AR ) is expressed as AH AH end groups AR 2 8m 8 (1) 10

11 The right hand side of the Eq. (1) is formulated from the chemical structure of LSMM (Fig. 1). The left hand side of the equation is equal to 1/ from 1 H NMR spectrum. The value of m for LSMM is calculated from the Equation (1) which is Therefore, the molecular weight of LSMM is 2.65 x 10 3 Daltons Membrane characterization (S1) (S2) (S3) (S4) (S5) (S6) (S6-Top) 11

12 (S6-Bottom) Fig. 5 SEM images of S1 to S Membrane performance by DCMD The casted membranes S1-S4 (with 100µm thickness) were tested to evaluate the membrane distillation performance by using a DCMD setup. Fig. 6ashows the DCMD flux of all four different membranes versus temperature. A trend of increasing flux is shown for all the membranes with the increase in temperature and S3 has the highest flux. Initialy the membrane distillation flux increased as SMM concentration increased and then the flux decreased from S3 to S4. Considering that the pore size decreases progressively from S1 to S4, the flux is governed not only by the pore size but also by the skin layer thickness and porosity. The average salt rejections of all four membranes were displayed in Fig. 6b. It is noted that the salt rejection of all the membranes is more than 99%. 12

13 Salt Rejection (%) J w (kg/m².hr) S1 S2 S3 S Temperature (ºC) Fig. 6a DCMD flux of all four different membranes versus temperature Temperature (ºC) S1 S2 S3 S4 Fig. 6b The average salt rejections of all four membranes. 13