Chemical Engineering Journal 197 (2012) 398–406
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The role of zeolite nanoparticles additive on morphology, mechanical properties and performance of polysulfone hollow fiber membranes Ganpat J. Dahe, Rohit S. Teotia, Jayesh R. Bellare ⇑ Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" We have used zeolite nanoparticles
A proposed microstructural model (a) uniform sized and nanoscale dispersion of zeolite nanoparticles in dope solution lead to small size nodules formation and result in small pore size with high pore density; (b) an agglomerate of zeolite nanoparticles in dope solution lead to large size nodules formation and result in large pore size with reduced pore density.
in dope solution for hollow fiber membrane preparation. " Effects of loading on morphology and pure water permeability and solute rejection studied. " Zeolite nanoparticles act as nucleating agent for nodule formation in skin as confirm from FESEM and EDAX study. " A microstructural model proposed to understand role of zeolite nanoparticles additive in morphology, subsequently the performance.
a r t i c l e
i n f o
Article history: Received 5 December 2011 Received in revised form 9 May 2012 Accepted 11 May 2012 Available online 18 May 2012 Keywords: Polysulfone HFM Zeolite nanoparticles Microstructural model
a b s t r a c t In ultrafiltration membranes, selective layer composed of nodular microstructure usually formed by nucleation and growth during phase separation. The nodule size and extent of nodule packing can be varied by the use of inorganic nanoparticles of zeolite to render hydrophilic and hydrophobic microdomain structure to the membrane which minimizes fouling. The zeolite nanoparticles are dispersed from 0.01 to 1 wt% in N-methyl-2-pyrrolidone (NMP) solvent with the compatibilizer (D-a-tocopheryl polyethylene glycol succinate, TPGS) to form nanocomposites of Psf/zeolite in the form of hollow fiber membranes (HFMs). High Resolution Scanning Electron Microscopy (HRSEM) and EDAX studies show that zeolite nanoparticles participate in the nucleation process during phase separation. An almost linear increase in the tensile modulus with nanoparticle concentration shows that the mechanical properties of the HFMs also get influenced. We observed that water permeability of HFMs increases from 15.92 to 21.31 mL/m2 h mm Hg, when zeolite loading increased from 0.01 to 0.1 wt% loading. Further, permeability decreases to 11.79 mL/m2 h mm Hg at 1 wt%. The molecular weight cut off of composite HFM shows a steady increase with loading concentration from 9500 Da to 54,000 Da. We proposed a microstructural model explaining the influence of zeolite addition on HFM properties which forms the basis for selection and optimization of such additives. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +91 22 2576 7207; fax: +91 22 2572 6895. E-mail address:
[email protected] (J.R. Bellare). 1385-8947/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.05.037
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1. Introduction Membrane technology has generated renewed interest in the last decade in many emerging applications due to its clean and environmentally benign nature [1]. Synthetic polymeric membranes are being widely used for ultrafiltration separations; however, their applicability is limited by low membrane formation ability, intrinsic flux and selectivity and fouling ability due to a hydrophobic nature. Inorganic material based membranes offer better chemical, thermal and antifouling resistance, but their application is limited by expensive, brittle and poor separation performance and, also, low membrane forming ability. These constraints can be overcome by preparing mixed-matrix membranes based on polymeric and inorganic materials so as to obtain favorable or required characteristics [2,3]. Taurozzi et al. prepared anti-microbial polysulfone silver nanocomposite membranes, by two methods – silver nanoparticles were either synthesized ex situ and then added to the dope solution as an organosol or produced in the casting solution via in situ reduction of ionic silver by the polymer solvent [4]. Another study showed that the fouling characteristics of polyethersulfone membranes could be improved by adding silica nanoparticles coated with polyvinylpyrrolidone (PVP) to the dope solution. This improved the surface coverage of PVP and, thus, the hydrophilicity of membranes leading to reversible membrane fouling in which the foulant was easily removed by simply washing with water [5]. On similar lines, Bae and Tak reported fouling resistant membranes by introducing titanium oxide nanoparticles at membrane surfaces [6]. Among the inorganic additives, zeolite nanoparticles are of particular interest because of their widespread applications in catalysis, adsorption [7] and membrane separations such as ultrafiltration [8], gas separation [9], pervaporation [10] and reverse osmosis [11]. Particularly, zeolite as an additive in polymer-based gas separation membranes has been widely studied [9,12] and has been utilized for improvement of O2/N2 selectivity due to its uniformly sized molecular pores [8,11]. Jeong et al. prepared thin film composites impregnated with zeolite nanoparticles using interfacial polymerization for reverse osmosis. The superhydrophilic and negatively charged three-dimensional pore network of zeolite formed the basis for enhancement of water permeability and selectivity through combination of steric and Donnan exclusion [11,13]. Han et al. showed that increase in concentration of NaA zeolite particles in poly(phthalazinone ether sulfone ketone) casting solution decreases water flux, while improving the separation performance. The polyethersulfone membranes showed increased water flux with increases in polyethylene glycol (PEG) content in dope solution, probably due to leaching of PEG and forming highly porous structure [14,15]. Ultrafiltration membranes widely used in water, food and pharmaceutical industries requires low fouling, high permeability and selectivity [16]. Ultrafiltration membranes consist of selective layer made up by nodular microstructure which are responsible for separation. Nodules are formed by nucleation and growth of polymer poor phase during phase inversion [17] and size ranges in few nanometer [18]. Thus, membrane flux and selectivity can be optimized by nodule size and its extent of packing which can be altered by means of nucleation and growth. The nucleation can be controlled using small quantities of nanoparticles as nucleating agents in the dope solution without changing the dope concentration much [19]. In present study, we have prepared polysulfone (Psf) based nanocomposite membranes using zeolite nanoparticles as additive and D-a-tocopheryl polyethylene glycol succinate (TPGS) as a compatibilizer. By using different additive concentrations, we have
assessed its effects on membrane morphology, properties and performance. Based on these, we have proposed a microstructural model to explain effects of additives on membrane pore characteristics. 2. Materials and methods 2.1. Preparation of Polysulfone/NZL hollow fiber membranes Polysulfone (UDEL™ P-3500 LCD MB7-BULK) was procured from M/s. Solvay Advanced Polymers, USA. Nanozeolite (LucidotÒ NZL 40) (particle size 40–60 nm) was procured from M/s. Clariant Produkte (Deutschland) GmbH, Germany. Polysulfone and nanozeolite were dried in vacuum oven for 1 day at 120 °C for removal of absorbed water. Zeolite nanoparticles of 0.01, 0.05, 0.1 and 1 wt% were suspended in N-methylpyrrolidone (NMP) using probe sonicator (Branson Sonifier 450, CT, USA) for 30 min. D-a-tocopheryl polyethylene glycol 1000 succinate (TPGS) was added to nanozeolite suspension to act as a primer, then polysulfone was added. HFMs were labeled as PT, PT-NZL-1, PT-NZL-2, PT-NZL-3 and PTNZL-4 corresponding to zeolite concentration 0, 0.01, 0.05, 0.1 and 1 wt% respectively as shown in Table 1. Membranes were prepared as described earlier [20] based on compositions listed in Table 1 and spinning parameters as in Table 2. For spinning, the dope solution was degassed prior to the start of spinning and the prepared fibers were kept in water for 1 day to remove the residual solvent before use in further studies. Dope solution viscosity was measured at 25 °C with a CC27/Q1 coaxial cylinder measuring system in an Anton Paar Physica MCR301 Rheometer. Shear rate was varied from 0.1 to 100 s1. 2.2. Nanoparticle characterization The crystalline structures and zeolite types were evaluated by powder X-ray diffraction (PANalytical X’Pert Pro X-ray Diffractometer) operated at 40 kV and 30 mA by using Cu Ka monochromatic Table 1 Dope solution compositions used for HFM preparation. Membrane type
Membrane composition (Psf/TPGS/solvent) wt%
PT PT-NZL-1 PT-NZL-2 PT-NZL-3 PT-NZL-4
25/1/74 25/1/74 25/1/74 25/1/74 25/1/74
NMP 0.01 wt% NZL suspension in NMP 0.05 wt% NZL suspension in NMP 0.1 wt% NZL suspension in NMP 1 wt% NZL suspension in NMP
Solvent
Table 2 Process parameters used for hollow fiber membrane preparation. Ambient temperature (°C) Relative humidity (%) Dope solution composition Bore solution composition Dope solution temperature (°C) Bore solution temperature (°C) Dope flow rate (ml/min) Bore flow rate (ml/min) Spinneret ID/OD (mm) Air gap (cm) Coagulation bath composition Rinse bath composition Coagulation bath temperature (°C) Rinse bath temperature (°C) Take-up drum velocity (m/min)
25 50–60 Psf/TPGS/zeolite Deionized water 25 25 2 2.5 0.8/1.4 45 RO water RO water 25 35 3.89
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radiation. The XRD patterns were recorded between 5° and 60° (2h) in step of 0.0167. Nitrogen adsorption–desorption measurements were carried out at 196 °C on a Micromeritics ASAP 2020 instrument (Micromeritics Instrument Corporation, GA, USA) to determine Langmuir surface area, micropore area and median pore width. Samples were evacuated overnight at 350 °C and 1 mm Hg before micropore analysis. The mean hydrodynamic diameter and zeta potential of zeolite nanoparticles were measured by dynamic light scattering (Malvern Instrument, UK). Morphological characterization of zeolite nanoparticles was carried out using Transmission Electron Microscopy (TEM, Philips/FEI Tecnai 12) and Field Emission Scanning Electron Microscopy (FE-SEM, JSM7600F, Jeol, Japan). 2.3. Characterization of nanocomposite hollow fiber 2.3.1. Hollow fiber morphology by SEM Morphology studies of HFMs were carried out using scanning electron microscope (SEM). Hollow fibers were fractured in liquid nitrogen, soaked overnight in isopropyl alcohol, followed by n-hexane and dried at room temperature. These HFMs were coated with gold/palladium using SC7640 Sputter Coater (Quorum Technologies Ltd., UK). Samples were observed under electron microscope at 10 kV. EDAX analysis for zeolite in HFM was carried out using JSM-7600F electron microscope. 2.3.2. Mechanical testing by UTM Mechanical properties of hollow fibers consisting Young’s modulus, tensile stress at break, and % breaking tensile elongation of single were measured with Material Testing Machines (H1KT UTM, Tinius Olsen, Inc., PA USA) using load cell of 50 N at room temperature. The HFMs of 50 mm gauge lengths with a constant elongation speed of 50 mm/min were used. The fiber mechanical properties such as tensile strength or stress at break and Young’s modulus were determined to gain indirect information about the molecular orientation of the hollow fibers produced. Tensile strength at break was calculated as the ratio of the breaking force divided to the cross sectional area of the fiber:
Tensile strength at break ¼
4F
pðD2o D2i Þ
! ð1Þ
where F is the load at break point, Do and Di are the outer and inner diameter of hollow fiber, respectively. The breaking tensile elongation was calculated as the ratio of the elongated length (DL) to the original length of the fiber (L0),
d¼
DL Lo
ð2Þ
2.3.3. Ultrafiltration experiment Cross flow filtration system was used for measurement of pure water permeability (PWP) as shown in Fig. 1. The prepared hollow fibers were kept in water bath for 24 h to remove residual solvent in fibers and then immersed in 50 wt% aqueous glycerine solution for 24 h in order to prevent damage due to the dried structure. Finally, they were dried in air at room temperature for making the test modules [21]. A typical hollow fiber bundle consisted of 16 fibers of 25 cm in length potted in nylon tube of 8 mm diameter with Araldite resin. The glued fiber modules were kept at room temperature for 12 h to cure the Araldite resin completely. After curing, both ends of module along with Teflon tube were cut in order to open the fibers. Residual glycerine was removed by immersing the modules in water for 24 h. For permeability measurements, the hollow fiber testing modules were fitted in water permeability setup in which deionized
Fig. 1. Schematic of pure water permeability setup.
water (Milli-Q system, 18 MX cm) was pumped at a constant flow rate of 100 mL/min through the lumen of the hollow fibers, using a QD2Q2CKC valve-less metering pump (Fluid Metering, Inc., USA) and the permeate was collected from shell side. Pressure difference across the membrane module was kept 50 kPa. A coiled Tygon tube was used between discharge point of pump and rotameter to nullify flow fluctuations. Pure water permeability (J) was calculated using following equation [1],
J¼
Q npDi LDP
ð3Þ
where Q is volumetric flow rate of permeate (mL/min), n is number of HFMs used, Di is inner diameter of hollow fiber (cm), L is length of hollow fiber (cm) and DP is trans-membrane pressure (bar). The PWP of each sample was calculated by averaging six readings while the PWP of a set was reported by calculating the average reading of three samples from same run. Hollow fiber testing modules and commercial HFM (Hemoflow F6, Fresenius Medical Care, Bad Homburg, Germany) were characterized for PWP. 2.3.4. Solute rejection A feed solution of mixed dextran fractions in deionized (DI) water was used for molecular weight cut off analysis on the PWP set up by replacing DI water with the dextran feed solution as per the compositions shown in Table 3. Dextran solution was pumped at a constant flow rate of 100 mL/min and pressure 50 kPa across the membrane module at 25 °C. The retentate and permeate were recycled through the module for 30 min, sampled and then analyzed using gel permeation chromatography (GPC – Waters 1525 binary pumps, Waters 2414 refractive index detector) using two PL aquagel-OH MIXED GPC columns (MIXED-H (8 lm, 300 7.5 mm)) with a guard column (M/s. Varian BV, Middleburg, Netherlands) in series. [22]. The GPC column was calibrated using standard dextran fractions ranging from 1–670 kDa (M/s. Pharmacosmos A/S, Holbaek, Denmark) to establish a calibration curve in terms of molecular weight (g/mol) vs retention time (min). Flow rate of mobile phase (DI water) was set at 1 mL/min. GPC chromatograph of dextrans in retentate and permeate was obtained in terms of RI (refractive index) signal vs retention time (min). The rejection curve was then obtained using procedure described earlier [23,24]. Solute rejection was calculated from the following equation,
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RðMWi Þ ¼
Dextran fraction
MW (kDa)
Suppliers
Concentration (g/L)
T1 T4 T10 T40 T70 T500 T2000
1 4 10 40 70 500 2000
Pharmacosmos(Holbaek,Denmark) Serva(Frankfurt,Germany) Pharmacosmos(Holbaek,Denmark) Pharmacosmos(Holbaek,Denmark) Pharmacosmos(Holbaek,Denmark) Pharmacosmos(Holbaek,Denmark) Pharmacosmos(Holbaek,Denmark)
0.74 1.22 0.54 0.74 0.34 0.27 3.65
SAp ðMWi Þ 1 SAr ðMWi Þ
ð4Þ
where SAp(MWi) and SAr(MWi) are the slice areas of permeate and retentate at molecular weight MWi, respectively. Hollow fiber testing modules and commercial HFM (Hemoflow F6, Fresenius Medical Care, Bad Homburg, Germany) were characterized for solute rejection. 3. Results and discussion 3.1. Characterization of nanozeolite Zeolite is known to exist in different types which can be distinguished by X-ray diffraction (XRD). The XRD pattern of the zeolite
shown in Fig. 2a exhibits that it is of the LTL type. Characteristic zeolite LTL diffraction pattern consisting of narrow characteristics Bragg peaks at 5.50°, 19.22°, 22.58°, 25.47°, 27.91°, 29.00°, and 30.61° 2h lines are evident. This pattern shows good matching with the JCPDS standard 39–224 (K6Na3Al9Si27O7221H20). The nitrogen adsorption isotherm of zeolite LTL is shown in Fig. 2b and represents a combination of type I and type IV isotherms, indicating micro and mesoporous solid structures, respectively [25]. Type I isotherm is observed due to the microporous structure of primary zeolite nanoparticles while type IV isotherm because of mesopores in between the nanoparticles or interparticle aggregates as reported in TEM and DLS study. The zeolite LTL possesses a Langmuir surface area of 599.70 m2/g, t-plot micropore area as 487.04 m2/g, t-plot external surface area as 112.66 m2/g and the median pore width as 0.668 nm (0.77 nm is ideal pore
Fig. 2. Zeolite nanoparticles characterization (a) X-ray diffraction pattern, (b) nitrogen adsorption–desorption isotherm, (c) HRSEM micrograph of zeolite nanoparticles and (d) TEM micrograph of zeolite nanoparticles showing agglomeration.
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The SEM micrograph of zeolite shows uniform primary particles of 20 nm width and 66 nm in length (Fig. 2c). The TEM micrograph of nanozeolite is shown in Fig. 2d and exhibits regularly shaped particles of 40–60 nm which agglomerate into aggregates of 150– 180 nm. DLS study revealed that mean hydrodynamic diameter of 191 nm with a narrow size distribution and supports the nanoparticles aggregate formation as observed by TEM. Zeta potential of suspended zeolite particle is 48.2 mV when immersed in deionised water of pH 6.9, indicating the stability of suspension. 3.2. Dope solution viscosity
Fig. 3. Viscosity shear rate plot of dope solutions with addition of zeolite nanoparticles.
diameter of LTL zeolite [26]). Since the microporous surface area is much higher than the mesoporous, it indicates a pre-dominance of microporous structure.
Dope solution viscosity is a vital parameter for optimization of spinning parameters since it directly affects the fiber-forming ability of the dope solution. The viscosity vs shear rate plot of dope solutions of Psf/TPGS with and without zeolite additives is shown in Fig. 3. Zeolite nanoparticles addition showed increase in the viscosity of Psf/TPGS solutions even at very low level of loading. However, this increase is not concentration-dependent and is maximum at a zeolite content of 0.05 wt%. This may be because of induced rigidity in Psf chains due to enhanced surface area resulting from better dispersion zeolite nanoparticles at 0.05 wt%. Reduction in viscosities of dope solutions for PT-NZL-3 and PT-NZL-4 observed (Fig. 3) because of the formation of zeolite agglomerates which reduce the effective surface area available
Fig. 4. SEM micrographs of cross section of whole HFM (a) PT, (b) PT-NZL-1, (c) PT-NZL-2, (d) PT-NZL-3 and (e) PT-NZL-4 [Inset: Micrographs of cross section of inner side of HFM showing nodular structure and nodules are showed by arrows].
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for interaction. An analogous viscosity trend has also been observed for zinc oxide nanoparticles suspension in polyacrylonitrile/N,N-dimethylacetamide solution earlier [27]. 3.3. Morphology study by SEM The SEM micrographs showing cross-sections of PT, PT-NZL-1, PT-NZL-2, PT-NZL-3 and PT-NZL-4 HFMs are shown in Fig. 4 and exhibit uniform wall thickness along the circumference with finger-like macrovoids. HFM dimensions in terms of inner diameter and wall thickness calculated from SEM micrographs are shown in Table 4. The inner diameters and wall thicknesses of HFMs increase slightly with zeolite loadings up to 0.05 wt% and reflect the gradual increase in viscosity of dope. Upon subsequent increases in of zeolite concentration, a decrease in fiber dimensions is observed which is not in correlation with the dope viscosity. However, inner diameter and wall thickness of PT-NZL-2 is observed higher as compared to other membrane types. This may be due to reduction in elongation in air gap because of highest viscosity of solution. Fig. 4 inset shows high magnification micrographs of cross-sections of the inner sides of HFMs. PT membranes (Fig. 4a inset) without additive show dense structure while a nodular structure is observed for nanocomposite membranes (Fig. 4b–e inset). These nodules are macromolecular aggregates of a few nanometers and their sizes increase with zeolite nanoparticle loading. Increase in nodule size with zeolite nanoparticle loading may be due to agglomeration of zeolite nanoparticles. Taurozzi et al. have observed aggregation of nanoparticle at membrane skin, when silver nanoparticles were externally added in dope solution [4]. Hence, zeolite nanoparticles may useful in constructing desirable membrane skin, which dictates selective separation. Presence of zeolite nanoparticles in membrane skin was confirmed by EDAX analysis using FESEM and discussed in the next section. The outer sides of these HFMs possess a porous structure and hence no skin formation.
Fig. 5. FESEM micrograph of cross section of inner side of hollow fiber.
Table 5 Elemental analysis of Psf/zeolite composite membrane showing embedding of polysulfone around the zeolite particles. Element
Weight %
Atomic %
C Al Si S Pd Au O
15.23 0.28 1.05 12.52 1.44 7.58 61.9
22.53 0.18 0.67 6.94 0.24 0.68 68.75
Totals
100
99.99
3.4. HRSEM study of nanocomposite membranes A high resolution SEM micrograph with elemental analysis of a representative PT-NZL-2 HFM is shown in Fig. 5 and exhibits agglomeration and fusing of nodules leading to a dense structure. The EDAX analysis clearly shows that zeolite particles are encapsulated by polysulfone (Table 5). The observed gold and palladium peaks arise due to the coating procedure which is performed to avoid charging. This behavior is observed for all polysulfone zeolite HFMs indicating that zeolite takes part in nucleation and growth of nodules leading to formation of a nanocomposite structure. Similarly, Cheng et al. have prepared mica-intercalated-Nylon 6 nanocomposite membranes by dispersing mica in dope solution of Nylon-6 and formic acid solution [28]. 3.5. Mechanical properties The influence of loading of zeolite nanoparticles on Young’s modulus, breaking strength and % breaking elongation are summa-
Table 4 Hollow fiber dimensions measured from electron micrograph. Membrane type
Inner diameter (lm)
Wall thickness (lm)
PT PT-NZL-1 PT-NZL-2 PT-NZL-3 PT-NZL-4
822.93 ± 7.201 825.47 ± 10.8 844.58 ± 12.61 759.23 ± 39.63 783.44 ± 12.61
124.20 ± 12.88 128.66 ± 3.29 132.48 ± 2.94 112.73 ± 5.64 121.65 ± 9.15
Fig. 6. Mechanical properties plot of nanocomposite Psf/zeolite HFMs.
rized in Table 6. Tensile modulus increases almost linearly with nanoparticle loading (Fig. 6) and no significant differences in yield stress and breaking strength are observed. However, the tensile modulus of pure Psf hollow fiber found higher than the zeolite nanocomposite HFMs indicating reduced interaction in the native membrane which may be due to the passive nature of zeolite additives. The linear increase in tensile modulus with the volume of packing materials in polymer matrix has been shown earlier and is a well known phenomenon. Ash et al. have reported an analogous trend of tensile modulus for PMMA/alumina nanocomposite [29]. The percentage elongation, however, decrease with increase in zeolite concentration which is attributable to reduction in ductility of the fibers and has been reported earlier for nanocomposites [30].
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Table 6 Mechanical properties of Psf/zeolite nanocomposites HFMs. Membrane type
Young’s/modulus (MPa)
Yield stress (MPa)
Breaking strength (MPa)
% Elongation
PT PT-NZL-1 PT-NZL-2 PT-NZL-3 PT-NZL-4
182.60 ± 8.64 126.73 ± 4.98 147.13 ± 3.20 148.27 ± 3.85 168.48 ± 3.22
6.58 ± 0.24 5.89 ± 0.28 6.01 ± 0.22 6.13 ± 0.02 6.37 ± 0.37
9.44 ± 0.17 8.15 ± 0.24 8.41 ± 0.50 8.46 ± 0.16 8.87 ± 0.49
80.08 ± 2.3 70.60 ± 4.25 68.90 ± 2.43 67.19 ± 0.44 63.92 ± 0.61
3.6. Pure water permeability Fig. 7 shows the water flux characteristics of nanocomposite membranes and commercial membrane (Hemoflow F6). The water flux increases with zeolite addition up to 0.05 wt% to 21.31 mL/ m2 h mm Hg, and subsequently decreases upon further addition of zeolite (1 wt%) to 11.79 mL/m2 h mm Hg. However, water flux of Hemoflow F6 is 4.76 mL/m2 h mm Hg, which is lower than native Psf and nanocomposite HFMs. The lower value of water flux of the Hemoflow F6 is not possible to explain, since it is a Psf membrane with undisclosed additives and unknown process spinning parameters. We observed in our DLS measurements of zeolite dispersion in NMP that zeolite suspends uniformly and has a smaller particle size (without agglomeration) at 0.05 wt% while particle size increases and agglomeration is observed at higher concentrations. Thus, zeolite particles should suspend in dope solution up to 0.05 wt% concentration while agglomeration is expected upon further increase in loading. Accordingly, we observed that the morphology of the nodules formed corresponds to the uniformity of the zeolite suspension. As membrane skin consists of packed nodular structure, the nodule size and packing dictate the final membrane performance. Thus, the pore numbers increase due to increase in nodule formation at 0.05 wt% loading which may be the reason for maximum water flux. However upon further increases in zeolite concentrations, the water flux decreases, due to decrease in pore number which occurs because of particle agglomeration leading to increased nodule size. 3.7. Solute rejection The rejection profiles of zeolite nanocomposite and commercial (Hemoflow F6) membranes are shown in Fig. 8. The rejection curves describe the type of pore size and their distribution with re-
Fig. 8. Solute rejection plot of nanocomposite Psf/zeolite and Hemoflow F6 HFMs measured by gel permeation chromatography.
spect to the nominal molecular weight cut-offs (NMWCO) and show that increases in zeolite concentration increase MWCO from 9500 Da to 54000 Da. This is because of the broadening of the pore size distributions above 0.05% zeolite concentration. By drawing inferences from water permeability and solute rejection studies, it can be implied that the nanocomposite membrane with 0.05 wt% zeolite shows increase in the number of pores with slight increase in pore size while for higher zeolite concentrations, the pore numbers decrease with significant increase in pore size. It has earlier observed that the nucleation and growth increases with increase in nanoparticle concentrations and primarily depends on uniformity of their suspensions [31]. Hemoflow F6 shows 4300 Da MWCO, which is much lower than nanocomposite membranes. This reveals that Hemoflow F6 possess smaller pore size and pore numbers than nanocomposite membranes. 3.8. Microstructural model proposed for zeolite nanoparticles in HFMs
Fig. 7. Bar graph showing pure water permeability of nanocomposite Psf/zeolite and Hemoflow F6 HFMs.
During HFM formation by phase inversion [32] of a polymer solution of Psf (Mw 79,000), the solvent (NMP) and non-solvent (water) molecules diffuse since both are highly miscible. This occurs at inner side of membranes, where membrane skin is formed. To modulate skin microstructure, zeolite nanoparticles (40– 60 nm), as additives, are suspended in Psf/NMP solution but their highly hydrophilic nature renders them incompatible with the hydrophobic Psf leading to the undesirable formation of zeolite aggregates in the polymer matrix [33]. To overcome this, TPGS (Mw 1513) is used as a compatibilizer since it contains both hydrophobic (tocopheryl) and hydrophilic (PEG) groups [34]. Zeolite has a microporous crystalline structure which aids in increased interaction with polysulfone/TPGS in the dope solution.
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Fig. 9. A proposed microstructural model: (a) Uniform sized and nanoscale dispersion of zeolite nanoparticles in dope solution lead to small size nodules formation and result in small pore size with high pore density; (b) an agglomerate of zeolite nanoparticles in dope solution lead to large size nodules formation and result in large pore size with reduced pore density.
The rapid solvent/non-solvent diffusion across the Psf solution film generates an incompatible environment with regard to Psf macromolecules. Due to the tendency to reduce this non-favorable interaction, the Psf macromolecules cluster around zeolite nanoparticles. These clusters are inter-connected together by entanglement due to shared Psf chains, and are also called as nodules as shown in the microstructural models in Fig. 9. The pore sizes in these composite membranes correspond to the interstices between nodules. We have shown in our current work that the pore size and pore number are the defining characteristics which dictate the flux and MWCO of an HFM, which can be controlled by the nature of the suspension of zeolite additive. In case of a uniform suspension (Fig. 9a), a large number of small and uniformly-sized pores are formed while for zeolite aggregates (Fig. 9b) large-sized nodules, and consequently, a small number of large-sized pores are formed. 4. Conclusions The membrane skin was found to consist of packed nodular structure formed by nucleation of polymer solution. Zeolite nanoparticles were used as additives to enhance the nucleation, thereby leading to control over skin microstructure by formation of Psf/
zeolite nanocomposite HFM. HRSEM and EDAX studies showed that zeolite nanoparticles participated and directed the nucleation process during phase separation. Consequently, the membrane skin consisted of nodules containing more zeolite nanoparticles. Increase in zeolite loading from 0.01 to 1 wt% resulted in initial increase in flux to 21.31 mL/m2 h mm Hg at 0.05 wt% loading and a subsequent decrease in flux to 11.79 mL/m2 h mm Hg at 1 wt%. The NMWCO increased from from 9500 Da to 54,000 Da. To explain these changes in performance of HFMs, we proposed a microstructural model based on the nature of zeolite suspension. The model postulated that the increase in flux of HFMs at initial loading may because of zeolite suspension at nanoscale leading enhancement in nucleation and nodule formation. Further decrease in flux could be due to reduction in pore number and increase in pore size because of agglomeration of zeolite at higher loading. It also explained the increase in pore size, which resulted in increase in NMWCO. Our studies on mechanical aspects showed that the addition of nanoparticles weaken the nanocomposite HFMs as compared pure Psf HFMs which may be due to weak interaction resulting from passive nature of zeolite and Psf. This study may be useful for tuning membrane properties using nanoparticles.
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