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Preparation, Morphology, Performance, and Hydrophilicity Studies of Poly(amide-imide) Incorporated Cellulose Acetate Ultrafiltration Membranes Sahadevan Rajesh,† Kavalapara H. Shobana,‡ Selvaraj Anitharaj,† and Doraiswamy R. Mohan*,† † ‡
Membrane Laboratory, Department of Chemical Engineering, Anna University Chennai, Chennai 600 025, India Department of Physics, Saveetha Engineering College, Thandalam, Chennai 602 105, India ABSTRACT: Fouling-resistant cellulose acetate (CA) membranes were prepared by the phase inversion technique using hydrophilic poly(amide-imide) (PAI) as the modification agent. The prepared membranes were characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), pure water flux, water content, porosity, and contact angle technique to investigate the influence of PAI on the final properties of the membranes. Intermolecular interactions between the components in blend membranes were established by ATR-FTIR, and semicrystalline nature was confirmed by XRD. SEM analysis showed that blend CA membranes have a thinner top layer and higher porosity in the sublayer. AFM surface roughness analysis data substantiate the enhanced surface porosity with an increase in PAI content, while the mean pore size decreases. The contact angle measurements indicated that the hydrophilicity of the CA membranes was improved by the addition of PAI due to the preferential orientation of functional groups towards the surface. Moreover, the surface free energy parameters of the membrane such as surface free energy, interfacial free energy, work of adhesion, and spreading coefficient were calculated. From the results, it was revealed that low interfacial free energy membranes prepared by the incorporation of PAI may be valuable in fouling-resistant industrial separations.
1. INTRODUCTION Membrane separation technology as a commercial separation process became practical after the introduction of the phase inversion technique for the preparation of synthetic membranes.1,2 During the past three decades, membranes attracted the attention of chemists, chemical engineers, and biotechnological engineers due to their unique separation principle, i.e, the selective transport and efficient separation in comparison with other unit operations. Separations using membranes do not require additives, and they can be performed isothermally at low temperatures with less energy consumption compared to other energy-intensive conventional processes. However, membrane fouling, which is caused by the deposition of particles on the membrane surfaces, results in a substantial decline in permeate flux with operation time and consequently limits its application. Current research and development efforts are directed towards drastic improvements in antifouling properties while maintaining the inherent high-throughput characteristics of the membranes.3 Ultrafiltration (UF) membranes, which have been largely developed and commercialized over the past three decades, are one of the promising technologies for separating extremely small suspended particles and dissolved macromolecules from fluids using asymmetric membranes.4 Cellulosic polymers, aromatic polyamides, and polysulfones are so far the most important membrane materials in ultrafiltration membrane technology today.5 However, the potentialities of these materials for making membranes are far more than what have already been realized in practice. Therefore it would be wise to give priority to these materials and their chemical modifications for creating new, r 2011 American Chemical Society
improved, and reliable membranes for a wide variety of industrial applications. Research and development using the above polymer materials is necessary for keeping them always in developing new membranes.6,7 Cellulose acetate (CA) is a potentially outstanding ultrafiltration membrane material, because of the advantages such as moderate flux, high salt rejection properties, relatively easy manufacture, cost effectiveness, and renewable source of raw material.8 The application of cellulose acetate in ultrafiltration application is limited because of the drawbacks such as a fairly narrow temperature range of usage (maximum 30 °C), a narrow pH range restricted to pH 28, poor chlorine resistance, greater compaction susceptibility, and high biodegradability, which reduces membrane lifetime and usage.911 Recently, aromatic polyimides have attracted interest as promising membrane materials because of their excellent chemical, mechanical, and thermal stabilities as well as good permselective properties.12,13 However, there are some restrictions on selecting suitable solvents in preparing asymmetric membranes via the traditional phase inversion technique, since the polyimide materials are normally very resistant to solvent dissolution.14 The processabilty and solubility of the polyimides can be improved by the inclusion of amide group in the polyimide backbone.15 Thus, aromatic poly(amide-imide) appears to be of particular interest as a membrane material in view of the fact that the aromatic imide units provide the high Received: September 24, 2010 Accepted: March 11, 2011 Revised: March 7, 2011 Published: March 25, 2011 5550
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Figure 1. Chemical structure of (Torlon 4000T-HV) poly(amideimide) units.
performance properties while the flexible amide linkages provide good processabilty.16 In general, the highly hydrophilic nature of cellulose materials slows the diffusion of the nonsolvent and delays coagulation during the phase inversion process, resulting in a denser skin layer and a lower flux.17 Common methods used for the modification of polymeric membranes such as surface modification, plasma treatment, and grafting are inappropriate for cellulose acetate due to its biological origin. On the other hand, blending of an appropriate polymer with cellulose acetate has been a versatile technique for modifying the CA membranes. Thus, in order to improve the permeability of CA membranes, it had been blended with several high performance polymers such as polysulfone, poly(ethersulfone), polyurethane, poly(ether ether ketone), and poly(ether imide) for improving the CA membrane properties, which was found to be successful.1720 However, an extensive literature survey revealed that there is no published document about the exploitation of poly(amideimide) in the modification of CA membranes. This is the first attempt in the literature that explores the usage of hydrophilic poly(amide-imide) in the modification of CA membranes for ultrafiltration applications. The objective of the present study was to develop low surface energy, highly permeable, and antifouling ultrafiltration membranes by incorporating poly(amide-imide) (PAI) into the casting solution of CA. It was expected that bringing together poly(amide-imide) (PAI) and cellulose acetate (CA) would conserve their superior properties in the final mixture while concurrently reducing their poor characteristics. Membranes were prepared by the phase inversion technique in different blend compositions of CA and PAI in the absence and presence of pore former polyethylene glycol 600 (PEG 600). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD) were used to characterize the intermolecular interactions between the components and crystalline structure of the membranes, respectively. The morphology of the membranes was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of polymer blend composition and additive on the membrane morphology, compaction, pure water flux, water content, porosity, and membrane hydrophilicity were investigated. An attempt has been made to correlate the changes in membrane morphology with the compaction, pure water flux, water content, and porosity of the blend membranes. Moreover, the changes in surface free energy and work of adhesion of the membranes were calculated by the Newman method and the results are discussed.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Commercial grade MYCEL cellulose acetate CDA5770 (glass transition temperature 214 °C and acetyl content 39.99%) was procured from Mysore Acetate and Chemicals Company Ltd., India, and was used after
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recrystallization from acetone. Commercial grade poly(amideimide), Torlon 4000T-HV (glass transition temperature 285 °C), supplied as gift sample by Solvay Advanced Polymers, LLC, Alpharetta, GA, USA, was used as received. The chemical structure of Torlon 4000T-HV unit is shown in Figure 1. AnalaR grade N-methyl-2-pyrrolidone (NMP) from SRL Chemicals Ltd., India, was sieved through molecular sieves (type 5 Å) to remove moisture and stored in dry condition prior to use. Acetone of analytical grade was purchased from SRL Chemicals Ltd., India, and used for the recrystallization of CA. Sodium lauryl sulfate (SLS) of AnalaR grade was obtained from Qualigens Fine Chemicals Ltd., India, and was used as a surfactant in the coagulation bath. Polyethylene glycol 600 (PEG 600) was procured from Merck (I) Ltd., and was used as supplied, as a nonsolvent additive for the whole study. Distilled water was employed for the ultrafiltration experiments and for the preparation of the gelation bath. 2.2. Preparation of Blend Membranes. The blend solutions based on CA and PAI (total polymer concentration = 17.5 wt %) were prepared by dissolving the two polymers in different compositions in the absence and presence of additive PEG 600 in NMP under constant mechanical stirring in a round-bottom flask for 4 h. A series of such polymer solutions were prepared by varying the composition of CA and PAI with PEG 600, as shown in Table 1. The homogeneous solution thus obtained was allowed to stand at room temperature for at least 6 h in an airtight condition to get rid of air bubbles.20 The preparation method involved is the same as that of the “phase inversion” method employed in our earlier works as reported by other researchers.21 The casting environment, namely, relative humidity (25 ( 2%) and temperature (30 ( 1 °C), were standardized and maintained for the preparation of membranes with better physical properties such as homogeneity, thickness, and morphology. The thickness of the membranes was maintained at 0.19 ( 0.20 mm and measured with a micrometer having a precision of 0.001 mm. The casting and gelation conditions were maintained constant throughout, since the thermodynamic conditions largely affect the morphology and performance of the resulting membranes.22 Prior to casting, a 2 L gelation bath, consisting of 2% (v/v) NMP (solvent) and 0.2 wt % surfactant, SLS in distilled water (nonsolvent), was prepared and kept at 20 ( 1 °C. The membranes were cast over a glass plate using a doctor blade. After casting, the solvent present in the cast film was allowed to evaporate for 30 s, and the cast film along with the glass plate was gently immersed in the gelation bath. After 2 h of gelation, the membranes were removed from the gelation bath and washed thoroughly with distilled water to remove all NMP and surfactant from the membranes. The membrane sheets were subsequently stored in distilled water, containing 0.1% formalin solution to prevent microbial growth. 2.3. Ultrafiltration Experimental Setup. The ultrafiltrationpermeation experiments were carried out in a batch-type, dead end cell (UF cell, Model 8400, Amicon, USA) with an internal diameter of 76 mm fitted with a Teflon-coated magnetic paddle. This cell was connected to a compressor with a pressure control valve and gauge through a feed reservoir. The schematic representation of the ultrafiltration experimental setup has given elsewhere.20,21 The effective membrane area available for ultrafiltration was 38.5 cm2. The solution filled in the cell was stirred at 300 rpm using a magnetic stirrer. All the experiments were carried out at 30 ( 2 °C and compaction at 414 kPa 5551
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Table 1. Composition and Casting Conditions of CA/PAI Blend Membranesa
follows:23 Jw ¼
blend composition cellulose acetate poly(amide-imide) additive, PEG 600 solvent, NMP (wt %)
(wt %)
(wt %)
(wt %)
100
0
0
82.5
90 80
10 20
0 0
82.5 82.5
70
30
0
82.5
100
0
2.5
80.0
90
10
2.5
80.0
80
20
2.5
80.0
70
30
2.5
80.0
Casting temperature = 30 ( 1° C; casting relative humidity = 25 ( 2%; solvent evaporation time = 30 s. Total polymer concentration at 17.5 wt %.
a
transmembrane pressure, with the pure water flux study at 345 kPa transmembrane pressure. 2.4. Membrane Characterization. 2.4.1. FTIR Analysis. FTIR spectra of pure CA and CA/PAI blend membranes were recorded using an attenuated total reflectance (ATR) technique with a spectrometer (Thermo Nicolet, Avatar 370) in the range 4000400 cm1. The IR spectrum for the pure PAI powder was obtained by the KBr pellet method. 2.4.2. X-ray Diffraction (XRD) Analysis. Wide angle X-ray diffraction (WAXD) patterns of the pure CA and CA/PAI blend membranes and PAI powder were recorded by a Bruker AXS D8 Advance X-ray diffractometer. The diffractograms were measured at diffraction angle 2θ in the range 550° using Cu KR radiation (λ = 1.5406 Å). A tube voltage of 40 kV and tube current of 30 mA were kept constant throughout the experiment. 2.4.3. Scanning Electron Microscopy (SEM) Analysis. The cross-sectional images of the CA and CA/PAI blend membranes were taken by a scanning electron microscope (SEM, Cam Scan MV2300). The membranes were cut into small pieces and cleaned with filter paper. The pieces were immersed in liquid nitrogen for 6090 s and were frozen. These frozen fragments of the membranes were stored in a desiccator. The dried samples were gold sputtered for producing electric conductivity, and photographs were taken in very high vacuum conditions operating at 15 kV. 2.4.4. Atomic Force Microscopy (AFM) Analysis. Atomic force microscopy was used to analyze the surface morphology and roughness of the prepared membranes (AFM device was Dual Scope scanning probe-optical microscope, DME Model C-21, Denmark). Small squares of the prepared membranes (approximately 1 cm2) were cut and glued onto a glass substrate. The membrane surfaces were imaged in a scan size of 1 μm 1 μm, and the surface roughness was measured by tapping mode, since contact mode AFM cannot be applied to polymeric membranes because of the excessive tracking forces applied to the sample by the probe. 2.4.5. Membrane Compaction. The hydraulic compaction of fresh membranes was carried out by loading the membranes of necessary size into the ultrafiltration test cell connected to the pressure reservoir, with water at a pressure of 414 kPa. The water flux was measured every hour until it leveled off after 45 h. The pure water flux (Jw) and compaction factor were determined as
Q AðΔtÞ
ð1Þ
where Jw is the water flux (in L m2 h1), Q is the quantity of permeate collected (in L), Δt is the sampling time (in h), and A is the membrane area (in m2). compaction factor ¼
steady state pure water flux initial pure water flux
ð2Þ
In order to understand the compaction effect, the thickness of the membrane before and after compaction study was measured with a micrometer sleeve having a precision of 0.001 mm. 2.4.6. Pure Water Flux. Membranes after compaction were subjected to a transmembrane pressure of 345 kPa to measure the pure water flux. The permeability was measured under steady state conditions, and the pure water flux was calculated by using eq 1. The pure water flux was measured three times and the average of it was reported for accuracy. 2.4.7. Equilibrium Water Content. Equilibrium water content of the membranes was obtained by soaking the membranes in water for 24 h and weighing them after mopping with blotting paper. These wet membranes were placed in a vacuum oven at 35 °C for 48 h, and the dry weights were determined. From these two values, the water content is derived as follows:24 m1 m2 water content ð%Þ ¼ 100 ð3Þ m1 where m1 is the weight of the wet membrane and m2 is the weight of the dry membrane. 2.4.8. Molecular Weight Cutoff, Mean Pore Radius, and Porosity. The molecular weight cutoff (MWCO), mean pore radius, and porosity of the pure CA and CA/PAI blend membranes were determined by ultrafiltration of polyethylene glycol (PEG) of different molecular weights. PEGs were used as the probe solutes since they are water-soluble and can be readily obtained commercially with narrow molar weight distributions. Solutions were prepared individually at a concentration of 500 mg/L using deionized water and used as standard solutions for the rejection studies. The UF cell was filled with PEG solution and pressurized at a constant pressure of 345 kPa throughout the experiments. During ultrafiltration, the permeate solutions of corresponding membranes were collected in a graduated tube and were analyzed for the solute concentration using a total organic carbon analyzer (Shimadzu, TOC-V CPH). A series of PEGs of different molecular weights such as 6, 8, 10, 12, 15, 20, 25, and 35 kDa were used for the estimation of MWCO and solute rejection studies. The percentage solute rejection (% SR) was calculated from the concentration of the feed (Cf) and the concentrate of the permeate (Cp) by the following equation:25 Cp SR ð%Þ ¼ 1 100 ð4Þ Cf The molecular weight has a linear relationship with the pore size of the membrane. In general, the MWCO of the membrane is determined by identifying an inert molecule of lowest molecular weight that has a solute rejection of 8090% in steady state UF experiments. The molecular weights (M) of the used PEGs were correlated with their StokesEinstein radii, and this enables the calculation of the mean pore radius 5552
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form γsl ¼ f ðγlv , γsv Þ
Figure 2. Schematic representation of contact angle measurement and various interactions during measurement.
of the membranes, r:25 r ¼ ð0:0262 nmÞ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M g=mol 0:03 nm
ð5Þ
where M is the PEG molecule of lowest molecular weight which has a rejection of more than 80% in the ultrafiltration experiments. The membrane porosity (ε) was determined by gravimetric method,26,27 defined as m1 m2 ð6Þ ε¼ Fw Al where m1 is the weight of the wet membrane, m2 is the weight of the dry membrane, Fw is the water density (0.998 g cm3), A is the effective area of the membrane (cm2), and l is the membrane thickness (cm). 2.4.9. Measurement of Contact Angle and Surface Free Energy. Contact angle measurements of water on the wet membrane surfaces were carried out by the sessile drop method at ambient temperature using a goniometer (GBX Instruments, Germany). Membrane samples of size of 3 3 cm2 were washed thoroughly with water and mopped with filter paper to remove the moisture content prior to the experiment. Then they were put on a sintered glass plate with the active layer up, to study the polar interactions between the membranewater interfaces. The sessile drop was formed on the membrane surface by depositing 5 μL of Milli-Q water slowly and steadily onto the membrane surface with a microsyringe. The contact angle was measured at membranewaterair interphase at room temperature within 30 s of the addition of water drop. For each sample, measurements were performed in six different locations and the average was considered for accuracy. 2.5. Theory and Calculation. The importance of contact angles is that they contain information about the surface tensions of the solid surfaces through Young’s equation:28 γlv cos θ ¼ γsv γsl
ð7Þ
where γlv is the liquidvapor, γsv is the solidvapor, and γsl is the solidliquid interfacial free energies, respectively, and θ is the measured angle with respect to the surface, as illustrated schematically in Figure 2. The calculation of interfacial free energy, γsl, from the contact angle of a liquid of surface tension γlv starts with Young’s equation, eq 7. Of the four quantities in Young’s equation, only γlv and θ are readily measurable. Thus, in order to determine γsl and γsv, further information is necessary. Conceptually, an obvious approach is to seek one more relation among the parameters of eq 7, such as an equation-of-state relation, of the
ð7aÞ
The simultaneous solution of eqs 7 and 7a would solve the problem. Neumann et al.28 modified Berthelot’s rule and put forward an equation-of-state relation of the form of eq 7a in order to calculate the surface free energy of the solid. rffiffiffiffiffiffi γsv βðγlv γsv Þ2 e cos θ ¼ 1 þ 2 ð7bÞ γlv This enables the calculation of solidvapor surface tension, γsv, from a single contact angle measurement with a liquid of known surface tension. According to Neumann such an equation of state in conjunction with Young’s equation is acceptable only when the results go with that obtained using another equation of state. An alternative formulation of Berthelot’s rule, put forward by Neumann, is rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi γsv ð1 β1 ðγlv γsv Þ2 Þ cos θ ¼ 1 þ 2 ð7cÞ γlv Equation 7c has been tested along with Young’s equation for the calculation of γsv and was found to be in good agreement with the results of eq 7b. This establishes the use of either of the proposed equations of state. Thus, the solidvapor surface tension can be determined when experimental contact angle (Young’s) and liquidvapor surface tensions are known. The constants β and β1 used in eqs 7b and 7c are based on a constant value of 0.000 102 4 mJ m2 and 0.000 105 7 mJ m2, respectively. Interfacial free energy, γsl, of the membranes is calculated from these γsv values by using Young’s equation: γsl ¼ γsv γlv cos θ
ð7dÞ
Thermodynamically, the work of adhesion, Wa, is the work required per unit area to separate the liquid from the membrane leaving an adsorbed film in equilibrium with the surface of membranes and is calculated by29 Wa ¼ ð1 þ cos θÞγlv
ð7eÞ
The spreading coefficient, Sc, is the work done in spreading the liquid over a unit area of surface.29 Sc ¼ γsv γsl γlv
ð7f Þ
3. RESULTS AND DISCUSSION 3.1. Fourier Transform Infrared Spectroscopy (FTIR) Analysis. FTIR spectra of the pure CA and CA/PAI blend membranes and PAI powder were recorded to investigate the intermolecular interactions between CA and PAI in the blend membranes. FTIR spectra obtained for all membranes and PAI powder are presented in Figure 3. The spectrum of pure CA membrane shows a broad band at 3485 cm1 due to the stretching frequency of OH group, and a sharp peak at 1752 cm1 ascertained to be the stretching frequency of —CdO group of CA. On the other hand, spectral bands of pure PAI provide a broad band at 3506 cm1 which corresponds to the stretching frequency of NH group in the amide linkage. Two other strong peaks with maxima at 1717 and 1652 cm1 are 5553
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Figure 3. ATR-FTIR spectra of CA/PAI blend membranes and pure PAI powder: (a) pure PAI; (b) 100/0; (c) 90/10; (d) 80/20; (e) 70/30.
Figure 4. Schematic representation of the intermolecular interactions between the components in CA/PAI blends. Changes in spectral peak positions are also indicated.
ascertained to belong to the stretching frequencies of —CdO group of the amide linkage and —CdO group in the fivemembered ring of PAI, respectively. However, in the CA/PAI blend membranes interesting interactions are revealed due to the shift in the stretching frequencies of the above functional groups to higher wavenumbers. The spectra of CA/PAI blend membranes indicated that the broad band at 3485 cm1 of pure CA is shifted to 3509 cm1. Further, the NH group stretching frequency at 3506 cm1 and the —CdO stretching frequency of at 1717 cm1 of pure PAI are shifted to 3590 and 1750 cm1, respectively, whereas the stretching frequency of —CdO group at 1652 cm1 of PAI and the —CdO stretching frequency of CA at 1752 cm1 are unaltered. This shift in the stretching frequencies of CA/PAI blend membranes confirms the establishment of hydrogen bonding between the amide group of PAI and the hydroxyl group of CA. It could be observed in the spectra of the CA/PAI blend membranes that, with the increase of PAI content, the band at 3485 cm1 became more pronounced with a shift to higher wavenumbers. The occurrence of such interactions is initiated by the transfer of lone pair of electrons present in the nitrogen of amide group. The possible interaction and the established hydrogen bond between CA and PAI in blend membranes are schematically represented in Figure 4. The presence of such —NdH—C—O 3 3 3 HO— interactions
Figure 5. XRD patterns of the CA/PAI blend membranes and pure PAI powder: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30; (e) pure PAI.
implied a good miscibility of CA and PAI in the blend membranes. The formation of this intermolecular hydrogen bonding favored the better compatibility and homogeneity at the molecular scale in blend membranes. This type of intermolecular interactions between a hydroxyl group of CA and a carbonyl group of polyvinylpyrrolidone has been reported in the literature.30 3.2. X-ray Diffraction (XRD) Analysis. The crystalline/amorphous ratio of the constituent polymer in a binary blend is of importance in understanding the development of membranes during gelation in the phase inversion process and permeability of the membranes in operation. Thus in order to study the crystalline structure, prepared membranes were analyzed using an X-ray diffractometer. The XRD patterns of pure CA and CA/ PAI blend membranes and pure PAI powder are shown in Figure 5. From the XRD spectra it was observed that the pure 5554
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Figure 6. Simple model indicating the formation of crystalline and amorphous fractions in a polymer.
CA membrane showed a broad semicrystalline peak at a diffraction angle (2θ) of 9.6° and three sharp crystalline peaks at diffraction angles of 18.9, 20.3, and 23.8°. The PAI powder exhibited a semicrystalline diffraction peak at 11.2°. The CA/PAI blend membranes displayed XRD patterns that integrated all the diffraction peaks of both CA and PAI. However, in contrast to the sharp diffraction peaks for the pure CA membrane, the intensity of peaks in the CA/PAI spectrum was reduced, indicating a decrease of crystallinity and a transition of the structure from crystalline to amorphous phase. This high crystalline nature of pure CA is attributed to the presence of acetyl groups and inside a highly acetylated membrane matrix the structural regularity would be relatively higher. In ultrafiltration, permeability depends upon the microcrystalline structure of membrane material and highly crystalline membranes are less permeable than amorphous ones.31 Usually a semicrystalline membrane may be regarded as a mixed matrix consisting of a crystalline region embedded in an amorphous matrix.32 Figure 6 shows a simple model indicating the formation of rigid amorphous phase in a polymer. Crystalline regions of the polymer chains are formed from polymer molecules which fold back on themselves in the shape of spirals, and they are too compact for material transport.33 However, the amorphous regions of polymer chains are loosely packed and material transport occurs in this region. Thus, in the case of pure CA membranes, increase in crystalline nature both decreases the volume of the amorphous material available for the material passage and increases the tortuosity of the path across the membrane. However, in CA/PAI blend membranes this tortuous path is decreased by the incorporation of PAI. This change in crystallinity with the addition of PAI corresponds to the improvement in morphology and performance of the CA/PAI blend membranes that will be discussed in subsequent sections. 3.3. Scanning Electron Microscopic Analysis. The surface and cross section of the thin film ultrafiltration membranes have a crucial role in identifying the role of the membrane in the mechanism of selectivity and permeability. The structure of the asymmetric or anisotropic membrane prepared by nonsolvent induced phase separation usually contains a relatively thin skin supported on a much thicker spongelike substructure. During operation, this top skin layer would offer high resistance to the material transport and the lower sponge-type structure would offer less resistance to the material transport besides providing mechanical strength to the membrane. Cross-sectional SEM images of the membranes prepared from 100/0, 90/10, 80/20, and 70/30 compositions of CA/PAI in the absence and presence of the pore former PEG 600 are shown in Figures 7 and 8, respectively. General structures of the membranes are found to be very similar consisting of a top skin layer, an intermediate layer with a spongelike substructure, and a
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bottom layer of fully developed open pores. However, the membranes prepared from CA without PAI exhibit a macrovoidal structure and the pores were not fully opened. During gelation, due to the strong interaction between NMP and water, NMP in the casting solution starts to desolvate rapidly into the coagulation bath.34 Consequently, molecules at the interface aggregate rapidly to form a dense skin without pores. As soon as the water is in contact with the casting solution, the affinity of water to CA leads to the formation of a relatively immobilized bound water layer at the polymernonsolvent interface. As a result, the nonsolvent inflow and solvent outflow from the polymer casting solution were restrained. In addition to this, the greater affinity of CA to water results in a longer time for the exchange between the nonsolvent in the bath and the solvent in the polymer casting film before gelation and vitrification. A highly open transition sublayer structure was difficult to obtain with pure cellulose acetate, because the diffusion process associated with the redissolution step was slowed down by the viscosity of the polymer solution and the immobilized water layer formed on the waterpolymer interface. The morphology of the blends of CA/PAI membranes at 90/10 and 80/20 compositions differ from that of pure CA membrane. Comparison between images indicates that incorporation of PAI in the casting solution produces higher porous membranes with spongelike structure in the sublayer. The changes in the morphology can be attributed to and explained by changes in the properties of the membrane polymer by addition of PAI. Experimentally, around 810 min was needed for the setting of the film made by pure CA, while CA/PAI blend membranes were set within 4 min. This longer settling time for the pure CA membrane was due to the “tortuous” path of the crystalline regions of the CA, and these regions were considerably decreased in blend films due to the presence of amorphous polymer PAI. The longer exchange time between the solvent and nonsolvent in the coagulation bath results in a more developed process of polymer-lean phase growth and coalescence. This results in the formation of macrovoids in pure cellulose acetate membranes. However, in the case of blends of CA/PAI, high diffusion rate and hydrophilicity result in the formation of tearlike structures with more spongelike areas in the resulting membranes. In contrast to other blend membranes, CA/PAI at 70/30 composition exhibits a fingerlike cavity in the sublayer and macrovoids in the bottom layer. Because of the larger driving force between solvent and nonsolvent, polymers clustered together to exclude solvent within their domains; small pores rapidly grow to form macrovoids. During the exclusion step, the droplets covered with polymer-lean regions were ruptured, which leads to the formation of open-type macropores. In addition to this, immiscible phase behavior of the blend components largely contributes to this macrovoidal structure, since the segmental gap between the components is high at the 70/30 blend composition. From Figure 8 it is also clear that, in the presence of PEG 600, at 70/30 composition macropores with a fingerlike structure were formed due to the leaching out of this nonsolvent additive during gelation. The formation mechanism of the fingerlike cavities had long been a controversial subject. Smolders et al.3537 had reported that, under the top layer of membrane, nuclei containing quite high solvent concentration could be created. As long as the solution remained stable, no new nuclei deeper in the membrane would be formed and the nucleus then opened to form a fingerlike cavity. Nonsolvent diffusing into the 5555
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Figure 7. SEM cross-sectional images of CA/PAI blend membranes (w/w) without additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.
Figure 8. SEM cross-sectional images of CA/PAI blend membranes (w/w) with 2.5 wt % additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.
membrane sublayer would almost contribute to the growth of the new nucleus as far as the polymer-rich phase reached the solidification concentration, and the resulted membrane structure was then fixed.38 The creation and growth of new nuclei
induced by this mutual diffusion competed with each other, since the growth of each nucleus consumes solvent. Thus during gelation the growth of every nucleus was limited by the other nearby nuclei. In the case of membranes with additive, 5556
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Figure 9. AFM three-dimensional images of CA/PAI membranes (w/w) without additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.
Figure 10. AFM three-dimensional images of CA/PAI blend membranes (w/w) with 2.5 wt % additive: (a) 100/0; (b) 90/10; (c) 80/20; (d) 70/30.
aggregation of polymer is relatively quicker, which restricts the creation of new nuclei beneath the skin layer and the existing nucleus then grows to form a fingerlike cavity. 3.4. Atomic Force Microscopic Analysis. AFM is considered to be more reliable in the characterization of hydrophilic/hydrophobic properties of the membranes, in particular surface roughness
of the membranes, which plays a major role in the determination of the value of fouling. Hence, in order to study the influence of PAI on the final blend membrane morphology and surface roughness, AFM analysis was carried out by tapping mode. Figures 9 and 10 indicate the three-dimensional AFM images of surfaces of pure CA and CA/PAI blend membranes in the 5557
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Table 2. Surface Roughness Parameters of CA/PAI Blend Membranesa roughness parameters
a
CA/PAI composition (wt %)
Ra (nm)
Rq (nm)
Rz (nm)
100/0
6.38 (0.58)
5.03 (0.31)
31.38 (1.73)
90/10
5.11 (0.36)
3.62 (0.11)
23.47 (2.23)
80/20 70/30
3.26 (0.53) 2.27 (0.27)
2.32 (0.23) 1.59 (0.29)
15.57 (1.29) 9.92 (0.91)
100/0
7.97 (0.82)
6.43 (0.16)
43.38 (1.94)
90/10
6.43 (0.41)
5.31 (0.24)
36.47 (2.49)
80/20
4.72 (0.27)
3.84 (0.37)
27.09 (0.64)
70/30
5.38 (0.59)
4.02 (0.11)
18.11 (1.31)
Values within the parentheses are standard deviations.
absence and presence of PEG 600, respectively. All membranes were scanned at a same scan size of 1 μm, since the scanned area is crucial in comparing the surface roughness of the membranes. As the scan area increases, the surface roughness increases due to the dependency of roughness on the spatial wavelength of the scanned area or the frequency.39 In these images, the brightest area represents the highest points or nodules of the membrane surface and dark regions indicate the valleys or membrane pores. From these images, it was clear that the surface properties of the CA membrane were improved considerably by means of blending it with PAI. It seems that, in the case of pure CA membrane, the surface contains a large number of “nodules” and as the concentration of PAI increases the “nodules” almost disappear at 80/20 blend composition. This is due to the high viscosity of the CA casting solution and its highly crystalline structure, which restricts the “valley” formation in pure CA membrane. These high peaks or nodules are almost completely absent or decreased to a significant level at 80/20 blend composition due to the presence of amorphous PAI. However, in the case of CA/PAI blend at 70/30 composition, the “nodules” reappear once again and are larger in size. This is due to the segmental gap formed between the polymer components in the blend system because of its incompatibility. The results obtained from this AFM analysis are in good agreement with SEM results. The pores beneath the skin layer are larger in size for pure CA membrane, while in the case of blends up to 80/20 concentrations pores are small beneath the skin and are equally distributed. Differences in surface morphology of the CA/PAI blend membranes, which was measured and expressed in terms of surface roughness, are tabulated in Table 2. Three important components of the surface roughness parameters were calculated: the mean roughness (Ra, the mean value of the surface relative to the center plane), the root mean square of the Z data (Rq), and the mean difference between the highest peaks and lowest valleys (Rz). From the results it was clear that the surface roughness of the membranes decreased with an increase in concentration of PAI in the casting solution. The decrease in surface roughness means that depressions and peaks become smaller, which in turn indicates a decrease in mean pore size. Similar trends for the surface roughness and molecular weight cutoff were observed in the literature.40,41 Bessieres et al.42 and Idris et al.43 reported that change in the surface roughness is proportional to the change in the pore size. The results obtained from the analysis of MWCO values of CA/PAI membranes are in accordance with this surface roughness. This may be explained by the fact that pure CA membrane with higher MWCO
has less tightly packed polymer aggregates in the skin layer, which in turn contribute to the increase in the surface roughness. The decrease in surface roughness in blend membranes could be understood on the basis of the reduction of the pore size due the migration of less viscous PAI toward the surface.44 This change is expected, because the roughness parameters depend on the Z value, which is the vertical distance that the piezoelectric scanner moves. When the surface consists of deep depressions (pores) and high peaks (nodules), the tip moves up and down over a wide range and the result should be a high roughness parameter. Elimelech and co-workers45 correlated the surface roughness of the membranes with colloidal fouling, and their experiments showed that as the roughness increases colloidal fouling increases. This was explained by the fact that, in the initial stages of fouling, the colloidal particles preferentially accumulated in the “valleys” of rough membranes, resulting in “valley clogging”, which in turn decreases the permeate flux. 3.5. Effect of PAI Composition on the Compaction of CA/PAI Blend Membranes. The compaction study was aimed at making the pores of the membranes uniform and rigid and to get steady state flux. Precompression of the membranes at a pressure higher than the operating pressure permits stress relaxation during operation. During compaction, any trace amount of additive or surfactant present inside the pores would also be eliminated. The results obtained for the compaction studies are shown in Table 3. From Table 3 it is observed that pure CA membrane without additives showed a steady state pure water flux of 109.1 L m2 h1 and the compaction factor was found to be 0.57. In the case of blend membranes the steady state pure water flux and compaction factor were increased considerably. Irrespective of the amount of CA and PAI content, all the membranes were compacted and steady state water flux was attained within 45 h of operation. When membranes operated under high external pressure, the thickness and pore size of membranes were decreased by the reorganization of polymeric chains, which lowers the porosity. This leads to an increase in the hydraulic resistance offered by the membrane as a result of the dense structure of the membrane and, consequently, lowers its flux. In compaction, due to an increased pressure drop with distance from the membrane surface, the load on the material is gradually increased with the distance from the surface.46 Thus the ultimate end of the material (the membrane downside) was exposed to the highest compaction pressure. Since most of the membrane space was found beneath the thin separating layer, the blend membrane with sponge-type structure was more exposed to structural disorder than pure CA membrane with continuous macrovoidal structure. Previous observations in the literature47 say that the separation properties of the membrane remain unchanged, which means that the separating layer was unchanged, but the total flux/ permeability was reduced, which indicates that the overall porosity was reduced. Thus it is important to note that distilled water did not cause any flux decline and observed flux reduction was due to the change in the thickness of the membranes by compaction and reorganization of the polymer chains. The comparatively higher water flux in membranes with higher PAI content was attributed to the presence of polar amide and imide linkages on the surface of the blend membranes, which in turn increased the hydrophilicity. The membranes with additive PEG 600 follow the same trend as that of membranes without additive. From the compaction studies, we can conclude that the CA/ PAI blend membranes were more susceptible to compaction than pure CA membranes. This was owing to the increase in the 5558
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Table 3. Initial Pure Water Flux, Final Pure Water Flux, and Compaction Factor of CA/PAI Blend Membranes pure water flux at 414 kPa (L m2 h1)
polymer blend composition (17.5 wt %) CA (wt %)
PAI (wt %)
additive, PEG 600 (wt %)
final
initial
compaction factor
membrane thickness (μm) before compaction after compaction
100
0
0
190.1
109.1
0.57
195
176
90
10
0
236.9
144.9
0.61
195
170
80
20
0
285.2
187.0
0.66
198
170
70
30
0
327.3
205.7
0.62
210
181
100
0
2.5
268.1
159.0
0.59
190
175
90
10
2.5
346.0
221.3
0.63
194
177
80
20
2.5
475.3
310.1
0.65
195
175
70
30
2.5
581.3
378.7
0.65
207
180
3.6. Effect of PAI Composition on the Pure Water Flux of CA/PAI Blend Membranes. After initial compaction of the
Figure 11. Effect of PAI Concentration on pure water flux and water content of CA/PAI blend membranes: —, pure water flux; ---, water content; 4, without additive; , with 2.5% additive.
porosity and change in the crystalline/amorphous ratio of the CA membranes by blending with PAI. This increase in porosity with the increase in concentration of PAI is in agreement with the AFM analysis presented earlier. In AFM data, the surface roughness decreased with the addition of PAI in the casting solution because of the decrease in mean pore size. In addition to its effects on material transport phenomena, the crystalline/amorphous ratio affects the various chemical accessibility and mechanical property parameters, which in turn will influence the membrane functional behavior in a time-dependent manner (compaction). Crystalline regions extend not more than a few hundred angstrom units, which is less than the completely extended length of the polymer chain; therefore, crystallites contain only a section of polymeric molecule. Semicrystalline structure thus consists of a composite single phase in which individual polymer chains participate in both crystalline and amorphous regions.48,49 Because of their rigidity, crystallites inhibit the compaction of membrane structure and the intervening amorphous region is subject to deformation. When PAI was introduced into the casting solution, due to its amorphous nature, the crystalline nature of CA was reduced accordingly, which in turn reduced its rigidity. Thus CA/PAI blend membranes were more susceptible to compaction than pure CA membranes.
membranes, the pure water flux was measured at a transmembrane pressure of 345 kPa. The pure water fluxes of all the CA and CA/PAI blend membranes were measured after an initial stabilization period of 2030 min, and the results are presented in Figure 11. From this it was clear that the pure water flux of CA/PAI blend membranes is higher than that of pure CA membranes. The pure water flux of membranes increased from 80.7 to 167.3 L m2 h1 with changes in the casting solution composition from 100/0 to 70/30 without additive. The membrane prepared in the presence of additive PEG 600 yielded enhanced flux values varying from 119.4 to 313.6 L m2 h1. This trend indicates the leachability of water-soluble additive in the gelation process, which leads to the formation of larger and higher numbers of pores in the membranes. The increase in flux is not only due to the hydrophilicity but is also attributed to the increasing immiscible phase behavior of the blend, because of the low molecular attractive forces between the blend components. This effect was more prominent in the presence of higher PAI content and led to increased flux due to the segmental gap formed between the polymer chains. The addition of PAI in the CA membrane preparation solution has brought three advantages to the final membrane structure: (i) a change in the skin layer of the membranes with increased porosity, (ii) an improvement in hydrophilicity of the CA membrane, and (iii) a change in the crystalline nature of the CA membrane. The porosity of the membranes increased with the addition of PAI as evidenced by the surface roughness analysis data. The results from the contact angle analysis (Table 5) show that the hydrophilicity of all the membranes prepared from different compositions of CA/PAI blend are higher than that of the membrane from CA without addition of PAI. In SEM analysis also, the more spongelike structure was formed and porosity was increased with the increment in the concentration of PAI. In accordance with these results, the pure water flux values from different compositions of CA and PAI were higher than that of the membrane without PAI. From this it is clear that the pore size did not have any profound effect on the membrane permeability. As explained by the SEM analysis, pure cellulose acetate membrane had low water flux due to the “tortuous” path of the crystalline regions of the cellulose acetate and this region was considerably decreased in blend films due to the presence of PAI. From the pure water flux studies it was clear that not only the porosity influences the water flux but also the hydrophilicity and properties of the constituent polymer in the membrane play a critical role. 5559
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Figure 12. Schematic representation of possible H-bond formation between CA, PAI, and water in CA/PAI blend membranes: (a) water approaches membranes; (b) tendency for H-bond formation; (c) destructuring of water clusters; (d) transport through the membranes.
In pure water permeation, water itself exists in the form of hydrogen bonded clusters of approximately 100 molecules with four to six nearest neighbors.49 The transport of water molecules through the membrane depends on the physical and chemical nature of the membrane barrier.50 In the case of the CA/PAI system, polar groups (amide and imide) may effectively compete with the tendency of water molecules to associate with others of their kind, thereby causing a destructuring of the original water complexes and facilitating their transport through the membrane. However, in the case of pure CA membranes, the repulsion between the bulky acetate groups considerably reduces the competency and destructuring of the water molecules, which results in lower water flux. In addition, the presence of intermolecular hydrogen bonding increases the internal energy of intermolecular motion and it restricts the free movement of the parent chain in pure CA membranes.51 However, compared to CA, PAI is more flexible due to the low steric effect, which in turn increases pure water flux. Schematic representation of possible hydrogen bond formation between CA, PAI, and water is shown in Figure 12. 3.7. Effect of PAI Composition on the Water Content of CA/PAI Blend Membranes. Water content of the membranes is an indirect indication of the hydrophilicity and flux behavior of membranes. The pure cellulose acetate membrane, in the absence of PEG 600, was found to have a water content of 58.21%, as shown in Figure 11. In the CA/PAI polymer blend, as the PAI content was increased, the water content improved considerably and at 30% poly(amide-imide) the water content was found to be 60.87% as indicated in Figure 11. As the PAI concentration increases, the immiscible nature of blend increases due to low adhesion properties between cellulose acetate and poly(amideimide) chains. Further, this leads to the creation of void volume in the membrane because of the increase in the number of
supermolecular aggregates in the casting solution, which results in the formation of a larger number of pores. This in turn increases the water intake of the pores and enhanced water content at higher PAI composition in the blend. However, the difference in water content between the pure CA and CA/PAI blend membranes is very small, due to the peculiar affinity of CA to water (even dried under P2O5, up to 1% is retained). The addition of PEG 600 to the casting solution of pure CA enhanced the water content of the membranes, and at 70/30 composition the membrane was found to have a water content of 64.17%. It is also evident from Figure 11 that in all the blend membranes the introduction of PEG increased the water content. This increase in water content, irrespective of the polymer blend composition, may be attributed to the addition of PEG 600 to the casting solution, which gets leached out upon gelation leading to pore formation, which becomes the domain of water molecules. The above reason confirms the PEG 600 activity in the formation of membranes. Water exists as a hydrogen bonded network structure because of its self-association, and therefore the water content of the polymeric membranes depends on the hydrophobic/hydrophilic balance of the polymer surface and intrinsic heterogeneity of the active layer of the membranes. Porosity is another important parameter influencing the water content, since the interaction of a water molecule with the surface functional groups can be strong or weak depending on the microstructure of the pores.52 In the literature Pinho et al.53 indicated that the hydrogen bonding strength of the water molecules in the active layer and possibly the average cluster size increased from the active layer to the downstream side of the asymmetric membranes. The size and stability of such clusters are presumably due to hydrogen bonding interactions between water molecules and the hydrophilic polar groups on the surface of the pores, and therefore a change in pore size should be reflected by the change in water 5560
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Table 4. Molecular Weight Cutoff (MWCO), Mean Pore Radius, and Porosity of CA/PAI Blend Membranes polymer blend composition (17.5 wt %) CA (wt %)
PAI (wt %)
additive, PEG 600 (wt %)
MWCO (kDa)
mean pore size (Å)
porosity (%)
100
0
0
25
41.13
37.34
90
10
0
15
31.79
39.75
80
20
0
12
28.40
43.18
70
30
0
12
28.40
42.96
100
0
2.5
35
48.72
45.13
90
10
2.5
20
36.75
48.48
80
20
2.5
12
28.40
50.90
70
30
2.5
10
25.90
50.37
content. The water content of the CA membranes was increased with an enhancement in PAI content, due to the preferential orientation of the polar groups toward water during the membrane formation process which leads to the enrichment of the surface with organic functional groups. From the water content values it was concluded that CA membranes adsorbed relatively small amounts of weakly hydrogen bonded clusters compared to the larger amounts of strongly hydrogen bonded clusters in the CA/PAI blend membranes. As the active layer of the blend membranes were more permeable, the water could enter the polymer structure more easily and form large bulk water clusters relative to the CA membrane, which has a low porosity. The amount of water present in the membrane depends on the heterogeneous morphology of the membranes, and the results obtained are in agreement with the SEM analysis data. Thus the introduced polar sites on the surface of the CA membranes by the incorporation of PAI support the hydrogen bonding interactions with water and lead to the formation of bigger water clusters in blend membranes. 3.8. Effect of PAI Composition on the Molecular Weight Cutoff, Mean Pore Radius, and Porosity. The MWCO, mean pore size, and porosity of the pure CA and CA/PAI blend membranes, in the presence and absence of the additive PEG 600, were determined, and the results are presented in Table 4. The analysis of the results showed that enhancement of PAI composition in the blend system from 10 to 30% yielded changes in pore statistics. Pure CA membranes exhibited a molecular weight cutoff of 35 kDa, and it was decreased to 12 kDa at 80/20 blend composition without additive. From the observed results it was clear that an increase in PAI composition in the blend membranes decreases the molecular weight cutoff of the membranes. The mean pore radius of the membranes decreased to 28.40 Å when the PAI composition reached 30%. It is evident from these results that the pure CA membrane prepared in the absence of additive has a relatively smaller porosity as discussed in compaction studies. This is because of the slow diffusion of the N-methylpyrrolidone and water during the phase inversion process of CA membrane preparation, resulting in a much denser skin layer and a lower porosity in the sublayer. This is in accordance with compaction study data, where the blend membranes were susceptible to compaction because of their high porosity in the sublayers of the membranes. The increase in the porosity with increasing PAI composition might have been due to the availability of more PAI for the formation of small networks with CA. The addition of water-soluble additive PEG 600 to the casting solution changed the pore sizes and porosity of the resulting
membranes. It is shown in Table 4 that, for a given blend composition, with addition of PEG 600, the porosity percentage increased proportionately compared to membranes without additive. This substantiates the role of the additive in the formation of pores and its interaction with the nonsolvent during gelation and membrane formation. Arthanareeswaran et al.16,54 prepared CA ultrafiltration membranes with polyethylene glycol 600 (PEG 600) as additive, and they observed that amphiphilic polymer can influence the penetration rate of the coagulation solution when the membrane is prepared through phase inversion. 3.9. Effect of PAI Composition on the Contact Angle and Surface Free Energy. The contact angle measurements of the prepared membranes were carried out to assess the effect of polarity of the monolayer surface functional groups (surface chemistry) on hydrophilicity. The interaction of liquid with membrane is referred to as wettability. When the interaction of liquid with the membrane is strong, liquid spreads spontaneously out as a thin film across the membrane and it is said to wet the surface. When the interactions are weak, liquid beads up on the surface and the liquid partially wets the surface. Measurements of the wettability of membrane, expressed by the contact angle, permit evaluation and comparison of the surface energy parameters of the membranes. The effect of PAI concentration on the hydrophilic properties of the blend membranes was studied by measuring the contact angles of water on the membranes of various blend compositions, and the results are tabulated in Table 5. As shown in Table 5, the contact angles of the blend membranes are smaller than those of the pure CA membranes, which indicates that the addition of PAI can be a useful way to improve membrane hydrophilicity. In general, contact angle measurement at thermodynamic equilibrium on a polymeric membrane measures its wettability and it is influenced by the chemical composition of the surface of the membrane, the membrane porosity, and its roughness. In the case of CA/PAI blend membranes, the presence of polar amide and imide functional groups effectively compete with water by hydrogen bonding and van der Waals interactions and lead to lower contact angles as explained in the pure water flux studies. The effectiveness of individual atoms and substituent groups in increasing the wettability of polymeric membranes is in the order N > O > I > Br > Cl > H > F.55 In pure CA membranes, the acetyl content (CH3CO) increases the hyhrophobicity due to the steric hindrance of these groups. Since nitrogen is completely absent in the pure cellulose acetate membranes, the trend shown in this case is satisfactory. In the case of CA/PAI membranes with additive PEG 600, the contact angle values were decreased 5561
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Table 5. Surface Free Energy, Interfacial Free Energy, Work of Adhesion, and Spreading Coefficient Values Calculated from Contact Angle Values surface free energy,a γsv (mJ/m2) contact angle (θ)
eq 7b,b
eq 7c
interfacial free energy, γsl (mJ/m2)
work of adhesion, Wa (mJ/m2)
spreading coefficient, Sc (mJ/m2)
100/0
38.9
59.68 (0.63)
59.70 (0.45)
3.02
57.66
16.14
90/10
34.4
62.11 (0.62)
62.11 (0.49)
2.04
61.07
12.73
80/20
31.1
63.80 (0.73)
63.80 (0.51)
1.46
63.34
10.46
70/30
28.4
65.11 (0.65)
65.11 (0.51)
1.07
65.04
8.76
100/0
33.38
62.63 (0.53)
62.64 (0.49)
1.84
61.79
12.01
90/10 80/20
30.6 25.3
64.04 (0.55) 66.54 (0.53)
64.05 (0.62) 66.54 (0.54)
1.37 0.72
63.66 66.82
10.14 6.98
70/30
22.7
67.65 (0.73)
67.65 (0.45)
0.49
68.16
5.64
CA/PAI composition (wt %)
γsl 72.8 mJ/m2. Numbers in the parentheses are standard deviations. b γsl, Wa, and Sc values are calculated based on the surface free energy (γsv) values obtained from eq 7b, since both eqs 7b and 7c have given almost the same values for γsv. a
compared to the membranes without additive. In this case, in addition to the surface functional groups, the surface porosity has an intense effect on the contact angle, since the water drop could penetrate into the pores gradually due to the capillary force, which decreases the contact angle compared to membrane without additive. From the measured contact angle values, various surface parameters, such as surface free energy, interfacial free energy, work of adhesion, and spreading coefficient were calculated, and the results are tabulated in Table 5. For pure CA membranes, the surface free energy, γsv, value was 59.68 mJ/m2 and it increased to 65.11 mJ/m2 for the blend membranes at 70/30 composition without additive. From the results it was clear that surface free energy increases with the increase in composition of PAI (lower contact angle) in the CA/PAI blend membranes. However, in the case of interfacial free energy, γsl shows an opposite trend to that of surface free energy, which decreases with increase in PAI composition. Interfacial free energy, γsl, for pure CA membranes was 3.02 mJ/m2 and it decreased to 1.07 mJ/m2 for the blend membranes at 70/30 composition without additive. In the polymeric membrane blend system, the surface composition may differ greatly from that in the bulk since the components of lower surface free energy always tend to enrich the surface in order to minimize the free energy of the system.56,57 A lowered equilibrium interfacial energy surface that results from the placement of lower surface energy component is achieved at the cost of maintaining a gradient between the surface and bulk composition. In most membrane processes, especially in bioseparations, lower interfacial free energy surface corresponds to lower fouling. Thus the development of such a low energy, low adhesive surface is accepted as the most promising alternative for the control of fouling.57,58 The work of adhesion, Wa, values calculated follow the same trend as that of surface free energy; that is, the Wa values increase with increase in PAI composition. Thus for pure CA membrane the Wa value is 57.65 mJ/m2 and the for the blend membranes without additive it has gone up to 65.03 mJ/m2. Adhesion on the low energy surfaces is very strong, whereas that on surfaces possessing a high polar contribution to their surface energy is very low.59 Thus, it can be concluded that the polar group introduced on the surface of the CA membrane by blending with PAI was good enough to modify its adhesive properties. The spreading coefficient values obtained reveal that, as the PAI composition increases in the blend system, the spreading
coefficient values become less negative, which shows an increase in wettability.60
4. CONCLUSION In the present investigation, high performance CA ultrafiltration membranes were prepared by the phase inversion technique using hydrophilic PAI as the modification agent and polyethylene glycol 600 as pore former. The effects of blend ratio on the morphology, permeation properties, porosity, and hydrophilicity of the resultant membranes were evaluated. ATR-FTIR measurements revealed the presence of hydrogen bonding interaction between the hydroxyl group of CA and carbonyl group of PAI in blend membranes. Morphological analysis of the blend membranes revealed that, as the weight percentage of PAI in the CA matrix increased, defect-free thin layer and spongy sublayer were formed. AFM studies revealed that the surface properties and porosity of the membranes were improved considerably by the addition of PAI to the casting solution. It was obvious that the less viscous component (PAI) forms smaller dispersed phase in the more viscous matrix (CA) due to comparatively restricted diffusion and increased shear stress resulting from the more viscous matrix phase. The pure water flux and water content of the CA membranes were increased with an enhancement in PAI content, due to the preferential orientation of the polar groups toward water during the membrane formation process which leads to the enrichment of surface with organic functional groups. Porosity of the CA/PAI blend membranes increased and mean pore radii decreased compared to those of the pure CA membranes. Surface energy parameters calculated from the contact angle measurements indicate that interfacial free energy of the blend membranes decreased compared to those of CA membranes. Overall results suggest that membrane morphology, pure water fluxes, water content, porosity, and hydrophilicity of the prepared CA/PAI blend membranes improved significantly by the incorporation of PAI. Therefore, PAI should be considered as an effective modification agent for the development of low energy, antifouling CA ultrafiltration membranes for various industrial separations. ’ AUTHOR INFORMATION Corresponding Author
* E-mail:
[email protected]. Tel.: 044-22359136. Fax: 044-22350299. 5562
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’ ACKNOWLEDGMENT The authors gratefully acknowledge Solvay Advanced Polymers, Alpharetta, GA, USA, for providing poly(amide-imide) (Torlon 4000T-HV). The authors also thank University Grants Commission (UGC), New Delhi, India, for financial assistance. ’ REFERENCES (1) Mohammadi, T.; Saljoughi, E. Effect of production conditions on morphology and permeability of asymmetric cellulose acetate membranes. Desalination 2009, 243, 1. (2) Idris, A.; Zain, N. M.; Noordin, M. Y. Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives. Desalination 2007, 207, 324. (3) Saxena, A.; Tripathi, B. P.; Kumar, M.; Shahi, V. K. Membranebased techniques for the separation and purification of proteins: An overview. Adv. Colloid Interface Sci. 2009, 145, 1. (4) Nagendran, A.; Vijayalakshmi, A.; Lawrence Arockiasamy, D.; Shobana, K. H.; Mohan, D. Toxic metal ion separation by cellulose acetate/sulfonated poly (ether imide) blend membranes: Effect of polymer composition and additive. J. Hazard. Mater. 2008, 155, 477. (5) Blanco, J. F.; Sublet, J.; Nguyen, Q. T.; Schaetzel, P. Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process. J. Membr. Sci. 2006, 283, 27. (6) Vogrin, N.; Stropnik, C.; Musil, V.; Brumen, M. The wet phase separation: the effect of cast solution thickness on the appearance of macrovoids in the membrane forming ternary cellulose acetate/acetone/water system. J. Membr. Sci. 2002, 207, 139. (7) Sourirajan, M.; Matsuura, T. Reverse Osmosis/Ultrafiltration Process Principles; National Research Council Canada Publications: Ottawa, Canada, 1985. (8) Cheryan, M. Ultrafiltration Handbook; Technomic Publications Co.: Lancaster, PA, USA, 1986. (9) Rahimpour, A.; Madaeni, S. S. Polyethersulfone (PES)/cellulose acetate phthalate (CAP) blend ultrafiltration membranes: Preparation, morphology, performance and antifouling properties. J. Membr. Sci. 2007, 305, 299. (10) Malaisamy, R.; Mahendran, R.; Mohan, D.; Rajendran, M.; Mohan, V. Cellulose acetate and Sulfonated polysulfone blend Ultrafiltration Membrane. I. Preparation and characterization. J. Appl. Polym. Sci. 2002, 86, 1749. (11) Mahendran, R.; Malaisamy, R.; Mohan, D. Cellulose Acetate and Polyethersulfone blend ultrafiltration membrane. Part I: Preparation and characterizations. Polym. Adv. Technol. 2004, 15, 149. (12) Vandezande, P.; Li, X.; Gevers, E. M. L.; Vankelecom, F. J. I. High throughput study of phase inversion parameters for polyimidebased SRNF membranes. J. Membr. Sci. 2009, 330, 307. (13) Vanherck, K.; Vandezande, P.; Aldea, S. O.; Vankelecom, F. J. Cross-linked polyimide membranes for solvent nanofiltration in aprotic solvents. J. Membr. Sci. 2008, 329, 468. (14) See Toh, Y. H.; Lim, F. W.; Livingstone, A. G. Polymeric membranes for nanofiltration in polar aprotic solvents. J. Membr. Sci. 2007, 301, 3. (15) Rahimpour, A.; Madaeni, S. S.; Mehdipour-Ataei, S. Synthesis of a novel poly (amide-imide) (PAI) and preparation and characterization of PAI blended Polyethersulfone (PES) membranes. J. Membr. Sci. 2008, 311, 349. (16) Arthanareeswaran, G.; Thanikaivelan, P.; Raajenthiren, M. Fabrication and characterization of CA/PSf/SPEEK ternary blend membranes. Ind. Eng. Chem. Res. 2008, 47, 1488. (17) Lv, C.; Su, Y.; Wang, Y.; Ma, X.; Sun, Q.; Jiang, Z. Enhanced permeation performance of cellulose acetate ultrafiltration membrane by incorporation of pluronic F127. J. Membr. Sci. 2007, 294, 68. (18) Sivakumar, M.; Mohan, D.; Rangarajan, R. Studies on cellulose acetate polysulfone ultrafiltration membranes. II. Effect of additive concentration. J. Membr. Sci. 2006, 268, 208.
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(19) Vijayalakshmi, A.; Lawrence Arockiasamy, D.; Nagendran, A.; Mohan, D. Separations of proteins and toxic heavy metal ions from aqueous solution by CA/PC blend ultrafiltration membranes. Sep. Purif. Technol. 2008, 62, 32. (20) Wu, L.; Sun, J.; Wang, Q. Poly(vinylidene fluoride)/polyethersulfone blend membranes: Effects of solvent sort, polyethersulfone and polyvinylpyrrolidone concentration on their properties and morphology. J. Membr. Sci. 2006, 285, 290. (21) Arthanareeswaran, G.; Thanikaivelan, P.; Jaya, N.; Mohan, D.; Raajenthiren, M. Removal of chromium from aqueous solutions using cellulose acetate and sulfonated poly (ether ether ketone) blend ultrafiltration membranes. J. Hazard. Mater. B 2007, 139, 44. (22) Chun, K. Y.; Jang, S. H.; Kim, H. S.; Kim, Y. W.; Han, H. S.; Joe, Y. I. Effects of solvent on the pore formation in asymmetric 6FDA-4,4’ ODA polyimide membrane: terms of thermodynamics, precipitation kinetics and physical factors. J. Membr. Sci. 2000, 169, 197. (23) Vidya, S.; Vijayalakshmi, A.; Nagendran, A.; Mohan, D. Effect of Additive Concentration on Cellulose Acetate Blend Membranes— Preparation, Characterization and Application Studies. Sep. Sci. Technol. 2008, 43, 1993. (24) Arthanareeswaran, G.; Mohan, D.; Raajenthiren, M. Preparation, characterisation and performance studies of ultrafiltration membranes with polymeric additive. J. Membr. Sci. 2010, 350, 135. (25) Mockel, D.; Staude, E.; Guiver, M. D. Static protein adsorption, ultrafiltration behaviour and cleanability of hydrophilized polysulfone membranes. J. Membr. Sci. 1999, 158, 63. (26) Li, J. F.; Xu, Z. L.; Yang, H.; Yu, L. Y.; Liu, M. Effect of TiO2 nanoparticles on the surface morphology and performance of micro porous PES membrane. Appl. Surf. Sci. 2009, 255, 4725. (27) Wua, G.; Gan, S.; Cui, L.; Xu, Y. Preparation and characterization of PES/TiO2 composite membranes. Appl. Surf. Sci. 2008, 254, 7080. (28) Kwok, D. Y.; Neumann, A. W. Contact angle interpretation in terms of solid surface tension. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 161, 31. (29) Sreekumar, P. A.; Leblanc, N.; Saiter, J. M. Characterization of bulk agro-green composites: Sisal fiber reinforced wheat flour thermoplastics. Polym. Compos. 2010, 31, 939. (30) Wu, H.; Fang, X.; Zhang, X.; Jiang, Z.; Li, B.; Ma, X. Cellulose acetatepoly(N-vinyl-2-pyrrolidone) blend membrane for pervaporation separation of methanol/MTBE mixtures. Sep. Purif. Technol. 2008, 64, 183. (31) Cao, S.; Shi, Y.; Chen, G Influence of acetylation degree of cellulose acetate on pervaporation properties for MeOH/MTBE mixture. J. Membr. Sci. 2000, 165, 89. (32) Thomas, P. S.; Thomas, S.; Bandyopadhyay, S.; Wurm, A.; Schick, C Polystyrene/calcium phosphate nanocomposites: Dynamic mechanical and differential scanning calorimetric studies. Compos. Sci. Technol. 2008, 68, 3220. (33) Ma, X.; Hu, C.; Guo, R.; Fang, X.; Wu, H.; Jiang, Z HZSM5filled cellulose acetate membranes for pervaporation separation of methanol/MTBE mixtures. Sep. Purif. Technol. 2008, 59, 34. (34) Wienk, I. M.; Boom, R. M.; Beerlage, M. A. M. Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers. J. Membr. Sci. 1996, 113, 361. (35) Smolders, C. A.; Reuvers, A. J.; Boom, R. M. Microstructures in phase inversion membranes. Part 1. Formation of macrovoids. J. Membr. Sci. 1992, 73, 259. (36) Shen, F.; Lu, X.; Bian, X.; Shi, L. Preparation and hydrophilicity study of poly (vinyl butyral)-based ultrafiltration membranes. J. Membr. Sci. 2005, 265, 74. (37) Young, T. H.; Chen, L. W.; Cheng, L. P. Membranes with microparticulate morphology. Polymer 1996, 37, 1305. (38) Barzin, J.; Sadatnia, B. Correlation between Macrovoid formation and the ternary phase diagram for polyethersulfone membranes prepared from two nearly similar solvents. J. Membr. Sci. 2008, 325, 92. (39) Boussu, K.; Bruggen, B. V.; Volodin, A.; Snauwaert, J.; Haesendonck, C. V.; Vandecasteele, C. Roughness and hydrophobicity 5563
dx.doi.org/10.1021/ie1019613 |Ind. Eng. Chem. Res. 2011, 50, 5550–5564
Industrial & Engineering Chemistry Research
ARTICLE
studies of nanofiltration membranes using different modes of AFM. J. Colloid Interface Sci. 2005, 286, 632. (40) Singh, S.; Khulbe, K. C.; Matsuura, T.; Ramamurthy, P. Membrane characterization by solute transport and atomic force microscopy. J. Membr. Sci. 1998, 142, 111. (41) Alsari, A. M.; Khulbe, K. C.; Matsuura, T. The effect of sodium dodecyl sulfate solutions as gelation media on the formation of PES membranes. J. Membr. Sci. 2001, 188, 279. (42) Bessieres, A.; Meireless, M.; Coratger, R.; Beauvillain, J.; Sanchez, V. Investigations of surface properties of polymeric membranes by near field microscopy. J. Membr. Sci. 1996, 109, 271. (43) Idris, A.; Zaina, N. M.; Noordin, M. Y. Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives. Desalination 2007, 207, 324. (44) Khayet, M.; Feng, C. Y.; Matsuura, T. Morphology study of fluorinated asymmetric polyetherimide ultrafiltration membranes by surface modifying macromolecules. J. Membr. Sci. 2003, 213, 159. (45) Vrijenhoek, E. M.; Hong, S.; Elimelech, M. Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and Nanofiltration membranes. J. Membr. Sci. 2001, 188, 115. (46) Tarnawski, V. R.; Jelen, P. Estimation of compaction and fouling effects during membrane processing of cottage cheese whey. J. Food Eng. 1986, 5, 75. (47) Persson, K. M.; Gekas, V.; Tragardh, G. Study of membrane compaction and its influence on ultrafiltration water permeability. J. Membr. Sci. 1995, 100, 155. (48) Fuls, P. F.; Dell, M. P.; Pearson, I. A. Non-linear flow through compressible membranes and its relation to osmotic pressure. J. Membr. Sci. 1992, 66, 37. (49) Kesting, R. Synthetic Polymeric Membranes; McGraw-Hill: New York, 1971. (50) Baker, R. W. Membrane Technology and Applications; McGrawHill: New York, 2000. (51) Schultz, J. M. Polymer Crystallization; American Chemical Society, Oxford University Press: New York, 2001. (52) Dias, C. R.; Rosa, J. R.; Pinho, M. N. Structure of water in asymmetric cellulose ester Membranes and ATR-FTIR study. J. Membr. Sci. 1998, 138, 259. (53) Murphy, D.; Pinho, M. N. An ATR-FTIR study of the water in cellulose acetate membranes prepared by phase inversion. J. Membr. Sci. 1995, 106, 245. (54) Arthanareeswaran, G.; Thanikaivelan, P.; Srinivasan, K.; Mohan, D.; Raajendran, M. Synthesis, Characterisation and thermal studies on Cellulose acetate membranes with additive. Eur. Polym. J. 2004, 40, 2153. (55) Brady, R. F., Jr. Polymer Characterisation and Analysis; American Chemical Society, Oxford University Press: New York, 2003. (56) Lewin, M. Some comments on the modes of action of nanocomposites in the flame retardancy of polymers. Fire Mater. 2003, 27, 1. (57) Kwok, D. Y.; Lin, R.; Mui, M.; Neumann, A. W. Low-rate dynamic and static contact angles and the determination of solid surface tensions. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 116, 61. (58) Xu, Z.; Huang, X.; Wan, L. Surface Engineering of Polymer Membranes; Springer: Berlin, and Zhejiang University Press: Hangzhou, China, 2009. (59) Rosa, M. J.; de Pinho, M. J. Membrane surface characterization by contact angle measurements using the immersed method. J. Membr. Sci. 1997, 131, 167. (60) Anadao, P.; Sato, L. F.; Wiebeck, H.; Rolando Valenzuela-Diaz, F. Montmorillonite as a component of polysulfone nanocomposite membranes. Appl. Clay Sci. 2010, 48, 127.
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dx.doi.org/10.1021/ie1019613 |Ind. Eng. Chem. Res. 2011, 50, 5550–5564
