International Journal of Advanced Chemical Science and Applications (IJACSA)
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Morphology, Performance and Application of carboxylated polyethersulfone incorporated cellulose acetate ultrafiltration
membrane
1R. Kalaivizhi, 2D. Mohan
1Department of chemistry, SRM University, Kattankulathur, Tamil nadu, India
2Membrane laboratory, Department of chemical engineering, Anna university Chennai , India Email: 1[email protected], 2[email protected]
[Received: 8th Feb.2016; Accepted: 13th Feb.2016; Available online from: 15th Feb.2016]
Abstract : Cellulose acetate (CA) and Carboxylated Polyethersulfone (CPES) blend ultrafiltration membranes were prepared by the precipitation phase-inversion technique in 100/0, 90/10, 80/20, and 70/30% polymer blend compositions in the absence and presence of a polymeric additive, poly(ethylene glycol) 600, at different additive concentrations and were used . The prepared membranes were characterized using, scanning electron microscopy(SEM), pure water flux, water content, and protein separation. SEM analysis showed that blend CA membranes have a thinner top layer and higher porosity in the sublayer. The rejection of proteins trypsin, pepsin, egg albumin, and bovine serum albumin; a maximum of 96%
rejection was achieved. Permeate flux studies of proteins were performed simultaneously with the rejection experiments. The rejection and permeate flux of the blend membranes were compared with those of pure cellulose acetate membranes.
Keywords: Carboxylated Polyethersulfone, additive, Scanning Electron Microscopy, protein Rejection.
I. INTRODUCTION
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.[1] Cellulosic polymers, aromatic polyamides, and polysulfones are so far the most important membrane materials in ultrafiltration membrane technology today.[2] 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, 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.[3,4] 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.[5]
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 2_8, poor chlorine resistance, greater compaction susceptibility, and high biodegradability, which reduces membrane lifetime and usage.[6,7,] Recently, carboxylated polyethersulfone have attracted interest as promising membrane materials because of their excellent chemical, mechanical, and thermal stabilities as well as good permselective properties. [9,10]. 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.[11] 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, polyurethane, poly(ether ether ketone), and poly(ether imide) for improving the CA membrane properties, which was found to be successful. [12,13,14].However, an extensive literature survey revealed that there is no published document about the exploitation of carboxylated polyethersulfone in the modification of CA membranes. This is the first attempt in the literature that explores the usage of hydrophilic carboxylated polyethersulfone 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 carboxylated polyethersulfone(CPES) into the casting solution of CA.
It was expected that bringing together carboxylated
polyethersulfone (CPES) 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 CPES in the absence and presence of pore former polyethylene glycol 600 (PEG 600). Fourier transform infrared spectroscopy. The morphology of the membranes was studied by scanning electron microscopy (SEM) The effects of polymer blend composition and additive on the membrane morphology, compaction, pure water flux, water content, and separation of proteins were investigated.
Figure 1. Chemical structures of polymers used in this study
II. EXPERIMENTAL
Materials
Commerical grade MYCEL cellulose acetate CDA 5770 (acetyl content 39.99 wt%) procured from Mysore Acetate and Chemicals Company Ltd., India and commercial grade polyethersulfone (Gafone 3300) obtained, as a gift sample, by Gharda Chemicals Pvt Ltd., India were used as supplied. Analar grade N,N- Dimethyl formamide (DMF) from Qualigens Fine Chemicals, Glaxo India Ltd. was sieved through molecular sieves (Type-4A° ) to remove moisture and stored in dry conditions prior to use. Other solvents of analar grade such as acetone and methanol from Qualigens Fine Chemicals Ltd., India were used as supplied. Sodium lauryl sulphate (SLS) of analar grade was obtained from Qualigens Fine Chemicals Ltd., India and was used as surfactant. Polyethylene glycol 600 (PEG 600) was procured from Merck (I) Ltd., and was used as supplied, as a non-solvent additive for the whole study. Proteins, namely, bovine serum albumin (Mw=69 kDa), pepsin (Mw=35 kDa), trypsin (Mw=20 kDa) were purchased from SRL Chemicals Ltd., India and used as received. Egg albumin (Mw=45 kDa) was obtained from CSIR Bio Chemical Centre, New Delhi, India.
Preparation and characterization of the membrane CA and CPES were blended in different proportions using DMF as a solvent with additive, PEG 600, by thoroughly mixing for 4 h at room temperature. The membranes were cast using a casting blade on a glass plate. The casting procedure as reported earlier [15] was
adopted. The thickness of the membrane maintained in this work was .2220mm. The membranes were initially pressurized with distilled water at 414 kPa for 4 h. The pre-pressurized membranes were used in sub-sequent UF experiments at 345 kPa using a UF kit supplied by Spectrum, USA. The membranes were characterized by pure water fux [16], molecular weight cut of (MWCO) [17], water content [18] and pore size [19].The effects of casting solution compositions on membrane performance were studied. These membranes were applied for protein separations by ultrafiltration technique at room temperature and at a pH of 7.2.
III. RESULTS AND DISCUSSION
The different compositions of cellulose acetate- carboxylated polyethersulfone blend membranes were prepared both in the presence and absence of different concentrations of additive, PEG 600. The membrane with polymer composition 80/20(CA/CPES) which gave the best results was chosen for further studies. The effect of different concentrations of dditive, PEG 600 on the performance of CA/CPES blend membranes are discussed.
Membrane compaction
At constant operating pressure (414 kPa), the pure water flux (PWF) through a CA/CPES (80/20) blend membrane with different concentrations of additive decreases with time due to compaction of membrane and is illustrated in Fig. 2. PWF as a function of compaction time has been measured at a fixed transmembrane pressure [20]. During compaction, initially the pure water flux declines gradually and after 2 h of compaction, reaches a steady state. This may be due to the fact that the membrane pores are compacted in course of time by the application of pressure (414 kPa).
Similar results have also been reported for cellulose acetate membranes. The pre-compressed membranes were used for further studies.
Pure water flux
Pure water flux of CA/CPES (80/20) blend membranes with different concentrations of PEG 600 from 0 to 2.5 wt% was observed to increase with increase in PEG 600 concentration.The PWF increased from 31.3 to 170.5 lm-2 h-1 and this may be attributed to leaching out of swelling agent, PEG 600, from nascent membrane to the coagulation bath [21].
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Fig. 2. Effect of compaction time on PWF of CA/CPES blend membranes.
PEG 600 is a water-soluble polymeric additive and it is assumed that it is totally leached out in the gelation step, leaving the highly porous membranes with larger pores on the membrane and thereby increasing the pure water flux at higher PEG 600 concentration. This is also evidenced by the SEM photographs.
Water content
Water content is considered to be an important parameter for membrane characterization since the PWF of the membrane can be predicted based on these results.
Variation in the concentration of additive from 0 to 2.5 wt% in the blend membranes resulted in an increase in the water content of the membranes from 75.06 to 78.72%. This is due to bigger pore size of the membranes as compared with those with lower concentration of PEG 600 and attributed by the faster diffusion of additive, PEG 600, from nascent membrane into the gelation bath at higher PEG 600 concentration.
Further, the hydrophilicity of the blend membranes also increases with increase in PEG 600 [22].
Morphological studies
In order to study the influence of CPES composition on the final membrane structure, cross sections of the pre- pared membranes were taken using scanning electron microscopy. The cross section of the thin film ultrafiltration membranes have a critical role in helping to identify the significance of membrane in
Table 1 Effect of Blend composition on water content Blend
Composition %
Water Content (%)
PEG 600 Concentration, wt %
CA APES 0 2.5 5 7.5 10
100 0 68.16 70.2 71.8 78.1 80.8 90 10 69.52 72.7 75.2 80.1 84.1 80 20 75.06 78.7 82.1 83.0 86.2 70 30 82.15 86.65 87.8 89.5 89.9 the mechanism of selectivity and permeability. It was known that, the top layer of membrane is responsible for the permeation or rejection, whereas the sub layer of the membrane acts as a mechanical support. In our preparation, dry–wet casting process was applied during which evaporation time was 30 s. During the exposure, DMF evaporates from the casting solution and simultaneously it absorbs water from the vapour surroundings. The rate of absorption of water was high in the case of pure CA membranes and it showed a tendency to form a liquid layer above the casting solution. The subsequent immersion of the polymer solution in the coagulation bath, leads to the formation of relatively immobilized bound layer of water at the polymer non-solvent interface, which would inhibit further flow of water into it. This immobilized bound layer in the pure CA leads to the formation of dense top layer without pores. But in the case of CA/CPES blend membranes water absorption was low and it did not shows the tendency to hold the water at the interface
[23,24]. Thus during the immersion process, the rapid driving force of the solvent with non-solvent at the interface leads to the formation of equally dispersed pores in the skin layer of the CA/CPES blend membranes. The SEM images of cross-sections of the membranes prepared from pure CA and CA/CPES blend membranes in the absence and presence of the pore former, PEG-600 were shown in Fig. 3. Gen eral structure was very similar for all the membranes consisting of a top skin layer, an intermediate layer with a sponge like structure and a bottom layer of fully developed open pores. However, the membranes prepared from pure CA exhibits a finger like cavities and the pores were not fully developed. The morphologies of the CA/CPES blend membranes differ from that of pure CA membranes. Comparison between images indicates that incorporation of CPES in the casting solution produces highly porous membranes with sponge like structure in the sub layer. The changes in the morphologies can be attributed to the changes in the properties of the membrane forming polymer by the addition of the amorphous polymer CPES. This difference in morphology of the blend membranes was mainly because of the presence of carboxylic groups in PES and effect of these groups in the membrane formaion. We can expect an enhancement of the surface hydrophilicity for a given content of CPES, due to the preferential orientation of these groups towards water during the membrane formation process [25].When we are comparing the CA membranes with CA/CPES, the spongier like structure was formed in the later and it was due to the difference in tolerance towards water without phase separation. Greater affinity of CA to water, leads to longer time for the solvent exchange between the water and the polymer solution before gelation and vitrification. Experimentally, several minutes of coagulation time was needed for the “setting” of the film made of pure CA, while blend polymers were set in 4–5 min. This longer settling time for the pure CA membrane was due to the “tortuous” path of the crystalline regions of the CA and this region was considerably decreased in blend films due to the presence of CPES. The longer exchange time between the solvent and non-solvent in the coagulation bath result in more developed process of polymer-lean phase growth and coalescence, thus larger the finger like pores in pure CA membranes. But in case of blends of CA/CPES, high diffusion rate and hydrophilicity, results in the formation of tear like structures with more sponge like areas in the sub layer. Highly open transition sub- layer structure were difficult to obtain with pure CA, because the diffusion process associated with the re- dissolution step were slowed down by the viscosity of the polymer solution and the immobilized water layer formed on the water–polymer interface [26,27].
A B
C D
E F
Fig. 3 Cross sectional SEM images of CA/CPES blend membranes (w/w): (a)100/0,without additive;
(b)100/0with additive;(c)80/20,without additive;
(d)80/20, with additive.
Protein rejection studies
The CA/CPES blend membranes with compositions of 90/10, 80/20, and 70/30% in the presence and absence of different additive concentrations of PEG
Table 2 Effect of blend composition on rejection of proteins
Blend Composition
PEG 600
Percent rejection of Proteins
CA CPES BSA
69 KDa
EA 45 kDa
Pepsin 35 kDa
Trypsin 20 kDa
100 0 0 96 93 84 76
90 10 0 95 88 75 68
80 20 0 85 83 63 60
70 30 0 85 80 61 57
100 0 2.5 95 90 77 71
90 10 2.5 90 85 70 64
80 20 2.5 81 83 66 58
70 30 2.5 81 76 63 56
100 0 5 88 80 72 67
90 10 5 81 77 63 58
80 20 5 74 75 63 53
70 30 5 71 62 54 46
100 0 7.5 81 75 65 60
90 10 7.5 75 70 56 52
80 20 7.5 68 65 50 48
70 30 7.5 60 54 45 41
100 0 10 78 72 55 48
90 10 10 73 62 47 42
80 20 10 65 61 42 40
70 30 10 58 51 40 37
600 were used for the rejection of proteins under a nitrogen atmosphere, and the results were compared with the rejection by the pure CA membranes. Initially, a protein of low molecular weight, trypsin, was used for the ultrafiltration experiments because we expected the use of a high-molecular- weight protein at the beginning would spoil the originality of the pores for the separation and comparison of low-molecular-weight proteins. Thus, the rejection of proteins were performed in the order trypsin, pepsin, EA, and BSA.
Role of the polymer blend composition and additive concentration on the rejection of proteins
The composition of the polymer blend membrane had the effect of altering the protein rejection efficiency.
The pure CA membrane exhibited rejections of 96% for BSA and 76% for trypsin. The higher rejection of BSA may have been due to the larger size of the BSA compared with trypsin. As the CPES composition was increased from 10 to 30% in the CA/CPES blend in the absence of any additive, the percentage Rejection decreased, as shown in Table 2. This may have been because the higher CPES content related inhomogeneity between the polymer matrices, resulting in the formation of aggregate pores in the membranes. Similar results were also observed for CA/sulfonated polysulfone (SPS) blend membranes.[28] For the 90/10% blend composition, the percentage rejection values were 95, 88, 75, and 68 % for BSA, EA, pepsin, and trypsin, respectively. The decrease in rejection may have been the decrease in the solute size of the proteins in the aforementioned order.
Figure 4 Effect of the PEG 600 concentration on the proteins for the 100% CA membranes.
Figure 5 Effect of the PEG 600 concentration on the flux of proteins for the 80/20 CA/CPES membranes.
% SR by CA/CPES Blend Membranes
The effects of the additive (PEG 600) concentration on the rejection of the blend membranes is shown in Table 2. The additive concentration was increased from 2.5 to
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10 wt %, in each blend composition, and the percentage rejection decreased. For the 100% CA membrane with 2.5 wt % additive, the BSA rejection was 95%, and it decreased to 78% with the increase of the additive concentration to 10 wt %. A similar trend was also observed for other proteins, with varying magnitudes.
This may have been due to the leaching out of the additive (PEG 600) from the membranes during gelation, which created pores proportionately on the membrane. In the CA/CPES blend membranes also, for a given polymer composition, when the additive concentration was increased, from 2.5 to 10 wt %, the separation efficiency decreased. All of the blend membranes with various additive concentrations showed similar trends for all of the protein molecules. The higher percentage rejection of BSA and the lower percentage rejection of trypsin was obviously due to their molecular sizes.
Protein flux studies
The permeate protein flux is the measure of the product rate of the membrane for the given protein solutions.
Role of the polymer blend composition and additive concentration on the product rate efficiency of the Proteins:
The permeate flux of the proteins BSA, EA, pepsin and trypsin by the 100/0, 90/10, 80/20, and 70/30%
CA/CPES blend membranes in the absence of the additive is shown in Figures 6–9. The pure 100% CA membrane, in the absence of additive, showed the lowest permeate flux of 24.5 lm-2 h-1 for BSA. The other proteins, EA, pepsin, and trypsin, showed comparatively higher fluxes with the pure CA Membranes. For the CA/CPES blend membranes, without additive ,for a given protein molecule (e.g., BSA), when the CPES content in the blend was increased, from 10 to 30%, the flux also increased from 32.2 to 70.9 lm-2 h-1 A similar trend was observed for all of the proteins. This trend may have been due to the hydrophilic CPES, which could have reduced the fouling of protein, thereby enhancing the product rate efficiency.
Figure 6 Effect of the PEG 600 concentration on the flux of proteins for the 100% CA membranes.
Figure 7 Effect of the PEG 600 concentration on the flux of proteins for the 90/10% CA/CPES blend membranes.
The presence of additive in the casting solution had a significant role in the morphology and, in turn, on the flux of the resulting membranes. Thus, the pure CA membrane for a given protein molecule had an enhanced flux when the additive was increased from 2.5 to 10 wt
%, In the 100% CA membrane, BSA had a flux of 56 lm-2 h-1 for 2.5 wt % PEG 600 and 92 lm-2 h-1 for 10 wt
% PEG 600.
Figure 8 Effect of the PEG 600 concentration on the flux of proteins for the 80/20% CA/PES blend
membranes.
Figure 9 Effect of the PEG 600 concentration on the flux of proteins for the 70/30% CA/CPES blend
membranes.
The other proteins also exhibited a similar trend. For the 90/10% CA/CPES blend membrane , the increase of additive from 2.5 to 10 wt % increased the protein permeate flux from 71.6 to 108 lm-2 h-1 for BSA. All of the other blend compositions also exhibited similar behavior when the additive was increased from 2.5 to 10 wt %, A similar trend was also observed for the other proteins. This may have been due to the formation of macrovoids in the membrane, due to the faster rate of leaching out of the additive during gelation. In all of the membranes, regardless of the additive concentration and polymer blend composition, the order of protein flux was trypsin > pepsin > EA > BSA. The reason for this
trend may be explained by the fact that the flux of the proteins was inversely proportional to their size
IV. CONCLUSION
In the present investigation, high performance CA ultrafiltration membranes were prepared by the phase inversion technique using hydrophilic CPES as the modification agent and polyethylene glycol 600 as pore former. The effects of blend ratio on the morphology, permeation properties, and hydrophilicity of the resultant membranes were evaluated. Morphological analysis of the blend membranes revealed that, as the weight percentage of CPES in the CA matrix increased, defect-free thin layer and spongy sublayer were formed.
The pure water flux and water content of the CA membranes were increased with an enhancement in CPES 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. Overall results suggest that membrane morphology, pure water fluxes, water content, of the prepared CA/CPES blend membranes improved significantly by the incorporation of CPES. Therefore, CPES should be considered as an effective modification agent for the development of low energy, antifouling CA ultrafiltration membranes for various industrial separations.
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