Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers

Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers

Separation and Purification Technology 57 (2007) 209–219 Effect of molecular weight and concentration of PEG additives on morphology and permeation p...

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Separation and Purification Technology 57 (2007) 209–219

Effect of molecular weight and concentration of PEG additives on morphology and permeation performance of cellulose acetate hollow fibers Wen-Li Chou a , Da-Guang Yu a , Ming-Chien Yang b,∗ , Chi-Hsiung Jou c a

b

Department of Materials and Fibers, Nanya Institute of Technology, Chung-Li, Tao-Yuan 320, Taiwan, ROC Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC c Department of Materials and Textiles, Oriental Institute of Technology, Pan-Chiao, Taipei County 220, Taiwan, ROC Received 30 September 2006; received in revised form 29 March 2007; accepted 12 April 2007

Abstract Asymmetric cellulose acetate (CA) hollow fiber membranes were prepared by dry/wet spinning process from a dope composed of cellulose acetate (CA), N,N-dimethylformamide (DMF), and polyethylene glycol (PEG). Herein, PEG was the additive; DMF was the solvent; whereas water was the nonsolvent. The spinning parameters in this study were the contents and molecular weights of PEG and the external coagulation temperature. The surface and cross-section morphology of the resulting hollow fibers were examined using scanning electron microscopy (SEM). The pure water permeability (PWP) and retention of dextran were also measured. The results showed that the addition of PEG would suppress the formation of macrovoids. The effect on the suppression was more obvious for PEG with higher molecular weight and higher content. When PEG was added, the outer surfaces changed from smooth to microporous, whereas the inner surface remained smooth and dense. The PWP increased with the additive content but slightly decreased with the increase of molecular weight. Oppositely, the retention of dextrans decreased with the increase of additive contents but increased with the molecular weight. When adding PEG and coagulating at higher temperature simultaneously, the outer surface and cross-section of CA/PEG blended membrane exhibited macrovoids on outer surface and finger-like voids near the inner and outer edges. The resulting membranes showed higher PWP (4–7 times) with slight decrease in the retention. Hence, adding PEG and elevating coagulant temperature will promote the permeation performance of CA hollow fibers. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Hollow fiber; Cellulose acetate; PEG additive; Dry–wet spinning

1. Introduction Since the successful development of cellulose acetate (CA) asymmetric membranes with a very dense and thin active layer on the top of a porous substrate [1], CA membranes have been applied in reverse osmosis for converting sea water into freshwater. Furthermore, CA hollow fiber membranes are widely used for clinical hemodialysis. For instance, Kell and Mahoney [2] invented a CA hollow fiber suitable for use in artificial kidneys to provide superior water and solute clearances. Hemodialysis is one of the most important methods for blood purification. In general the characteristics of a hemodialyzing material are ultrafiltration rate, solute permeability, mechani-



Corresponding author. Tel.: +886 2 2737 6528; fax: +886 2 2737 6544. E-mail address: [email protected] (M.-C. Yang).

1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.04.005

cal strength, and hemocompatibility. Because of CA hollow fiber membranes have an excellent performance in the removal of low molecular weight toxic substances such as urea and creatinine from patients and can reduce the activation of the complement system and the leucopenia observed in the early period of blood extracorporeal circulation. However, CA hollow fiber membranes have demonstrated insufficient removal of lower molecular weigh proteins such as ␤2 -microglobulin (␤2 -MG, 11,800 Da) which can cause amyloidosis. Besides, CA membranes can easily cause serious protein adsorption and clot formation when the membrane contacts blood as do other blood purification membranes. These phenomena induce a decrease in the water flux and solute permeability of the membrane and require infusion of an anticoagulant into the patient during blood purification therapy and these factors make serious complications for chronic renal failure patients who require continuous and long time treatment [3–5]. Therefore, many researchers or

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companies have pay attention to improve its performance by added additive or plasticizer [2,6–8]. Cellulose acetate hollow fibers are produced by the dry–wet spinning technique, and often have asymmetric structure with dense inner and outer skin layers, therefore its performance for water treatment or hemodialysis is always undesirable. Furthermore, the dry/wet spinning is a complex process since it involves many process variables, such as the dope extrusion rate [9–11], the flow rate of core liquid [9,12,13], the composition and temperature of the core liquid and coagulant [12,14–18], post-drawing and on-line drawing [19,20], the length of the air gap [9,10,13,21–26], and additive agent or nonsolvent to dope [8,27], that would influence both the geometrical characteristics and the permeation properties of the hollow fibers. The addition of organic or inorganic components as a third component to a casting solution has been one of the important techniques used in membrane preparation. However, the role of organic and inorganic additives, such as poly(vinylpyrrolidone) (PVP), polyethylene glycol (PEG), water, LiCl, and ZnCl2 , has been reported as a pore-forming agent to enhance the permeation properties. This behavior was explained in terms of their water-soluble characteristics [3,28]. Although their works are important and interesting, their theories might need further modifications to be applied to hollow fiber membrane fabrication. It is due to the fact that it is difficult to simulate the hollow-fiber spinning process by adopting the process conditions developed for asymmetric flat membranes. In addition, Merrill et al. [29] reported that added PEG additive in the polymer solution or grafted PEG on the polymer surface would improve anticoagulation. In order to suppress macrovoids in the hollow fiber membranes, Liu et al. added high concentration of PEG400 to the dope of polyethersulfone (PES) and using PEG400 aqueous solution as the coagulant to control the morphology of the resulting hollow fiber [30]. Li et al. employed a co-extrusion spinneret to prepare dual-layer Matrimid/PES hollow fibers [31]. They also added high concentration (66%) of PEG (MW 100 kDa) to the spinning dope in order to suppress the macrovoids of the PES layer. In this paper, in order to increase the removal of lower molecular weight proteins such as ␤2 -microglobulin, CA asymmetric hollow fiber membranes were prepared from polymer solutions composing of CA/PEG/DMF via dry–wet spinning method. By varying the coagulant temperature and the molecular weight and content of PEG, the morphological and transport properties were systemically investigated. The method of this research not only added additive to the dope, but also varied the coagulant temperature. In so doing, the permeation performance can be improved with a single-pass manufacture procedure. 2. Experimental 2.1. Materials Cellulose acetate (number average molecular weight 50,000 with acetyl content 39.8%) was purchased from Aldrich, USA. N,N-dimethylformamide (DMF), sulfuric acid, and phenol were

purchased from Acros, USA. PEG (number average molecular weight 1, 10 and 40 kDa) were purchased from Merck, USA. Dextran with molecular weights of 10.2, 35.0, and 66.7 kDa were purchased from Sigma, USA and stored at a suitable temperature before use. 2.2. Spinning conditions A homogenous spinning solution was prepared by adding 25 wt% CA polymer powder in 75 wt% solvent of DMF. In the dope, the PEG was added 7, 14 and 21% with respect to the weight of CA, respectively. All the ingredients were added in the flask and were stirred at room temperature for 24 h until the CA polymer and PEG additive were entirely dissolved to form a homogenous spinning dope. Afterward, the dope was degassed in a vacuum oven at room temperature for 2 h and extruded under 100 kPa through a hollow fiber spinneret with an inner diameter of 0.4 mm and an outer diameter of 0.6 mm. Spinning of the CA hollow fiber was based on the dry–wet technique. Both the core liquid and the external coagulant were pure water, and the external coagulant temperature was controlled at either 25, 50, or 75 ◦ C. The air gap which is the distance between the tip of spinneret and the surface of external coagulant was kept 20 cm. After coagulation, the resulting CA hollow fiber was wound up with a take-up roller and rinsed with water for 2 days to remove residual DMF. Then the fibers were immersed in 50 wt% glycerol solution for another 24 h and then were dried in air at room temperature for ultrafiltration testing and morphology examination. The spinning conditions and detailed parameters were listed in Table 1. 2.3. Morphology studies In order to examine the morphology of hollow fiber membranes, dried samples of hollow fibers were broken in liquid Table 1 Process parameters and spinning conditions Process parameters/spinning conditions

Value

1. Polymer composition 2. PEG molecular weight (Dalton) 3. PEG concentration (PEG vs. CA by weight) 4. CA concentration (by weight) 5. Spinneret temperature (◦ C) 6. Dope extrusion rate (g/min) 7. Spinneret OD/ID 8. Bore liquid 9. Bore liquid flow rate (ml/min) 10. Dope pressure (kPa) 11. Air-gap distance (cm) 12. Take-up velocity (m/min) 13. Coagulant composition 14. Coagulant temperature (◦ C) 15. Drying procedure

CA/DMF/PEG 1 k, 10 k, 40 kDa 7%, 14%, 21% 25% Room temperature 11 0.6 mm/0.4 mm H2 O 7.6 100 20 35 Tap water 25–70 A few days in deionized water. One day in 50% (w/w) glycerin aqueous solution at room temperature and dried at room temperature

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nitrogen and then sputtered with a thin layer of gold using Jeol JFC-11-E sputtering device. The outer and inner surfaces of the hollow fibers were examined using a field emission scanning electrical microscopy (FESEM) (S-800, Hitachi, Tokyo, Japan). 2.4. Ultrafiltration experiments The experimental setup for measuring the pure water permeability (PWP) and retention of dextran for CA hollow fiber was shown in our previous work [20]. Hollow fibers were tested in bundles of 10 fibers of 15 cm in length. Each end of the module was a piece of Teflon tubing, 6.35 mm (1/4 in.) in diameter and 2.5 cm in length. The fibers were potted at both ends with epoxy resin and mounted in a test module. While not taking measurements, the modules were preserved in 50 wt% glycerol. The preserving solution was flushed out first with water before taking data. The feed was held in the hopper under a pressure of 50 kPa. To prevent concentration polarization [32], the feed was recirculated at a flow rate of 25 ml/min with a metering pump (RH1CKC, Fluid Metering, Inc., USA) through the lumen of the hollow fibers, and the permeate was collected from the shell side to simulate hemodialysis. The PWP was determined when the flow rate stabilized. The pore size distribution was determined using dextran of different molecular weights (10.2k, 35k, 66.7k) to simulate the ␤2 -MG (MW: 11,800), pepsin (MW: 35,000) and bovine serum albumin (MW: 68,000) protein. The dextran solutions concentration in the feed solution was kept at 1000 ppm. The concentrations of dextran solutions were determined by sulfuric acid phenol solution titration [33,34] and the absorbance at 490 nm was determined using an ultraviolet-visible spectrophotometer (CE7200, Aquarius, England). The PWP, or the ultrafiltration rate (UFR), was calculated by the following equation: PWP =

Q Q = AP nπDi LP

(1)

where Q is the volumetric flow rate (L/h), A is the inner surface area of the hollow fiber membrane (m2 ), n is the numbers of fibers, Di is the inner diameter of hollow fiber, L is the length of the hollow fiber and P is the applied pressure of ultrafiltration

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experiment. The retention R was calculated as follows: R=1−

Cp Cf

(2)

where Cf and Cp are the solute concentrations in the feed and permeate, respectively. All experiments were performed in hollow-fiber modules. Three modules were prepared for each hollow-fiber sample. 3. Result and discussion 3.1. The effect of molecular weight and content of PEG on morphology Fig. 1 shows the SEM photos of CA and CA/PEG hollow fiber membranes via dry–wet spinning under the same spinning conditions. The molecular weight and content of PEG were 40 kDa and 21 wt%-CA, respectively. The CA membrane in Fig. 1(a) exhibits an asymmetrical structure consisting of a dense top-layer and a porous sub-layer with droplet morphologies near the inner edge, whereas the CA/PEG membrane in Fig. 1(b) also has an asymmetrical structure but without observable droplet morphologies. In order to understand the reasons that the macrovoids of membranes would be reduced by the addition of PEG, the molecular weight and content of PEG in the dope were varied. Fig. 2 shows the SEM photos of partial cross-section. Those hollow fiber membranes were prepared with three different contents (7, 14, 21 wt%-CA) of the same molecular weight (10 kDa). These images indicated that the droplet morphologies enclosed in the inner edge gradually disappeared with increasing PEG content and that the thickness of the membranes increased with the PEG content. The reason may be that the solid content of the dope was increased due to the addition of PEG. Because PEG can increase the viscosity of the solvent, this hindered the exchange of solvent and nonsolvent, thus higher PEG content led to denser structure. The other possible reason is that some part of the PEG chains on the membranes surface penetrate into the pores and are attached inside of the pores [35].

Fig. 1. SEM photograph of hollow fiber membrane: (a) CA/DMF/water system (magnification: 140×) and (b) CA/PEG/DMF/water system (magnification: 150×).

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Fig. 2. Effect of the PEG (MW 10 kDa) contents (PEG vs. CA weight ratio) on the morphology of cross-section of CA hollow fiber membranes: (a) 0%; (b) 7%; (c) 14% and (d) 21%.

In general, macrovoid formation during phase separation occurs from freshly formed nuclei of the diluted phase when the composition in front of the nuclei remains stable for a relatively long period of time [36,37]. Due to diffusion of solvent expelled from the surrounding polymer solution the macrovoid grows. Macrovoids are generally formed in systems where instantaneous demixing takes place, except when the polymer additive (e.g. PVP) concentration and/or the nonsolvent concentration in the polymer solution exceed a certain minimum value [36–39]. In addition, some researchers [29,40,41] have reported that the macrovoid formation disappears as adding hydrophilic additive to the casting solution [41] and that the macrovoids are suppressed by adding organic acids, because the acids form acid–base complex with basic polar solvents such as NMP, DMF, and DMAc [40]. Polymeric additives such as PVP and PEG are widely used for the structure control for the fabrication of ultrafiltration and microfiltration membranes [29]. Based on those SEM photos in the Fig. 2, our results are in accordance with their reports. The effect of molecular weight of PEG (1 k, 10 k and 40 kDa) on the morphologies was also investigated, as shown in Fig. 3.

Fig. 3 indicates that the droplet macrovoids was obviously suppressed as higher molecular weight of PEG was added. Although those hollow fiber membranes were spun from the same PEG content (21%-CA), Fig. 3(a) and (b) (1 kDa PEG was added) apparently show a few macrovoids near the inner edge, whereas Fig. 3(c) and (d) (10 kDa PEG was added) and Fig. 3(e) and (f) (40 kDa PEG was added) show that macrovoids were inhibited. The difference between Fig. 3(c)–(f) was only in the thickness, and the thickness of membrane increased with the molecular weight (Fig. 3(f)). The effect of the contents and molecular weights of PEG on the thickness of membranes were also exhibited in the Table 2. Table 2 lists the dimensions of the resulting CA/PEG hollow fibers. In general, higher PEG content and higher molecular weight of PEG led to larger wall thickness of the hollow fiber, since the total polymer content (CA and PEG) was higher. In addition, larger PEG molecules would be more difficult to be leached, and leading to thicker hollow fibers. It has been known that the formation of macrovoid is influenced by the molecular weight of additives [42,43]. Lower molecular weight PEG is more soluble in water than higher

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Fig. 3. SEM overall and partial cross-section photographs of membranes made from of CA solutions containing various molecular weight and same content of PEG (21 wt%): (a and b) PEG (1 kDa); (c and d) PEG (10 kDa) and (e and f) PEG (40 kDa).

molecular weight, thus low molecular weight PEG can be easily washed out quickly by water during the formation of membrane. Therefore, the thickness and the droplet-like macrovoids were dependent on the solubility of PEG. In fact, intensive studies on the effect of water-soluble additives to a dope solution on the morphology of asymmetric membranes have been performed [37,44,45]. Jung et al. [46] suggested that the top-layer thicknesses is not varied at low concentration and low molecular weight of PVP, but the top layer thickness changes distinctly at high concentration and high molecular weight of PVP. The main reason is caused by residual content increase with increasing

concentration and molecular weight of PEG during the membrane formation. In summary, as more PEG is added, the number of macrovoids gradually disappears, regardless of the molecular weight of PEG. When the same amount of PEG is added, the wall thickness increased with the molecular weight of PEG, as shown in Figs. 2 and 3 and Table 2. Fig. 4 shows that the CA hollow fiber membranes with or without PEG additive exhibited a dense and sponge-like structure. Fig. 4(b)–(d) clearly show that CA/PEG membranes were denser than original CA membrane in Fig. 4(a). This is because CA/PEG membranes were spun from a dope with higher poly-

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Table 2 The dimensions of CA/PEG blend hollow fiber membranes PEG content (wt%-CA)

PEG (Da)

Do (␮m)

Di (␮m)

δdry (␮m)

δwet (␮m)

0 7 7 7 14 14 14 21 21 21

– 1k 10k 40k 1k 10k 40k 1k 10k 40k

473.5 (5.1) 444.2 (3.3) 416.7 (5.1) 449.0 (4.9) 475.2 (6.2) 487.9 (7.3) 500.0 (4.6) 433.9 (3.3) 512.0 (3.8) 515.1 (4.3)

350.0 (4.3) 315.8 (2.4) 293.3 (4.8) 300.0 (2.3) 336.6 (5.1) 330.3 (4.9) 329.6 (6.3) 283.5 (6.5) 341.2 (4.0) 322.1 (5.3)

61.6 (2.0) 64.2 (2.1) 70.0 (4.1) 74.5 (3.4) 69.3 (2.6) 78.8 (3.7) 85.2 (3.5) 73.2 (2.1) 83.5 (2.2) 95.5 (2.2)

81.3 (2.7) 83.7 (2.5) 86.3 (3.3) 94.0 (2.6) 89.3 (2.3) 94.3 (3.1) 100.3 (2.8) 90.3 (2.3) 96.9 (2.9) 110.7 (3.7)

Note: The numbers in the brackets are the standard deviation (n = 4–6).

mer concentration (CA and PEG). As shown in Fig. 4, the porosity of the membrane decreased with the increase of molecular weight of PEG. In general, the radius of gyration (Rg ) of PEG chains increases with the molecular weight [47]: Rg = 0.0215M0.583 (nm) w

(3)

Smaller PEG molecules can quickly diffuse out to the coagulant during the dry–wet spinning process which would lead to

a looser structure, while larger PEG molecules were more difficult to be removed which would retard the exchanging rate of solvent and nonsolvent and result in a denser structure. Furthermore, when PEG and CA polymer chains entangled with each other, higher molecular weight would make the entanglements tighter. Hence the wall structure of membrane appeared denser when adding higher molecular weight PEG. On the other hand, because higher molecular weight PEG would diffuse out slower and were entrapped in the CA matrix, after leaching out the resi-

Fig. 4. SEM image of partial cross-section of outer edge of membranes made from various molecular weight and same content of PEG (14 wt%): (a) original; (b) PEG (1 kDa); (c) PEG (10 kDa) and (d) PEG (40 kDa).

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Fig. 5. SEM image of inner surface of membranes made from various contents and same molecular of PEG (10 kDa): (a) original (magnification: 50,000×); (b) PEG (7%, magnification: 1000×); (c) PEG (14%, magnification: 1000×); (d) PEG (21%, magnification: 1000×); (e) PEG (14%, magnification: 10,000×) and (f) PEG (21%, magnification: 10,000×).

dual PEG, the space originally occupied by PEG would become larger pores in the membranes. 3.2. Effect of the external coagulant temperature on the morphology of CA/PEG hollow fiber membranes Figs. 5–7 show the SEM micrographs of the inner skin, outer skin and cross-section of the CA and CA/PEG hollow fibers spun at various coagulant temperatures, respectively. Fig. 5 depicts the inner surface of various contents of PEG with a molecular weight of 10 kDa. Those SEM images show that the inner surface was generally smooth except the ripple of water in various

contents of PEG by magnification 1k–50k times. This is because the bore liquid used pure water at room temperature and was injected 7.6 ml/min by a gear pump. When the spinning dope was extruded from the spinneret, the polymer solution immediately contacted with the bore liquid that led to the immediate occurrence of phase inversion. Because the bore liquid flow rate (4.03 m/s) is faster than dope extrusion rate (1.27 m/s), the friction exerted by the bore liquid to the nascent hollow fiber led to the ripples at the inner surface. Fig. 6 depicts the coagulant temperature on the morphology of outer surface of the both pure CA and CA/PEG blended hollow fiber membranes. Apparently, the temperature is a strong

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Fig. 6. SEM image of outer surface of membranes made from various coagulant temperature and same content molecular weight of PEG (14%, 10 kDa): (a) pure CA, 25 ◦ C, magnification: 50,000×; (b) pure CA, 75 ◦ C, magnification: 50,000×; (c) 25 ◦ C, magnification: 10,000×; (d) 50 ◦ C, magnification: 10,000×; (e) 75 ◦ C, magnification: 5000×.

influence on the morphology of membranes, especially, the CA/PEG blended hollow fiber membranes clearly exhibited macrovoids at higher coagulant temperature. This is because the nascent hollow fiber plunged into a coagulation bath of higher temperature. Higher coagulant temperature cannot only increase the exchange rate of the solvent and nonsolvent but also would increase the humidity in the air gap region (the vapor pressure of water was 3.17, 12.34, and 38.56 kPa at 25, 50, and 75 ◦ C, respectively). Both would accelerate the phase separation. The molecular mobility would be higher at higher temperature, and hence the extensibility of the hollow fibers and the humidity induced phase-separation process seems to be a much faster

process at higher external coagulant temperature. Therefore, raising the coagulant temperature could result in porous surface due to faster phase separation at higher temperature. In general, macrovoid formation occurs under rapid precipitation conditions and the precipitation is faster at higher temperature [36]. Besides, higher temperature would increase the effusion rate of the solvent from the spinning jet, hence the polymer molecules precipitated faster that shortened the time for molecular rearrangement. The other important parameter affecting the outer surface morphologies is that PEG additive is water-soluble, especially at higher temperature. Higher coagulant temperature could

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Fig. 7. SEM image of partial cross-section of membranes made from various coagulant temperature with same content and molecular weight of PEG (14%, 10 kDa): (a) pure CA, 25 ◦ C, magnification: 1000×; (b and c) 25 ◦ C, magnification: 700×; (d) 50 ◦ C, magnification: 700×; (e) 75 ◦ C, magnification: 700×.

increase the exchange rate of the solvent and nonsolvent, hence the PEG was more quickly effused out when the nascent hollow fiber membrane was immersed in the coagulation bath of higher temperature. The outer skin layer of the resulted hollow fiber membranes would have less pore density but higher pore sizes when coagulated at higher temperature. Those SEM images in Fig. 6 were spun at various temperatures. Fig. 6(a) and (b) are the images of pure CA hollow fiber membranes spun at room temperature and 75 ◦ C, respectively. Fig. 6(b) shows higher roughness and micropore surface than Fig. 6(a) under a magnification of 50k times, whereas Fig. 6(c)–(e) show the CA/PEG hollow fiber membranes whose spun at various coagulant temperature. It is clearly that the pores or macrovoids and pore size of membranes increased with increasing temperature. In other words, either adding PEG or increasing coagulant temperature would be limited to changing or improving morphology of membranes, but combining these two factors, the morphology of membranes would be controlled more precisely. Fig. 7 shows the cross-section of the CA/PEG hollow fibers. In Fig. 7(a), when coagulating at 25 ◦ C without the addition of PEG, CA hollow fiber appeared as a sponge-like structure with few macrovoids near the inner surface. When blending with 14% PEG, there was no macrovoids observable when coagulating at 25 ◦ C, as shown in Fig. 7(b). Macrovoids increased with the

coagulant temperature, as shown in Fig. 7(c) and (d). This can be attributed to that higher coagulant temperature would lead to lower viscosity and higher exchanging rate of DMF and water. This gave rise to more macrovoids. 3.3. Effect of molecular weights and content of PEG and coagulant temperature on the performance Our previous work reported that a higher coagulant temperature would strongly affect the permeation performance of CA hollow fiber membranes [18]. Based on these results, the effect of PEG additive combining with spinning parameters were further investigated in this present work. Most membrane manufactures characterize their products by the pore size or molecular weight cut-off value which is usually obtained by measuring the retention (R) of macromolecules with a series of hydrodynamic diameters or molecular weights. The pure water permeability (PWP) and the retention of dextran can be also regarded as the indices for the permeation performance of the hollow fiber [48]. Thus, we measured these two terms to evaluate the permeation performance of CA/PEG hollow fiber membranes. As mentioned in previous section, the thickness of the membrane and the morphology depend on the molecular weight and

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Table 3 Effect of various content and molecular weight of PEG additive on the performance (the membranes were spun at 25 ◦ C) PEG content (PEG vs. CA ratio)

0 7% 7% 7% 14% 14% 14% 21% 21% 21%

PEG (MW)

Pure CA 1k 10k 40k 1k 10k 40k 1k 10k 40k

PWP (L/m2 h atm)

– 5.6 4.4 3.9 12.4 10.1 8.6 18.4 15.9 14.0

concentration of PEG in the dope. Table 3 shows that the PWP and R also depended on the molecular weight and concentration of PEG in the dope. The PWP and R of pure CA could not be measured with our ultrafiltration experimental device. This is because of the original CA has a sponge-like structure in sub-layer and it is also found that both the inner and outer skin layers are dense and smooth, as shown in the SEM images in Figs. 5(a) and 6(a). Figs. 5(b) and 6(b) show that the CA/PEG hollow fiber coagulated at 25 ◦ C exhibit an outer skin layer with micropores and rough surface and an inner skin with dense and smooth surface. For CA, water is a weak nonsolvent, which means that coagulation occurs slowly when the polymer solution is brought into contact with water, and the resulting membranes have a spongelike sub-layer and almost no pore in on the surface. Although PEG additive is water-soluble, the bore liquid in the lumen of nascent hollow fiber membranes cannot dissolve all PEG at once. When the nascent hollow fiber plunged into the coagulation bath, a great quantity of water would replace DMF in the nascent hollow fiber and dissolve part of PEG on the outer surface of nascent hollow fiber, hence the outer skin exhibited more micropores and rough surface than the inner skin. Table 3 also shows that PWP increases with increasing PEG content and decreased with increasing molecular weight. It

Retention (dextran MW) 10.2k

35k

66.7k

– 89.5 88.8 90.3 85.8 87.3 89.3 83.0 84.1 85.9

– 96.2 94.7 95.2 94.6 93.5 94.4 89.3 90.5 90.4

– 99.5 97.4 97.7 95.7 94.5 95.3 95.1 94.5 95.2

seems that the PEG content is more influential on the permeation performance than the molecular weight of PEG. Higher PEG content would lead to higher PWP. This is because that PEG is a pore-forming agent that creates pores in the polymer matrix of CA. On the other hand, higher molecular weight would be less soluble and more difficult to effuse out than a lower molecular weight of PEG. Therefore the resulting CA hollow fiber would have less but larger pores. It has been generally accepted that performance of hollow fiber membranes were mainly affected by both of inner and outer skin. Micropore was observed on the outer surface but not on the inner surface, even though the PWP increased from 0 to 18.4 L/m2 h atm. The retention of dextran slightly decreased with the increase of PEG content but increased with the increase of the molecular weight of PEG. This implies that the size of micropores increased with the PEG content. However, the number of micropore decreased with the increase of the molecular weight of PEG. During the coagulation of hollow fiber, a substantial portion of PEG may be entrapped in the pores and the entrapping amount depends on the molecular weight of PEG. When the hollow fibers were coagulated at lower temperature, the resulting membranes exhibited dense structure, higher tensile force, and lower PWP. By destroying the dense skin in either surface, the PWP can be improved while retaining the retention. This can be achie-

Table 4 Effect of the coagulant temperature on the performance of CA/PEG blend hollow fiber membrane PEG (MW and content) (PEG vs. CA ratio)

Coagulant temperature (◦ C)

PWP (L/m2 h atm)

Retention (dextran MW) 10.2k

35k

66.7k

Pure CA

25 50 75

– 8.4 15.6

– 87.6 78.7

– 93.8 88.2

– 97.2 93.1

CA + PEG (1k, 14%)

25 50 75

12.4 25.8 45.9

85.8 83.5 72.3

94.6 90.2 85.3

95.7 93.1 90.5

CA + PEG (10k, 14%)

25 50 75

10.1 30.1 55.3

87.3 75.3 59.3

93.5 84.6 74.9

94.5 92.4 90.2

CA + PEG (40k, 14%)

25 50 75

8.6 35.6 62.5

89.3 74.9 41.9

94.4 80.8 65.3

95.3 90.6 88.5

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ved by using higher coagulant temperature combining with the addition of PEG additive. Table 4 illustrates the effect of the external coagulant temperature on the performance of. CA/PEG hollow fiber membranes. The PWP was progressively increased to 62.5 L/m2 h atm at the highest coagulant temperature (75 ◦ C), which was about 4–7 times of that coagulated at 25 ◦ C. It is also found that the PWP was decreased with increasing molecular weight. This is because that larger PEG additive in the CA/PEG membranes can be extracted readily at 75 ◦ C than at 25 ◦ C. The results also show that the increase in PWP did not sacrifice to much the retention for 66.7 kDa dextran, which dropped from 97% to 88%. 4. Conclusion

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Asymmetric cellulose acetate hollow fiber membranes were prepared by dry/wet spinning process from dope composed of cellulose acetate, N,N-dimethylformamide, and polyethylene glycol. The effect of the molecular weight and the content of PEG in the dope combining with external coagulant temperature on the morphology and performance were investigated. The addition of PEG can suppress the formation of macrovoids. These phenomena became obvious with increasing content and molecular weight of PEG. The PWP increased with the PEG content and decreased with the increase of the molecular weight of PEG. In addition, higher coagulant temperature also increased the PWP and decreased the retention of membranes. Combining simultaneously the addition of PEG and higher coagulant temperature, the resulting CA hollow fiber membranes would improve greatly the permeation performance that would effectively remove the toxic substances such as ␤2 -microglobulin from the blood.

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