Effect of Molecular Weight and Concentration of P

8/10/2019 Effect of Molecular Weight and Concentration of P

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Separation and Purification Technology 57 (2007) 209219

Effect of molecular weight and concentration of PEG additiveson 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

aDepartment of Materials and Fibers, Nanya Institute of Technology, Chung-Li, Tao-Yuan 320, Taiwan, ROCbDepartment of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC

cDepartment 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 (47 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; Drywet 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 fresh-

water.Furthermore, CAhollow fibermembranesare 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 oneof themost 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).

cal strength, and hemocompatibility. Because of CA hollow

fiber membranes have an excellent performance in the remo-

val 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 hol-

low fiber membranes have demonstrated insufficient removal

of lower molecular weigh proteins such as 2-microglobulin

(2-MG, 11,800Da)which cancause 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 thepatient duringblood

purification therapy and these factors make serious complica-

tions for chronic renal failure patients who require continuous

and long time treatment[35].Therefore, many researchers or

1383-5866/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.seppur.2007.04.005
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.seppur.2007.04.005http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.seppur.2007.04.005mailto:[email protected]


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210 W.-L. Chou et al. / Separation and Purification Technology 57 (2007) 209219

companies have pay attention to improve its performance by

added additive or plasticizer[2,68].

Cellulose acetate hollow fibers are produced by the drywet

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. Fur-

thermore, the dry/wet spinning is a complex process since it

involves many process variables, such as the dope extrusion rate

[911],the flow rate of core liquid[9,12,13],the composition

and temperature of the core liquid and coagulant[12,1418],

post-drawing and on-line drawing[19,20],the length of the air

gap[9,10,13,2126],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 per-meation 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 fabri-

cation. It is due to the fact that it is difficult to simulate the

hollow-fiber spinning process by adopting the process condi-

tions developed for asymmetric flat membranes. In addition,

Merrill et al.[29]reported that added PEG additive in the poly-

mer solution or grafted PEG on the polymer surface would

improve anticoagulation.

In order to suppress macrovoids in the hollow fiber mem-

branes, Liu et al. added high concentration of PEG400 to thedope of polyethersulfone (PES) and using PEG400 aqueous

solution as the coagulant to control the morphology of the resul-

tinghollow fiber[30]. Lietal.employeda co-extrusion spinneret

to prepare dual-layer Matrimid/PES hollow fibers [31].They

also added high concentration (66%) of PEG (MW 100kDa) 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 mole-

cular weightproteins such as2-microglobulin,CA asymmetric

hollow fiber membranes were prepared from polymer solutions

composing of CA/PEG/DMF via drywet spinning method. By

varying thecoagulant temperature and themolecularweightand

content of PEG, themorphological andtransport properties weresystemically investigated. The method of this research not only

added additive to the dope, but also varied the coagulant tempe-

rature. In sodoing,the permeation performancecanbe 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.

Dextranwith molecularweightsof10.2,35.0, and66.7 kDawere

purchasedfrom Sigma,USAand storedat 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

ina vacuumoven at roomtemperature for 2 h and extrudedunder

100 kPa through a hollow fiber spinneret with an inner diameter

of 0.4 mm and an outer diameter of 0.6mm. Spinning of the

CA hollow fiber was based on the drywet technique. Both the

core liquid and the external coagulant were pure water, and theexternal 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 thefibers 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 inTable 1.

2.3. Morphology studies

In order to examine the morphology of hollow fiber mem-

branes, dried samples of hollow fibers were broken in liquid

Table 1

Process parameters and spinning conditions

Process parameters/spinning

conditions

Value

1. Polymer composition CA/DMF/PEG

2. PEG molecular weight (Dalton) 1 k, 10k, 40kDa

3. PEG concentration (PEG vs. CA by

weight)

7%, 14%, 21%

4. CA concentration (by weight) 25%5. Spinneret temperature (C) Room temperature

6. Dope extrusion rate (g/min) 11

7. Spinneret OD/ID 0.6 mm/0.4 mm

8. Bore liquid H2O

9. Bore liquid flow rate (ml/min) 7.6

10. Dope pressure (kPa) 100

11. Air-gap distance (cm) 20

12. Take-up velocity (m/min) 35

13. Coagulant composition Tap water

14. Coagulant temperature (C) 2570

15. Drying procedure 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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W.-L. Chou et al. / Separation and Purification Technology 57 (2007) 209219 211

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 per-

meability (PWP) and retention of dextran for CA hollow fiber

was shown in our previous work[20].Hollow fibers were tes-

ted in bundles of 10 fibers of 15 cm in length. Each end of the

module was a piece of Teflon tubing, 6.35mm (1/4 in.) in dia-

meter 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 25ml/min with a metering pump

(RH1CKC, Fluid Metering, Inc., USA) through the lumen of thehollow fibers, and thepermeate was collected from theshell 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 the2-MG(MW: 11,800), pepsin(MW: 35,000) and

bovine serum albumin (MW: 68,000) protein. The dextran solu-

tions 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 absor-

bance at 490 nm was determined using an ultraviolet-visible

spectrophotometer (CE7200, Aquarius, England). The PWP, or

the ultrafiltration rate (UFR), was calculated by the followingequation:

PWP =Q

AP=

Q

nDiLP(1)

whereQis the volumetric flow rate (L/h),Ais the inner surface

area of the hollow fiber membrane (m2),n is the numbers of

fibers,Diis the inner diameter of hollow fiber,Lis the length of

the hollow fiber and Pis the applied pressure of ultrafiltration

experiment. The retentionRwas 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 eachhollow-fiber sample.

3. Result and discussion

3.1. The effect of molecular weight and content of PEG on

morphology

Fig. 1shows the SEM photos of CA and CA/PEG hollow

fiber membranes via drywet spinning under the same spin-

ning conditions. Themolecular weight andcontent of PEGwere

40 kDa and 21 wt%-CA, respectively. The CA membrane in

Fig.1(a) exhibits anasymmetricalstructure consisting of a densetop-layeranda poroussub-layerwith droplet morphologiesnear

the inner edge, whereas the CA/PEGmembrane 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 mole-

cular 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 contentand 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 andnonsolvent, thus higherPEGcontent ledto 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 instanta-

neous 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[3639].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 suppres-

sed by adding organic acids, because the acids form acidbase

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 ultra-

filtration and microfiltration membranes[29].Based on those

SEM photos in theFig. 2,our results are in accordance with

their reports.

The effect of molecular weight of PEG (1k, 10k and 40kDa)

on the morphologies was also investigated, as shown inFig. 3.

Fig. 3indicates that the droplet macrovoids was obviously sup-

pressedashighermolecular weightof PEGwas 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, whe-

reasFig. 3(c) and (d) (10 kDa PEG was added) andFig. 3(e)

and (f) (40 kDa PEG was added) show that macrovoids were

inhibited. The difference betweenFig. 3(c)(f) was only in the

thickness, and the thickness of membrane increased with themolecularweight (Fig.3(f)).The effect of thecontentsand mole-

cular weights of PEG on the thickness of membranes were also

exhibited in theTable 2.

Table 2lists the dimensions of the resulting CA/PEG hol-

low 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 (1kDa); (c and d) PEG (10kDa) and (e and f) PEG (40 kDa).

molecular weight, thus low molecular weight PEG can be easily

washed outquickly by water during the formationof 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 thick-

nesses is not varied at low concentration and low molecular

weight of PVP, but the top layer thickness changes distinctly at

high concentration andhigh molecularweightof PVP. Themain

reason is caused by residual content increase with increasing

concentration and molecular weight of PEG during the mem-

brane formation. In summary, asmore PEGis added, thenumber

of macrovoidsgradually 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 inFigs. 2 and 3andTable 2.

Fig. 4shows that the CA hollow fiber membranes with or

without PEG additive exhibited a dense and sponge-like struc-

ture.Fig. 4(b)(d) clearly show that CA/PEG membranes were

denser than original CA membrane inFig. 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) dry(m) wet(m)

0 473.5 (5.1) 350.0 (4.3) 61.6 (2.0) 81.3 (2.7)

7 1k 444.2 (3.3) 315.8 (2.4) 64.2 (2.1) 83.7 (2.5)

7 10k 416.7 (5.1) 293.3 (4.8) 70.0 (4.1) 86.3 (3.3)

7 40k 449.0 (4.9) 300.0 (2.3) 74.5 (3.4) 94.0 (2.6)

14 1k 475.2 (6.2) 336.6 (5.1) 69.3 (2.6) 89.3 (2.3)14 10k 487.9 (7.3) 330.3 (4.9) 78.8 (3.7) 94.3 (3.1)

14 40k 500.0 (4.6) 329.6 (6.3) 85.2 (3.5) 100.3 (2.8)

21 1k 433.9 (3.3) 283.5 (6.5) 73.2 (2.1) 90.3 (2.3)

21 10k 512.0 (3.8) 341.2 (4.0) 83.5 (2.2) 96.9 (2.9)

21 40k 515.1 (4.3) 322.1 (5.3) 95.5 (2.2) 110.7 (3.7)

Note: The numbers in the brackets are the standard deviation (n = 46).

mer concentration (CA and PEG). As shown in Fig. 4, the

porosity of the membrane decreased with the increase of mole-

cular weight of PEG. In general, the radius of gyration (Rg) of

PEG chains increases with the molecular weight[47]:

Rg = 0.0215M0.583w (nm) (3)

Smaller PEG molecules can quickly diffuse out to the coagu-

lant during the drywet spinning process which would lead to

a looser structure, while larger PEG molecules were more dif-

ficult to be removed which would retard the exchanging rate of

solvent and nonsolvent and result in a denser structure. Further-

more, 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 (1kDa); (c) PEG (10kDa) 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. 57show the SEM micrographs of the inner skin, outer

skin andcross-sectionof theCA andCA/PEG hollow fibers spun

at various coagulant temperatures, respectively.Fig. 5depicts

the inner surface of various contents of PEG with a molecular

weight of 10 kDa. Those SEM images show that the inner sur-

face was generally smooth except the ripple of water in various

contents of PEGby magnification 1k50k 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 imme-

diately 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 fric-

tion exerted by the bore liquid to the nascent hollow fiber led to

the ripples at the inner surface.

Fig. 6depicts the coagulant temperature on the morphology

of outer surface of the both pure CA and CA/PEG blended hol-

low 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%, 10kDa): (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, rai-

sing the coagulant temperature could result in porous surface

due to faster phase separation at higher temperature. In general,

macrovoid formation occurs under rapid precipitation condi-

tions 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 mole-

cules 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 hol-

low fiber membrane was immersed in the coagulation bath of

higher temperature. The outer skin layer of the resulted hol-

low fiber membranes would have less pore density but higher

pore sizes when coagulated at higher temperature. Those SEM

images inFig. 6were 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 thanFig. 6(a) under

a magnification of 50k times, whereasFig. 6(c)(e) show the

CA/PEG hollow fiber membranes whose spun at various coagu-lant 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 tem-

perature wouldbe limited to changing or improvingmorphology

of membranes, but combining these two factors, themorphology

of membranes would be controlled more precisely.

Fig. 7shows the cross-section of the CA/PEG hollow fibers.

InFig. 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 theinnersurface.When blending with 14%

PEG, there was no macrovoids observable when coagulating at

25

C, as shown inFig. 7(b). Macrovoids increased with the

coagulant temperature, as shown inFig. 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 tempe-

rature would strongly affect the permeation performance of CA

hollow fibermembranes[18]. Basedontheseresults, theeffectof

PEG additive combining with spinning parameters were furtherinvestigated 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 theretention(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 mem-

brane 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) PEG (MW) PWP (L/m2 hatm) Retention (dextran MW)

10.2k 35k 66.7k

0 Pure CA

7% 1k 5.6 89.5 96.2 99.5

7% 10k 4.4 88.8 94.7 97.47% 40k 3.9 90.3 95.2 97.7

14% 1k 12.4 85.8 94.6 95.7

14% 10k 10.1 87.3 93.5 94.5

14% 40k 8.6 89.3 94.4 95.3

21% 1k 18.4 83.0 89.3 95.1

21% 10k 15.9 84.1 90.5 94.5

21% 40k 14.0 85.9 90.4 95.2

concentration of PEG in the dope.Table 3shows that the PWP

andR also depended on the molecular weight and concentra-

tion 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 insub-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 coa-

gulated 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 coagu-

lation occurs slowly when the polymer solution is brought into

contact with water, and the resulting membranes have a sponge-

like 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 membranescannotdissolveallPEG atonce.

When thenascenthollow fiber plunged into thecoagulationbath,

a great quantity of water would replace DMF in the nascent hol-

low fiber anddissolve part of PEGon theoutersurface of nascent

hollow fiber,hencethe outer skin exhibited more micropores and

rough surface than the inner skin.

Table 3also shows that PWP increases with increasing PEG

content and decreased with increasing molecular weight. It

seems that the PEG content is more influential on the permea-

tion performancethan themolecular weightof PEG. HigherPEG

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 beless soluble and more difficult to effuse out than a lower mole-

cular 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 membraneswere mainly affec-

ted 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 18.4L/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 PEGmay beentrappedin theporesandthe entrapping amount

depends on themolecular weightof PEG. When thehollow 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 hatm) Retention (dextran MW)

10.2k 35k 66.7k

Pure CA 25

50 8.4 87.6 93.8 97.2

75 15.6 78.7 88.2 93.1

CA + PEG (1k, 14%) 25 12.4 85.8 94.6 95.7

50 25.8 83.5 90.2 93.1

75 45.9 72.3 85.3 90.5

CA + PEG (10k, 14%) 25 10.1 87.3 93.5 94.5

50 30.1 75.3 84.6 92.4

75 55.3 59.3 74.9 90.2

CA + PEG (40k, 14%) 25 8.6 89.3 94.4 95.3

50 35.6 74.9 80.8 90.6

75 62.5 41.9 65.3 88.5


11/11


ved by using higher coagulant temperature combining with the

addition of PEG additive.Table 4illustrates the effect of the

external coagulant temperature on the performance of. CA/PEG

hollow fiber membranes. The PWP was progressively increased

to 62.5L/m2 h atm at the highest coagulant temperature (75C),

which was about 47 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 66.7kDa dextran, which dropped from

97% to 88%.

4. Conclusion

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 tempe-rature 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 coagulanttemperature alsoincreased

the PWP and decreased the retention of membranes. Combi-

ning simultaneously the addition of PEG and higher coagulant

temperature, the resulting CA hollow fiber membranes would

improve greatly the permeation performance that would effecti-

vely remove the toxic substances such as2-microglobulin from

the blood.

References

[1] G.S. Loeb, S. Sourirajan, Adv. Chem. Ser. 38 (1963) 117.

[2] M.J. Kell, R.D. Mahoney, GB Pat., 2,000,722 (1979).

[3] R.E. Kesting, Synthetic Polymeric MembranesA Structure Perspective,

Wiley, New York, 1985.

[4] T. Tsuruta, T. Hayashi, K. Kataoka, K. Ishihara, Y. Kimura (Eds.), Biome-

dical Applications of Polymeric Materials, CRC Press, Boca Raton, FL,

1993.

[5] R. Deppisch, M. Storr, R. Buck, H. Gohl, Sep. Purif. Technol. 14 (1998)

241.

[6] D. Gustav, S. Lothar, A. Wolfgang, US Pat. 5,403,485 (1995).

[7] Teijin Ltd., EP 0,697,242 (1996).

[8] J.J. Qin, Y. Li, L.S. Lee, H. Lee, J. Membr. Sci. 218 (2003) 173.

[9] S.A. Mckelvey, D.T. Clausi, W.J. Koros, J. Membr. Sci. 124 (1997) 223.

[10] T. Liu, D. Zhang, S. Xu, S. Sourirajan, Sep. Sci. Technol. 27 (1992) 161.

[11] J.J. Qin, T.S. Chung, J. Membr. Sci. 157 (1999) 35.

[12] T.S. Chung, E.R. Katchniski, J. Appl. Polym. Sci. 65 (1997) 1555.

[13] T.S. Chung, S.K. Teoh, X. Hu, J. Membr. Sci. 133 (1998) 161.

[14] T.S. Chung, E.R. Kafchinski, P. Foley, J. Membr. Sci. 75 (1992) 181.

[15] M. Henmi, T. Yoshioka, J. Membr. Sci. 85 (1993) 129.

[16] T.S. Chung, E.R. Kafchinski, R. Vora, J. Membr. Sci. 88 (1994) 21.[17] W.L. Chou, M.C. Yang, Polym. Adv. Technol. 16 (2005) 1.

[18] W.L. Chou, D.G. Yu, M.C. Yang, J. Polym. Res. 12 (2005) 219.

[19] W.L. Chou, M.C. Yang, J. Membr. Sci. 250 (2005) 259.

[20] M.C. Yang, M.T. Chou, J. Membr. Sci. 116 (1996) 279.

[21] P. Aptel, N. Abidine, F. Ivaldi, J.P. Lafaille, J. Membr. Sci. 22 (1985)

199.

[22] X. Miao, S. Sourirajan, H. Zhang, W.W.Y. Lau,Sep. Sci.Technol.31 (1996)

141.

[23] G.C. East, J.E. McIntyre, V. Rogers, S.C. Senn, Proceedings of the Fourth

BOC Priestly Conference, Royal Society of Chemistry, London, 1986, p.

63.

[24] H. Kim, Y.I. Park, J. Jagel, K.H. Lee, J. Appl. Polym. Sci. 57 (1995) 1637.

[25] H.A. Tsai, D.H. Huang, S.C. Fan, Y.C. Wang, C.L. Li, K.R. Lee, J.Y. Lai,

J. Membr. Sci. 198 (2002) 245.

[26] M. Khayet, Chem. Eng. Sci. 58 (2003) 3091.[27] Z.L. Xu, F.A. Qusay, J. Membr. Sci. 233 (2004) 101.

[28] J.H. Kim, K.H. Lee, J. Membr. Sci. 138 (1998) 153.

[29] E.W. Merrill, S. Wan, E.W. Salzman, Trans. Am. Soc. Artif. Intern. Organs

20 (1986) 1517.

[30] Y. Liu, G.H. Koops, H. Strathmann, J. Membr. Sci. 223 (2003) 187.

[31] D. Li, T.S. Chung, R. Wang, J. Membr. Sci. 243 (2004) 155.

[32] H. de Balmann, V. Sanchez, Int. Chem. Eng. 32 (1992) 665.

[33] M. Dubois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Nature 168

(1951) 167.

[34] M. Dubois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Anal.Chem.

28 (1956) 350.

[35] Y.S. Park, J. Won, Y.S. Kang, Langmuir 16 (2000) 9662.

[36] C.A. Smolders, A.J. Reuvers, R.M. Boom, I.M. Wienk, J. Membr. Sci. 73

(1992) 259.

[37] R.M.Boom,I.M.Wienk,T. vanden Boomgaard, C.A.Smolders, J. Membr.

Sci. 73 (1992) 277.

[38] J.Y. Lai, F.C. Lin, C.C. Wang, D.M. Wang, J. Membr. Sci. 118 (1996) 49.

[39] K. Kimmerle, H. Strathmann, Desalination 79 (1990) 283.

[40] M.J. Han, Desalination 121 (1999) 31.

[41] I.M. Wienk,R.M. Boom, M.A.M. Beerlage,A.M.W. Bulte, C.A.Smolders,

H. Strathmann, J. Membr. Sci. 113 (1996) 361.

[42] Z.L. Xu, T.S. Chung, Y. Huang, J. Appl. Polym. Sci. 74 (1999) 2220.

[43] Z.L. Xu, F.A. Qusay, J. Appl. Polym. Sci. 91 (2004) 3398.

[44] M.J. Han, S.T. Nam, J. Membr. Sci. 202 (2002) 55.

[45] L.Y. Lafreniere, F.D.F. Talbot, T. Matsuura, S. Sourirajan, Ind. Eng. Chem.

Res. 26 (1987) 2385.

[46] B. Jung, J.K. Yoon, B. Kim, H.W. Rhee, J. Membr. Sci. 243 (2004) 45.

[47] K. Devanand, J.C. Selser, Macromolecules 24 (1991) 5943.

[48] H. de Balmann, P. Aimar, V. Sanchez, J. Membr. Sci. 45 (1989) 17.

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Effect of Molecular Weight and Concentration of P