Effect of nonsolvents on properties of spinning solutions and polyethersulfone hollow fiber...

Effect of nonsolvents on properties of spinning solutions andpolyethersulfone hollow ®ber ultra®ltration membranes

B. Torrestiana-Sancheza,*, R.I. Ortiz-Basurtoa, E. Brito-De La Fuenteb

aChemical and Biochemical Engineering Department, Technological Institute of Veracruz, Calle 9 No. 50-2, Costa Verde,

Boca del Rio, 94294 Veracruz, MexicobFood Science and Biotechnology Department, Chemistry Faculty `̀ E'', National Autonomous University of Mexico,

UNAM, 04510 Mexico DF, Mexico

Accepted 22 April 1998

Abstract

The relationship among the presence of nonsolvent additives, the rheological behavior of spinning solutions and properties

of hollow ®ber membranes was studied. The additives tested were water, polyvinylpyrrolidone (PVP) and polyethylene glycol

(PEG), and the base mixture was polyethersulfone/N-methyl-2-pyrrolidone (PES±NMP). In addition the effect of combining

water and PVP or PEG was also studied. Membranes were prepared using a spinneret having two concentric ori®ces. The

internal coagulant used as well as the nonsolvent from the coagulation bath were both water at 288C and 308C, respectively.

Rheological properties of polymer solutions were evaluated using a rheometer Haake RV 20. Changes on composition of

spin-solutions were also evaluated in terms of membrane water permeability, solute rejection and membrane structure

observed using scanning electron microscopy (SEM). Experimental results from this work showed that spinning solutions

containing any of the three additives behave as Newtonian ¯uids in the range of shearing rates tested. The addition of

water, PVP or PEG to the base PES±NMP solution increased its viscosity and this effect was independent of the type of

additive used. A direct relation between viscosity of casting solutions and membrane thickness was found. However,

rheological properties (viscosity and normal stress difference) could not be used to explain differences on membrane

water ¯ux (MWF) when using different additives at the same concentration. The addition of any of the three additives

generally increased MWF. The extent of this increment seemed to be more related to changes on membrane porosity than

changes on pore sizes induced by the nature and concentration of the additive used. # 1999 Elsevier Science B.V. All rights

reserved.

Keywords: Fiber membranes; Membrane preparation and structure; Ultra®ltration; Nonsolvent effects; Rheology of spinning

solutions

1. Introduction

One important goal in membrane technology is to

control membrane structure and thus membrane per-

formance. This objective is not easy to achieve

Journal of Membrane Science 152 (1999) 19±28

*Corresponding author. Fax: +52-2922-3814; e-mail: btorres-

[email protected]

0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 3 7 6 - 7 3 8 8 ( 9 8 ) 0 0 1 7 2 - 0

because membrane structure and performance depend

upon different factors like polymer choice, solvent and

nonsolvent choice, composition and temperature of

coagulant and casting solution among others. Further-

more, by changing one or more of these variables,

which are dependent on each other, membrane struc-

ture may be affected quite signi®cantly.

For example, in the preparation of polysulfone

membranes, polymeric additives like polyvinylpyrro-

lidone (PVP) have been used to increase the casting

solution viscosity and improve membrane perfor-

mance. Several authors [1±4] have reported that add-

ing a second polymer, like PVP to solutions of

polysulfone and polyethersulfone (PES), produces

membranes with higher porosity, well-interconnected

pores and surface properties that are different from the

properties of the pure membrane forming polymer.

These differences could not be understood from the

theories existing for ternary forming systems.

Recently, Boom [5] proposed a model to explain

the formation of the characteristic structures of mem-

branes from quaternary systems like PES/PVP/NMP/

water. Boom regarded this system as a quasi-ternary

system and used `̀ virtual'' binodals to describe the

strong reduction of demixing gaps observed for dif-

ferent PES/PVP ratios. This situation was proposed to

be only valid for a very short timescale, during which

the diffusion of solvent and nonsolvent governs the

membrane formation process. On this scale, at the

beginning of the demixing process, the inter diffusion

of the two polymers relative to each other is consid-

ered negligible. But as the membrane formation pro-

cess proceeds further, in a long timescale, the diffusion

of the two polymers with respect to each other

becomes more important. This model was also used

by Wienk et al. [6] to explain the role of low weight

molecular additives, such as dicarboxylic acid on

polyimide/dimethylformamide solutions during form-

ation of ultra®ltration membranes. A similar situation

may be found for other nonsolvent additives such as

polyethylene glycol (PEG) and water.

Properties of hollow ®ber membranes including

structure formation are controlled not only by the

composition of the polymer±additives±solvent formu-

lation used, but also by the preparation method. In this

regard, there is still room for discussion concerning

the role of ¯ow properties (e.g. steady shear viscosity)

on the properties of UF asymmetric membranes.

Several reports on zero shear casting solution visc-

osities have been reported in literature [3,7,8]. Other

workers [9] have increased the viscosity of the ®nal

polymeric mixture to better control the spinning of

®ber membranes. From these studies, the consensus

seems to be that casting solution viscosity affects

membrane properties. However, most of these studies

have not quantitatively related viscosity results with

membrane performance. One thing might be clear in

literature; high solution viscosities are needed to

produce more mechanical resistant ®bers whereas

low viscosities are undesirable because they may

produce macrovoids [7,10]. Unfortunately, the com-

plete meaning of high or low viscosities is still not

clear.

For the type of additives, casting solutions and

procedures involved in the preparation of asymmetric

UF hollow ®ber membranes, we feel that important

questions still are left regarding their effect on spin-

ning conditions, and membrane performance charac-

teristics. Thus, the aim of this work is to gain insight

into the role of nonsolvent additives like PEG and

water on membrane formation and performance since

most of the work has been done with PVP. The strategy

to do so is to analyze more carefully the steady ¯ow

rheological properties of the polymer solution and

their impact on membrane structure and physical

properties. Also, elastic properties of the polymer

solution measured as the ®rst normal stress difference

(N1) and their relationship with membrane properties

are reported for the ®rst time to our knowledge.

2. Experimental

2.1. Materials

PES, with the commercial name of Radel A-200

kindly donated from Amoco, was used as membrane

material. Reagent grade N-methyl-2-pyrrolidone

(NMP) supplied by Aldrich was used as solvent.

The polymeric additives, PVP and PEG both having

molecular weights (MW) of 10 000 were from Sigma.

Deionized water was the low molecular weight

additive tested. Distilled water was used as the internal

coagulant as well as the nonsolvent from the coagula-

tion bath. As organic reference solutes for UF

experiments, bovine serum albumin (BSA), MW

20 B. Torrestiana-Sanchez et al. / Journal of Membrane Science 152 (1999) 19±28

64 000 and lysozyme, MW 13 000 both from Sigma

were used.

2.2. Design

The effect of type and additive concentration on

spinning conditions and membrane properties was

studied according to the experimental matrix shown

in Table 1. Experiments were also carried out combin-

ing water with PVP or PEG at the concentration ratios

summarized in Table 1. The degree of saturation of the

casting solution, de®ned as the ratio between the

amount of precipitant added to the system and that

causing phase separation [11], for every additive

concentration tested are shown in Table 2. The cloud

point value of solutions were determined visually.

2.3. Membrane preparation

The membranes were laboratory prepared using a

spinning line developed in our laboratory which is

shown schematically in Fig. 1. The polymer solution

was gas pumped (under nitrogen pressure) into a

coaxial conic bottom tube spinneret, while water,

the core coagulant liquid, was forced through the

inner tube. The ®ber extrusion pressure and the inter-

nal coagulant ¯ow rate were both set by considering

the viscosity of the polymeric mixture (Table 3). The

annular gap of the coaxial tube was of 0.85 mm. The

spinneret was maintained at a distance of 600 mm

from the coagulant bath. All the experiments were

conducted at room temperature (288C). The coagula-

tion bath was maintained at 308C while the core

Table 1

Experimental matrix followed to study the effect of the type and

concentration of nonsolvents on properties of the system and

ultrafiltration membranes

Type of additive Additive (%) PES (%) NMP (%)

H2O 5.0 20 75.0

7.5 20 72.5

PVP 5.0 20 75.0

10.0 20 70.0

PEG 5.0 20 75

10.0 20 70

H2O/PVP 5/5 20 70

5/10 20 65

H2O/PEG 5/5 20 70

5/10 20 65

Table 2

Degree of saturation (DS) of spinning solutions obtained at the

nonsolvent concentrations tested

Additive (%) Water PVP PEG

5 0.53 0.12 0.26

7.5 0.79 ± ±

10 ± 0.23 0.51

Cloud point (%) 9.5 43 19.5

DS�grams of additive used/grams of additive causing phase

separation.

Fig. 1. Schematics of the experimental set-up used to prepare

hollow fibers: (1) nitrogen tank, (2) polymer solution reservoir, (3)

spinneret, (4) rotameter, (5) internal coagulant reservoir, and (6)

coagulation bath.

Table 3

Viscosity range of spinning solutions and the corresponding range

of operating conditions used during fiber preparation

Viscosity range

(Pa s)

Fiber extrusion

pressure range

(kPa)

Internal coagulant

flow rate range

(ml/min)

1.0±1.9 5.5±13.1 13.5

2.0±3.9 13.8±26.9 13.5±18.0

4.0±4.5 27.5±41.4 13.5±24.0

B. Torrestiana-Sanchez et al. / Journal of Membrane Science 152 (1999) 19±28 21

coagulant liquid at 288C. The hollow ®bers formed

were kept in the coagulant bath for 24 h. After this

period, the ®bers were washed for two periods of

24 h each. For the ®rst period into fresh water added

with 4000 ppm of NaOCl, and for the second into

water added with 30% of glycerol, both periods

at 708C. Finally, the ®bers were dried at room

temperature.

2.4. Fiber characterization

Inner and outer diameters of hollow ®bers were

estimated by means of both an optical microscope and

a micrometer. Membrane morphology was observed

by using a scanning electron microscope (SEM).

Cross-sections of hollow ®bers for the SEM were

prepared by breaking the ®bers at the temperature

of liquid nitrogen. A test system similar to that

reported by Liu et al. [12] was used to evaluate

membrane performance. Hollow ®ber bundles were

prepared collecting three pieces of 40 cm length and

covering the surface of the two ends with epoxy glue.

Each end was then inserted into a 0.63 cm diameter

polypropylene tube of 5 cm length.

Membrane performance measurements were car-

ried out at an average transmembrane pressure of

68.95 kPa. Membrane rejection properties were eval-

uated using 1 and 0.2 g/l of BSA and lysozyme,

respectively, both dissolved into 0.2 M sodium chlor-

ide solution, as reference solutes.

2.5. Rheological measurements

The steady shear material functions, viscosity (�)

and ®rst normal stress difference (N1), were evaluated

using a rotational rheometer (Haake CV-20N) pro-

vided with a cone and plate geometry (PK20-4). All

measurements were performed at 308C in a range of

shear rate from 1 to 400 sÿ1.

3. Results and discussion

3.1. Membrane formation

Typical results of the steady shear viscosity function

are summarized in Table 4Table 5. The viscosity of

the base casting solution (i.e., PES 20%±NMP 80%)

was independent of the rate of deformation. In

other words, the base solution behaves as a Newtonian

¯uid at least in the range of shearing rates studied here.

The addition of PVP, PEG or water to the base PES±

NMP casting solution increases its viscosity as shown

in Table 4. It can be seen from this table that for a

5 wt% of additive the viscosity increases up to about

50% and this effect is independent of the type of

Table 4

Effect of the type and additive concentration on viscosity of the spin-solutions and membrane properties

Spin-solution composition

(%)

� (Pa s) Di/Do �(mm)

MWPa

(l/m2 h)

BSA rejection

(%)

Lysozyme

rejection (%)

PES/NMP

20/80 0.70 0.96 0.02 18�9 98�2 68�15

PES/H2O/NMP

20/5/75 1.25 0.66 0.17 34�18 96�3 81�11

20/7.5/72.5 1.76 0.52 0.31 50�13 99�1 88�5

PES/PVP/NMP

20/5/75 1.44 0.81 0.09 61�20 97�2 66�10

20/10/70 2.75 0.52 0.36 9�1 98�2 49�14

PES/PEG/NMP

20/5/75 1.46 0.78 0.11 254�99 97�2 45�13

20/10/70 2.75 0.59 0.27 117�11 99�1 62�13

Di/Do: membrane diameter ratio; �: membrane thickness.aMWP: membrane water performance, measured at 68.95 kPa.


additive. The addition of 10% of any of the additives

studied here had a more signi®cant effect on the

casting solution viscosity, increasing it up to more

than 200% (see Table 4). In addition, the presence of

PVP, PEG or water does not change the Newtonian

characteristics of the base casting solution as seen in

Fig. 2. When combining water with either PVP or

PEG, the increase in viscosity is even more consider-

able, up to 400% as summarized in Table 5. The

results of this study regarding the PES±PVP±NMP

system are in agreement with those reported by Tam

et al. [7].

The viscosity of the casting solution also affects

physical membrane characteristics. In this study, visc-

osity effects on membrane thickness were quite impor-

tant as summarized in Tables 4 and 5. For the same

operational conditions (i.e., pressure in the spinneret) as

the viscosity increases, membrane thickness increases.

Membranes formed using only the base casting solution

had a maximum thickness of 0.02 mm. On the other

hand, for the most viscous solutions, the maximum

thickness was 0.44 mm, this last means about 400%

increase in membrane thickness which is similar to the

increase in polymer casting solutionviscosity. It must be

Table 5

Effect of combining the presence of water with either PVP or PEG on the viscosity of the polymer solution and membrane properties

Spin-solution composition

(%)

�(Pa s)

Di/Do �(mm)

MWPa

(l/m2 h)

BSA rejection

(%)

Lysozyme

rejection (%)

PES/H2O/PVP/NMP

20/5/5/70 2.04 0.61 0.28 13�3 91�5 52�10

20/5/10/65 4.00 0.55 0.40 29�8 97�2 69�12

PES/H2O/PEG/NMP

20/5/5/70 2.46 0.55 0.30 40�6 99.9 57�7

20/5/10/65 4.12 0.46 0.44 68�6 100 73�5

Di/Do: membrane diameter ratio; �: membrane thickness.aMWP: membrane water performance, measured at 68.95 kPa.

Fig. 2. Steady shear viscosity (�) of spinning solutions having the following compositions: (a) PES 20%/PEG 5%/NMP 75%, and (b) PES

20%/PVP 5%, NMP 75%.


mentioned here that an adequate hollow ®ber diameter

ratio, Di/Do, close to 0.5 has been suggested to obtain

good mechanical strength properties [10]. Furthermore,

it has been reported [13] that the Di/Do ratio is strongly

affected by the type of additive used. In this study, our

results summarized in Tables 4 and 5 suggest that when

using any of the additives at 10%, the Di/Do ratio is very

close to 0.5 and this corresponds to the higher viscosity

solutions which are independent of the type of additive

used.

The measurement of the primary normal stress

difference, N1, in steady shear ¯ow is a common

method to asses ¯uid's elasticity. Trying to explain

the signi®cant increase in membrane water ¯ux

(MWF) observed when using PEG (see Table 4), in

this study we performed N1 measurements on both

type of polymer solutions. Fig. 3 illustrates the experi-

mental results of N1 versus shear rate for two polymer

solutions. In this case both solutions were formulated

at the same concentration of PES and NMP, respec-

tively, one with PVP and the other with PEG, these two

last components also at the same concentration. We

®nd a similar level of elasticity for both ¯uids at

constant shear rate. These results suggest that ¯uid

elastic properties do not explain differences on MWF,

at least in the range of variables studied here.

It is well known that regarding the effect of polymer

concentration on N1, at constant shear stress and

molecular weight, the primary normal stress differ-

ence decreases as the polymer concentration increases

[14]. In this study both PEG and PVP have similar

molecular weights and were formulated at the same

concentration. This may explain the results of Fig. 3.

It is important tomentionhere thatelasticpropertiesmay

signi®cantly in¯uence ¯ow processes (e.g. tilted surface

troughentranceandexiteffects,®nalproductdiameterin

extrusionprocesses, etc.). Themagnitudeof theeffect of

elasticity depends on the ¯uid properties. In this study

not de®nitive conclusions can be drawn about the role of

elasticity on membrane formation because, when for-

mulated at equal additive concentration, all polymeric

solutions showed a similar level of elasticity. In order to

clarify the role of both viscous and elastic properties on

membrane formation process, a different experimental

design should be followed. This will be the subject of a

future communication.

3.2. Membrane performance

Membrane performance evaluated in terms of

MWF could not be related to the rheological proper-

ties of spinning solutions. This parameter was rather

related to the type and concentration of the additive

used. Experimental data from Table 4 show that the

addition of 5 wt% of any of the three additives to

casting solutions produced membranes with an impor-

tantly increased water performance. This effect was

stronger when using PEG than when using water or

PVP. Further increments on the additive concentration

increased MWF only in the case of water. In the case

of using 10 wt% of either PVP or PEG a signi®cant

reduction on MWF was observed. The last one was

more pronounced for PVP. Otherwise, combining the

presence of water with PVP or PEG on the spin-

solution, generally, tend to decrease MWF (Table 5).

The above MWF changes seem to be related not

only to differences on the surface porosity but also to

variations on the pore size. Experimental results

(Table 4) from rejection experiments show that the

addition of water produced membranes with higher

retentions, i.e., lower pore sizes. These results agree

with those reported by Tweddle et al. [2] and suggest

that increments on MWF are due to an increase in

surface porosity. The presence of PVP had two dif-

Fig. 3. Primary normal stress difference profiles of spinning

solutions with the following concentration of polymeric additives:

5 wt% PEG (*), and 5 wt% PVP (~).


ferent effects. For 5 wt%, solute retention was not

affected but for 10 wt% it was slightly lowered. These

results are in qualitative agreement with the ®ndings

of Tweddle et al. [2] and Lafreniere et al. [3] for ¯at

membranes prepared at similar PVP/PES ratios. From

MWF and rejection data (Table 4) it may be con-

cluded that PVP at 5 wt% increased porosity, but at

10 wt%, it rather decreased it.

The high MWF values obtained when using both 5

and 10 wt% PEG (Table 4) seem to be also due to

increments on porosity. Lysozyme retention was

slightly lowered for 5 wt% PEG while it remained

the same for 10 wt%. Differences on solute retention

induced by the presence of PEG or PVP on the spin-

solution were masked when combining water with

PVP or PEG. Similar protein retentions were observed

when adding 5 wt% of water to solutions containing

either 5 or 10 wt% of PVP or PEG (Table 5).

3.3. Membrane morphology

ItwasobservedusingSEMthat thepresenceof5 wt%

PEG induced changes on membrane structure from a

partial to a complete cross-sectioned ®nger like voids

(Fig. 4), typical of instantaneous demixing [15,16].

Otherwise, adding 5 wt% water or PVP leads to a

structure with macro voids somewhat deeper in the

membrane followed by a ®nger like structure formed

from the interphase of the coagulation bath. This

structure suggests a decreased rate of solvent±nonsol-

vent exchange before ®bers reach the coagulation

bath. For PVP, this phenomenon has been explained

[5,17] in terms of the formation of a denser skin which

seems to appear as a consequence of a faster interfacial

mass transfer in the ®rst stage (desolvation), of the

membrane formation process. For water a similar effect

may be occurred considering the fact that these

Fig. 4. Scanning electron micrographs of membrane cross-sections prepared from spinning solutions with: (a) no additive, (b) 5 wt% water,

(c) 5 wt% PEG, and (d) 5 wt% PVP.


membranes presented also increased retentions

(Table 4).

Using 10 wt% PEG or PVP induced the appearance

of skinned macro voids in the middle of the substruc-

ture (Fig. 5). However, when the nonsolvent used was

PVP, macro voids were surrounded by a thick dense

layer, while for PEG they were followed by a ®nger

like structure similar to that formed when using 5 wt%

PVP or water. The formation of skinned macro voids

may be related to a decrease on demixing rates due to

viscous effects [18]. Regarding the differences on the

surrounding structures, these may suggest that the

overall demixing rates for systems with PVP are even

lower than for those containing PEG. This result may

be explained by the fact that the degree of saturation of

solutions containing PEG is higher and therefore the

system is thermodynamically less stable than that

containing PVP at the same concentration (Table 2).

The decreased effect of viscosity on demixing rates is

more evident when adding 7.5 wt% water (Table 2). In

this case solutions have similar viscosities than those

containing 10 wt% PVP or PEG but they produced

structures typical of fast demixing (Fig. 5). Combin-

ing water with PVP or PEG induced the appearance of

a thick dense layer on all cases (Fig. 6) suggesting a

decrease on the overall rate of membrane formation.

This was probably associated to the increased visc-

osity obtained under these experimental conditions.

4. Conclusions

During membrane formation, as the viscosity of

spinning solutions increases, membrane thickness

increases. This effect was independent of the additive

used. Rheological properties (i.e., steady shear visc-

osity and ®rst normal stress differences) of spinning

Fig. 5. Scanning electron micrographs of membrane cross-sections prepared from polymer solutions having the following compositions: (a)

PES 20%/PEG 10%/NMP 70%, (b) PES 20%/PVP 10%/NMP 70%, (c) PES 20%/7.5% water/NMP 72.5%.


solutions could not be used to explain differences on

membrane water performance (MWF), when using the

additives studied here at the same concentration,

because all polymeric solutions showed a similar level

of both viscous and elastic properties. In order to

clarify the role of rheological properties on membrane

formation process, a different experimental design

should be followed. Differences on MWF were found

to be related mainly to increments on the skin porosity

induced by the nature and concentration of the addi-

tive used. In addition, the presence of any of the

nonsolvents used provoked changes on the internal

membrane structure which were apparently a result of

variations on demixing rates. Considering membrane

water performance, experimental data from this study

suggest that PEG promotes more convenient mem-

brane structures for ultra®ltration purposes than PVP

and water under the conditions tested.

Acknowledgements

The authors wish to acknowledge the ®nancial

support provided by COSNET-SEP (MeÂxico) and

CONACyT (MeÂxico). Mr. Luis Medina (FQ-UNAM,

MeÂxico) is also acknowledged for his assistance with

the rheological measurements.

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