Effect of wet and dry-jet wet spinning on the shea

Journal of Membrane Science 182 (2001) 57–75
Effect of wet and dry-jet wet spinning on the
shear-induced orientation during the formation of
ultrafiltration hollow fiber membranes
Jian-Jun Qin a,b , Juan Gu a , Tai-Shung Chung a,c,∗
a
Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
b Membrane Research Technology Singapore, Ltd, 10 Science Park Road #02–27, Singapore 117684, Singapore
c Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
Received 21 February 2000; received in revised form 28 July 2000; accepted 8 August 2000
Abstract
We have demonstrated the effect of wet and dry-jet wet spinning on the shear-induced orientation during hollow fiber
membrane formation by characterizing the permeability, separation performance and thermomechanical properties of hollow
fiber ultrafiltration membranes. Both wet-spun and dry-jet wet spun fibers were prepared by using the phase inversion process.
An air gap of 1 cm was chosen for the dry-jet wet spinning process in order to minimize gravity effect. To generalize our
conclusion, various hollow fiber UF membranes with different structures were prepared using six spinning dopes with different
kinds of polymers, solvents and additives under different shear rates. Experimental results show that pure water flux, coefficient
of thermal expansion (CTE) and elongation of the wet spun fibers are lower than that of the dry-jet spun fibers but separation
performance, storage modulus, loss modulus and tensile strength of the wet spun fibers are higher. The results indicate that
the wet spun fiber has smaller pore size and/or a denser skin than the dry-jet wet spun fiber. These results also confirm our
hypothesis that the molecular orientation induced at the outer skin of the nascent fiber by shear stress within the spinneret
can be frozen into the wet-spun fiber but relax in a small air gap region for the dry-jet wet-spun fiber. Experimental results
strongly indicate that an air gap of 1 cm does make significant impact to the membrane performance. This conclusion arises
from the fact that wet spun fibers has the greater shear-induced molecular orientation (from spinneret) than the dry-jet wet
spun fiber and there is strong molecular relaxation in air gap. A hollow fiber UF membrane with high flux of 1220 L/h m2 bar
can be prepared by the approach proposed in this paper. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Shear-induced orientation; Air gap effect; Ultrafiltration membranes; Membrane formation; Pore size
1. Introduction
It is well known that the preparation of hollow fiber
membranes involves more controlling parameters
than that of flat sheet membranes during membrane
formation. Such parameters include the structure
and dimension of the spinneret [1–6], the viscosity
∗ Corresponding author. Tel.: +65-874-6645; fax: +65-779-1936.
E-mail address: [email protected] (T.-S. Chung).
and spinnability of the dope [7–10], the temperature
difference between the dope and quench medium
[11–12], the dope extrusion rate [5,13–17], the flow
rate of the bore fluid [5,18–21], the composition
and temperature of both the bore fluid and external coagulant [19,22–27], the relative exchange rate
between the solvent and coagulant at the inner and
outer surfaces of the nascent fiber [28–30], the fiber
take-up speed [5,31–33], the length of the air gap
[5,9,15,19,21,26,27,34–36], etc. As a result, the for-
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 5 5 2 - 4
58
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
mer is much more complex than the later. However,
hollow fiber membrane configuration is the preferred
option in most membrane applications nowadays
because hollow fiber membranes have several advantages over flat sheet membranes [37,38]. From the
fundamental viewpoint, there is lack of systematic
studies on the effect of shear-induced orientation
coupling with air gap distance during hollow fiber
formation on UF membrane performance, thus this is
the subject of our focus in this paper.
There may be different stresses that induce molecular orientation during hollow fiber formation. When
a polymer dope is extruded through a spinneret with
a tube inserting in an orifice, shear stress will be produced within the thin annual [39,40]. The highest shear
stress is usually resulted at the walls of the spinneret
as shown in the work of Bird et al. [40] and Shilton
[41]. In addition, polymer macromolecules while exiting from the spinneret may experience die swell and
relax which will change their orientation immediately
if there is an air gap between the spinneret and the
external coagulation bath (this statement is more suitable at the outer surface of the nascent fiber) [17,38].
Furthermore, the elongation stress outside of the spinneret will increase with an increase in the air gap due
to the gravity of the dope [15,34,35] and the spin line
stress [42–44] will also enhance if the fiber draw ratio
is increased, resulting an increase in the molecular orientation. As a consequence, the various stresses may
dramatically affect the molecular orientation of polymer chains at the surface of the nascent fiber, eventually the stresses may couple and influence the skin
formation and separation performance as well as thermomechanical properties of final fibers, depending on
which stress is dominant.
Among the study of hollow fiber membrane formation by the dry-jet wet spinning, many researchers
[5,9,15,19,21,26,27,34–36] have investigated the effect of the air gap length on performance of final
membranes. For example, Liu et al. [15] reported
that the average size of pores on the polysulfone/polyethersulfone membrane surface in the bore
decreased with an increase in the length of the air
gap from 30 to 200 cm. Their explanation was that
the nascent fiber was stretched and elongated by its
own weight and the polymer aggregates moved closer
together and rearranged themselves into a state of
greater stability. Similarly, Miao et al. [34] demon-
strated that the PEG separation increased or the average pore size on the bore surface of the polyethersulfone hollow fiber UF membrane decreased when the
air gap length increased from 50 to 120 cm. Ekiner
and Vassilatos [9] studied the polyaramide hollow
fibers for hydrogen/methane separation by varying
the air gap length for more than 10 times and revealed that the selectivity of the membrane increased
but the flux decreased as the air gap length increased.
They explained their results by speculating that with
an increase in the air gap length a more defect-free
and perhaps thicker skin was formed due to the increase in amount of evaporated solvent. Chung and
Hu [35] investigated the effect of air gap distance
on morphology and properties of polyethersulfone
hollow fibers for gas separation. They reported that
an increase in air gap distance from 0 to 14.4 cm resulted in a significant decrease in permeance of the
membrane. They interpreted that the most likely reason was because the fibers spun from a high air gap
distance may have a greater orientation and tighter
molecular packing due to a high gravity-induced
elongational stress on the fibers than that of wet-spun
fiber.
In the case of shear-induced orientation during hollow fiber formation, Chung and co-workers
[6,17,38,45] have investigated the effect of shear
stress within a spinneret on morphology and properties of hollow fiber membranes for liquid and gas
separation. Their results suggested that the hollow
fiber membranes spun with enhanced shear had a
lower flux but a higher separation due to the greater
molecular orientation induced in the high-sheared
fibers. They had a hypothesis and believed that the
orientation induced at the outer surface of nascent
fiber by shear stress within the spinneret could be
frozen into the wet-spun fibers but might relax in the
air gap region if one used dry-jet wet spinning process. The latter statement on relaxation has not been
proven yet.
To our knowledge, very rare membrane scientists
paid attention to the effect of dope relaxation exiting
from the spinneret or what would happen if the air
gap length is very small such as about 1 cm compared
to 0 cm (wet spinning). Is it true for the above hypotheses given by Chung et al. [6,17,38,45]? The aim
of this study is trying to answer these questions. To
study this, for each particular polymer dope, we pur-
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
posely spun hollow fiber membranes by only changing
air gap (0 and 1 cm). In other words, we only fabricated wet spun (air gap of 0 cm) and dry-jet wet spun
(air gap of 1 cm) hollow fibers but maintained other
spinning parameters the same and then compared the
performance of final membranes between these two
different conditions. The air gap of 1 cm was purposely
chosen for the dry-jet wet spinning process in order
to minimize the complexity of gravity effect. For the
sake of validating the generality of our hypothesis, we
designed our experiments by utilizing different polymers (polyethersulfone and polysulfone), various solvents (NMP, DMAc, DMF) and varieties of additives
such as organic (diethylene glycol, 1,2-propanediol),
water solvable polymer (PVP40K) and strong precipitant (water) in the present work. These various dope
compositions provided us with the opportunity to investigate hollow fiber membranes with different morphology (typical sponge-like and finger-like structure).
Hollow fiber membranes not only with an outer skin
but also with double skins were studied in order to confirm that an inner dense skin does influence the data
analysis of molecular orientation on the outer surface
of hollow fibers. This work will not only characterize the hollow fiber membranes in terms of morphology, flux and separation but also try to relate the thermomechanical properties of fibers with the molecular
orientation.
2. Experimental
2.1. Materials
Polyethersulfone (PES) RADEL A-300 and
Polysulfone (PSf) P-1800 were purchased from
Amoco Performance Products Inc. Ohio, USA.
N-methyl-2-pyrrolidone (NMP, >99%), N,N-dimethylacetimide (DMAc, >99%), N,N-dimethylformamide,
diethylene glycol (DG) and 1,2-propanediol (1,2-PD)
were supplied by Merck. Three types of poly(vinyl
pyrrolidone) (PVP) from Acros Organics New Jersey
USA, PVP10K (average M.W. 10 000), PVP29K (average M.W. 29 000), PVP40K (average M.W. 40 000)
and bovine serum albumin (BSA) (M.W. 67 000)
from Fluka were used to characterize the separation
performance of fibers. Other organic solvents were
reagent grade and used as received.
59
2.2. Measurements of dope rheological
characteristics and preparation of hollow fiber UF
membranes
A homogeneous spinning solution was prepared by
the following procedure: first the solvent and additive were mixed well in a glass bottle, then the polymer dried in an oven was added into the mixture in
the bottle. Finally, the bottle containing the mixture
was rolled on a roller machine for one week at room
temperature.
In order to study the rheological characteristics of
the dope within the spinneret, an ARES Rheometric
Scientific rheometer was used to determine the shear
stress of the dope solution as a function of shear rate.
The experiment was carried out with 25 mm cone-plate
at 25◦ C and the steady-state shear was measured in the
range of 1–100 s−1 . The power law model [40,41,46]
was applied to fit these rheological data and a relationship between shear stress τ (N/m2 ) and shear rate
γ (s−1 ) was obtained as following
τ = m|γ |n
where n is the power law index and m is the rheological
constant.
The above equation was assumed to be applicable
to higher shear rates and to be able to describe the
rheological behaviour of the dope solution within the
spinneret during spinning. According to axial annular flow of a Power-Law fluid in Bird’s work [40,46],
the shear rates of the spinning dope near the external
wall of the annular region within the spinneret were
evaluated using the power law model. And then the
shear stresses induced were calculated by the above
equation. For Newtonian fluids or non-Newtonian fluids with n very close to 1, such as dopes 1, 2 and 5,
the errors created by the extrapolation of the obtained
rheological parameters to higher shear rates may be
small and acceptable. For dopes 3, 5 and 6 the extrapolation can only give us approximate values of shear
rates and stresses.
The process of hollow fiber spinning is schematically shown in Fig. 1 and its detail description was
given in our previous work [17]. All the nascent fibers
produced were spun without any extra drawing. As a
result, the effect of elongation stresses on fiber formation could be significantly minimized. Water is a
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J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 1. Schematic diagram of a hollow fiber spinning line.
powerful coagulant and was used as the external coagulant in order to yield an outer selective layer and
to freeze immediately the orientation induced by shear
stress within the spinneret into the solidified polymer at the outer surface when the polymer solution
emerges from the spinneret. In addition, a mixture of
high concentration solvent in water was purposely employed as the internal coagulant in order to create an
open porous inner surface without a dense skin and to
minimize the effect of the inner skin on fiber performance. To evaluate the effect of an inner skin on the
data analysis of fiber performance, we also spun hollow fiber membranes with double skins by applying
water as the bore fluid.
Table 1 lists the experimental parameters of spinning UF hollow fiber membranes and Table 2 summarizes the rheological behavior of the spinning dopes.
Table 3 shows dope flow rate, calculated shear rates,
shear stresses induced at the outer surface and inner
surface of nascent hollow fibers and residence time
in the air gap during spinning. The as-spun fibers
were rinsed in flowing water at room temperature
for 16 h to remove the residual solvents and organic
additives. After rinsing, the fibers were post-treated
by two methods. One was that they were dipped in
a 50 wt.% glycerol aqueous solution for 48 h and
dried in air at room temperature for making test modules. The other was that they were post-treated by so
called ethanol–hexane-air drying procedure [47] in
order to minimize the shrinkage effect by gradually
reducing the surface tension during the fiber drying
process for the study of thermomechanical properties
of fibers.
2.3. Morphology study of hollow fibers by scanning
electron microscope (SEM)
The rinsed wet fibers were immersed in liquid nitrogen and fractured to obtain tidy cross-section of fibers,
followed by the ethanol–hexane-air drying procedure
and then sputtered with gold by use of a BAL-TEC
Sputtering coater for SEM study. The fiber samples
were observed under PHILIPS XL 30 SEM to investigate the morphology.
2.4. Measurement of water flux and separation
performance of hollow fiber UF membranes
Our experiments were carried out in a cross-flow
filtration set-up. For the fibers with an outer skin, the
feed was pumped into the shell side of the module and
the permeate came out from the lumen of the fibers
and the details is introduced elsewhere [17]. For the
hollow fibers with double skins, the feed was pumped
into the lumen side of the fibers and the permeate
was collected at the bottom of U type modules. The
schematic diagram of the testing apparatus to measure
the permeability and separation performance of hollow fiber UF membranes was illustrated in Fig. 2. Ten
fibers with a length of 30 cm were assembled into a
test module. Three modules were tested for each sample and the average of their performance was reported.
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
61
Table 1
Experimental parameters of spinning hollow fiber UF membranesa
Dope solution composition
PES/NMP/DG
PES/DMAc/DG
PES/DMAc/H2 O
PSf/DMF/
PVP40K/1,2 PD
Dope flow rate (cc/min)
Bore fluid composition
Ratio of dope flow rate to bore fluid flow rate
Air gap distance (cm)
External coagulant
Spinneret and coagulant temperature (◦ C)
Dimensions of spinneret (mm)
The die length (L) to flow channel gap (1D) ratio
Room relative humidity (%)
3.8–4.5
86/14 NMP:H2 O
0.5
0 and 1b
Water
25
ID/OD 0.60/0.90
6.7
50–60
4.0
83/17 DMAc:H2 O
0.5
0 and 1b
Water
25
ID/OD 0.60/0.90
6.7
50–60
4.0
H2 O
0.5
0 and 1b
Water
25
ID/OD 0.60/0.90
6.7
50–60
4.0
H2 O
0.5
0 and 1b
Water
25
ID/OD 0.60/0.90
6.7
50–60
a PES, polyethersulfone; NMP, N-methyl-2-pyrrolidone; DG, diethylene glycol; DMAc, N, N-dimethylacetimide; DMF, N,
N-dimethylformamide; PVP40K, poly(vinyl pyrrolidone), M.W., 40 000; 1,2 PD, 1,2-propanediol.
b 0, wet spinning; 1, dry-jet wet spinning.
Table 2
Composition and rheological properties of spinning dopes
Dope ID
Dope composition
Power-law equation
τ = m|γ |n (N/m2 )
Index n
Rheological
constant m
1
2
3
4
5
6
PES/NMP/DG 13/45/42
PES/NMP/DG 18/42/40
PES/NMP/DG 21/40/39
PES/DMAc/DG 17/43/40
PES/DMAc/water 16/78/6
PSf/DMF/PVP40K/1,2 PD 15/67.5/15/2.5
τ
τ
τ
τ
τ
τ
0.9465
0.8974
0.8729
0.7485
0.9520
0.5996
3.2904
6.0445
10.090
2.6118
0.2754
3.6417
= 3.2904|γ |0.9465
= 6.0445|γ |0.8974
= 10.090|γ |0.8729
= 2.6118|γ |0.7485
= 0.2754|γ |0.9520
= 3.6417|γ |0.5996
Table 3
Dope flow rate, shear rate, shear stress induced at the outer surface of hollow fibers, fiber take-up speed and residence time in the air gap
during spinning
Dope ID Dope flow
rate (cc/min)
Shear rate (s−1 )
Shear stress (N/m2 )
At outer
wall
At outer
wall
At inner
wall
1
3.8
6875
7907
14100
16096
2
4.5
8272
9567
19820
22583
3
4.0
7424
8601
24131
27438
4
4.0
7817
9197
2142
2420
5
4.0
7219
8304
1298
1483
6
4.0
8513
10236
828
924
At inner
wall
Fiber ID Spun type Take-up
Residence time
speed (m/min) in the air gap (s)
1w
1dw
2w
2dw
3w
3dw
4w
4dw
5w
5dw
6w
6dw
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
9.22
11.2
12.1
13.2
7.18
8.13
7.60
8.63
8.40
10.5
9.34
10.2
0
0.054
0
0.045
0
0.074
0
0.070
0
0.057
0
0.059
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J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 2. Schematic diagram of measurement for water permeability and separation performance for hollow fiber UF membranes with double
skins.
Double distilled water was used to prepare the feed
solutions containing 200 ppm of the solute. The transmembrane pressure was average 1.0 bar and the feed
velocity was 1.0 m/s. The solute concentration in the
feed or in the solution of permeate was determined by
a TOC-5000A Analyser (SHIMADZU). The normalized pure water flux and the percent separation were
calculated by Eqs. (1) and (2) separately.
Flux =
Q
A × 1P
(1)
where Q is volume flow rate (L/h), A is membrane
area (m2 ), and 1P is transmembrane pressure (bar).
1 − Cp
× 100
(2)
Separation (%) =
Cf
In which, Cf and Cp represent the solute concentrations in the feed and permeate, respectively.
2.5. Study of thermomechanical properties of hollow
fibers
Thermomechanical properties of the as-spun fibers
were characterized in terms of coefficient of thermal
expansion (CTE), Tg , storage modulus and loss modulus. To measure CTE of hollow fiber membranes,
the dried sample was heated from room temperature
to 250◦ C at a heating rate of 5◦ C/min and the axial
dimension change of the hollow fiber versus temperature was observed using Thermal Instruments’
TMA 2940. The length of fiber was about 15 mm.
The CTE for all fibers was calculated in the range
of 50–100◦ C of temperature. To measure the storage
modulus and loss modulus, the hollow fiber sample
with the length of about 10 mm was heated from room
temperature to 250◦ C at a heating rate of 3◦ C/min
and a frequency of 10 Hz with Thermal Instruments’
DMA 2980. At least three fibers for each sample
were tested for both TMA and DMA to see the reproducibility. Tensile stress and elongation at break of
the fibers were measured by use of an Instron 5542
tensile test machine with a constant elongation velocity of 50 mm/min at room temperature (24 ± 1◦ C)
and relative humidity of 75 ± 2%. At least 10 fibers
for each sample were measured and the average data
were reported. The fiber thermomechanical properties were determined to gain indirect information
about the molecular orientation of the as-spun hollow
fibers.
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
63
Table 4
Bore fluid composition, dimension and morphology of hollow fibers
Dope ID
Bore fluid composition
Fiber ID
Spun type
i.d. (mm)
o.d. (mm)
Sub-structure
Skin type
1
NMP/H2 O 86/14
Outer dense skin
NMP/H2 O 86/14
Finger-like
Outer dense skin
4
DMAc /H2 O 83/17
Sponge-like
Outer dense skin
5
Water
Finger-like
Double dense skins
6
Water
0.90
0.85
0.86
0.70
0.96
0.90
1.10
0.96
1.20
1.10
1.30
1.24
Finger-like
3
0.60
0.55
0.53
0.43
0.58
0.56
0.75
0.70
0.85
0.80
0.96
0.87
Outer dense skin
NMP/H2 O 86/14
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Wet
Dry-jet
Finger-like
2
1w
1dw
2w
2dw
3w
3dw
4w
4dw
5w
5dw
6w
6dw
Sponge-like
Double dense skins
3. Results and discussion
3.1. Effect of wet and dry-jet wet spinning on the
morphology of hollow fiber UF membranes
The dimensions and morphology of wet and dryjet wet spun hollow fibers with different dopes are
summarized in Table 4. In all cases, the diameter of the
wet spun fiber is bigger than that of the corresponding
dry-jet wet spun fiber. Fig. 3 shows an example. It
is due to the fact that polymer macromolecules may
experience die swell when exiting from the spinneret
as shown in Fig. 4. In the case of wet-spun process, the
die swell may be immediately coagulated by water and
the orientation induced within the spinneret may be
rapidly frozen into the solidified outer skin of the wet
spun fiber. The swell may disappear before the nascent
fiber enters the coagulation bath and the shear-induced
orientation may relax for the dry-jet wet spun fiber
even if the air gap is 1 cm. In addition, gravity induced
elongation occurs and stretches the fiber. As a result,
the wet spinning may result in hollow fiber membranes
with a denser and/or smaller pore size outer skin that
shows a lower flux and higher separation performance
in comparison with that of dry-jet wet spun fibers. This
will be confirmed by later results.
Wet and dry-jet wet spun hollow fiber membranes
with different morphology have been prepared by
choosing various kinds of polymers, solvents and additives in the dopes (Table 4) in order to generalize
the conclusion of our study. For example, Fig. 5a
(Fiber 1w) shows an example of wet spun fiber with
the structure of finger-like and an outer skin and
Fig. 5b (Fiber 4dw) indicates a dry-jet wet spun
fiber with the structure of sponge-like and an outer
skin. While Fig. 5c (Fiber 6dw) shows a dry-jet wet
spun fiber with sponge-like and double skins, and
Fig. 5d (Fiber 5w) illustrates a wet spun fiber with
the structure of finger-like and double skins. The
SEM pictures of inner and outer surfaces separately
shown in Figs. 6 and 7 confirm the statement above.
The inner surfaces of Fiber 1w (Fig. 6a) and Fiber
4dw (Fig. 6b) are uniformly microporous due to the
delayed liquid–liquid demixing process taking place
at the fiber inner surface because of high solvent
concentration. In addition, a dense inner surface can
be observed from Fig. 6c and d even at a magnification of 10 000 and 35 000. From Fig. 7, it also
can be seen that all of the outer surfaces are dense
even at a magnification of 10 000 or 35 000 regardless of finger-like or sponge-like structure because an
outer dense skin was formed on top of a porous substructure due to instantaneous liquid–liquid demixing
process. Since both skins of hollow fiber UF membranes play a role in the separation performance
[37], the morphology of the fibers with an outer
dense (selective) layer may confirm our objective to
decouple the effect of inner skin on membrane separation performance. The morphology of the fibers
with double skins may confirm our hypothesis that
the inner skin does influence the data analysis for
the molecular orientation on the outer surface of
hollow fiber. This also will be confirmed by later
results.
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J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 3. Die swell schematic diagram of the nascent fiber extruding a spinneret.
Fig. 4. The overall cross-section of wet and dry-jet wet spun hollow fiber (a, wet spun Fiber 3w; b, dry-jet wet spun Fiber 3dw, magnification
×100, respectively).
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
65
Fig. 5. The cross-section of wet and dry-jet wet spun hollow fibers with different structure and skin types (a, b, c and d are corresponding
to identity of Fiber: 1w and 4dw, magnification ×1000; 6dw and 5w, magnification ×500, respectively).
3.2. Effect of wet and dry-jet wet spinning on pore
size of hollow fiber UF membranes
In order to support the hypothesis that the orientation induced at the outer surface by shear stress within
the spinneret could be frozen into the wet-spun fibers
but might relax in the air gap region for dry-jet wet
spun fibers, pore size of hollow fiber UF membranes
was characterized. Furthermore, it is to confirm our
design that the inner skin of hollow fiber with double
skins does influence the data analysis for the molecular orientation on the outer surface of membrane.
Table 5 clearly shows that the pure water permeability (PWP) of the wet spun hollow fiber UF membranes
is lower than that of dry-jet wet spun fibers. But the
separation of the wet spun fiber for a particular solute
is higher than the dry-jet spun fiber for all spinning
dopes studied in this work, no matter what polymers,
solvents and additives in the dope and also no matter
what shear rates and kinds of final membrane morphology. These results strongly indicate that an air
gap of 1 cm does make significant impact to the membrane performance. Wet spun hollow fiber UF membranes have smaller pore size and/or denser skin than
the dry-jet spun fibers because the former has higher
resistance for water permeation and better rejection
for a particular solute than the latter. The results in
this work seem to be contrary to that of former work
[9,15,19,26,34,35] for the effect of high air gap. How
to explain?
The most probable reason is due to the relaxation
phenomenon at the air gap region. As mentioned in
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J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 6. Inner surface of hollow fibers (a, b, c and d are corresponding to identity of Fiber: 1w and 4dw, magnification ×500; 6dw,
magnification ×10 000 and 5w, magnification ×35 000, respectively).
the Introduction, various stresses induce molecular orientation during hollow fiber formation. If no extra
stretch of the fiber is applied, the shear-induced orientation within the spinneret and the gravity-induced
orientation in the air gap may dominate skin structure of final membranes. If powerful coagulants are
employed as the internal and external precipitants, the
shear-induced orientation within the spinneret may be
frozen into the nascent solidified inner and outer skins
of wet-spun fibers because the dope meets precipitants
immediately as it exits from the spinneret. With an
increase in the air gap to 1 cm, the shear-induced orientation at the outer skin relaxes and loosens its tight
structure, thus yields membranes with a low separation but a high flux. Similar phenomenon takes places
for a hollow fiber that has one shear-induced dense
outer skin. Thus, a wet spun process results in fibers
with smaller pore size and/or denser skin than that of
a dry-jet wet spun process. Clearly, there are many
processes occurred in molecular level during the 1 cm
air gap. They may consist of: (1) the relaxation of the
shear-induced orientation, (2) the elongation induced
pore formation and (3) elongation induced orientation.
Based on our data, the first two factors tend to dominate the system and loosen hollow fiber structure, thus
yield membranes with a low separation but a high
flux.
Table 5 suggests that the effect of relaxation on
membrane performance is also dependent on the number of dense skins. Wet spun fibers that have only one
outer skin have a dramatically much lower flux but
much higher separation than that of dry-jet wet spun
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
67
Fig. 7. Outer surface of hollow fibers (a, b, c and d are corresponding to identity of Fiber: 1w, 4dw and 6dw, magnification ×10 000 and
5w, magnification ×35 000, respectively).
Table 5
Effect of wet spinning and dry-jet wet spinning on pure water permeability (PWP) and separation performance of hollow fiber UF membranes
Fiber ID Polymer
(wt.%)
Solvent Additives
1w
1dw
2w
2dw
3w
3dw
4w
4dw
5w
5dw
6w
6dw
PES (13)
NMP
PES (18)
NMP
PES (21)
NMP
PES (17)
DMAc
PES (16)
DMAc
PSf (15)
DMF
DG
Spun type PWP
(L/h m2 bar)
Wet
Dry-jet
DG
Wet
Dry-jet
DG
Wet
Dry-jet
DG
Wet
Dry-jet
H2 O
Wet
Dry-jet
PVP40K, 1,2-PD Wet
Dry-jet
214
720
90
680
62
100
303
1220
280
400
146
347
PVP10K
(%)
PVP29K
(%)
PVP40K
(%)
BSA67K
(%)
39
0
58
0
64
16
9
1
11
8
7
5.3
76
1
82
2
87
54
54
1
79
65
65
55.8
77
2
88
8.5
90
57
70
2
90
84
78
67.3
93
58
95
74
98
95
100
64
98
96
98
98
68
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 8. Effect of wet and dry-jet wet spinning on PWP of fibers with an outer skin as a function of polymer concentration.
fibers (Table 5, Figs. 8–10). This statement is more
applicable in case that the concentration of PES in
the spinning dope is less than 21 w.%. However, for
the membranes with double skins, the wet spun fibers
have an obviously lower flux but only slightly higher
separation than the dry-jet wet spun fibers (Table 5
and Fig. 11). These results indicate that both skins
play important roles for the flux but the inner skin of
the two-skin membrane plays main role for the separation. This is due to the fact that the shear-induced
orientation at the inner surface may be immediately
frozen into the inner fiber skin no matter wet-spinning
or dry-jet wet spinning because the dope meets precipitants as soon as it exits from the spinneret. As
a consequence, the shear-induced inner surface will
dominate the separation performance of whole membrane because the shear-induced at the inner surface
is higher than that at the outer surface, as shown in
Table 3. Thus, the shear-induced at the outer surface
only plays a part role for the flux of whole membranes.
Figs. 8 and 9 separately show that the flux of a
hollow fiber membrane decreases but the separation
increases with an increase in polymer concentration
in the spinning dope. On the other hands, these figures indicate that the difference in flux and separation between wet spun and dry-jet wet spun membrane
decrease with increasing polymer concentration. This
phenomenon is possible due to the fact that the relaxation process of a low concentration dope is much
more rapid than that of a high concentration dope and
1 cm air gap may not provide enough time for the relaxation of concentrated dopes although the resident
time for Fiber 3dw is longer than that for Fiber 2dw
(Table 3). The result of Fiber 4dw in Table 5 also suggests a hollow fiber UF membrane with high flux of
1220 L/h m2 bar can be prepared by dry-jet wet spinning with the relaxed molecular orientation.
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
69
Fig. 9. Effect of wet and dry-jet wet spinning on separation performance of hollow fiber UF membranes with an outer skin as a function
of polymer concentration.
Fig. 10. Effect of wet and dry-jet wet spinning on separation performance of hollow fiber UF membranes (Fiber 4w and 4dw) with an
outer skin.
70
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 11. Effect of wet and dry-jet wet spinning on separation performance of hollow fiber UF membranes with double skins.
3.3. Effect of wet and dry-jet wet spinning on
thermomechanical properties
Thermomechanical analysis was used in order further to confirm the hypothesis that the orientation induced at the outer surface by shear stress within the
spinneret could be frozen into the wet-spun fibers but
might relax in the air gap region for dry-jet wet spun
fibers.
The calculated CTE between 50 and 100◦ C of wet
spun and dry-jet wet spun hollow fibers from different dopes is shown in Table 6. The data illustrate that
CTE of the wet spun hollow fiber UF membrane is
lower than that of dry-jet wet spun fibers. CTE has
been used as a parameter to monitor the fiber molecular orientation [48–50]. A decrease in CTE indicates
an increase in molecular orientation, thus the CTE results in Table 6 imply that the wet spun hollow fibers
have a greater molecular orientation than dry-jet wet
spun fibers. Previous work [4,17,38] has suggested that
Tg measured by DSC and/or TMA is not sensitively
enough to characterize the effect of shear-induced ori-
entation because the skin is ultra thin. Similar conclusion can be made in this work. Fig. 12 illustrates an example of the TMA curves for wet spun and dry-jet wet
spun hollow fibers. It is important to point out that Tg
measured by TMA/DMA is different from that mea-
Table 6
Effect of wet spinning and dry-jet wet spinning on CTE measured
by TMA of hollow fiber UF membranes
Fiber ID
Spun type
1w
1dw
2w
2dw
3w
3dw
4w
4dw
5w
5dw
6w
6dw
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
CTE (␮m/m ◦ C)
spun
spun
spun
spun
spun
spun
116
140
82.2
87.0
65.9
78.0
93.2
119
105
147
173
285
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
71
Fig. 12. Dimension changes of wet spun and dry-jet wet spun fibers versus temperature measured by TMA.
sured by DSC because of different responses as the
temperature is raised during tests. For reader information, Tg of polyethersulfone hollow fibers is observed
to be around 212◦ C measured by DSC, around 209◦ C
measured by TMA, and around 236◦ C measured by
DMA. Similar results have been reported elsewhere
[51].
The storage modulus and the loss modulus of wet
spun and dry-jet wet spun hollow fibers as a function of temperature measured by DMA at a frequency
of 10 Hz are illustrated in Figs. 13 and 14, respectively. It can be seen that the storage modulus and the
loss modulus of the wet spun hollow fibers are higher
than that of the dry-jet wet spun fibers. Based on the
explanation of Jaffe and co-workers [48–50], Miller
and Murayama [52], the storage modulus is related to
the storage of energy as potential energy in molecular chains, while the loss modulus is associated with
the dissipation of energy as heat when the material is
deformed. They indicated that the increased storage
modulus and loss modulus might be due to the enhanced orientation of molecular chain. Therefore, the
result in Fig. 13 may imply that the wet spun hollow
fibers have more oriented molecular chain arrangement than the dry-jet wet spun fibers. While Fig. 14
may indicate that the wet spun fibers undergo a more
irreversible molecular chain rearrangement and relaxation due to higher molecular orientation when they
are subjected to heat and deformation than dry-jet wet
spun fibers. Similar loss modulus curve has been published elsewhere [53].
Polyethersulfone and polysulfone hollow fiber UF
membranes in this study show a yield point and elongation before failure and break on the stress-strain
curve since they are amorphous and ductile. Table 7
shows that the wet spun fibers have greater tensile
strength at break but smaller elongation at break than
the dry-jet wet spun fibers. Smith and Lemstra [54],
Yang and Chou [55] reported that the mechanical
properties of fibers increased with an increase in
draw ratio. As we know, an increase in draw ratio
during spinning results in enhanced spin line stress
that substantially increases the orientation. Therefore,
the results in this work may also indirectly confirm
72
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Fig. 13. Storage modulus of wet spun and dry-jet wet spun fibers versus. temperature measured by DMA.
that the higher mechanical properties for the wet spun
fibers might be due to the greater orientation and more
closely packed molecular chains at the outer surface
in the direction of shear along the fiber. As such, wet
spun fibers exhibit a higher resistance to elongation
before yield than the dry-jet wet spun fibers and a
higher stress is required to stretch and break the wet
spun fibers.
Table 7
Effect of wet spinning and dry-jet wet spinning on tensile strength and elongation of hollow fiber UF membranesa
Fiber ID
Spun type
1w
1dw
2w
2dw
3w
3dw
4w
4dw
5w
5dw
6w
6dw
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
Wet spun
Dry-jet wet
a
spun
spun
spun
spun
spun
spun
The values in the brackets were standard deviation.
Tensile stress at break (MPa)
Elongation at break (%)
2.24
1.70
3.27
2.66
3.50
3.05
1.97
1.81
3.96
2.67
1.45
1.32
17.5
21.3
25.0
27.9
25.4
34.0
28.3
31.8
50.0
53.4
23.4
24.9
(0.13)
(0.10)
(0.16)
(0.17)
(0.15)
(0.12)
(0.14)
(0.17)
(0.38)
(0.20)
(0.14)
(0.12)
(2.5)
(2.0)
(2.9)
(2.1)
(4.0)
(3.6)
(3.1)
(3.9)
(2.3)
(3.6)
(4.6)
(3.5)
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
73
Fig. 14. Loss modulus of wet spun and dry-jet wet spun fibers vs. temperature measured by DMA.
4. Conclusion
By using the phase inversion process, we have investigated the effects of wet spinning and dry-jet wet
spinning on permeability, separation performance and
thermomechanical properties of hollow fiber ultrafiltration membranes. We prepared hollow fiber UF
membranes with different structure using different
kinds of polymers, solvents and additives. Experimental results showed that the molecular orientation
induced at the outer surface of the nascent fiber by
shear stress within the spinneret could be frozen
into the wet-spun fibers, but might relax in the air
gap region. As a consequence, the wet spun fibers
have smaller pore size and/or denser skin than the
dry-jet wet spun fibers because the greater molecular
orientation was resulted in for the wet spun fibers.
Furthermore, pure water flux, CTE and elongation of
the wet spun fibers are lower than that of dry-jet wet
spun fibers, but separation performance, storage modulus, loss modulus and tensile strength of the former
are higher. It was found that for the membranes with
an outer skin, the wet spun fibers have a dramatically
lower flux but much higher separation than the dry-jet
wet spun fibers. However, for the membranes with
double skins, the wet spun fibers have an obviously
lower flux but only slightly higher separation than
the dry-jet wet spun fibers. Furthermore, the result
suggested that a hollow fiber UF membrane with
high flux of 1220 L/h m2 bar could be prepared by the
approach proposed in this paper.
Acknowledgements
The authors would like to thank the National University of Singapore (NUS) for funding this project
(research fund no.: 960609A) and the support from
Membrane Research Technology Singapore, Ltd. Special thanks to Dr. R. Wang for her help in shear rate
calculation. Thanks are also due to Ms. S. Chen, Ms.
W. Lin, Mr. S.L. Liu, Mr. K.C. Toh and Ms. Agnes
74
J.-J. Qin et al. / Journal of Membrane Science 182 (2001) 57–75
Lim and the Institute of Materials Research and Engineering of Singapore for using their equipment.
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