JMS2002

Journal of Membrane Science 204 (2002) 153–171
High flux polyethersulfone–polyimide blend hollow
fiber membranes for gas separation
G.C. Kapantaidakis, G.H. Koops∗
Membrane Technology Group, Faculty of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Received 8 November 2001; received in revised form 17 January 2002
Abstract
In this work, the preparation of gas separation hollow fibers based on polyethersulfone Sumikaexcel (PES) and polyimide
Matrimid 5218 (PI) blends, for three different compositions (i.e. PES/PI: 80/20, 50/50 and 20/80 wt.%), is reported. The
dry/wet spinning process has been applied to prepare asymmetric hollow fibers by using blends of two different polymers with
a common solvent. Dope viscosity measurements were performed to locate the blend concentrations where significant chain
entanglement occurs. Cloud point measurements were carried out to estimate the tolerance of both pure components and blends
in water. Scanning electron microscopy (SEM) was used to investigate the morphological characteristics and the structure of
asymmetric hollow fibers. The permeation rates of CO2 and N2 were measured by the variable pressure method. In all cases,
hollow fibers exhibit a typical asymmetric structure with a dense skin layer and a finely porous substructure. Macrovoids
in the membrane substructure were observed only for the fibers spun at high PES concentration (80 wt.%). After coating
with a silicone rubber solution, the developed hollow fibers exhibit a CO2 permeance varying from 31 to 60 gas permeation
units (GPU) and a CO2/N2 selectivity varying from 40 to 35, at room temperature. The thickness of the skin layer, which
corresponds to these permeation rates, varies from 0.1 to 0.15 ␮m. The effect of air-gap distance on hollow fibers structure and
permeation performance is examined. The aforementioned permeation properties, establish PES/PI hollow fibers as excellent
candidates membranes for the separation of gaseous mixtures in industrial level. © 2002 Published by Elsevier Science B.V.
Keywords: Polymer blends; Dry/wet spinning; Hollow fibers; Asymmetric membranes
1. Introduction
The performance of membrane units in the sep-
aration of gaseous mixtures is highly dependent on
the intrinsic physicochemical characteristics of the
utilized polymeric material. The most important prop-
erties which are taken into account when selecting
a polymer membrane material are the: (1) gas per-
meability and selectivity coefficients, (2) mechanical
strength, (3) glass transition temperature, (4) critical
∗ Corresponding author. Tel.: +31-53-4894611;
fax: +31-53-4894185.
E-mail address: g.h.koops@ct.utwente.nl (G.H. Koops).
pressure of plasticization, (5) material availability,
processability and (6) cost. In general, polymeric
materials used in industrial level for the preparation
of gas separation membranes do not meet absolutely
and simultaneously all of these criteria. For example,
highly permeable polymers exhibit moderate to low
selectivity values while materials with high resistance
to harsh chemical environments or plasticizing gases
are either hardly processed or are very expensive
[1,2]. Considering the complementary properties of
glassy polymers, research has been directed to the
preparation of novel materials by polymer blending
[3]. Compared with other modification techniques
or even with the synthesis of entirely new materials,
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V.
PII: S0376-7388(02)00030-3

154 G.C. Kapantaidakis, G.H. Koops / Journal of Membrane Science 204 (2002) 153–171
polymer mixing is preferred due to its simplicity,
reproducibility and commercial character. It is worth-
while to mention that more than 500 scientific papers
and more than 5000 patents are published every year
in the area of polymer blending, while about 40% of
the total world wide polymer production is referred
to polymer mixtures [4].
In general, polymer blends are classified as homo-
geneous (miscible) or heterogeneous (immiscible or
alloys) [5]. In most of the cases and due to thermo-
dynamical reasons polymeric blends belong to the
second category. However, phase separated systems
are not suitable for the preparation of gas separation
membranes since they cannot be fabricated in the form
of ultrathin and defect-free asymmetric structures. In
the open literature, various polymer blends have been
studied [6–13]. The vast majority of these studies
deal with the permeation, sorption and compatibility
characteristics of flat and dense blend membranes
while only few report results in the preparation and
characterization of gas separation hollow fibers. In
such a work, Chung and Xu studied the thermal and
mechanical properties, miscibility and morphology,
and permeation characteristics of polybenzimidazole
(PBI)–polyetherimide (PEI) composite hollow fiber
membranes [14]. It was stated that when the interac-
tions between the pure polymers are strong enough
then the miscibility is independent of bore fluid chem-
istry, bore fluid flow rate and post treatment steps.
However, the prepared asymmetric PBI/PEI hollow
fibers exhibited a thick skin layer and a tight sub-
structure resulting therefore in low permeability and
selectivity values.
Among the polymers that are used as standard
membrane materials are polysulfone of bisphenol A
(PSF), polyethersulfone (PES), and aromatic poly-
imides (PI) [15–18]. Typical commercial polysulfone
and polyimide materials are Udel P-1700/P-3500
(Amoco Chemical) and Matrimid 5218 (Ciba-Geigy),
respectively. Polysulfone is a high performance engi-
neering thermoplastic with resistance to degradation,
good gas permeability and selectivity values, low
cost and high critical pressure of plasticization. On
the other hand, Matrimid 5218 is a thermally stable
polymer with excellent mechanical properties, good
correlation between permeability and selectivity but
with high cost and low carbon dioxide pressure of
plasticization. In previous works, these two materials
were prepared in the form of dense flat membranes
and tested for their gas permeation properties, rhe-
ological and thermal characteristics and miscibility
behavior [19–21]. It was concluded that homoge-
neous PSF/PI polymer blends could be viewed as
new economical, high performance materials suitable
for the preparation of gas separation membranes with
advanced permeation and thermophysical properties.
Furthermore, the critical partial pressure of plasti-
cization for CO2 was increased appreciably by using
moderate PSF concentration (<50%) in the PSF/PI
blend, as compared to that of pure polyimide. The
only disadvantage of PSF/PI blends is that they can
be prepared in the form of miscible membranes only
when chloroform is used as the common solvent [22].
In a typical spinning process, however, the applica-
tion of halogenated hydrocarbons is undesirable since
only highly flammable and toxic alcohols can be used
as a coagulation medium.
Another pair of polymers with high potential of
industrial application and complementary properties,
is the analogous polyethersulfone (Sumikaexcel)–
polyimide (Matrimid 5218) system. Table 1 summa-
rizes the main physicochemical characteristics and
cost of these two polymers. This blend was exten-
sively studied by Liang et al. [23] and proved to
be miscible over the whole range of composition.
However, this work focuses mainly on the thermal
and rheological properties of PES/PI blends and does
not examine their gas permeation properties. Like
polysulfone, polyethersulfone is an excellent candi-
date for the preparation of gas separation membranes
since it exhibits high chemical resistance, thermal
and dimensional stability and high selectivity values.
However, due to its high degree of chain rigidity,
Table 1
Physicochemical properties and cost of PI Matrimid 5218 and PES
Sumikaexcel
Parameter PES
Sumikaexcel
PI Matrimid
5218
Molecular weight, Mw 88000 80000
Glass transition
temperature (◦C)
230 320
CO2 permeability
(Barrer), 25 ◦C
3.8 7.0
Plasticization pressure (bar) 25 15
Cost ( /kg, 10/2001) 12 360

G.C. Kapantaidakis, G.H. Koops / Journal of Membrane Science 204 (2002) 153–171 155
polyethersulfone is less permeable than polysulfone
and polyimides [24].
The production of gas separation hollow fibers
based on polyethersulfone–polyimide blends is re-
vealed in a patent of Air Liquid, by Ekiner [25]. This
patent provided actually a three-component blend of:
(1) an aromatic polyethersulfone, (2) an aromatic poly-
imide and (3) an alkyl-substituted aromatic polyimide,
polyamide or polyamide–imide or mixtures thereof.
The spinning dope consisted of the blend polymers,
in various compositions, N-methylpyrrolidone, acetic
anhydride and tetramethylensulfone. The permeation
rate of O2 for silicone rubber coated PES/PI fibers
varied from 8 to 17 gas permeation units (GPU) while
the O2/N2 selectivity factor varied from 4.7 to 6.9. As
in the case of PSF/PI blends, the utilization of toxic
and harmful chemical additives such as acetic an-
hydride and tetramethylenesulfone could prevent the
application of these hollow fibers at industrial level.
The most common way to prepare gas separation
hollow fiber membranes is by using the so-called
dry/wet phase inversion process. In the simple case
that the polymer solution consists of one polymer
and one solvent, asymmetric structures are usually
obtained by increasing locally the polymer concen-
tration in the surface layer of the fiber. The latter can
be achieved by: (1) solvent evaporation taking place
at the air-gap distance between the spinneret and the
coagulation bath, (2) fast solvent outflow relative to
the coagulant inflow or (3) higher surface tension of
the solvent compared to the polymer. In all cases, the
ultimate goal of these methods is to produce asym-
metric hollow fibers with a thickness of the separating
layer equal or even less than 1000 Å.
Usually, gas separation hollow fiber membranes
with an ultrathin skin layer are prepared at industrial
level by using a solvent/non-solvent or multisolvent
systems. The addition of a non-solvent in the spinning
dope aims to bring the solution composition closer to
the point of phase separation. Permea, produced its
second generation polysulfone membranes by using
an N-methyl-2-pyrrolidone (NMP)–propionic solvent
system [26]. In their US patent 4,871,494 it was re-
vealed that the Lewis acids, Lewis bases, and Lewis
acid: base complex solvent system could result in
polymer structures with higher frozen-free volume
among nodules in the skin layer and therefore high
permeation rates were observed. After coating the
fibers with a dilute (1 wt.%) silicone rubber solution in
isopentane, intrinsic selectivity values were achieved.
A multisolvent/non-solvent system was used also
by Clausi and Koros to produce ultrathin polyimide
Matrimid 5218 membranes [27]. They actually used a
dope solution consisting of EtOH as the non-solvent
and either pure NMP or mixtures of NMP and THF
as the solvent. They surprisingly found that nearly
defect-free fibers could be produced even without
no-volatile solvent (THF). They concluded that phase
separation in the air gap is not critical to the formation
of defect-free integrally-skinned hollow fibers. Phase
separation via nucleation and growth or spinodal
decomposition were proposed as the two possible
mechanisms to explain the formation of defect-free
skin layers when a locally high polymer concentra-
tion enters the coagulation bath and phase separates.
In another approach, polyimide asymmetric hollow
fibers were prepared by using a dope consisting of
Matrimid 5218, NMP and acetone [28]. By adjusting
the polymer and the acetone concentration in the spin-
ning dope, fibers were produced with defect-free
skins and an effective top layer thickness in the range
of 0.3–0.4 ␮m.
Chung et al. [29] demonstrated that ultrathin skin
layer polyethersulfone hollow fiber membranes with
a skin layer of about 500 Å can be prepared by using
only one polymer and one solvent. Despite the fact
that the manufactured fibers were not defect-free and
a coating step with a silicone rubber material was
required, the achieved skin thickness was one of the
lowest values reported in the open literature. They
showed that the addition of a non-solvent in the spin-
ning dope is not the precondition to form asymmetric
hollow fibers with ultrathin skin layers. It was pro-
posed that the key factors that control skin thickness
is: (a) the viscosity of the spinning dope and (b) the
chemistry of the bore fluid and the bore fluid flow
rate. It was hypothesized that a polymer dope starts
to exhibit significant chain entanglement at a critical
polymer concentration. The exact concentration value
was determined by the intersection of the two asymp-
totic lines of the viscosity–concentration curve. It is
believed that fibers spun from this critical concentra-
tion exhibit theoretically the thinnest skin layer with
minimum to no surface defects.
Taking into account the undeniable promising fea-
tures of polyethersulfone Sumikaexcel and polyimide

Matrimid 5218 blends, it seems reasonable to examine
the feasibility of preparing such hollow fiber mem-
branes by using only NMP as the common solvent.
NMP has a strong interaction with both polymers, is a
low cost solvent, is miscible with water and finally is
benign from a health viewpoint. Gas permeation ex-
periments were focused mainly on carbon dioxide and
nitrogen, since this gas pair offers the advantages of
high selectivity values, acceptable permeation rates,
safety and low cost. Finally, there are a lot of litera-
ture references dealing with the permeation of these
two gases in pure polyethersulfone and polyimide
membranes.
2. Experimental
2.1. Materials
Hollow fiber blend membranes were prepared by
using the commercially available materials polyimide,
Matrimid 5218 (Ciba-Geigy) and polyethersulfone
Sumikaexcel (Sumitomo). Polyimide (PI) Matrimid is
composed of 3,3 ,4,4 -benzophenone tetracarboxylic
dianhydride and diaminophenylindane and has the
following repeating chemical structure [23]:
PES is an amorphous thermoplastic polymer with
the following repeating chemical structure [23]:
NMP (99% Acros) was used as a solvent and tap
water as the external coagulant in the hollow fiber spin-
ning. Finally, polydimethylsiloxane (PDMS, Sylgard-
184) commercialized by DOW Corning Corp., was
used as the rubber coating material to heal surface
defects of the prepared hollow fibers. N-Hexane was
used as a solvent for the Sylgard-184.
2.2. Viscosity measurements
Three different PES/PI blend compositions were
examined namely, PES/PI 80/20 wt.% (Dope A),
PES/PI 50/50 wt.% (Dope B) and PES/PI 20/80 wt.%
(Dope C). Solution viscosities were measured using
a Brabender® cone and plate viscometer (Viscotron)
at three different temperatures, 40, 50 and 60 ◦C.
Various concentrations (20–39 wt.%) of each dope
in NMP were prepared. The viscosity value of each
polymer solution was determined by the magnitude
of torque needed to overcome the viscous resistance
when a cone-shape spindle rotates in the solution.
The critical polymer concentration where signifi-
cant chain entanglement occurs in the spinning dope
was estimated by the intercept of the tangent line at
the lowest polymer concentration with the tangent line
at the highest polymer concentration.
2.3. Cloud point measurements
Cloud point measurements were performed for pure
PES, PI and the three different PES/PI blend composi-
tions (Dopes A, B and C), at three polymer concentra-
tions (26, 30, 35%) by titration with water. Actually, a
solution of the polymer (20–30 g) is placed in a vessel
which is thermostated at 50 ◦C. Small quantities of
water (0.05 g) were slowly added to the polymer solu-
tion and phase separation was locally observed. After
thorough stirring, solution homogeneity was achieved
again. The same procedure was repeated until perma-
nent turbidity was visually detected indicating thus the
final cloud point of the polymer solution [30].
2.4. Hollow fiber preparation
PES/PI hollow fibers were prepared by the dry/wet
spinning process in a spinning set up shown in Fig. 1.
Dopes consisting of PES/PI blends and NMP were

Fig. 1. Hollow fiber spinning set up: (1) spinning dope tank; (2) bore liquid vessel; (3) spinneret; (4) air gap; (5) coagulation bath; (6)
fiber guiding wheel; (7) pulling wheel; (8) spinning line; (9) fiber collecting reservoir.
mixed in a 2 l thermostated stainless steel vessel by a
150 W IKA® Labortechnik laboratory stirrer (Janke
& Kunkel Gmbh & Co.). The mixture was stirred for
at least 7 h in order to achieve a homogeneous solution
and then filtered through a 15 ␮m metal filter (Bekaert)
to remove impurities existing in the raw polymers.
Both vessels and spinneret were thermostated at 50 ◦C
in order to facilitate the flow of the polymer solu-
tion. After filtering, the dopes were allowed to de-
gas inside a second stainless steel vessel for 2 days.
The bore liquid was a degassed mixture of NMP and
deionized water (Milli-Q, 18 M cm). Polymer so-
lution and bore fluid were simultaneously pumped
through a tube-in-orifice spinneret using gear pumps.
The i.d. of the spinneret was 200 ␮m and the o.d.
500 ␮m. The extruded fibers passed first trough an air
gap varying from 1 to 31 cm before entering to the
coagulation bath filled with tap water at room temper-
ature. The nascent fibers were oriented by means of
two guiding wheels and pulled by a third wheel into a
collecting reservoir. In order to remove residual NMP,
produced fibers were washed with tap water overnight
and then solvent exchanged in plastic containers with
ethanol for 4 h.
2.5. Scanning electron microscopy (SEM)
A Joel JSM-T220A scanning electron microscope
was used to determine the asymmetric structure
and the dimensions of the fibers. Membrane sam-
ples were first immersed in ethanol, fractured in
liquid nitrogen and then sputtered with a thin layer
of gold using a Balzers Union SCD 040 sputtering
apparatus.
2.6. Modules preparation and gas permeation
experiments
The permeation characteristics of hollow fiber
membranes were measured in a high pressure set up
by using the variable pressure method. Carbon diox-
ide (99.996%) and nitrogen (99.999%) were chosen
as test gases. The effective permeation area for the
hollow fiber membrane modules varied between 9 and
14 cm2 depending on fibers geometrical characteris-
tics. Actually, five fibers, each one 10–15 cm long,
were potted from the one side into a 3/8 in. stainless
steel holder while sealing the other side by a regular
epoxy resin. Pure gases were applied to the shell side

at a pressure of 4 bar and at temperature of 25 ± 2 ◦C
while the permeate side was kept at vacuum pressure.
The permeances, P/L, of pure gases through the hollow
fibers were calculated from the steady state pressure
increase with time in a calibrated volume on the per-
meate side. Permeance values are expressed in GPU,
where 1 GPU = 10−6 cm3 (STP) cm−2 s−1 cmHg−1.
In order to check reproducibility, three to six differ-
ent modules, prepared from the same batch of fibers,
were tested. Both average values and experimental
error were subsequently calculated. The permselectiv-
ity coefficient of hollow fibers was determined by the
ratio of pure gas permeances while the thickness of
the skin layer was calculated by dividing the intrinsic
gas permeability with the obtained permeance value.
At 25 ◦C the reported permeability coefficients of
CO2 and N2 for pure PI are 7 and 0.18 Barrer, respec-
tively, while those of pure PES are 3.9 and 0.1 Barrer
[27,31–33]. Since polyimide Matrimid 5218 and
polyethersulfone Sumikaexcel are completely misci-
ble [23], the gas permeability coefficients for each
blend composition were calculated from the values of
pure polymers and by using simple mixing equations
[34].
2.7. Hollow fiber post treatment
Defects on the membrane surface were healed by
a coating technique. Hollow fiber modules were first
immersed in a solution of 3 wt.% PDMS in N-hexane
and then cured in an oven for 4 h at 65 ◦C.
Table 2
Spinning conditions and process parameters
Parameter Dope A Dope B Dope C
Blend composition (PES/PI wt.%) 80/20 50/50 20/80
Blend concentration in NMP (wt.%) 35 30 26
Viscosity (cP) 37371 28420 20707
Dope fluid rate (ml/min) 1.92 1.92 1.92
Actual spinning speed (m/min) 12.5 8.9 8.9
Spinning temperature (◦C) 50 50 50
Bore fluid composition (NMP/H2O wt.%) 80/20 80/20 80/20
Bore fluid flow rate (ml/min) 1.12 1.46 1.46
Type of coagulant Water Water Water
Coagulant bath temperature (◦C) 21 26 25
Air gap (cm) 1–10 5–20 6–31
Room temperature (◦C) 20 23 23
Room humidity (%) 53 63 63
3. Results and discussion
Table 2 summarizes the detailed spinning conditions
for the three different dopes examined in this work.
Fig. 2a–c shows the effect of blend polymer concentra-
tion on viscosity for the three examined dopes (Dopes
A, B and C, respectively) at three different tempera-
tures, 40, 50 and 60 ◦C. As expected, increase of poly-
mer concentration and decrease of temperature results
in higher dope viscosity values. However, in all cases
a significant increase occurs at a critical concentration
of about 35 wt.% for Dope A, 30 wt.% for Dope B and
26 wt.% for Dope C. Increase of the polyimide con-
centration in the blends results in lower values of crit-
ical concentrations due to the high intrinsic viscosity
of pure polyimide. Therefore, viscosity measurements
provided an indication for the polymer concentration
in the spinning dope. For the initial Dope A, values
for the rest of the experimental parameters (air-gap
length, bore fluid flow rate, take up speed) were chosen
according to previous spinning experience in our lab
while for Dopes B and C they were suitably adjusted.
3.1. PES/PI 80 /20 wt.%, Dope A
The structure and the geometrical characteristics of
the produced hollow fiber membranes were studied by
scanning electron microscopy (SEM). Fig. 3 shows the
cross-section of the fibers spun at an air-gap distance
of 1 cm. In this case, the fibers exhibit a typical asym-
metric structure; a dense skin layer supported by a

Fig. 2. Effect of polymer concentration on dope viscosity: (a)
Dope A; (b) Dope B; (c) Dope C.
spongy porous substructure, which contains also spo-
radic macrovoids. The direction of macrovoids forma-
tion is always from the outer side of the hollow fibers.
Fig. 4 shows the structure of the bore side, for a fiber
spun at an air-gap distance of 10 cm while Fig. 5 de-
picts the respective top layer. From this SEM picture,
the thickness of the skin layer is estimated to be in the
range of 0.1–0.2 ␮m.
The effect of air-gap distance on the permeance of
N2 and CO2 for uncoated and PDMS coated fibers
is shown in Fig. 6. In the case of uncoated fibers in-
crease of the air-gap length from 1 to 10 cm results in
higher permeation rates for both gases. Actually, the
permeance of CO2 is increased from 57 to 130 GPU
while that of N2 increases from 5 to 65 GPU. For
PDMS coated fibers spun at 1 cm, the permeance of
CO2 is decreased somewhat (from 57 to 38 GPU), but
a significant decrease is observed for N2 (from 5 to
1 GPU). A careful inspection of Fig. 6 could reveal a
slight increase of CO2 and N2 permeation rates with
the air-gap distance even for the PDMS coated fibers.
For instance, fibers spun at an air gap of 10 cm exhibit
13% higher permeance rates of CO2 (43.5 GPU) than
that spun at 1 cm (38 GPU). A possible reason for this
behavior could be the fact that a longer air-gap distance
results in protracted coagulation from the bore side
and therefore in a more open membrane substructure.
Fig. 7 shows the effect of air-gap distance on
the respective ideal selectivity factor, α(CO2/N2), at
the same operating conditions (T = 25 ◦C and P =
4 bar). For uncoated fibers, these values are lower
than the intrinsic selectivity of PES/PI 80/20 wt.%
blends (α(CO2/N2) = 40), but are higher than the
selectivity predicted by the Knudsen diffusion mech-
anism (α(CO2/N2) = 0.8). This means that the skin
layer contains a small fraction of tiny pores and this
fraction increases with air-gap distance. The latter
was verified by conducting N2 and CO2 permeation
experiments at different feed pressures. Generally,
increasing the feed pressure resulted in higher gas
permeation rates thus indicating the existence of sur-
face porosity. After coating, Dope A hollow fibers
exhibit CO2/N2 ideal selectivity values which are
almost equal to the intrinsic selectivity of PES/PI
80/20 wt.% blends. The effective top layer thickness
was then calculated by dividing the blend CO2 and
N2 permeability coefficients (Q(CO2) = 4.52 Barrer
and Q(N2) = 0.11 Barrer) with the experimental gas

Fig. 3. SEM pictures of Dope A hollow fibers: cross-section (air gap = 1 cm).
permeance values. By this way, it was found that the
thickness of the hollow fiber skin layer varies from
1029 Å (air-gap distance of 10 cm) to 1198 Å (air-gap
distance of 1 cm).
3.2. PES/PI 50/50 wt.%, Dope B
From the previous results (especially Fig. 6),
it seems that the parameter of air-gap length can
Fig. 4. SEM pictures of Dope A hollow fibers at the bore side (air gap = 10 cm).
influence significantly the gas permeation properties
of both uncoated and PDMS coated fibers. Therefore,
Dope B was spun at even longer air-gap distances,
actually from 5 to 20 cm. The structure of the asym-
metric hollow fibers was observed by using SEM.
Fig. 8 shows the cross-section of the fibers spun from
an air-gap distance of 20 cm. It is evident that contrary
to Dope A, blend fibers spun from Dope B exhibit no
macrovoids in their substructure. Fig. 9a and b shows

Fig. 5. SEM pictures of a Dope A hollow fiber at the skin layer (air gap = 10 cm).
also the structure of the sublayer as well as the skin
layer of fibers spun at an air-gap length of 20 cm. In
this case a uniform microporous substructure is ob-
tained with a skin layer thickness of about 0.1–0.2 ␮m.
The effect of air gap on the permeation proper-
ties of these fibers is given in Fig. 10. For uncoated
Fig. 6. Effect of air-gap distance on the permeation properties of Dope A fibers.
fibers, the permeance of CO2 is increased from 30
to 77 GPU while that of N2 from 2 to 42 GPU.
Compared with Dope A, the maximum permeance
values are somehow similar (Dope A, N2: 65 GPU;
Dope B, N2: 42 GPU) but they are achieved in a
twofold air-gap distance (20 cm). For PDMS coated

Fig. 7. Effect of air-gap distance on the CO2/N2 selectivity of Dope A fibers.
fibers it is clear that by increasing the air-gap length
from 5 to 20 cm the permeance of CO2 increases
from 17 to 30 GPU and that of N2 from 0.45 to
0.9 GPU.
Fig. 8. SEM pictures of Dope B hollow fibers: cross-section (air gap = 20 cm).
The effect of air gap on the selectivity values is
shown in Fig. 11. Similarly with the results obtained
for Dope A uncoated fibers, increase of the air gap re-
sults in a decrease of selectivity from 14.9 to 1.8 while

Fig. 9. SEM pictures of Dope B hollow fibers at the: bore side (a); skin layer (b) (air gap = 20 cm).
for coated fibers it is substantially higher and varies
from 35 to 40. By taking into account the blend intrin-
sic CO2 and N2 permeability coefficients (Q(CO2) =
5.46 Barrer and Q(N2) = 0.14 Barrer) and the perme-
ance values, the thickness of the skin layer was found
equal to 1570 Å (at an air-gap distance of 20 cm).
The observed effect of air-gap length on the mor-
phology and the permeation properties of the blend
hollow fibers could be rationalized by consider-
ing the fundamental physicochemical phenomena,
which take place during dry/wet spinning. In this
process, the nascent hollow fiber membrane could

Fig. 10. Effect of air-gap distance on the permeation properties of Dope B ﬁbers.
Fig. 11. Effect of air-gap distance on the CO2/N2 selectivity of Dope B ﬁbers.

Fig. 12. Effect of polyimide concentration on the cloud point of pure polymers and blends.
face diffusion induced phase separation due to: (1)
penetration of air moisture (non-solvent for the poly-
mer) at the outer surface of the ﬁber, (2) internal
coagulation from the bore side, (3) precipitation at
Fig. 13. SEM pictures of Dope C hollow ﬁbers: cross-section (air gap = 6 cm).
the skin side, when the membrane is immersed in
the non-solvent bath and (4) temperature quench of
the dope solution (50 ◦C) in the coagulation bath
(21–26 ◦C).

Depending on the residence time of the spinning
line and the humidity in the air-gap region, water
molecules can indeed cause precipitation inside the
nascent membrane. In our case, the residence time of
the spinning line in the air-gap region varied from
0.34 s (air gap: 5 cm) to 1.34 s (air gap: 20 cm) while
the maximum room humidity encountered was 63%
(Table 2). The cloud point measurements, shown in
Fig. 12, indicate that a substantial amount of water
(∼4 g H2O/100 g of Pol. solution) can penetrate the
nascent membrane before demixing occurs. It is also
worthwhile to mention that this value is independent
of blend composition and polymer concentration. The
longer the nascent hollow fiber membrane is exposed
to the humid atmosphere, the higher the water content
in the membrane top layer before coagulation starts in
the water bath. This leads to an increase of the surface
porosity and therefore higher permeability values. In
other words, the utilization of an air gap during spin-
ning could be considered as equivalent to the well-
known method of adding small amounts of water to
the dope in order to increase porosity.
Besides, the increase of gas permeance with the
air-gap distance could also be explained by taking into
account the internal coagulation in the bore side of the
fiber. The NMP content in the bore liquid (80 wt.%) is
higher than that in the dope (70 wt.%). Therefore, the
direction of solvent diffusion is from the bore liquid
into the polymer solution. This process of dilution
mainly affects the substructure close to the bore side.
Whether it influences the top layer remains speculative
since analytical estimations of diffusion coefficients of
NMP in the polymer dope have not been performed.
Again, the increase of the air-gap distance results in
longer contact times between the nascent fiber and the
bore fluid and therefore in membranes with a looser
substructure.
3.3. PES/PI 20/80 wt.%, Dope C
Since pure polyimide membranes are more perme-
able than those of polyethersulfone, higher permeation
rates would be expected for the blend fibers with the
highest PI concentration, namely PI 80 wt.%, Dope
C. In order to check once more the effect of air-gap
distance on the gas permeation properties, this blend
composition was also spun at air-gap distances vary-
ing from 6 to as long as 31 cm.
The structure and the dimensions of hollow fibers
produced from Dope C are shown in Fig. 13 for an
air-gap distance of 6 cm. In this case, the hollow
fibers have an outer diameter of about 510 to 520 ␮m
and a wall thickness of about 80 ␮m. Similar to Dope
B, the substructure is uniformly microporous without
any macrovoids. Since only fibers spun form Dope A
(80 wt.% PES) exhibit macrovoids in their substruc-
ture it appears that the composition of the polymer
blend affects the morphology of the membrane struc-
ture. Boom et al. [35] observed a similar behavior to
membranes prepared from a casting solution of PES,
polyvinylpyrrolidone (PVP), N-methylpyrrolidone
and water. It was found that the addition of PVP
to the ternary system suppresses the formation of
macrovoids in the sublayer. The reason is that the
phase separation of a miscible blend involves in a first
step the demixing of the entangled and interwined
polymer chains. However, the diffusion of the two
components (high molecular weight) with respect to
each other is considerably slower than the exchange of
NMP and water (low molecular weight) between the
casting solution and the coagulation bath occurring
immediately upon immersion. As long as two dif-
ferent time scales are distinguished, local conditions
for delay demixing cannot take place. Apparently, the
growth of nucleus is effectively blocked by the for-
mation of new nuclei and as a result the macrovoid
formation is hindered. To achieve this, a certain mini-
mum of molecular weight and polymer concentration
is required. In our case, adding 20 wt.% of PI to
PES is not sufficient enough to suppress macrovoid
formation.
In Fig. 14a and b, a closer view of Dope C fibers,
especially in the bore region, is depicted. The sub-
structure of the fibers spun at the highest air-gap
distance, 31 cm (Fig. 14b) is more open than the re-
spective structure of fibers spun at 6 cm (Fig. 14a).
Finally, Fig. 15 depicts the top layer for a fiber spun at
an air-gap distance of 31 cm. From this SEM picture
it can be estimated that the skin layer has a thickness
of about 0.1–0.2 ␮m.
Fig. 16 shows the effect of air-gap distance on the
permeation rates of CO2 and N2. Once more, increase
of air gap from 6 to 31 cm results in higher gas per-
meation rates. For uncoated fibers, the permeance of
CO2 is increased from 79 to 193 GPU while that of N2
increases from 36 to 146 GPU. After PDMS coating,

Fig. 14. SEM pictures of Dope C hollow fibers at the bore side: (a) air gap = 6 cm; (b) air gap = 31 cm.
the permeance of CO2 is now reduced to values vary-
ing between 40 and 60 GPU and the permeance of N2
between 1 and 1.5 GPU. For uncoated fibers, the in-
crease of permeance with air gap is counterbalanced
by a decrease of selectivity from 2.2 to 1.3 while
after coating the CO2/N2 selectivity is almost equal
to 39 (Fig. 17). By taking into account the intrin-
sic permeability coefficients (Q(CO2) = 6.4 Barrer
and Q(N2) = 0.17 Barrer), it can be calculated that
the skin layer thickness varies from 1760 Å (air-gap
distance of 6 cm) to 1120 Å (air-gap distance of
31 cm).

Fig. 15. SEM pictures of a Dope C hollow fiber at the skin layer.
Similarly to Dopes A and B, fibers spun from Dope
C exhibit permeation properties which depend on the
air-gap distance. Compared to the values obtained for
Dope B, the selectivity coefficients of the uncoated
fibers of Dope C are even lower (2.2–1.3). This means
that hollow fibers with a high PI concentration have
Fig. 16. Effect of air-gap distance on the permeation properties of Dope C fibers.
a more porous skin layer and open structure, which
could be justified by the relatively low polymer con-
centration.
For comparison reasons, Table 3 summarizes the
achieved CO2 permeance values (after coating with
PDMS), the CO2/N2 selectivity coefficients and the

Fig. 17. Effect of air-gap distance on the CO2/N2 selectivity of Dope C fibers.
calculated thickness of the hollow fiber skin layers for
the three different spinning dopes examined in this
work. Fig. 18 shows also the comparison of PES/PI
blends with commercial gas separation membranes in
terms of CO2 permeance and CO2/N2 selectivity [36].
Fig. 18. Comparison of PES/PI hollow fibers with commercial gas separation membranes.
PES/PI blends exhibit the highest CO2/N2 selectivity
values and competitive CO2 permeance values indi-
cating thus the perspective for future commercial ap-
plication. We mention, for instance, the separation of
CO2 from power plant exhaust gases or the separation

Table 3
Permeation properties of PDMS coated PES/PI hollow fibers
Dope PI
(wt.%)
PES
(wt.%)
α(CO2/N2) P(CO2)
(GPU)
δ (␮m)
A 20 80 40 40 0.1
B 50 50 35–38 31 0.16
C 80 20 39 60 0.11
of CO2 from hydrocarbons in natural gas streams.
Finally, future work on the developed PES/PI blend
asymmetric hollow fibers includes the study of: (1)
plasticization behavior for CO2 and/or CxHy, (2)
aging phenomena and (3) the separation performance
of binary gaseous mixtures.
4. Conclusions
Polyethersulfone–polyimide hollow fibers can be
prepared by the dry/wet spinning process using only
NMP as the single common solvent. The hypothesis
that a dope showing significant chain entanglement
may be one of the criteria to prepare ultrathin high
performance gas separation hollow fibers has been
validated for polymer blends. In this work, it is
demonstrated that the air-gap distance in the dry/wet
spinning process affects both membrane structure and
permeation properties. Porous skin layer, loose sub-
structure and high permeance values were obtained
for fibers spun at longer air-gap distances. After coat-
ing with a silicone rubber solution, the developed
blend hollow fibers exhibit CO2 permeance varying
from 31 to 60 GPU and CO2/N2 selectivities varying
from 40 to 35, at room temperature. The thickness of
the skin layer is as low as 1030 Å. The morpholog-
ical characteristics and the permeation properties of
polyethersulfone–polyimide hollow fibers are directly
comparable to existing commercial gas separation
membranes and therefore exhibit good perspective
for future industrial application.
Acknowledgements
This research was supported through a European
Community Marie Curie B-30 Fellowship (Contract
No. HPMFCT-2000-0475).
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