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Research Article
Received: 3 September 2019 Revised: 17 January 2020 Accepted article published: 6 February 2020 Published online in Wiley Online Library: 11 March 2020
(wileyonlinelibrary.com) DOI 10.1002/pi.5984
Fabrication of membranes of polyethersulfone
and poly(N-vinyl pyrrolidone): influence of
glycerol on processing and transport
properties
Ulrich A Handge,a
*
Oliver Gronwald,bMartin Weber,bJoachim Koll,a
Clarissa Abetz,aBirgit Hankiewiczcand Volker Abetza,c
Abstract
In this study, we focus on membranes of polyethersulfone and poly(N-vinyl pyrrolidone) and elucidate the influence of compo-
sition on the rheological, diffusion and precipitation properties of solutions which are used for membrane preparation via a
non-solvent-induced phase separation process. The low-molar-mass component of the solution is a mixture of the solvent
N-methyl-2-pyrrolidone and the non-solvent glycerol. Cloud point, viscosity and diffusion measurements as well as precipita-
tion experiments were performed in order to achieve a comprehensive understanding of the time dependence of the precipi-
tation process. The addition of glycerol yields an increase of viscosity and a stronger tendency for demixing. The enhanced
tendency for demixing causes a more rapid precipitation process. The average relaxation time of the solution as a function
of glycerol concentration follows a similar trend to its viscosity. The increase of viscosity is associated with the increase of
the monomeric friction coefficient. Two diffusive processes with clearly separated time scales appear in dynamic light scatter-
ing experiments in the presence of glycerol. This phenomenon is discussed taking into account the phase behaviour of the solu-
tion and the quality of the solvent. The addition of glycerol yields a lower pure water permeance whereas the molecular weight
cut-off is not altered in the ultrafiltration range.
© 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
Supporting information may be found in the online version of this article.
Keywords: polymer membranes; cloud point; phase inversion process; rheology; dynamic light scattering
INTRODUCTION
Membranes of poly(aryl ethers) and poly(N-vinyl pyrrolidone)
(PVP) are frequently used for water treatment applications.
1–3
Per-
meability, retention, molecular weight cut-off and resistance
against fouling are key properties which characterize a mem-
brane. Essential morphological parameters which determine
membrane performance are pore size distribution and porosity.
Consequently, an in-depth knowledge of the mechanisms which
determine morphology formation in membrane fabrication is cru-
cial for production of tailored membranes, for both porous as well
as dense polymer membranes.
4
However, until now a profound
understanding of morphology formation during membrane prep-
aration has been only partially achieved. An example is the pio-
neering work of Strathmann et al.
5
where the influence of the
precipitation rate on the membrane morphology was investi-
gated. Nowadays, a large number of investigations aims to
establish structure–processing–property relations for polymer
membranes which are prepared using the non-solvent-induced
phase separation process; see, for example, the work of Hopp-
Hirschler et al.
6
For example, in the work of Bakeri et al.
7
the phase
separation behaviour of polyetherimide solutions for different
non-solvents and coagulants was investigated. An essential ques-
tion is the choice of the appropriate composition of the casting or
spinning solution, the bore fluid in the case of hollow fibre spin-
ning and the coagulation bath. A variety of solvents has been cho-
sen for membrane preparation. However, glycerol has been used
to a much lesser extent.
8
Various previous studies have focused on the effect of
processing conditions on the performance of polysulfone and
*Correspondence to: UA Handge, Helmholtz-Zentrum Geesthacht, Institute of
Polymer Research, Max-Planck-Strasse 1, 21502 Geesthacht, Germany.
E-mail: ulrich.handge@hzg.de
aHelmholtz-Zentrum Geesthacht, Institute of Polymer Research, Geesthacht,
Germany
bBASF SE, Advanced Materials & Systems Research, Performance Polymer
Blends & Membranes RAP/OUB, Ludwigshafen, Germany
cUniversität Hamburg, Institute of Physical Chemistry, Hamburg, Germany
© 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
502
polyethersulfone (PESU) membranes.
9
The study of Barth et al.
9
revealed that an increasing polymer concentration in the solution
leads to a smaller pore size. The general mechanism of structure
formation in membrane preparation using PESU, PVP and N-
methyl-2-pyrrolidone (NMP) is elucidated in the work of Wienk
et al.
10
Generally, PESU has a higher tendency to form finger-like
structures than polysulfone because of the polarity of PESU.
9
However, in technological applications a uniform morphological
structure is preferred. Another study focused on the effect of
the dope extrusion rate.
1
A higher dope extrusion rate led to a
lower water flux. Theoretical investigations were carried out by
Boom et al. who analysed the phase diagram of dope solu-
tions.
11,12
In the work of Guillen et al.
13
it was shown that a
sponge-like morphology is created by a slow exchange of solvent
and non-solvent, whereas a fast solvent/non-solvent exchange
yields a finger-like morphology. Hopp-Hirschler et al. discussed
in detail the role of viscous fingering in membrane formation.
6
The role of shear fields has been addressed in the work of Gor-
deyev et al.
14
Under practical conditions, shear flow at high shear
rates mainly influences the molecular orientation of polymer
chains, but does not induce demixing.
14
Several publications are devoted to specific aspects of mem-
brane preparation. Increasing the viscosity of the casting solution
leads to a pronounced hindrance against phase separation.
15
In
the work of Han and Nam
15
the influence of PVP concentration
was investigated and an optimum PVP concentration with respect
to membrane permeability was found. The authors emphasized
the role of thermodynamics (reduction of miscibility by addition
of PVP) and rheology (kinetic hindrance because of high solution
viscosity). An essential aspect is the mutual diffusion of the differ-
ent components. A direct correlation between thickness of the
membrane and the viscosity of the casting solution was found
by Torrestiana-Sanchez et al.
16
Furthermore, the addition of PVP
or poly(ethylene oxide) (PEO) increased the porosity and conse-
quently membrane permeability. A comparison between linear
and hyperbranched PESU membranes was made by Yang
et al.
17
Hyperbranched PESU led to a smaller average pore diam-
eter and a narrower pore size distribution than the linear counter-
part. The influence of PVP on PESU membranes for hemofiltration
has been also studied.
18
It was shown that PVP with a molecular
weight of 360 kg mol
−1
led to a negatively charged, hydrophilic
inner surface of a hollow fibre membrane. Hollow fibre mem-
branes were also prepared by Alsalhy et al.
19
The authors applied
a so-called steam/wet/dry process for membrane fabrication and
analysed the influence of processing parameters on the separa-
tion performance and the mechanical properties of the mem-
brane. A strong influence of PEO concentration in the dope
solution on the membrane properties was found. Different types
of additives for membrane fabrication were tested in the work
of Susanto and Ulbricht.
20
The triblock copolymer Pluronic® seems
to be the most efficient additive for practical applications. Conse-
quently, it and related additives have been used in a variety of
studies.
3,21–23
In the study reported here, we focused on the effect of solution
composition on membrane properties. The objective was to ana-
lyse the role of glycerol in the fabrication of membranes of PESU
and PVP, since glycerol strongly affects the membrane structure.
The change in the thermodynamic properties as a result of the
addition of glycerol is studied. Rheological and dynamic light scat-
tering (DLS) experiments were carried out in order to characterize
the dynamical properties of the polymer solutions. Furthermore,
precipitation experiments were performed in order to compare
qualitatively these experimental results with the analysis of
Strathmann et al.
5
MATERIALS AND METHODS
Materials
The polymeric components of the solutions were commercial
grades of PESU (Ultrason® E 3010, BASF SE, Ludwigshafen) and
PVP (Luvitec® K90, BASF SE, Ludwigshafen). Ultrason® E 3010
and Luvitec® K90 were used as received. Sodium hypochlorite
(14% chlorine in aqueous solution) was obtained from VWR Inter-
national GmbH (Darmstadt, Germany). The weight-averaged
molecular weight of PESU was M
w
=58 000 g mol
−1
as deter-
mined using gel permeation chromatography in dimethylaceta-
mide (calibration with a poly(methyl methacrylate) standard).
The dispersity was equal to 3.3. PVP had a viscosity characterized
by a K-value of 90, determined according to the method of
Fikentscher.
24
The number-averaged molecular weight of this
PVP is of the order of 1.4 ×10
6
g mol
−1
with a dispersity of the
order of 4.3.
25
The low-molar-mass fluids were NMP (solvent)
and glycerol (non-solvent) and were also used as received. NMP
was supplied by BASF SE (Ludwigshafen). Glycerol (purity greater
than 99.5%) was purchased from Cremer Oleo GmbH & Co. KG
(Hamburg, Germany). The chemical structures of the polymers
and the solvents are shown in Fig. 1.
Preparation of polymer dope solutions
In this study, two different methods (A and B) for solution prepa-
ration were used. Before mixing, the commercial polymers were
dried at 130 °C for at least 24 h under vacuum (method A). Then
the polymers and the organic solvent were mixed using a mag-
netic stirrer at a temperature of 60 °C. After mixing, the solutions
were degassed under vacuum for 2 h.
The second method (method B) involved homogenizing the
components PESU and PVP in a mixture of NMP and glycerol
using a SpeedMixer™DAC 600 (Hauschild & Co. KG) with increas-
ing mixing speeds of 800, 1100, 1500 and 2000 rpm within
60 min.
Figure 1. Chemical structure of (a) polyethersulfone, (b) poly(N-vinyl pyrrolidone), (c) N-methyl-2-pyrrolidone and (d) glycerol.
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503
The composition of the solutions of method B was 19 wt%
PESU, 6 wt% PVP and 75 wt% solvent. The composition of the sol-
vent (NMP or its mixtures with glycerol with a glycerol content up
to 10 wt%) was varied by changing the glycerol concentration in
the complete solution from 0 to 10 wt%. A similar composition
of the polymer components was used in a related study.
15
Fur-
thermore, additional solutions for DLS experiments were pre-
pared using method A.
Cloud point measurements
Cloud point measurements were performed as follows. The poly-
mer solution was put into a glass flask which was heated using a
water bath at 20 and 60 °C. Then water or a water/NMP mixture
(60/40 wt%), respectively, was added to the solution using a titra-
tion device until the solution changed from transparent to turbid
(point of precipitation). After each addition of a liquid drop (water
or water/NMP mixture) the polymer solution was shaken again for
at least 1 h in order to verify whether a transparent state had been
achieved. The mass of added water was determined and the
weight fraction of added water at each step was calculated.
Rheological experiments
All rheological measurements were performed at 60 °C. The vis-
cosity of mixtures of NMP and glycerol was measured using an
Ubbelohde viscosimeter (iVisc LMV 830, LAUDA-Scientific GmbH,
Lauda-Königshofen). The kinematic viscosity was directly mea-
sured and then converted into the dynamic viscosity using the
measured value of the solution density (Density Meter DMA™
4100M, Anton Paar, Graz, Austria).
The rheological experiments were performed using a rotational
rheometer (MCR 502, Anton Paar, Graz, Austria). A concentric cyl-
inder geometry (Searle type) was chosen for the investigations.
First a defined amount (14.0 mL) of bubble-free solution was
inserted into the gap of the Searle geometry. A waiting time of
10 min was chosen in order to guarantee that temperature equil-
ibration was achieved. So-called frequency sweeps were per-
formed. In order to determine the linear viscoelastic range of
the oscillatory shear measurements, an amplitude sweep was per-
formed at an angular frequency of ω=10 rad s
−1
where the shear
amplitude γ
0
was varied between 0.2 and 20%. A shear amplitude
of γ
0
=10% was chosen for the frequency sweeps. The frequency
was incrementally reduced from 100 to 0.01 rad s
−1
(five points
per decade).
Dynamic light scattering
In order to study diffusion phenomena and to determine the dif-
fusion coefficient which is associated with these diffusion pro-
cesses, DLS experiments were performed with an ALV/CSG-3
Compact Goniometer System (ALV-Laser Vertriebsgesellschaft
GmbH, Langen, Germany) which was equipped with an
ALV/LSE-5003 multiple tau digital correlator (for variation of the
glycerol concentration). The light source was a He–Ne laser with
a wavelength ⊗of 632.8 nm. The experiments to vary the PVP
and PESU concentration were performed with the ALV/CSG-3
which was equipped with an ALV-7004 multiple tau correlator.
Figure 2. Scheme of the experimental set-up of the precipitation experiment, performed using a light microscope.
Figure 3. Results of cloud point measurements using (a) pure water and (b) a water/NMP (60/40 wt%) mixture as non-solvent. ϕ
non −solvent
is the con-
centration of water and the water/NMP mixture at the cloud point and c
Glycerol
is the concentration of glycerol (both in wt%). The temperature of the solu-
tion is indicated.
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The light source was a Nd:YAG laser with a wavelength ⊗of
532.0 nm. The scattering angle ⊔was varied between 40°and
140°in increments of 5°. The magnitude qof the Bragg wave vec-
tor is related to the wavelength ⊗and the solvent refractive index
nby
q=4πn
⊗sin ⊔=2ðÞ ð1Þ
The sample vials (glass) were placed into a measurement cell
which was filled with toluene. The toluene bath was tempered
using a Julabo F25 thermostat working with a mixture of water
and ethylene glycol. The accuracy of the temperature was
0.01 °C. Each sample was measured at two temperatures,
i.e. 20 and 60 °C. The time for a measurement at a constant
angle was 120 s.
Afitting procedure was applied in order to determine the relax-
ation time spectrum. A sum of two exponential functions was
used as fitting function for the field autocorrelation function to
determine the mean relaxation rate Γand the fast relaxation pro-
cess. The fitting was implemented as a nonlinear cumulant analy-
sis in Matlab (see Mailer et al.
26
for details). The series expansion
was taken until the second order. In order to determine the relax-
ation time spectrum, the software NLREG
27
was used which is
based on a regularization routine.
28
Precipitation experiments
A special precipitation experiment was carried out in order to elu-
cidate the time dependence of the precipitation process. The
experimental set-up is similar to the technique proposed by
Frommer and Lancet
29
(Fig. 2). The experiment focuses on the
time scale of diffusion of a water/NMP (60/40 wt%) mixture in
the polymer solution and on the morphological features of the
membrane.
In our experiments, two tapes were fixed on the longer edges of
a glass slide. A drop of the polymer solution was placed on the
bottom glass slide. Then another glass slide was immediately
put onto the droplet of the polymer solution with a distance of
30 mm to a short edge. The two tapes guarantee that the gap
between the two glass slides is constant (approx. 50 μm). A
Figure 4. Dynamic viscosity of mixtures of NMP and glycerol at a temper-
ature of 60 °C. The inset shows the range of concentrations from 0 to
10 wt% glycerol. A linear curve was fitted to these data.
Figure 5. (a) Storage modulus G0and loss modulus G00 as functions of angular frequency ωfor solutions of 19 wt% PESU, 6 wt% PVP, xwt% glycerol and
(75 –x) wt% NMP. (b) Magnitude of complex viscosity η
*
for these solutions as a function of ω. (c) Zero shear rate viscosity η
0
and (d) average relaxation
time τversus concentration of glycerol based on the fit shown in (b).
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mixture of Millipore water and NMP (weight ratio of 60/40) was
placed using a syringe on the free part of the bottom glass plate.
Driven by capillary forces, the water/NMP mixture flows in the gap
between the bottom and top glass plates which leads to precipi-
tation of the solution droplet. Using an optical microscope (Leica
DM LM, Wetzlar, Germany) equipped with a digital camera (Leica
Type DFC320), the time dependence of this precipitation process
was monitored using image acquisition software (IMS, IMAGIC AG,
Glattburg, Switzerland). By analysis of the video images, the dis-
placement of the diffusion front of the water/NMP mixture was
measured as a function of time. This experiment mimics the real
precipitation process which takes place during membrane cast-
ing. These experiments were performed at room temperature
similar to fabrication of the flat sheet membranes.
Casting of flat sheet membranes
The casting solutions were prepared by dissolving 19 g of PESU
with 6 g of PVP and 0 to 10 g of glycerol as additional dope addi-
tive in 75 to 65 g of NMP. Under gentle stirring the mixture was
heated at 60 °C until a homogeneous and clear viscous solution
was obtained. To remove air bubbles the solution was subse-
quently degassed overnight at room temperature. Then the poly-
mer dope solution was cast onto a glass plate by means of a knife
with a thickness of 300 μm with a casting speed of 5 mm s
−1
.
Before immersion in the aqueous coagulation bath containing
40 wt% NMP at 25 °C for 10 min the polymer dope film could rest
for 30 s. After the membrane had detached from the glass plate, it
was carefully transferred into a water bath for 12 h. Subsequently
the membrane was transferred into a bath containing 2000 ppm
sodium hypochlorite at 60 °C for 1.5 h and then washed with
water at 60 °C and once with a 0.5 wt% solution of sodium bisul-
fite to remove active chlorine. Until characterization regarding
pure water permeability (PWP) and minimum pore size, the mem-
brane was stored wet in distilled water.
Membrane characterization
The PWP of the membranes was determined with sample speci-
mens of 60 mm in diameter using a pressure cell with ultrapure
water (salt-free water, filtered by a Millipore UF-system).
The molecular weight cut-off was measured with PEO ultrafiltra-
tion and gel permeation chromatography sieving curve analysis
according to a total PEO concentration of 2.5 g L
−1
in water.
30
The concentration polarization effect on the rejection was kept
minimal by using the lowest possible pressure of 0.15 bar. The
value of the molecular weight cut-off was defined as the molecu-
lar weight from which 90% rejection takes place.
The ultrafiltration membrane samples were subjected to solvent
exchange with ethanol/water mixtures of increasing alcohol con-
tent and subsequently dried overnight at 40 °C in vacuum. Then
the samples were characterized using mercury intrusion porosi-
metry with an Autopore V (Micromeritics, Norcross, GA, USA).
The membrane morphology was determined by means of SEM.
Liquid nitrogen was used to immerse the membranes obtained
before fracture. Each specimen was mounted on a sample holder
and sputter-coated with a gold layer of 5 nm thickness to confer
electrical conductivity. The cross-sectional morphology was
Figure 6. (a) Normalized intensity auto-correlation function ^
g2q,tðÞ=g2q,tðÞ−1½=⊎, (b) relaxation time spectrum H(τ) for different qvalues and
(c) relaxation rate Γversus q
2
at 60 °C for the reference solution with 19 wt% PESU, 6wt% PVP and 75 wt% NMP.
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506
determined using a scanning electron microscope (Phenom Pro X,
Phenom-World) with a magnification of ×1500 and ×5000 at an
acceleration voltage of 15 kV.
RESULTS AND DISCUSSION
Cloud point measurements
The results of cloud point measurements are shown in Fig. 3. Pure
water and a water/NMP mixture (60/40 wt%) were used as non-
solvents. An increasing fraction of glycerol leads to a lower
amount of non-solvent until demixing takes place. The data in
Fig. 3(a) reveal that a linear relationship holds with only minor dif-
ferences between the data at 20 and at 60 °C. If a water/NMP
(60/40 wt%) mixture is used as non-solvent (Fig. 3(b)), generally
higher concentrations of the non-solvent are necessary in order
to cause precipitation, since NMP is a solvent for PESU and PVP.
Thus a linear relation also holds in the case of the water/NMP mix-
ture. The data in Fig. 3(b) indeed are approximately equal to the
data in Fig. 3(a) if only the added water concentration is plotted
versus c
Glycerol
. In conclusion, the addition of glycerol yields a
stronger tendency for phase separation. Consequently, phase
separation during membrane casting should take place more rap-
idly with increasing fraction of glycerol if the solution viscosity
remains constant.
Rheology
The viscosity of glycerol is larger than that of NMP because of the
hydrogen bonding (see also the review of Ferreira et al.
31
and the
data in Fig. 4). Consequently, the addition of glycerol leads to an
overall increase of viscosity which directly influences the viscosity
of the polymer solution. The increase of viscosity follows in total a
strongly nonlinear trend in the whole concentration range. How-
ever, in this study, the concentration range of 0 to 10 wt% for
glycerol is of relevance where a linear dependence (low glycerol
concentrations) of the dynamic viscosity as a function of glycerol
concentration was observed at the measurement temperature of
60 °C (inset of Fig. 4).
Figure 5 presents the storage and loss moduli G0and G00 of the
polymer solutions as a function of the angular frequency ωat a
measurement temperature of 60 °C. In this regime of a semi-
dilute polymer solution, typical features of a Zimm fluid become
apparent. The addition of glycerol increases the magnitude of
the complex viscosity η
*
at low angular frequencies ω. This effect
is caused by the relatively high value of viscosity of glycerol and
the polymer–glycerol interactions. The data can be described by
the Cross model:
η*ωðÞ
=η0
1+ τωðÞ
mð2Þ
with zero shear rate viscosity η
0
, power-law exponent mand aver-
age relaxation time τ. Equation (2) was fitted to the experimental
data. The fitting parameters are shown as a function of glycerol
concentration in Figs 5(c) and (d). In the investigated range of
concentrations, the increase of the magnitude of complex viscos-
ity is linear, and the addition of glycerol is associated with a linear
trend with a slope of 0.44 Pa s/1 wt%. The characteristic relaxation
time τis of the order of 20 to 30 ms depending on the
Figure 7. (a) Normalized intensity auto-correlation function for q=23.2 μm
−1
and different glycerol concentrations, (b) relaxation time spectrum H(τ) for
different qvalues and (c) relaxation rate Γversus q
2
at 60 °C for solutions with 19 wt% PESU, 6 wt% PVP, xwt% glycerol and (75 –x) wt% NMP.
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concentration of glycerol. The zero shear rate viscosity and the
average relaxation time follow a similar (linear) trend which indi-
cates that the average elastic modulus remains constant within
experimental scatter (±10%). Our data reveal that the viscosity
increase is mainly determined by the increase of the average
monomeric friction coefficient. The polymer–solvent (mixture of
NMP and glycerol) interactions are described by the value of the
monomeric friction coefficient. Because of its relatively high vis-
cosity, the addition of glycerol changes the average value of the
monomeric friction coefficient as determined by our rheological
experiments.
Dynamic light scattering
DLS of polymer solutions can be used to analyse diffusion phenom-
ena.
32
In the semi-dilute regime which is considered here, interac-
tions between polymer chains are of relevance and strongly
influence the diffusion of the polymer chains. In a conventional
DLS experiment, the time-dependent intensity auto-correlation
function g
2
(q,t) is measured which is used for calculation of the nor-
malized field correlation function g
1
(q,t) via the Siegert relation:
g2q,tðÞ=1+⊎g1q,tðÞ
2
ð3Þ
with the coherence factor ⊎(an instrument-specific constant
which ranges between 0 and 1). Generally, the field auto-
correlation function g
1
(q,t) decreases with time twith a distribu-
tion H(τ) of relaxation times τ:
33
g1q,tðÞ=ð∞
0
HτðÞexp −t=τðÞdlnτð4Þ
The relaxation rate Γis defined by Γ=1/τ. In the case of a Brow-
nian diffusion process, this quantity is related to the translational
diffusion coefficient Dby
Γ=Dq2ð5Þ
In the following, the experimental data are discussed using the
normalized auto-correlation function which is denoted by
^
g2q,tðÞ=g2q,tðÞ−1½=⊎.
Figure 6 shows the normalized auto-correlation function, the
relaxation time spectrum and the relaxation rate as a function of
q
2
for the reference solution with 19 wt% PESU, 6 wt% PVP and
75 wt% NMP. The normalized auto-correlation function rapidly
decays on the time scale of 10
−5
to 10
−4
s (Fig. 6(a)). The principal
relaxation processes can be seen in Fig. 6(b). The most rapid pro-
cess is the most dominant process. The relaxation rate Γis propor-
tional to q
2
which indicates a diffusive process (Fig. 6(c)). This
process is associated with the total polymer concentration (coop-
erative diffusion). The influence of glycerol on the DLS data is
shown in Fig. 7. The experimental data reveal that the addition
Figure 8. Concentration dependence of the diffusion coefficient Das determined by DLS experiments. The measurement temperature is indicated.
(a) The concentration of PESU is 19 wt% and the concentration of PVP is 6 wt%. The concentration of glycerol is varied. (b) The concentration of PESU
is 19 wt%. The concentration of PVP is varied. The glycerol concentration is 0 wt%. (c) The concentration of PVP is 6 wt%. The concentration of PESU is
varied. The glycerol concentration is 0 wt%.
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508
of glycerol leads to the visual appearance of a second relaxation
process (Fig. 7(a)). This process is gradually more pronounced with
increasing glycerol concentration and most clearly seen at a glyc-
erol concentration of 10 wt%. The analysis of the relaxation time
spectrum also indicates that the second relaxation process
appears for all qvalues (Fig. 7(b)). The relaxation rate again is pro-
portional to q
2
and thus also indicates a diffusion process. Further-
more, the data of the relaxation rate reveal that the addition of
glycerol reduces the value of the diffusion coefficient because of
the viscosity increase caused by the addition of glycerol. The
DLS data convincingly demonstrate that the two main diffusive
processes appear on two clearly separated time scales. This result
shows that a new mechanism must be responsible for the slower
Figure 9. SEM micrographs of cross-sections of polymer membranes prepared using different dope solutions. The polymer concentrations in the dope
solution were 19 wt% PESU and 6 wt% PVP. The concentration of glycerol (in wt%) is (a) 0, (b) 2, (c) 4, (d) 6, (e) 8 and (f) 10.
Table 1. Membrane thickness obtained from polymer dope solu-
tions and coagulation in a water/NMP (60/40 wt%) mixture
Glycerol concentration (wt%) Thickness (μm)
0 149
2 166
4 161
6 166
8 168
10 166
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diffusion process. Since glycerol changes the solvent quality to a
poor solvent, concentration fluctuations may lead to the forma-
tion of clusters of polymer chains because of the stronger ten-
dency to precipitation. The diffusion of these clusters of polymer
chains leads to the second, slower diffusion process. An increasing
glycerol concentration causes on the average larger clusters of
polymer chains which leads to a lower diffusion coefficient D.
In order to elucidate the origin of these two relaxation processes
in more detail, various different dope solutions were prepared
without glycerol and analysed using DLS. Dope solutions with a
constant PESU concentration of 19 wt% and a varying PVP con-
centration were prepared. In addition, solutions with a constant
concentration of PVP of 6 wt% and a varying PESU concentration
were also prepared. The results of these measurements are
Figure 10. (a) Pore area and (b) median of pore diameter as determined by porometry measurements of the membranes of this study.
Figure 11. SEM micrographs of the lower side of the flat sheet membrane which was prepared using (a) 0 wt% glycerol and (b) 10wt% glycerol in the
dope solution.
Figure 12. (a) PWP and (b) molecular weight cut-off (MWCO) of the polymer membranes of this study.
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presented in Fig. 8 which shows the dependence of the diffusion
coefficient on glycerol, PESU and PVP concentrations. Figure 8
(a) reveals that the diffusion coefficient decreases with glycerol
concentration which can be explained by the increase of the
monomeric friction coefficient caused by the addition of glycerol
(higher value of zero shear rate viscosity). In the absence of glyc-
erol the slow process is much less pronounced than the rapid pro-
cess. Figures 8(b) and (c) show that in the absence of glycerol the
diffusion coefficient of the rapid process is almost independent of
the total polymer concentration. The diffusion coefficient of the
(less pronounced) slow process increases with polymer concen-
tration which indicates that a cooperative diffusion process takes
place.
Membrane morphology
PVP simultaneously acts as a pore former
34
and as a viscosity
enhancer. SEM micrographs of the cross-sections of the mem-
branes are shown in Fig. 9. Only in the case of 0 wt% glycerol does
the membrane thickness attain a significantly lower value than in
the case of the other membranes (Table 1). Generally, the phase
inversion process leads to an asymmetric membrane structure
with smaller pores (nanometre range) in the top layer and larger
pores (micrometre range) in the bottom layer, since the water/
NMP mixture diffuses into the cast polymer solution starting from
the top layer. If the solvent purely consists of NMP, a large number
of macrovoids can be seen. At a concentration of glycerol of up to
4 wt% such macrovoids are still visible. The macrovoids disappear
in our series of experiments at glycerol concentrations of 6 wt%
and higher. Consequently, already at a glycerol concentration of
approximately 6 wt% the conditions for macrovoid formation
are unfavourable. It seems that the addition of glycerol acceler-
ates phase separation such that no time for the development of
macrovoids is available. Furthermore, the addition of glycerol
causes a deeper quench into the region of spinodal decomposi-
tion which is associated with a strong driving force to maintain
this mechanism of phase separation. The surface morphology
(lower and upper side of the membrane) was investigated using
SEM. The diameter of the pores on the upper side of the mem-
brane was smaller than the experimental resolution and thus
the pores on the upper side could not be detected (supporting
information). It seems that the morphology on the upper side of
the membrane is caused by a deeper quench into the spinodal
region which is associated with a smaller wavelength of spinodal
decomposition. The pore area and the median of the pore diam-
eter of the membranes of this study were determined by porome-
try measurements. The pore area increases with glycerol
concentration which indicates a higher surface porosity with
increasing concentration of glycerol (Fig. 10(a)). On the other
hand, the median of the pore diameter decreases with glycerol
concentration (Fig. 10(b)).
The SEM micrographs of the lower side of the membranes also
show a more uniform porous structure with smaller pore sizes
and almost no macrovoids for a glycerol concentration of 10 wt
% in comparison to the membrane prepared by a solution without
glycerol (Fig. 11). The PWP as a function of glycerol concentration
is shown in Fig. 12(a). The decrease of PWP with increasing glyc-
erol fraction reflects the fact that the number of macrovoids and
the average pore diameter decrease with increasing glycerol frac-
tion while the molecular weight cut-off values range from 41.0 to
47.3 kDa regardless the glycerol concentration (Fig. 12(b)) and
thus remains approximately constant. Hence the decrease of
PWP with concentration of glycerol is also in agreement with
the results of the porometry measurements (median of pore
diameter). The PWP value linearly varies with glycerol concentra-
tion. The results of our characterization experiments reveal that
the addition of glycerol simultaneously increases the polymer
solution viscosity and decreases the necessary amount of non-
solvent for precipitation. The phase separation process is influ-
enced by the thermodynamic properties (phase diagram) and
viscosity which both depend on the concentration of glycerol.
The increase of viscosity from the dope solution with 0 wt% glyc-
erol to the one with 10 wt% only is approximately 50% in the
Newtonian regime and less at higher shear rates. Hence we
assume that the change of the phase diagram by the addition of
glycerol essentially triggers the formation of a sponge-like
Figure 13. Evolution of the diffusion front (precipitated region) with time
in precipitation experiments. The position of the diffusion front is denoted
by x
front
with the left boundary of the drop of the polymer solution at
x
front
=0.
Figure 14. Optical micrographs of the morphology of the precipitated solution at time t=60 s. The non-solvent diffuses into the solution and causes
precipitation starting from the left-hand side of the micrograph. The different types of morphology for different glycerol concentrations can be
clearly seen.
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511
structure. Furthermore, after phase separation each of the two
phases is associated with different viscosities. The superposition
of these two effects (change of thermodynamic and viscous prop-
erties) complicates a firm conclusion about the mechanism of
morphology formation. The present data for these membranes
based on different glycerol concentrations show that the forma-
tion of a compact sponge-type substructure is favoured at a large
glycerol concentration, although the precipitation of the poly-
meric components in the solution is not slowed down. This result
is in contrast to the analysis of Strathmann et al.
5
In summary, the
formation of defects such as macrovoids is suppressed with
higher glycerol concentrations. At a glycerol concentration of
10 wt%, a well-established sponge-like substructure covered by
a compact filtration layer is clearly visible.
Precipitation experiments
In order to get more insight into the precipitation process, a funda-
mental precipitation experiment was performed. The experimental
set-up is shown in Fig. 2. This experiment elucidates the time depen-
dence of the precipitation process. The experiments were performed
at room temperature similar to the condition for membrane casting.
The diffusion of the water/NMP mixture (i.e. the coagulation bath)
into the casting solution was analysed by measurement of the pre-
cipitation front as a function of time. The results of these measure-
ments are presented in Fig. 13 which shows the evolution of the
water/NMP front with time. We note that these experiments are
associated with experimental scatter which might be caused by
humidity in the air. However, a clear trend can be observed for the
membranes prepared using 0 and 10 wt% glycerol. The addition of
glycerol into the dope solution is associated with a faster movement
of the front of the precipitated region with time. Therefore, the addi-
tion of glycerol changes the demixing rate which has a strong influ-
ence on the membrane properties; see also Torrestiana-Sanchez
et al.
16
In addition, the membrane morphology after 1 min of diffu-
sion time is shown in Fig. 14. The micrographs also reveal the forma-
tion of macrovoids in the absence of glycerol, whereas a more
uniform structure is formed for the solution with 10 wt% glycerol.
CONCLUSIONS
We focused on membrane preparation using commercial grades
of PESU and PVP at a specific polymer composition. We studied
in detail the influence of adding glycerol to the casting solution
on the processing and membrane properties. Generally, the addi-
tion of glycerol yields an increase of solution viscosity (roughly
40% at a glycerol concentration of 10 wt%) and precipitation at
a lower water content (2 wt% in case of pure water) as revealed
by cloud point measurements. Consequently, precipitation pro-
ceeds faster at a higher glycerol concentration as shown by the
analysis of a precipitation experiment. The change of rheological
properties is much less pronounced than the alteration of the
phase diagram by the addition of glycerol. In DLS experiments,
two diffusive processes were observed. The addition of glycerol
yields a change from a morphology with many macrovoids to a
sponge-like morphology with a smaller pore size. This effect was
explained by the change of thermodynamic and viscous proper-
ties caused by the addition of glycerol.
ACKNOWLEDGEMENTS
The financial support of the Federal Ministry of Education and
Research (BMBF project MABMEM, grant no. 03XP0043E) is
gratefully acknowledged. The authors also thank Melanie Reyes
(oscillatory rheology), Kristian Buhr (viscosimetry) and Anke-Lisa
Höhme (image acquisition) for experimental support. The DLS
experiments of Margarethe Fritz and Nina Schober are gratefully
acknowledged.
SUPPORTING INFORMATION
Supporting information may be found in the online version of this
article.
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