Molecular engineering of high-performance nanofiltration membranes from intrinsically microporous poly(ether-ether-ketone)

Article abstract of DOI:10.1039/d0ta08194a

Poly(ether-ether-ketone) has received increased attention due to its high thermal and chemical stability, and high performance in various applications. However, it suffers from a semi-crystalline morphology, low fractional free volume, and poor processability, requiring the use of harsh acidic solvents, which leads to undesired sulfonation. In this work, three intrinsically microporous poly(ether-ether-ketones) (iPEEKs), incorporating spirobisindane, Tr?ger's base, and triptycene contorted structures, were developed for organic solvent nanofiltration. Molecular dynamics simulations have assisted the molecular engineering of the polymers and the understanding of the improved membrane performance through the binding energies between solvents and polymers. Application of the design principles of polymers of intrinsic microporosity has led to a paradigm shift with a notable enhancement in both the polymer properties and the subsequently fabricated nanofiltration membranes' performance. The iPEEKs showed excellent solution processability, a high surface area of 205-250 m2 g-1, and excellent thermal stability. Mechanically flexible nanofiltration membranes were prepared from N-methyl-2-pyrrolidone dope solution at iPEEK concentrations of 19-35 wt%. The molecular weight cutoff of the membranes was fine-tuned in the range of 450-845 g mol-1 displaying 2-6 fold higher permeance (3.57-11.09 L m-2 h-1 bar-1) than previous reports. The long-term stabilities were demonstrated by a 7 day continuous cross-flow filtration. This journal is

Full text of DOI:10.1039/d0ta08194a

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DOI: 10.1039/D0TA08194A.
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DOI: 10.1039/D0TA08194A
ARTICLE
Molecular engineering of high-performance nanofiltration
membranes from intrinsically microporous poly(ether-ether-
ketone)
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
b
c
Mahmoud A. Abdulhamid, Sang-Hee Park, Hakkim Vovusha, Faheem Hassan Akhtar, Kim Choon
c
b
*a
Ng, Udo Schwingenschlögl and Gyorgy Szekely
Poly(ether-ether-ketone) has received increased attention due to its high thermal and chemical stability, and high
performance in various applications. However, it suffers from semi-crystalline morphology, low fractional free volume,
and poor processability, requiring the use of harsh acidic solvents, which leads to undesired sulfonation. In this work,
three intrinsically microporous poly(ether-ether-ketone) (iPEEK), incorporating spirobisindane, Tröger’s base, and
triptycene contorted structures, were developed for organic solvent nanofiltration. Molecular dynamics simulations have
assisted the molecular engineering of the polymers and the understanding of the improved membrane performance
through the binding energies between solvents and polymers. Application of the design principles of polymers of intrinsic
microporosity has led to a paradigm shift with a notable enhancement in both the polymer properties and the
subsequently fabricated nanofiltration membranes’ performance. The iPEEKs showed excellent solution processability,
2
-1
high surface area of 205–250 m g , and excellent thermal stability. Mechanically flexible nanofiltration membranes were
prepared from N-methyl-2-pyrrolidone dope solution at iPEEK concentrations of 19–35 wt%. The molecular weight cutoff
-
1
of the membranes was fine-tuned in the range of 450–845 g mol displaying 2–6 fold higher permeance (3.57–11.09 L
m-2 h-1 bar ) than previous reports. The long-term stabilities were demonstrated by a 7-day continuous cross-flow
filtration.
-1
Poly(ether-ether-ketone) (PEEK) has attracted the attention of
membrane scientists in recent years due to its excellent solvent
Introduction
Organic solvent nanofiltration (OSN) is an energy-efficient separation
technology that can distinguish solute concentrations in the range of
resistance and thermal stability.3–8 PEEK is a semi-crystalline polymer
with very low fractional free volume (FFV). PEEK is insoluble in
organic solvents and can only be dissolved in methanesulfonic acid
1
00–2,000 g mol-1 using solvent-resistant membranes in organic
media.1,2 OSN has found numerous applications in the petrochemical
and fine chemical industries, such as product or catalyst purification
and recovery, and solvent recycling or exchange. One of the key
challenges in OSN is to develop new membrane materials with
improved performance, manifested in higher permeance, better
selectivity, and better chemical and thermal stability.
(
MSA) and sulfuric acid (SA).6 However, this advantage of OSN
applicability results in poor processability. The harsh acidic
conditions required for making PEEK membranes are undesirable,
and also lead to the unavoidable sulfonation of the polymer. The
degree of sulfonation can be minimized to less than 7% by using a
5
solvent mixture of MSA and SA (3:1) at 20 °C. Nonetheless, the
3
presence of –SO H groups adversely affects the properties of the
membrane materials by decreasing the chemical and thermal
stability, the adsorption of solutes, and even the potential to
transform fine chemicals.9–10
a. Advanced Membranes and Porous Materials Center, Physical Science and
Engineering Division (PSE), King Abdullah University of Science and Technology
(
KAUST), Thuwal, 23955-6900, Saudi Arabia. E-mail:
gyorgy.szekely@kaust.edu.sa, Web: www.szekelygroup.com, @SzekelyGroup
Physical Science and Engineering Division (PSE), King Abdullah University of
Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia.
Water Desalination and Reuse Center, King Abdullah University of Science and
Technology, Thuwal 23955-6900, Saudi Arabia.
b.
c.
A previously proposed solution includes converting the carbonyl
group of the PEEK to a ketal in a two-step synthesis and fabricating a
membrane from the resulting soluble precursor, followed by an acid
†
Electronic Supplementary Information (ESI) available: materials and methods;
8
treatment, thus allowing to retain the original PEEK structure. In
instrumental procedures; NMR, FTIR spectra; DSC, BET, nanoindentation graphs;
SEM, AFM images; filtration tests, swelling experiments and MD simulation. See
DOI: 10.1039/x0xx00000x
another approach, carboxylic acid moieties were introduced into the
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DOI: 10.1039/D0TA08194A
Scheme 1 a) The concept of molecular fusion of PEEK and PIM into intrinsically microporous poly(ether-ether-ketone) (iPEEK) by the substitution of the
phenyl ring in PEEK by contorted structures to enhance the fractional free volume (FFV), porosity and the solution-processability. b) Synthetic route for the
contorted, diol monomers and their corresponding polymers.
PEEK framework, which improved its solubility and allowed fusion into intrinsically microporous poly(ether-ether-ketone)
crosslinking of the resulting membrane. 11 The negligible aging of (iPEEK) is explored. Three solution-processable iPEEKs were
PEEK in organic solvents was also recently demonstrated, which prepared by nucleophilic aromatic substitution reaction using three
triggered further interest in this class of polymers for OSN different contorted monomers and 4,4'-difluorobenzophenone
applications.7
(Scheme 1). The phenyl ring in the commercial PEEK was substituted
by three different kinked structures, namely spirobisindane (SBI),
Tröger’s base (TB), and triptycene (Trip), resulting in iPEEK-SBI, iPEEK-
TB, and iPEEK-Trip, respectively.
Polymers of intrinsic microporosity (PIMs) have emerged as high
surface area, thermally stable, solution-processable polymers.12 The
design concept of these materials is based on the insertion of non-
planar, contorted structures into a polymer backbone that can inhibit
polymer chain packing, and therefore increase the FFV of the
material.13–15 A plethora of applications have successfully exploited
Experimental
Monomer synthesis
PIMs, including gas separation,16 hydrogen storage,17 sensors,18 Synthesis
of
3,3,3',3'-tetramethyl-2,2',3,3'-tetrahydro-1,1'-
electrochemistry,19 catalysis,20 and printed electronics.21 PIM-1 is the spirobi[indene]-6,6'-diol (SBI) (2)
first choice for the preparation of OSN membranes, either
4
,4′-Isopropylidenediphenol (Bisphenol A) (20 g, 88 mmol) and
unmodified,22–24 modified,25 or as a polymer blend.26 Subsequent
studies have also explored PIM-7 and PIM-8.27 In general, these
membranes demonstrate i) high permeance compared to non-
intrinsically porous polymer membranes, and ii) a wide range of
molecular weight cut-off (MWCO) values between 190 and 650 g
methanesulfonic acid (3 ml) were mixed and heated at 135 °C for 2.5
h. The obtained brown sticky oil was poured onto iced water and left
to stir for an hour before filtrating the fine brown powder. White
powder (6.3 g, 70% yield) was obtained after recrystallizing the crude
product by water/methanol (60/40, wt/wt). Further purification was
-
1 26, 28–30
mol .
Consequently, both PIM and PEEK have great potential
1
done to get high purity product for polymerization reaction. H NMR
as engineering membrane materials, and herein their molecular
2
| J. Name., 2012, 00, 1-3
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(
400 MHz, DMSO-d
J=12.92 Hz), 2.24-2.28 (d, 2H, J=12.96 Hz), 6.11 (d, 2H, J=2.24 Hz), 6.6 prepared by one-step high-temperature DaOrIo: m10a.1t0ic39/nDu0cTlAe0o8p1h9i4liAc
dd, 2H, J=8.16 Hz), 7.0 (d, 2H, J=8.16 Hz), 9.01 (s, 2H). 13C NMR (100 substitution reaction (S
Ar) using an equimolar amount of the
MHz, DMSO-d , δ): 31, 32.1, 42.8, 57.4, 59.8, 110.4, 114.8, 122.8, commercially available 4,4'-Difluorobenzophenone and the
42.6, 151.9, 157.2; MS-HESI (m/z): [M+CH
6
, δ): 1.25 (s, 6H), 1.32 (s, 6H), 2.09-2.12 (d, 2H, As shown in Scheme 1, iPEEK-SBI, iPEEK-TB and iPEViEeKw-ATrrtiicple Ownelirnee
(
N
6
-
1
[
3
COO ] calcd for corresponding diol compounds (i.e. SBI, TB, and Trip) in DMAc and in
the presence of K CO . The general procedure of this polymerization
-
C
21
H
24
O
2
+ CH
3
COO ]: 367.19; Found: 367.00.
2
3
reaction is as following: 4,4'-difluorobenzophenone and the diol
compounds were added to a two-necked 200 ml round bottom flask
equipped with a Dean-Stark apparatus under nitrogen atmosphere.
The reagents were dissolved in anhydrous DMAc and anhydrous
Synthesis of 6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-
diol (TB) (3)
p-Anisidine (10 g, 82.5 mmol) was added to a 500 ml dry round
bottom flask, followed by the addition of 80 ml CF COOH and
3
2 3
toluene (4/1: DMAc/toluene). 1.2 equivalent of K CO was added and
paraformaldehyde at room temperature. The reaction was left to stir
for 24 h before pouring it onto iced water and ammonium hydroxide
solution. The pH of the medium was adjusted to 6, and the obtained
precipitates were collected by filtration. Silica gel column was used
to further purify this intermediate using hexane/ethyl acetate: 1/2 as
eluent to obtain dimethoxy Tröger’s base as a light yellow crystals (8
g, 70% yield).
the reaction was heated to 140 °C and kept for few hours for
azeotropic distillation to totally remove water that was produced
during the reaction. After total collection of toulene, the reaction
was then heated to 165 °C and left for about 14–16 hours. Then
reaction was then poured onto distilled water and stirred for 6 hours,
filtered and then refluxed for 24 hours with water, and 24 hours with
methanol before drying in the vacuum oven at 180 °C for 24 hours.
1
The dry dimethoxy Tröger’s base (7 g, 24.8 mmol) was dissolved The polymers were fully characterized by H NMR, FT-IR, GPC, BET
3
in anhydrous dichloromethane and cooled down to 0 °C.BBr was surface area and demonstrated good solution processability, which
added dropwise and then the reaction was left to stir overnight at allowed the fabrication of robust membranes using phase inversion
room temperature. The reaction mixture was poured onto water and method.
stir for 6 hours under nitrogen flushing, before adjusting the pH of
the medium between 5-6 using 10% of Na CO . The obtained
1
3
iPEEK-SBI. Yield: 94%. H NMR (500 MHz, CDCl , δ): 1.37 (s, 6H),
2
3
1
6
7
.38 (s, 6H), 2.26-2.29 (d, 2H, J=15 Hz), 2.40-2.43 (d, 2H, J=15 Hz),
.56 (d, 2H, J=2.1 Hz), 6.89 (dd, 2H, J=6.9 Hz), 6.94 (d, 4H, J=8.55 Hz),
.14 (d, 2H, J=8.2 Hz), 7.71 (d, 4H, J=8.65 Hz). 13C NMR (125 MHz,
, δ): 30.3, 31.8, 43.2, 57.6, 59.6, 115.9, 116.6, 119.2, 123.2,
31.9, 132.2, 148.5, 152.3, 154.8, 161.7, 194.1; FT-IR (ν, cm ): 2950-
precipitates were collected and dried in the oven at 110 °C for 24
1
hours to afford white solid (5.5 g, 90%). H NMR (400 MHz, DMSO-
6
d , δ): 3.88 (d, 2H, J=16.72 Hz), 4.1 (s, 2H), 4.45 (d, 2H, J=16.68 Hz),
CDCl
3
6
.30 (d, 2H, J=2.52 Hz), 6.54 (dd, 2H, J=8.6 Hz), 6.88 (d, 2H, J=8.6 Hz).
-1
1
3
1
3
C NMR (100 MHz, DMSO-d6, δ): 58.7, 67.2, 112.6, 114.8, 126,
000 (C-H, str), 1655 (C=O asym, str), 1596 (C=O sym, str), 1229 (C-
+
+
1
2
29.2, 140, 153.7. MS-HESI (m/z): [M+H] calcd for [C15
H
15
N
2
O
2
] :
= 40,000 g mol ; PDI = 2.60; ρ = 1.161 g ml ; SBET = 205 m2
-1
-1
O, str); M
n
55.11; Found 255.00.
g-1; TGA analysis: Td,5% = 495 °C.
Synthesis of (9s,10s)-9,10-dihydro-9,10-[1,2]benzenoanthracene-
,4-diol (Trip) (4)
1
iPEEK-TB. Yield: 90 %. H NMR (500 MHz, CDCl
3
, δ): 4.15 (d, 2H,
1
J=15 Hz), 4.37 (s, 2H), 4.72 (d, 2H, J=15 Hz), 6.68 (s, 2H), 6.92-6.98 (m,
13
Antharacene (35.65 g, 200 mmol) and benzoquinone (21.63 g, 200 6H), 7.2 (d, 2H, J=7.65 Hz), 7.75 (d, 4H, J=6.85 Hz). C NMR (125 MHz,
o
mmol) were refluxed in toluene at 115 C for 6 hours and then cooled CDCl
3
, δ): 58.4, 66.8, 117.2, 117.4, 118, 119.6, 126.6, 129, 132.2,
-1
down to RT. Triptycenequinone was precipitated as greenish-yellow 132.3, 143.4, 152.2, 161.3, 194.1; FT-IR (ν, cm ): 2846-2969 (C-H,
precipitates which was collected by filtration and washed few times str), 1652 (C=O asym, str), 1596 (C=O sym, str), 1235 (C-O, str); M
by toluene. The obtained crude product was placed in the vacuum 49,600 g mol ; PDI = 1.92; ρ = 1.302 g ml ; SBET = 220 m g ; TGA
n
=
-1
-1
2
-1
oven at 75 °C for 6 hours. 45 grams (79% yield) of the analysis: Td,5% = 405 °C.
triptycenequinone was obtained and used without any further
purification for the second step. In which 45 grams was suspended in
1
iPEEK-Trip. Yield: 96 %. H NMR (500 MHz, CDCl
3
, δ): 5.62 (s, 2H),
6
(
1
(
.79 (s, 2H), 6.95 (d, 4H, J=8.65 Hz), 6.99 (m, 4H), 7.24 (m, 4H), 7.85
d, 4H, J=8.6 Hz); 13C NMR (125 MHz, CDCl
24.1, 125.5, 132.1, 132.4, 146.5, 161.9, 194.1; FT-IR (ν, cm ): 1655
3
00 ml glacial acetic acid and heated it to reflux 118 °C. Then 1.4 ml
3
, δ): 48.2, 116.2, 119.5,
(
33% HBr) was added drop wise to the solution and then left to stir
-1
for 3 minutes at reflux. The system was cooled down and filtered
then washed with toluene and placed in the oven at 100 °C for 18
hours. 80 % yield was obtained. Further purification was done to
C=O asym, str), 1596 (C=O sym, str), 1222 (C-O, str); M
n
= 40,700 g
mol ; PDI = 3.17; ρ = 1.261 g ml ; SBET = 250 m g ; TGA analysis: Td,5%
526 °C.
-1
-1
2 -1
=
1
afford white crystals with high purity for polymerization reaction. H
Membrane fabrication
NMR (400 MHz, DMSO-d
7
6
, δ): 5.81 (s, 2H), 6.32 (s, 2H), 6.98 (m, 4H),
.40 (m, 4H), 8.83 (s, 2H). 13C NMR (100 MHz, DMSO-d
, δ): 47.1,
The iPEEKs were dissolved at different concentrations (Table 2) in
NMP by overhead mechanical stirring (IKA RW 20 digital) at 22 °C
(Fig.1). The dope solutions were then placed in an IKA KS 4000
6
®
+
1
13.4, 124, 125.1, 132.4, 145.3, 146.3. MS-HESI (m/z): [2M-H ] calcd
®
+
14 2
for [2C20H O -H ]: 571.20; Found: 571.00.
incubator shaker for 24 h at 22 °C to degas the solution. The dope
solution was poured onto a Novatexx 2471 polypropylene non-
woven support (Freudenberg Filtration Technologies, Germany). A
Polymer synthesis
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Fig. 1 Schematic overview of iPEEK membranes preparation through the phase inversion method.
bench top casting machine (Elecometer 4340 Automatic Film with 5 nm of iridium. The dried membranes were fixed on a slide
Applicator) was used to cast a film using a blade film applicator glass using a both-side tape to obtain its flat surface. Surface
(
1
Elcometer 3700) set at 250 µm thickness, with a transverse speed of roughness of the iPEEK membranes were obtained from atomic force
50 m h . The temperature was 22 °C with relative humidity of 56%. microscope (AFM, Agilent 5500) and calculated as an average of four
-1
The membrane was phase inverted immediately by immersion in times scanning, 5 x 5 µm images were taken and plotted in the ESI†.
deionized Type II water with resistivity of 18.2 MΩ cm sourced from Swelling ratio (SR) was calculated from:
a Milli-Q Reference. The DI water in the bath was changed three
times, finally the membranes were cut and saved in DI water with 1
vol% acetonitrile to prevent any bacterial growth. Refer to section
(
)
푳풘풂풕풆풓−푳풔풐풍풗풆풏풕
푳풘풂풕풆풓
푺푹(%) =
∙ ퟏퟎퟎ
(1)
where Lwater and Lsolvent represents the thickness of the membrane
soaked in water and selected solvents (24 h), respectively. Water
contact angle of membranes was measured by the sessile drop
method using a drop shape analyzer (Easy drop, KRUSS) equipped
with a video camera. The average values were obtained from at least
five measurements for each sample. The carbon dioxide adsorption
isotherms of the powder samples of the three polymers were
performed by using a surface area and porosimetry analyzer
Micrometrics ASAP 2050) at zero degree up to 10 bar after degassing
the samples at 180 °C for 12 h with pressure lower than 10 μmHg.
Apparent Brunauer-Emmett-Teller (BET) surface area were
calculated from CO adsorption data by multi-point BET analysis.
2
1
.2 of the ESI† for the membrane testing.
Polymer and membrane characterization
1H and 13C NMR spectra of the synthesized monomers and polymers
were recorded with a Bruker AVANCE-III spectrometer at a frequency
of 400 MHz in either deuterated chloroform (CDCl
3
) or deuterated
dimethylsulfoxide (DMSO-d ) and recorded in ppm. Molecular
6
(
weight and molecular weight distribution (PDI) of iPEEK-SBI, iPEEK-
TB and iPEEK-Trip were obtained by high temperature (140 °C) gel
permeation chromatography (GPC) (Agilent PL-GPC 220) using
trichlorobenzene as solvent and polystyrene as external standard.
FT-IR of the obtained membranes were acquired using a Varian 670-
IR FT-IR spectrometer. Thermal gravimetric analysis (TGA) was
carried out using a TGA Q5000 (TA Instruments); all analyses entailed
A Mettler-Toledo balance equipped with a density measurement
kit was used to determine the polymer density based on buoyancy
method using iso-octane as the reference liquid. Water vapor
sorption was conducted as a function of relative humidity (RH) at 25
-1
a drying step at 100 °C for 30 min followed by a ramp of 5 °C min to
00 °C. The glass transition temperature of all iPEEK polymers were
8
°C
using
a
gravimetric sorption analyzer on Q5000-SA
obtained via differential scanning calorimetry (DSC) (TA Instruments,
Model Q2000) with a ramp rate of 10 °C min-1 to 500 °C. The d-
spacing between the polymers chains were measured by wide-angle
X-ray scattering which is conducted on a Bruker D8 Advance
diffractometer from 8 to 50° with a scanning rate of 0.5° min . The
surface and cross-sectional images of membranes were collected
using the scanning electron microscopy (SEM Merlin, ZEISS) which is
operated at 5 kV with 5 mm of the working distance. The samples for
the cross-sectional image were prepared by fracturing the frozen
membranes in liquid nitrogen. All membranes were sputter-coated
(
TA Instruments, USA). The membrane samples were dried inside the
sorption analyzer at 70 °C for 3 h to attain a constant weight prior to
the sorption measurements. Sorption measurement were recorded
at different RH in the range from 0 to 95% RH. Once the vapor uptake
reached to a certain humidity, it was equilibrated for at least 3 h to
ensure steady state data. The mechanical properties were obtained
using Nano Test Vantage instrument. The dried membranes were
fixed on a silicon wafer surface and the test were done three times
for each membrane to confirm the obtained results.
-1
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Table 1 Physical properties of the iPEEKs and their comparison to those of the commercial PEEK.
View Article Online
DOI: 10.1039/D0TA08194A
a
a
PDI a
b
c
d
M
w
M
n
T
d,5%
T
g
S
BET
2
Density
(g cm )
-
1
-1
-1
-3
Polymer
iPEEK-SBI
iPEEK-TB
iPEEK-Trip
PEEK 5
(g mol )
104,600
98,000
129,200
-
(g mol )
40,000
49,600
40,700
39,000
(°C)
495
405
526
550
(°C)
243
197
256
143
(m g )
205±7
220±10
250±6
27±5
2.60
1.98
3.17
-
1.161±0.004
1.302±0.027
1.261±0.01
1.3
a
b
Measured by high temperature GPC with polystyrene as the calibration standard and trichlorobenzene as a solvent at 140 °C; Measured by TGA with a
ramp rate of 3 °C min-1 to 800 °C; Measured by DSC with a ramp rate of 10 °C min ; Measured using Archimedes principle and Metler Toledo density
c
-1 d
kit in iso-octane solution.
solubility. In this work, the semi-crystalline polymer was converted
to an amorphous morphology and thus the solubility (Fig. 2e) and the
Results and discussion
Molecular design and synthesis
FFV were enhanced by up to 104% (Fig. 2d) at the same time, by
The iPEEKs were prepared by a one-step, high-temperature aromatic
incorporating kinked structures that prevented the packing of
N
nucleophilic substitution reaction (S Ar) between 4,4’-
polymer chains, thereby enhancing the porosity.
difluorobenzophenone (1) and three diol compounds (2–4) in the
presence of anhydrous K CO in anhydrous DMAc at 165 °C (Scheme
). The chemical structures of the monomers and polymers were
The wide-angle X-ray diffraction (WXRD) results demonstrated
that incorporating SBI, TB, or Trip monomers led to an amorphous
morphology and increased the d-spacing, confirmed by the peaks
shifting toward lower 2 theta by 2 degrees (Fig. 2c, Table S10). The
obtained results were consistent with the Brunauer-Emmett-Teller
2
3
1
confirmed by H and 13C nuclear magnetic resonance (NMR) (Fig. S2–
S13) and mass spectroscopy (MS). Fourier transform infrared (FTIR)
spectroscopy was used to detect the characteristic absorption bands
1
(
BET) surface area and the predicated FFV from MD simulations. The
-1
for the iPEEKs (Fig. 2a). The ether linkage was obtained at 1229 cm
BET surface area of iPEEK was about 7–10 fold higher than that of
-1
(
C-O, str) and the keto group was obtained at, 1655 cm (C=O asym,
str), and 1596 cm-1 (C=O sym, str). The thermogravimetric analysis
TGA) confirmed high thermal stability for all polymers (Fig. 2b, Table
2
-1
PEEK at 27 m g (Fig. S17–S18). The FFV values of iPEEKs were found
to be 0.103, 0.136, and 0.156 as a result of substituting the phenyl
ring of PEEK (FFV = 0.075) by TB, Trip, and SBI, respectively. Dope
solutions of the novel polymers, having different concentrations,
were used to fabricate OSN membranes.
(
1
). In particular, the decomposition temperature values for iPEEK-SBI
(
Td,5% = 495 °C) and iPEEK-Trip (Td,5% = 526 °C) were comparable with
the commercial PEEK (Td,5% = 550 °C), while iPEEK-TB (Td,5% = 405 °C)
showed a moderate decrease due to the early decomposition of the
Tröger’s base bridge.31 The glass transition temperatures (T
polymers, obtained by differential scanning calorimetry (DSC), were
significantly higher than that of the conventional PEEK (T = 143 °C).
For instance, iPEEK-Trip displayed the highest T of 256 °C, followed
by iPEEK-SBI of 243 °C, and iPEEK-TB of 197 °C (Fig. S14–S16). For the
novel polymers, high-temperature gel permeation chromatography
) of the
g
Membrane fabrication and characterization
Three series of nanofiltration membranes were prepared by phase
inversion of the polymer dope solutions. The membranes are
g
g
o
distinguished as: open (iPEEK-X ), with the lowest polymer
a
concentration and dope solution viscosity; ajar (iPEEK-X ), with
medium polymer concentration and dope solution viscosity; and
(
(
GPC) was used to measure the number average molecular weight
) and the polydispersity index (PDI). The three polymers revealed
value and moderate PDI. iPEEK-SBI, iPEEK-TB, and iPEEK-Trip
t
tight (iPEEK-X ), with the highest polymer concentration and dope
M
n
solution viscosity (Table 2 and Fig. S19). In the membrane
designation, X refers to the polymer series, which are SBI, TB and
Trip. The FTIR spectra of the nine membranes were identical to that
of the polymer powders, as shown in Fig. S20, proving that there
were no changes in the molecular structure of the polymer after
phase inversion. The iPEEK dope solutions were prepared in a way
that the resulting viscosities fall within the same range for
performance comparison purposes (Fig. S19). The thickness of the
polymer layer was obtained from the SEM cross-sectional images for
the nine iPEEK membranes with increasing values from 67 to 151 m
as a function of increasing viscosity of the dope solutions.
high M
showed M
n
-1
-1
-1
n
of 40,000 g mol , 49,600 g mol , and 40,700 g mol with
PDI values of 2.6, 1.98 and 3.17, respectively.
The solution processability of the polymers was improved by
replacing the hydroquinone monomer (which is used for commercial
PEEK synthesis) with contorted structures (Fig. 2e, Tables S2–S9).
PEEK is insoluble in organic solvents and can only be dissolved in
2 4
strong acids such as H SO , whereas iPEEKs are soluble in some
organic solvents. Plotting the Hildebrand solubility parameters (HSP)
of the solvents as a function of their dielectric constant (DC) showed
that iPEEKs are soluble in some solvents that have HSP between 17
and 25 MPa0.5 and DC between 0 and 40. On the contrary, the
polymers are insoluble in solvents that have HSP lower than 17 and
higher than 25 MPa0.5, and DC above 40. Commercial PEEK
demonstrates a semi-crystalline morphology, which governs the
chain packing, leading to very low FFV (Fig. 2d) and thus very poor
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Fig. 2 Characterization of iPEEK-SBI, iPEEK-TB, and iPEEK-Trip: (a) Fourier transform infrared (FTIR) spectra. (b) thermogravimetric analysis (TGA) under N
2
atmosphere. (c) X-ray diffraction (XRD) spectra. (d) Calculated fractional free volumes (FFV) and geometry optimized 3D structures of the polymers obtained
from Materials Studio software. (e) Polymer solubility as a function of Hildebrand solubility parameter (HSP) versus dielectric constant of organic solvents.
The filled and empty circles refer to the polymers being soluble and insoluble in the given solvent, respectively (see also Tables S2–S9).
t
Nanoindentation revealed that the membranes have good and the lowest modulus was obtained for iPEEK-TB of 0.703 GPa. The
mechanical properties, flexibility, and high reduced modulus, as hardness of all membranes lay between 0.005 GPa and 0.05 GPa. All
shown in Table 2 and Fig. S21–S24. For instance, taking the tightest membranes were sufficiently robust to be used for OSN at pressures
t
membrane series as an example, iPEEK-Trip displayed the highest up to 30 bar. The surface roughness of the membranes ranged
t
reduced modulus of 1.1 GPa, followed by iPEEK-SBI with 0.89 GPa, between 4 and 9 nm (Table 2 and Fig. S25).
6
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Table 2 Dope solution viscosity and concentration; membrane thickness, swelling ratio and surface properties. There are membranes wiVtihew3AdrtiifcfleerOennltine
DOI: 10.1039/D0TA081 o9 4A
polymer series: SBI, TB and Trip; and each polymer series can be broken down into 3 further series based on the tightness of the membranes: open ( ),
a
t
ajar ( ) and tight ( ). The latter designation corresponds to the increase in the dope solution viscosity. See also Fig. S19.
Surface properties
Viscosity
cP)
Concentration
(wt%)
Thickness
(m)
Reduced
modulus
Membrane
Hardness
GPa)
Contact
angle ()
Roughness,
(nm)
(
(
R
q
(GPa)
iPEEK-SBIo
iPEEK-SBIa
iPEEK-SBIt
iPEEK-TBo
iPEEK-TBa
iPEEK-TBt
iPEEK-Tripo
iPEEK-Tripa
iPEEK-Tript
6737±15
7704±18
9927±24
6481±17
7454±22
9516±23
6660±19
7592±21
9803±26
27
31
35
19
23
27
26
30
34
1074
1176
1453
1333
1381
1512
673
0.5130.069
0.9530.143
0.8920.072
0.7250.034
0.7620.127
0.7030.024
0.9290.127
1.1530.029
1.1140.04
0.00530.002
0.04550.016
0.01750.001
0.05560.009
0.01760.002
0.02630.002
0.01450.001
0.03260.002
0.02920.001
902
931
951
831
902
951
712
752
781
8.971.26
5.820.53
4.400.36
5.240.35
4.210.42
3.890.61
5.630.71
4.740.51
3.940.50
1004
1122
The surface roughness decreased as the membranes became in the range of 0.84–0.90, 0.67–0.74, and 0.44–0.52 nm for the open,
tighter within the same iPEEK series, and the water contact angle ajar, and tight membranes, respectively (Fig. 4b). The MWCO values
(
WCA) increased simultaneously. Since the chemical structure of the showed good correlation with the pore size diameter, which was
polymer within a series was the same, the increase in WCA can be obtained from the pore flow model using the styrene rejection
attributed to the decrease in the surface roughness.
values.33–35 The tightest membranes displayed lower MWCO and
lower pore size diameters, and vice versa in the case of the more
open membranes.
Typical surface and cross-sectional morphologies of iPEEK
membranes are shown in Fig. 3. The surface morphology revealed a
uniform structure for all membranes. However, the cross-sectional
The tightness of the membranes was analyzed and expressed
images show that the finger-like macrovoid structures changed to through their MWCO, permeance, and pore diameter, while the
sponge-like structures with an increase in the concentration of the tightness of the new polymers was expressed by the FFV and BET
dope solution (Fig. 3h–j). The viscous polymer dope solution delayed surface area. A correlation between these parameters was observed
the formation of the asymmetric membrane during the phase as the tightness of the membranes followed the tightness of the
inversion.32 Therefore, the formation of the more dense surface polymers; iPEEK-SBI, iPEEK-TB, and iPEEK-Trip polymers resulted in
morphology and sponge-like structures for iPEEK-Trip can be open, ajar, and tight membranes, respectively. Therefore, the
explained by the delayed demixing rate at high dope solution MWCO, and subsequently the separation performance of the iPEEK
viscosity. The same trend and morphology changes were obtained membranes, can be fine-tuned either by adjusting the dope solution
for the two other polymers (iPEEK-SBI, and iPEEK-TB), indicating the concentration or by incorporating different kinked structures.
consistency of this process, as shown in Fig. S26–S28.
Five solvents, with varying polarity, were used to further
Separation Performance
characterize the nanofiltration performance and stability of the
membranes (Table S11). The separation performance was directly
related to the interactions between the solvents and the polymer
matrix. These interactions were expressed through binding energies
The MWCO of the membranes was determined in acetonitrile at 30
bar (Fig. 4a, Fig. S29–S31). The MWCO and the permeance decreased
with increasing membrane tightness, i.e., higher dope solution
viscosity and concentration. In the case of iPEEK-SBI, the MWCO
decreased from 770 to 450 g mol . In contrast, the acetonitrile
permeance decreased from 7.8 to 3.6 L m-2 h-1 bar-1 as a result of
increasing the dope solution concentration from 27 wt% to 35 wt%,
respectively. In general, the highest concentrations afforded the
tightest membranes with MWCO values of 450, 480, and 528 g mol
(
BE) obtained from molecular dynamic simulations using Materials
-1
Studio (Fig. S32–S34, and Table S13). The correlation between the
solvent permeance and the BEs is shown in Fig. 4c. As the absolute
value of the binding energy between the solvents and the polymers
increased, the solvent permeance increased. The non-polar hexane
with the lowest absolute BE value displayed the lowest permeance
-
-2
-1
-1
1,
-2
-1
of 3.4–4.9 L m h bar . On the contrary, the polar acetonitrile with
with corresponding permeance values of 3.6, 3.9, and 4.9 L m h
bar-1 for iPEEK-SBI , iPEEK-TB , and iPEEK-Trip , respectively. On the
t
t
t
the highest absolute BE value displayed the highest permeance up to
-2
-1
-1
1
1.1±0.4 L m h bar . Consequently, in general, the higher the BE
contrary, the lowest concentrations exhibited the most open
membranes with MWCO values of 770, 845, and 820 g mol , with
-
1
between the solvent and the polymer, the higher the permeance. As
-1 a function of BE, the swelling ratio of the membranes showed the
same trend as the permeance; as the BE increased, the swelling ratio
-2
-1
corresponding permeance values of 7.8, 9.1, and 11.1 L m h bar
o
o
o
for iPEEK-SBI , iPEEK-TB , and iPEEK-Trip , respectively. The pore
diameter distribution analysis revealed that the pore diameters were
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Fig. 3 Typical iPEEK membrane surface morphology at high (a–c) and low (d–f) magnification, and cross-sectional images (g–i). The inset images are water
contact angles (WCA) of the membrane surface. Surface roughness shown by AFM images (j–l).
t
of the membranes increased linearly (Fig. S35–S39). The solvent using iPEEK-Trip in acetonitrile at 30 bar (Fig. 4g). The MWCO value
-1
permeance was also plotted as a function of common solvent was found to be approx. 420 g mol , which is 20% lower than for the
parameters, such as the Hansen Solubility Parameter (HSP) and the polystyrene markers (Fig. S31). In general, these results indicate that
dielectric constant and density. However, no strong correlation was polar solutes have higher rejection and the membranes are tighter.
found (Fig. S48–S53). Moreover, the membranes showed stable
performance both at 10–30 bar (Fig. S40) and over seven days of
Conclusions
continuous cross-flow operation in acetonitrile under 30 bar (Fig. In summary, three intrinsically porous poly(ether-ether-ketone)
4
d).
polymers (iPEEKs) were successfully designed, synthesized, and
utilized for the fabrication of organic solvent nanofiltration (OSN)
membranes. The polymers comprised triptycene, spirobisindane,
and Tröger’s base with contorted structures in their backbone. The
obtained polymers successfully overcame the solution processability
issues of commercial PEEK, displaying high thermal and chemical
stability, and excellent mechanical flexibility. A notable boost in the
fractional free volume and surface area were obtained. Fine-tuning
of the iPEEK membranes separation performance was demonstrated,
with molecular weight cutoff values in the range of 450–845 g mol ,
by either varying the contorted structure or the dope solution
concentration. Molecular dynamic simulations were successfully
used to reveal the relationship between the polymer structures and
the membrane performance. The long-term stability of the
membranes was successfully demonstrated in five solvents at
pressures up to 30 bar and up to seven days of continuous filtration.
The presented work paves the way for molecular engineering of high-
performance membranes.
Owing to the unavoidable presence of the –SO
3
H moieties in the
state-of-the-art SPEEK membrane obtained from commercial PEEK,5
SPEEK displayed 33-fold (169%) higher water vapor uptake at 95%
relative humidity with 3-fold higher BE than the iPEEKs (Fig. 4e).
These results demonstrate that even a small percentage of undesired
sulfonation from the solvent significantly changes the characteristics
of the membrane. Among the new polymers, iPEEK-TB exhibited the
highest water vapor sorption (4.6%) and BE (-17.9 kJ mol ) due to
the presence of the tertiary amine group in the polymer matrix (Fig.
S41 and Table S14).32 Fig. 4f compares the performance of the iPEEK
membranes with the reported SPEEK membrane. The acetonitrile
permeance of the iPEEK membranes was found to be 2–6 fold higher
than that of the SPEEK. The iPEEK-SBI membrane exhibited the same
rejection (57.4%) as SPEEK but the corresponding permeance was
found to be 5.51 L m-2 h-1 bar , which is 184% higher than that of
SPEEK. Moreover, in comparison to SPEEK, both the rejection and the
-1
-1
5
a
-1
t
permeance for the iPEEK-SBI membrane increased by 26.4% and
8
4%, respectively.
In addition to the polystyrene rejections (Fig. S29–31), three dyes
and five active pharmaceutical ingredients (API) were also filtered
8
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DOI: 10.1039/D0TA08194A
Fig. 4 a) Molecular weight cutoff (MWCO) and permeance of the membranes. b) Pore size distribution of the membranes. c) Solvent permeances of the open
membranes as a function of binding energies between the solvents and the polymers obtained from MD simulations. d) Long-term stability during continuous
operation of the tight membranes. Comparison of e) the water vapor uptake as a function of relative humidity, f) the nanofiltration performance for the novel
t
iPEEKs and the previously reported SPEEK, and g) rejection of some dyes and APIs on iPEEK-Trip . All nanofiltration experiments were carried out in acetonitrile
at 30 bar using cross-flow rig unless otherwise stated.
5
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6
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8
1
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Acknowledgements
The graphical abstract and Fig. 1 were created by Heno Hwang,
scientific illustrator at King Abdullah University of Science and
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Table of contents entry
Poly(ether-ether-ketone) gets intrinsic microporosity with spirobisindane, Tröger’s base, and
triptycene contorted structures for organic solvent nanofiltration.