Design, synthesis, and gas permeation properties of polyimides containing pendent imidazolium groups

Article abstract of DOI:10.1002/pola.29053

Film-forming polymers containing ionic groups have attracted considerable attention as emerging materials for gas separation applications. The aim of this article was to synthesize new film-forming polyimides containing imidazolium groups (PI-IMs) and est

Full text of DOI:10.1002/pola.29053

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Design, Synthesis, and Gas Permeation Properties of Polyimides
Containing Pendent Imidazolium Groups
1,2
Bharat Shrimant,1,2 Ulhas K. Kharul,1,2 Prakash P. Wadgaonkar
1Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune 411008, India
2Academy of Scientific and Innovative Research, Delhi-Mathura Road, New Delhi 110025, India
Correspondence to: P. P. Wadgaonkar (E-mail: pp.wadgaonkar@ncl.res.in) or U. K. Kharul (E-mail: uk.kharul@ncl.res.in)
Received 25 January 2018; accepted 22 April 2018; published online in Wiley Online Library
DOI: 10.1002/pola.29053
ABSTRACT: Film-forming polymers containing ionic groups have
attracted considerable attention as emerging materials for gas
separation applications. The aim of this article was to synthesize
new film-forming polyimides containing imidazolium groups
(PI-IMs) and establish their structure–performance relationship.
In this context, a new aromatic diamine, namely, N1-(4-amino-
phenyl)-N1-(4-(2-phenyl-1H-imidazol-1-yl)phenyl)benzene-1,4-
diamine (ImTPADA), was synthesized and polycondensed with
three aromatic dianhydrides, namely, 4,40-(hexafluoroisopropyli-
dene)diphthalic anhydride, 4,4-(4,4-isopropylidenediphenoxy)
bis(phthalic anhydride), and 4,40-oxydiphthalic anhydride to
form the corresponding polyimides containing pendent 2-
phenylimidazole groups (PI-IEs). Next, PI-IMs were prepared by
N-quaternization of pendent 2-phenylimidazole groups present
in PI-6FDA using methyl iodide followed by anion exchange with
bis(trifluoromethane)sulfonimide lithium salt (LiTf2N). PI-IEs and
PI-IMs exhibited reasonably high molecular weights, amorphous
nature, good solubility, and could be cast into self-standing films
from their DMAc solutions. Thermogravimetric analysis showed
that 10% weight loss temperature of PI-IEs and PI-IMs were in
the range 545–475 8C and 303–306 8C, respectively. Gas perme-
ability analysis of films of PI-IEs and PI-IMs was investigated by
variable-volume method and it was observed that incorporation
of ionic groups into PI-6FDA resulted in increased permeability
while maintaining selectivity. In particular, polymer bearing
Tf2N2 anion exhibited high CO2 permeability (33.3 Barr) and
C
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high selectivity for CO2/CH4 (41.1) and CO2/N2 (35.4).
2018
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018,
56, 1721–1729
KEYWORDS: gas permeation; ionic liquids; membrane; polyi-
mides; polyimides-containing imidazolium groups; structure–
property relationship
separation membranes enhances their gas separation
performance.8,9
INTRODUCTION Owing to their excellent thermal and
mechanical stability, structural tunability, and film-forming
ability, polymeric membranes have attracted considerable
attention for various applications such as hydrogen recov-
ery, O2 enrichment, purification of natural gas, and so
forth.1–5 The performance of membranes for gas separation
applications is judged by permeability and selectivity.6 The
high permeability and selectivity leads to the lower capital
area and higher product purity, respectively. For polymeric
membranes, there exists a tradeoff relationship between
gas permeability and selectivity, wherein increased perme-
ability is accompanied by the decrease in selectivity or vice
versa.2,7 Therefore, the current research efforts in the field
of membrane technology are directed toward overcoming
the limit of Robeson upper bound by designing polymers
with high permeability and good selectivity. It has been
demonstrated that increasing polymer chain rigidity and
incorporation of ionic groups into polymers used for gas
In the recent years, polymers containing ionic groups have
gained significant attention for various applications in the
fields such as gas separation, lithium ion batteries, proton-
exchange membrane fuel cells (PEMFC), supercapacitors, and
so forth.10–20 It has been demonstrated that polymers con-
taining ionic groups exhibit excellent gas permeation proper-
ties and their gas permeation properties can be tuned by
varying compositions of ionic groups.13 For CO2 separation,
polymers containing ionic groups showed high selectivity
and high permeability because of interaction of ionic coun-
terpart with CO2.4,10 To date, various examples of polymers
containing ionic groups have been reported for gas separa-
tion applications including polybenzimidazoles, polyethers,
polyimides, and so forth.8,21–26 Taking into consideration the
attractive performance offered by polymers containing ionic
groups based on high-performance polymers, it is of further
Additional Supporting Information may be found in the online version of this article.
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interest to synthesize new film-forming polymers containing
ionic groups and investigate their gas permeation properties
so as to improve the understanding of structure–property
relationships.
spectrometer at the frequency of 200 MHz in CDCl3 or
DMSO-d6. High-resolution mass spectrometry (HR-MS) analy-
sis of monomers was performed on Thermo Scientific Q-
exacative with Accela 1250 Pump. Molecular weights of poly-
mers were measured using GPC analyses in DMF containing
LiBr (0.25 M) with polystyrene as a standard. TGA of PI-IEs
and PI-IMs were performed in N2 on Perkin Elmer TGA-7
system. Polymer samples were heated from 30 to 900 8C at
a heating rate of 10 8C/min under nitrogen atmosphere. Dif-
ferential scanning calorimetry (DSC) analysis was performed
with DSC Q10 at a heating rate of 10 8C/min from 30 to
380 8C under nitrogen. X-Ray diffractograms of PI-IEs and
PI-IMs were obtained on Rigaku Dmax 2500 with a scanning
rate of 28/min and d-spacing values of polymers were calcu-
lated using Bragg’s equation.
Due to their highly rigid structure, high thermo-chemical sta-
bility and possibilities of structural variations via choice of
appropriate monomers, polyimides are widely studied poly-
mers for gas separation applications.27–29 Generally, most of
the polyimides exhibited low permeability and high selectiv-
ity of one gas over another. Various moieties have been
incorporated into polyimides that include bulky groups,
spiro-center, cardo groups, propeller, and ionic groups to
improve gas separation performance.3,27,28,30–35 The incorpo-
ration of ionic groups into polyimide was first reported by Li
et al. in 2010 and it was demonstrated that there was an
increase in CO2-based selectivity by the incorporation of
ionic groups.36 The postmodification of polyimides and eval-
uation of obtained polymers containing imidazolium groups
as membrane materials for gas separation applications was
firstly reported by Shaplov et al. They found an improvement
in gas separation performance compared to parent poly-
mer.37 These results point out the favorable effect of incor-
poration of ionic groups into polymers for obtaining both
high permeability and selectivity.
Synthesis of 1-(4-Nitrophenyl)-2-Phenyl-1H-Imidazole
(ImN)
Into a 500 mL two-necked round-bottom flask equipped
with a magnetic stirring bar, a reflux condenser and a nitro-
gen inlet were charged dry dimethyl sulfoxide (250 mL),
sodium hydride (5 g, 208 mmol) and 2-phenylimidazole
(20 g, 138.7 mmol). The reaction mixture was stirred under
nitrogen at room temperature for 1 h and then 1-fluoro-4-
nitrobenzene (17.67 mL, 166.4 mmol) in dimethyl sulfoxide
(30 mL) was added dropwise over a period of 30 min. The
reaction mixture was heated at 140 8C for 48 h under nitro-
gen, cooled to room temperature and precipitated into ice-
cold water (1000 mL). The crude product was collected by
filtration, washed several times with water and dried under
vacuum at 70 8C. ImN was purified by recrystallization from
methanol.
In this work, a new aromatic diamine, namely, N1-(4-amino-
phenyl)-N1-(4-(2-phenyl-1H-imidazol-1-yl) phenyl)benzene-
1,4-diamine (ImTPADA), was synthesized. ImTPADA was pol-
ycondensed with commercially available aromatic dianhy-
drides, namely, 6FDA, BPADA, and ODPA, to obtain
polyimides containing pendent 2-phenylimidazole groups
(PI-IEs). Polyimides containing imidazolium groups (PI-IMs)
were obtained by N-quaternization of 2-phenylimidazole
groups present in PI-6FDA and its anion exchange reaction.
PI-IEs and PI-IMs were characterized by IR and 1H NMR
spectroscopy, GPC, XRD, TGA, and DSC. The effects of imida-
zolium groups and anion variation on physical and gas per-
meation properties of polyimides were investigated.
Yield: (28 g, 76%); melting point: 110 8C; IR (CHCl3, cm21):
1597 (ANO2 asymmetric stretching) and 1345 (ANO2 sym-
metric stretching); 1H NMR (200 MHz, CDCl3, d/ppm): 8.23
(d, 2H), 7.42–7.33 (m, 8H), 77.26 (d, 1H); 13C NMR (50 MHz,
CDCl3, d/ppm): 146.8, 146.6, 143.4, 129.8, 129.4, 129.0,
128.7, 128.4, 126.0, 124.8, 122.0.; HRMS (ESI): calcd. for
C
15H12N3O2 ([M 1 H]1): 266.0924; found 266.0922.
EXPERIMENTAL
Synthesis of 4-(2-Phenyl-1H-Imidazol-1-Yl) Aniline (ImA)
Into a 500 mL three-necked round-bottom flask equipped
with a magnetic stirring bar, a nitrogen inlet and a reflux
condenser were charged ImN (26 g, 98 mmol), 10 wt % Pd/
C (1 g) and ethanol (200 mL). To the reaction mixture,
hydrazine hydrate (10 mL) was added dropwise over a
period of 30 min at room temperature under nitrogen atmo-
sphere and then refluxed overnight. After completion of reac-
tion (monitored by TLC), the reaction mixture was filtered,
cooled, and precipitated into ice-cold water (1000 mL). The
crude product was collected by filtration and dried under
vacuum at 70 8C. The product was recrystallized from etha-
nol to afford ImA as colorless crystals.
Materials
2-Phenylimidazole (Alfa Aesar), sodium hydride (Alfa Aesar),
10 wt % Pd/C (Aldrich), isoquinoline (Aldrich), 1-fluoro 4-
nitrobenzene (Aldrich), iodomethane (Aldrich), bis(trifluoro-
methane) sulfonimidelithium salt (LiTf2N, Aldrich), and
hydrazine hydrate (Alfa Aesar) were used as received. N,N-
Dimethylformamide (DMF) and m-cresol (Alfa Aesar) were
distilled before use. 4,40-(Hexafluoroisopropylidene) diph-
thalic anhydride (6-FDA), 4,4-(4,4-isopropylidenediphenoxy)-
bis (phthalic anhydride) (BPADA), and 4,40-oxydiphthalic
anhydride (ODPA) were procured from Aldrich and were
sublimed prior to use.
Characterizations
Yield: (18 g, 78%); melting point: 170 8C; IR (CHCl3, cm21):
3463 (NAH asymmetric stretching) and 3387 (NAH sym-
metric stretching); 1H NMR (200 MHz, CDCl3, d/ppm): 7.46
(d, 2H), 7.28–7.24 (m, 4H), 7.11 (s, 1H) (d, 2H), 6.67 (d, 2H).
IR spectra were recorded using Perkin Elmer spectrum GX
spectrophotometer in the range 4000–400 cm21. NMR (1H
and 13C) spectra were recorded using
a
Bruker-AV
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3.71 (s, 2H); 13C NMR (50 MHz, CDCl3, d/ppm): 146.5, 146.4,
129.9, 129.0, 128.4, 128.3, 128.1, 127.8, 126.9, 123.3, 115.2.;
HRMS (ESI): calcd. for C15H14N3 ([M 1 H]1): 236.1182;
found 236.1181.
General Procedure for the Synthesis of Polyimides
Into a 100 mL three-necked round-bottom flask equipped
with a magnetic stirring bar, a nitrogen inlet and a calcium
chloride guard tube were charged ImTPADA (2.5 g, 6 mmol)
and freshly distilled m-cresol (35 mL). After complete disso-
lution of ImTPADA, 6-FDA (2.66 g, 6 mmol) and isoquinoline
(1 mL) were added. The reaction mixture was initially
stirred at room temperature for 4 h followed by heating at
140 8C for 12 h. Finally, the reaction temperature was
increased to 200 8C and held at that temperature for 8 h.
The viscous polymer solution was poured into methanol
(1000 mL), filtered, washed with methanol, and dried at 80
8C for 48 h under reduced pressure.
Synthesis of 4-Nitro-N-(4-Nitrophenyl)-N-(4-(2-Phenyl-1H-
Imidazol-1-Yl)Phenyl) Aniline (ImTPADN)
Into a 500 mL two-necked round-bottom flask equipped
with a magnetic stirring bar, a reflux condenser and a nitro-
gen inlet were charged ImA (16 g, 68 mmol), sodium
hydride (4.9 g, 204 mmol), and dry dimethyl sulfoxide
(150 mL). The reaction mixture was stirred under nitrogen
atmosphere at room temperature for 1 h and then 1-fluoro-
4-nitrobenzene (17.34 mL, 163.2 mmol) in dimethyl sulfox-
ide (30 mL) was added dropwise. The reaction mixture was
heated at 140 8C for 72 h, cooled to room temperature and
precipitated into ice-cold water (1000 mL). The solids were
collected by filtration, washed several times with water and
dried under vacuum at 70 8C. The product was dissolved in
ethyl acetate. The ethyl acetate solution was washed with
brine and water, dried over sodium sulfate, and filtered.
Ethyl acetate was evaporated on a rotary evaporator. The
crude product was purified by silica gel column chromatog-
raphy using pet ether: ethyl acetate (50:50, v/v) as an eluent
to afford ImTPADN as a faint yellow solid.
IR (Film, cm21): 1784 (C@O asymmetric stretching) and
1722 (C@O symmetric stretching); 1H-NMR (200 MHz,
CDCl3, d/ppm): 8.06 (d, 2H), 7.92 (d, 4H), 7.38–7.16 (m,
19H).
The other polyimides, namely, PI-BPADA and PI-ODPA were
synthesized following the similar procedure by polycondesa-
tion of ImTPADA with aromatic dianhydrides, namely, BPADA
and ODPA, respectively.
Postmodification
N-Quaternization of PI-6FDA
Yield: (15 g, 46%); Melting point: above 300 8C; IR (CHCl3,
cm21): 1580 (ANO2 asymmetric stretching) and 1312
(ANO2 symmetric stretching); 1H NMR (200 MHz, CDCl3, d/
ppm): 8.21 (d, 4H), 7.40 (d, 2H), 7.31-7.27 (m, 7H), 7.24-
7.19 (m. 6H); 13C NMR (50 MHz, CDCl3, d/ppm): 151.2,
146.0, 145.4, 143.4, 135.1, 129.9, 129.0, 128.6, 127.8, 127.2,
126.7, 125.7, 123.1, 122.7.; HRMS (ESI): calcd. for C27H20
N5O4 ([M 1 H]1): 478.1510; found 478.1511.
Into a 250 mL two-necked round-bottom flask equipped
with a magnetic stirring bar and a nitrogen inlet were
charged PI-6FDA (2.9 g, 3.5 mmol) and DMSO (30 mL). To
the reaction mixture, methyl iodide (0.16 mL, 2.5 mmol) was
added dropwise and stirred at room temperature for 48 h.
The polymer solution was precipitated into methanol, fil-
tered, washed with methanol (2 3 500 mL), and dried at
100 8C for 4 days.
Synthesis of N1-(4-Aminophenyl)-N1-(4-(2-Phenyl-1H-
Imidazol-1-Yl)Phenyl)Benzene-1,4-Diamine (ImTPADA)
Into a 500 mL three-necked round-bottom flask equipped
with a magnetic stirring bar, a nitrogen inlet and a reflux
condenser were charged ImTPADN (14 g, 29.3 mmol), 10
wt % Pd/C (1 g) and ethanol (150 mL). To the reaction mix-
ture, hydrazine hydrate (30 mL) was added dropwise over a
period of 1 h at room temperature under nitrogen atmo-
sphere and then refluxed overnight. After completion of reac-
tion (monitored by TLC), the reaction mixture was filtered
while hot, cooled, and precipitated into ice-cold water
(800 mL). The crude product was collected by filtration,
washed with water and dried under vacuum at 70 8C. The
product was purified by recrystallization from ethanol to
afford ImTPADA as colorless crystals.
IR (Film, cm21): 1784 (C@O asymmetric stretching) and
1722 (C@O symmetric stretching); 1H-NMR (200 MHz,
DMSO-d6, d/ppm): 8.21–8.12 (m, 4H), 7.95 (s, 2H), 7.78 (s,
2H), 7.62–7.44 (m, 6H), 7.37–7.09 (m, 13H), 3.8 (s, 3H).
Anion Exchange of [PI-6FDA] [I]
Into a 250 mL two-necked round-bottom flask equipped
with a magnetic stirring bar and a nitrogen inlet were
charged [PI-6FDA] [I] (1.5 g) and DMF (20 mL). To the reac-
tion mixture, LiTFSI (0.89 g, 3.1 mmol) in DMF (5 mL) was
added and stirred at room temperature for 48 h. The poly-
mer solution was precipitated into water, washed thoroughly
with methanol, and dried at 100 8C for 4 days.
IR (Film, cm21): 1784 (C@O asymmetric stretching) and
1722 (C@O symmetric stretching); 1H-NMR (200 MHz,
DMSO-d6, d/ppm): 8.20–8.09 (m, 4H), 7.76 (s, 2H), 7.60 (s,
2H), 7.47–7.08 (m, 19H), 3.79 (s, 3H).
Yield: (9 g, 74%); melting point: 204 8C; IR (CHCl3, cm21):
3463–3400 (NAH stretching); 1H NMR (200 MHz, CDCl3, d/
ppm): 7.52 (d, 2H), 7.32–7.27 (m, 4H), 7.11 (d, 1H), 6.99–
6.93 (m, 6H), 6.83 (d, 2H), 6.65 (d, 4H), 3.38 (s, 4H); 13C
NMR (50 MHz, CDCl3, d/ppm): 149.4, 146.1, 143.1, 138.1,
128.9, 128.8, 128.6, 128.5, 128.2, 127.4, 126.8, 126.4, 123.3,
118.2, 116.1.; HRMS (ESI): calcd. for C27H23N5 ([M 1 H]1):
417.1948; found 417.1945.
The degree of N-quaternization was determined by 1H NMR
spectroscopy. Volhard’s method was used to determine the
percentage of the iodide exchanged with Tf2N2 anion.23 PI-
IM (0.1 g) was added into 0.01 M AgNO3 solution (10 mL)
and stirred at room temperature for 30 h. An excess of
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SCHEME 1 Synthesis of N1-(4-aminophenyl)-N1-(4-(2-phenyl-1H-imidazol-1-yl)phenyl)benzene-1,4-diamine (ImTPADA).
AgNO3 was titrated with 0.01 M KSCN, from which the
iodide exchange in the polyimide containing imidazolium
groups was estimated.
finally afford N1-(4-aminophenyl)-N1-(4-(2-phenyl-1H-imida-
zol-1-yl)phenyl)benzene-1,4-diamine (ImTPADA).
The structural characterization of ImTPADA and other inter-
mediates was carried out by IR, 1H and 13C NMR spectros-
copy and HR-MS. IR spectrum of ImTPADA (Supporting
Information, Fig. S7) showed absorption bands of NAH
stretching of amino group in the range 3463–3400 cm21. In
1H NMR spectrum of ImTPADA (Fig. 1), the broad peak
appeared at d 5 3.58 ppm corresponding to four protons of
amino group. The peaks due to aromatic protons of
ImTPADA appeared in the range d 5 6.64–7.53 ppm. The
structure of ImTPADA was further supported by 13C NMR
(Supporting Information, Fig. S8) and HR-MS analysis (Sup-
porting Information, Fig. S9).
Preparation of Dense Membranes
The dense membranes were prepared via solution casting
method. The polymer was dissolved in DMAc, the solution
was filtered, and poured onto a leveled glass surface, which
was then kept in a vacuum oven at 90 8C for 20 h. Further,
these films were dried at 100 8C in a vacuum oven for one
week. The average membrane thickness was in the range
60–70 microns as measured by digital micrometer.
Gas Permeability Measurements
The pure gas permeability measurements on films of PI-IEs
and PI-IMs were carried out by the variable-volume method
at pressure of 6–7 bar and at room temperature (28 8C) for
He, H2, N2, O2, CH4, and CO2.19,38 The active area of mem-
brane was 12.4 cm2. Gas permeability coefficient was deter-
mined using the following equation:
Synthesis and Properties of PI-IEs and PI-IMs
Three PI-IEs were synthesized by polycondensation of
ImTPADA with three commercially available aromatic dianhy-
drides namely, 4,40-(hexafluoro isopropylidene)diphthalic
anhydride (6-FDA), 4,4-(4,4-isopropylidenediphenoxy)bis
(phthalic anhydride) (BPADA), and 4,40-oxydiphthalic anhy-
dride (ODPA). Polycondensation was performed using one-
step high-temperature method in m-cresol containing cata-
lytic amount of isoquinoline (Scheme 2).
J:l
P5
p12p2
where P is the permeability coefficient expressed in Barr (1
Barr 5 10210 cm3 (STP) cm cm22 s21 cmHg21), J is the
steady-state penetrant flux (cm3.cm22.s), l is the membrane
thickness (cm), while p1 and p2 are the feed and permeate
side pressures (cmHg). Three membrane samples of each
polymer were taken for gas permeability analysis prepared
under similar conditions and the data were averaged.
Depending on the gas, the variation in permeability measure-
ment from average value was in the range 1–14% (2–6% for
He, 1–5% for H2, 3–6% for N2, 4–8% for O2, 6–14% for CH4,
and 3–10% for CO2).
PI-IMs were synthesized from PI-6FDA (Scheme 3). The N-qua-
ternization of PI-FDA containing pendent 2-phenylimidazole
units was carried out with methyl iodide at room temperature
to obtain [PI-6FDA] [I].
To synthesize film-forming PI-IMs with the optimum content
of ionic character, PI-6FDA was reacted with various equiva-
lent quantities (0.5, 0.7, and 1) of methyl iodide. PI-IMs pre-
pared using 0.5 and 0.7 equivalents of methyl iodide formed
strong and flexible films while the film was brittle in case of
PI-IMs prepared using 1 equivalent of methyl iodide. The
exchange of iodide anion in [PI-6FDA] [I] was performed
using bis(trifluoromethane) sulfonimidelithium salt (LiNTf2)
in DMF to form [PI-6FDA] [Tf2N]. The degree of N-quaterni-
zation and anion exchange was measured by 1H NMR spec-
troscopy and Volhard’s method and the results of both these
methods were in good agreement.
RESULTS AND DISCUSSION
Monomer Synthesis and Characterization
The route for synthesis of N1-(4-aminophenyl)-N1-(4-(2-phe-
nyl-1H-imidazol-1-yl)phenyl)benzene-1,4-diamine (ImTPADA)
comprised four steps as depicted in Scheme 1. These four
steps involved N-arylation of 2-phenylimidazole with 4-nitro-
1-fluorobenzene, reduction of nitro groups of 1-(4-nitro-
phenyl)-2-phenyl-1H-imidazole (ImN), aromatic nucleophilic
substitution reaction (SNAr) of ImA with 4-nitro-1-
fluorobenzene, and reduction of nitro groups of ImTPADN to
The solubility tests of PI-IEs and PI-IMs were carried out at
3 wt % concentration in various organic solvents at room
temperature and the results are summarized in Supporting
Information, Table S1. PI-6FDA and PI-BPADA were soluble
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FIGURE 1 1H NMR spectrum (in CDCl3) of ImTPADA. [Color figure can be viewed at wileyonlinelibrary.com]
in various organic solvents such as chloroform (CHCl3),
dichloromethane (DCM), N,N-dimethylacetamide (DMAc),
N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP),
and dimethyl sulfoxide (DMSO). PI-ODPA was insoluble in
these organic solvents, thus the solution-based characteriza-
tion of PI-ODPA by GPC and NMR was not possible. PI-IMs
were found to be soluble in DMAc, NMP, DMSO, and DMF.
The good solubility of PI-IMs could be attributed to the pres-
ence of propeller-shaped triphenylamine, bulky ACF3 groups
and ionic character. Except for PI-ODPA, films of PI-IEs and
PI-IMs could be cast from their DMAc solution (Supporting
Information, Fig. S15).
(Supporting Information, Fig. S13) and [PI-6FDA][Tf2N] (Sup-
porting Information, Fig. S14), the peak observed at d 5 3.8
ppm is attributed to methyl group of N-quaternized polymer,
indicating N-quaternization reaction was successful.
Inherent viscosity of PI-IEs and PI-IMs were in the range
0.62–0.72 dL/g (Table 1). Molecular weights of PI-IEs and
PI-IMs were measured by GPC in DMF using polystyrene as
a standard. PI-IEs and PI-IMs showed number average
molecular weight and dispersity values in the range 79,200–
35,000 g/mol and 2.1–1.6, respectively (Table 1). The mea-
sured GPC molecular weights of PI-IMs were lower than that
of corresponding parent polyimide which could be attributed
to the differences in solubility parameters and hydrodynamic
volume due to variation in their chemical structures. Similar
observations have been reported in the literature for poly-
mers containing ionic groups.37
IR spectra (Supporting Information, Fig. S10) of PI-IEs and
PI-IMs exhibited characteristics bands of C@O stretching at
1784 and 1722 cm 21. In addition, bands of CAN and imide
21
ring deformation were observed at 1364 and 820 cm
,
1
respectively. H NMR spectra of PI-IEs and PI-IMs along with
assignments are presented in Supporting Information,
Figures S11–S14. In 1H NMR spectra of [PI-6FDA][I]
The broad halo in wide-angle X-ray diffraction patterns
(Fig. 2) confirmed that PI-IEs and PI-IMs were amorphous in
SCHEME 2 Synthesis of polyimides containing pendent 2-phenylimidazole groups (PI-IEs).
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SCHEME 3 Synthesis of polyimides containing imidazolium groups (PI-IMs).
nature. The amorphous nature could be attributed to the
loosening of polymer chain packing due to the presence of
bulky triphenylamine and 2-phenylimidazole in the polymer.
demonstrated excellent thermal stability with 10% weight
loss and % char yield in the range 475–545 8C and 55–60,
respectively. The 10% weight loss of PI-IMs was in the range
303–306 8C. PI-IMs exhibited lower T10 values compared to
corresponding polyimide due to the decreased rigidity of
polymer chains and the higher propensity of ionic moieties
to degrade at a lower temperature compared to the parent
heterocyclic moiety.33 Glass transition temperature (Tg) of
PI-IEs were in the range 248–303 8C, as measured by DSC
(Table 1). In case of PI-IEs with varying dianhydrides, Tg was
in accordance with the order of rigidity of dianhydrides:
6FDA > ODPA > BPADA. No Tg was detected in DSC analysis
of PI-IMs. Similar observations have been reported for poly-
mers containing ionic groups in the literature.37
The interchain spacing (d-spacing) was calculated using
Bragg’s equation from peak maxima of X-ray diffractogram.
The d-spacing values of PI-IEs and PI-IMs were in the range
5.43–5.67 Å (Table 1). The order of d-spacing is as follows:
PI-ODPA < PI-BPADA < PI-6FDA < [PI-6FDA] [I] < [PI-6FDA]
[Tf2N]. In the series of PI-IEs, PI-6FDA showed higher d-
spacing compared PI-ODPA and PI-BPADA. This could be
attributed to the bulky hexafluoroisopropylidene linkages
which disturbed polymer chain packing more effectively as
compared to PI-ODPA and PI-BPADA which contain isopropy-
lidene and ether linkages, respectively.39,40 PI-IMs exhibited
higher d-spacing as compared to parent polyimide which
could be due to the presence of bulky ionic groups. In the
case of PI-IMs, the d-spacing of PI-6FDA [TF2N] is higher
compared to [PI-6FDA] [I], which indicated more disordered
chain packing due to the bulkier nature of anion in the for-
mer. These results implied that both bulk of hinge groups
present in the dianhydride and bulk of anion play an impor-
tant role in governing the polymer chain packing.
For gas separation applications, membranes should exhibit
acceptable mechanical properties and sustain pressure dur-
ing measurements. The films of PI-IEs and PI-IMs withstood
pressure of 5–6 bar during gas permeability measurement
experiments.
PI-IEs and PI-IMs exhibited tensile strength, Young’s modu-
lus, and elongation at break in the range 43.5–49.3 MPa,
0.91–1.38 GPa, and 7.9–31.5%, respectively. It was observed
that the mechanical properties of PI-IMs were inferior to
that of parent polyimide. This could be attributed to the
The thermal properties of PI-IEs and PI-IMs were deter-
mined by TGA and DSC (Table 1 and Fig. 3). PI-IEs
TABLE 1 Synthesis and Properties of PI-IEs and PI-IMs
Char
Yieldg
Tensile
Young’s
Elongation
e
f
Sr.
ginh
(dL/ g)a Mn
d-Spacing T10
Tg
Strength Modulus at Break
b
b
d
No. Polymer
Mw
Dispersityc (A)
(8C)
(8C) (%)
(MPa)
(GPa)
(%)
˚
1
2
3
PI-6FDA
0.72
79200 128000 1.6
5.49
5.63
5.67
545
303
306
303 55
ND 38
ND 49
47.1
43.5
44.6
1.34
0.91
1.0
31.5
9.2
[PI-6FDA] [I] 0.65
40200 85000
35000 60000
2.1
1.7
[PI-6FDA]
[Tf2N]
0.62
7.9
4
5
PI-BPADA
PI-ODPA
0.70
ns
73000 117000 1.6
ns ns ns
5.46
5.43
475
508
248 56
298 60
49.3
-
1.38
-
22.7
-
a
f
Measured at a concentration of 0.5 g/dL in DMAc at 35 6 8C.
Molecular weights were obtained from GPC in DMF (Polystyrene
Glass transition temperature (Tg) recorded by DSC.
Char yield at 900 8C.
b
g
standard).
Abbreviations: ND, not detected; ns, inherent viscosity and GPC meas-
urements could not be carried out due to insolubility of polyimide in
chloroform, tetrahydrofuran, and DMF.
c
Dispersity 5 Mw/Mn.
d-Spacing was calculated from X-ray diffractograms.
d
e
Temperature at which 10% weight loss recorded by TGA at a heating
rate of 10 8C/min.
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Gas Permeability
The gas permeability measurements of films of PI-6FDA, PI-
BPADA, [PI-6FDA] [I], and [PI-6FDA] [Tf2N] were carried out
by variable-volume method19,38 and the data are summa-
rized in Table 2. The sequence of gases studied is as follows:
He, H2, N2, O2, CH4, and CO2. For easy comparison, the gas
permeability data of commercially available polymers such
as Matrimid9 and polysulfone,41 which are widely studied for
gas separation applications is included in Table 2 (entries 5
and 6). Also included in Table 2 is the reported gas perme-
ability data of polyamide42 and polyimide32 containing tri-
phenylamine units (entries 7 and 8). The present polymers
exhibited higher permeability than Matrimid and polysulfone.
The selectivity data of PI-IEs and PI-IMs are comparable
with the reported polyimide32 and polyamide42 containing
triphenylamine units. The gas permeability order in this
series is as PI-BPADA < PI-6FDA < [PI-6FDA] [I] < [PI-6FDA]
[Tf2N]. PI-IEs exhibited CO2 permeability in the range 11.9–
19.7 Barr and selectivity for CO2/CH4 and CO2/N2 gas pairs
was in the range 35.2–42.5 and 27.0–34.0, respectively. The
relatively higher permeability of PI-6FDA could be due to
presence of bulky hexafluoroisopropylidene linkages which
FIGURE 2 X-ray diffractrograms of PI-IEs and PI-IMs. [Color fig-
ure can be viewed at wileyonlinelibrary.com]
weaker interchain interactions between polymer chains in
PI-IMs as compared to parent polyimide. Similar observation
has been reported in the literature for polybenzimidazole.23
FIGURE 3 (a) TG curves of PI-IEs and PI-IMs and (b) DSC curves of PI-IEs. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 2 Gas Permeability and Selectivity Data of PI-IEs and PI-IMsa
Permeabilityb
Selectivityc
Sr.
No.
Polymer
He
H2
N2
O2
CH4
CO2
H2/N2
O2/N2
CO2/N2
CO2/CH4
1
2
3
4
5
6
7
8
PI-6FDA
31.1
23.0
9.9
24.8
29.6
18.0
14.0
-
0.73
0.35
0.78
0.94
0.32
0.25
3.94
0.89
3.5
0.56
0.28
0.65
0.81
0.28
0.25
3.18
0.50
19.7
11.9
26.2
33.3
10.0
5.6
31.5
28.3
31.8
31.5
56.0
56.0
-
4.8
5.1
5.0
4.9
6.6
5.6
8.47
4.81
27.0
34.0
33.6
35.4
31.2
22.4
35.78
18.9
35.2
42.5
40.3
41.1
36.0
22.0
44.33
33.43
PI-BPADA
[PI-6FDA] [I]
[PI-6FDA] [Tf2N]
Matrimid9
Polysulfone41
PA-A42
14.5
1.8
33.1
3.9
44.3
4.6
-
-
-
-
2.1
1.40
33.4
4.28
141
PI-IIb32
-
16.8
-
a
b
c
Gas permeability measurements were carried out at room temperature
Units of permeability (P): 1 Barr 5 10210 cm3 (STP) cm/cm2 s cmHg.
Selectivity (a) 5 P1/P2.
(28 8C) and 5–7 bar.
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FIGURE 4 Robeson plots of (1) PI-6FDA, (2) [PI-6FDA] [I], (3) [PI-6FDA] [Tf2N], and (4) 6.5 mol % mRTIL [36], (5) 14.8 mol % mRTIL
[36], (6) 25.8 mol % mRTIL [36], (7) 8 mol % diRTIL [36], (8) PI-1–50 IL [37], (9) PI-1.TFSI [37], (10) PI-1.TFSI-50 IL [37] for (a) CO2/
CH4 and (b) CO2/N2 gas pairs. [Color figure can be viewed at wileyonlinelibrary.com]
disturbed polymer chain packing more effectively compared
to PI-BPADA which contains isopropylidene linkages.
weights, exhibited good solubility in organic solvents, and
film-forming ability. The gas permeability increased by incor-
poration of ionic moieties into PI-6FDA and with the
increase in the bulkiness of anion. Overall, the gas perme-
ability data demonstrated that the present PI-IMs are attrac-
tive candidates for gas separation applications, particularly
for CO2 separation.
PI-6FDA was chosen for the synthesis of PI-IMs due to its
higher permeability than PI-BPADA. It was observed that
incorporation of ionic groups into polyimide resulted in
increased permeability while selectivity was maintained. PI-
6FDA] [Tf2N] showed higher permeability compared to [PI-
6FDA] [I]. For instance, [PI-6FDA] [Tf2N] exhibited CO2 per-
meability of 33.3 Barr and selectivity for CO2/N2 and CO2/
CH4 was 35.4 and 41.1, respectively. The increment of gas
permeation properties of PI-IMs could be attributed to the
presence of ionic moieties which are known to interact with
carbon dioxide and loosening of packing of polymer chains.
This trend was also in good agreement with the d-spacing
values. Overall, these results indicated that permeability of
PI-IEs and PI-IMs increased with the bulkiness of hinge
groups, ionic character, and bulk of anion.
ACKNOWLEDGMENT
Bharat Shrimant would like to thank University of Grant Com-
mission (UGC), New Delhi, India for fellowship.
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