Polyetherimides for gas separation membranes

Article abstract of DOI:10.1080/15421400490478687

A series of polyetherimides were synthesized by polycondensation reaction at high temperature of 2,2-bis[4,4-(3,4-dicarboxyphenoxy) phenyl]propane dianhydride with various aromatic diamines. These polymers are easy soluble in polar aprotic solvents such as N-methylpyrrolidinone, dimethylformamide or dimethylacetamide or even in less polar liquids such as chloroform. They show high thermal stability, with decomposition temperature being above 400°C and glass transition temperature in the range 200-275°C. Polymer solutions in chloroform were processed into thin films which were tested as gas separation membranes. Transport parameters for light gases were measured. The dependence of glass transition and decomposition temperature on conformational rigidity parameters was calculated.

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Polyetherimides for Gas
Separation Membranes
a
a
b
M. Bruma , E. Hamciuc , Yu. Yampolskii , A.
b
c
c
Alentiev , I. A. Ronova & E. M. Rojkov
a
Institute of Macromolecular Chemistry , Iasi,
Romania
b
Institute of Petrochemical Synthesis , Moscow,
Russia
c
Institute of Element-Organic Compounds , Moskow,
Russia
Published online: 18 Oct 2010.
To cite this article: M. Bruma , E. Hamciuc , Yu. Yampolskii , A. Alentiev ,
I. A. Ronova & E. M. Rojkov (2004) Polyetherimides for Gas Separation
Membranes, Molecular Crystals and Liquid Crystals, 418:1, 11-19, DOI:
10.1080/15421400490478687
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Mol. Cryst. Liq. Cryst., Vol. 418, pp. 11=[739]–19=[747], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490478687
POLYETHERIMIDES FOR GAS SEPARATION
MEMBRANES
M. Bruma and E. Hamciuc
Institute of Macromolecular Chemistry, Iasi, Romania
Yu. P. Yampolskii and A. Yu. Alentiev
Institute of Petrochemical Synthesis, Moscow, Russia
I. A. Ronova and E. M. Rojkov
Institute of Element-Organic Compounds, Moskow, Russia
A series of polyetherimides were synthesized by polycondensation reaction at
high temperature of 2,2-bis[4,4-(3,4-dicarboxyphenoxy) phenyl]propane di-
anhydride with various aromatic diamines. These polymers are easy soluble
in polar aprotic solvents such as N-methylpyrrolidinone, dimethylformamide
or dimethylacetamide or even in less polar liquids such as chloroform. They
show high thermal stability, with decomposition temperature being above
ꢀ
ꢀ
4
00 C and glass transition temperature in the range 200–275 C. Polymer solu-
tions in chloroform were processed into thin films which were tested as gas sep-
aration membranes. Transport parameters for light gases were measured. The
dependence of glass transition and decomposition temperature on conforma-
tional rigidity parameters was calculated.
1. INTRODUCTION
Among high performance polymers, the greatest interest was payed to aro-
matic polyimides due to their outstanding thermal stability, good chemical
and electroinsulating properties, low dielectric constant and good adhesion
to common substrates [1–3]. But, the application of fully aromatic poly-
imides is limited in many fields because they are normally insoluble and
intractable after conversion from the polyamidic acid to the polyimide
We acknowledge with great pleasure the financial support provided through the INTAS-01–
172 Project. Our warm thanks also go to T. Kopnick at the Institute of Thin Film Technology
0
and Microsensors, Teltow, Germany, for GPC analyses.
Address correspondence to M. Bruma, Institute of Macromolecular Chemistry, Iasi,
Romania. E-mail: mbruma@icmpp.tuiasi.ro
11=[739]

1
2=[740]
M. Bruma et al.
form. One approach to increase the solubility and processability of polyi-
mides is the introduction of flexible linkages or bulky units into the poly-
mer chain. It has been generally recognized that aromatic ether linkages
provide the respective polymers with a significantly lower energy of inter-
nal rotation, which leads to the decrease of the melting points or glass tran-
sition temperature without significant effect on thermal stability [4,5]. On
the other hand, polyethers are well known for their good mechanical
properties. Thus, the combination of good solubility and robust mechanical
properties makes poly(ether-imide)s especially interesting for membrane
application and ultimately can allow easy fabrication.
Poly(ether-imide)s are a particularly attractive class of polymers for
structure-property investigation. Our aim was to study structural features
which influence combination of permeabilities and selectivities for various
gas separations and to examine the well known trade-off between gas
permeability which is normally associated with reduced selectivity. Thus,
a series of three polyetherimides have been synthesized by polycondensa-
tion reaction of certain aromatic diamines with a dianhydride containing
two ether linkages, namely 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]pro-
pane dianhydride (I).
2
2
. EXPERIMENTAL
.1. Synthesis of the Monomers
The bis(ether-anhydride) I was prepared by a multistep reaction, starting
with the nitro displacement reaction of 4-nitro-phthalodinitrile with isopro-
pylidene-bisphenol which is named bisphenol A, as shown in scheme 1
[
6,7]. Such a reaction is possible because the cyano group is one of the
strongest activating groups in the nucleophilic displacement process. The
reaction is performed in the presence of K CO which produces the potass-
2
3
ium bisphenolate. Nitro displacement by phenoxide on nitrobenzonitrile
proceeds under mild conditions, at room temperature, by using dimethyl-
0
formamide (DMF) as solvent. The bis(ether dinitrile) I resulting at room
temperature condition was subsequently hydrolyzed in an alkaline solution
0
0
and acidified to yield the tetracarboxylic acid I . Dehydration with acetic
anhydride afforded the bis(ether anhydride) I. The yield in each step
was satisfactory and the structures of the intermediate bis(ether dinitrile)
0
00
I , tetracarboxylic acid I and the final bis(ether anhydride) I were con-
0
firmed by IR spectra and melting point (m.p.) measurements. M.p. of I :
ꢀ
ꢀ
00
ꢀ
1
95–196 C; m.p. of I : 208–216 C; m.p. of I: 189–190 C.
Aromatic diamines (II) which were used in the synthesis of polymers
0
are: hexafluoro-isopropylidene-4,4 -dianiline, 2,3,5,6-tetramethyl-p-phenyl-

Polyetherimides for Gas Separation Membranes
13=[741]
SCHEME 1 Synthesis of bis(ether-anhydride) I.
ene-diamine and p-phenylene diamine. These three diamines were
provided by commercial sources and used as received.
2.2. Synthesis of Polyetherimides
The polyetherimides III were prepared by polycondensation reaction of
equimolar amounts of 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane
dianhydride (I) and an aromatic diamine, in NMP as solvent. The reaction
was performed in two steps, without isolating the polyamidic acid resulting
in the first step (Scheme 2). Thus, the solid diamine was added to the sol-
ution of dianhydride in NMP at room temperature and the reaction was run
0
for 2 h, resulting the polyamidic acid III . The second step was performed
ꢀ
by heating the polyamidic acid solution at 185–190 C for 3–4 h. The poly-
etherimides III were isolated by precipitation in water. The details on poly-
mers’ preparation are given in Table 1.

1
4=[742]
M. Bruma et al.
TABLE 1 Preparation of Polyetherimides III
Amount Amount
Reaction Reaction
of di-
anhydride diamine NMP Conc.
of
time at
room
(mL) (%) temp. (h) temp. (h) (dL=g)
time at
high
g
inh
Polymer
Ar
(g)
2.6
(g)
IIIa
1.67
25 14.5
2
4
0.5
IIIb
IIIc
3.12
3.9
0.984
0.81
25 14.0
35 12.0
2
2
3
3
0.6
0.7
2.3. Measurements
Melting points of the monomers were measured on a Melt-Temp II (Labora-
tory Devices) apparatus without correction. Infrared spectra were recorded
with a Specord M 80 spectrometer in transmission mode by using KBr pel-
lets. The inherent viscosities (ginh) of the polymers were determined with
ꢀ
an Ubbelohde viscometer, by using polymer solutions in NMP, at 20 C, at a
concentration of 0.5 g=dL. The molecular weight was determined by gel
permeation chromatography (GPC) using a Waters GPC apparatus pro-
vided with refraction and UV detectors and PL Mixed C Column. Measure-
ments were carried out with polymer solutions of 2% concentration in
DMF containing 0.1 M NaNO and by using DMF with 0.1 M NaNO as
3
3
eluent. Standard polystyrene of known molecular weight was used for cali-
bration. Thermogravimetric analyses (TGA) were performed on a MOM
ꢀ
derivatograph made in Budapest, Hungary, at a heating rate of 12 C=min
in air. The initial decomposition temperature (IDT) is characterized as
the temperature at which the sample achieves a 5% weight loss. The tem-
perature of 10% weight loss (T ) was also recorded. The glass transition
10
temperatures (T ) of the precipitated polymers were determined with a
g
Mettler differential scanning calorimeter DSC 12E, at a heating rate of
ꢀ
1
0 C=min under nitrogen. The mid-point of the inflection curve resulting
from the typical second heating was assigned as the glass transition tem-
perature of the respective polymers. The Kuhn segments were calculated
by using the Monte Carlo method [8]. The Kuhn segment (A ), the
fr

Polyetherimides for Gas Separation Membranes
15=[743]
characteristic ratio (C ) and the number of aromatic rings in the Kuhn
1
segment (p) were chosen as the conformational parameters and were
discussed in correlation with some physical properties of the present
polymers. As is known, the Kuhn statistical segment can be written as
!
2
hRi
A ¼ lim
n!1 nl
0
2
where hR i=nl is the ratio of the average square end-to-end distance of a
0
chain to its contour length; n is the number of repeating units; l is the con-
0
tour length of a repeating unit. In the case of polyheteroarylenes, when the
structure of the repeating units consists of virtual bonds of different bond
length and angles, the contour length of polymer is the length of broken
line connecting the centers of virtual bonds. The term ‘‘virtual bond’’ is
taken to mean a rigid section of a chain, approximated by a straight line
about which a rotation is possible. In a particular case it may be an ordinary
valence bond; more generally, it may contain rings as well. A comparison
between the experimental values of the Kuhn segment (Aexp) for poly-
heteroarylenes and those calculated under assumption of a free rotation
(
Afr) showed them to be nearly equal [9–11].
3. RESULTS AND DISCUSSION
Polycondensation of equimolar amounts of dianhydride I and diamine II in
NMP, at high temperature, yielded polyetherimides III (Scheme 2).
The structure of polyetherimides III was identified by IR spectra. Strong
ꢁ
1
ꢁ1
bands at 1780 cm and 1730 cm are commonly attributed to the sym-
metrical and asymmetrical stretching vibrations of carbonyl groups of
ꢁ
1
imide. The absorption band at 1375 cm is due to CꢁN stretching in imide
ꢁ
1
ring and the absorption at 750 cm is due to imide ring deformation. All
ꢁ
1
the polymers exhibited IR bands at 1225 cm due to ether linkages.
These polymers are easily soluble in polar amidic solvents such as NMP
and DMF, and even in less polar liquids such as chloroform. The inherent
viscosities measured in NMP solution, at a concentration of 0.5% were in
the range of 0.5–0.7 dL=g (Table 1). The good solubility is due to the rela-
tively high flexibility of macromolecular chains which was obtained by the
introduction of ether and isopropylidene linkages into the structure of the
polymer. Also, the hexafluoroisopropylidene and the methyl groups intro-
duced a factor of asymmetry and steric hindrance in the macromolecules
IIIa and IIIb that prevent a dense packing of the chains. The disturbed
packing of macromolecular chains facilitates the diffusion of small mole-
cules of solvents between the polymer chains which leads to better

1
6=[744]
M. Bruma et al.
SCHEME 2 Synthesis of polyetherimides III.
solubility. The good solubility makes the present polymers potential candi-
dates for practical applications in spin coating and casting processes for the
preparation of thin polymer films and membranes.
The molecular weight of polymers was determined by gel permeation
chromatography. The values of weight-average molecular weight Mw are
in the range of 83000–94800, the number-average molecular Mn are in
the range of 64300–70900 and the polydispersity Mw=Mn is in the range
of 1.29–1.34 (Table 2). These values show that the present polymers have
fairly high molecular weights and narrow molecular weight distribution.
All these polymers possess film-forming ability. Their solutions in NMP
or in chloroform having a concentration of about 10% were cast onto glass
substrates and dried to yield thin transparent free-standing films having a
thickness of tens of micrometers. The films obtained from polymers IIIb
and IIIc were quite brittle and only the film made from polymer IIIa
was enough flexible in order to be tested for gas separation membrane.

Polyetherimides for Gas Separation Membranes
TABLE 2 The Properties of Polyetherimides III
17=[745]
General formula:
ꢀ
ꢀ
ꢀ
˚
˚
Polymer
T
g
( C) IDT ( C)
T
10 ( C)
Mw
Mn
Mw=Mn
A
fr (A)
A
hin (A)
p
IIIa
IIIb
IIIc
205
275
220
490
400
500
510
420
520
94800 70900
83000 64300
1.34
1.29
24.95
29.04
29.04
26.45
29.91
35.18
4.92
5.44
6.40
a
a
a
–
–
–
T
g
¼ Glass transition temperature.
IDT ¼ Initial decomposition temperature ¼ temperature of 5% weight loss.
10 ¼ Temperature of 10% weight loss.
T
a
The polymer was not soluble in DMF.
fr ¼ Kuhn segment value calculated under the assumption of free rotation.
hin ¼ Kuhn segment value calculated under the assumption of hindered rotation.
p ¼ parameter of conformational rigidity.
A
A
The permeability (P) of a membrane made from polymer IIIa was tested
ꢀ
for four gases at 20 C. The following results were obtained: P(H ) ¼ 13.40
2
Barrer; P(He) ¼ 15.60 Barrer; P(O2) ¼ 1.28 Barrer; P(CO ) ¼ 6.62 Barrer.
2
These polymers exhibit high thermal stability, as evidenced by thermo-
gravimetric analyses. The polymer IIIb begins to decompose above 400 C,
ꢀ
ꢀ
while the polymers IIIa and IIIc are stable up to 470 C. The temperature
ꢀ
ꢀ
at which the weight loss is 10% is 420 C for polymer IIIb and 510–520 C
for the polymers IIIa and IIIc (Table 2). By comparing the present
FIGURE 1 Dependence of glass transition temperature (T
fr) -left and on conformational rigidity parameter p-right.
g
) on Kuhn segment
(A

1
8=[746]
M. Bruma et al.
FIGURE 2 Dependence of decomposition temperature (IDT) on Kuhn segment
A )-left and on conformational rigidity parameter p-right.
(
fr
polyetherimides with related wholly aromatic polyimides it can be seen that
their decomposition temperatures are similar. This shows that the presence
of ether linkages preserves the high thermal stability while improving the
solubility.
The glass transition temperatures (Tg) of these polyetherimides IIII
ꢀ
were in the range of 205–275 C (Table 2). It can be noticed that the poly-
mer IIIIa containing the flexible hexafluoroisopropylidene linkage coming
ꢀ
from the diamine segment has the lowest Tg value (205 C), while the poly-
mer IIIIb having four voluminous methyl substituents on the phenylene
ꢀ
ring shows the higher Tg value (275 C). It can also be noticed that there
is a large interval between Tg and the decomposition temperature, which
could be advantageous in the processing of these polymers by a thermo-
forming technique. The glass transition temperature and the initial
decomposition temperature values are known to be significantly dependent
on the presence of the aromatic rings in the polymeric chain. Therefore,
one can try to find a correlation between these parameters and the number
of aromatic rings in the fragment of the polymeric chain equivalent to the
Kuhn segment. This correlation is described by the equation:
p ¼ ðAfr=loÞn
where p is the parameter of conformational rigidity, n is the number of
aromatic rings and A =l is the number of repeating units per Kuhn seg-
fr
0
ment. Such dependencies for the present polymers are shown in Table 2
and Figures 1 and 2.
4. CONCLUSIONS
The incorporation of ether linkages together with isopropylidene groups
into the main chain of aromatic polyimides gave products with substantially

Polyetherimides for Gas Separation Membranes
19=[747]
improved solubility in polar aprotic solvents, and even in less polar liquids
such as chloroform. These polymers maintain high thermal stability, with
ꢀ
the decomposition temperature being above 400 C and glass transition in
ꢀ
the range 205–275 C. The large interval between glass transition and
decomposition temperature may be advantageous for their processing by
a thermoforming technique. The polymers can also be processed from
NMP- or chloroform-solutions into thin and ultrathin, pinhole-free films.
All these properties, associated with easy processability, make the present
polymers potential candidates for practical applications as gas separation
membranes.
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[