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Original Article
Structure–property relationship of
polyetherimide based on aromatic
dianhydride and long alkyl chain
containing aromatic diamines
R. Balasubramanian, X. Jayaseeli and M. Sarojadevi
Abstract
A series of new poly(ether imide)s (PEIs) were synthesized from different commercially available dianhydrides like
3,30,4,40-benzophenonetetracarboxylic dianyhride (BTDA) and pyromellitic dianhydride (PMDA) and diamines like 1,4-
bis(4-aminophenoxy)propane, 1,4-bis(4-aminophenoxy)butane, 1,4-bis(4-aminophenoxy)pentane, and 1,4-bis(4-aminophe-
noxy)hexane by a typical two-step polymerization method. The structure of the monomers and PEIs prepared were con-
firmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and carbon-13 nuclear magnetic
resonance spectral analysis. Solubility of the PEIs was tested in various organic solvents. Thermal properties of the PEIs
were investigated by thermogravimetric analysis and differential scanning calorimetry. The temperature corresponding to
5% (T
5%
) and 10% (T
10%
) weight loss are from 318Cto418
C and from 380Cto480
C, respectively. These PEIs exhi-
bit glass transition temperature in the range of 195–245C, very low water absorption (0.37–0.62%), and low dielectric
constant (2.68–3.17) at 1 MHz. The PEIs film possessed good mechanical properties with tensile strength of 77–98 MPa,
elongation at break of 8–13%, and tensile modulus of 1.5–2.2 GPa. These results imply that the synthesized PEIs are well
suited to be used as dielectric materials. Based on these studies the structure–property relationships were established.
Keywords
Polyetherimide, dielectric constant, thermal properties, structure–property relationship, mechanical properties
Introduction
Aromatic polyimides are considered to be a class of high-
performance polymers and find a wide range of applica-
tions in advanced technologies. However, most of them
have high melting or softening temperature, strong inter
chain interactions, and are insoluble in most organic sol-
vents because of the rigidity of the backbone and hence
have limited use for many applications.
1–7
The difficulties
in processing conventional aromatic polyimides are due to
their inherent molecular features, which include molecular
stiffness, high polarity, and high intermolecular association
forces (high density of cohesive energy), which make these
polymers virtually insoluble in any organic medium and
shift the transition temperatures to well above the decom-
position temperatures. Thus, the strategies to prepare novel
processable aromatic polyimides have focussed on chemi-
cal modifications, mainly by preparing new monomers that
provide less molecular order, torsional mobility, and lower
intermolecular bonding.
8–11
Structural modifications were
envisioned early to incorporate these features. Of the
various alternatives to design novel processable polyi-
mides, some general approaches have been universally
adopted, that is, introduction of aliphatic or other kind of
flexible segments, which reduce chain stiffness, introduc-
tion of bulky side substituents, which helps in separation
of polymer chains and hinder molecular packing and crys-
tallization, use of /enlarged monomers containing angular
bonds, which suppress coplanar structures, use of 1,3-
substituted instead of 1,4-substituted monomers, and/or
asymmetric monomers, with lower regularity and molecu-
lar ordering, and preparation of co-polyimides from two
or more dianhydrides or diamines.
11–16
Aliphatic polyi-
mides have attracted much attention in recent years for
Department of Chemistry, Anna University, Chennai, Tamilnadu, India
Corresponding author:
M. Sarojadevi, Department of Chemistry, Anna University, Chennai
600025, Tamilnadu, India.
Email: msrde2000@yahoo.com
High Performance Polymers
2015, Vol. 27(6) 758–771
ªThe Author(s) 2014
Reprints and permission:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0954008314561130
hip.sagepub.com
potential applications as liquid crystal orientation layers,
nonlinear optical buffer layer, and low dielectric materials.
An increase in aliphatic content reduces intermolecular
interaction and increases the flexibility of the polymer back-
bone the amorphous nature of the polymer, which lead to an
enhancement in the desired properties. However, factors
leading to better solubility or lower glass transition tempera-
ture (T
g
) or melting temperature (T
m
) in a polymer often
conflict with other important requirements, such as mechan-
ical properties, thermal resistance, or chemical resistance.
Therefore, an adjusted degree of modification should be
applied to optimize the balance of properties.
Most studies of polyimides in electronics applications
have looked at ways to reduce the dielectric constant of the
materials. The reduction of the dielectric constant has been
achieved by lowering the polarizability of the polymer
through modification of the backbone with the incorpora-
tion of bulky, space-filling groups, or with the replacement
of hydrogen with fluorine atoms or a combination of
both.
17–20
The introduction of space filling groups such
as aromatics increases the free volume and thus decreases
dipolar and atomic polarizability, whereas the incorpora-
tion of fluorine will reduce the total polarizability. This
along with its hydrophobic character (low moisture uptake)
results in low dielectric constant.
21–26
This study is concerned with the synthesis and charac-
terization of a series of new highly soluble poly(ether
imide)s (PEIs) based on the dianhydrides namely pyro-
mellitic dianhydride (PMDA), 3,30,4,40-benzophenonete
tracarboxylic-dianyhride (BTDA), and diamines contain-
ing 1,4-bis(4-aminophenoxy)alkane linkage (Figure 1).
The PEIs contain alkyl and ether linkage (to increase the
processability) as well as aromatic group (to increase the
thermal stability). The properties of the PEIs such as
solubility, thermal properties, mechanical properties, and
dielectric constant have been studied and the structure–
property relationships of the monomers were also studied
in detail. Thus PEIs synthesized via polymerization of a
common aromatic dianhydride with various aromatic
alkyl diamines results in overall reduction of the free vol-
ume as well as maintaining a high imide functional group
density in the polymer backbone.
27–29
Figure 1. Synthesis of 1,4-bis(4-aminophenoxy)alkane.
Balasubramanian et al. 759
Experimental
Materials
4-Acetamidophenol (98%) was purchased from Alfa Aesar
(Hyderabad, Telangana, India), N,N-dimethylformamide
(DMF) (Merck Limited; Chennai, Tamilnadu, India) was
purified by stirring with sodium hydroxide (NaOH), fol-
lowed by distillation from phosphorus pentoxide under
reduced pressure. Anhydrous potassium carbonate was
dried for 12 h at 120C before use. Ethanol (Spectrochem,
Mumbai, Maharashtra, India) was purified by stirring with
calcium carbonate for 12 h, followed by distillation under
atmospheric pressure. BTDA, (Sigma Aldrich, Chennai,
Tamilnadu, India), PMDA (Sigma Aldrich, Chennai,
Tamilnadu, India) were recrystallized from acetic anhy-
dride and dried in a vacuum oven at 150C overnight
prior to use. N,N-Dimethylacetamide (DMAc),triethylamine,
dibromobutane, dibromo pentane, dibromohexane, NaOH,
and methanol were purchased from Spectrochem India Pvt
Ltd (Chennai, Tamilnadu, India) and used as received.
Characterization methods
Fourier Transform Infrared (FTIR) spectra of the samples
were obtained using an ABB Bomem (model MB3000;
Quebec, Canada) spectrometer. The cured samples were
ground with spectroscopy grade potassium bromide (KBr)
and made into pellets. Proton (
1
H) (500 MHz) and carbon
13 (
13
C) (125 MHz) nuclear magnetic resonance (NMR)
spectra were recorded on a Jeol spectrometer (Tokyo,
Japan) with tetramethylsilane as the internal standard.
Solutions were prepared in deuterated dimethyl sulfoxide
(DMSO-d
6
). Differential scanning calorimetry (DSC) was
performed in a TA instruments Q
10
model instrument (New
Castle, Delaware, USA) using 5–10 mg of the sample at a
heating rate of 10 C min
1
in under nitrogen (N
2
) atmo-
sphere. Dielectric constant and dielectric loss measure-
ments were carried out with the help of an impedance
analyzer (Solatron 1260 Impedance/Gain-phase Analyzer,
UK) at room temperature. The polymer samples were made
in the form of pellets (1 mm thickness 12 mm diameter)
using a platinum electrode sandwich model in the fre-
quency range of 1 kHz–1 MHz at room temperature. The
dielectric constant and dielectric loss of the samples were
determined using "0and "00 as the standard relations. The
inherent viscosities of all polyimides were measured using
a 0.5 g dl
1
Ubbelohde viscometer. Solubility of the poly-
mers was determined at a concentration of 5%(w/v) in var-
ious organic solvents and kept aside for 24 h with
occasional shaking. If the mixture was insoluble at ambient
temperature, then it was heated and cooled. For the mea-
surement of moisture uptakes, PEI film specimens were
immersed in deionized water at 25C, and the weight differ-
ence was calculated after 1 week. Molecular weights and
molecular weight distributions of the prepared polymers
were determined using a Polymer Laboratories PL-GPC-
50 integrated gel permeation chromatography (GPC) sys-
tem interfaced with a WellChrom (Berlin, Germany) K-
2301 refractive index detector. A 5 mm PL gel mixed-C col-
umn with tetrahydrofuran (THF 0.01 mol/lithium bromide)
as the eluent was used. Polystyrene was used for standards.
The polymer solutions were filtered through 0.2 mm Teflon
membranes before analysis. Mechanical properties of the
films were measured with an Instron model 1130 tensile
tester with a 5-kg load cell at a crosshead speed of 5 cm
min
1
on strips approximately 30–40 mm and 0.5 cm width
with 2 cm gauge length. Thermogravimetric analysis
(TGA) was performed using a TA Q 600 thermal analyzer.
Cured samples were analyzed in an open silicon pan at a
heating rate of 20Cmin
1
under N
2
atmosphere, up to a
maximum temperature of 800C.
Monomer synthesis
Synthesisof 1,4-bis(4-acetamidophenoxy) propane.4-Acetamido
phenol (3.02 g, 0.02 mol) and NaOH (0.80 g, 0.02 mol)
dissolved in 30 ml of distilled water were taken in a
round-bottomed flask fitted with a reflux condenser and a
magnetic stirrer. Then, tertabutylammonium bromide
(0.64 g, 0.002 mol) dissolved in 10 ml of distilled water
was added into the reaction mixture. Sequentially a solution
of dibromopropane (2.02 g, 0.01 mol) was added dropwise
through the funnel into the reaction mixture, and stirring
was continued for 12 h at 60C after which the solution was
cooled to room temperature; the precipitate thus formed
was collected by filtration, washed repeatedly with water,
and dried in vacuum oven at 100C for 12 h. Yield: 95%,
melting point: 230C.
Synthesis of 1,4-bis(4-aminophenoxy)propane. 1,4-Bis (4-acet-
amidophenoxy)propane (3.40 g, 0.01 mol) and NaOH (0.80
g, 0.02 mol) in 100 ml of ethanol were taken in a 250 -ml
round-bottomed flask. The reaction was continued at reflux
temperature for 24 h. The reaction mixture was cooled and
poured onto crushed ice. The precipitate formed was col-
lected by filtration, recrytallized in ethanol, and dried in a
vacuum oven at 100C for 12 h. Yield: 90%, melting point:
190C.
FTIR (KBr, cm
1
): 3070 (C–H aromatic stretching),
1520 (C¼C aromatic stretching), 1242 (C–O–C stretching),
3286 and 3472 (symmetric and asymmetric stretching
vibrations of –NH
2
group), 2939 (C–H aliphatic stretch-
ing), 833 (C–N bending vibrations);
1
H-NMR (deuterated
chloroform (CDCl
3
), ppm): 2.60 (m, 2 H, a), 3.81 (t, 4 H,
b) 6.76 (d, 2 H, c), 6.65 (d, 2 H, d), 3.96 (s, 4 H, e);
13
C-
NMR (CDCl
3
, ppm): 29.44 (C
1
), 68.41 (C
2
), 152.51(C
3
),
116.20 (C
4
), 115.75 (C
5
), 139.58 (C
6
).
All other diamines were prepared using a similar procedure.
Synthesis of 1, 4-bis (4-aminophenoxy)butane. 1,4-bis (4-ami-
nophenoxy)butane (BAPBu) was synthesized in a similar
760 High Performance Polymers 27(6)
way to 1,4-bis (4-aminophenoxy)propane (BAPPr) using
dibromobutane instead of dibromopropane.
Melting point of BAPBu: 190C, yield: 90%
FTIR (KBr, cm
1
): 3055 (C–H aromatic stretching),
1528 (C¼C aromatic stretching), 1273 (C–O–C stretching),
3317 and 3448 (symmetric and asymmetric stretching
vibration of –NH
2
group), 2932 (C–H aliphatic stretching),
813 (C–N bending vibration);
1
H-NMR (CDCl
3
, ppm):
1.63 (m, 4 H, a), 3.80 (t, 4 H, b), 6.75 (d, 2 H, c), 6.65
(d, 2 H, d), 3.92 (s, 4 H, e);
13
C-NMR (CDCl
3
, ppm):
22.79 (C
1
), 66.41 (C
2
), 148.51(C
3
), 112.20 (C
4
), 111.75
(C
5
), 147.58 (C
6
).
Synthesis of 1,4-bis (4-aminophenoxy)pentane. BAPPt was
synthesized in a similar way to BAPPr using dibromopen-
tane instead of dibromopropane.
Melting point of BAPPt: 170C, yield: 92%
FTIR (KBr, cm
1
): 3055 (C–H aromatic stretching), 1520
(C¼C aromatic stretching), 1242 (C–O–C stretching), 3294
and 3456 (symmetric and asymmetric stretching vibration
of –NH
2
group), 2947 (C–H aliphatic stretching);
1
H-NMR
(CDCl
3
, ppm): 1.60 (m, 2 H, a), 1.75 (m, 4 H, b), 3.90 (t, 4
H, c), 6.76 (d, 2 H, d), 6.65 (d, 2 H, e), 4.10 (S, 4 H, f);
13
C-
NMR (CDCl
3
, ppm): 23.82(C
1
), 28.71 (C
2
), 67.41
(C
3
),150.51 (C
4
),114.20 (C
5
),113.75 (C
6
), 138.58 (C
7
).
Synthesis of 1, 4-bis(4-aminophenoxy)hexane. 1,4-bis(4-ami-
nophenoxy) hexane (BAPHx) was also synthesized in a
similar way by following the procedure from BAPPr where
dibromohexane was used instead of dibromopropane.
Melting point of BAPHx: 142C, yield: 90%
FTIR (KBr, cm
1
): 3070 (C–H aromatic stretching),
1512 (C¼C aromatic stretching), 1250 (C–O–C stretching),
3286 and 3387 (symmetric and asymmetric stretching
vibration of –NH
2
group), 2939 (C–H aliphatic stretching);
1
H-NMR (CDCl
3
, ppm): 1.77 (m, 4 H, a), 1.98 (m, 4 H, b),
3.96 (t, 4 H, c), 6.75 (d, 2 H, d), 6.67 (d, 2 H, e), 4.00 (s, 4 H,
f);
13
C-NMR (CDCl
3
, ppm): 22.82(C
1
), 29.32 (C
2
), 64.72
(C
3
), 152.51(C
4
), 111.53 (C
5
), 110.39 (C
6
), 139.77 (C
7
).
Polymer synthesis
The PEIs were synthesized from diamines and dianhy-
drides via a two-step method. The synthesis of PEI-1 is
used as an example to illustrate the general synthetic route
used to yield the PEIs. The polymerization was conducted
in a three-necked, N
2
flushed, 100 ml round-bottomed flask
equipped with a magnetic stirrer, reverse Dean–Stark trap,
and reflux condenser. BTDA (0.322 g, 0.001 mol) was
added to BAPPr (0.258 g, 0.001 mol), which was predis-
solved in freshly distilled N-methyl-2-pyrrolidone (NMP)
to make 20%solid concentration. The reaction mixture was
stirred under N
2
at room temperature for over 15 h to make
poly(amic) acid (PAA) solution, which was then imidized
to form (PEI-1).
The cyclization can be achieved by either thermal or
chemical method. For the thermal imidization method, the
PAA solution was cast onto a clean glass plate and heated at
different temperatures (60C/20 min
1
, 120C/30 min
1
,
165C/2 h
1
, 200C/30 min
1
, and 250C/30 min
1
) under
vacuum to convert PAA into polyimide films. The film was
stripped from the plate by soaking in water. This film was
then used to the test mechanical property, dielectric prop-
erty, and moisture uptake. In chemical imidization, a mix-
ture of acetic anhydride (0.204 g, 0.002 mol) and
triethylamine (0.101 g, 0.001 mol) was added to the PAA
solution. The reaction mixture was stirred for 1 h at room
temperature and then heated at 80C for 4 h. The polymer
solution was finally poured into methanol to obtain the pre-
cipitate, which was collected by filtration and washed thor-
oughly with methanol and finally cold water. The
precipitate was dried at 100C under vacuum for 24 h.
Yield: 97%.
FTIR (KBr, cm
1
): 1782 and 1720 (asymmetric and
symmetric stretching vibrations of the imide carbonyl),
3070 (C–H aromatic stretching), 1512 (C¼C aromatic
stretching), 2939 (C–H aliphatic stretching), 725 and
1111 (C–N–C bending vibration), 1373 (C–N–C stretching
vibtation), 1242 (–C–O–C– stretching vibration);
1
H-NMR
(DMSO-d
6
, ppm): 6.39–8.68 (m, 14H, aromatic protons),
2.82 (m, 2H, a), 4.20 (t, 4H, b).
PEI-2 (BTDA–BAPBu), PEI-3 (BTDA–BAPPt), PEI-
4 (BTDA–BAPHx), PEI-5 (PMDA–BAPPr), PEI-6
(PMDA–BAPBu), PEI-7 (PMDA-BAPPt), and PEI-8
(PMDA-BAPHx) were synthesized using a similar
method with high yield.
PEI-2 (BTDA–BAPBu): yield 97%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching vibra-
tion of the imide carbonyl), 3070 (C–H aromatic stretching),
1512 (C¼C aromatic stretching), 2947 (C–H aliphatic
stretching), 725 and 1119 (C–N–C bending vibration),
1389 (C–N–C stretching vibration), 1250 (–C–O–C– stretch-
ing vibration);
1
H-NMR (DMSO-d
6
, ppm): 6.39–8.80 (m,
14H, aromatic protons), 2.20 (m, 4H, a), 4.10 (t, 4H, b).
PEI-3 (BTDA–BAPPt): yield 94%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching
vibration of the imide carbonyl), 3070 (C–H aromatic
stretching), 1520 (C¼C aromatic stretching), 2939 (C–H
aliphatic stretching), 717 and 1119 (C–N–C bending vibra-
tion), 1389 (C–N–C stretching vibration), 1242 (–C–O–C–
stretching vibration);
1
H-NMR (DMSO-d
6
, ppm): 6.84–
8.88 (m, 14H, aromatic protons), 2.14 (m, 2H, a), 2.42
(m,4H,b),4.07(t,4H,c).
PEI-4 (BTDA–BAPHx): yield 95%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching
vibration of the imide carbonyl), 3070 (C–H aromatic
stretching), 1512 (C¼C aromatic stretching), 2939 (C–H
aliphatic stretching), 725 and 1111 (C–N–C bending
vibration), 1373 (C–N–C stretching vibration), 1242 (–C–
O–C– stretching vibration);
1
H-NMR (DMSO-d
6
,ppm):
Balasubramanian et al. 761
6.39–8.22 (m, 14 H, aromatic protons), 1.51 (m, 2H, a), 2.42
(m,4H,b),4.41(t,4H,c).
PEI-5 (PMDA–BAPPr): yield 94%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching vibra-
tion of the imide carbonyl), 3063 (C–H aromatic stretching),
1520 (C¼C aromatic stretching), 2986 (C–H aliphatic
stretching), 725 and 1111 (C–N–C bending vibration),
1373 (C–N–C stretching vibration), 1242 (–C–O–C– stretch-
ing vibration).
1
H-NMR (DMSO-d
6
, ppm): 6.56–7.43 (m,
10H, aromatic protons), 1.80 (m, 2H, a), 3.80 (t, 4H, b).
PEI-6 (PMDA–BAPBu): yield 95%.FTIR(KBr,cm
1
):
1790 and 1720 (asymmetric and symmetric stretching vibra-
tion of the imide carbonyl), 3070 (C–H aromatic stretching),
1512 (C¼C aromatic stretching), 2924 (C–H aliphatic
stretching), 725 and 1111 (C–N–C bending vibration),
1373 (C–N–C stretching vibration), 1242 (–C–O–C– stretch-
ing vibration);
1
H-NMR (DMSO-d
6
, ppm): 6.86–8.23 (m,
10H, aromatic protons), 2.70 (m, 4H, a), 3.98 (t, 4H, b).
PEI-7 (PMDA–BAPPt): yield 96%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching
vibration of carbonyl of the imide C¼O), 3047 (C–H aro-
matic stretching), 1520 (C¼C aromatic stretching), 2939
(C–H aliphatic stretching), 717 and 1111 (C–N–C bending
vibration), 1366 (C–N–C stretching vibration), 1242 (–C–
O–C– stretching vibration);
1
H-NMR (DMSO-d
6
, ppm):
6.86–7.70 (m, 10H, aromatic protons), 2.00 (m, 2H, a),
2.75 (m, 4H, b), 3.93 (t, 4H, c).
PEI-8 (PMDA–BAPHx): yield 96%. FTIR (KBr, cm
1
):
1782 and 1720 (asymmetric and symmetric stretching
vibration of carbonyl of the imide C¼O), 3063 (C–H aro-
matic stretching), 1520 (C¼C aromatic stretching), 2986
(C–H aliphatic stretching), 725 and 1111 (C–N–C bending
vibration), 1373 (C–N–C stretching vibration), 1242 (–C–
O–C– stretching vibration);
1
H-NMR (DMSO-d
6
, ppm):
6.86–7.94 (m, 10H, aromatic protons), 2.45 (m, 2H, a),
2.96 (m, 4H, b), 3.92 (t, 4H, c).
Results and discussion
Synthesis of diamines
The diamines BAPPr, BAPBu, BAPPt, and BAPHx con-
taining alkyl chain were synthesized by a two-step process.
In the first step, a colorless crystal was obtained by the
reaction between 4-acetamidophenol, NaOH, and dibromo
alkane in the presence of phase-transfer catalyst. In the sec-
ond step, the product obtained was subjected to hydrolysis
using NaOH in ethanol. The diamine was precipitated in
ice-cold water. This procedure gives considerably higher
yield than the alkylation of p-nitrophenol compounds.
Therefore, this is a more efficient and convenient method
for the synthesis of diamines avoiding costly and long alter-
native routes.
30,31
The FTIR spectra (Figure 2) of the diamines exhibit
characteristic absorptions of the amino group at 3472 and
3286 cm
1
. The absorptions observed at 3070, 2939, and
1242 cm
1
may be attributed to that of C–H (aromatic
stretching), C–H (aliphatic stretching), and C–O–C (asym-
metric stretching), respectively. Figure 3 shows the
1
H-
NMR spectra of the diamines. The peak at 3.96 ppm is
associated with the resonance of the amino group and the
peaks between 6.65 ppm and 6.76 ppm are assigned to the
aromatic protons. The peak at 2.60 and 3.81 ppm are
assigned to the alkyl protons. Figure 4 shows the
13
C-
NMR spectra of all the diamines. The characteristic carbon
resonances of the aromatic ring are found between 115.75
and 152.51 ppm, and the peaks at 68.41 and 29.44 ppm are
assigned to alkyl carbons. These results indicate that the
chemical structure of diamine were confirmed by FTIR and
NMR spectra.
Synthesis of PEIs
All the polyimides were synthesized using a conventional
two-step method with the polycondensation reactions of the
commercially available aromatic dianhydrides such as
PMDA and BTDA reacted with equimolar amounts of
BAPPr, BAPBu, BAPPt, and BAPHx in NMP at room tem-
perature under N
2
atmosphere to form PAA. The reaction
mixture became viscous within 10–15 min, and the poly-
merization reaction was continued for 15 h. Then, the PAA
were cyclized to form PEI by chemical and thermal
dehydration.
The FTIR spectra of the PEIs (Figure 5) exhibit charac-
teristic absorptions of imide at 1782 and 1720 cm
1
(asym-
metrical and symmetrical stretching vibration of carbonyl
group of the imide ring), 1373 cm
1
(C–N–C stretching
Figure 2. FTIR spectrum of (a)BAPPr, (b) BAPBu, (c) BAPPt,and (d)
BAPHx. FTIR: Fourier transform infrared spectroscopy; BAPPr: 1,4-
bis(4-aminophenoxy)propane; BAPBu: 1,4-bis(4-aminophenoxy)
butane; BAPPt: 1,4-bis(4-aminophenoxy)pentane; BAPHx: 1,4-bis(4-
aminophenoxy)hexane.
762 High Performance Polymers 27(6)
vibration), 1242 cm
1
(C–O–C stretching vibration), 1111
and 725 cm
1
(imide ring deformation). Figure 6 shows the
1
H-NMR spectra of PEIs derived by chemical imidization.
The peaks between 6.39 ppm and 8.68 ppm are assigned to
the aromatic protons. The peaks at 2.82 ppm and 4.20 ppm
are assigned to alkyl chain protons. All results illustrate that
the PEIs have been successfully synthesized as shown in
Figure 7.
Inherent viscosity and molecular weight
Inherent viscosity of the prepared PEIs was obtained
using a Ubbelohde viscometer at 0.5 g dl
1
concentra-
tioninNMPat30+0.1C, which is shown in Table
3. This is a most commonly used technique. The inher-
ent viscosities of the PEIs are in the range of 0.59–0.88
dl g
1
, indicating the formation of high molecular
Figure 3.
1
H-NMR spectra of (a) BAPPr, (b) BAPBu, (c) BAPPt, and (d) BAPHx.
1
H-NMR: proton nuclear magnetic resonance; BAPPr:
1,4-bis(4-aminophenoxy)propane; BAPBu: 1,4-bis(4-aminophenoxy)butane; BAPPt: 1,4-bis(4-aminophenoxy)pentane; BAPHx: 1,4-
bis(4-aminophenoxy)hexane.
Balasubramanian et al. 763
weight polymer. However, the PMDA series polymers
have higher viscosity than the BTDA series polymers.
This is in accordance with the general observation that
the highly rigid PMDA moiety in the polymer back-
bone leads to increased chain–chain interaction or in
other words, increased close packing of the polymer
chains.
The number-average molecular weight (M
n
), weight-
average molecular weight (M
w
), and polydispersity
index(PDI)ofthePEIsweredeterminedbyGPCin
THF as the eluent (Table 3). Both M
w
and M
n
are in the
range of 38,480–53,000 and 19,750–28,620 respectively,
with PDIs of about 1.76–2.50. These results indicate that
all the synthesized polymers have high good inherent
Figure 4.
13
C-NMR spectra of (a) BAPPr, (b) BAPBu, (c) BAPPt, and (d) BAPHx.
13
C-NMR: carbon-13 proton nuclear magnetic
resonance; BAPPr: 1,4-bis(4-aminophenoxy)propane; BAPBu: 1,4-bis(4-aminophenoxy)butane; BAPPt: 1,4-bis(4-aminophenoxy)pen-
tane; BAPHx: 1,4-bis(4-aminophenoxy)hexane.
764 High Performance Polymers 27(6)
viscosity and high molecular weight with narrow mole-
cular weight distributions, indicating that a high degree
of polymerization was achieved, which is necessary for
achieving a sufficient level of desired properties of poly-
mer for electronics and aerospace applications.
Thermal properties
Thermal properties of the PEIs were determined by TGA
and DSC. The T
g
of the polymer was studied by means
of DSC at a heating rate of 10Cmin
1
under N
2
atmo-
sphere from 30C to 300C and the thermal analysis data
are summarized in Table 1. The T
g
of the PEIs are in the
range of 195–245C. No melting endotherm peak was
observed from DSC traces. This is the evidence for the
amorphous nature of the polymers. The introduction of
ether linkage into the polymer backbone resulted in poly-
mers with moderate T
g
values in comparision with T
g
val-
ues of polyimides as reported in other studies so far.
32–34
As expected, the T
g
values depend on the structure of the
dianhydride and diamine component in the polymer chain
and decreased with increasing flexibility of the PEIs. It is
evident that the bridging of carbonyl group between two
phenyl rings present in BTDA facilitated bond rotation and
reduced T
g
. Among the synthesized PEIs, the PEIs based on
BTDA (PEI-1, PEI-2, PEI-3, and PEI-4) have lower T
g
val-
ues than that of the PMDA based PEIs (PEI-5, PEI-6, PEI-
7, and PEI-8). This may be attributed to the presence of
rigid phenyl groups in the polymer backbone, which inhibit
the molecular motion and thus results in increase in T
g
value.
The introduction of long alkyl chains along the PEI
backbone would impart increased C–C bond rotation
thereby increasing the segmental mobility of the PEIs, thus
resulting in a reduction in the T
g
. All PEIs have the same
aromatic part, so the observed difference should only be
due to the different mode of package of the alkyl/aliphatic
part in the final polymer structure. A linear decrease in T
g
value is observed by increasing the number of methylene
groups (–CH
2
–) in the PEI backbone.
Figure 8 shows the TGA curves of the PEIs, evaluated
under N
2
atmosphere. The polymers show 5%weight loss
at temperatures ranging from 318Cto418
Casshownin
Table 1. Polyimides containing alkyl/aliphatic chains in
their backbone were found to be the less stable in higher
temperature. Thermal stability decreases as the length of
thermally fragile alkyl chain increased. Flexible oxygen
linkage also decreased the thermal stability. The presence
of phenoxy group containing long alkyl chain structure
showed properties in between those of aromatic and ali-
phatic polyimides. Due to low degree of intermolecular
interaction, thermal stability of the PEIs derived from dia-
mines BAPPt and BAPHx were found to be low. PMDA
(rigid structure of aromatic dianhydride)-containing PEIs
show better thermal stability than BTDA-based PEIs
because of the carbonyl bridging group between the two
phenyl rings in the backbone in BTDA, which increased
the flexibility of the polymer. The thermal analysis
showed that PEI-2 and PEI-6 exhibit superior thermal
properties due to the short alkyl chain presence in the
polymer structure.
In general, the char yield decreases with increasing alkyl
character of the polyimide structure.
35
The char yield at
800C under N
2
atmosphere varies from 29%to 43%for all
the PEIs, the highest char yield of PEIs is due to aromatic
rigid structure and shorter alkyl chain.
The limiting oxygen index (LOI) values which can be
taken as an indicator to evaluate the polymer’s flame
retardancy were measured and are shown in Table 1.
Char yield of a material can be used to estimate LOI
according to Van Krevelen and Hoftyzer equation.
36
The
char yield of the PEIs was found to be high in the range
of 29.1–34.7%indicating high flame retardancy, which
is expressed in terms of their LOI value. The LOI values
of the polymers should be above the threshold value of
26 to render them self-extinguishing and for their quali-
fication for many applications requiring good flame
resistance. It was found that the LOI increases with
increasing char yield as expected. The prepared PEIs
show LOI value greater than 26 confirming their good
flame-retardant properties.
Solubility
The solubility behavior of the PEIs via both thermal and
chemical imidization was determined by dissolving pow-
dery polymer samples at a concentration of 5%(w/v) in a
number of organic solvents and the results are summar-
ized in Table 2. It was observed that the polyimides
Figure 5. FTIR spectra of PEIs. FTIR: Fourier transform infrared
spectroscopy; PEI: poly(ether imide).
Balasubramanian et al. 765
prepared by chemical imidization have better solubility
than those prepared by thermal imidization. The lower
solubility of the thermally imidized PEIs could be
explained by the imidization reaction mechanism. PAA
should be a linear polymer with aromatic amide, but this
PAA is an unstable polymer. The hydroxyl group of the
carboxylic acid and the amino group of amide in PAA
either formed aromatic imide by further dehydration or
again formed aromatic anhydride and amine groups by
a nucleophilic substitution reaction.Thearomaticamine
formed however could react with the electrophilic carbo-
nyl group present in the PAA chain during the heating
process to form an imine structure that results in
cross-linked PEI.
37–40
In addition, intermolecular imidi-
zation reaction among PAA would easily occur and may
result in the intermolecular cross-linking. Therefore, the
PEIs obtained by thermal imidization would show inso-
lubility in organic solvents at room temperature because
Figure 6.
1
H-NMRspectraof(a)PEI-1,(b)PEI-2, (c) PEI-3, and (d) PEI-4.
1
H-NMR: proton nuclear magnetic resonance; PEI: poly(ether imide).
766 High Performance Polymers 27(6)
Figure 7. Synthesis of PEIs. PEI: poly(ether imide).
Table 1. Thermal properties, dielectric properties, and water absorption of PEIs.
Polymer T
g
(C) T
5%
T
10%
Char yield (%) LOI values
Dielectric constant
("000: 1 MHz)
Dielectric loss
("00: 1 MHz) Water absorption (%)
PEI-1 230 353 388 35.0 31.5 3.10 0.35 0.37
PEI-2 216 393 423 32.0 30.3 2.88 0.14 0.42
PEI-3 204 318 352 29.0 29.1 2.80 0.17 0.62
PEI-4 195 323 358 34.1 31.1 2.68 0.12 0.43
PEI-5 245 388 428 38.3 33.0 3.17 0.24 0.45
PEI-6 232 418 448 43.0 34.7 3.06 0.19 0.48
PEI-7 224 353 393 30.3 29.6 2.82 0.31 0.56
PEI-8 202 373 408 36.0 32.0 2.77 0.26 0.40
PEI: poly(ether imide); T
g
: glass transition temperature; LOI: limiting oxygen index; T
5%
: 5% weight loss temperature; T
10%
: 10% weight loss temperature.
Balasubramanian et al. 767
of some cross-link. In chemical imidization using acetic
anhydride and triethylamine as a dehydrating agent and
catalyst, respectively, acetic anhydride would react with
carbonyl group in PAA to form new anhydride group
that reacts with the amide group in PAA to form a linear
PEI structure by releasing acetic acid via nucleophilic
reaction. The presence of flexible ether bridge segments
in PEIs enhances the rotational freedom thereby enhan-
cing the solubility. PMDA-based PEIs (PEI 5–8) show
lower solubility than the BTDA based PEIs (PEI 1–4)
due to the inherent rigidity and strong intermolecular
interaction.
Mechanical properties
All the PEIs afford good quality and creasable films. The
mechanical properties of the flexible films are summarized
in Table 3. They showed an appreciable tensile strength
varying from 77 MPa to 98 MPa, elongation to break from
8%to 13%and tensile modulus in the range of 1.5–2.2
GPa. The PMDA-based PEIs exhibit higher modulus due
to the inherent rigidity of the pyromellitic unit. PEI-3,
PEI-4, PEI-7, and PEI-8 with flexible, linear long alkyl
chain containing diamine exhibited the lowest tensile
strength and highest elongation at break. These results indi-
cate that PEIs films have good mechanical properties.
41,42
Dielectric properties and water absorption
The dielectric constant, dielectric loss, and water absorp-
tion were measured, and the results are summarized in
Table 1, Figures 9 and 10. The dielectric constant of the
synthesized PEIs is in the range of 2.68–3.17 at 1 MHz
and dielectric loss is in the range of 0.12–0.35 at 1 MHz,
which is fairly low in comparison to that of fluorinated
polyimides and hetrocyclic group containing polyimides.
1,43
The dielectric properties are directly related to the polari-
zability and strongly dependent on the chemical structure
of the PEI. Polarization is the alignment of permanent or
induced atomic or molecular dipole moments with an
externally applied electric field. All materials possess
electric polarization by contributions by dipole or ions
or both and will depend on the chemical structure of the
materials.
Structure–property relationships for the dielectric con-
stant were also investigated. The dielectric constant ("0)is
defined as the ratio of the dielectric constant of the material
to that of free space; its value varies with frequency, tem-
perature, and water content within the material. Both the
dielectric constant and dielectric loss of the synthesized
PEIs decrease with increase in the alkyl chain length in the
polymer. This is because of the presence of fewer polariz-
able functional groups. In addition, the phenylene ether
units in diamine that induced the dilution effect of the polar
imide ring may have contributed to the decrease in the
dielectric constant.
12,44
The moisture absorption properties of polymers are of
great importance with regard to their practical use in micro-
electronics. The absorbed water in the polymer structure
affects their performance and long-term stability. The PEI
film placed in water bath at 25C for 1 week, after that each
sample was first removed from water and dried with a tis-
sue before weighing. The percentage of weight change for
the specimen was determined using the following equation:
Weight change ð%Þ¼W2W1
W1
100;
where, W
1
and W
2
are the weight (in grams) of the speci-
men before and after immersion in water, respectively. The
moisture absorption values for synthesized PEI films under
atmospheric condition were found to be in the range of
0.37–0.62%(Table 1) respectively. The obtained values are
lower than those of non-fluorinated PEIs, such as Kapton
1
H.
45
This is due to the presence of water proofing alkyl
groups and ether linkage. These results indicate that the
PEIs possess an outstanding property to resist moisture
uptake.
Conclusions
A series of new PEIs have been synthesized by the reac-
tion between aromatic dianhydride (BTDA and PMDA)
and various aromatic diamines (BAPPr, BAPBu, BAPPt,
and BAPHx). As the alkyl content increased, the T
g
,
thermal stability and dielectric constant decreased. The
synthesized PEIs show high thermal stability as their
T
5%
are in the range of 336–411C with 29–43%of char
yield and the T
g
are in the range of 195–245C. They
have low dielectric constant in the range of 2.68–3.17
at1MHzandalsogoodmechanicalpropertieswithten-
sile strength of 77–98 MPa, elongation at break 8–13%,
Figure 8. TGA thermograms of PEIs. TGA: thermogravimetric
analysis; PEI: poly(ether imide).
768 High Performance Polymers 27(6)
and tensile modulus of 1.5–2.2 GPa. The prepared PEIs
arequitesolubleincommon organic solvents. These
results open a wide range of options for the optimization
of these PEIs to tailor their functionality to different
industrial applications.
Funding
The authors acknowledge the University grant commission, for
funding this project. The authors also acknowledge DST (FIST)
and UGC (SAP) for the financial support extended to procure
instrumental facilities.
Table 2. Solubility of chemically/thermally imidized polyetherimides.
Polymers NMP DMAc DMSO DMF Conc. H
2
SO
4
THF Acetone CHCl
3
PEI-1 þ/+þ/+þ/+þ/+þ/+þ/þ//
PEI-2 þ/+þ/+þ/+þ/+þ/+þ///
PEI-3 þ/+þ/+þ/+þ/+þ/+þ/þ//
PEI-4 þ/+þ/+þ/+þ/+þ/+þ///
PEI-5 þ/+þ/+þ/+þ/+þ/+þ///
PEI-6 þ/+þ/+þ/+þ/+þ/+þ/þ//
PEI-7 þ/+þ/+þ/+þ/+þ/+þ///
PEI-8 þ/+þ/+þ/+þ/+þ/+þ///
NMP: N-methyl-2-pyrrolidone; DMAc: N,N-dimethylacetamide; DMSO: dimethyl sulfoxide; DMF: N,N-dimethylformamide; conc. H
2
SO
4
: concentrated
sulfuric acid; THF: tetrahydrofuran; CHCl
3
: chloroform; PEI: poly(ether imide); þ: soluble at ambient temperature; : insoluble; +: partially soluble
upon heating.
Table 3. Inherent viscosity, molecular weight, and mechanical properties.
Polymer
inh
(dl g
1
)M
w
M
n
PDI
Tensile strength
(MPa)
Elongation at
break (%)
Tensile modulus
(GPa)
PEI-1 0.66 38,480 19,750 1.94 88 8 1.8
PEI-2 0.79 48,060 26,000 1.84 91 11 2.0
PEI-3 0.77 50,590 28,620 1.76 82 10 1.6
PEI-4 0.59 44,060 21,300 2.06 77 12 1.5
PEI-5 0.76 48,000 23,800 2.01 98 10 2.2
PEI-6 0.88 53,000 22,400 2.50 92 10 1.9
PEI-7 0.80 45,480 24,400 1.86 79 12 1.7
PEI-8 0.73 46,100 20,900 2.20 83 13 1.6
M
n
: number-average molecular weight; M
w
: weight-average molecular weight; PDI: polydispersity index.
Figure 9. Dielectric constant of PEIs. PEI: poly(ether imide). Figure 10. Dielectric loss of PEIs. PEI: poly(ether imide).
Balasubramanian et al. 769
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